Physicochemical Treatment Processes

PhysicochemicalTreatment Processes

Edited by

Lawrence K. Wang, PhD, PE, DEEZorex Corporation, Newtonville, NY

Lenox Institute of Water Technology, Lenox, MAKrofta Engineering Corporation, Lenox, MA

Yung-Tse Hung, PhD, PE, DEEDepartment of Civil and Environmental Engineering

Cleveland State University, Cleveland, OH

Nazih K. Shammas, PhDLenox Institute of Water Technology, Lenox, MA

VOLUME 3HANDBOOK OF ENVIRONMENTAL ENGINEERING

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Physicochemical treatment processes / edited by Lawrence K. Wang, Yung-Tse Hung, Nazih K. Shammas.p. cm. — (Handbook of environmental engineering)

Includes bibliographical references and index.ISBN 1-58829-165-0 (v. 3 : alk. paper)1. Water—Purification. 2. Sewerage—Purification. I. Wang, Lawrence K. II. Hung, Yung-Tse. III.Shammas, Nazih K. IV Series: Handbook of environmental engineering (2004) ; v. 3.TD170 .H37 2004 vol. 3[TD430]628 s—dc22 [628.1/ 2004002102

Preface

v

The past 30 years have seen the emergence of a growing desire worldwide to takepositive actions to restore and protect the environment from the degrading effects of allforms of pollution: air, noise, solid waste, and water. Because pollution is a direct orindirect consequence of waste, the seemingly idealistic demand for “zero discharge”can be construed as an unrealistic demand for zero waste. However, as long as wasteexists, we can only attempt to abate the subsequent pollution by converting it to a lessnoxious form. Three major questions usually arise when a particular type of pollutionhas been identified: (1) How serious is the pollution? (2) Is the technology to abate itavailable? and (3) Do the costs of abatement justify the degree of abatement achieved?The principal intention of the Handbook of Environmental Engineering series is tohelp readers formulate answers to the last two questions.

The traditional approach of applying tried-and-true solutions to specific pollution prob-lems has been a major contributing factor to the success of environmental engineering, andhas accounted in large measure for the establishment of a “methodology of pollution con-trol.” However, realization of the ever-increasing complexity and interrelated nature ofcurrent environmental problems makes it imperative that intelligent planning of pollutionabatement systems be undertaken. Prerequisite to such planning is an understanding of theperformance, potential, and limitations of the various methods of pollution abatement avail-able for environmental engineering. In this series of handbooks, we will review at a tutoriallevel a broad spectrum of engineering systems (processes, operations, and methods) cur-rently being utilized, or of potential utility, for pollution abatement. We believe that theunified interdisciplinary approach in these handbooks is a logical step in the evolution ofenvironmental engineering.

The treatment of the various engineering systems presented in PhysicochemicalTreatment Process shows how an engineering formulation of the subject flows natu-rally from the fundamental principles and theories of chemistry, physics, and math-ematics. This emphasis on fundamental science recognizes that engineering practicehas in recent years become more firmly based on scientific principles rather than itsearlier dependency on empirical accumulation of facts. It is not intended, though, toneglect empiricism when such data lead quickly to the most economic design; certainengineering systems are not readily amenable to fundamental scientific analysis, and inthese instances we have resorted to less science in favor of more art and empiricism.

Because an environmental engineer must understand science within the context of appli-cation, we first present the development of the scientific basis of a particular subject, fol-lowed by exposition of the pertinent design concepts and operations, and detailedexplanations of their applications to environmental quality control or improvement.Throughout this series, methods of practical design calculation are illustrated by numericalexamples. These examples clearly demonstrate how organized, analytical reasoning leadsto the most direct and clear solutions. Wherever possible, pertinent cost data have beenprovided.

Our treatment of pollution-abatement engineering is offered in the belief that thetrained engineer should more firmly understand fundamental principles, be more awareof the similarities and/or differences among many of the engineering systems, and ex-hibit greater flexibility and originality in the definition and innovative solution of envi-ronmental pollution problems. In short, environmental engineers should by convictionand practice be more readily adaptable to change and progress.

