Biofilm Reactors
Prepared by the Biofilm Reactors Task Force of the Water Environment Federation® Rhodes R. Copithorn, P.E., BCEE, Chair Joshua P. Boltz, Ph.D., P.E. Christine deBarbadillo, P.E. Paul Dombrowski, P.E., BCEE John R. Harrison, P.E. Sarah Hubbell Joseph A. Husband, P.E., BCEE Samuel Jeyanayagam, Ph.D., P.E., BCEE Ishin Kaya, P.Eng. Carl M. Koch, Ph.D., P.E., BCEE David J. Krichten Frank M. Kulick III
James P. McQuarrie, P.E. Robert Nerenberg, Ph.D., P.E. Heather M. Phillips, P.E. Dana W. Rippon, P.E. Frank Rogalla Edward D. Schroeder, Ph.D. Dipankar Sen, Ph.D., P.E. Spencer Snowling George Sprouse, Ph.D., P.E. Christopher W. Tabor, P.E. Stephen Tarallo Ifetayo Venner, P.E. Thomas E. Wilson, Ph.D., P.E., BCEE Stefan Wuertz, Ph.D.
Under the Direction of the Municipal Design Subcommittee of the Technical Practice Committee 2010 Water Environment Federation® 601 Wythe Street Alexandria, VA 22314–1994 USA http://www.wef.org
Biofilm Reactors
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WEF Manual of Practice No. 35 Prepared by the Biofilm Reactors Task Force of the Water Environment Federation®
., BCEE
WEF Press Water Environment Federation Alexandria, Virginia
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About WEF Formed in 1928, the Water Environment Federation® (WEF ®) is a not-for-profit technical and educational organization with members from varied disciplines who work toward WEF’s vision to preserve and enhance the global water environment. For information on membership, publications, and conferences, contact: Water Environment Federation® 601 Wythe Street Alexandria, VA 22314-1994 USA (703) 684-2400 http://www.wef.org
Manuals of Practice of the Water Environment Federation® The WEF Technical Practice Committee (formerly the Committee on Sewage and Industrial Wastes Practice of the Federation of Sewage and Industrial Wastes Associations) was created by the Federation Board of Control on October 11, 1941. The primary function of the Committee is to originate and produce, through appropriate subcommittees, special publications dealing with technical aspects of the broad interests of the Federation. These publications are intended to provide background information through a review of technical practices and detailed procedures that research and experience have shown to be functional and practical. Water Environment Federation Technical Practice Committee Control Group R. Fernandez, Chair J. A. Brown, Vice-Chair B. G. Jones, Past Chair A. Babatola L. W. Casson K. Conway V. D’Amato R. P. Dominak A. Ekster R. C. Johnson S. Moisio T. Page-Bottorff S. J. Passaro R. C. Porter E. P. Rothstein A. T. Sandy A. Tyagi A. K. Umble
Contents List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxv List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxxiii Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxxvii
Chapter 1 Introduction 1.0 Background and Purpose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 2.0 Characteristics of Fixed-Growth Processes . . . . . . . . . . . . . 2 3.0 History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3.1 Contact Beds. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3.2 Trickling Filters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3.3 Rotating Biological Contactors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3.4 Coupled Trickling Filter/Activated Sludge Process. . . . . . . . . . . . . . . . 6 3.5 Biological Filters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 3.6 Hybrid Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 4.0 Organization of Manual . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 5.0 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 6.0 SUGGESTED READINGS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Chapter 2 Biology of Fixed-Growth Process 1.0 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 2.0 CLASSIFICATION OF LIVING ORGANISMS . . . . . . . . . . . . . . . . . . . . . . 18 3.0 MICROORGANISMS OTHER THAN BACTERIA . . . . . . . . . . . . . . . . . . 21 3.1 Fungi. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 3.2 Algae. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 3.3 Protozoa. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 3.4 Multicellular Invertebrates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 vii
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3.5 Viruses. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 3.6 Consortia. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 4.0 CHARACTERISTICS OF BACTERIA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 4.1 Structure of the Bacterial Cell. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 4.2 Chromosome and Plasmids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 4.3 Cytoplasm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 4.4 Cell Membrane. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 4.5 Cell Wall. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 4.6 Pili. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 4.7 Flagella. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 4.8 Extracellular Polymeric Substances. . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 4.9 Chemical Composition of Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 4.10 Example 1—Theoretical Oxygen Demand of Bacterial Cells. . . . . . . 32 4.10.1 Solution. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 4.10.2 Comment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
5.0 B ACTERIAL METABOLISM, NUTRITION, AND RESPIRATION . . . . 33 5.1 Energy Source. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 5.2 Chemoheterotrophic Metabolism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 5.3 Chemoautotrophic Metabolism. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 5.4 Photosynthetic Metabolism. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 5.5 Nutrient Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 5.6 Bacterial Energy Metabolism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 5.7 Aerobic Growth and Respiration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 5.8 Anoxic Conditions and Respiration. . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 5.9 Anaerobic Respiration and Fermentative Metabolism. . . . . . . . . . . . 44 5.10 Energetics of Respiration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 5.11 Example 2—Calculation of Electrode Reduction Potentials and ∆G for Half-Reactions ��������������������������������������������������������������������48 5.12 Solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 5.13 Co-Metabolism. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
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6.0 BACTERIAL GROWTH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 6.1 The Bacterial Growth Curve. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 6.2 Growth in Mixed Cultures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 6.3 Enrichment Cultures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 6.4 Stability of Mixed Cultures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 6.5 Effects of Environmental Variables. . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 7.0 BACTERIAL GROWTH KINETICS IN BIOFILMS . . . . . . . . . . . . . . . . . 55 7.1 Rate of Bacterial Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 7.2 Note . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 7.3 Physical and Chemical Changes in Biofilms Resulting from Growth��������������������������������������������������������������������������������������������59 7.4 Structured Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 7.5 Temperature Effects. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 7.6 Example 3—Effect of Temperature on Organic Removal . . . . . . . . . 61 7.7 Solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 7.8 Inhibition and Toxicity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 7.9 Mass-Transfer-Rate Limitations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 8.0 KEY TRANSFORMATIONS IN BIOFILMS . . . . . . . . . . . . . . . . . . . . . . . 65 8.1 Chemoheterotrophic Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 8.2 Chemoautotrophic Processes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 8.3 Biology of Nitrogen Transformations. . . . . . . . . . . . . . . . . . . . . . . . . . 67 8.4 Denitrification. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 8.5 Aerobic Nitrification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 8.6 Anoxic Nitrification/Denitrification. . . . . . . . . . . . . . . . . . . . . . . . . . . 69 8.7 Biological Phosphorus Removal. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 8.8 Sulfide and Sulfur Oxidation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 8.9 Hydrogen Oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 9.0 FEATURES OF MICROBIAL COMMUNITIES IN BIOFILMS . . . . . . . 72 10.0 REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
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Chapter 3 Trickling Filter and Combined Trickling Filter Suspended-Growth Process Design and Operation 1.0 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 2.0 General Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 2.1 Distribution System. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 2.2 Biofilm Carriers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 2.3 Containment Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 2.4 Underdrain System and Ventilation. . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 2.5 Trickling Filter Pumping Stations: Influent and Recirculation. . . . . . 91 2.6 Hydraulic and Contaminant Loading. . . . . . . . . . . . . . . . . . . . . . . . . . . 92 3.0 Process Flow Sheets and Bioreactor Configuration . . 93 3.1 Standard Process Flow Diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 3.2 Bioreactor Classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 3.3 Hydraulic Application: Effect on Media Wetting, Flow Distribution, and Control ��������������������������������������������������������������98 4.0 Ventilation and Air Supply Alternatives . . . . . . . . . . . . . . 100 4.1 Natural Draft. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 4.2 Mechanical Ventilation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 5.0 Trickling Filter Process Models . . . . . . . . . . . . . . . . . . . . . . . . 103 5.1 National Research Council. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 5.2 Galler and Gotaas. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 5.3 Kincannon and Stover. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 5.4 Velz. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 5.5 Schulze. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 5.6 Germain. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108 5.7 Eckenfelder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 5.8 Chartered Institution of Water and Environmental Management ����������������������������������������������������������������110 5.9 Logan Trickling Filter Model. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
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5.10 Selecting a Trickling Filter Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 5.11 Method for Combining Trickling Filter and Suspended-Growth Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113 6.0 Process Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 6.1 Combined Carbon Oxidation and Nitrification. . . . . . . . . . . . . . . . . . 115 6.2 Nitrifying Trickling Filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120 6.2.1 Gujer and Boller Nitrifying Trickling Filter Model. . . . . . . . . . . . . . . . . . . . . 122 6.2.2 Okey and Albertson Nitrifying Trickling Filter Model. . . . . . . . . . . . . . . . . . 124 6.2.2.1 Application of the Gujer and Boller Model. . . . . . . . . . . . . . . . . . . . . . 126 6.2.2.2 Application of the Albertson and Okey Model . . . . . . . . . . . . . . . . . . . 128
6.3 Temperature and Hydraulic Application Effects . . . . . . . . . . . . . . . . 131 7.0 Design Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 7.1 Distribution System. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133 7.1.1 Hydraulic Drive Rotary Distributors.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134 7.1.2 Electronic or Mechanical Drive Rotary Distributors. . . . . . . . . . . . . . . . . . . . 136 7.1.3 Optimizing Rotary Distributor Operation. . . . . . . . . . . . . . . . . . . . . . . . . . . . 138
7.2 Construction of Rotary Distributors . . . . . . . . . . . . . . . . . . . . . . . . . . . 138 7.3 Trickling Filter Media Selection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139 7.3.1 Depth. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141 7.3.2 Structural Integrity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
7.4 Trickling Filter Pumping Station or Dosing Siphon. . . . . . . . . . . . . . 144 7.5 Control Mechanisms for Trickling Filter Macro Fauna . . . . . . . . . . . 144 7.5.1 Operational Strategies and Facility Improvements for Macro Fauna Control . . . 145 7.5.2 Spülkraft. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147 7.5.3 Flooding.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148 7.5.4 Chemical Treatment.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149 7.5.5 Physical Control����������������������������������������������������������������������������������������������152
7.6 Trickling Filter Startup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153 7.7 Combined Trickling Filter and Suspended-Growth Processes. . . . . 155 7.7.1 Activated Biofilter.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156 7.7.2 Trickling Filter/Solids Contact. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
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7.7.3 Roughing Filter/Activated Sludge.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162 7.7.4 Biofilter/Activated Sludge.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162 7.7.5 Trickling Filter/Activated Sludge. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162
8.0 REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163
Chapter 4 Rotating Biological Contactors 1.0 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174 2.0 Process design considerations . . . . . . . . . . . . . . . . . . . . . . . . . 178 2.1 Media Surface Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179 2.2 pH and Nutrient Balance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180 2.3 Oxygen Transfer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180 2.4 Flow and Loading Variability. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182 2.5 Operating Temperature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183 2.6 Solids Production. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183 2.7 Toxic and Inhibitory Substances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184 3.0 Rotating biological contactor design methods . . . . . 184 3.1 Monod Kinetic Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184 3.2 Second-Order Model. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186 3.3 Empirical Model. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187 3.4 Manufacturers’ Design Curves. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189 3.5 Comparison of Model Predictions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190 3.6 Predicted Performance versus Full-Scale Data . . . . . . . . . . . . . . . . . . 191 3.7 Temperature Correction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192 4.0 Rotating biological contactor nitrification models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194 5.0 Denitrification Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197 6.0 Physical design features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198 6.1 Physical Layout. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198 6.2 Tank Volume. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198 6.3 Hydraulics and Flow Control. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198 6.4 Media. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199
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6.5 Drive Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200 6.6 Covers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201 6.7 Biomass Control. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201 7.0 Rotating biological contactor design examples . . . 202 7.1 Secondary Treatment Design Example. . . . . . . . . . . . . . . . . . . . . . . . 202 7.2 Advanced Secondary Treatment Design Example. . . . . . . . . . . . . . 203 8.0 Problems and corrective actions . . . . . . . . . . . . . . . . . . . . . 204 8.1 Inadequate Treatment Capacity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205 8.2 Excessive First-Stage Loadings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205 8.3 Excessive Biomass Growth. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206 8.4 Loping of Air-Drive Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206 8.5 High Clarifier Effluent Suspended Solids. . . . . . . . . . . . . . . . . . . . . . 207 8.6 Corrosion of Media Supports. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207 9.0 Pilot-plant studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207 10.0 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208
Chapter 5 Moving-Bed Biofilm Reactors 1.0 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212 2.0 Moving-Bed Reactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213 3.0 Design Considerations for Moving-Bed Reactors . . . . . 217 3.1 Carrier Biofilms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 218 3.1.1 Carbonaceous Matter Removal. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220 3.1.2 High-Rate Designs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220 3.1.3 Normal-Rate Designs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221 3.1.4 Low-Rate Designs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221 3.1.5 Nitrification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224 3.1.6 Denitrification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229 3.1.6.1 Pre-Denitrification Moving-Bed Biofilm Reactors.. . . . . . . . . . . . . . . . 229 3.1.6.2 Post-Denitrification Moving-Bed Biofilm Reactors . . . . . . . . . . . . . . . 230 3.1.6.3 Combined Pre-/Post-Denitrification Moving-Bed Biofilm R eactors������������������������������������������������������������������������������������230
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3.2 Mixers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231 3.3 Pretreatment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232 4.0 Solids Separation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233 5.0 General considerations for moving-bed biofilm reactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234 5.1 Approach Velocity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234 5.2 Foaming. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234 5.3 Media Transfer and Inventory Management. . . . . . . . . . . . . . . . . . . . 234 6.0 CASE STUDIES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235 6.1 Moa Point Wastewater Treatment Plant, Wellington, New Zealand��������������������������������������������������������������������������������������������235 6.2 Harrisburg Wastewater Treatment Plant, Harrisburg, Pennsylvania��������������������������������������������������������������������������������������������238 6.3 Moorhead Wastewater Treatment Facility, Moorhead, Minnesota (Adapted from Zimmerman et al., 2004)��������������������������240 6.4 Williams Monaco Wastewater Treatment Plant, Henderson, Colorado ��������������������������������������������������������������������������������������������������241 6.5 Klagsham Wastewater Treatment Plant, Malmö, Sweden (Adapted from Taljemark et al., 2004)��������������������������������������������������246 6.6 Gardemoen Wastewater Treatment Plant, Gardemoen, Norway. . . . . 250 7.0 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253
Chapter 6 Hybrid Processes 1.0 OVERVIEW OF INTEGRATED FIXED-FILM ACTIVATED SLUDGE SYSTEMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260 1.1 Advantages. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 261 1.2 Disadvantages. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262 2.0 Media Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262 2.1 Fixed-Media Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263 2.2 Free-Floating Media Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264 2.2.1 Plastic. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264 2.2.2 Sponge. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264
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3.0 History of Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265 4.0 Application of INTEGRATED FIXED-FILM ACTIVATED SLUDGE Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267 4.1 Fixed Media. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267 4.1.1 General Requirements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267 4.1.2 Growth on Media. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 268 4.1.3 Kinetics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 269 4.1.4 Worm Growth. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 270 4.1.5 Media Breakage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 270 4.1.6 Dissolved Oxygen Level. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 270 4.1.7 Mixing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271 4.1.8 Access to Diffusers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271 4.1.9 Odor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271
4.2 Free-Floating Media—Sponge Media. . . . . . . . . . . . . . . . . . . . . . . . . . 272 4.2.1 General Requirements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272 4.2.2 Screen Clogging. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272
4.3 Control of Biomass Growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273 4.3.1 Loss of Sponges. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273 4.3.2 Taking Tank Out-of-Service. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274 4.3.3 Loss of Solids. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274 4.3.4 Air Distribution System. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274 4.3.5 Plastic Media . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274 4.3.5.1 General Requirements.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274 4.3.5.2 Biomass Growth.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275 4.3.5.3 Media Mixing.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276 4.3.5.4 Screens. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276 4.3.5.5 Foaming. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276 4.3.5.6 Media Replacement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276 4.3.5.7 Taking Tank Out-of-Service.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276 4.3.5.8 Worm Growth.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 277 4.3.5.9 Startup.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 277
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5.0 Process Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 277 5.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 277 5.2 Parameters Influencing Organics Removal in the Biofilm of Integrated Fixed-Film Activated Sludge Systems������������������������������278 5.2.1 Biofilm Flux Rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278 5.2.2 Removals in Biofilm per Unit of Tank Volume . . . . . . . . . . . . . . . . . . . . . . . . 278
5.3 Parameters Influencing Removals in the Mixed-Liquor Suspended Solids������������������������������������������������������������������������������������281 5.4 Interaction Between the Mixed-Liquor Suspended Solids and the Biofilm����������������������������������������������������������������������������������������282 5.5 Interaction Between Heterotrophs and Nitrifiers. . . . . . . . . . . . . . . . 284 5.6 Design Tools/Procedures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284 5.6.1 Empirical Methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285 5.6.1.1 Equivalent-Sludge-Age Approach. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285 5.6.1.2 Quantity (Length or Web Surface Area) of Media Approach. . . . . . . . 286 5.6.2 Rates Based on Pilot Studies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 286 5.6.3 Biofilm Rate Model. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287 5.6.3.1 Define Range of Flux Rates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287 5.6.3.2 Quantify Removal at Different Mixed-Liquor Suspended Solids Mean Cell Residence Times ������������������������������������������������������287 5.6.3.3 Select Flux Rates Based on Location Along Aerobic Zone. . . . . . . . . . 287 5.6.3.4 Calculate the Quantity of Media Required. . . . . . . . . . . . . . . . . . . . . . 288 5.6.3.5 Additional Analysis to Finalize a Design. . . . . . . . . . . . . . . . . . . . . . . 288 5.6.3.6 Application of Kinetics-Based Approach with Integrated Fixed-Film Activated Sludge Design Software������������������������������������288
6.0 Case Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 288 6.1 Annapolis Water Reclamation Facility, Anne Arundel County, Maryland��������������������������������������������������������������������������������������������������288 6.1.1 Original Wastewater Treatment Plant. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 289 6.1.2 Pilot Study (1993 to 1996). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 289 6.1.3 Full-Scale Upgrade for Biological Nutrient Removal (1997 to 2000). . . . . . . 291 6.1.3.1 Pilot Study. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291
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6.1.3.2 During Construction (1997 to 2000) .. . . . . . . . . . . . . . . . . . . . . . . . 293 6.1.3.3 Post-Construction (2000 to 2003). . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299
6.2 Westerly Wastewater Treatment Plant, Westerly, Rhode Island . . . . . 299 6.2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299 6.2.2 Description of Original Facilities. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299 6.2.3 Description of Upgrade. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299 6.2.4 Design Criteria. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 302 6.2.5 Performance of the Integrated Fixed-Film Activated Sludge System. . . . . . . 302 6.2.6 Operational Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306 6.2.7 Costs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307
6.3 Broomfield Wastewater Treatment Plant, Broomfield, Colorado . . . 307 6.3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307 6.3.2 Full-Scale Plant Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 308
6.4 Colony Wastewater Treatment Plant, Colony, Texas. . . . . . . . . . . . . 308 6.4.1 Introduction and Background. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 308 6.4.2 Changing Design Conditions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 316 6.4.3 Plant Construction and Operation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 318 6.4.4 System Flexibility. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 320 6.4.5 Redworm Predation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321
7.0 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321
Chapter 7 Biological Filters 1.0 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 327 2.0 Descriptions of BIOLOGICALLY ACTIVE FILTER Reactors and Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 329 2.1 Brief History of Biologically Active Filters. . . . . . . . . . . . . . . . . . . . . . 329 2.2 Downflow Biologically Active Filter with Sunken Media. . . . . . . . . 331 2.3 Upflow Biologically Active Filter with Sunken Media. . . . . . . . . . . . 334 2.4 Upflow Biologically Active Filter with Floating Media. . . . . . . . . . . 335 2.5 Moving-Bed, Continuous Backwash Filters. . . . . . . . . . . . . . . . . . . . . 337 2.6 Non-Backwashing, Open-Structure Media Filters . . . . . . . . . . . . . . . 339
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3.0 Media for Use in BIOLOGICALLY ACTIVE FILTERS . . . . . . . . . . . 341 3.1 Mineral Media. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 341 3.2 Random Plastic Media. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 343 3.3 Modular Plastic Media. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 345 4.0 Backwashing and Air-Scouring . . . . . . . . . . . . . . . . . . . . . . . . . 345 5.0 BIOLOGICALLY ACTIVE FILTER Process Design . . . . . . . . . . . . . 349 5.1 Process Design for Secondary Treatment. . . . . . . . . . . . . . . . . . . . . . . 351 5.1.1 Volumetric Biochemical Oxygen Demand Loading. . . . . . . . . . . . . . . . . . . . . 351 5.1.2 Hydraulic Loading. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 351 5.1.3 Backwashing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 351 5.1.4 Design Example: Design of a Submerged, Upflow Biological Aerated Filter System for Secondary Treatment (No Nitrification) ��������������353 5.1.5 Solution. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 353
5.2 Process Design for Nitrification. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355 5.2.1 Influence of Hydraulic Filtration Rates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355 5.2.2 Effect of Process Air Velocity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 357 5.2.3 Dependence on Loading Conditions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 359 5.2.4 Temperature Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 360 5.2.5 Design Example: Design of a Submerged, Upflow Biological Aerated Filter System for Nitrification Following Secondary Treatment��������������������360 5.2.6 Solution. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 361
5.3 Process Design for Combined Nitrification and Denitrification. . . . 362 5.4 Process Design for Tertiary Denitrification . . . . . . . . . . . . . . . . . . . . . 365 5.4.1 Volumetric Mass Loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 365 5.4.2 Half-Order Kinetic Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 367 5.4.3 Hydraulic Loading. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 368 5.4.4 Solids Removal and Sludge Production. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 368 5.4.5 Supplemental Carbon Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 369 5.4.6 Tertiary Denitrification Typical Operations Issues and Corrective Actions�������������������������������������������������������������������������������������������� 370 5.4.6.1 Excess Backwashing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 370 5.4.6.2 Gas (Nitrogen) Accumulation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 371
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5.4.6.3 Solids Breakthrough. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 371 5.4.6.4 Nitrate/Nitrite Breakthrough ��������������������������������������������������������������371 5.4.6.5 Carbon Breakthrough. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 372 5.4.6.6 Phosphorus Management.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 372 5.4.6.7 Operation During Peak Flow Events . . . . . . . . . . . . . . . . . . . . . . . . . . 372
5.5 Phosphorus Removal Considerations for Biologically Active Filter Processes����������������������������������������������������������������������������������������373 6.0 Design Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 374 6.1 Preliminary and Primary Treatment. . . . . . . . . . . . . . . . . . . . . . . . . . . 374 6.2 Backwash Handling Facilities. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 374 6.3 Biologically Active Filter Process Aeration . . . . . . . . . . . . . . . . . . . . . 375 6.3.1 Oxygen-Transfer Efficiency. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375 6.3.2 Process Air Distribution Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 377 6.3.3 Process Air Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 377
6.4 Supplemental Carbon Feed Requirements. . . . . . . . . . . . . . . . . . . . . . 378 7.0 Biologically active filter Case Studies . . . . . . . . . . . . . . . . 379 7.1 Chemically Enhanced Primary Treatment Followed by Two-Stage Biologically Active Filter for Total Nitrogen Removal: VEAS Wastewater Treatment Plant, Oslo, Norway ��������379 7.2 Chemically Enhanced Primary Treatment Followed by Three-Stage Biologically Active Filter for Total Nitrogen Removal: Siene Centre Wastewater Treatment Plant, Colombes, France��������������������������������381 7.3 Total Nitrogen Removal in a Single-Stage Biologically Active Filter: Frederikshavn Wastewater Treatment Plant, Denmark��������384 7.4 Nitrification and Denitrification: West Warwick, Rhode Island��������������������������������������������������������������������������������������������387 7.5 Post-Denitrification Sand Filters: Havelock, North Carolina. . . . . . 389 8.0 REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 391
Chapter 8 New and Emerging Fixed-Film Technologies 1.0 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 401 2.0 BIOFILM REACTORS WITH SUSPENDED CARRIERS OR GRANULES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 402
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2.1 Biofilm Airlift Suspension Reactor . . . . . . . . . . . . . . . . . . . . . . . . . . . 402 2.2 Upflow Anaerobic Sludge Blanket . . . . . . . . . . . . . . . . . . . . . . . . . . . 404 2.3 Expanded Granular Sludge Blanket . . . . . . . . . . . . . . . . . . . . . . . . . . 404 2.4 Internal Circulation Reactor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 404 3.0 ANAMMOX BIOFILM REACTORS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 405 4.0 MEMBRANE BIOFILM REACTORS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 406 5.0 REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 408
Chapter 9 Clarification 1.0 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 414 2.0 Solids-Separation Choices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 416 3.0 Design Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 417 3.1 Types of Settling Regimes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 418 3.1.1 Type I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 419 3.1.2 Type II. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 419 3.1.3 Type III. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 419 3.1.4 Type IV. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 419
3.2 Special Considerations for Nutrient Removal Sludges. . . . . . . . . . . 3.3 Clarifier Enhancements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 Wastewater Flocculation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5 Flocculation Criteria. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6 Clarifier Design Details. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
419 420 422 424 429
3.6.1 Influent Column. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 431 3.6.2 Energy-Dissipating Inlet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 431 3.6.3 Feed Well (Flocculating Type). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 433 3.6.4 Side Water Depth, Clear Water Zone, and Overflow Rate . . . . . . . . . . . . . . . 435 3.6.5 Floor Slope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 438 3.6.6 Effluent Weir and Launder. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 440 3.6.7 Sludge Collectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 443 3.6.8 Sludge Hopper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 445
3.7 Rectangular versus Circular Clarifiers . . . . . . . . . . . . . . . . . . . . . . . . . 445
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3.8 Design Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 446 3.9 Clarifier Following Moving-Bed Biofilm Reactor, Trickling Filter, Rotating Biological Contactor, and Biotower��������������������������448 3.9.1 Secondary (Integrated Fixed-Film Activated Sludge) Clarifiers. . . . . . . . . . . 451 3.9.2 Sludge Hopper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 454 3.9.3 Process Performance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 455
3.10 Other Considerations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 458 3.10.1 Modeling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 458 3.10.2 Interaction with Other Facilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 458 3.10.3 International Practices. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 458
4.0 REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 459
Chapter 10 Effluent Filtration 1.0 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 463 2.0 Process performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 465 3.0 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 469
Chapter 11 Development and Application of Models for Integrated Fixed-Film Activated Sludge, Moving-Bed Biofilm Reactors, Biological Aerated Filters, and Trickling Filters 1.0 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 473 2.0 MODELING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 478 2.1 Numerical Approach Using Semi-Empirical Equations for Biofilm (Steady-State and Dynamic Simulation)��������������������������������478 2.1.1 Ammonium-Nitrogen Uptake Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 480 2.1.1.1 Ammonium-Nitrogen Uptake Rate by Nitrifiers in Biofilm. . . . . . . . . 480 2.1.1.2 Biofilm Nitrification Rates from Pilot Studies . . . . . . . . . . . . . . . . . . . 482 2.1.1.3 Ammonium-Nitrogen Uptake Rate by Nitrifiers in Mixed-Liquor Volatile Suspended Solids ��������������������������������������������485 2.1.1.4 Mass Balance for Ammonium-Nitrogen in Each Reactor. . . . . . . . . . . 490 2.1.2 Chemical Oxygen Demand Removal. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 494
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2.1.3 Biomass Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 499 2.1.3.1 Mixed-Liquor Volatile Suspended Solids . . . . . . . . . . . . . . . . . . . . . . . 500 2.1.3.2 Biofilm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 501 2.1.4 Fraction of Nitrifiers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 502 2.1.5 Denitrification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 503 2.1.6 Oxygen. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 504
2.2 Numerical Approach to Solve One- and Two-Dimensional BiofilmDiffusion Models (Steady-State and Dynamic Simulation)��������������504 2.2.1 Ammonium-Nitrogen. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 506 2.2.2 Linkage to Equations 11.1 to 11.42 Presented Earlier. . . . . . . . . . . . . . . . . . . 508 2.2.3 Chemical Oxygen Demand, Biomass (Volatile and Total Suspended Solids), Dissolved Oxygen, and NOx-N ��������������������������������������������������������510 2.2.4 Biofilm Thickness, Growth, and Fraction Nitrifiers. . . . . . . . . . . . . . . . . . . . . 510
3.0 MODEL APPLICATIONS TO FULL-SCALE FACILITIES . . . . . . . . . . . 512 3.1 Integrated Fixed-Film Activated Sludge Plant Description and Modeling ������������������������������������������������������������������������������������������513 3.1.1 Integrated Fixed-Film Activated Sludge Plant Description. . . . . . . . . . . . . . 514 3.1.2 Integrated Fixed-Film Activated Sludge Plant Operation. . . . . . . . . . . . . . . . 515 3.1.2.1 Data from December 2006. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 515 3.1.2.2 Flow and Recycle. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 515 3.1.2.2.1 Primary Effluent.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 515 3.1.2.2.2 Aerobic Cells. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 518 3.1.2.2.3 Secondary/Plant Effluent. . . . . . . . . . . . . . . . . . . . . . . . . . . . 518 3.1.2.2.4 Discussion of the Data.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 518 3.1.3 Modeling Integrated Fixed-Film Activated Sludge in Aquifas . . . . . . . . . . . . 518 3.1.3.1 Results from Aquifas. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 519 3.1.3.2 Key Inputs to Aquifas Biofilm One-Dimensional Model . . . . . . . . . . . 526 3.1.3.3 Discussion of Aquifas Model and Accuracy of Results. . . . . . . . . . . . . 526 3.1.4 Modeling in BioWin. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 527 3.1.4.1 Framework. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 527 3.1.4.2 Results from BioWin.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 531 3.1.4.3 Discussion of Results from BioWin. . . . . . . . . . . . . . . . . . . . . . . . . . . . 531