Coverage of the unusually broad field of environmental engineering has demandedan expertise that could only be provided through multiple authorships. Each author (orgroup of authors) was permitted to employ, within reasonable limits, the customarypersonal style in organizing and presenting a particular subject area, and, consequently,it has been difficult to treat all subject material in a homogeneous manner. Moreover,owing to limitations of space, some of the authors’ favored topics could not be treatedin great detail, and many less important topics had to be merely mentioned or com-mented on briefly. All of the authors have provided an excellent list of references at theend of each chapter for the benefit of the interested reader. Because each of the chap-ters is meant to be self-contained, some mild repetition among the various texts wasunavoidable. In each case, all errors of omission or repetition are the responsibility ofthe editors and not the individual authors. With the current trend toward metrication,the question of using a consistent system of units has been a problem. Wherever pos-sible the authors have used the British system along with the metric equivalent or viceversa. The authors sincerely hope that this doubled system of unit notation will provehelpful rather than disruptive to the readers.

The goals of the Handbook of Environmental Engineering series are: (1) to cover theentire range of environmental fields, including air and noise pollution control, solid wasteprocessing and resource recovery, biological treatment processes, water resources, natu-ral control processes, radioactive waste disposal, thermal pollution control, and physico-chemical treatment processes; and (2) to employ a multithematic approach toenvironmental pollution control because air, water, land, and energy are all interre-lated. The organization of the series is mainly based on the three basic forms in whichpollutants and waste are manifested: gas, solid, and liquid. In addition, noise pollutioncontrol is included in one of the handbooks in the series.

This volume, Physicochemical Treatment Processes, has been designed to serve as abasic physicochemical treatment text as well as a comprehensive reference book. Wehope and expect it will prove to be of high value to advanced undergraduate or gradu-ate students, to designers of water and wastewater treatment systems, and to researchworkers. The editors welcome comments from readers in all these categories. It is ourhope that this book will not only provide information on the physical, chemical, andmechanical treatment technologies, but will also serve as a basis for advanced study orspecialized investigation of the theory and practice of the individual physicochemicalsystems covered.

The editors are pleased to acknowledge the encouragement and support receivedfrom their colleagues and the publisher during the conceptual stages of this endeavor.We wish to thank the contributing authors for their time and effort, and for having

vi Preface

patiently borne our reviews and numerous queries and comments. We are very gratefulto our respective families for their patience and understanding during some rather try-ing times.

Lawrence K. WangYung-Tse Hung

Nazih K. Shammas

Preface vii

ix

Contents

Preface ............................................................................................................................ vContributors ................................................................................................................. xix

1 Screening and ComminutionFrank J. DeLuise, Lawrence K. Wang, Shoou-Yuh Chang,

and Yung-Tse Hung....................................................................................... 1

1. Function of Screens and Comminutors ....................................................................................................... 12. Types of Screens ........................................................................................................................................... 2

2.1. Coarse Screens .................................................................................................................................... 22.2. Fine Screens ........................................................................................................................................ 2

3. Physical Characteristics and Hydraulic Considerations of Screens .......................................................... 34. Cleaning Methods for Screens ..................................................................................................................... 55. Quality and Disposal for Screens ................................................................................................................ 66. Comminutors ................................................................................................................................................ 77. Engineering Specifications and Experience ................................................................................................ 8

7.1. Professional Association Specifications ............................................................................................ 87.2. Engineering Experience .................................................................................................................... 11

8. Engineering Design .................................................................................................................................... 128.1. Summary of Screening Design Considerations ............................................................................... 128.2. Summary of Comminution Design Considerations ........................................................................ 14

9. Design Examples ........................................................................................................................................ 159.1. Example 1: Bar Screen Design ......................................................................................................... 159.2. Example 2: Bar Screen Head Loss ................................................................................................... 169.3. Example 3: Plugged Bar Screen Head Loss .................................................................................... 179.4. Example 4: Screen System Design .................................................................................................. 17

Nomenclature .................................................................................................................................................... 18References ......................................................................................................................................................... 18

2 Flow Equalization and NeutralizationRamesh K. Goel, Joseph R. V. Flora, and J. Paul Chen ............................... 211. Introduction ................................................................................................................................................. 212. Flow Equalization ....................................................................................................................................... 21

2.1. Flow Equalization Basin Calculations ............................................................................................. 232.2. Mixing and Aeration Requirements ................................................................................................. 252.3. Mixer Unit ......................................................................................................................................... 26