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3.2 Moving-Bed Biofilm Reactor Plant Description and Modeling. . . . . 534 3.2.1 Moving-Bed Biofilm Reactor Modeling with GPS-X. . . . . . . . . . . . . . . . . . . . 537 3.2.1.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 537 3.2.1.2 Example. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 542 3.2.2 Moving-Bed Biofilm Reactor Modeling with Aquifas. . . . . . . . . . . . . . . . . . . 542 3.2.3 Moving-Bed Biofilm Reactor Modeling—General Comments. . . . . . . . . . . . . 543 3.2.4 Integrated Fixed-Film Activated Sludge and Moving-Bed Biofilm Reactor Modeling—General Observations������������������������������������������������������552
4.0 REFERENCEs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 553
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 559
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List of Figures Figure 2.1 2.2 2.3 2.4 2.5 2.6
2.7 2.8
2.9
2.10
2.11 3.1 3.2
Page
Coenocytic cell structure typical of many fungi, in which cell contents, or cytoplasm, is multinucleate and continuous throughout the organism. . . . . . . . . . . . . 22 Schematic representation of the bacterial cell.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 Examples of organic compounds that are toxic and/or difficult to biooxidize.. . . . . . 37 Repeating glucose units of starch and glycogen connected by α-glycosidic bonds and cellulose connected by β-glycosidic bonds. . . . . . . . . . . . . . . . . . . . . . . . . . . 38 Schematic of reactions through which pyrite [FeS2(s)] is oxidized in mine drainage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 Bacterial growth curve for a batch system in which nutrients initially are not limiting. As growth begins, the increase in cell number (or mass) is dependent only on the number (or mass) of cells present, and the system behaves in a first-order autocatalytic manner.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 Relationships between organic removal, cell mass production, and OUR.. . . . . . . . . . 58 Structured models of metabolism include reactions or steps in the metabolic process that can be identified or separated: (a) the rate of metabolism is separated from the rate of organic removal from the liquid and (b) a somewhat more complex storage mechanism is shown, which could be used to account for several observed rates of growth and oxygen uptake.. . . . . . . . . . . . . . . . . . . . . . . . . . . 60 Effect of temperature on organic removal rate and microbial growth in example 3. Initial BODU and suspended solids concentrations were 400 mg/L and 30 mg/L, respectively. The initial conditions provide for near-exponential growth because K << Co.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 Effect of inhibitant concentration on specific removal rate for two conditions: (a) substrate inhibition by a metabolizable inhibitant, represented by eq 2.22, and (b) a nonmetabolizable inhibitant, such as a heavy metal, represented by eq 2.23.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 Steps in the removal and breakdown of an organic particle in a biofilm.. . . . . . . . . . . 66 Typical trickling filter components and cross-section.. . . . . . . . . . . . . . . . . . . . . . . . . . . 85 Hydraulically driven rotary distributors use variable frequency drive controlled gates that either open or close distributor orifices, which adjust with varying pumped flowrates to maintain a constant preset rotational speed, and electrically driven rotary distributor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
xxv
xxvi List of Figures
3.3
3.4
3.5
3.6 3.7
3.8
3.9 3.10 3.11 3.12 3.13 3.14 3.15 3.16
3.17 3.18
3.19 3.20 3.21 3.22
Adjustable plastic stanchions and FRP grating on the concrete floor of a bolted steel containment structure, and an HDPE mat used to support random synthetic media.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 Typical flow diagrams for the trickling filter process: (a) and (b) single-stage trickling filter process, (c) two-stage trickling filter process, and (d) two-stage trickling filter process with intermediate clarification. . . . . . . . . . . . . . . . . . . . . . . . . . . 94 Typical flow diagrams for trickling filter-suspended growth processes: (a) activated biofilter, (b) trickling filter/solids contact or trickling filter/activated sludge, (c) activated biofilter/solids contact or activated biofilter/activated sludge, and (d) trickling filter and activated sludge.. . . . . . . . . . . . 95 Three-dimensional representation of liquid flow distribution in cross-flow media, both (a) with and (b) without biofilm.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 (a) Trickling filter underdrain and media supports with air distribution piping throughout the underdrain, and (b) typical low-pressure fan applied to provide process air.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 Effect of SRT in a suspended-growth reactor and nitrification efficiency in an upstream trickling filter on effluent ammonia-nitrogen concentration in a combined TF/SG reactor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 Nitrification efficiency versus organic loading in rock-media trickling filters. . . . . . 116 Nitrification efficiency as a function of filtered, or soluble, BOD5 in the effluent of a vertical-flow media trickling filter at Stockton, California.. . . . . . . . . . . 117 Nitrification efficiency versus organic loading in trickling filters.. . . . . . . . . . . . . . . . 118 Nitrification efficiency versus organic loading in trickling filters.. . . . . . . . . . . . . . . . 119 Effect of bulk liquid TSS concentration on nitrification in a pilot-scale NTF.. . . . . . . 122 Transition NH3-N concentrations as function of temperature.. . . . . . . . . . . . . . . . . . . 125 Actual and predicted effluent from an NTF. Predicted effluent was calculated using the modified Gujer and Boller (1986) model.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131 (a) Typical hydraulically driven rotary distributor with reverse-thrusting jets and electric drive, and (b) modern hydraulically driven rotary distributor with variable-frequency controlled gates that close or open orifices (therefore, the flowrate contributes to reverse thrust).. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134 (a) Modern hydraulically driven rotary distributor, (b) gate controller, (c) orifices, and (d) full-length arm and distributor.. . . . . . . . . . . . . . . . . . . . . . . . . . . . 137 (a) “Pouch” snail typically found in trickling filters treating m unicipal wastewater, and (b) a modular plastic media sheet with biofilm and snail-grazing pathways.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145 Snail shells covering fine-bubble diffusers in suspended-growth reactors that are located in (a) Pueblo, Colorado; and (b) Garland, Texas.. . . . . . . . . . . . . . . . . . . . . 146 Biofilm and filter fly control with low-frequency dosing.. . . . . . . . . . . . . . . . . . . . . . . 148 Nitrifying trickling filter operating modes for high-concentration undissociated aqueous ammonia dosing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151 Nitrification rate and biomass accumulation in pilot-scale NTFs after startup.. . . . . 155
List of Figures
3.23 4.1 4.2 4.3
4.4
4.5 4.6 4.7 4.8
4.9 4.10 5.1 5.2 5.3
5.4 5.5 5.6
5.7 5.8
Three modes of TF/SC process operation.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160 General representation of RBC process.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175 RBC process general flow arrangements.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177 Clark RBC predictive model, Monod kinetics, BOD5 removal: (1) values are total BOD5; (2) 50% sBOD5 assumed; (3) >13°C (>55°F) temperature; (4) three stages, with 50% of media area in the first stage; and (5) standard-density media. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186 Opatken RBC predictive model, second-order kinetics, BOD5 removal: (1) values are total BOD5; (2) >13°C (>55°F) temperature; (3) 50% sBOD5 assumed; (4) three stages, with 50% of media area in the first stage; and (5) standard-density media. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187 Benjes RBC predictive model, BOD5 removal: (1) values are total BOD5; (2) >13°C (>55°F) temperature; and (3) standard-density media.. . . . . . . . . . . . . . . . . 188 Manufacturers’ RBC design basis, BOD5 removal: (1) values are total BOD5; (2) >13°C (>55°F) temperature; and (3) 50% sBOD5 assumed. . . . . . . . . . . . . . . . . . . . 189 Equipment manufacturers’ predictions versus full-scale plant data, BOD5 removal.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191 Benjes RBC model predictions versus full-scale plant data, BOD5 removal: (1) values are total BOD5; (2) k = 0.30; (3) >13°C (>55°F) temperature; and (3) standard-density media. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192 Benjes RBC predictive model, effect of k rate on predicted performance (standard-density media assumed).. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193 Manufacturers’ design basis, nitrification of domestic wastewater. . . . . . . . . . . . . . . 196 Vertical mounted flat-panel sieves with air sparge system and horizontal sieves located over aeration grid.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216 (a) Aeration grid and distribution piping, and (b) stainless diffuser with 4-mm aeration-holes along underside.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216 Representative carriers removed from each of four moving-bed reactors in series showing the variation in biofilm color (specialization and biofilm concentration (active biofilm) dependent on the operating condition and treatment function of each reactor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 218 (a) COD removal efficiency at high loading conditions, and (b) poor settling character of biofilm slough under high loading rates. . . . . . . . . . . . . . . . . . . . . . . . . . . 221 Soluble BOD removal rate as a function of total BOD loading in roughing MBBR application. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222 (a) Effect of BOD loading and dissolved oxygen on nitrification rates at 15°C (adapted from Hem et al. [1994]), and (b) difference in nitrification rate between multiple MBBRs in series.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223 Influence of dissolved oxygen at low ammonia concentrations. . . . . . . . . . . . . . . . . . 225 (a) Seasonal biomass concentration and temperature in tertiary nitrifying MBBR, and (b) nitrification activity grouped by temperature condition as a function of dissolved oxygen level.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228
xxvii
xxviii List of Figures
5.9 5.10
5.11 5.12 5.13 5.14 5.15 5.16 5.17 5.18 5.19 5.20 5.21 5.22 5.23 5.24 5.25 5.26 5.27 6.1 6.2
6.3 6.4 6.5 6.6 6.7 6.8 6.9 6.10 6.11 6.12 6.13
Denitrification rate as a function of temperature with different external carbon types.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 230 (a) ABS 123K mixers installed toward the surface of the reactor oriented with a 30-degree negative inclination to “push” the media down into the depths of the reactor, and (b) post-denitrification MBBR reactor in operation at the Sjölunda WWTP (Malmö, Sweden).. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232 Simplified liquid treatment schematic of Moa Point WWTP (New Zealand). . . . . . . 236 BOD profile results at Moa Point WWTP (New Zealand).. . . . . . . . . . . . . . . . . . . . . . . 237 (a) Test periods 1, 2, and 3, and (b) test periods 4 through 9. . . . . . . . . . . . . . . . . . . . . 238 Ammonia removal rate as function of loading rate in a tertiary MBBR.. . . . . . . . . . . 239 Simplified liquid treatment schematic of Moorhead WWTF (Minnesota).. . . . . . . . . 240 Ammonia profile results across nitrifying MBBR at the Moorhead WWTF (Minnesota).. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241 Simplified liquid treatment schematic of Williams Monaco WWTP (Colorado).. . . . 242 Two MBBR trains with four reactors each: anoxic and aerated.. . . . . . . . . . . . . . . . . . 243 BOD profile results across MBBRs at Williams Monaco WWTP (Colorado) (2004 to 2007). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244 Settled effluent TSS at Williams Monaco WWTP (Colorado) (2004 to 2007). . . . . . . . 245 Simplified liquid treatment schematic of Klagsham WWTP (Sweden).. . . . . . . . . . . . 247 Effluent BOD7 results at Klagsham WWTP (Sweden).. . . . . . . . . . . . . . . . . . . . . . . . . . 249 Effluent total nitrogen results at Klagsham WWTP (Sweden).. . . . . . . . . . . . . . . . . . . 249 Effluent total phosphorus results at Klagsham WWTP (Sweden).. . . . . . . . . . . . . . . . 249 Simplified liquid treatment schematic of Gardemoen WWTP (Norway).. . . . . . . . . . 251 Ammonia profile results across MBBRs at Gardemoen WWTP (Norway) (2001).. . . 252 Nitrogen profile results across MBBRs at Gardemoen WWTP (Norway) (2001).. . . . 252 IFAS process versus conventional activated sludge process. . . . . . . . . . . . . . . . . . . . . 260 Effect of biofilm thickness on locations of growth and surface area of biofilm: (a) magnitude of difference in biofilm surface area for “thin” and “thick” biofilms on the same carrier particle of moving-bed or IFAS system, and (b) magnitude of difference in biofilm surface area for “thin” and “thick” biofilms on the same carrier particle of moving-bed or IFAS system. . . . . . . . . . . . . . 279 Typical layout of plastic-carrier-media IFAS systems.. . . . . . . . . . . . . . . . . . . . . . . . . . 282 Typical layout of sponge-media IFAS system.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283 Typical layout of cord or web fixed-bed-media IFAS systems.. . . . . . . . . . . . . . . . . . . 283 Pilot study configuration.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 290 Schematic of BNR upgrade to IFAS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 292 Process configuration for BNR upgrade. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 292 Westerly, Rhode Island WWTF IFAS system flow diagram.. . . . . . . . . . . . . . . . . . . . . 300 Westerly, Rhode Island, WWTF IFAS system reactor train plan.. . . . . . . . . . . . . . . . . 301 Full-scale flow diagram. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309 Monthly operating data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 314 Polyester fabric media.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 316
List of Figures
6.14 6.15 6.16 6.17 6.18 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9 7.10 7.11 7.12 7.13 7.14
7.15 7.16 7.17 7.18 7.19 7.20 8.1 9.1 9.2
9.3 9.4 9.5
Existing and proposed aeration basin configuration.. . . . . . . . . . . . . . . . . . . . . . . . . . . 317 Proposed process flow diagram. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317 Installation of 10 IFAS media modules in existing basin.. . . . . . . . . . . . . . . . . . . . . . . . 319 Effluent cBOD concentration after train B startup.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319 Effluent ammonia concentration (7-day average).. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .320 BAF process flow diagrams.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 328 BioCarbone® biological filter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 332 Downflow denitrification filter.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333 Biofor® upflow biological filter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334 Biostyr® process arrangement for nitrification and denitrification. . . . . . . . . . . . . . . . 336 Schematic of moving-bed denitrification filter.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 338 Schematic of non-backwashing, open-structure media filter.. . . . . . . . . . . . . . . . . . . . 339 Mineral media upflow SAF with block underdrain.. . . . . . . . . . . . . . . . . . . . . . . . . . . . 340 Carrier diameter versus media specific surface area.. . . . . . . . . . . . . . . . . . . . . . . . . . . 344 Relation between total solids of backwash liquor and time for different backwash airflow rates.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 347 Relationship between total solids of backwash liquor and time for d ifferent backwash water flowrates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 347 Nitrification rate for differently pretreated raw wastewaters as a function of C:N ratio. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 356 Maximum nitrification versus long-term average loading.. . . . . . . . . . . . . . . . . . . . . . 359 BAF configurations for combined nitrification and denitrification: (a) pre-denitrification/nitrification and optional post-denitrification, and (b) pre-denitrification/nitrification in one reactor.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 363 Nitrification with flocculated raw wastewater.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 363 Denitrification filter design curves using EBDT with additional data points. . . . . . . 369 VEAS WWTP schematic.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 380 Seine Centre WWTP liquid treatment schematic.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 382 Frederikshavn Central WWTP liquid treatment schematic. . . . . . . . . . . . . . . . . . . . . . 386 Nitrification and denitrification BAF reactors at West Warwick, Rhode Island.. . . . 388 Reactor configurations: (a) UASB, (b) EGSB, (c) BAS, and (d) internal circulation. . . . . 403 BOD5 content of effluent versus TSS and trickling filter loading.. . . . . . . . . . . . . . . . . 415 (a) Suspended solids concentration and turbidity data collected throughout the study with a linear relationship function and 95% suspended solids concentration prediction interval estimate; (b) turbidity/time data collected from the Camp Creek plant on August 13, 1990, with batch flocculation curve fit; (c) turbidity/time data collected at the Utoy Creek plant on July 19, 1990, with batch flocculation curve fit; and (d) turbidity/time data collected at the Coneross plant on May 30, 1991, with batch flocculation curve fit.. . . . . . . . . . . . . . . 425 Effect of coarse-bubble aeration rate on effluent solids.. . . . . . . . . . . . . . . . . . . . . . . . . 426 Flocculation procedures, test number 1.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 427 Flocculation procedures, test number 2a.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 428
xxix
xxx List of Figures
9.6 9.7 9.8 9.9 9.10 9.11 9.12 9.13 9.14 9.15 10.1 10.2 10.3 10.4
EDI and hydraulic flocculating feed well. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 432 General arrangement of the improved clarifier inlet structure. . . . . . . . . . . . . . . . . . . 433 Dense sludge blanket profile: average and peak flows.. . . . . . . . . . . . . . . . . . . . . . . . . 435 Clarifier detention: efficiency versus CWD.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 436 Recommendations for primary clarifier floor configuration.. . . . . . . . . . . . . . . . . . . . . 439 Clarifier improvement with Stamford baffle.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 441 McKinney (Lincoln)/Stamford baffle arrangements.. . . . . . . . . . . . . . . . . . . . . . . . . . . 443 General arrangement of sludge hopper.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 446 General arrangement of improved wastewater clarifier.. . . . . . . . . . . . . . . . . . . . . . . . 447 EDI primary clarifier.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 450 Cloth filters.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 465 Traveling-bridge filter.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 465 Compressible media filters.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 466 Performance data for six different types of granular-medium filters tested on the same activated sludge effluent at a filtration rate of 2.7 L/m2 s (4 gpm/sq ft).. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 467 11.1 Plant configurations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 474 11.2 Schematic of IFAS and MBBR system with 10 cells (reactors) in series operating in enhanced nutrient removal configuration.. . . . . . . . . . . . . . . . . . . . . . . . . 475 11.3 Modeling biofilm in (a) trickling filter, (b) MBBRs, and (c) BAF.. . . . . . . . . . . . . . . . . 476 11.4 Nitrification rates for biofilm in IFAS and MBBR systems: (a) biodegradable COD limiting nitrifiers in biofilm, ammonium-nitrogen is not limiting, liquid temperature = 12°C, soluble biodegradable COD >10 mg/L, NH4-N > 3 mg/L, measured at a dissolved oxygen concentration of 8 to 9 mg/L; (b) liquid temperature = 12°C, soluble biodegradable COD < 10 mg/L, NH4-N < 5 mg/L, measured at dissolved oxygen concentration of 8 to 9 mg/L.. . . . . . . . . . . . . . . . . . . . 484 11.5 COD uptake rates for biofilm in IFAS and MBBR systems: liquid temperature = 12°C, measured at a dissolved oxygen concentration of 8 to 9 mg/L.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 496 11.6 Layout of Broomfield WWTP, Colorado.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 514 11.7 Aquifas output: The figures in the left column are from the semi-empirical model; the figures in the right column are from the biofilm one-dimensional model. The diffusion model can offer a higher degree of precision in its ability to predict day-to-day variations in a dynamic simulation, but takes substantially longer to run. Both models are able to predict the diurnal and 31-day average. . . . . . . . . . . 522 11.8 Substrate profiles for COD, NH4-N, and NO3N. The left column is from the semi-empirical model; the right column is from the biofilm one-dimensional model.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 523 11.9 COD uptake, NH4-N uptake, and denitrification in the biofilm and MLVSS. . . . . . . 524 11.10 Substrate profiles (dissolved oxygen, NH4-N, NOx-N, SCODbio, and percent VSS) inside the biofilm in aerobic cells 1 and 2.. . . . . . . . . . . . . . . . . . . . . . . . . 525
List of Figures
11.11 11.12 11.13 11.14 11.15 11.16 11.17 11.18
11.19 11.20 11.21 11.22 11.23 11.24
Typical raw influent COD fractions in BioWin. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 528 Screen shot of IFAS model in BioWin.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 531 BioWin IFAS calibration results for effluent ammonia and nitrate.. . . . . . . . . . . . . . . 532 BioWin IFAS calibration results for MLSS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 532 BioWin IFAS calibration results for effluent BOD and TSS. . . . . . . . . . . . . . . . . . . . . . 533 Schematic of one of two MBBR trains at the South Adams plant, Adams County, Colorado���������������������������������������������������������������������������������������������������������������������������������� 534 Layout of MBBR as modeled in GPS-X������������������������������������������������������������������������������ 543 Comparison of MBBR effluent NH3-N and NOx-N from GPS-X model, with actual data. The NH3-N represents ammonium-nitrogen + ammonia-nitrogen; at pH ~7, >99% is as ammonium-nitrogen. ���������������������������������������������������������������������� 546 Comparison of MBBR secondary effluent BOD5 and TSS model results from GPSX model with actual data �������������������������������������������������������������������������������������������������������� 546 Comparison of MBBR effluent MLSS (before settling in secondary clarifier) from GPSX model with actual data.������������������������������������������������������������������������������������ 547 Layout of South Adams MBBR as modeled in AquaNET (Windows.NET) version of Aquifas.������������������������������������������������������������������������������������������������������������������������������ 547 Comparison of MBBR effluent NH3-N and NOx-N from Aquifas model with actual data������������������������������������������������������������������������������������������������������������������������������ 550 Comparison of MBBR secondary effluent BOD5 from Aquifas model with actual data������������������������������������������������������������������������������������������������������������������������������������������ 551 Comparison of MBBR effluent MLSS (before settling in secondary clarifier) from Aquifas model with actual data�������������������������������������������������������������������������������� 551
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List of Tables Table 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 3.1 3.2 3.3 3.4 3.5 4.1 4.2 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 5.10 5.11 5.12 5.13 5.14 5.15
Page
Subdivisions of organisms and principal groups found in biological treatment systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Common human diseases caused by viruses. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Typical composition of Escherichia coli.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 Typical elemental composition of bacterial cells.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .28 Classification of living organisms by carbon and energy sources.. . . . . . . . . . . . . . . . . 34 Bacterial metabolism and respiration.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 Standard reduction potentials at 25°C for selected environmentally important redox couples.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 BODU and suspended solids concentrations for the three temperature values.. . . . . . 63 Properties of some trickling filter media.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88 Trickling filter classification.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 Operating and flushing dosing rates for distributors.*. . . . . . . . . . . . . . . . . . . . . . . . . . 100 Reported zero-order nitrification rates for vertical- and cross-flow media. . . . . . . . . 121 Design criteria for selected combined TF/SG processes.. . . . . . . . . . . . . . . . . . . . . . . . 157 Comparison of organic loadings for models.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190 Media surface adjustment factors.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194 Plastic biofilm carriers.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215 General summary of treatment schemes offered by MBBRs.. . . . . . . . . . . . . . . . . . . . .219 Typical BOD design loading criteria.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220 Average results from normal-rate MBBRs with chemical phosphorus removal.. . . . 222 Examples of reported O2:NH4-N.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226 Examples of reported temperature-dependency coefficients.. . . . . . . . . . . . . . . . . . . . 227 Examples of typical pre-denitrification rates observed with municipal wastewater.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229 Screening examples at MBBR installations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233 Solids separation examples at MBBR installations.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233 MBBR/solids contact process at Moa Point WWTP (New Zealand).. . . . . . . . . . . . . . 236 Tertiary MBBR process at Moorhead WWTF (Minnesota).. . . . . . . . . . . . . . . . . . . . . . 240 Multistage MBBR process at Williams Monaco WWTP (Colorado). . . . . . . . . . . . . . . 243 Post-denitrification MBBR process at Klagsham WWTP (Sweden).. . . . . . . . . . . . . . . 247 Multistage MBBRs at Gardemoen WWTP (Norway).. . . . . . . . . . . . . . . . . . . . . . . . . . . 251 Summary of annual performance showing that the process meets performance requirements.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253
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xxxiv List of Tables
6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9 6.10 6.11 6.12 6.13 6.14 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9 7.10 7.11 7.12 7.13 7.14 7.15 9.1 9.2 9.3 9.4 9.5 9.6 9.7
Summary of media characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 280 Biofilm SSA of various types of media.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 281 Design criteria, original Annapolis WRF.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 290 Design criteria, BNR upgrade.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293 Operating MLSS MCRTs, HRTs, media, and average performance for IFAS. . . . . . . 293 Performance data.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294 Summary of discharge permit limits. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303 Design criteria for the Westerly LINPOR-CN system.. . . . . . . . . . . . . . . . . . . . . . . . . . 303 Summary of operating data.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 304 Effluent requirements.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307 Specific design criteria for the upgraded plant.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309 Influent data.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310 Effluent data.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312 Colony, Texas, 2004 historical influent loadings and effluent discharge. . . . . . . . . . . 315 Commercially available BAF reactor systems and media.. . . . . . . . . . . . . . . . . . . . . . . 342 Summary of baf backwashing requirements.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 348 Typical BAF loading rates for secondary treatment.. . . . . . . . . . . . . . . . . . . . . . . . . . . . 352 Effect of water velocity on nitrification rate for three types of biofilters. . . . . . . . . . . 356 Influence of grain size and shape on performance from testing in Oslo, Norway. . . 357 Typical BAF loading rates for nitrification. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 358 Typical biologically active filter loading rates (BAF) for pre-denitrification.. . . . . . . 365 Typical biologically active filters (BAF) loading rates for post-denitrification. . . . . . 366 Performance results for two-stage BAF in Oslo, Norway.. . . . . . . . . . . . . . . . . . . . . . . 381 Year 2000 average performance for three-stage BAF system at Seine Center WWTP.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383 Year 2000 average performance for first-stage BAF system at Seine Center WWTP.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 384 Year 2000 average performance for second-stage BAF system at Seine Center WWTP.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 384 Year 2000 average performance for third stage BAF system at Seine C enter WWTP.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 385 Year 1998 average performance for the two-stage BAF at F rederikshavn Central WWTP.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 387 Havelock WWTP comparison of effluent BOD and total nitrogen average and variation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 390 Clarification characteristics of trickling filter systems in municipal WWTPs. . . . . . . 421 Energy gradient (G) values as a function of aeration type. . . . . . . . . . . . . . . . . . . . . . . 427 Clarifier design features/interactions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 430 Example minimum floor slopes versus diameter for clarifiers with scrapers.. . . . . . 440 Results of full-scale study of launder positioning. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 442 Design basis for primary and secondary clarifiers.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 447 Review of secondary clarifier operating conditions.. . . . . . . . . . . . . . . . . . . . . . . . . . . . 452
List of Tables
9.8 9.9 10.1 11.1 11.2 11.3 11.4 11.5 11.6 11.7 11.8 11.9 11.10 11.11 11.12 11.13 11.14 11.15 11.16
Effluent quality of single-stage trickling filter nitrification plants.. . . . . . . . . . . . . . . . 456 Clarified/filtered effluent quality of Ohio nitrifying trickling filter plants.. . . . . . . . 457 Full-scale filtration of settled trickling filter effluent.. . . . . . . . . . . . . . . . . . . . . . . . . . . 468 Biofilm specific areas and modeling trickling filter, TF/AS, MBBR, and BAFs within models.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 478 Typical biofilm yields. Yields are generated by running a biofilm one-dimensional model or from measurements in pilot studies.. . . . . . . . . . . . . . . . . 480 Values of coefficients measured in pilot studies and the default values for the semi-empirical model.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 486 Matrix showing kinetic coefficients for MLVSS in semi-empirical and biofilm models.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 493 Semi-empirical modeling of the biofilm—equations for semi-empirical computation of biofilm flux.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 495 Actual plant data used for IFAS process model.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 516 Comparison of 31-day average from dynamic simulation against plant data Aquifas semi-empirical and biofilm one-dimensional models.. . . . . . . . . . . . . . . . . . . 520 Measured and computed values of biofilm growth (Aquifas biofilm one-dimensional model).. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 525 Summary of the BioWin influent fractions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 529 Steady-state BioWin calibration results for a full-scale IFAS facility.. . . . . . . . . . . . . . 530 SA-T1: MBBR influent data and cell data for dissolved oxygen, January 2007.. . . . . 535 SA-T2: MBBR effluent data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 538 SA-T3: Principal GPS-X biofilm parameters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 540 SA-T4: Summary of GPS-X influent fractions (Mantis and ASM2d).. . . . . . . . . . . . . . 540 South Adams County (Colorado) steady-state results for January 2007 with GPS-X.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 544 South Adams County—steady state results for January 2007 with Aquifas. . . . . . . . 548
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Preface The purpose of this Manual of Practice (MOP) is to provide a detailed understanding of all aspects of wastewater treatment using fixed-film processes. By way of background, an overview of the historical development of fixed-film treatment systems is provided. The microbiology of attached growth biofilms is presented as background to better understand the design and operation of the various fixed-film technologies. The following types of pure fixed-film and hybrid treatment systems are discussed in detail: • Trickling filters, • Rotating biological contactors, • Moving-bed biofilm reactors, • Integrated fixed-film activated sludge, and • Biological filters. Both aerated and anoxic applications of the biological filter systems are included. The chapter on trickling filters discusses combined fixed-film and suspended-growth processes. There is a chapter dedicated to new and emerging fixed-film treatment processes, including such processes as upflow anaerobic sludge blanket reactors and anammox biofilm reactors. Factors that affect the design of the various processes, appropriate design criteria and procedures, modelling techniques, equipment available, and construction features are discussed. Operational issues associated with each type of process are presented, including potential problems and corrective actions. Case studies are included to illustrate the application of these technologies. Finally, there is a detailed discussion of modeling techniques as applied to fixed-film and hybrid processes. The chapter discusses the use of several commercially available models.
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xxxviii
Preface
The overall objective of this book is to provide the reader with a complete understanding of the types of pure and hybrid fixed-growth treatment processes available and an understanding of their design, performance, and operational issues. This Manual of Practice was produced under the direction of Rhodes R. Copithorn, P.E., BCEE, Chair. The principal authors of this Manual of Practice are as follows: Chapter 1 Chapter 2 Chapter 3 Chapter 4 Chapter 5 Chapter 6 Chapter 7 Chapter 8 Chapter 9 Chapter 10 Chapter 11
Rhodes R. Copithorn, P.E., BCEE, Chair Stefan Wuertz, Ph.D. Edward D. Schroeder, Ph.D. Joshua P. Boltz, Ph.D., P.E. Joshua P. Boltz, Ph.D., P.E. James P. McQuarrie, P.E. Rhodes R. Copithorn, P.E., BCEE, Chair Dipankar Sen, Ph.D., P.E. Christine deBarbadillo, P.E. Joseph A. Husband, P.E., BCEE Frank Rogalla Christopher W. Tabor, P.E. Stephen Tarallo Robert Nerenberg, Ph.D., P.E. Thomas E. Wilson, Ph.D., P.E., BCEE Joseph A. Husband, P.E., BCEE Dipankar Sen, Ph.D., P.E. Heather Phillips, P.E.
Authors’ and reviewers’ efforts were supported by the following organizations: AECOM, Alexandria, Virginia Aqualia, Madrid, Spain Black & Veatch Corporation, Gaithersburg, Maryland, and Kansas City, Missouri Brentwood Industries, Reading, Pennsylvania CDM, Newport News, Virginia, and Portland, Oregon CH2M Hill, Philadelphia, Pennsylvania; Tampa, Florida; Redding, California Earth Tech, Alexandria, Virginia Entex Technology, Inc., Chapel Hill, North Carolina
Preface
GHD, Bowie, Maryland Greeley and Hansen, LLC, Wilmington, Delaware Hydromantis, Inc., Hamilton, Ontario, Canada Malcolm Pirnie, Inc., Columbus, Ohio, and White Plains, New York Regional Municipality of Waterloo, Ontario, Canada Thomas E. Wilson Environmental Engineers LLC, Barrington, Illinois University of California, Davis, California University of Notre Dame, Notre Dame, Indiana Woodard & Curran, Cheshire, Connecticut, and Dedham, Massachusetts
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Biofilm Reactors
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Chapter 1
Introduction 1.0 Background and Purpose
3.4 Coupled Trickling Filter/ Activated-Sludge Process 6 3.5 Biological Filters 7 3.6 Hybrid Processes 7
1
2.0 Characteristics of FixedGrowth Processes 2 3.0 History 3.1 Contact Beds 3.2 Trickling Filters 3.3 Rotating Biological Contactors
3 4 4
4.0 Organization of Manual
10
5.0 References
11
5
6.0 SUGGESTED READINGS
14
1.0 Background and Purpose This manual of practice is an update of the special publication titled Aerobic FixedGrowth Reactors, previously published by the Water Environment Federation® (Alexandria, Virginia) (WEF) in the year 2000. A number of changes in our understanding of biofilm processes and the technology and regulations that drive the wastewater treatment industry prompted this decision to rewrite the manual. Techniques developed in the field of molecular biology have expanded our understanding of biofilm kinetics and their microbial ecology. Regulations are imposing increasingly more stringent limits on the concentration of pollutants that may be discharged, including, most importantly, nutrients. The imposition of nitrogen and phosphorus limits has dramatically changed the design and operation of wastewater treatment facilities. These changes have brought about advances in the technology applied to 1
2
Biofilm Reactors
wastewater treatment, including many significant changes to the type of fixed-film treatment processes that are available today. The change in the title of this manual from Aerobic Fixed-Growth Reactors to Biofilm Reactors acknowledges some of these advances. The manual now incorporates both aerobic and anoxic fixed-film treatment processes that are applied for biological carbon and nitrogen removal.
2.0 Characteristics of Fixed-Growth Processes Although the basic metabolic processes that biological systems use to remove carbon and nutrients in wastewater treatment plants (WWTPs) are the same for fixed-film and suspended-growth systems, there are some inherent differences that provide several advantages and some challenges for the application of fixed-film processes. It is difficult to generalize, because each specific application has its own set of advantages and disadvantages, as will become apparent in the discussion of the various fixed-growth technologies discussed in this manual. However, a general summary of the differences would be useful at this point. Suspended-growth systems are comprised of biological flocs, but, theoretically, all of the dissolved substrate is available to all of the cells. With fixed-growth systems, the substrates must diffuse through the biofilm layers to become available; thus, the transport of substrate from the bulk liquid through the stagnant boundary layer and into the biofilm through the process of diffusion becomes another limiting factor. End products of the metabolic reactions must diffuse in the reverse direction. Thus, a cross-section through a fully developed biofilm will exhibit varying environmental and kinetic characteristics. A single biofilm may have aerobic, anoxic, and anaerobic processes occurring, and the substrate that becomes limiting will change through the depth of the biofilm. Thus, fixed-film processes are quite complex to model. Some advantages generally associated with fixed-film processes include the following: • Reduced operating and energy costs, • Smaller reactor volume, • Minimized need for settling capacity, and • Operational simplicity.
Introduction
Disadvantages are more specific to each technology, and they will be discussed in subsequent chapters. However, some general concerns associated with fixed-film processes include the following: • Potential clogging of the media system as a result of inadequate screening; • Excessive growth, which could plug the media system or cause free-floating media to sink; and • Inadequate mixing or short-circuiting, resulting in inefficient use of the media.
3.0 History Biological wastewater treatment technology has advanced tremendously since its early roots as a primitive science in the late 1800s. Much of the impetus was provided by increasingly crowded urban centers and the outbreak of typhoid resulting from a lack of understanding of public health issues. As the science developed, there was an early understanding that aeration of the wastewater was beneficial. An excellent presentation of the origins and development of fixed-film treatment processes has been provided by Alleman and Peters (1982). Initial experiments with this practice yielded nominal results, most likely because of a lack of understanding regarding the need for an active biomass. Aeration of fixed-film processes provided better results because of the presence of an established biomass. This was understood more fully later, as Gilbert J. Fowler of the University of Manchester, England, and his associates, Edward Ardern and William T. Lockett, in the first few decades of the 1900s, experimented with various processes, which led to our understanding of activated sludge (Cooper, 2001). Fixed-film processes continued to evolve as experimenters substituted wooden lathes in place of coarse rock, in an effort to increase the available surface area. Much of this work was advanced in the United Kingdom at the Manchester Sewage Works and at the Lawrence Experiment Station in Lawrence, Massachusetts (now known as the Senator William X. Wall Experiment Station). Biological filters originally were developed by the Lawrence Experiment Station in Massachusetts (Mills, 1890), using gravel for the biological media. These results proved that the process was not merely mechanical filtration, but also involved removal of pollutants by biological growth on the media. The results showed a correlation between the media volume and the volume of wastewater that could be treated.
3
4
Biofilm Reactors
From this beginning, the process evolved to be the dominant secondary wastewater treatment process in the United States and elsewhere by the 1950s. The introduction of synthetic media in the 1950s resulted in extending the use of fixed-growth concepts. Continued research led to the development of high-rate systems, such as biological aerated filters, moving-bed biofilm reactors, and various hybrid systems that combine the advantages of both suspended-growth and fixed-film processes.
3.1 Contact Beds The 1890s studies at the Lawrence Experiment Station in Massachusetts were viewed with interest in the United Kingdom. Corbett (1902) developed a trickling filter design that was the forerunner of today’s trickling filter systems. There was provision for distributing the influent across the surface of the gravel bed and ventilation of the underdrain. Independently, Crimp (1890) and Dibdin (1903) conducted studies that led to the development and use of a process known as a contact bed. Crimp and Dibdin constructed a basin, filled it with slag media, provided a means of flooding the basin, and then slowly drained it after approximately 1 hour of contact. The tank then was allowed to stand empty for 4 to 6 hours, which permitted the organics on the surface of the media to be oxidized. To prevent plugging of the slate media, the wastewater was screened or chemically precipitated before application. As a result, 75% removal of the oxidizable organic matter in the wastewater at a hydraulic loading rate of 1.2 m3/m2·d was obtained. The process evolved further by adding stages. However, clogging remained a continuing problem that was not resolved easily. The demise of the contact bed was initiated when the Royal Commission on Sewage Disposal report (1908) was published, which showed that a trickling filter could process twice as much wastewater per unit of volume as a two-stage contact bed.
3.2 Trickling Filters The emergence of the trickling filter was aided by the development of an effective means of distributing the flow to the filter media. Caink (1897) and Candy (1898) advanced Mills’ Lawrence Experiment Station concept into a rotating arm distribution system driven by water jets. Around the same time, a reciprocating distributor was developed for the rectangular filters, and mechanical drives (engine and electrically driven) were in use by 1904 (Stanbridge, 1972). The trickling filter process
Introduction
evolved further, and many different connotations were used to describe the process configurations. Following the initial evaluation by Dow Chemical Company (Midland, Michigan) from 1954 to 1955 (Bryan, 1955; Dow Chemical Company, 1955), the combined efforts of the Mead Corporation (Dayton, Ohio), Fluor Corporation (Irving, Texas), and Dow Chemical Company resulted in the development and application of both random and bundle synthetic media in the late 1950s. The history of the developmental stage of plastic media is described in reviews by Bryan (1982) and Peters and Alleman (1982). Different media configurations, producing different surface areas, continue to evolve today.
3.3 Rotating Biological Contactors Rotating biological contactors (RBCs) were an outgrowth of trickling filters and were influenced by the desire to reduce power consumption for wastewater treatment. Steels (1974) credits the first RBC concept to have been around 1900. A wooden cylinder with slatted walls was filled with brushwood and slowly rotated while approximately 50% submerged. Extensive testing of the RBC process was conducted in Germany. In 1900, Weigand patented a rotary cylinder made of wooden slats (Alleman and Peters, 1982). In the 1950s, researchers first used asbestos sheets, and, by the early 1960s, expanded polystyrene media came into use. The use of this lightweight material allowed the shafts holding the media to be lengthened. This resulted in a major growth of the RBC in the 1960s and 1970s, with more than 700 plants put into operation in Europe and the United States. This growth subsequently was stymied by several problems that developed, including performance of less than design expectations, excess biomass accumulations, shaft breakage, loping of disks (caused by unbalanced biomass weight), and undesirable biological growths. Most of the early developmental problems have been resolved, and there are many systems operating successfully, but the acceptance of RBCs as an effective treatment process has not returned to its former level among design engineers and owners. A submerged RBC, or submerged biological contactor (SBC), appeared in the 1980s, in which the discs are 70 to 90% submerged, and the shaft is driven by air. The intent was to decrease the loading on the shafts, improve biomass control, and provide an opportunity to retrofit existing activated sludge basins. The SBCs have been piloted for denitrification in an anoxic reactor. However, SBCs have seen only limited application.
5
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Biofilm Reactors
3.4 Coupled Trickling Filter/Activated-Sludge Process The use of coupled process designs likely precedes the 1950s, although definitive reports are lacking. The development of plastic media led to the use of trickling filters for highly loaded (roughing) modes of operation. These units were found to be suited particularly for strong municipal and industrial wastewater. Bryan and Moeller (1960) reported that the initial coupled process installation was designed to improve the quality of an activated sludge (AS) effluent during upset conditions. Soon after, the roughing trickling filter (RTF) found a better process location, which was before the activated sludge process. Here, the RTF smoothed the loading and stabilized the overall process. It also was used to protect activated sludge systems against toxic and shock loads from industrial contributors. Gehm and Gellman (1965) demonstrated that the coupled RTF/AS process could provide for control of bulking organisms and thus enhance the overall performance of WWTPs. The reduction of the sludge volume index (Gehm and Gellman, 1965) often was cited as the reason for using a trickling filter before an activated sludge system. Lower energy costs and the ability to handle shock loads also were cited as advantages of the coupled process. Many WWTPs receiving food, beverage, and other highcarbohydrate wastewater found it beneficial to provide partial treatment of the wastewater with trickling filters before the activated sludge process, to enhance the overall treatment performance. In the early 1970s, a modified trickling filter concept using wood slat media was marketed. This concept was called the activated biofilter (ABF) process. The first reference to this concept was reported by Egan and Sandlin (1960), where the settled sludge from a plastic media effluent was recycled over the tower. Bryan (1962) reported on the design of the Saginaw Bay and Cities Service facilities in Saginaw, Michigan, where provision was made to recirculate the return activated sludge through a media filter. In this process, the sloughed trickling filter biomass was recirculated at a high rate through the filter. The filter used an open media, and wood stacks were the most common media. This trickling filter concept was later coupled with a short-term (15 to 30 minutes) activated sludge system (ABF/AS) (Slechta and Owen, 1974), at Corvallis, Oregon, and found wide acceptance for treating municipal and industrial wastewater. Research in Corvallis, Oregon, during the late 1970s led to the development of the trickling filter/solids contact process in 1979 (Norris et al., 1982). This process uses a trickling filter sized for the removal of the majority of the soluble organic matter
Introduction
followed by an aerated solids contact channel or basin and a secondary clarifier with recycle of return activated sludge to the aerated solids channel. The primary purpose of the aerated solids contact channel is to flocculate suspended solids contained in trickling filter effluent for removal in the secondary clarifier.
3.5 Biological Filters The basic concept of the biological filter is to provide a high specific surface area for the growth of biomass in a compact volume. The biological filter provides for both the removal of substrates biologically and the retention of solids, which are removed periodically through a backwash cycle. The basic technology has been applied to aerobic, anoxic, and anaerobic processes. Biological aerated filters (BAFs) were developed in the 1980s in Europe and have found extensive application since then for carbon and nitrogen removal. Since 1982, over 500 installations have been built using a variety of technologies (Stephenson et al., 2004). A wide variety of configurations and media systems have been developed, but, generally, the advantages that have come to be recognized for this type of technology include the following: • Relatively small space requirement, because they are typically high-rate systems; • Ability to treat dilute wastewaters; • No concern for sludge settleability; and • Relatively easy to contain potential odors. The technologies may be categorized according to whether they are upflow or downflow processes; packed bed or fluidized bed; and aerobic, anoxic, or anaerobic.
3.6 Hybrid Processes The use of fixed-film media in aerated reactors is an old concept. In recent years, the concept has been referred to as integrated fixed-film activated sludge (IFAS) and has been used increasingly as a means of improving the treatment capacity and nutrient removal capabilities of existing activated sludge facilities. Before the 1940s, the Hays and Griffith processes (Wilford and Conlon, 1957) used baffles, cement asbestos, wood, or other construction materials in the aeration tank to enhance performance. The Hays and Griffith processes were improved over
7
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Biofilm Reactors
the original process (Hays, 1931), in which submerged rock filters were force-aerated. These processes were called submerged contact aeration processes. Approximately 60 Hays contact plants were constructed for United States Army camps between 1940 and 1945 (Packham, 1988). Wood, asbestos, and plastic sheeting were fixed in the aeration basin to provide a surface for bacterial growth. The plant treatment trains consisted of primary clarification followed by two equally sized aeration basins with intermediate and final clarification. Sludge return was not practiced. Asbestos sheets were suspended vertically in the aeration basins, such that they extended from 10 cm below the liquid surface to just above the horizontal grid of aeration pipes. The asbestos sheets were spaced 3.8 cm apart throughout the length of the aeration tanks and permitted a serpentine pathway for floating material. The aerators provided oxygen transfer and mixing between the asbestos sheets, and treatment was affected by the attached fixed-film growth on the sheets. Typical combined hydraulic retention times for the two aeration basins ranged from 1.7 to 3.0 hours. The National Research Council Report (1946) developed design criteria for the contact aeration process simultaneously with the development of design criteria for activated sludge and trickling filter processes. The contact aeration process eventually failed because of the additional cost of aeration and because it produced a lower quality effluent compared with the activated sludge process. The process essentially ceased to be used in the 1960s. Steels (1974) reported on previous attempts in the 1920s to improve plant operations by retaining various types of small particulates in the aeration basins. These particulates included grit, brushwood, wood, and cork and were effective in treating strong wastewater. The Hays and Griffith processes resurfaced in a number of variant forms, including a similar concept (Hamoda and Abd-El-Bary, 1987) referred to as an activated submerged fixed-growth bioreactor. The use of submerged RBCs in aeration basins is another form of this concept. The basic concept of these systems, as currently applied, is to increase the amount of biomass available for treatment per unit volume. By retaining the biomass on a fixed-film media within the suspended-growth reactor, treatment performance can be enhanced without increasing the solids loading on the secondary clarifiers. The IFAS systems, in their various forms, combine the apparent advantages of both fixedfilm and suspended-growth biomass systems. Another major effort to use fixed-film media in aeration tanks originated in Japan during the 1960s. Kato and Sekikawa (1967) developed a process that they called fixed
Introduction
activated sludge and applied primarily to the treatment of industrial wastes. This process vertically suspended an open plastic matrix in an aerated reactor and typically was operated without a return sludge. More than 60 such installations were in place in Japan during the 1960s. Both fixed and free-floating types of media systems have been developed. A rope-type media, which is installed on racks placed in the activated sludge basins, originally was developed in Japan for the purpose of achieving greater levels of carbonaceous biochemical oxygen demand removal within the volume of an existing activated sludge basin (Iwai et al., 1990). The product then was applied in Germany to upgrade treatment plants for nitrification and subsequently in North America for both purposes. The majority of applications have been in aerobic basins or in basins that alternate between aerobic and anoxic conditions. Pilot testing of the rope-type media began in the United States in the early 1990s, followed by full-scale installations (Randall and Sen, 1996). Several different types of rope media have been developed and are available commercially. Free-floating types of systems use the sponge of plastic media. Processes using sponge-type media originally were developed in Europe in the late 1970s, and two basic systems emerged. One system, the Captor process (Atkinson et al., 1979), was developed by Simon-Hartley through work completed at the University of Manchester in the United Kingdom. The concept was commercialized by Simon-Hartley in conjunction with several universities, Severn Trent Water Authority (Birmingham, United Kingdom), and the Water Research Center in Swindon, Wiltshire, United Kingdom. A second sponge-type system was developed by the LindeAG Corporation in the mid-1970s (Hegemann, 1984) and was introduced commercially in Europe and North America as the Linpor System by the Lotepro Corporation (Mount Kisco, New York) (Morper and Wildmoser, 1990; Reimann, 1990). A plastic media system using small cylindrical biofilm carrier elements originally was developed by Kaldnes Miljoteknologi in Norway (Odegaard and Rusten, 1990; Odegaard et al., 1994). There currently are several manufacturers of this type of media system. The free-floating media systems have been applied in both IFAS and movingbed biofilm reactor (MBBR) configurations. The MBBR process differs from IFAS in that there is no return activated sludge; thus, the MBBR is a pure fixed-film process and not a hybrid. The plastic media by Kaldnes Mitjoteknologi originally was developed as an MBBR process and was patented (AnoxKaldnes MBBR) (Odegaard, 2006).
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Biofilm Reactors
4.0 Organization of Manual A brief description of the organization of this manual and the material contained in each chapter is presented in this section. Chapter 2 presents the microbiology of wastewater treatment as applied to biofilms and is intended to provide a foundation for understanding the discussion of kinetics presented in the individual technology chapters and in the final chapter on modeling. Trickling filter processes and RBC processes are discussed in Chapters 3 and 4, respectively. Moving-bed biofilm reactors are discussed in Chapter 5. In each case, the process and mechanical design considerations and the operational issues are presented. Various design approaches are presented, including both empirical and kinetic-based models specific to the technology. Application of the technologies to carbon removal and nutrient removal are presented, and case studies are included. The focus of Chapter 6, Hybrid Processes, is on IFAS processes. The various types of media incorporated to IFAS systems are discussed with design considerations relevant to each type of media. Case studies are presented to illustrate their application. Chapter 7, Biological Filters, presents the design, construction, and operational issues associated with aerobic and anoxic biological filters in their various configurations, including downflow and upflow BAFs with sunken media, upflow with floating media, open-structure media filters, and continuous backwash filters. The application of biological filters for carbon removal, nitrification, and denitrification is discussed. New and emerging technologies that show promise in the laboratory or through small pilot-scale studies, but that are not yet commercially available, are discussed in Chapter 8. Clarification, as it relates to meeting advanced levels of treatment following fixed-film systems, is discussed in the following two chapters. Chapter 9 discusses gravity settling and membrane separation. Chapter 10 discusses effluent filtration technologies. Finally, a comprehensive discussion of modeling concepts is presented in Chapter 11. Each technology chapter presents modeling techniques specific to that process, but this chapter covers the kinetics in general, modeling techniques, and application of models to various types of pure fixed-film and hybrid systems.
Introduction
5.0 References Alleman, J.; Peters, R (1982) The History of Fixed Film Wastewater Treatment Systems. Proceedings of the International Conference of Fixed Film Biological Processes, Kings Island, Ohio. http://web.deu.edu.tr/atiksu/ana52/biofilm4. pdf (accessed March 2010). Atkinson, B.; Black, G. M.; Lewis, P. J. S.; Pinches, A. (1979) Biological Particles of Given Size, Shape and Density for Use in Biological Reactors. Biotechnol. Bioeng., 21 (2), 193–200. Bryan, E. H. (1982) Development of Synthetic Media for Biological Treatment of Municipal and Industrial Wastewater. Paper presented at the 1st International Conference on Fixed-Film Biological. Processes, Vol. 1, Kings Island, Ohio; Sponsored by University of Pittsburg, U.S. Army Corps of Engineers, U.S. Environmental Protection Agency, and U.S. National Science Foundation), 89. Bryan, E. H. (1955) Molded Polystyrene Media for Trickling Filters. Proceedings of the 10th Purdue Industrial Waste Conference, West Lafayette, Indiana, May 9–11; Purdue University: West Lafayette, Indiana, 164. Bryan, E. H. (1962) Two-Stage Biological Treatment: Industrial Experience. Proceedings of the 11th South Municipal Industrial Waste Conference; North Carolina State University, North Carolina. Bryan, E. H.; Moeller, D. H. (1960) Aerobic Biological Oxidation Using Dowpac. Proceedings of the Conference on Biological Waste Treatment; Manhattan College: Riverdale, New York. Caink, T. (1897) Specifications of Inventions. Br. Patent 19153. Candy, F. P. (1898) Specifications of Inventions. Br. Patent 2749. Cooper, P. F. (2001) Historical Aspects of Wastewater Treatment, Decentralized Sanitation and Reuse: Concepts, Systems and Implementation, Chapter 2; International Water Association Publishing: London, United Kingdom. Corbett, J. (1902) Some Sewage Purification Treatment Experiments. J. Sanit. Inst., 23, 601–602. Crimp, S. (1890) The Construction of Works for the Prevention of Pollution by Sewage of Rivers and Estuaries; Charles Griffin & Company: London, United Kingdom.
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Biofilm Reactors
Dibdin, W. J. (1903) The Purification of Sewage and Water, 3rd ed.; The Sanitary Publishing Company: London, United Kingdom. Dow Chemical Company (1955) Dowpac™ FN-90 and Dow HCS™. Plastics Technical Services., Dow Chemical Company: Midland, Michigan. Egan, J. T.; Sandlin, M. (1960) The Evaluation of Plastic Trickling Filter Media. Proceedings of the 15th Purdue Industrial Waste Conference, West Lafayette, Indiana; Purdue University: West Lafayette, Indiana, 107–-115. Gehm, H. W.; Gellman, I. (1965) Practice, Research and Development in Biological Oxidation of Pump and Paper Effluents. J. Water Pollut. Control Fed., 57, 1392–1398. Hamoda, M. F.; Abd-El-Bary, M. F. (1987) Operating Characteristics of the Aerated Submerged Fixed-Film (ASFF) Bioreactor. Water Res., 21, 939–947. Hays, C. C. (1931) Sewage Treatment Process. U.S. Patent 1,991,896. Hegemann, W. (1984) A Combination of the Activated Sludge Process with FixedFilm Bio-Mass to Increase the Capacity of Wastewater Treatment Plants. Water Sci. Technol., 16, 119–130. Iwai, S.; Oshino, Y.; Tsukada, T. (1990) Design Operation of Small Wastewater Treatment Plants by the Microbial Film Process. Water Sci. Technol., 22, 139–144. Kato, K.; Sekikawa, Y. (1967) FAS (Fixed Activated Sludge) Process for Industrial Waste Treatment. Proceedings of the 22nd Purdue Industrial Waste Conference, West Lafayette, Indiana; Purdue University: West Lafayette, Indiana, 926–949. Mills, H. F. (1890) Purification of Sewage and Water. Special report to the Massachusetts State Board of Health: Boston, Massachusetts, 25. Morper, M.; Wildmoser, A. (1990) Improvement of Existing Wastewater Treatment Plant Efficiencies Without Enlargement of Tankage by Application of the Linpor Process—Case Studies. Water Sci. Technol., 37, 207–215. Norris, D. P.; Parker, D. S.; Daniels, M. L.; Owens, E. L. (1982) High Quality Trickling Filter Effluent Without Tertiary Treatment. J. Water Pollut. Control Fed., 54, 1087–1098. Odegaard, H. (2006) Innovations in Wastewater Treatment: the Moving Bed Biofilm Process. Water Sci. Technol., 53, 17–33.
Introduction
Odegaard, H.; Rusten, B. (1990) Upgrading of Small Municipal Wastewater Treatment Plants with Heavy Dairy Loadings by Introduction of Aerated Submerged Biofilters. Water Sci. Technol., 22 (7/8), 191–198. Odegaard, H.; Rusten, B.; Westrum, T. (1994) A New Moving Bed Biofilm Reactor—Application and Results. Proceedings of the 2nd International Specialized Conference on Biofilm Reactors, Paris, France, Sep 29–Oct 1; International Association on Water Quality: London, United Kingdom, 221–229. Packham, R. F. (1988) Biological Filtration. Manuals of British Practice in Water ollution. Control, Institute of Water Pollution Control: London, United Kingdom. Peters, R. W.; Alleman, J. E. (1982) The History of Wastewater Treatment Systems. Paper presented at 1st International Conerence on Fixed-Film Biological Processes, Vol. 1, Kings Island, Ohio (sponsored by University of Pittsburg, U.S. Army Corps of Engineers, U.S. Environmental Protection Agency, and U.S. National Science Foundation), 60. Randall, C.; Sen, D. (1996) Full-Scale Evaluation of an Integrated Fixed-Film Activated Sludge (IFAS) Process for Enhanced Nitrogen Removal. Water Sci. Technol., 33 (12), 155–162. Reimann, H. (1990) The Linpor Process for Nitrification and Denitrification. Water Sci. Technol., 22, 297–298. Royal Commission on Sewage Disposal (1908) Fourth Report. Royal Commission on Sewage Disposal: London, United Kingdom. Slechta, A. E.; Owen, W. F. (1974) ABF Short-Term Aeration-Pilot Plant Results, Corvallis, OR. Technical Bulletin, Neptune, Microfloc, Inc.: Corvallis, Oregon. Stanbridge, H. H. (1972) The Introduction of Rotating and Traveling Distributors for Biological Filters. Water Pollut. Control, 44, 573. Steels, I. H. (1974) Design Basis for the Rotating Disc Process. Effluent Water Treat. J., 14 (9), 434–445. Stephenson, T.; Cornel, P.; Rogalla, F. (2004) Biological Aerated Filters (BAF) in Europe: 21 Years of Full Scale Experience. Proceedings of the 77th Annual Water Environment Federation Technical Exposition and Conference, New Orleans, Louisiana, Oct 2–6; Water Environment Federation: Alexandria, Virginia. Wilford, J.; Conlon, T. P. (1957) Contact Aeration Sewage Treatment Plants in New Jersey. Sew. Ind. Wastes, 29, 845–855.
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6.0 SUGGESTED READINGS Sen, D.; Randall, C. W. (1996) Mathematical Model for a Multi-CSTR Integrated Fixed Film Activated Sludge (IFAS) System. Proceedings of the 69th Annual Water Environment Federation Technical Exposition and Conference, Dallas, Texas, Oct 5–9; Water Environment Federation: Alexandria, Virginia. Stensel, H. D.; Brenner, R.; Lee, K.; Melcer, H.; Rakness, K. (1988) Biological Aerated Filter Evaluation. J. Environ, Eng., 114 (6), 1352–1358.
Chapter 2
Biology of Fixed-Growth Process 1.0 INTRODUCTION
17
4.7 Flagella 4.8 Extracellular Polymeric Substances 4.9 Chemical Composition of Cells 4.10 Example 1—Theoretical Oxygen Demand of Bacterial Cells 4.10.1 Solution 4.10.2 Comment
2.0 CLASSIFICATION OF LIVING ORGANISMS 18 3.0 MICROORGANISMS OTHER THAN BACTERIA 21 3.1 Fungi 22 3.2 Algae 22 3.3 Protozoa 23 3.4 Multicellular Invertebrates 23 3.5 Viruses 24 3.6 Consortia 26 4.0 CHARACTERISTICS OF BACTERIA 4.1 Structure of the Bacterial Cell 4.2 Chromosome and Plasmids 4.3 Cytoplasm 4.4 Cell Membrane 4.5 Cell Wall 4.6 Pili
5.0 BACTERIAL METABOLISM, NUTRITION, AND RESPIRATION 5.1 Energy Source 5.2 Chemoheterotrophic Metabolism 5.3 Chemoautotrophic Metabolism 5.4 Photosynthetic Metabolism 5.5 Nutrient Requirements 5.6 Bacterial Energy Metabolism
26 26 28 30 30 30 31
31 31 32
32 32 33
33 36 36 38 39 40 41
(continued) 15
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Biofilm Reactors
5.7 Aerobic Growth and Respiration 5.8 Anoxic Conditions and Respiration 5.9 Anaerobic Respiration and Fermentative Metabolism 5.10 Energetics of Respiration 5.11 Example 2—Calculation of Electrode Reduction Potentials and ∆G for Half-Reactions 5.12 Solution 5.13 Co-Metabolism 6.0 BACTERIAL GROWTH 6.1 The Bacterial Growth Curve 6.2 Growth in Mixed Cultures 6.3 Enrichment Cultures 6.4 Stability of Mixed Cultures
42 43
44 46
48 48 50 51 51 53 53 54
6.5 Effects of Environmental Variables 54 7.0 BACTERIAL GROWTH KINETICS IN BIOFILMS 7.1 Rate of Bacterial Processes 7.2 Note 7.3 Physical and Chemical Changes in Biofilms Resulting from Growth
55 56 56
59
7.4 Structured Models 7.5 Temperature Effects 7.6 Example 3—Effect of Temperature on Organic Removal
59 60
7.7 Solution 7.8 Inhibition and Toxicity 7.9 Mass-Transfer-Rate Limitations
61
61
63 64
8.0 KEY TRANSFORMATIONS IN BIOFILMS 65 8.1 Chemoheterotrophic Processes 66 8.2 Chemoautotrophic Processes 67 8.3 Biology of Nitrogen Transformations 67 8.4 Denitrification 67 8.5 Aerobic Nitrification 68 8.6 Anoxic Nitrification/ Denitrification 69 8.7 Biological Phosphorus Removal 70 8.8 Sulfide and Sulfur Oxidation 71 8.9 Hydrogen Oxidation 71 9.0 FEATURES OF MICROBIAL COMMUNITIES IN BIOFILMS 72 10.0 REFERENCES
74
Biology of Fixed-Growth Process
1.0 INTRODUCTION The purpose of this chapter is to provide a general introduction to the biology of fixedgrowth processes used in water and wastewater treatment. Although the fundamental principles of biological processes are the same for all process configurations, the constraints of fixed-growth processes result in particular characteristics of microbial communities and advantages and disadvantages relative to alternative systems. In fixed-growth processes, fluid containing necessary nutrients passes over the microorganisms growing in a biofilm on a support surface. Nutrients diffuse into the biofilm and are metabolized by the immobilized microorganisms. A significant result is that microbial populations can function well, even when fluid-phase nutrient concentrations are extremely low (Schroeder, 2002; Schroeder et al., 2000). Correspondingly, at high fluid-phase nutrient concentrations, oxygen transfer may become limiting, and anoxic zones may develop within the biofilm. The most basic considerations in microbial treatment process design and operation are that microorganisms use contaminants in water or wastewater as sources of carbon and energy for growth, as nutrients required to sustain growth, or as electron acceptors in the respiratory process. In all cases, microorganisms remove contaminants from the dissolved state; that is, particulate material must first be dissolved and then metabolized. Engineering of microbial systems requires consideration of the requirements for microbial growth and determination of the conditions necessary to carry out the desired biochemical reactions. Particular constraints encountered in biological wastewater treatment include the necessity to remove a wide range of organic and inorganic materials, consistent production of finished waters having extraordinarily low organic and suspended solids concentrations, and the presence of organic materials that are highly toxic to most forms of life. The environmental requirements for biological growth generally are inconsistent with the production of potable waters. If biological treatment is a step in the production of potable waters, a number of abiotic treatment steps must be incorporated downstream. In a sense, most rivers form minimally controlled treatment links between wastewater and potable water treatment systems. Treated wastewater discharged to rivers is diluted and contaminants are further stabilized before extraction and treatment of water for use by municipalities.