3. Neutralization ............................................................................................................................................. 283.1. pH ....................................................................................................................................................... 283.2. Acidity and Alkalinity ...................................................................................................................... 293.3. Buffer Capacity ................................................................................................................................. 303.4. Hardness ............................................................................................................................................ 31

4. Neutralization Practices ............................................................................................................................. 324.1. Neutralization of Acidity .................................................................................................................. 324.2. Neutralization of Alkalinity ............................................................................................................. 334.3. Common Neutralization Treatments ................................................................................................ 34

5. pH Neutralization Practices ....................................................................................................................... 365.1. Passive Neutralization ...................................................................................................................... 365.2. In-Plant Neutralization ..................................................................................................................... 365.3. Influent pH Neutralization ................................................................................................................ 365.4. In-Process Neutralization ................................................................................................................. 375.5. Effluent Neutralization ..................................................................................................................... 385.6. Chemicals for Neutralization ........................................................................................................... 38

5.7. Encapsulated Phosphate Buffers for In Situ Bioremediation ......................................................... 396. Design of a Neutralization System ............................................................................................................ 397. Design Examples ........................................................................................................................................ 40Nomenclature .................................................................................................................................................... 43References ......................................................................................................................................................... 44

3 MixingJ. Paul Chen, Frederick B. Higgins, Shoou-Yuh Chang,

and Yung-Tse Hung..................................................................................... 471. Introduction ................................................................................................................................................. 472. Basic Concepts ........................................................................................................................................... 48

2.1. Criteria for Mixing ............................................................................................................................ 502.2. Mixing Efficiency ............................................................................................................................. 522.3. Fluid Shear ........................................................................................................................................ 54

3. Mixing Processes and Equipment .............................................................................................................. 553.1. Mixing in Turbulent Fields ............................................................................................................... 553.2. Mechanical Mixing Equipment ........................................................................................................ 583.3. Impeller Discharge ............................................................................................................................ 693.4. Motionless Mixers ............................................................................................................................ 713.5. Mixing in Batch and Continuous Flow Systems ............................................................................. 733.6. Suspension of Solids ......................................................................................................................... 773.7. Static Mixer ....................................................................................................................................... 84

4. Design of Facilities ..................................................................................................................................... 864.1. Pipes, Ducts, and Channels .............................................................................................................. 864.2. Self-Induced and Baffled Basins ...................................................................................................... 894.3. Mechanically Mixed Systems .......................................................................................................... 90

Nomenclature .................................................................................................................................................... 99References ....................................................................................................................................................... 100

4 Coagulation and FlocculationNazih K. Shammas ......................................................................................... 1031. Introduction ............................................................................................................................................... 1032. Applications of Coagulation .................................................................................................................... 104

2.1. Water Treatment ............................................................................................................................. 1042.2. Municipal Wastewater Treatment .................................................................................................. 1042.3. Industrial Waste Treatment ............................................................................................................ 1042.4. Combined Sewer Overflow ............................................................................................................ 1042.5. Factors to be Considered in Process Selection .............................................................................. 105

3. Properties of Colloidal Systems .............................................................................................................. 1053.1. Electrokinetic Properties ................................................................................................................ 1053.2. Hydration ......................................................................................................................................... 1063.3. Brownian Movement ...................................................................................................................... 1063.4. Tyndall Effect .................................................................................................................................. 1063.5. Filterability ...................................................................................................................................... 107

4. Colloidal Structure and Stability ............................................................................................................. 1075. Destabilization of Colloids ...................................................................................................................... 109

5.1. Double-Layer Compression ............................................................................................................ 1105.2. Adsorption and Charge Neutralization .......................................................................................... 1105.3. Entrapment of Particles in Precipitate ........................................................................................... 1115.4. Adsorption and Bridging between Particles .................................................................................. 111

6. Influencing Factors ................................................................................................................................... 1126.1. Colloid Concentration ..................................................................................................................... 1126.2. Coagulant Dosage ........................................................................................................................... 1126.3. Zeta Potential .................................................................................................................................. 1126.4. Affinity of Colloids for Water ........................................................................................................ 1136.5. pH Value .......................................................................................................................................... 1136.6. Anions in Solution .......................................................................................................................... 114

x Contents

6.7. Cations in Solution .......................................................................................................................... 1146.8. Temperature .................................................................................................................................... 114

7. Coagulants ................................................................................................................................................ 1147.1. Aluminum Salts ............................................................................................................................... 1157.2. Iron Salts .......................................................................................................................................... 1167.3. Sodium Aluminate .......................................................................................................................... 1167.4. Polymeric Inorganic Salts ............................................................................................................... 1177.5. Organic Polymers ............................................................................................................................ 1177.6. Coagulation Aids ............................................................................................................................. 118

8. Coagulation Control ................................................................................................................................. 1188.1. Jar Test ............................................................................................................................................. 1198.2. Zetameter ......................................................................................................................................... 1208.3. Streaming Current Detector ............................................................................................................ 121

9. Chemical Feeding ..................................................................................................................................... 12110. Mixing ....................................................................................................................................................... 12211. Rapid Mix ................................................................................................................................................. 12412. Flocculation .............................................................................................................................................. 12513. Design Examples ...................................................................................................................................... 127

Nomenclature ............................................................................................................................................ 137References ................................................................................................................................................. 138

5 Chemical PrecipitationLawrence K. Wang, David A. Vaccari, Yan Li, and Nazih K. Shammas ... 1411. Introduction ............................................................................................................................................... 1412. Process Description .................................................................................................................................. 1423. Process Types ........................................................................................................................................... 142

3.1. Hydroxide Precipitation .................................................................................................................. 1423.2. Sulfide Precipitation ....................................................................................................................... 1443.3. Cyanide Precipitation ...................................................................................................................... 1453.4. Carbonate Precipitation .................................................................................................................. 1453.5. Coprecipitation ................................................................................................................................ 1463.6. Technology Status ........................................................................................................................... 146

4. Chemical Precipitation Principles ........................................................................................................... 1464.1. Reaction Equilibria ......................................................................................................................... 1464.2. Solubility Equilibria ........................................................................................................................ 1474.3. Ionic Strength and Activity ............................................................................................................ 1484.4. Ionic Strength Example .................................................................................................................. 1494.5. Common Ion Effect ......................................................................................................................... 1504.6. Common Ion Effect Example ......................................................................................................... 1504.7. Soluble Complex Formation ........................................................................................................... 1514.8. pH Effect ......................................................................................................................................... 1524.9. Solubility Diagrams ........................................................................................................................ 152

5. Chemical Precipitation Kinetics .............................................................................................................. 1525.1. Nucleation ....................................................................................................................................... 1535.2. Crystal Growth ................................................................................................................................ 1535.3. Aging ............................................................................................................................................... 1545.4. Adsorption and Coprecipitation ..................................................................................................... 154

6. Design Considerations ............................................................................................................................. 1556.1. General ............................................................................................................................................ 1556.2. Chemical Handling ......................................................................................................................... 1556.3. Mixing, Flocculation, and Contact Equipment ............................................................................. 1566.4. Solids Separation ............................................................................................................................ 1576.5. Design Criteria Summary ............................................................................................................... 157

7. Process Applications ................................................................................................................................ 1587.1. Hydroxide Precipitation .................................................................................................................. 1587.2. Carbonate Precipitation .................................................................................................................. 1597.3. Sulfide Precipitation ....................................................................................................................... 1607.4. Cyanide Precipitation ...................................................................................................................... 1617.5. Magnesium Oxide Precipitation ..................................................................................................... 162

Contents xi

7.6. Chemical Oxidation–Reduction Precipitation ............................................................................... 1627.7. Lime/Soda-Ash Softening .............................................................................................................. 1627.8. Phosphorus Precipitation ................................................................................................................ 1627.9. Other Chemical Precipitation Processes ........................................................................................ 163

8. Process Evaluation ................................................................................................................................... 1638.1. Advantages and Limitations ........................................................................................................... 1638.2. Reliability ........................................................................................................................................ 1648.3. Chemicals Required ........................................................................................................................ 1658.4. Residuals Generated ....................................................................................................................... 1658.5. Process Performance ....................................................................................................................... 165

9. Application Examples .............................................................................................................................. 165Nomenclature .................................................................................................................................................. 169References ....................................................................................................................................................... 170Appendices ...................................................................................................................................................... 174