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2.0 CLASSIFICATION OF LIVING ORGANISMS Living organisms currently are classified into three general domains—Eucarya, Bacteria, and Archaea (see Table 2.1). Viruses—nonliving obligate infectious particles—are significant factors in microbial processes and also are discussed in this chapter. Two types of cell organization exist—eukaryotic and prokaryotic. Plants; animals; and the algae, protozoa, and fungi have eukaryotic cells, while members of the Bacteria and Archaea domains have the structurally simpler prokaryotic cell type. Viruses do not have a cellular structure, being composed of one or two strands of genetic material (deoxyribonucleic acid or ribonucleic acid) and a protein coat or
Table 2.1 Subdivisions of organisms and principal groups found in biological treatment systems. Domain or group
Cell structure Properties
Constituent groups
Eukaryotes
Eukaryotic
Nematodes, Plants (seed plants, rotifers ferns, mosses), algae, fungi Animals (invertebrates, vertebrates)
Multicellular, extensive differentiation of cells and tissues
Groups commonly found in biological treatment
Algae, fungi, protozoa Unicellular, mycelial or filamentous: little or no differentiation of cell type
All groups
Bacteria
Prokaryotic
Cell chemistry similar to Eukaryotes
Most bacteria
Most Gram-positive genera, some Gram-negative genera
Archaea
Prokaryotic
Distinctive cell chemistry
Methanogens, halophiles, thermoacidophiles
Methanogens
Viruses
None
Non-living, obligate parasites composed of nucleic acid strand(s) and protein coat or capsid
Biology of Fixed-Growth Process
capsid. Eukaryotes are divided into two general groups—multicellular organisms, in which cells have specific functions, and unicellular organisms, in which all cells carry out the same range of functions. Multicellular organisms are classified into two general categories—plants and animals—according to their energy and carbon sources, structure, type of growth, and movement. Unicellular organisms are classified into three general categories—protozoa, fungi, and algae—in a manner analogous to that for multicellular eukaryotic organisms. The two domains characterized by the prokaryotic cell have distinctive differences in cellular chemistry. All of the organisms in these two divisions are microscopic, and most of the individual cells are smaller than 5 µm. Bacteria are divided into 17 major lineages (or phylums) of cultured organisms and are estimated to contain many more phylums if uncultured microbial communities are included. Classification is based on the similarity of ribosomal ribonucleic acid (RNA) sequences that have been retrieved from various environments. As will be explained later, RNA comprises approximately 20% of the dry weight of cells. The most commonly used method of categorizing bacteria by genus and species is to establish similarities in a specific type of RNA, 16S ribosomal RNA, which has a molecular weight of approximately 500 000. Bacteria include organisms of relevance in biological treatment of water and wastewater, most of the organisms of importance in soils, and most of the organisms pathogenic to humans and other mammals. A relatively small fraction of known bacterial species are agents of disease (Schroeder and Wuertz, 2003), and pathogens may be harbored within biofilms in drinking water distribution systems (Szewzyk et al., 2000) and wastewater treatment systems (Skraber et al., 2007). Three phylums have been classified as Archaea. Euryarchaeota include Extremely Halophilic Archaea in environments having salinities of 1 to 5.5 M; Methanogens, which produce methane (CH4); Thermoplasmatales, which include thermophilic and extremely acidophilic genera that require low-pH (0.5 to 4.5) and high-temperature (55 to 85°C) environments; and Hyperthermophilic Euryarchaeota, which thrive at temperatures ranging from 80 to 110 °C. Crenarchaeota, the second phylum, contains members capable of existing at either very hot or very cold temperatures. A presumptive third phylum is Korachaeota; it consists of members found at high-temperature hydrothermal environments. In most cases, organisms are identified by their genus and species, with the genus capitalized and the species non-capitalized, and both genus and species in italics or underlined. For example, organisms of the genus Pseudomonas are extremely common in both soil and biological treatment processes. Species of Pseudomonas frequently
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Biofilm Reactors
found are P. aeruginosa, P. cepacia, P. putida, and P. stutzeri. Within a species, strains often are identified that behave slightly differently. For example, some strains of Pseudomonas putida may metabolize toluene, and others may not (metabolism is the sum of the processes in a living organism; materials that are metabolized are used to fuel cellular activities and incorporate to cellular molecular components). The differences between strains might be considered analogous to characteristics in humans such as height, left- or right-handedness, or ability to distinguish color. Microorganisms are defined loosely as organisms that cannot be seen by the naked eye, a definition that includes members of all of the divisions of organisms, as shown in Table 2.1. Biological treatment involves the use of microbial communities to remove or modify contaminants in water and wastewater. In most cases, the microbial communities are dominated by mixed (heterogeneous) bacterial populations (cultures made up of a number of bacterial species). A few species generally may make up the bulk of the bacterial population on either a number or mass basis. The reason that predominant species develop is the existence of a competitive advantage over less numerous groups, as a result of the particular environmental conditions or the available nutrient sources. For example, a biological process designed to remove a chlorinated aromatic compound and operating at 10 °C would almost always have at least one strain of the genus Pseudomonas among the dominant species, because this genus of bacteria is competitive at relatively low temperatures, and some species have a particular capacity to degrade aromatic compounds (Evans and Fuchs, 1988; Levin and Shapiro, 1967; Reineke and Knackmuss, 1988). Mixed bacterial populations also should be thought of in terms of interacting communities and symbiotic relationships. Overall growth and contaminant removal rates are enhanced by the interaction of the various species making up the population. In addition, interactions can be exploited further by controlling specific operational parameters in reactors. For example, some genera responsible for biological phosphorus removal require low-molecular-weight (volatile) organic acids that would not occur in significant concentrations in most wastewaters. Operating biological processes with an anaerobic segment results in fermentation (production) of volatile acids by acid-fermenting bacteria. In this manner, the substrates required by the phosphorus-removing bacteria are produced (substrate is a general term for microbial energy and/or carbon sources; the term comes from cultures grown on the surface of solid growth media, because nutrients are extracted from the substratum) (Fuhs and Chen, 1975; Levin and Shapiro, 1967, Shapiro et al., 1967). In a mixed-culture system that has not been engineered to remove nutrients, it would be expected that
Biology of Fixed-Growth Process
the various species in a mixed bacterial population produce necessary growth factors for other species as a matter of course, but this might not result in the desired reactions (e.g., phosphorus removal) taking place. In such mixed bacterial populations, species that grow most rapidly are better adapted to the particular environment (e.g., temperature, pH, or salinity) and are most efficient in energy use predominate. However, a type of hierarchy develops in which species depend on each other and in which species occupy ecological niches. For example, in a system treating gasoline production wastewaters, species that break down aromatic molecules may leave residues that serve as carbon and energy sources for species unable to break the aromatic ring. The predominant use of biological treatment is for the removal of organic compounds from water or wastewater, but removal of inorganic ions, such as ammonium, nitrate, nitrite, and phosphate, or even materials that are present in trace concentrations, such as selenium, arsenic, and mercury, is both possible and increasingly common. In addition, mixed microbial cultures can be used to produce desired end products, such as CH4. Descriptions of the structure and growth characteristics of bacterial cells and the other organisms found in biological treatment processes can be found elsewhere (Madigan et al., 2003).
3.0 MICROORGANISMS OTHER THAN BACTERIA Both microscopic multicellular and unicellular organisms have roles in fixed-growth biological treatment processes. Microscopic plants serve as surfaces on which smaller organisms grow and, through photosynthesis, as sources of oxygen. Microscopic animals serve as scavengers that remove floating debris. A few biological treatment processes have been designed that take advantage of plant growth characteristics, but the role of microscopic animals is largely uncontrolled. Unicellular eukaryotic organisms are present in most biological treatment processes. Protozoans serve a particulate scavenging role similar to multicellular animals and also may have a role in making particulate material more bioavailable to bacteria. Fungi have roles similar to the bacteria, but rarely compete well in treatment systems. Algae, like plants, can be used to provide oxygen to microbial systems and to remove inorganic nutrients, although most fixed-growth processes do not have enough light to maintain significant photosynthesis. A few biological treatment processes that take advantage of the potential contributions of algae and fungal treatment processes are under development for use with some hazardous materials (Woertz, Kinney, McIntosh, and Szaniszlo, 2001; Woertz, Kinney, and Szaniszlo, 2001).
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Figure 2.1 Coenocytic cell structure typical of many fungi, in which cell contents, or cytoplasm, is multinucleate and continuous throughout the organism.
3.1 Fungi Fungi have cell walls, typically are coenocytic (multinucleate with no cellular subunits [see Figure 2.1]), are immotile, and use organic material for both energy and carbon sources. Inorganic contaminants, such as nitrogen, phosphorus, and other nutrients, are incorporated to cell tissue in stoichiometric amounts, as in the case of bacterial growth. Fungi grow at considerably lower rates than bacteria and do not compete well in most treatment process environments. A fungus that appears to have considerable potential in the treatment of hazardous organic compounds is Phanerochaete chrysporium, a white rot fungus. This organism produces an extracellular peroxidase that degrades lignin in the presence of peroxide. The reaction is relatively nonspecific, with respect to the type of carbon–carbon bond attacked, and has been found to be effective in initiating the degradation of a variety of highly chlorinated compounds, including the dioxins (Aust et al., 1988; Hackett et al., 1977). The use of Phanerochaete chrysporium is limited to conditions in which nitrogen is limiting, because the peroxidase is not produced otherwise. A possible application of genetic engineering would be in the transfer of the gene responsible for the synthesis of the peroxidase to bacteria that grow well in wastewater treatment processes.
3.2 Algae Algae, like the fungi, are immotile eukaryotes with cell walls. Most species are unicellular, and some form aggregates of cells, including filaments. Not all algae are microscopic. Carbon dioxide (CO2) serves as the carbon source for algal growth, and energy is derived from the absorption of light by photosynthetic pigments, with oxygen being produced as a byproduct. Because the algae are aerobic organisms, a portion of the oxygen produced is used in their metabolism. Excess oxygen accumulates in the surrounding water, as long as enough light is available. The principal wavelength range in which algae absorb light is between 300 and 700 nm.
Biology of Fixed-Growth Process
Although algae are sometimes used in nutrient removal systems, they are extremely difficult to separate from water and often are troublesome contaminants themselves. Nutrient-rich ponds, sloughs, and lakes often have algal “blooms” over short periods of time, which result in unaesthetic green mats on the water surface. The mats may be blown onto beaches, where they decay, or are problems for people using the waters for recreation. Some algal species produce organic compounds that cause taste and odors in drinking water supplies. Certain marine flagellated algae (called dinoflagellates) produce toxins that are harmful to humans and fish.
3.3 Protozoa The protozoa do not contain cell walls and use bacteria, and presumably other organisms, and particulate and soluble organic materials for food. Thus, protozoa are heterotrophic predators and, in a sense, are higher on the trophic ladder than bacteria or algae. A large number of protozoan species exist, and a number typically are seen in the microbial communities found in fixed-growth processes. Protozoa ingest some organic contaminants and use inorganic ions also. However, the numbers of protozoa present are relatively small, their growth rates are substantially lower than those of bacteria, and the amount of contaminants removed by protozoa is negligible relative to that removed by bacteria, fungi, and algae. However, they may serve to pre-process particulates for bacteria, by producing exudates of increased solubility.
3.4 Multicellular Invertebrates Higher forms of microorganisms—those with complex, invertebrate, multicellular bodies, such as rotifers and nematodes—generally act as a higher step in the food chain, but overall perform a role that is similar to the protozoa. It is quite possible for a fixed-growth process to operate successfully without the presence of protozoa or higher forms, but the presence of scavengers generally is believed to be beneficial. These organisms are present because the environmental conditions necessary for satisfactory contaminant removals often are suitable for protozoa and higher forms. Their absence in fixed-film processes often is associated with sloughing events—that is, the uncontrollable detachment of whole segments of biofilm from the support medium. The upper limit on the size of the higher forms (e.g., nematodes rather than carp) is the result of several factors, including sensitivity to oxygen concentration, physical space requirements, and growth/replication rates.
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3.5 Viruses A group that stands alone as being nonliving, but having an effect on the growth of organisms, is the viruses. The viruses are particles composed of one or two strands of genetic material and a protective protein coat, or capsid. All viruses are obligate parasites; they are unable to carry out any form of metabolism and are completely dependent on host cells for replication. The mechanism of viral action is composed of the following four steps:
(1) Adsorption of the viral particle (viron) onto a receptor site on the cell membrane; (2) Infection—the insertion of the genetic material to the host cell; (3) Redirection of host cell functions to replication of viral particles; and (4) Release of the viral particles into the environment (often by disruption of the cell membrane, with consequent death of the cell).
Viral particles typically are 0.01 to 0.2 µm in size and are species-specific, with respect to infection. Most viral groups attack only one host species, and individuals within a species may become resistant. Viruses that attack bacteria are termed bacteriophage (or simply phage). A number of important human diseases are caused by viruses, and examples are listed in Table 2.2. Modes of transmission of hepatitis B viruses, polioviruses, and picornaviruses are known to include water, although the principal pathways typically are through food, personal contact, or exchange of body fluids. Some viruses, such as the retroviruses (including the HIV group), appear to be too fragile for water transmission to be a significant danger to public health. Viruses are important in water and wastewater treatment as contaminants to be removed. Discharge of human viruses into drinking water supplies is completely unacceptable, and the discharge of significant concentrations of human viruses with treated wastewaters is a matter of increasing concern. The role of bacteriophages in biological treatment processes is not currently understood, although phages are known to influence bacterial population dynamics in marine systems (Steward et al., 1996). Methods for monitoring the presence of human viruses are not commonly applied because of the small size of the particles and the difficulty in culturing. Some important environmental viruses cannot be grown by cell culture, at present. Increasingly, molecular biology methods based on the amplification of deoxyribonucleic acid (DNA) and, indirectly, RNA using the polymerase chain reaction (PCR) are applied to water samples (Rajal, McSwain, Thompson, Leutenegger, Kildare, and
Biology of Fixed-Growth Process
Table 2.2 Common human diseases caused by viruses (adapted from Stanier et al., 1986). Viral group and type
Disease
Herpesviruses Cytomeglovirus
Respiratory infections
Epstein–Barr virus
Mononucleosis
Herpes simplex viruses
Oral and genital cold sores
Varicella virus
Chickenpox, shingles
Hepatitus B virus
Serum hepatitus
Influenza viruses
Viral influenza and viral pneumonia
Polioviruses
Poliomyelitis
Poxviruses Orf virus
Contagious pustular dermatitis
Variola virus
Smallpox
Picornaviruses Coxsackie viruses
Herpangina
Hepatitis A virus
Infectious hepatitis
Poliomyelitis virus
Poliomyelitis
Rhinoviruses
Most colds
Parainfluenza viruses
Measles, mumps, rubella
Rhabdoviruses
Rabies
Reoviruses
Diarrheal diseases
Retroviruses Human T-cell leukemia virus
T-cell leukemia
Human immunodeficiency viruses
Acquired immunodeficiency syndrome (AIDS)
Rotaviruses
Diarrhea
Wuertz, 2007). This research now allows a more rapid and quantitative detection of viruses. A differentiation between viable or infectious and inactivated viruses by quantitative PCR is possible, in principle, but is difficult to perform at this time. At present, disinfection methods are used that have been demonstrated to be effective in
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Biofilm Reactors
laboratory and field experiments. Regular monitoring of process effluents for viruses is not a conventional practice, but may become so once molecular detection methods have been validated sufficiently.
3.6 Consortia It is important to see the microorganisms used in biological treatment as a community living in consortia. Each of the organisms present occupies an ecological niche, and the relative numbers of the species will change as conditions change. For example, increases or decreases in temperature will modify the competitive advantages among the species present and result in changes in species predominance. It must be assumed that cyclic operations are more favorable to some groups than others. The ability to use particular materials, such as benzene rings or ammonia, as energy sources is not widespread among microbial species, and the presence of these materials may give some species either a niche, allowing them to survive, or a competitive advantage over other species.
4.0 CHARACTERISTICS OF BACTERIA Because bacteria are the dominant organisms in biological treatment systems, they will be discussed here in greater detail. Additional information on the other organisms commonly present in fixed-growth processes will be given as appropriate. General structural characteristics of the bacterial cell are presented here, and more detailed information can be found in other references (Ingraham et al., 1983; Madigan et al., 2003; Stanier et al., 1986).
4.1 Structure of the Bacterial Cell An overall understanding of the typical composition of bacterial cells can be obtained from Tables 2.3 and 2.4. Although the composition given is for a particular bacterial species, Escherichia coli, grown under specific conditions, the relative makeup of most bacteria is similar. The physical structure of the bacterial cell can be characterized by shape (i.e., spherical, rod, and spiral) and components, including their chemical makeup, size, and the manner in which they grow (i.e., individual cells, colonies, and filaments). Shape and size are somewhat variable with stage of growth. A species listed as a rod may appear to be spherical under some growth conditions, and cell size changes during the growth cycle. Bacteria that typically are found in groups, such as the filamentous species (which often are characterized by coenocytic cell structure),
Biology of Fixed-Growth Process
27
Table 2.3 Typical composition of Escherichia coli (adapted from Ingraham et al., 1983). Macromolecule
Percentage of Mass/cell* Molecular Number of Different kinds total dry weight grams (×1015) mass molecules per cell of molecules
Protein
55.0
155.0
RNA (ribonucleic acid)
20.5
59.0
4.0 x 104
2 360 000
1050
23 S rRNA
(31.0)
1.0 x 106
18 700
1
16 S rRNA
(16.0)
5.0 x 10
18 700
1
5 S rRNA
(1.0)
3.9 x 10
18 700
1
transfer
(8.6)
2.5 x 10
205 000
60
messenger
(2.4)
1.0 x 10
1380
400
5 4 4 6
2.13
1
DNA (deoxyribonucleic acid)
3.1
9.0
2.5 x 10
Lipid
9.1
26.0
705
22 000 000
4
Lipopolysaccharide
3.4
10.0
4346
1 200 000
1
Peptidoglycan
2.5
7.0
(904)n
1
1
Glycogen
2.5
7.0
1.0 x 10
4360
1
96.1
273.0
Soluble pool
2.9
8.0
Inorganic ions
1.0
3.0
Total macromolecules
Total dry mass/cell
284.0
Water
670.0
Total mass/cell
9
6
954.0
*Note that grams times 10 means that the actual mass has been multiplied by 10n. For example, the protein mass of an E. coli cell is approximately 155 x 10-15 g. n
grow as single cells also. Thus, visual observation does not provide a method of species identification. Current molecular methods of identification and classification are based primarily on similarity of 16S ribosomal RNA or detection of specific genes. For example, intact cells belonging to the Bacteria and Archaea domains can be tested with fluorescently labeled genetic probes (short sequences of DNA that can penetrate most microbial cells). The probes are single-stranded and bind to 16S rRNA in the cell. Only perfect matches of sequences will lead to retention of the probe in the cell after a sequence of hybridization and washing steps. With the aid of a good fluorescent
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Table 2.4 Typical elemental composition of bacterial cells (Stanier et al., 1986). Element Carbon Oxygen Nitrogen Hydrogen Phosphorus Sulfur Potassium Sodium Calcium Magnesium Chlorine Iron ΣTrace elements
Percent of dry weight General physiological function 50 20 14 8 3 1 1 1 0.5 0.5 0.5 0.2 0.3
Constituent of organic cell materials Constituent of organic cell materials and cellular water Constituent of proteins, nucleic acids, coenzymes Constituent of cellular water and organic cell materials Constituent of nucleic acids, phospholipids, coenzymes Constituent of proteins and coenzymes Major cation in cell processes Major cation in cell processes Major cation in cell processes and enzyme cofactor Major cation in cell processes, cofactor in ATP reactions Major anion in cell processes Constituent of cytochromes and other proteins, enzyme cofactor Inorganic contituents of special enzymes
microscope, it is possible to verify in situ the presence or absence of certain groups of prokaryotes in a biofilm (Wilderer et al, 2002). Other methods include immunoassays and fatty acid methyl ester analysis and phospholipid fatty acid analysis, which are used to develop unique “fingerprints” of species and consortia, bacterial metabolism, resistance to antibiotics, and chemical characteristics of the cell wall. The nature of the structural components of the cell is of considerable interest in biological treatment. Reference can be made to Figure 2.2 in discussing the most important components relative to contaminant removal—the genetic component, enzymes, storage bodies, cytoplasmic (cell) membrane, cell wall, and bound extracellular polymeric substances (EPSs, often referred to as capsule). Bound EPS is associated closely with cells, but its polymers are not anchored in the cell envelope (Nielsen and Jahn, 1999). Strictly speaking, EPSs are not part of the cell structure but are included here because of their importance in biofilms and the difficulty of physically separating bound EPSs from cells (Spaeth and Wuertz, 2000).
4.2 Chromosome and Plasmids The genetic component of bacterial cells (the genome) includes both the single DNA molecule, called chromosome, and relatively small DNA circlets, called plasmids, located in the cytoplasm. The chromosome is circular and double-stranded, with a length greater than 1000 µm and a molecular weight of approximately 109. Because bacterial
Plasmid
Ce ll Membrane
Cell Wall Nucleoid DNA
Capsule Pilus
Biology of Fixed-Growth Process
Flagellum
Cytoplasm Includes: RNA, protein, plasmids (DNA), volutin granules (polyphosphates, sulfur), storage products (glycogen, lipids)
Figure 2.2 Schematic representation of the bacterial cell. Note that particular bacterial species are characterized by general shape and the tendency to occur singly, in clumps, or as chains or filaments. Typical cell sizes, excluding the capsule, range from 0.5 to 2 µm. The capsule is considered part of the EPS, and its size tends to be very small during rapid growth.
cells are approximately 1 to 2 µm in length or diameter, and the nuclear region is only a small part of the cell, the chromosome must be folded tightly. The chromosomal DNA is essential to the life of a cell. Without it, the information required for the production of necessary enzymes and other structures necessary for growth is missing. Damage to the DNA can result in the loss of an essential activity and death. Plasmids consist of extrachromosomal, typically circular, double-stranded DNA and typically are not a required component of the cell, but they seem to provide particular capabilities that make a strain more or less competitive in a given environment. These include resistance to specific antibiotics, toxin production, ability to metabolize unusual compounds or ions, and possibly resistance to attack by bacteriophage.
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4.3 Cytoplasm The tightly packed, granular region within the cytoplasmic membrane is the cytoplasm. Principal components of the cytoplasm are the ribosomes, made up of RNA and proteins, where other cell components are synthesized; enzymes (proteins), which catalyze necessary chemical reactions; plasmids, organic and inorganic compounds involved in metabolism; and granular inclusions containing storage materials, such as glycogen and poly-β-hydroxybuterate (PHB), polyphosphate (volutin granules), and sulfur. Carbon storage as glycogen or PHB appears to be speciesdependent. Enteric bacteria, cyanobacteria (blue-green algae), and spore-forming bacteria tend to store the glucose polymer, glycogen, while groups commonly found in soil, such as pseudomonads and rhizobia, tend to store PHB. Some groups (e.g., Acinetobacter and possibly the filamentous bacteria) do not store carbon, which is a factor in their competitiveness under feast/famine conditions. Glycogen granules are detectable only using electron microscopy (Ingraham et al., 1983) and are relatively evenly distributed. Poly-β-hydroxybuterate, polyphosphate (or volutin), and sulfur granules often can be seen, with special stains applied to slides, as refractile bodies under conventional light or fluorescent microscopy (Serafim et al., 2002). The intracellular accumulation of large amounts of polyphosphate granules in some bacterial species under cyclic anaerobic or anoxic/aerobic operation is used in enhanced biological phosphorus removal processes (Kong et al., 2005).
4.4 Cell Membrane Surrounding the cytoplasm is a bilayer unit membrane composed primarily of phospholipids and proteins. The cell membrane acts as an osmotic barrier (phospholipid function) and has specific transport functions (protein function), in which transferase enzymes carry out transport reactions for specific molecules. In aerobic bacterial cells—those that use oxygen as a terminal electron acceptor—the electron transport system of enzymes, in which the energy-rich compound ATP is produced and oxygen is consumed, is attached to the cell membrane. The typical thickness of the cell membrane is approximately 7 to 8 nm.
4.5 Cell Wall Structural strength is provided by the cell wall. However, this structure also acts as a molecular sieve, which screens out toxic molecules and antibiotics, and contains binding and hydrolytic enzymes, which aid in nutrient gathering and transport.
Biology of Fixed-Growth Process
In bacteria, two general types of cell envelopes are identified by the Gram-stain process, in which an applied dye is found to be permanently fixed (positive) or washed out of the cell (negative). Both types have cell walls made of peptidoglycan. Gram-positive bacteria have a thicker cell wall, and many species also have acidic polysaccharides, called teichoic acids. They often are resistant to desiccation and are found more commonly in soil and other oligotrophic environments. Many bacteria found in wastewater reactors are Gram-negative. Their cell envelope includes a thinner cell wall made of peptidoglycan and a second (outer) membrane, in addition to the cell membrane. Archaea have cell walls made up of a variety of components, but not peptidoglycan.
4.6 Pili Single strands of protein originating in the cell membrane and extending approximately 10 µm from the cell wall are the pili. These hairlike apparati appear to have a function of binding the cell to specific structures. For example, pili are required to establish cell-to-cell contact in the transfer of plasmids from one cell to another, in the process of bacterial conjugation. The presence of pili on the cell surface can aid the initial cell adhesion to interfaces. A specific type of pilus, type IV, also has been shown to be involved in cell movement (Mattick, 2002) and movement of biofilms across solid surfaces by retracting the pilus and pulling the cell forward (HallStoodley et al., 2004).
4.7 Flagella The long (15 to 20 µm) filament called the flagella moves the bacterial cell with a whiplike rotational motion. Movement of motile bacteria (not all species have such structures) toward favorable environments is a complex response to chemical gradients and “attractant solutes” in the cell’s surroundings.
4.8 Extracellular Polymeric Substances A polysaccharide layer is secreted by many bacteria. The size of this amorphous capsule or glycocalyx may be larger than the cell, in some cases. In addition, there are many other macromolecules that are either actively secreted or released through cell leakage and cell death. They include proteins and nucleic acids (DNA and RNA). The EPS can act as a binding agent, attaching cells onto surfaces, such as fixed-growth packing media, rocks, pipes, and teeth, or to other cells to form microbial aggregates
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in the form of biofilms and flocs. Initial adhesion to interfaces also is facilitated by the cell appendages, such as the pili discussed above. Once a biofilm is developing, the cellular EPS production is increased (Venugopalan et al., 2005). An operational definition of EPS distinguishes between bound and soluble EPS (Nielsen and Jahn, 1999).
4.9 Chemical Composition of Cells In most engineering problems, the fundamental composition and structure of microbial cells is of less importance than overall size (typically 1 to 5 µm for bacterial cells) and elemental chemical composition. The principal elements of all living cells, on a mass basis, are carbon, oxygen, nitrogen, hydrogen, phosphorus, and sulfur. A number of metals (iron, manganese, potassium, cobalt, calcium, copper, and zinc) are essential to life, because they serve as cofactors (or mediators) to the electron transport that takes place in specific enyzyme-catalyzed reactions. Empirical cell formulas, or molar ratios, of the chemical constituents of microbial cells are used to estimate nutrient requirements for growth and convert gravimetric cell mass measurements into theoretical oxygen demand of cell tissue. The most widely used empirical cell formula, C5H7NO2 (Porges et al., 1953, Rittman and McCarty, 2001), omits the essential nutrient phosphorus. Inclusion of phosphorus results in considerably more complex formulas, such as C42H100N11O13P (McCarty, 1965). Empirical cell formulas are representative of growth under specific environmental conditions, and care must be used in general application of the relationships, as can be seen in the following example.
4.10 Example 1—Theoretical Oxygen Demand of Bacterial Cells Determine the theoretical oxygen demand of 1 g (dry) of microbial cells using the two empirical cell formulas given above. Assume that organic nitrogen in the cells is not oxidized and remains in the -3 oxidation state.
4.10.1 Solution The solution can be obtained as follows:
(1) Write a stoichiometric equation for the oxidation of C5H7NO2. C5H7NO2 + 5O2 → 5CO2 + NH3 + 2H2O (2) Determine the ThOD of 1 g of C5H7NO2 Empirical molecular weight = 113
Biology of Fixed-Growth Process
1 g = 0.008 85 mol 5 mol of O2 is required per empirical mole of cells (0.008 85 mol cells)(5 moles O2/mol cells)(32 g O2/mol O2) = 1.42 g
(3) Write a stoichiometric equation for the oxidation of C42H100N11O13P
2C42H100N11O13P + 107 O2 = 84CO2 + 22NH3 + 64H2O + 6H+ + 2PO4– 3 Empirical molecular weight = 997 1 g = 0.001 00 mol 53.5 mol of O2 are required per empirical mole of cells (0.001 mol cells)(53.5 mol O2/mol cells)(32 g O2/mol O2) = 1.71 g
4.10.2 Comment Note the large difference in oxygen demand that would be predicted for the same mass of cells from application of the two empirical formulas. The empirical formula used most commonly, C5H7NO2, was developed based on cells grown on a case in medium, and the general validity is not well-established.
5.0 B ACTERIAL METABOLISM, NUTRITION, AND RESPIRATION Microbial metabolism of interest in water and wastewater treatment is divided into the following two major categories: • Catabolism—the breakdown or degradation of materials with the concomitant release of energy, and • Anabolism—the synthesis of new cellular materials using the energy released in catabolism and the breakdown products of catabolism. Metabolism also can be described by the type of energy source used (e.g., organic compounds, inorganic ions, and light). Emphasis in this section will be placed on catabolism, while anabolism will be discussed in the Viruses and Consortia sections, dealing specifically with microbial growth. Nutrition, as defined here, is related to the chemical requirements associated with microbial metabolism. Most of the bacteria found in soils and natural waters and those used in wastewater treatment are not very fastidious, with respect to nutrition. Inorganic nutrient sources, such as ammonia and nitrate for nitrogen or phosphate for phosphorus, are satisfactory, and a relatively
33
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wide variety of organic materials can be used as energy sources. A few microorganisms found in natural systems, such as the nitrifiers and the methanogenic bacteria, have tightly constrained nutrient requirements. Microbial respiration is defined by the type of terminal electron acceptor used in the energy-yielding reaction processes and can be classified generally as aerobic, anoxic, or anaerobic and by the electron acceptor used (aerobic, anoxic, or anaerobic). A summary of bacterial metabolism and respiration is given in Tables 2.5 and 2.6, and more detailed discussions can be found in Lengeler et al. (Eds., 1999) and Madigan et al. (2003).
Table 2.5 Classification of living organisms by carbon and energy sources. Classification
Carbon source
Energy source
Autotroph
CO2
—
Heterotroph
Organic compounds
—
Chemotroph
—
Chemical compounds
Chemolithotroph
—
Inorganic compounds
Chemoorganotroph
—
Organic compounds
Phototroph
—
Light
Chemoautotroph
CO2
Chemical compounds
Photoautotroph
CO2
Light
Chemoheterotroph
Organic compounds
Chemical compounds
Photoheterotroph
Organic compounds
Light
Methylotroph
1-carbon compounds
1-carbon compounds
General
Other terms Eutroph
Use high concentrations for carbon and energy
Oligotroph
Use low concentrations for carbon and energy
Zymogenous
Grow rapidly when carbon and energy source added
Saprophyte
Live off of dead organic matter
Table 2.6 Bacterial metabolism and respiration. Electron acceptor
Principal non-cell product
Microbial group
O2
CO2, H2O
NO3–, NO2– SO4–2 Fe3+ , ClO4–
CO2, , N2, N2O CO2, H2S, S0 CO2, Fe+2 CO2, Cl-
CO2 Organic metabolites
CO2, CH4 Volatile acids, alcohols
Aerobic heterotrophs Denitrifiers Sulfate reducers Iron reducers Perchlorate reducers Methanogens Fermenters
O2
CO2, NO2
NH3
NO2–
N2, H2O, NO3–
NO2– H2 H2S H2 H2 Light (≈870 nm)*, H2S
O2 NO3–, NO2– O2 CO2 SO4–2 CO2
CO2, NO3– CO2, , N2, N2O CO2, SO3–2 CH4 H2S, H2O
Type of metabolism Electron donor Chemoheterotrophic
35
Chemoautotrophic
–
Photoheterotrophic Photoautotrophic *Energy source
Organic compounds
Inorganic compounds NH3
Light (450 to 650 nm)*, H2O Light (≈870 nm)*, H2S
Carbon source Organic compounds
CO2
Organic compounds CO2 O2 CO2 CO2
SO3–2, H2O O2, H2O SO3–2, H2O
Ammonia oxidizers Anaerobic ammonium oxidizers Nitrite oxidizers Denitrifiers Sulfide oxidizers Methanogens Sulfate reducers Phototrophs Phototrophs Phototrophs
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Biofilm Reactors
5.1 Energy Source Three types of energy source are used by bacteria—organic compounds (chemoheterotrophic metabolism), inorganic compounds (chemoautotrophic metabolism), and radiant energy (photosynthetic metabolism). In general, a single species does not use more than one type of energy source, and, for this reason, bacteria often are classified as chemoheterotrophic, chemoautotrophic, or photosynthetic. For example, the Pseudomonads, a common soil bacterial genus, are all chemoheterotrophic. In wastewater treatment, the most important groups of bacteria are chemoheterotrophic. However, the chemoautotrophic nitrifying bacteria are essential in processes where ammonia-nitrogen removal is required. The photosynthetic Cyanobacteria (formerly blue-green algae) are essential components of wastewater treatment processes, such as oxidation ponds and artificial marshes.
5.2 Chemoheterotrophic Metabolism Organic material serves as both an energy and carbon source in chemoheterotrophic (typically shortened to heterotrophic) metabolism. Bacteria are able to use a wide variety of organic materials as food or “substrate,” including complex molecules, such as protein and starches, and many anthropogenic compounds, such as chlorobenzoate and pentachlorophenol. Most bacterial species are capable of metabolizing, or partially metabolizing, a relatively large number of compounds. However, the rates of metabolism of specific compounds vary significantly among species, and, in cases where a mixture of organics are available, a large number of species will grow as a consortium that, in some manner, appears to provide the most efficient biodegradative community. Nearly all naturally occurring organic compounds can be degraded by at least one species of bacteria. However, a number of synthetic organic compounds that have no naturally occurring counterparts (termed xenobiotics after the Greek word xeno, or stranger) have proven difficult to degrade biologically (Madigan et al., 2003; Reineke and Knackmuss, 1988; Rittman and McCarty, 2001). Examples include multi-ring compounds, such as the pesticides Dieldrin, a chlorinated hydrocarbon originally produced in 1948 by J. Hyman & Co., Denver, Colorado, and toxephene; the polychlorinated biphenyls; and chlorinated volatile organic compounds, such as the solvents trichloroethylene (TCE), tetrachloroethylene, and 1,1,2-trichloroethane, which are shown in Figure 2.3. Chlorendic acid is used as a “builder” for other chemicals, many of which are pesticides. Over 200 polychlorinated biphenyls exist. The compounds were used principally as insulators in electrical transformers, but currently are not manufactured in the
Biology of Fixed-Growth Process
Cl
Cl
Cl
Cl
Cl Cl
Cl
COOH Cl Cl
COOH
Chlorendic Acid
Cl Cl A Polychlorinated Biphenyl
Benzene
OH
Napthalene
Phenol
Phenanthrene
Cl
Cl
Cl
Cl
C
C
C
C
Cl Cl Cl H Trichloroethylene Tetrachloroethylene
H
Cl
Cl
C
C
H
Cl H 1,1,2-Trichloroethane
Figure 2.3 Examples of organic compounds that are toxic and/or difficult to biooxidize.
United States because of their carcinogenicity. Benzene is easily degraded biologically, but is highly carcinogenic. Phenol is biodegradable at low concentrations. Napthalene and Phenanthrene are among the most common polynuclear aromatic hydrocarbons. The chlorinated aliphatic solvents TCE, tetrachloroethylene (perchloroethylene, perc, or PCE), and 1,1,2-trichoroethane (1,1,2-TCA) are commonly used chlorinated solvents. Non-degradable xenobiotic compounds are a matter of current concern because of potential toxicity to living organisms. Many have been accumulating in the food chain, and effects of the accumulation, such as decreases in bird reproduction rates, loss of fisheries, and increases in specific malignant tumors, are being discovered continually. Materials that are slow or impossible to break down biologically are referred to as recalcitrant or refractive. Some naturally occurring compounds are refractive. These include cellulose and lignin, both of which are associated with plant fiber. Cellulose is a particularly interesting compound because it is, like starch, a polymer of glucose. Both starch and glucose are easily broken down by a wide variety of bacteria and most other heterotrophic organisms. Starch is broken up into small subunits in reactions catalyzed by enzymes, such as α-amylase and maltase, which break the α-glycosidic bond (Figure 2.4)
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Biofilm Reactors
HOCH2 H
H
H
OH
O
HOCH O
H
H O
H
2
H
OH
O H
OH
H
H
OH
O
α − glycosidic bond a. Repeating unit of starch and glycogen
HOCH
O
H
2
H
HOCH O
OH
H
H
OH
H
O
H
2
H OH H
O
H
H
O
OH
β − glycosidic bond
b. Repeating unit of cellulose
Figure 2.4 Repeating glucose units of starch and glycogen connected by α-glycosidic bonds and cellulose connected by β-glycosidic bonds.
between the glucose molecules. The bond between the glucose molecules of cellulose is a β-glycosidic bond (a mirror image of the α-glycosidic bond), and the enzymes that catalyze the cleavage reaction are unusual in bacteria but common in the fungi. Bacteria in the aerobic Actinomycete group and a few anaerobic heterotrophic bacteria, notably some Clostridia, found in such places as the intestines of termites and certain anaerobic environments, produce the enzyme, but it is lacking in most aerobic species.
5.3 Chemoautotrophic Metabolism Organisms capable of using CO2 or bicarbonate (HCO3–) as a carbon source and a reduced inorganic chemical, such as hydrogen (H2), ammonia (NH3) (or ammonium [NH4+]), nitrite (NO2–), Fe2+, hydrogen sulfide (H2S), or elemental sulfur (S0), as an energy source are referred to as chemolithotrophic, chemoautotrophic, or, more commonly, autotrophic. Specific groups of bacteria oxidize each of the above materials. The ammonia oxidizers (Nitosomonas, Nitrosospira, and Nitrosococcus are the predominant genera) and nitrite oxidizers (Nitrobacter and Nitrospira are the predominant
Biology of Fixed-Growth Process
H O + O2 + FeS 2(s) 2
-2 + H+ 2
Fe +2 + S
_ +2 2 Fe + SO4
+3 Fe + H2 O O 2 + H+
H 2O
Fe(OH)
3(s) +
H+
Figure 2.5 Schematic of reactions through which pyrite [FeS2(s)] is oxidized in mine drainage. Note that the stoichiometry is not shown. genera) are the autotrophs of major interest in biological treatment. Sulfur-oxidizing bacteria (the aerobic Thiobacillus, Thiotrix, and Beggiatoa and the anaerobic photosynthetic green and purple bacteria) are important in many environments, including wastewater collection systems. Sulfides and thiosulfate are oxidized by these bacteria in the aerobic liquid film that forms on the upper surface of open-channel pipes, and the oxidation product is sulfuric acid. The resulting corrosion is difficult to control and results in severe maintenance problems. Iron bacteria cause corrosion in many industrial systems, and species such as Thiobacillus ferrooxidans, Thiobacillis thiooxidans, and Ferobacillis ferrooxidans are responsible for the production of acid mine drainage through catalysis of the reaction Fe+2 → Fe+3 + e-. Oxidation of Fe+2 in mine drainage occurs spontaneously under some conditions, but the bacterially mediated corrosion reaction is 106 times faster (Singer and Stumm, 1970). Acid mine drainage is a major problem in most coal and copper mining areas. The reactions are summarized in Figure 2.5. Molecular hydrogen is a common product in the anaerobic bacterial breakdown of organic materials. If H2 accumulates in a reaction system, organic breakdown will cease and, therefore, a mechanism for removal must be present. Autotrophic conversion of molecular hydrogen and CO2 to CH4 by anaerobic methane-fermenting Archaea is an important process in the treatment of organic sludges and is essential to the functioning of ruminant animals.
5.4 Photosynthetic Metabolism A small number of bacterial species obtain their energy for growth from light and their carbon for growth from CO2 or HCO3–. One group, the Cyanobacteria, formerly known as blue-green algae, carries out oxygenic photosynthesis, in which molecular
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Biofilm Reactors
oxygen is a product. Blue-green bacteria are important in oxidation ponds, a type of biological wastewater treatment process. Other photosynthetic bacteria may occur in overloaded oxidation ponds but are not of importance in biological treatment systems under normal operating conditions. Cyanobacteria have chlorophyll that absorbs light in the 800- to 900-nm wavelength range, and the principal mode of growth is light adsorption with concomitant lysis of H2O. Oxygen is a byproduct, and CO2 is assimilated as the carbon source. Ammonia and nitrate both are used as nitrogen sources. A number of Cyanobacteria species can fix nitrogen. Simple organic compounds can be used as carbon, but not as energy sources, by a number of Cyanobacteria species, and a few species can grow heterotrophically. Thus, the Cyanobacteria are extraordinarily adaptable microorganisms, in terms of their nutrition, and it is not surprising that they predominate in many natural environments. Fixed-growth biological treatment processes do not provide suitable conditions for their growth, however, and the use of Cyanobacteria essentially is limited to faculative and aerobic oxidation ponds. Anoxygenic photosynthesis is carried out by two other groups of phototrophic bacteria—the Rhodospirillaceae, or purple nonsulfur bacteria, and the Chlorobiaceae, or green bacteria. Anoxygenic photosynthesis does not result in oxygen generation and does not occur in the presence of oxygen. The principal wavelengths absorbed by the phototrophic bacteria are between 400 and 500 nm and 700 and 1000 nm. A number of species can grow heterotrophically on simple sugars, amino acids, and organic acids. Although these bacteria typically are considered anaerobes, most are not extremely sensitive to oxygen, and many can grow in micro-aerophilic environments. Neither the green or purple bacteria are of great importance in fixed-growth systems.
5.5 Nutrient Requirements Microbial metabolism is directed toward reproduction of the organisms, and this requires that chemical constituents of cell components are available for assimilation and incorporation to new cell components. The elements listed in Table 2.4 are required in the approximate proportions given. Note that the two empirical formulas given above in the Chemical Composition of Cells section have approximately the same percentages of the principal elements. For example, in the empirical formula C5H7NO2, the percentages of carbon, hydrogen, nitrogen, and oxygen on a mass basis are 53, 6, 12, and 28, respectively. Nutrients typically are assimilated from a limited number of elemental states; therefore, both the amount of nutrients present and the state of the nutrients are
Biology of Fixed-Growth Process
important. For example, heterotrophic bacteria require carbon in the organic form. Many bacterial species are able to use only a limited range of organic compounds. Only a few species are capable of metabolizing the 5-carbon sugar lactose in the absence of oxygen, and this property is used to determine the presence of coliform organisms (species that are common in the gut of warm-blooded animals) in water samples. The most probable number test for coliform bacteria is constructed on the basis of this fact. Most bacteria can assimilate nitrogen in the ammonia (-3), nitrite (+3), and nitrate (+5) oxidation states. Sulfur assimilation typically is from the sulfate (+6) oxidation state. Metals are assimilated from the ionic state, in almost all cases. Often one or two nutrients in the environment limit microbial growth. The limiting nutrient concept is useful in predicting the effects of pollutants on receiving waters and in designing and operating biological treatment processes. The Great Lakes in the United States and Canada are phosphorus-limited, and the increases in phosphorus inflow rates that resulted from the introduction of biodegradable detergents in the late 1960s greatly increased the rates of eutrophication. The removal of phosphorus from wastewater discharges to tributaries of the Great Lakes was an application of the limiting-nutrient concept. Many industrial wastewaters are unbalanced, with respect to nutrients, and stoichiometric additions of the growth-limiting nutrients (typically nitrogen and/or phosphorus) must be made.
5.6 Bacterial Energy Metabolism Respiration is the mechanism through which the electrons removed from energy sources are discharged to the cell’s external environment and when use of a terminal electron acceptor is coupled to energy production. Typically, respiration involves a series of oxidation–reduction (redox) reactions, but the final step involving a terminal electron acceptor defines the type of respiration occurring. Two types of respiration occur—aerobic, in which molecular oxygen serves as a terminal electron acceptor, and anaerobic, in which inorganic compounds or ions, such as nitrate (NO3–), sulfate (SO4–2), S0, CO2, Fe3+, Mn4+, SeO4–2, AsO33–, and ClO4–, or organic compounds, such as fumarate, chlorobenzoate, and dimethyl sulfoxide, serve as the terminal electron acceptor. The terminal electron acceptor is discharged by the cell to the environment; thus, the use of organic compounds as electron acceptors results in the discharge of organics to the liquid outside the cell. Fermentation reactions are used by bacteria to catabolize organic compounds when there is no external electron acceptor available. The organic substrate is used as the electron donor, and the electron acceptor is a high-energy intermediate product
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of the transformation of the donor that can yield ATP (the energy currency of life systems) via substrate-level phosphorylation. There are several types of fermentations, and some, such as the production of CH4 from acetate, are of importance in fixed-film processes.
5.7 Aerobic Growth and Respiration The use of oxygen as a required terminal electron acceptor is the principal characteristic of aerobic metabolism. Heterotrophic metabolism will be used as the model in this section, although important autotrophic organisms, such as the nitrifiers, are also aerobes. Biological treatment systems nearly always are made up of mixed microbial populations, and, because of the metabolic interactions of the various species, the principal end-products of heterotrophic aerobic growth and respiration are new microbial cells, CO2, and water (H2O). Other products may be produced in significant quantities, but this is not the general case. Models of aerobic metabolism generally are based on the conceptual expression given in eq 2.1.
New Organic + O 2 + Nutrients → bacterial + CO 2 + H 2 O matter cells
(2.1)
The new microbial mass term in eq 2.1 includes all synthesized materials, including microbial exopolymers. Experimentally, the value of the new microbial mass term is measured as the increase in either suspended or volatile suspended solids (VSS). Volatile suspended solids measure only organic solids and are better suited to describe biomass in a reactor. Writing eq 2.1 in the form of a stoichiometric balance is not possible in the general sense, because conversions are related to the characteristics of the organic material, the species of organisms present, and the treatment system operating characteristics. However, development of stoichiometric expressions is a necessary step in treatment process design, because estimates of both oxygen requirements and excess solids production must be made. In nearly all applications of biological treatment, a mixture of organic compounds is present, some of which are soluble and some of which are in the particulate form. This situation complicates the application of stoichiometric relationships, because the mixed microbial populations generally attack a wide variety of soluble and particulate materials simultaneously. The result is that surrogate variables for the organic reactant concentration must be used.
Biology of Fixed-Growth Process
The most common surrogate variable is the biochemical oxygen demand (BOD), although chemical oxygen demand (COD) and total organic carbon (TOC) are used, to some extent. In most cases, the BOD value reported in laboratory analyses is the 5-day, 20 °C value. However, the BOD value that represents the concentration of biodegradable organic material present is the ultimate BOD (UBOD). If the systems used in biological treatment were mathematically linear, the BOD5 value could be used, but this is not the case. Thus, care should be taken in process design to ensure that UBOD values are estimated and used in calculations. Models used to evaluate and design wastewater treatment processes generally use COD as a basis for quantifying the strength of the wastewater. This is because COD provides a consistent and convenient method for computing mass balances, which are fundamental to calculating, for example, the carbon entering and leaving a process, waste sludge produced, and oxygen consumed. The COD measures the electron-donating capacity of organic material, and, because electrons cannot be created or destroyed, it provides a conservative basis for these calculations. The issue of how much of the COD represents biodegradable organic matter is dealt with by characterizing the COD in terms of its different fractions. This is discussed further in Chapter 11 on modeling.
5.8 Anoxic Conditions and Respiration Anoxic conditions will be defined to occur when oxygen is absent from the liquid environment, or present at very low levels, and either NO3– or NO2– are available as the exogenous (from outside the cell) terminal electron acceptor. Anaerobic respiration is applied to situations where CO2, SO4–2, or certain organic compounds are available as terminal electron acceptors. While commonly used in environmental engineering practice, this differentiation is not made in the discipline of microbiology, and metabolism is termed aerobic, anaerobic, or fermentative. The term anoxic then is reserved to describe an environment that is free of molecular oxygen. We will adhere to the usage in environmental and wastewater engineering, but caution the reader about conflicting definitions and uses in the literature. A large number of heterotrophic bacterial species are capable of reducing NO3– to NO2– in a manner analogous to the use of oxygen as a terminal electron acceptor. These bacteria are all members of a group termed facultative anaerobes; that is, they can metabolize and grow under aerobic or anaerobic environments. A smaller number of heterotrophic facultative anaerobes are able to reduce NO2– to gaseous N2O and N2, with the end product distribution determined by the pH of the liquid. The organic
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Biofilm Reactors
materials used to drive the reduction processes (termed denitrification or dissimilatory nitrate reduction) are quite varied. Single carbon compounds, such as methanol, CH3OH, can be used, in addition to the more conventional types of organics found in domestic wastes. A second type of NO3– and NO2– reduction, termed assimilatory reduction, is used by many species of bacteria to produce a –3 oxidation state nitrogen for incorporation to organic molecules, such as the amino acids that make up protein molecules. Assimilatory reduction is not tied to energy metabolism, is not analogous to the use of oxygen in aerobic respiration, and occurs under both aerobic and anaerobic conditions. Because energy is expended in reducing NO3– and NO2–, the mass yield of microorganisms will be lower than for systems where NH3 is the nitrogen source. Both obligate aerobes and facultative anaerobes can carry out assimilatory reduction. Nitrogen compounds also can be removed from wastewater using anoxic ammonia oxidation (anammox), with NH4+ acting as an electron donor and NO2– as a terminal electron acceptor. This process is carried out by slow-growing bacteria of the Planctomycetes phylum of Bacteria, which are autotrophic. In biofilms, there exist both oxic and anoxic zones; hence, aerobic nitrifiers produce the NO2– that becomes the electron acceptor necessary for the anammox process.
5.9 Anaerobic Respiration and Fermentative Metabolism The common anaerobic terminal electron acceptors of importance are SO4–2 and CO2. Ferric (Fe+3) ion also is used by some bacteria, including many of those that reduce NO3–. However, the insolubility of Fe+3 makes the ion an insignificant factor as an electron acceptor in the biological treatment of water. The bacterial species capable of dissimilatory SO4–2 reduction are not numerous (limited to approximately 8 genera), and the reaction product, hydrogen sulfide (H2S) typically is noticeable because of its odor. All of the SO4–2 reducers are obligate anaerobes; that is, they do not grow in the presence of oxygen. Common energy sources for dissimilatory SO4–2 reduction are H2, lactic acid, and pyruvic acid. As in the case of NO3–, a large number of bacterial species, both aerobic and anaerobic, can reduce SO4–2 in an assimilatory manner. The ability to reduce SO4–2 is almost essential for organisms growing in soil and water, because nearly all of the available sulfur is in the SO4–2 form. Carbonate respiration involves H 2 as an electron donor and CO 2 as a terminal electron acceptor; it can lead to the formation of CH4 (in methanogenesis),
Biology of Fixed-Growth Process
carried out by a limited number of genera belonging to the Archaea, or acetate (in acetogenesis), which is performed by homoacetogens belonging to the Bacteria. Both groups are strictly anaerobic. Similar to NO3– and SO4–2, there also are a great number of species belonging to all three domains of life that can assimilate CO 2 as a nutrient source without concurrent use as an electron acceptor under aerobic or anaerobic conditions. The use of electron acceptors—those produced within the cells or by other cells— results in the fermentation of a variety of relatively low-molecular-weight organic compounds. Typical products are low-molecular-weight (volatile) organic acids, alcohols, and aldehydes. The principal volatile acids produced are formic, acetic, propionic, and butyric, with acetic being the most important. Methanogenic members of the Archaea, such as Methanosaeta and Methanosarcina, oxidize acetic and formic acids with the concomitant production of CH4 and CO2. Hence, they are distinct from autotrophic methanogens using H2 as an energy source and reducing CO2 (or HCO3–) as a terminal electron acceptor to CH4. Anaerobic growth by fermentation and respiration can be used to completely remove organic materials from water and wastewater only if methane fermentation is included as a step in the process. Methanogens are the final link in a series of interdependent biochemical reactions involving different organisms (termed syntrophic reactions), which characterize anaerobic environments in fixed-growth reactors treating biological waste. The concept is attractive, because the methane has a very low solubility and can be collected and used as a fuel. However, the energy for growth derived from methane fermentation is relatively small, and the rates associated with growth are low. Economic operating conditions generally require that operating temperatures be in the range 30 to 35 °C, which requires that the reactors be heated. Generation of sufficient methane to heat the systems requires that strong, or high-concentration, organic feed solutions are used. Organic sludges produced in wastewater treatment plants (WWTPs) and industrial wastewaters, such as those from food-processing industries, are the principal candidates for application of anaerobic fermentation treatment processes. Anaerobic sludge digestion is one of the most common methods of stabilizing sludges resulting from wastewater treatment, and sufficient excess methane often is produced to allow heating of buildings and, in some cases, production of electricity. In addition, low-temperature (<20 °C) anaerobic treatment of wastewater is possible using fixed-growth processes, but more insights to microbial community changes are needed to ensure stable maintenance of reactor operations (Enright et al., 2007).
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Biofilm Reactors
5.10 Energetics of Respiration The molar free energy released or consumed in a reversible reaction is given by eq 2.2.
∆G = ∆Go + RTlnQ
(2.2)
Where ∆G = free energy change, kJ/mol; ∆Go = standard free energy change, kJ/mol; T = absolute temperature, Kelvin (K); R = universal gas constant, J/K mol; and Q = reaction quotient (Q ≈ K, the equilibrium coefficient in dilute solutions). Electrical potential (in volts) can be related to the free energy change by the Nernst equation, as follows:
∆G = –nFE
(2.3)
Where n = number of electrons transferred in the reaction, and F = Faraday’s constant (9.649 × 104 J/mol∙V). In practice, ∆E is referenced to the standard hydrogen electrode for which a standard free energy change, ∆Go, of 0 is assigned to the half-reaction when the reaction quotient, Q, is equal to 1. 1/ 2 [H2 ] 1 + H + e– = 2 H2(gas) Q = = 1 (2.4) H + Note that, by convention, the activity of a gas is 1 and, therefore, [H+] = 1. Electrical potentials relative to the standard hydrogen electrode are designated by EH and, therefore,
∆G = -nFEH
(2.5)
Substitution of eq 2.5 into eq 2.2 gives the following:
–nFEH = –nF EHo + RTlnQ EH = EHo –
RT ln Q nF
(2.6) (2.7)
For the hydrogen ion reaction of eq 2.2, the standard free energy, ∆Go, is set equal to zero by convention; therefore, the standard electrode potential is also zero. Reactions in the environment typically occur under nonstandard conditions, that is, when the reaction quotient for the hydrogen ion reaction of eq 2.2 is not equal to 1
Biology of Fixed-Growth Process
and the temperature is not equal to 0. Comparisons typically are made for pH = 7 and 25 °C, where the standard potential is as follows: 1/ 2 RT RT [1] EH(W) = EHo – ln Q = 0 – ln −7 nF nF 10 (8.31J / mol ⋅ K )(293K ) ln 107 (1)(9.649x10 4 J / mol ⋅ V )
= −
= 0.0257(16.12) = –0.407 V
Standard potentials for selected environmentally important half-reactions are given in Table 2.7. In each case, the value of EH(W) is obtained by subtracting the functional term RTlnQ/nF from the standard potential. Table 2.7 Standard reduction potentials at 25°C for selected environmentally important redox couples. E (V)
E(W) (V)
∆G(W)/na (kJ/mol)
1.59
1.18
–113.5
1.22
0.81
–78.3
1.24
0.74
–71.4
0.52b
–50.2b
0.83
0.42
–40.5
NO3–+ 10H+ + 8 e– = NH4+ + 3H2O FeOOH(s) + HCO3– + 2H+ + e– = FeCO3(s) + 2H2O Pyruvate + 2H+ + 2e– = lactate SO –2 + 9H+ + 8e– = HS– + 4H O
0.88
0.36
–34.7
S(s) + 2H+ + 2e– = H2S(g) CO2(g) + 8H+ + 8e– = CH4(g) + 2H2O CO2 + HCO3– + H+ + e– = CH3COO– + 3H2O 2H+ + 2e– = H
Half-reaction 2HOCL + 2H+ + 2e– = Cl + H O 2
2
O2(g) + 4H+ + 4 e– = 2H2O 2NO3– + 12H+ + 10 e– = N2 + 6H2O Cl + 2e– = 2Cl– 2
MnO2(s) + HCO3– + 3H + 2e– = MnCO3(s) + 2H2O NO – + 2H+ + 2 e– = NO – + H O 3
2
4
2
2
2
6CO2 + 24 H+ + 24 e– = C6H12O6 + 6H2O a
Based on [HCO3–] = 10 M. Number of moles transferred.
b
–3
1.40
–0.05b
4.6b
–0.19
18.3
0.25
–0.22
21.3
0.17
–0.24
23.5
0.17
–0.25
23.5
–0.29
27.65
0.00
–0.41
39.6
–0.01
–0.43
41.0
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Biofilm Reactors
5.11 Example 2—Calculation of Electrode Reduction Potentials and ∆G for Half-Reactions Determine the electrode potential for the reduction of NO3– to NO2– at temperatures of 15 and 25 °C and pH values of 7 and 10.
5.12 Solution The solution can be obtained as follows:
(1) The half-reaction and standard electrode potential are NO3– + 2H+ + 2 e– = NO2– + H2O EH(W) = 0.83
(2) Determine the electrode potential at 15°C. pH = 7 NO −2 }{H 2 O} { RT EH(W) = 0.83 − ln 2 nF {NO −3 }{H+ }
For equal molar concentrations of NO3– and NO2–, that is {NO3–} = {NO2–}, and remembering that the activity of water is 1.0,
EH(W) = 0.83 −
EH(W) = 0.83 −
pH = 10
RT 1 RT ln = 0.83 − In (107)2 2 + nF nF H
(8.314J / mol ⋅ K)(285K) ln 1014= 0.83 − 0.40 = 0.43 V (2)(9.649 × 104 J / mol ⋅ V) EH(W) = 0.83 −
EH(W) = 0.83 −
RT ln(1010)2 nF
(8.314J / mol ⋅ K)(285K) ln 1020 = 0.83 − 0.57 = 0.26 V (2)(9.649 × 104 J / mol ⋅ V)
(3) Determine the electrode potential at 25 °C.
pH = 7
EH(W) = 0.83 −
(8.314J / mol ⋅ K)(285K) ln 1014 = 0.83 − 0.41 = 0.42 V (2)(9.649 × 104 J / mol ⋅ V)
Biology of Fixed-Growth Process
pH = 10
EH(W) = 0.83 −
(8.314J / mol ⋅ K)(285K) ln 1020 = 0.83 − 0.59 = 0.24 V (2)(9.649 × 104 J / mol ⋅ V)
Thus, the electrode potential is not very sensitive to the temperature value (within the typical environmental operating range). However, the potential value is quite sensitive to the operating pH. A sense of the relative attractiveness of electron acceptors in biochemical reactions can be attained by comparing the free energy change of the overall redox equations. An example is the oxidation of glucose (C6H12O6) using oxygen (O2) and NO3– as the electron acceptors. Determination of the free energy change is accomplished by application of the Nernst equation (eq 2.3) using the standard reduction potentials available in the literature. For the glucose –O2 and glucose NO3– reaction systems, the relevant half-reactions are as follows:
6CO2 + 24 H+ + 24 e– = C6H12O6 + 6H2O
(2.8)
O2(gas) + 4H+ + 4 e– = 2H2O
(2.9)
2NO3– + 12H+ + 10 e– = N2 + 6H2O
(2.10)
The free energy change for the two reaction systems is compared by writing the half-reactions on a per-electron basis, as follows: 1 1 1 CO2 + H+ + e– = C6H12O6 + H2O E(W) = − 0.43 (2.11) 4 24 4
1 1 O (gas) + H+ + e– = H2O E(W) = 0.81 4 2 2
(2.12)
1 6 1 3 NO3– + H+ + e– = N2 + H2O E(W) = 0.74 5 5 10 5
(2.13)
The ∆G values for the total reactions then are calculated, as follows: Glucose/O2
1 C H O + O2(gas) = CO2 + H2O 6 6 12 6
∆E(W) = E(W)reductant – E(W)oxidant = 0.81 − (−0.43) = 1.24 V ∆G(W) = -F∆E(W) = –119.6 kJ/mol
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Glucose/NO3–
C6H12O6 + NO3– + H+ = CO2 + N2 + H2O
∆E(W) = E(W)reductant – E(W)oxidant = 0.74 − (−0.43) = 1.17 V
∆G(W) = –F∆E(W) = –112.9 kJ/mol
Oxygen is a more attractive electron acceptor than nitrate, on the basis of available energy from the reaction, and this is one reason that oxygen dominates among the range of electron acceptors used by bacteria.
5.13 Co-Metabolism Organic compounds that do not serve as growth substrates are sometimes broken down through a process commonly called co-metabolism or incidental metabolism. The biodegradation process typically is initiated through the action of an enzyme having little specificity, which typically attacks growth substrates. In some cases, carbon from the breakdown of materials co-metabolized has been shown to be incorporated to cells, which may indicate that products of the initial reactions may be growth substrates. However, often, the product of co-metabolism is a dead end, that is, another non-growth substrate. The most widely studied co-metabolic reactions are associated with the breakdown of chlorinated organic compounds and, in particular, the widely used solvent TCE. Aerobic biodegradation of TCE is initiated by methane monooxygenase enzymes used to break down methane (methanotrophic bacteria) (AlvarezCohen and McCarty, 1991); ammonia monooxygenase (Hyman et al., 1988), which catalyzes the conversion of ammonia to hydroxyl ammine in members of the genus Nitrosomonas; and phenol monooxygenase and toluene dioxygenase, which catalyze the first steps in the breakdown of phenol and toluene, respectively, in members of the genus Pseudomonas (Fan and Scow, 1993). Co-metabolism of TCE presents the following two interesting problems:
(1) TCE strongly competes for the active site with the growth substrate (see the Consortia section); and (2) The reaction product, TCE epoxide, apparently sorbs onto the oxygenase enzyme and destroys the catalytic activity.
Incorporation of co-metabolic processes to treatment systems presents real challenges.
Biology of Fixed-Growth Process
6.0 BACTERIAL GROWTH The objective of biological treatment is the development of a process that will transform contaminants in a water or wastewater into an acceptable form or state. However, from the viewpoint of the microorganisms used in treatment processes, the objective is simply to replicate themselves. The result of replication is an increase in total microbial mass, or mass concentration, and this increase is defined as growth. In some cases, the microbial objective is modified to surviving austerity until replication is possible again. Examples of austerity include dry and low-temperature periods in soil and the segments of biological wastewater treatment systems in which the primary organic substrates are exhausted. Bacterial replication, or growth, requires energy; carbon (to build the organic compounds making up the cell); and inorganic nutrients, such as nitrogen, phosphorus, sulfur, and iron, which are components of organic compounds or serve in other ways in the chemical reaction processes carried out within the cell. Heterotrophic metabolism, where organic compounds serve as both the energy source and the carbon source, is the most important type of metabolism in biological treatment and will be used as a model.
6.1 The Bacterial Growth Curve When bacteria are grown in a batch culture with the initial condition that nutrients are not limiting, a characteristic pattern of growth occurs, which is commonly divided into the six segments shown in Figure 2.6—lag, increasing growth, logarithmic growth, declining growth, stationary, and death. The bacterial growth curve shown in Figure 2.6 is representative of pure (i.e., single species) cultures only, but the concepts generally are applied to other systems in a modified form. In the experimental process of producing a growth curve, the number of cells is determined by sampling at selected time intervals and using standard microbial plate-count techniques to estimate the population. A similar curve can be generated by measuring the mass concentration of cells present, and, because each cell can be presumed to have approximately equal mass, the two curves should be proportional. The limiting nutrient typically is the energy source (an organic compound for heterotrophic species), but could be an inorganic nutrient, such as nitrogen or phosphorus. Typically, the cells used in growth-curve experiments are taken from cultures in the stationary phase, and a period of time, or lag, is required before growth is initiated. The lag can be eliminated by transferring cells from a culture in the logarithmic
51
Stationary
Decreasing Growth
Lag No
Log Growth
Co
Increasing Growth
Limiting Nutrient Conc., mg/L
Biofilm Reactors
Log10 Number of Cells
52
Death
Figure 2.6 Bacterial growth curve for a batch system in which nutrients initially are not limiting. As growth begins, the increase in cell number (or mass) is dependent only on the number (or mass) of cells present, and the system behaves in a firstorder autocatalytic manner.
growth phase. A short period of time, in which the growth rate changes from zero to the maximum value, exists (the increasing growth phase), which is probably the result of an uneven conversion of the population from non-growth to growth conditions. During the logarithmic growth phase, the rate-limiting variable is the number of cells present. Because the cells are a reaction product, the overall rate typically is observed to be first-order autocatalytic, with respect to the cell number or mass concentration. As the limiting nutrient concentration decreases to the point where the individual cells must compete for the material, the overall growth rate slows (decreasing growth), and, when the limiting nutrient concentration approaches zero, the culture begins to convert to a resting state, in which cell division essentially stops, and cell functions are maintained by catabolizing unnecessary materials and storage compounds. The unnecessary materials include transferase enzymes necessary for transporting nutrients across the cell membrane and internal metabolic enzymes
Biology of Fixed-Growth Process
used in processing externally derived nutrients. Often, the capsular layer increases in size, possibly to limit desiccation in dry soils. After a period of time extending from several hours to days, the culture begins to die. Upon death, cells typically breakup (lyse), and many cell components become nutrient sources for surviving cells.
6.2 Growth in Mixed Cultures Mixed microbial cultures typically are dominated by a few species of bacteria. However, a large number of microbial groups typically are present, and all of the groups present occupy ecological niches. In most cases, several species present will be able to at least partially metabolize organic compounds initially present. As breakdown progresses, other species may be able to metabolize partially degraded materials or excreted waste products of the first metabolizing groups. Higher forms of organisms will begin to graze on the heterotrophs as the population increases. An interlocking set of growth curves is produced that cannot adequately be described by cell counts of a limited number of species. Total mass concentration commonly is used to follow the process, but the values measured are the sum of all species present, and, as time increases, they increasingly represent the growth of secondary organisms. This interaction of microbial groups in mixed cultures is one reason that kinetic and stoichiometric parameters measured under one set of conditions often are difficult to extrapolate to other conditions.
6.3 Enrichment Cultures When mixed cultures are operated under constant conditions, either steady-state feed and physical environment or repeated identical cycles, species with the highest growth rates will have a competitive advantage and will gradually predominate. In some cases, the procedure results in cultures limited to one or two species. Such operating conditions are called enrichment cultures, and the technique is used in the laboratory to isolate and grow species having particular desired characteristics. Enrichment culture conditions often exist in industrial wastewater treatment because of the limited number of biodegradable compounds present, the high temperature, or the low pH of the wastewater. In biological water and wastewater treatment, enrichment culture principles are used to develop flocculant cultures for suspended-growth systems and polyphosphate-storing cultures for phosphate-removal systems and also to control the growth of filamentous bacteria.
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Biofilm Reactors
6.4 Stability of Mixed Cultures The use of mixed cultures for water and wastewater treatment results in more stable process operation. In most treatment operations, characteristics of the influent stream are changing constantly. Flowrate, chemical constituents concentrations, and temperature change on a continuous basis. Such varying conditions help maintain the mixed culture, by providing environmental conditions in which a broad spectrum of bacterial species can compete. The principal advantages of maintaining mixed cultures are that a more complete breakdown of organic compounds present occurs and that changes in influent characteristics typically do not greatly affect effluent quality. Bio-oxidation by single species of bacteria often results in some compounds being left unoxidized. In many cases, certain branched aliphatic compounds or substituted rings are attacked by a limited number of bacterial species. An organic polymer may be broken down partially by several species in pure culture and completely broken down in mixed culture. Similarly, when influent characteristics change, we might expect different microbial species to have competitive advantages. For example, the predominant bacterial species present may change from summer to winter in municipal treatment plants or with the products being produced in an industrial operation. Maintaining mixed cultures allows the most competitive species for a given set of conditions to predominate in a matter of days.
6.5 Effects of Environmental Variables Environmental variables affecting bacterial growth include temperature, pH, chemicals in the water, and the presence of surfaces on which microbial films can be established. Temperature strongly affects the reaction rates; has minor, if any, effects on reaction stoichiometry (Flegal and Schroeder, 1976); and, under conditions where molecular diffusion is important, mildly affects the rate of transport of nutrients to the cells (Kehrberger et al., 1964). The relationship between temperature and reaction rate differs with microbial species, and, in mixed cultures, temperature changes may result in significant changes in predominant species. Temperature-growth rate relationships are discussed in greater detail in the following section. Bacteria typically function well within broad pH ranges. Most of the bacterial species commonly found in wastewater treatment processes grow between pH 6 and 9, but activity falls off rapidly outside of that range. Nitrifying bacteria grow best under slightly alkaline conditions, such as pH 8 to 9.5. The sulfur-oxidizing bacteria
Biology of Fixed-Growth Process
are quite acid-tolerant, and some species grow well at pH values as low as 1, but do not grow at all at pH values above 6. When the growth rate is reported as a function of pH (Grady et al., 1999), clear optimum pH values are rarely apparent, and the curves are virtually flat within the region of growth. The role of dissolved organic and inorganic chemicals as nutrients, energy sources, and toxins is discussed elsewhere in this chapter. Total solute concentration, typically approximated satisfactorily by the salinity of water, is of some significance, because many industrial wastes are quite saline, and because the discharge of treated wastewater to saline environments is common. Biological treatment of saline wastewaters may be affected by differing microbial sensitivities to salinity; that is, species predominance may be affected. In discharging treated wastewater to saline environments, bacterial survival may be related to salinity, but the available data are not adequate to suggest modification of discharge requirements for ocean versus freshwater conditions. Most bacterial species are able to adjust to broad ranges in salinity (e.g., from less than 100 mg/L to 30 000 to 34 000 mg/L in seawater) and osmotic pressures if changes are made incrementally. A few species of bacteria, known as halophiles, grow at concentrations as high as 360 000 mg/L (Niedhardt et al., 1990). Large, rapid decreases in salinity may result in the leaching of water from cells and damage to the cytoplasmic membrane. Large, rapid increases in salinity may result in the accumulation of water within cells to the point that they burst. In general, salinity is not a major factor in the design or operation of biological treatment processes.
7.0 BACTERIAL GROWTH KINETICS IN BIOFILMS Bacterial growth is the result of a large number of extra- and intracellular reactions that provide energy, carbon, and nutrients for the biosynthesis process. Design and operation of biological treatment processes are related strongly to the rate of bacterial growth, and, for this reason, discussion of bacterial growth kinetics is essential. However, bacterial growth theory was developed from experiments with pure cultures, while biological treatment processes are almost always composed of mixed cultures. Rates of growth and nutrient removal typically are measured using lumped or surrogate parameters, such as suspended or volatile solids, BOD, COD, or TOC, and global rates observed cannot be extrapolated or generalized. Even with this limitation, the application of rate process concepts is an essential component of biological process design and operation.
55
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7.1 Rate of Bacterial Processes Rate expressions used to describe the removal of organic material, growth of the microbial population, and use of oxygen, the three terms of greatest importance in eq 2.1, typically are based on the modified Monod model given in eqs 2.14 through 2.16.
ro = −
kC X K +C
kC − kd X rg = − Yro − kd X = Y K +C
rO2 = ro + γ rg = ro − γ (Yro + kdX)
= ro(1 − γY) − γkdX
(2.14) (2.15)
(2.16)
Where ro = rate of removal of organic material (g/m3·d); rg = rate of production of new microbial mass (g/m3·d); rO2 = rate of oxygen use (g/m3·d); k = rate of removal coefficient (d–1); C = organic concentration, often measured as UBOD or COD (g/m3); X = microbial mass concentration measured as suspended solids or volatile suspended solids (g/m3); K = saturation coefficient (g UBOD/m3); Y = mass yield coefficient (g solids produced/g UBOD removed); kd = maintenance energy rate coefficient (d–1); and γ = ratio of UBOD removed to cell mass produced (g/g).
7.2 Note
C X, where µm is the maximum K +C specific growth rate or the maximum mass of cells produced per unit mass present per unit of time. This nomenclature is awkward from the process analysis viewpoint because of the way that the maintenance energy must be introduced. The solids production rate, rg, and oxygen uptake rate (OUR), rO2, are shown as functions of the organic removal rate, and more specifically the organic (UBOD) and microbial mass concentrations. These expressions (eqs 2.14 through 2.16) will be used to describe overall metabolism in all types of biological processes. The growth rate often is written as rg = µm
Biology of Fixed-Growth Process
Often, the specific rates, or rates per unit mass of microbial mass present, are reported rather than the overall rate. For example, the specific removal and growth rates are stated as follows:
Ro = Rg =
ro kC =− X K +C
rg X
=Y
kC − kd K +C
(2.17) (2.18)
In some commonly used biodegradation and growth models, rate expressions are written for two or more components of the organic mixture. For example, the UBOD may be divided into soluble and particulate portions, and separate expressions may be written for each component (Henze et al., 1995; Metcalf and Eddy, 2003). The ratio of UBOD removed to cell mass produced, γ, is based on chemical stoichiometry and empirical cell formulations. The value also can be referred to the theoretical BOD of the cells produced. The most commonly used empirical formula for cells is C5H7NO2, but a number of other formulas are in use (McCarty, 1965; Porges et al., 1953). A value of γ is estimated by calculating the stoichiometry of the oxidation of the empirical cell formulation, as shown in eqs 2.19 and 2.20.
C5H7NO2 + 5 O2 → 5 CO2 + NH3 + 2H2O γ=
5(32) = 1.42 g/g 113
(2.19) (2.20)
Other suggested empirical formulas result in similar values of γ. For example, the formula C60H87O23N12P has a theoretical UBOD of 1.39 g/g. It is important to remember that eqs 2.14, 2.15, and 2.16 are global, nonstructured, empirical models, which sum up the results of a large number of coupled reactions, including interphase transport from the liquid to the solid cell surface. The assumption that growth of new cell mass and uptake of oxygen are coupled directly to nutrient removal is not completely true. Organic material may be removed from the liquid by adsorption onto/into biofilms without immediate production of new cell material and without oxygen uptake (Dobbs et al., 1989). If the sorbed material is biodegradable, a fraction of the sorbed mass will show up later in time, as new cell mass and a corresponding oxygen uptake would result. These events are illustrated in Figure 2.7, and descriptions of these more complex phenomena are termed structured models.
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Concentration, mg/L and Rate, mg/L-s
58
Total Organic Carbon mg/L
Cell Mass, mg/L
Oxygen Uptake Rate, mg/L-s
Time, min
Figure 2.7 Relationships between organic removal, cell mass production, and OUR.
As shown in Figure 2.7, organic carbon removal occurs more rapidly than the actual breakdown or degradation of the compounds. As a result, oxygen uptake remains above background levels for a substantial period following carbon removal. Following organic removal, the cell mass concentration may continue to increase somewhat, may be approximately constant (as shown in Figure 2.7), or may decrease slowly, as conversion of the externally stored organic material occurs. When stored organics are used completely, the OUR decreases to the “basal” or “endogenous” value, and cell mass begins to decrease, as cell components, such as ribosomes and unneeded enzymes, are broken down. The use of lumped parameters as measures of the organic concentration (UBOD and COD) and the microbial mass (suspended solids or VSS) in eqs 2.14, 2.15, and 2.16 generally is required because of the complexity of the organic mixtures and microbial populations present in most situations. Compounds are taken up in parallel rather than sequence, and a single microbial species can metabolize more than one type of compound simultaneously. A problem exists where a specific compound must be removed, particularly if the target compound is present at a low concentration. In such cases, removal of the BOD or COD to satisfactory levels may not result in satisfactory degradation of the target compound. In addition, the equations, in the
Biology of Fixed-Growth Process
form presented, do not include either mass transport limitations or consideration of the reactions or reaction sequences that are involved in the biological treatment process. Lumping the organic species present into a single concentration term generally is satisfactory. In most cases, it would be impossible to monitor individual organic contaminants, and, perhaps more importantly, the use of overall concentration parameters based on oxygen consumption (UBOD and COD) provide direct relationships to other parameters of importance, such as growth. Finally, in a given source, the contaminant makeup typically is relatively consistent, and surrogate parameters, such as UBOD, COD, and TOC, provide the necessary rate information. An argument similar to that for the use of overall parameters for organic concentration can be made for the use of an overall mass concentration for the microbial mass. The assumption is made that the actual cell mass concentration is proportional to the particulate filtered solids mass concentration (suspended solids or VSS) measured. This assumption would mean that the EPS-mass-to-cell-mass ratio was constant, for example. Although this and other similar assumptions are somewhat shaky, the models have had considerable utility, and laboratory and field measurements typically follow the patterns shown in Figure 2.7.
7.3 Physical and Chemical Changes in Biofilms Resulting from Growth The growth of bacterial cultures is defined by increases in cell number rather than cell size. In biofilms, daughter cells separate as EPS is produced, keeping the density approximately constant. Because growth is constrained latterly and by the support surface, depth increases as cells divide. Cells at the liquid or atmospheric surface have greater access to nutrients and oxygen. Deeper in the biofilm, cells may starve and die, or anoxic or anaerobic conditions may develop. Thus, gradients in oxygen and nutrient concentration and pH develop as biofilms become thicker (Lewandowski and Beyenal, 2003). In thick biofilms, gas formation at the support surface may result in detachment of the biofilm (Atkinson, 1974), and, where a liquid is flowing over the biofilm, as in trickling filters, weakened portions may be sloughed off.
7.4 Structured Models Inclusion of reaction sequences or reaction process sequences in reaction models is termed structuring. Flow diagrams for two structured models are shown in Figure 2.8. In both cases, a storage term is included, which accounts for non-stoichiometric
59
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Organic materital
r1
→
Storage r2 w ithin → cell
B iodegradation and synthesis of new cell mass
a) Simple structured model of organic metabolism based on intracellular storage Organic material
r1
Intracellular metabolite pool
r
r2
3
Biodegradation and synthesis of new cell material
r4
Intracellular Storage
b) Structured model of organic metabolism based on maximum rates and intracellular storage. An assumption is made that the rate of transport into the cell can exceed the rate of conversion of organics. Materials not reacted are stored, presumably as glycogen or PHB. The stored materials are used during periods when the intracellular metabolite pool is low.
Figure 2.8 Structured models of metabolism include reactions or steps in the metabolic process that can be identified or separated: (a) the rate of metabolism is separated from the rate of organic removal from the liquid and (b) a somewhat more complex storage mechanism is shown, which could be used to account for several observed rates of growth and oxygen uptake.
differences in organic removal and OURs that have been observed. More complicated structured models have been proposed to describe the rates of individual cellular processes, such as protein synthesis. Because each rate term requires knowledge of additional rate coefficients, the use of structured models is difficult, even in research. Application of structured models in biological treatment processes composed of a wide range of microbial species and degrading a mixture of organics currently is not feasible.
7.5 Temperature Effects Removal and growth rates are affected strongly by temperature. Rate–temperature relationships typically follow the van’t Hoff-Arrhenius relationship up to approximately 40 °C (Flegal and Schroeder, 1976). In most cases, a modified form of this relationship is used to relate rate constants at a temperature, T, to those at a reference temperature, TR (typically 20 °C).
Biology of Fixed-Growth Process
kT = kTRθT − TR
(2.21)
Values of the temperature coefficient, θ, typically are in the range 1.02 to 1.06. Including temperature effects in biological treatment process design is critical. Most laboratory studies reported in the literature are run at 20 °C, but winter temperatures may drop to 5 °C in cold climates. Summer operating temperatures in municipal plants may reach 30 °C in hot climates, and some industrial wastewaters may approach 40 °C. Achieving satisfactory removals is a problem at low temperatures, unless the decrease in the value of the rate constant has been considered, while oxygen transfer may be limiting at high temperatures because of the decreased solubility and the increased removal rates. For a θ of 1.04, a 10 °C temperature differential from the reference condition results in an increase in the rate constant of 48%.
7.6 Example 3—Effect of Temperature on Organic Removal A batch biological reactor has an initial cell concentration of 30 mg/L and an initial organic concentration of 400 mg UBOD/L. Oxygen and inorganic nutrients are present in excess throughout the reaction period. The rate and stoichiometric coefficients for the system are given below. Compare the performance of the system at 10, 20, and 30 °C, assuming that the maintenance energy requirement (characterized by -kdX) is negligible. Use a temperature coefficient, θ, of 1.02.
k20 °C = 4.0 d–1, K = 20 mg/L Y = 0.40
7.7 Solution The solution can be obtained as follows:
(1) Determine a relationship between the cell concentration and the change in BODU. Because the maintenance energy requirement is negligible, in this case,
∆X = –Y∆C Xt – X0 = –Y (Ct – C0) Xt = –Y (Ct – C0) + X0 = 0.4(400 mg/L – Ct) + 30 mg/L
(2) Write the mass-balance equation for the BODU of the batch reaction system. In − Out + Generation = Accumulation
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0 - 0 + Vr0 = V
∫
C C0
t K +C = – k ∫ dt 0 C(Xo + YCo − YC )
CXo K Xo + YCo − YC 1 In In = − kt Co (Xo + YCo − YC ) Y Xo + YCo Xo
dC dt
dC kC =− (Y − (C0 − C )+ X0 ) dt K +C
(3) Determine the rate constant values at 10 and 30 °C
kT = K20 °CθT − 20 = 4.0 (1.02T − 20)
k10 °C = 3.28
k30 °C = 4.88
(4) The equation can be solved and tabulated and plotted for the three temperature values (Figure 2.9 and Table 2.8). 500
200
400 300 200 10°C
100 0 0.0
30°C
150 SS, mg/L
BOD, mg/L
62
30°C
0.2
0.4
0.6
0.8
Time, days
20°C
1.0
20°C 10°C
100
50 0
1.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Time, days
Figure 2.9 Effect of temperature on organic removal rate and microbial growth in example 3. Initial BODU and suspended solids concentrations were 400 mg/L and 30 mg/L, respectively. The initial conditions provide for near-exponential growth because K << Co.
Biology of Fixed-Growth Process
Table 2.8 BODU and suspended solids concentrations for the three temperature values. Time (days)
BODU (mg/L)
Suspended solids (mg/L)
10°C
20°C
30°C
10°C
20°C
30°C
0
400
400
400
30
30
30
0.2
377
372
364
39
41
44
0.4
348
333
310
51
57
66
0.6
310
280
234
66
78
96
0.8
260
205
120
86
108
142
0.9
232
160
43
97
126
173
1.0
197
100
111
150
1.1
158
40
127
174
1.15
138
135
190
0.1
7.8 Inhibition and Toxicity Inhibition of microbial metabolism occurs in the presence of materials toxic to the microorganisms or when non-metabolizable materials present adhere to cell surfaces and block the attachment of substrates. Note that metals are toxic in the ionic form and that the oxidation state can be expected to affect the toxicity. Ammonia in the dissolved gas form is toxic to fish at very low concentrations (<0.5 mg/L, in most cases) and to microorganisms at concentrations that rarely occur in WWTPs. The effects of inhibitory substances on growth and removal rates typically are modeled by adding a term to the rate expressions, which results in a maximum removal rate occurring at a critical inhibitant concentration. The Haldane expression (Yu and Semprini, 2004), originally developed as a model for enzyme inhibition by the reaction substrate, often is used to describe rate inhibition that occurs when otherwise biodegradable compounds are present at high concentrations (Figure 2.10). kC 2 ro = − K + C + C Ki
where
i
X
= Haldane inhibition coefficient (mg/L).
(2.22)
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3.5 3.0 Specific Removal Rate, d–1
64
Nonmetabolizable inhibitant k = 4 d–1 K = 30 mg/L K'i = 50 mg/L CSubstrate = 100 mg/L
2.5 2.0 1.5
Substrate Inhibition k = 4 d–1 K = 30 mg/L Ki = 50 mg/L
1.0 0.5 0
0
20
40
60
80
100
Concentration, mg/L
Figure 2.10 Effect of inhibitant concentration on specific removal rate for two conditions: (a) substrate inhibition by a metabolizable inhibitant, represented by eq 2.22, and (b) a nonmetabolizable inhibitant, such as a heavy metal, represented by eq 2.23.
The application of eq 2.22 results in modified forms of eqs 2.17 and 2.18. In cases where inhibition results from the presence of non-metabolizable compounds, such as heavy metals, the inhibition relationship can be expected to be inversely proportional to the inhibitant concentration.
kC K'i ro = − X K + C K'i + CT
Where
K’i = inhibition coefficient (mg/L) and
CT = inhibitant concentration (mg/L).
(2.23)
7.9 Mass-Transfer-Rate Limitations Mass-transfer-rate limitations have been studied by a number of researchers (Atkinson, 1974; de Beer et al., 2004; Logan, 1993; Schroeder and Tchobanoglous, 1976; Swilley
Biology of Fixed-Growth Process
et al., 1964; Williamson and McCarty, 1976). Suggestions have been made that various rates are limited in both suspended- and attached-growth systems and that the limitations affect both process performance and microbial species makeup. Oxygen or one or more nutrients may be mass-transport-limited in the biofilms of fixed-growth processes. Such limitations result in changes in the type of metabolism (e.g., from O2 to NO3– as the terminal electron acceptor) and species predominance and removal and growth rates. A biofilm that is aerobic near the air interface and anaerobic near the supporting surface may have a significant pH gradient with depth.
8.0 KEY TRANSFORMATIONS IN BIOFILMS Chemical transformations in biofilms, as in all microbial processes, are associated with microbial growth. Sources of cell constituents are extracted from the liquid phase. Particulate matter is broken into soluble subunits through reactions catalyzed by extracellular enzymes, and soluble matter is transported across the cell membrane and converted into the necessary cell components in a complex series of enzymecatalyzed reactions. A parallel process exists for generating the energy required for cell functions. In nearly all fixed-growth processes used for wastewater treatment, chemical rather than photosynthetic energy is used, and the energy is released through redox reactions. Organic compound removal in biological processes occurs in a series of steps (Figure 2.11). The mechanism of nutrient transport to the biofilm–liquid interface differs somewhat with the type of unit. The first step, transport of the organic material to the biofilm surface, is brought about by turbulent transport in most reactor types. Only trickling filters are characterized by laminar flow in the liquid film. The mechanism of large molecule and particle attachment is not fully understood and probably involves sequential sorption and desorption. Biofilm surfaces are not smooth on the microscale, and the mechanisms of particle removal, such as interception and impaction, also may be significant. Smaller molecules pass into the spongelike biofilm structure until they come in contact with a bound enzyme or cell surface. A number of bacterial species typically are present, and the predominant species change with environmental conditions or the type of contaminants present. Changes in species predominance would be expected to take some time because of the low average growth rate imposed in most biological treatment processes and because the position of the bacteria in the biofilm would tend to make direct competition for substrate a slow process to develop.
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Transport of particles to biofilm surface Organic particle
Attachment of particles to biofilm
Biofilm
Extracellular breakdown and intrabiofilm transport
Biofilm
Transport into cells and microbial degradation
Biofilm
Figure 2.11 Steps in the removal and breakdown of an organic particle in a biofilm.
The biofilm serves as a storage unit for extracellular enzymes and enzymes released from dead cells and as a concentrator for nutrients. Biological treatment processes do not provide rich growth conditions for bacteria and in fact are operated with just the opposite objective—to produce an environment with minimal concentrations of necessary growth nutrients. Bacteria growing under such conditions need to minimize energy expenditures made in gathering nutrients and maximize the energy channeled toward reproduction. Evidently, the biofilm structure is one response to these constraints.
8.1 Chemoheterotrophic Processes As noted above, in chemoheterotrophic processes, organic compounds serve as both energy and carbon sources for growth. Aerobic processes are of principal interest, because a broader range of compounds can be used, and the majority of compounds
Biology of Fixed-Growth Process
can be oxidized to CO2 and, therefore, completely stabilized. In all cases, new microbial cells are a product, and only a portion of the organics metabolized are oxidized.
8.2 Chemoautotrophic Processes When carbon dioxide or bicarbonate ion, HCO3–, serves as the carbon source and an inorganic ion (e.g., NH3, NO2–, H2S, S, and H2) serves as the energy source for growth, the metabolism is termed chemoautotrophic or simply autotrophic. The oxidation of nitrogen is important in wastewater treatment, because the reaction provides a method of eliminating ammonia toxicity to fish in receiving waters. Sulfide oxidation presents a serious problem in collection systems and treatment plants, because the products (H2SO3 and H2SO4) are extremely corrosive. Hydrogen serves as an energy source in some of the methane-producing reactions that produce methane in anaerobic sludge treatment processes.
8.3 Biology of Nitrogen Transformations Nitrogen exists in oxidation states from −3 to +5, with the most common forms found in wastewater being nitrite (NO2–) and nitrate (NO3–). Cellular nitrogen is entirely in the organic (the majority of compounds can be oxidized to CO2 and, therefore, completely stabilized −3 oxidation state) form. Ammonium is used directly in the growth process, while oxidized forms of nitrogen must be reduced from the +3 or +5 oxidation states using energy supplied by heterotrophic or autotrophic redox reactions in a process referred to as assimilatory nitrate reduction. Assimilatory nitrate reduction is virtually ubiquitous among microorganisms and, beyond resulting in small decreases in cell yield, because of the energy requirement, has little effect on processes.
8.4 Denitrification Under oxygen-limited or anaerobic conditions, many species of bacteria are able to use NO3– and NO2– as terminal electron acceptors, a process referred to as dissimilatory metabolism. Nitrate is reduced to N2 via NO2–, nitric oxide (NO), and nitrous oxide (N2O). This is a beneficial process in wastewater treatment, because it results in an innocuous end product, whereas both NO and N2O are greenhouse gases, and their accumulation is undesirable. The reductase enzymes are repressed at oxygen concentrations greater than 2.5 mg/L, yet, at low concentrations of oxygen (tenths of mg/L), they are only inhibited (Rittman and McCarty, 2001). As a result, it is possible for
67
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Biofilm Reactors
enitrification to occur, even when residual oxygen is present. Biofilms particularly are suited to supporting multiple types of respiration because of their heterogenous structure.
8.5 Aerobic Nitrification The most important autotrophic bacteria in biological treatment systems are the nitrifiers—those organisms that oxidize NH3 and NO2–. Reduced forms of nitrogen are a source of oxygen demand in water, as shown in eqs 2.24 and 2.25.
NH4+ +
3 O2 → NO2– + 2 H+ + H2O 2
(2.24)
NO2– +
1 O2 → NO3– 2
(2.25)
Equations 2.24 and 2.25 are not completely correct, because carbon dioxide consumption and cell production are not included. Because very little energy is produced from the oxidation of NH3 and NO2–, and production of new cell material from CO2 requires a relatively large amount of energy, the cell mass yield per unit mass of nitrogen oxidized is small (typically approximately 0.05 g cells/g N). Estimates of oxygen requirements typically are based on the above equations. Thus, when nitrification occurs, approximately 4.57 g O2 must be supplied per gram of nitrogen oxidized. Domestic wastewaters typically contain 30 to 50 g/m3 of total Kjeldahl nitrogen (TKN), which includes organic nitrogen, NH3, and NH4+. Biodegradation of nitrogen-containing organic compounds results in release of the nitrogen as NH3. Thus, for the purpose of estimating oxygen demand, organic nitrogen can be treated as ammonia-nitrogen. Therefore, complete oxidation of TKN (nitrogen in the −3 oxidation state) in domestic wastewater will result in an oxygen demand of 130 to 190 g/m3. Estimates of the complete stoichiometry of the nitrification reactions have been made using “reasonable” values for the cell yield coefficients (Metcalf and Eddy, 2003); however, supporting data are difficult to obtain. Typical expressions are given in eqs 2.26 and 2.27. 55 NH4+ + 76 O2 + 109 HCO3– → C5H7NO2 + 54 NO2– + 57 H2O + 104 H2CO3
(2.26)
400 NO2– + 195 O2 + NH4+ + 4 H2CO3 + HCO3– → C5H7NO2 + 3 H2O + 400 NO3–
(2.27)
An important feature to note is that, for every mole of ammonia oxidized, approximately 2 mol H+ are produced. This means that 2 mol alkalinity are destroyed in
Biology of Fixed-Growth Process
the process, and, in many cases, nitrification results in significant decreases in pH. Because most biological processes work best in the pH range 6 to 8.5, nitrification can result in decreased efficiency or even process failure, if care is not taken to control alkalinity. The yields predicted from eqs 2.23 and 2.24 are 0.15 and 0.02 g cells/g N, respectively. The nitrifying bacteria are divided into two groups—those that oxidize NH3 and those that oxidize NO3–. The most widely studied NH3-oxidizing bacteria are of the genus Nitrosomonas, but several other genera exist. Nitrosococcus spp. and Nitrosomonas spp. commonly are found in wastewater, and members of the genus Nitrosospira may be the most common in soil (MacDonald, 1978; Stephen et al., 1996). Nitrosomonas also has an anaerobic metabolism and can use dinitrogen tetroxide (N2O4), a dimer of NO2–, as a terminal electron acceptor. Activity is low under anoxic conditions. N. eutropha also is capable of simultaneous denitrification; that is, it uses NO 2– as a terminal electron acceptor at O2 concentrations below 0.8 mg/L. It is estimated to denitrify at least 10% of the converted ammonia (Lipschultz et al., 1981). During this process, NO and N2O are produced. Nonetheless, all of the NH3-oxidizing bacteria are classified as obligate aerobes, because the oxidation of ammonia via hydroxylamine appears to be the only source of reducing power for biosynthesis in these organisms. Nitrobacter and Nitrospira are believed to be the most common genera among the – NO2 -oxidizing autotrophs. At least some of the Nitrobacter species also can denitrify under anoxic conditions, by reducing NO3– to NO2–. In addition, species of Nitrobacter are capable of growing mixotrophically, using organic carbon as a carbon source and NO2– as an energy source and yielding higher growth rates than when they grow lithoautotrophically. Even heterotrophic metabolism is possible under both oxic and anoxic conditions. Such growth is slower than lithoautotrophic growth (Bock, 1976), but can yield higher cell densities (Schmidt et al., 2003). The importance of heterotrophic metabolism by the nitrifying bacteria for fixed-growth systems has not been established, but the rates of nitrification reported in biological treatment are considerably greater than those reported for Nitrobacter growing heterotrophically or mixotrophically in pure cultures. It is entirely feasible that Nitrobacter uses a range of different strategies in biofilms. Nitrification generally is modeled using the modified Monod expressions.
8.6 Anoxic Nitrification/Denitrification Nitrification with concomitant denitrification is carried out under anoxic conditions by bacteria unknown to exist until the late 1980s. The bacteria belong to several genera
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of the order Planctomycetales and are referred to as anaerobic ammonium oxidizers, or anammox bacteria, for short. They have been found in over 30 freshwater and marine environments around the world (Op den Camp et al., 2006) Anammox bacteria use NH4+ as an energy source and CO2 as a carbon source; thus, the process is autotrophic. Both NO2– and NO3– can be used as electron acceptors, with N2 as a product. Anammox bacteria are unable to use oxygen and do not function at oxygen concentrations above approximately 0.5 g/m3. Growth rates and yields of anammox bacteria are very low, even compared with aerobic nitrifiers. However, the saturation (affinity) coefficients, KSNH4+, are less than 0.1 g/m3, resulting in operation at maximum rates at very low NH4+ concentrations. Anammox bacteria are believed to be responsible for over 50% of the nitrogen transformations in the oceans and for most of the N2 in the Earth’s atmosphere. The late discovery of the anammox bacteria is explained partially by the fact that the organisms do not grow under conventional laboratory culturing conditions. Nitrogen transformations that could not be explained by the concentrations of known denitrifiers led to the search for unidentified groups using molecular biological techniques. Anoxic nitrification/denitrification can be carried out in wastewater treatment, in conjunction with organic oxidation, by providing aerobic and anoxic zones spatially (plug-flow) or temporally (sequencing batch operation) (Strous et al., 1997). Aerobic nitrification occurs in the aerobic zones, providing the NO2– necessary for anammox in the anoxic zones. A stoichiometric expression for the anammox nitrification/denitrification process is given below (Strous et al., 1998).
NH +4 + 1.32 NO 2− + 0.066 HCO3− + 0.13 H + →
1.02 N 2 + 0.26 NO3− + 0.066 CH 2 O0.5 N 0.15 + 2.03 H 2 0
(2.28)
The term CH2O0.5N0.15 is an empirical formula for cell products. Note that the predicted cell yield is 0.11 g cells/g N oxidized. Domestic wastewaters have relatively low concentrations of ammonium and organic nitrogen, and maintaining an anammox population in a conventional treatment process is difficult. Application of anammox to industrial wastewaters containing high concentrations of ammonia or urine streams separated from domestic wastewater is promising.
8.7 Biological Phosphorus Removal Biological phosphorus removal provides an excellent example of the relationships that can be developed in microbial consortia treating wastewater. Several uncultured
Biology of Fixed-Growth Process
heterotrophic bacteria are capable of producing storage materials in the form of polyphosphates when volatile fatty acids (VFAs) are present in the feed and maintaining 2 to 5 times higher levels of phosphorus in the cell than typically are present in the biomass. Candidatus Accumulibacter phosphatis or Rhodocyclus-related bacteria have been found in several enriched laboratory-scale reactors and also in full-scale plants (Oehmen et al., 2007). Other organisms appear to be involved in enhanced biological phosphorus removal (EBPR), including Actinobacteria (Kong et al., 2005). Polyphosphateaccumulating organisms use this material as an energy source when exogenous terminal electron acceptors are unavailable. Polyphosphate use does not occur unless the fluid is devoid of oxygen, nitrate, and nitrite. The presence of sulfate probably makes little difference, because sulfate-reducing organisms do not store polyphosphate and do not seem to compete with polyphosphate-storing bacteria under the conditions that occur in biological treatment. When full anaerobic conditions exist and bacteria with stored polyphosphate are present, the stored polyphosphate is hydrolyzed, and the energy released is used by the cells to store organic material, principally as PHB (Chiesa, 1982). The orthophosphate produced is released into the liquid. If the cells are then transferred into an aerobic or anoxic environment, the stored PHB is used as an energy and carbon source for growth and polyphosphate storage. Because a net increase in cell mass occurs as a result of the process, a net uptake of phosphorus also occurs. Removal of phosphorus from the system then can be achieved by wasting the cells produced. A competing group of organisms, the glycogen-accumulating organisms, also is capable of taking up VFA under anaerobic conditions, but without accumulating polyphosphate under aerobic conditions. The EBPR is used worldwide, and there are several processes that successfully have integrated EBPR (Oehmen at al., 2007).
8.8 Sulfide and Sulfur Oxidation Chemolithoautotrophic sulfur-oxidizing bacteria exist in many different groups of prokaryotes and include acidophiles (optimum pH <6) and neutrophiles (optimum pH 7 to 8). Acidophiles play an important role in acidic metal leaching processes, and neutrophiles tend to dominate in wastewater treatment processes. In addition, there are sulfur-oxidizing bacteria, which thrive at pH values above 9 (Sorokin et al., 2006).
8.9 Hydrogen Oxidation Hydrogen is a source of energy, and reductant and hydrogen-oxidizing bacteria are a fairly diverse group (Schwartz and Friedrich, 2006). Nearly all of the hydrogen
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oxidizers also are able to metabolize simple organic compounds, and their taxonomic characteristics would place them in a variety of chemoheterotrophic genera. Molecular hydrogen is not a common product in aerobic environments; therefore, the hydrogen oxidizers are not major players in aerobic biological treatment. Autotrophic denitrification of drinking water can be accomplished with biofilms using hydrogen as an electron donor, either NO3– or NO2– as an electron acceptor, and CO2 as a carbon source. Hydrogen also is a key component of anaerobic metabolism in methanegenerating Archaea called methanogens and in anoxygenic phototrophs. Finally, hydrogen production allows cells to release excess reductant produced during fermentative metabolism.
9.0 FEATURES OF MICROBIAL COMMUNITIES IN BIOFILMS Bacteria most often are studied in pure cultures, but biological treatment processes use bacterial communities where species work together. This situation presents problems in translation of information gathered in bacterial research into applications in biological treatment problems, because it must be assumed that individual species behave differently as members of a consortium than in pure cultures. For example, the micro-environment surrounding a bacterium in a biofilm is affected by the activities of neighboring cells. The local oxygen concentration or tension may be low, although the average concentration in the medium is relatively high. In many cases, low oxygen concentrations result in the production of partially oxidized metabolites, such as organic acids, which are used by other species of bacteria or allow establishment of micro-aerophilic species, which would not be competitive at a higher oxygen concentration. Similar situations occur for local pH concentration effects. A number of important biological treatment processes are dependent on the activities of bacterial consortia consisting of different species or strains. Among these processes are nitrification and phosphorus removal (where acid-fermenting organisms provide substrate for the polyphosphate-storing organisms) and methane fermentation (in which acid and hydrogen fermenters produce substrate for the methane fermenters, and the methane fermenters remove compounds that would inhibit growth of the acid fermenters). In addition, many organic removal processes involve multiple bacterial strains or species where each may only be able to execute certain enzymatic steps in the degradation or transformation of a specific substrate.
Biology of Fixed-Growth Process
Microbial communities in fixed films (biofilms) and suspended systems used to be considered a black box, with respect to process engineering. In this line of thinking, there is sufficient microbial functional redundancy in a given community to lead to stable process performance (Briones and Raskin, 2003; Curtis and Sloan, 2006; Rittman and McCarty, 2001), such as COD removal, nitrification, and others. Curtis et al. (2006) further illustrated this concept by relating functional redundancy directly to diversity, where a more specialized function (e.g., nitrification) involves a less diverse pool of microbes or metacommunity than that associated with a less specialized task (e.g., simple carbohydrate breakdown). However, a long-term study revealed that stable biological treatment performance does not necessarily correlate to stable community dynamics (Fernandez et al., 1999). The relationship between microbial community structure and performance characteristics in fixed-film systems is not well-understood (Wuertz et al., 2004). What is known is that microbial community dynamics are reproducible in laboratory-scale reactors for at least 3 months, when operational parameters are kept unchanged (Falk et al., 2009). Advances in molecular microbial ecology and environmental biotechnology have opened the door for a more systems-based approach to the study of microbial consortia (Raes and Bork, 2008). It is known that many bacterial species can communicate via synthesis, release, and detection of diffusible signal molecules. They do so to monitor their own population density, in a process referred to as quorum sensing (Ryan and Dow, 2008), and thus modulate the level of expression of subsets of genes, such as those controlling polymer secretion (Nadell et al., 2008). Using pure culture biofilms, it has been shown that quorum sensing can influence the development and physical structure of biofilms. Bacteria also are capable of eavesdropping on signaling that occurs in their immediate environment. Such interspecies signaling is likely to play a role in fixed-film applications. Microbial consortia now can be studied using metagenomic approaches, even when it is not possible to cultivate individual members (Steele and Streit, 2005). Metagenomics refers to the study of mixed microbial populations from a specific habitat at the DNA level (Riesenfeld et al., 2004) and is made possible by the ability of researchers to clone large DNA fragments from an environmental sample and rapidly sequence the DNA. Increased knowledge about metabolic capacities at the genetic level, about the way intracellular communication organizes function in microbial consortia, and about natural fluctuations in microbial communities is expected to open the door to engineering microbial consortia for the optimization of fixed-growth
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processes. In the future, biofilms may be engineered as synthetic consortia, with specific metabolic properties, and monitored with high-throughput molecular screening methods using inexpensive gene-chip-based technology.
10.0 REFERENCES Alvarez-Cohen, L.; McCarty, P. L. (1991) Effects of Toxicity, Aeration, and Reductant Supply on Trichloroethylene Transformation by a Mixed Methanotrophic Culture. Appl. Environ. Microbiol., 57, 228–235. Atkinson, B. (1974) Biochemical Reaction Engineering; Pion Press: London, United Kingdom. Aust, S.; Fernando, T.; Brock, B.; Tuisel, H.; Bumpus, J. (1988) Biological Treatment of Hazardous Wastes by Phanerochaete Chrysoporium. Proceedings of the Conference on Biotechnology Applications in Hazardous Waste Treatment, Longboat Key Florida, Oct 30–Nov 4, Lewandowski, G., Baltzis, B., Armenante, P. (Eds.); Engineering Foundation: New York. Bock, E. (1976) Growth of Nitrobacter in Presence of Organic Matter: 2. Chemoorganic Growth of Nitrobacter agilis. Arch. Microbiol., 108, 305–312. Briones, A.; Raskin, L. (2003) Diversity and Dynamics of Microbial Communities in Engineered Environments and Their Implications for Process Stability. Curr. Opin. Biotechnol., 14 (3), 270–276. Chiesa, S. C. (1982) Growth and Control of Filamentous Microbes in Activated Sludge. Ph.D. Dissertation, University of Notre Dame, Notre Dame, Indiana. Curtis, T. P.; Head, I. M.; Lunn, M.; Woodcock, S.; Schloss, P. D.; Sloan, W. T. (2006) What is the Extent of Prokaryotic Diversity? Phil. Trans. R. Soc. B, 361, 2023–2037. Curtis, T. P.; Sloan, W. T. (2006) Towards the Design of Diversity: Stochastic Models for Community Assembly in Wastewater Treatment Plants. Water Sci. Technol., 54 (1), 227–236. de Beer, D.; Stoudley, P.; Roe, F.; Lewandowski, Z. (2004) Effects of Biofilm Structure on Oxygen Distribution and Mass Transport. Biotechnol. Bioeng., 43, 1132–1138.
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Dobbs, R. A.; Wang, L. P.; Govind, R. (1989) Sorption of Toxic Organic Compounds on Waste Water Solids—Correlation with Fundamental Properties. Environ. Sci. Technol., 23, 1092–1097. Enright, A. M.; Collins, G.; O’Flaherty, V. (2007) Low-Temperature Anaerobic Biological Treatment of Toluene-Containing Wastewater. Water Res.: 41, 1465–1472. Evans, W. C.; Fuchs, G. (1988) Anaerobic Degradation of Aromatic Compounds. Annual Review of Microbiology, Ornston, L. N., Balows, A., Baumann, P. (Eds.); Annual Reviews, Inc.: Palo Alto, California. Falk, M. W.; Song, K. G.; Matiasek, M. G.; Wuertz, S. (2009) Microbial Community Dynamics in Replicate Membrane Bioreactors—Natural Reproducible Fluctuations. Water Res., 43, 842–852. Fan, S.; Scow, K. M. (1993) Biodegradation of Trichloroethylene and Toluene by Indigenous Microbial Populations in Soil. Appl. Environ. Micriobiol., 59, 1911–1918. Fernandez, A.; Huang, S. Y.; Seston, S.; Xing, J.; Hickey, R.; Criddle, C.; Tiedje, J. (1999) How Stable is Stable? Function Versus Community Composition. Appl. Environ. Microbiol., 65 (8), 3697–3704. Flegal, T. M.; Schroeder, E. D. (1976) Temperature Effects on BOD Stoichiometry and Oxygen Uptake Rate. J. Water Pollut. Control Fed., 49, 2700–2707. Fuhs, G. W.; Chen, M. (1975) Microbiological Basis of Phosphate Removal in the Activated Sludge Process for the Treatment of Wastewater. Microb. Ecol., 2, 119–138. Grady, C. P. L., Jr.; Daigger, G. T.; Lim, H. C. (1999) Biological Wastewater Treatment, 2nd ed.; Marcel Dekker, Inc.: New York. Hackett, W. F.; Connors, W. J.; Kirk, T. K.; Zeikus, J. G. (1977) Microbial Decomposition of Synthetic 14C-Labeled Lignins in Nature: Lignin Biodegradation in a Variety of Natural Materials. Appl. Environ. Microbiol., 33, 43–51. Hall-Stoodley, L.; Costerton, J. W.; Stoodley, P. (2004) Bacterial Biofilms: From the Natural Environment to Infectious Diseases. Nat. Rev. Microbiol., 2, 95–108.
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Henze, M.; Gujer, W.; Mino, T.; Matsuo, T.; Wentzel, M. C.; Marais, G. v. R. (1995) Activated Sludge Model No. 2, IAWQ Scientific and Technical Reports, No. 3; International Association for Water Quality: London, United Kingdom. Hyman, M. R.; Murton, I. B.; Arp, D. J. (1988) Interaction of Ammonia Monooxygenase from Nitrosomonas europea with Alkanes, Alkenes, and Alkynes. Appl. Environ. Microbiol., 54, 3187–3188. Ingraham, J. L.; Maaloe, O.; Neidhardt, F. C. (1983) Growth of the Bacterial Cell; Sinauer and Associates, Inc.: Sunderland, Massachusetts. Kehrberger, G. J.; Norman, J. D.; Schroeder, E. D.; Busch, A. W. (1964) BOD Progression in Soluble Substrates. VII. Temperature Effects. Proceedings of the 19th Purdue Industrial Waste Conference, West Lafayette, Indiana, May 5–7; Purdue University: West Lafayette, Indiana. Kong, Y. H.; Nielsen, J. L.; Nielsen, P. H. (2005) Identity and Ecophysiology of Uncultured Actinobacterial Polyphosphate-Accumulating Organisms in Full-Scale Enhanced Biological Phosphorus Removal Plants. Appl. Environ. Microbiol., 71, 4076–4085. Lengeler, J. W. ; Drews, G. ; Schlegel, H. G. (Eds.) Biology of the Prokaryotes (1999) Thieme Verlag: Stuttgart, Germany. Levin, G. V.; Shapiro, J. (1967) Metabolic Uptake of Phosphorus by Wastewater Organisms. J. Water Pollut. Control Fed., 37, 800–821. Lewandowski, Z.; Beyenal, H. (2003) Mass Transport in Heterogeneous Biofilms. In Biofilms in Wastewater Treatment: An Interdisciplinary Approach, Wuertz, S., Bishop, P. L., Wilderer, P. A. (Eds.); IWA Press: London, United Kingdom. Lipschultz, F.; Zafiriou, O. C.; Wofsky, S. C.; McElroy, M. B.; Valois, F.W.; Watson, S. W. (1981) Production of NO and N2O by Soil Nitrifying Bacteria. Nature, 294, 641–643. Logan, B. E. (1993) Oxygen Transfer in Trickling Filters. J. Environ. Eng., 119, 1059–1076. MacDonald, R. M. (1978) Population Dynamics of the Nitrifying Bacterium Nitrosolubus in Soil. J. Appl. Ecol., 16, 529–535. Madigan, M. T.; Martinko, J. M.; Parker, J. (2003) Brock Biology of Microorganisms, 9th ed.; Prentice-Hall: Englewood Cliffs, New Jersey.
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Mattick, J. S. (2002) Type IV Pili and Twitching Motility. Annu. Rev, Microbiol., 56, 289–314. McCarty, P. L. (1965) Thermodynamics of Biological Synthesis and Growth. Proceedings of the 2nd International Conference on Water Pollution Research; New York, 169. Metcalf and Eddy, Inc. (2003) Wastewater Engineering: Treatment and Reuse, Tchobanoglous, G., Burton, F. L., Stensel, H. D. (Eds.); McGraw-Hill: New York. Nadell, C. D.; Xavier, J. B.; Levin, S. A.; Foster, K. R. (2008) The Evolution of Quorum Sensing in Bacterial Biofilms. PLoS Biol., 6, 0171–0179. Neidhardt, F. C.; Ingraham, J. L.; Schaechter, M. (1990) Physiology of the Bacterial Cell: A Molecular Approach; Sinauer Associates, Inc.: Sunderland, Massachusetts. Nielsen. P. H.; Jahn, A. (1999) Extraction of EPS. In Microbial Extracellular Polymeric Substances, Wingender, J., Neu, T. R., Flemming, H.-C. (Eds.); Springer: Berlin, Germany, 49–72. Oehmen, A.; Lemos, P. C.; Carvalho, G.; Yuan, Z. G.; Keller, J.; Blackall, L. L.; Reis, M. A. M. (2007) Advances in Enhanced Biological Phosphorus Removal: From Micro to Macro Scale. Water Res., 41, 2271–2300. Op den Camp, H. J. M.; Kartal, B.; Guven, D.; van Niftrik, L.; Haaijer, S. C. M.; van der Star, W. R. L.; van de Pas-Schoonen, K. T.; Cabezas, A.; Ying, Z.; Schmid, M. C. (2006) Global Impact and Application of the Anaerobic AmmoniumOxidizing (Anammox) Bacteria. Biochem. Soc. Trans., 34, 174–178. Porges, N.; Jaiswicz, L.; Hoover, S. R. (1953) Biological Oxidation of Dairy Waste, VII. Proceedings of the 24th Purdue Industrial Waste Conference, West Lafayette, Indiana, May 6–8; Purdue University: West Lafayette, Indiana. Raes, J.; Bork, P. (2008) Molecular Eco-Systems Biology: Towards an Understanding of Community Function. Nat. Rev. Microbiol., 6, 693–699. Rajal, V. B.; McSwain, B. S.; Thompson, D. E.; Leutenegger, C. M.; Kildare, B.; Wuertz, S. (2007) Validation of Hollow Fiber Ultrafiltration and Real Time PCR Using Bacteriophage PP7 as Surrogate for the Quantification of Viruses from Water Samples. Water Res., 41, 1411–1422.
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Rajal, V. B.; McSwain, B. S.; Thompson, D. E.; Leutenegger, C. M.; Wuertz, S. (2007) Molecular Quantitative Analysis of Human Viruses in California Storm Water. Water Res., 41, 4287–4298. Reineke, W.; Knackmuss, H. J. (1988) Microbial Degradation of Haloaromatics. Annu. Rev. Microbiol., 42, 263–287. Riesenfeld, C. S.; Schloss, P. D.; Handelsman, J. (2004) Metagenomics: Genomic Analysis of Microbial Communities. Ann. Rev. Genet., 38, 525–552. Rittman, B. E.; McCarty, P. L. (2001) Environmental Biotechnology: Principles and Applications; McGraw-Hill: New York. Ryan, R. P.; Dow, J. M. (2008) Diffusible Signals and Interspecies Communication in Bacteria. Microbiology, 154, 1845–1858. Schmidt, I.; Sliekers, O.; Schmid, M.; Bock, E.; Fuerst, J.; Kuenen, J. G.; Jetten, M. S. M.; Strous, M. (2003) New Concepts of Microbial Treatment Processes for the Nitrogen Removal in Wastewater. FEMS Microbiol. Rev., 27, 481–492. Schroeder, E. D. (2002) Trends in Application of Gas-Phase Bioreactors. Rev. Environ Sci. Biotechnol., 1, 65–74. Schroeder, E. D.; Eweis, J. B.; Chang, D. P. Y.; Veir, J. K. (2000) Biodegradation of Recalcitrant Compounds. Water Air Soil Pollut., 123, 133–146. Schroeder, E. D.; Tchobanoglous, G. (1976) Mass Transfer Limitations in Trickling Filters. J. Water Pollut. Control Fed., 48, 771–775. Schroeder, E. D.; Wuertz, S. (2003) Bacteria. In The Handbook of Water and Wastewater Microbiology, Mara, D., Horan, N. (Eds.); Elsevier: Amsterdam, Netherlands, 57–68. Serafim, L. S.; Lemos, P. C.; Levantesi, C.; Tandoi V.; Santos H.; Reis M. A. M. (2002) Methods for Detection and Visualization of Intracellular Polymers Stored by Polyphosphate-Accumulating Microorganisms. J. Microbiol. Meth., 51, 1–18. Schwartz, E.; Friedrich, B. (2006) The H 2-Metabolizing Prokaryotes. In The Prokaryotes: An Evolving Electronic Resource for the Microbiological Community, 3rd ed.; Springer: New York, 496–563. Shapiro, J.; Levin, G. V.; Zea, H. G. (1967) Anoxically Induced Release of Phosphate in Wastewater Treatment. J. Water Pollut. Control Fed., 39, 1810–1818.
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Singer, P. C.; Stumm, W. (1970) The Solubility of Ferrous Iron in CarbonateBearing Waters. J. Am. Water Works Assoc., 62, 198–202. Skraber, S.; Helmi, K.; Ferreol, M.; Gantzer, C.; Hoffmann, L.; Cauchie, H. M. (2007) Occurrence and Persistance of Bacterial and Viral Faecal Indicators in Wastewater Biofilms. Water Sci. Technol., 55 (8–9), 377–385. Sorokin, D. Y.; Banciu, H.; Robertson, L. A.; Kuenen, J. G. (2006) Haloalkaliphilic Sulfur-Oxidizing Bacteria. In The Prokaryotes: An Evolving Electronic Resource for the Microbiological Community, 3rd ed.; Springer: New York, 969–984. Spaeth, R.; Wuertz, S. (2000). Extraction and Quantification of Extracellular Polymeric Substances from Wastewaters. In Biofilms. Investigative Methods & Applications, Flemming, H.-C., Szewzyk, U., Griebe. T. (Eds.), Technomic Publishers: Lancaster, Pennsylvania, 51–68. Stanier, R. Y.; Ingraham, J. L.; Wheelis, M. L.; Painter, P. R. (1986) The Microbial World; Prentice-Hall: Englewood Cliffs, New Jersey. Steele, H. L.; Streit, W. R. (2005) Metagenomics: Advances in Ecology and Biotechnology. FEMS Microbiol. Lett., 247, 105–111. Stephen, J. R.; McCaig, A. E.; Smith, Z.; Prosser, J. L.; Embley, T. M. (1996) Molecular Diversity of Soil and Marine 16S rRNA Gene Sequences Related to β-Subgroup Ammonia-Oxidizing Bacteria. Appl. Environ. Microbiol., 62, 4147–4154. Steward, G. F.; Smith, D. C.; Azam, F. (1996) Abundance and Production of Bacteria and Viruses in the Bering and Chukchi Seas. Mar. Ecol. Prog. Ser., 131, 287–300. Strous, M.; Heijnen, J. J.; Kuenen, J. G.; Jetten, M. S. M. (1998) The Sequencing Batch Reactor as a Powerful Tool for the Study of Slowly Growing Anaerobic Ammonium-Oxidizing Microorganisms. Appl. Microbiol. Biotechnol., 50, 589–596. Strous, M.; Van Gerven, E.; Ping, Z.; Kuenen, J. G.; Jetten, M. S. M. (1997) Ammonium Removal from Concentrated Waste Streams with the Anaerobic Ammonium Oxidation (Anammox) Process in Different Reactor Configurations. Water Res., 31, 1955–1962. Swilley, E. L.; Bryant, J. O.; Busch, A. W. (1964) The Significance of Transport Phenomena in Biological Oxidation Processes. Proceedings of the 19th Purdue
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Industrial Waste Conference, West Lafayette, Indiana, May 5–7; Purdue University: West Lafayette, Indiana. Szewzyk, U.; Szewzyk, R. ; Manz, W. ; Schleifer, K.-H. (2000) Microbiological Safety of Drinking Water. Annu. Rev. Microbiol., 54, 81–127. Venugopalan, V. P.; Kuehn, M.; Hausner, M.; Springael, D.; Wilderer, P. A.; Wuertz, S. (2005) Architecture of a Nascent Sphingomonas sp. Biofilm under Varied Hydrodynamic Conditions. Appl. Environ. Microbiol., 71, 2677–2686. Wilderer, P. A.; Bungartz, H.-J.; Lemmer, H.; Wagner, M.; Keller, J.; Wuertz, S. (2002) Modern Scientific Methods and Their Potential in Wastewater Science and Technology. Water Res., 36, 370–393. Williamson, K.; McCarty, P. L. (1976) A Model of Substrate Uptake by Bacterial Films. J. Water Pollut. Control Fed., 48, 9–24. Woertz, J. R.; Kinney, K.A.; McIntosh, N. D. P.; Szaniszlo, P. J. (2001) Removal of Toluene in a Vapor-Phase Bioreactor Containing a Strain of the Dimorphic Black Yeast Exophiala lecanii-corni. Biotechnol. Bioeng., 75, 550–558. Woertz, J. R.; Kinney, K. A.; Szaniszlo, P. J. (2001) A Fungal Vapor-Phase Bioreactor for the Removal of Nitric Oxide from Waste Gas Streams. J. Air Waste Manag. Assoc., 51, 895–902. Wuertz, S.; Okabe, S.; Hausner, M. (2004) Microbial Communities and Their Interactions in Biofilm Systems: An Overview. Water Sci. Technol., 49 (11–12), 327–336. Yu, S.; Semprini, L. (2004) Kinetics and Modeling of Reductive Dechlorination at High PCE and TCE Concentrations. Biotechnol. Bioeng., 88, 451–464.
Chapter 3
Trickling Filter and Combined Trickling Filter Suspended-Growth Process Design and Operation 1.0 INTRODUCTION
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2.0 General Description
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4.0 Ventilation and Air Supply Alternatives
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2.1 Distribution System
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4.1 Natural Draft
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2.2 Biofilm Carriers
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4.2 Mechanical Ventilation
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2.3 Containment Structure 90 2.4 Underdrain System and Ventilation
5.0 Trickling Filter Process Models 103
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2.5 Trickling Filter Pumping Stations: Influent and Recirculation 91
5.1 National Research Council
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2.6 Hydraulic and Contaminant Loading
5.2 Galler and Gotaas
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5.3 Kincannon and Stover 106
3.0 Process Flow Sheets and Bioreactor Configuration 3.1 Standard Process Flow Diagrams
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5.4 Velz
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5.5 Schulze
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5.6 Germain
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5.7 Eckenfelder
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5.8 Chartered Institution of Water and Environmental Management 110
3.2 Bioreactor Classification 96 3.3 Hydraulic Application: Effect on Media Wetting, Flow Distribution, and Control 98
5.9 Logan Trickling Filter Model 81
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(continued)
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5.10 Selecting a Trickling Filter Model 112 5.11 Method for Combining Trickling Filter and Suspended-Growth Models 113 6.0 Process Design
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6.1 Combined Carbon Oxidation and Nitrification
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6.2 Nitrifying Trickling Filters
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6.2.1 Gujer and Boller Nitrifying Trickling Filter Model 122 6.2.2 Okey and Albertson Nitrifying Trickling Filter Model 124 6.2.2.1 Application of the Gujer and Boller Model 126 6.2.2.2 Application of the Albertson and Okey Model 128 6.3 Temperature and Hydraulic Application Effects 131 7.0 Design Considerations 132 7.1 Distribution System
133
7.1.1 Hydraulic Drive Rotary Distributors 134
7.1.2 Electronic or Mechanical Drive Rotary Distributors 136 7.1.3 Optimizing Rotary Distributor Operation 138 7.2 Construction of Rotary Distributors 138 7.3 Trickling Filter Media Selection 139 7.3.1 Depth 141 7.3.2 Structural Integrity 142 7.4 Trickling Filter Pumping Station or Dosing Siphon 144 7.5 Control Mechanisms for Trickling Filter Macro Fauna 144 7.5.1 Operational Strategies and Facility Improvements for Macro Fauna Control 145 7.5.2 Spülkraft 147 7.5.3 Flooding 148 7.5.4 Chemical Treatment 149 7.5.5 Physical Control 152 7.6 Trickling Filter Startup 153 7.7 Combined Trickling Filter and Suspended-Growth Processes 155 7.7.1 Activated Biofilter 156 7.7.2 Trickling Filter/Solids Contact 158 (continued)
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7.7.3 Roughing Filter/ Activated Sludge 162 7.7.4 Biofilter/Activated Sludge 162
7.7.5 Trickling Filter/ Activated Sludge 8.0 REFERENCES
162 163
1.0 INTRODUCTION Until the 1950s, trickling filter (TF) design protocol was scattered and empirical in nature. Then, during the 1950s and 1960s, the Dow Chemical Company (Midland, Michigan) began experimentation with modular plastic media (Bryan, 1955). Numerous trickling filter process studies were conducted during the same period (Eckenfelder, 1961; Galler and Gotaas, 1964; Germain, 1966; Schulze, 1960), which led to the development of generally accepted design criteria. After the U.S. Environmental Protection Agency (Washington, D.C.) issued its definition of secondary treatment standards in the early 1970s, the trickling filter process was regarded as being unable to consistently produce effluent water quality that met the published standards, in part, as a result of poor secondary clarifier design (Parker, 1999). Norris et al. (1982) described development of the trickling filter/solids contact process (TF/ SC) in response. The first full-scale TF/SC process included a rock-media trickling filter followed by a small aeration basin (receiving return sludge) and flocculator clarifier. The researchers demonstrated that wastewater treatment plant (WWTP) effluent water quality could be improved greatly by bioflocculation in the solids contact basin and improved secondary clarifier design. Combined trickling filter/suspended-growth (TF/SG) processes preceding the TF/SC process were designed with the suspended-growth reactor (SGR) primarily for oxidation. The modern trickling filter typically includes the following major components:
(1) Rotary distributors, with speed control; (2) Modular plastic biofilm carriers, or media (typically cross-flow [XF] media, unless the bioreactor is treating high-strength wastewater, which warrants the use of vertical-flow [VF] media); (3) A mechanical aeration system (which consists of air distribution piping and low-pressure fans); (4) Trickling filter effluent recirculation pumping station; and (5) Covers that aid with the uniform distribution of process air and foul air containment (for odor control).
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Covers may be equipped with sprinklers that discharge in-plant recycle to cool plastic media during emergency shutdown. Trickling filter mechanics are poorly understood. Consequently, there is a general lack of mechanistic mathematical models and design approaches, and the design and operation of trickling filter and TF/SG processes is empirical or semi-empirical. Benefits inherent to the trickling filter process (when compared with activated sludge processes) include operational simplicity, resistance to toxic and shock loads, and low energy requirements. However, trickling filters are susceptible to nuisance conditions that are primarily caused by macrofauna. Process mechanical components dedicated to minimizing the accumulation of macrofauna, such as filter flies, worms, and snail (shells), are now standard. Unfortunately, information on the selection and design of these process components is fragmented and has been poorly documented. The TF/SC process is the most common TF/SG process. This chapter describes state-of-the art trickling filter and TF/SC process design and operation.
2.0 General Description The trickling filter is a three-phase biofilm reactor, with fixed carriers. Wastewater enters the bioreactor through a distribution system, trickles downward over the biofilm surface, and air moves upward or downward in the third phase, where it diffuses through the flowing liquid and into the biofilm. Trickling filter components generally include an influent water distribution system, containment structure, rock or plastic media, underdrain, and ventilation system. Figure 3.1 illustrates a trickling filter crosssection and these typical bioreactor components. Wastewater treatment using the trickling filter results in a net production of total suspended solids (TSS). Therefore, liquid–solid separation is required and typically is achieved with circular or rectangular secondary clarifiers. The trickling filter process generally includes an influent/ recirculation pumping station, trickling filter, and liquid–solid separation unit.
2.1 Distribution System Primary effluent (or screened and degritted wastewater—fine screens with a maximum 3-mm bar spacing or perforated plate orifice diameter—should be applied for preliminary treatment, if primary clarification is excluded from the process flow sheet) is either pumped or flows by gravity to the trickling filter distribution system. Essentially, there are two types of trickling filter distribution systems—fixed-nozzle and rotary distributors. Because their efficiency is poor, distribution with fixed nozzles should not be used (Harrison and Timpany, 1988). Rotary distributors may be
ROTARY DISTRIBUTOR DISTRIBUTOR ARM CONTAINMENT STRUCTURE
BIOFILM SUPPORT MEDIUM
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FRP GRATING UNDERDRAIN
AIR DISTRIBUTION HEADER
INFLUENT
Figure 3.1 Typical trickling filter components and cross-section.
EFFLUENT
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driven hydraulically or electrically. A properly designed rotary distribution system allows for effective media-wetting and the intermittent application of wastewater to biofilm carriers. The intermittent application of influent wastewater (i.e., typically primary effluent diluted with recirculated trickling filter effluent) allows the biofilm to have periods of “resting,” which primarily serves as a process aeration mechanism. Poor media-wetting may lead to dry pockets, ineffective treatment zones, and odor. Hydraulically driven rotary distributors have discharge orifices along the front and back of each arm. The distributor is driven forward by energy imparted by the influent stream. To the extent possible, distributor rotational speed is controlled by flow passing through the orifices, which are situated along the distributor arm face that points to the direction of rotation. These back-spray orifices, or reverse-thrusting jets, slow rotational speed, but the designer must determine the extent of speed control based on site-specific operating conditions and process hydraulics. An electrically driven or modern hydraulically driven rotary distributor may be required if reverse-thrusting jets are incapable of achieving the degree of speed control required to maintain the desired trickling filter dosing rate(s). A modern hydraulically driven rotary distributor is depicted in Figure 3.2. This system uses gates (controlled by variable-frequency drive [VFD]) to open or close distributor orifices and adjust
Figure 3.2 (Left) Hydraulically driven rotary distributors use variable frequency drive controlled gates that either open or close distributor orifices, which adjust with varying pumped flowrates to maintain a constant preset rotational speed; and (right) electrically driven rotary distributor (Photographs courtesy WesTech, Inc., Salt Lake City, Utah).
Trickling Filter and Combined Trickling Filter Suspended-Growth Process
rotational speed. Figure 3.2 also depicts an electrically driven rotary distributor. The use of a VFD allows for the more precise control of distributor-arm location when stopping rotation. Electrically driven rotary distributors have motorized drive units that control distributor speed independent of the wastewater pumped flow.
2.2 Biofilm Carriers Ideal trickling filter media provides a high specific surface area, low cost, high durability, and porosity that is high enough to avoid clogging and promote ventilation (Metcalf & Eddy, 2003). Trickling filter media types include rock, random (synthetic), vertical-flow (synthetic), and cross-flow (synthetic). Both vertical- and cross-flow media are constructed with smooth and/or corrugated plastic sheets. Another commercially available synthetic media, although not commonly used, is vertically hanging plastic strips. Horizontal redwood or treated wooden slats also have been used, but generally are no longer considered viable because of high cost or limited supply. Modules of plastic sheets (i.e., self-supporting vertical- or cross-flow modules) are used almost exclusively for new and improved trickling filters, but several trickling filters with rock media exist and have proven capable of meeting treatment objectives, when properly designed and operated. Table 3.1 compares the characteristics of some trickling filter media types. The higher specific surface area and void space in modular synthetic media allows for higher hydraulic loading, enhanced oxygen transfer, and biofilm thickness control, compared with rock media. Rock media ideally has a 50-mm diameter, but may range in size. Rounded (river) rock helps mitigate some issues associated with rigid rock (slag) media. The slag rock contains crevices that can retain water and accumulate biomass. As a result of structural requirements associated with the large unit weight of the rock media, rock media are shallow compared with synthetic-media trickling filters and are more susceptible to excessive cooling. Trickling filter performance aside, the excessive cooling can subject media to freeze–thaw cycles. Water retained inside slag rock crevices may expand and sever rock fragments. This can result in fine material accumulation which, with retained biomass, is a primary contributor to rock-media trickling filter clogging (Grady et al., 1999). Generally, rock media is considered to have a low specific surface area, void space, and high unit weight. Although recirculation is common, the low void ratio in rock-media trickling filters limits hydraulic application rates. Excessive hydraulic application can result in ponding, limited oxygen transfer, and poor bioreactor performance. Performance of existing rock-media trickling filters sometimes may be improved by providing mechanical ventilation, solids contact channels, and/or deepened secondary clarifiers, which include energy-dissipating inlets
87
Table 3.1 Properties of some trickling filter media. Nominal size (m [ft])
Bulk density (kg/m3 [lb/cu ft])a
Specific surface area (m2/m3 [sq ft/cu ft])b
River
0.024 to 0.076 (0.08 to 0.25)
1442 (90)
62 (19)
50
Slag
0.076 to 0.128 (0.25 to 0.42)
1600 (100)
46 (14)
60
0.61 × 0.61 × 1.22 (2 × 2 × 4)
24 to 45 (1.5 to 2.8)
100, 138, and 223 (30, 42, and 68)
95
0.61 × 0.61 × 1.22 (2 × 2 × 4)
24 to 45 (1.5 to 2.8)
102 and 131 (31 and 40)
95
Media type
Material
Void space (%)
Rock
Plasticc
Cross-flow PVC
Vertical flow PVC
0.185 ø × 0.051 H (7.3 in. ø × 2-in. H)
Randomd
27 (1.7)
98 (30)
95
Polypropylene a
lb/cu ft × 16.02 = kg/m3.
b
sq ft/cu ft × 3.281 = m2/m3.
Manufacturers of modular plastic media: (formerly) BF Goodrich (Greenville, South Carolina), American Surfpac Corporation (Downingtown, Pennsylvania), NSW (Nordenham, Germany), Munters (Aachen, Germany), (currently) Brentwood Industries (Reading, Pennsylvania), Jaeger Environmental (Houston, Texas), and SPX Cooling (Overland Park, Kansas).
c
d
Manufacturers of random plastic media: (formerly) NSW and (currently) Jaeger Environmental.
88
Trickling Filter and Combined Trickling Filter Suspended-Growth Process
and flocculator-type feed wells. Replacement of the rock media (with synthetic media) may be required in cases where rock quality is poor. If WWTP expansion is expected, existing rock-media trickling filters may be deepened if the media is replaced with modular synthetic media. Rock-media trickling filter containment structures typically are capable of providing structural support for the deeper modular synthetic media, because the rock media is substantially heavier than synthetic media. A well-designed and -operated rock-media trickling filter can provide high-quality effluent. Grady et al. (1999) suggested that, under low organic loading (i.e., less than 1 kg BOD5/m3∙d), rock- and synthetic-media trickling filters are capable of equivalent performance. However, as organic loading increases, synthetic-media trickling filters are less susceptible to operational problems and have reduced potential for plugging. Synthetic trickling filter media has a higher specific surface area and void space and lower unit weight than rock media. Because of the reduced unit weight, synthetic-media trickling filters can be constructed at depths exceeding 3 times that of a comparably sized rock-media trickling filter. Modular synthetic media generally is manufactured with the following specific surface areas: 223 m2/m3 (68 sq ft/cu ft) as high-density, 138 m2/m3 (42 sq ft/cu ft) as medium-density, and 100 m2/m3 (30 sq ft/cu ft) as low-density. Both vertical- and cross-flow media are reported to remove 5-day biochemical oxygen demand (BOD5) and ammonia-nitrogen (NH3-N) (Aryan and Johnson, 1987; Harrison and Daigger, 1987), but sufficient scientific evidence exists to surmise that there is a difference in the treatment efficiency of trickling filters constructed with cross- and vertical-flow media, even when manufactured with virtually identical specific surface areas. Plastic modules with a specific surface area in the range of 89 to 102 m2/m3 are wellsuited for carbon oxidation and combined carbon oxidation and nitrification. Parker et al. (1989) recommended medium-density cross-flow media and recommended against the use of high-density cross-flow media in nitrifying trickling filters. This is supported by observations from a pilot-scale nitrifying trickling filter application data and the conclusions of Boller and Gujer (1986) and Gujer and Boller (1984), which show lower nitrification (flux) rates for lower density modular synthetic media. The researchers claim that lower rates occur with high-density media, as a result of the development of dry spots below the flow interruption points (i.e., higher-density media has more flow interruptions and therefore less effective wetting). Using medium-density media also reduces plugging potential. Vertically oriented modular synthetic (verticalflow) media generally is accepted as being ideally suited for high-strength wastewater (perhaps industrial) and high organic loadings, such as with a roughing trickling filter.
89
90
Biofilm Reactors
In some cases, cross-flow media has been placed in the top layer to enhance wastewater distribution, and vertical-flow media comprises the remainder of the trickling filter media. Typically, the top layer of a trickling filter’s modular plastic media is covered with fiberglass-reinforced plastic (FRP) or high-density polyethylene (HDPE) grating. The grating is skid-resistant, thereby reducing the potential for slip and fall occupational hazards. In addition, the grating protects modular plastic media from deterioration by UV light and potential structural damage, which may result from water-induced load exerted during periods of high-intensity dosing.
2.3 Containment Structure Rock and random synthetic media are not self-supporting and require structural support to contain the media within the bioreactor. These containment structures are typically precast or panel-type concrete tanks. When self-supporting media, such as plastic modules, is used, other materials, such as wood, fiberglass, and coated steel have been used as containment structures. The containment structure serves to avoid wastewater splashing and provide media support, wind protection, and flood containment. In some cases, trickling filter containment structures have been designed to allow flooding of the media, which increases operator flexibility in controlling macrofauna accumulation. A complete list and description of macrofauna control alternatives is presented later in this chapter.
2.4 Underdrain System and Ventilation The trickling filter underdrain system is designed to meet two objectives—collect treated wastewater for conveyance to downstream unit processes and create a plenum that allows for the transfer of air throughout the trickling filter media (Grady et al., 1999). Clay or concrete underdrain blocks commonly are used for rock-media trickling filters because of the required structural support. A variety of support systems, including concrete piers and FRP grating are used for other media types. Figure 3.3 illustrates field-adjustable plastic stanchions and FRP grating on the concrete floor of a trickling filter containment structure. The volume created between the concrete and media bottom creates the underdrain. The vertical flow of air through the media can be induced mechanically or by natural air draft. Natural air ventilation results from a difference in ambient air temperature outside and inside the trickling filter. The temperature causes air to expand when warmed or contract when cooled. The net result is an air-density gradient throughout the trickling filter, and an air front either rises or sinks, depending on the differential
Trickling Filter and Combined Trickling Filter Suspended-Growth Process
Figure 3.3 Adjustable plastic stanchions (top right) and FRP grating (bottom left) on the concrete floor of a bolted steel containment structure, and an HDPE mat used to support random synthetic media (top and bottom right).
condition. This rising or sinking action results in a continuous airflow through the bioreactor. Natural ventilation may become unreliable or inadequate in meeting process air requirements, when neutral temperature gradients do not produce air movement. Such conditions may be daily or seasonal and can lead to odorous anaerobic biofilm conditions and poor trickling filter performance. Therefore, mechanical ventilation generally is included to enhance and control airflow. Mechanical ventilation is accomplished with low-pressure fans to continuously circulate air throughout the trickling filter. When using mechanical ventilation, the designer must ensure that air is distributed uniformly across the trickling filter cross-section.
2.5 Trickling Filter Pumping Stations: Influent and Recirculation A critical unit in the trickling filter process is the pumping station that lifts primary effluent (or screened raw wastewater) and recirculates unsettled trickling effluent (referred to here as underflow) to the influent stream. Generally, trickling filter underflow is recirculated to the distribution system to achieve the hydraulic load (influent + recirculation) required for proper media-wetting and biofilm thickness control and decouple hydraulic and organic loading. Although effluent from the secondary
91
92
Biofilm Reactors
clarifier can be recirculated, this not common practice, because it may lead to the hydraulic overloading of secondary clarifiers. Trickling filter influent generally is pumped to allow trickling filter underflow to flow by gravity to the SGR (or solids contact basin), secondary clarifier, or other downstream of the trickling filter. When fit with weirs, a single pumping station can be used to convey both influent and recirculation streams.
2.6 Hydraulic and Contaminant Loading Trickling filters typically are classified by the intended mode of pollutant degradation and pollutant loading, namely carbon oxidation, combined carbon oxidation and nitrification, or nitrification. Organic loading typically is expressed as kilograms per day per cubic meter (kg/d·m3) of filter media as carbonaceous 5-day biochemical oxygen demand (cBOD5). General practice is to ignore the organic load imparted by recirculation streams, but the designer should be aware of the effects that recycle flow and pollutant loading (specifically ammonia-nitrogen) may have on treatment efficiency. Total organic load (TOL) may be calculated by eq 3.1.
Q in ⋅ S S,in BOD 5 applied = TOl = media volume VM
(3.1)
where VM = trickling filter media volume (m3). Nitrifying trickling filter data typically are expressed in terms of the surface-based ammonia-nitrogen loading rate. Dividing eq 3.1 by the media specific surface area (m2/m3) produces the surface loading rate. The trickling filter hydraulic loading rate can be calculated with and without recirculation. The wastewater hydraulic load, WHL (in cubic meters per square meters per day), excludes recirculation and can be calculated mathematically by eq 3.2, which has the units cubic meters per square meters per day. Q in (3.2) a The total hydraulic load (THL) is relevant for media-wetting and biofilm thickness control and considers trickling filter influent, Qin, and recirculation stream, QR. The total hydraulic load can be expressed mathematically by eq 3.3. wHl =
THl =
Q in + Q R a
(3.3)
Trickling Filter and Combined Trickling Filter Suspended-Growth Process
3.0 Process Flow Sheets and Bioreactor Configuration The wastewater treatment train of a trickling filter process typically consists of preliminary treatment (including screening and grit removal), primary clarification, trickling filter influent/recirculation pumping station, secondary clarification, and disinfection unit processes. The TF/SG processes also include an aeration basin and return activated sludge pumping station. This section describes standard process flow sheets, different trickling filter classifications, and hydraulics.
3.1 Standard Process Flow Diagrams The trickling filter recirculation stream influences process flow. Generally, there are two types of trickling filter recirculation. The first allows for direct recirculation to the trickling filter, and the second passes flow through a primary clarifier before entering the trickling filter. Four trickling filter process flow diagrams, including both singleand two-stage trickling filters, are shown schematically in Figure 3.4. Combined TG/ SG process flow sheets are illustrated in Figure 3.5. Recirculation of trickling filter underflow or settled effluent dilutes influent wastewater and dampens the influent organic loading variability, as a result of diurnal fluctuations. Clarifying trickling filter effluent may enhance the performance of a subsequent trickling filter in twostage operation, but the designer must ensure that the recirculation flow required for trickling filter wetting and biofilm thickness control does not exceed the limiting hydraulic loading rate for the sedimentation tank. Intermediate clarification may be beneficial for systems treating wastewaters containing high concentrations of soluble cBOD5 (likely from an industrial source) because of excessive biomass production. These solids can adversely affect the second-stage trickling filter, if not removed by intermediate settling. The design of settling tanks in two-stage trickling filter systems also is affected by the recirculation pattern. The practice of alternating the lead trickling filter in a primary–secondary trickling system is referred to as an alternating double-filtration (ADF) system. This concept is beneficial when applied to modular synthetic-media trickling filters. Gujer and Boller (1986) and Parker et al. (1989) observed patchy biofilm growth in the lower section of pilot-scale nitrifying trickling filters (NTFs). The researchers attributed the patchy growth to dry spots. Boller and Gujer (1986) recommended the use of ADF to enhance pilot-scale NTF performance. Aspegren (1992) also observed improved nitrification using ADF compared with a single-stage NTF and reported that the
93
TFINF
RS
PE
TF
PC TFRCY
PE
TF
PC PS
TFEFF
WS
TFINF-2
TF
TF
(c)
TFEFF
PC
SE SC
TFRCY
PS
WS
TFINF-1
PE
TF
PC PS
(b)
SE SC
PE
RS
(a)
WS
TFRCY
TFINF-1
RS
SE SC
PS
RS
TFEFF
TFINF-2
TFEFF
ICE
TF
IC TFRCY-1
WS
(d)
TFEFF
SE SC
TFRCY-1
WS
Figure 3.4 Typical flow diagrams for the trickling filter process: (a) and (b) singlestage trickling filter process, (c) two-stage trickling filter process, and (d) two-stage trickling filter process with intermediate clarification. RS = raw sewage, PC = primary clarifier, PS = primary sludge, PE = primary effluent, TFINF = trickling filter influent, TF = trickling filter, TFEFF = trickling filter effluent, TFRCY = trickling filter recycle, SC = secondary clarifier, WS = waste sludge, SE = secondary effluent, IC = intermediate clarifier, and ICE = intermediate clarifier effluent. 94
TFINF
RS
PE
ABF
TFEFF
PC
SE
TFRCY
PS
RAS
RS
PE
TF
PC
WS
TFEFF
MLSS
TFRCY
PE
ABF
PC
RAS
TFEFF
(b)
WS
MLSS
SE
(c)
SecC
SC/AS
TFRCY
PS
SE SecC
SC/AS
PS
RS
(a)
SecC
RAS
WS
TFINF-1 (d) RS
PE
TF
PC PS
TFRCY-1
TFEFF
MLSS
ICE IC WS
SE SecC
AS RAS
WS
Figure 3.5 Typical flow diagrams for trickling filter-suspended growth processes: (a) activated biofilter, (b) trickling filter/solids contact or trickling filter/activated sludge, (c) activated biofilter/solids contact or activated biofilter/activated sludge, and (d) trickling filter and activated sludge. ABF = activated biofilter, AS = activated sludge, SC = solids contact, RAS = return activated sludge, and SecC = secondary clarifier; other variables as defined in Figure 3.4. 95
96
Biofilm Reactors
improved performance resulted from the reduced biofilm patchiness. Use of the ADF approach with two trickling filters in series encourages full-depth biofilm development in both bioreactors. The lead trickling filter should be switched every 3 to 7 days, to ensure that healthy biofilms, which maximize media coverage, are developed along the entire bioreactor depth. The primary drawback of ADF is an increased power requirement, which may be in excess of 50%, as a result of double pumping. In addition to increased operating cost, capital costs associated with pipes and valves also will increase. Sludge handling also affects the trickling filter process. Each of the process-flow diagrams illustrated in Figures 3.4 and 3.5 implies that waste biological solids are removed by co-settling the biological sludge with the primary sludge before withdrawal from the system. Many facilities exist that separately handle the primary and secondary sludge. The benefits of each must be evaluated by the designer. This practice does require that operators conscientiously withdraw the solids from the process and that designers provide equipment and means to maintain a near-zero sludge blanket, if necessary. A common operational problem resulting from improper maintenance of biosolids is “rising sludge.” Any trickling filter application that results in nitrification produces nitrate, which is reduced to nitrogen gas (N2) in an anoxic sludge blanket macroenvironment (and the presence of an electron donor). The N2 (g) can become entrained in the sludge blanket and float biomass to the sedimentation basin water surface. This biomass can float over the weirs and degrade secondary effluent water quality. Another issue that designers must consider is the improper maintenance of a primary clarifier sludge blanket. When combined with waste biological sludge, the soluble cBOD5 that exists in primary sludge may generate odor. Alternatively, particulate BOD5 may hydrolyze and cause soluble BOD5 to enter the bulk liquid. This can result in an increased TOL (on the trickling filter) and diminish bioreactor performance.
3.2 Bioreactor Classification Trickling filters can be categorized by four modes of operation or application—(1) roughing, (2) carbon oxidation, (3) carbon oxidation and nitrification, and (4) nitrification. Table 3.2 summarizes generally accepted criteria defining each operational mode. Roughing filters receive high hydraulic and high organic loadings and generally are constructed with vertical-flow media to minimize the potential for excess biofilm accumulation. Although they may provide a high quantity of organic load removal per unit volume, their settled effluent still contains substantial BOD5. Roughing filters provide approximately 50 to 75% soluble BOD5 conversion and may receive total
Trickling Filter and Combined Trickling Filter Suspended-Growth Process
97
Table 3.2 Trickling filter classification. Carbon oxidizing cBOD5 and (cBOD5 removal) nitrification
Design parametera
Roughing
Media typically used
Vertical flow
Rock, cross-flow, or vertical flow
Rock, cross-flow, or vertical flow
Cross-flow
Wastewater source
Primary effluent
Primary effluent
Primary effluent
Secondary effluent
Hydraulic loading
52.8 to 178.2 (0.9 to 2.9)
14.7 to 88.0 (0.25b to 1.5)
14.7 to 88.0 (0.25b to 1.5)
35.2 to 88.0 (0.6 to 1.5)
1.6 to 3.52 (100 to 220)
0.32 to 0.96 (20 to 60)
0.08 to 0.24 (5 to 15)
NAc
NAc
NAc
0.2 to 1.0 (0.04 to 0.2)
0.5 to 2.4 (0.1 to 0.5)
Effluent quality (mg/L) (unless noted)
50 to 75% filtered cBOD5 conversion
15 to 30 mg/L cBOD5 and TSSd
<10 cBOD5; <3 NH3-Nd
0.5 to 3 NH3-Nb
Predation
No appreciable growth
Beneficial
Detrimental Detrimental (nitrifying biofilm)
Filter flies
No appreciable growth
No appreciable growth
No appreciable growth
No appreciable growth
Depth (m [ft])
0.91 to 6.10 (3 to 20)
≤12.2 (40)
≤12.2 (40)
≤12.2 (40)
m 3 (gpm/sq ft) d.m 2
Nitrification
BOD5 and NH3-N load kg m 3 .d g m 2 .d
(lb BOD5/d·1000 cu ft) (lb NH3-N/d·1000 sq ft)
gpm/sq ft × 58.674 = m3/m2·d (cubic meter per day per square meter of trickling filter plan area); lb BOD5/ d/1000 cu ft × 0.0160 = kg/m3∙d (kilograms per day per cubic meter of media); and lb NH3-N/d/1000 sq ft × 4.88 = g/m2∙d (grams per day per square meter of media). a
b Applicable to shallow trickling filters. gpm/sq ft = gallons per minute per square foot of trickling filter plan area. c
Not applicable.
d
Concentration remaining in the clarifier effluent stream.
BOD5 loadings of 1.5 to 3.5 kg BOD5/m3∙d. Carbon-oxidizing trickling filters may provide settled effluent concentrations of 15 to 30 mg/L for BOD5 and TSS, respectively, and may receive BOD5 loadings of 0.7 to 1.5 kg/m3∙d. Combined carbon oxidation and nitrification trickling filters may provide an effluent BOD5 concentration less than 10 mg/L and NH3-N less than or equal to 0.5 to 3 mg/L (after solids separation). These trickling filters may receive BOD5 loadings less than 0.2 kg/m3∙d and TKN loadings
98
Biofilm Reactors
of 0.2 to 1.0 g/m2∙d. The NTFs provide 0.5 to 3 mg/L effluent NH3-N when receiving a clarified secondary effluent and NH3-N loadings of 0.5 to 2.5 g N/m2∙d.
3.3 Hydraulic Application: Effect on Media Wetting, Flow Distribution, and Control Recirculation and distributor operation are important to good trickling filter performance and may be used to achieve proper media-wetting, flow distribution, biofilm thickness control, and prevent macrofauna accumulation. Albertson and Eckenfelder (1984) postulated that the active biofilm surface area in a trickling filter is dependent on biofilm thickness and media configuration and that the active biofilm surface area decreases with increasing biofilm thickness. The researchers stated that, for mediumdensity cross-flow media with 98 m2/m3 specific surface area, a 4-mm increase in biofilm thickness would cause a 12% reduction of active biofilm area (assuming that the entire media has been appropriately wetted). Poor trickling filter media-wetting results in reduced effluent water quality. In a study of rotary distributor efficiency, Crine et al. (1990) found that the wetted-area-to-specific-surface-area ratio ranged from 0.2 to 0.6, with the lowest values for high-density random-pack media. Figure 3.6 shows a threedimensional representation of liquid-flow distribution in cross-flow media, both with and without biofilm (Lekhlif et al., 1994). Many of the later mentioned design formulations incorporate a term that allows for specific surface area reduction, as a result of
(a)
(b)
Figure 3.6 Three-dimensional representation of liquid flow distribution in crossflow media, both (a) with and (b) without biofilm (Lekhlif et al., 1994).
Trickling Filter and Combined Trickling Filter Suspended-Growth Process
distributor inefficiency in trickling filter media-wetting. The interrelationship of liquid residence time, dosing, and media configuration on BOD5 removal kinetics has not been addressed, and additional research is required. Increasing the average hydraulic application rate reduces the liquid residence time, but has been proven to increase wetting efficiency. The recirculation ratio (Q/QR) typically is in the range 0.5 to 4.0. Data obtained by Dow Chemical Company (Bryan, 1955, 1962; Bryan and Moeller, 1960) demonstrate that vertical-flow media require an average application rate exceeding 1.8 m3/m2·h to maximize the BOD5 removal efficiency. Shallow towers using crossflow media have used hydraulic rates in the range 0.4 to 1.1 m3/m2·h. Slowed distributor operation has benefited trickling filter facilities because of interrupted flow (periodicity of dosing), increasing wetting efficiency (percent of media wetted), and biofilm thickness control. The designer should consider recirculation capabilities, the effect of reverse-thrusting jets, or the use of speed control on the distributor, to enhance performance or improve operation. A German-developed (ATV, 1983) process control parameter, referred to as Spülkraft, allows for the calculation of a dosing rate (mm/pass) by eq 3.4. mm m Sk = mi N a ⋅ ω d ⋅ 60 hour THl ⋅ 1000
Where SK = Spülkraft (mm/pass), Na = number of arms on the distributor, and ωa = distributor rotational speed (rpm).
(3.4)
The typical hydraulically driven distributor in North America operates in the range 2 to 10 mm/pass. Table 3.3 lists recommended dosing rates for rotary distributors. Higher dosing rates are recommended for higher organic loading rates, to enhance biofilm thickness control. Besides a normal operating dosing rate, it may be beneficial to periodically use a higher flushing dosing rate for 5 to 10% of the 24-hour operating period. Albertson (1989a, 1989b, 1995) and Parker and coworkers (Parker, 1999; Parker et al., 1995, 1997) demonstrated that biofilm thickness control enhanced nitrification in the trickling filter process, reduced odors, reduced power use for recycling, reduced the accumulation of nuisance organisms, and eliminated heavy sloughing cycles. Parker and coworkers (Parker, 1999; Parker et al., 1995, 1997) recommended the use of VFD-controlled recirculation pumps and distributor speed control
99
100
Biofilm Reactors
Table 3.3 Operating and flushing dosing rates for distributors.* Total organic load (kg/m3·d [lb BOD5/d·1000 cu ft])
Operating dosing rate (mm/pass [in./pass])
Flushing dosing rate (mm/pass [in./pass])
<0.4 (<25)
25 to 75 (1 to 3)
100 (4)
0.8 (50)
50 to 150 (2 to 6)
150 (6)
1.2 (75)
75 to 225 (3 to 9)
225 (9)
1.6 (100)
100 to 300 (4 to 12)
300 (12)
2.4 (150)
150 to 450 (6 to 18)
450 (18)
3.2 (200)
200 to 600 (8 to 24)
600 (24)
*Actual values are site-specific and vary with media type.
(if necessary) to maintain constant trickling filter hydraulic application. Parker et al. (1995) conducted pilot studies, which demonstrated that mechanically driven distributor dosing alone did not improve the performance of an NTF. Parker (1999) has pointed out that there is little research describing the effect of hydraulic transients on synthetic trickling filter media and their effect on media life.
4.0 Ventilation and Air Supply Alternatives Trickling filters require oxygen to sustain aerobic biochemical transformation processes. Several researchers have demonstrated that at least some portion (if not the entire bioreactor) of roughing, carbon-oxidizing, combined carbon-oxidizing and nitrification, and NTFs operates under oxygen-limited conditions (Kuenen et al., 1986; Okey and Albertson, 1989a; Schroeder and Tchobanoglous, 1976). Ventilation is essential to maintain aerobic conditions in a trickling filter. The vertical flow of air through trickling filter biofilm carriers can be induced by natural air draft or mechanical ventilation. Mechanical ventilation enhances and controls airflow with low-pressure fans, which continuously circulate air throughout the trickling filter. Current design practice requires the provision of adequate underdrain and effluent channel sizing to permit free airflow. Passive devices for ventilation include vent stacks on the trickling filter periphery, extensions of underdrains through trickling filter side walls, ventilating manholes, louvers on the side wall of the tower near the underdrain, and discharge of trickling effluent to the subsequent settling basin in an open channel or partially filled pipes.
Trickling Filter and Combined Trickling Filter Suspended-Growth Process
4.1 Natural Draft Naturally occurring airflow results from a difference in ambient air temperature and humidity outside and inside the trickling filter. The temperature causes air to expand when warmed or contract when cooled. The result is an air-density gradient throughout the trickling filter, and an air front rises or sinks, depending on the differential condition. This rising or sinking action results in a continuous airflow through the bioreactor. If the air inside the trickling filter is colder than the ambient air, the air will flow downward. Alternatively, if the ambient air is colder than the air inside trickling filter, air will flow upward. Schroeder and Tchobanoglous (1976) state that upward airflow is the worst case from a mass-transfer perspective, because the dissolved oxygen driving force is lowest in the region of highest oxygen demand (i.e., the top of the trickling filter). Natural ventilation may become unreliable or inadequate in meeting process air requirements when neutral temperatures do not produce air movement. Such conditions may be daily or seasonal and can lead to the development of anaerobic biofilms and poor trickling filter performance. Modular synthetic-media trickling filters that rely on natural draft to provide process oxygen for municipal wastewater treatment should include the following design features: • Drains, channels, and pipes should be sized sufficiently to prevent submergence greater than 50% of their cross-sectional area under design hydraulic loading. • Ventilating access ports with open grating covers should be installed at both ends of the central collection channel. • Large-diameter trickling filters typically have branch channels (to collect the treated wastewater). • These branches also should include ventilating manholes or vent stacks installed at the trickling filter periphery. • The open area of the slots in the top of the underdrain blocks should not be less than 15% of the trickling filter area. • Approximately a 1-m2 gross area of open grating in ventilating manholes and vent stacks should be provided for each 23 m2 of trickling filter area. Typically, 0.1 m2 of ventilating area is provided for every 3 to 4.6 m of trickling filter periphery, and 1 to 2 m2 of ventilation area is provided in the underdrain area per 1000 m3 of trickling filter media. • Another criterion for rock-media trickling filters is the provision of a vent area