6 Recarbonation and SofteningLawrence K. Wang, Jy S. Wu, Nazih K. Shammas,

and David A. Vaccari ................................................................................. 1991. Introduction ............................................................................................................................................... 1992. Process Description .................................................................................................................................. 1993. Softening and Recarbonation Process Chemistry ................................................................................... 2014. Lime/Soda Ash Softening Process .......................................................................................................... 2035. Water Stabilization ................................................................................................................................... 2056. Other Related Process Applications ........................................................................................................ 206

6.1. Chemical Coagulation Using Magnesium Carbonate as a Coagulant .......................................... 2066.2. Recovery of Magnesium as Magnesium Carbonate ...................................................................... 2076.3. Recovery of Calcium Carbonate as Lime ...................................................................................... 2076.4. Recarbonation of Chemically Treated Wastewaters ..................................................................... 208

7. Process Design .......................................................................................................................................... 2087.1. Sources of Carbon Dioxide ............................................................................................................ 2087.2. Distribution Systems ....................................................................................................................... 2107.3. Carbon Dioxide Quantities ............................................................................................................. 2127.4. Step-by-Step Design Approach ...................................................................................................... 212

8. Design and Application Examples .......................................................................................................... 215Nomenclature .................................................................................................................................................. 226Acknowledgments .......................................................................................................................................... 227References ....................................................................................................................................................... 227

7 Chemical OxidationNazih K. Shammas, John Y. Yang, Pao-Chiang Yuan,

and Yung-Tse Hung................................................................................... 2291. Introduction ............................................................................................................................................... 229

1.1. Dissolved Oxygen and Concept of Oxidation ............................................................................... 2301.2. The Definition of Oxidation State .................................................................................................. 231

2. Theory and Principles .............................................................................................................................. 2332.1. Stoichiometry of Oxidation–Reduction Processes ........................................................................ 2342.2. Thermodynamics of Chemical Oxidation ...................................................................................... 2362.3. Kinetic Aspects of Chemical Oxidation ........................................................................................ 240

3. Oxygenated Reagent Systems .................................................................................................................. 2433.1. Aeration in Water Purification and Waste Treatment ................................................................... 2433.2. Hydrogen Peroxide and Peroxygen Reagents ............................................................................... 2463.3. High-Temperature Wet Oxidation ................................................................................................. 248

4. Transition-Metal Ion Oxidation Systems ................................................................................................ 2564.1. Chromic Acid Oxidation ................................................................................................................ 2564.2. Permanganate Oxidation ................................................................................................................. 258

5. Recent Developments in Chemical Oxidation ........................................................................................ 2615.1. Ozone (O3) Processes ..................................................................................................................... 2615.2. Ultraviolet (UV) Processes ............................................................................................................. 2625.3. Wet Oxidation ................................................................................................................................. 263

xii Contents

5.4. Supercritical Water Oxidation ........................................................................................................ 2645.5. Biological Oxidation ....................................................................................................................... 264

6. Examples ................................................................................................................................................... 264Nomenclature .................................................................................................................................................. 268References ....................................................................................................................................................... 269

8 Halogenation and DisinfectionLawrence K. Wang, Pao-Chiang Yuan, and Yung-Tse Hung .................... 2711. Introduction ............................................................................................................................................... 2712. Chemistry of Halogenation ...................................................................................................................... 274

2.1. Chlorine Hydrolysis ........................................................................................................................ 2742.2. Chlorine Dissociation ..................................................................................................................... 2752.3. Chlorine Reactions with Nitrogenous Matter ................................................................................ 2752.4. Chlorine Reactions with Other Inorganics .................................................................................... 2792.5. Chlorine Dioxide (ClO2) Applications .......................................................................................... 2812.6. Chlorine Dioxide Generation ......................................................................................................... 2812.7. Chlorine Dioxide Reaction with Nitrogenous Matter ................................................................... 2822.8. Chlorine Dioxide Reactions with Phenolic Compounds and Other Substances .......................... 2832.9. Bromine Hydrolysis ........................................................................................................................ 283

2.10. Bromine Dissociation ..................................................................................................................... 2832.11. Bromine Reactions with Nitrogenous Matter ................................................................................ 2842.12. Iodine Hydrolysis ............................................................................................................................ 2842.13. Iodine Dissociation ......................................................................................................................... 2842.14. Iodine Reactions with Nitrogenous Matter .................................................................................... 285

3. Disinfection with Halogens ...................................................................................................................... 2853.1. Modes and Rate of Killing in Disinfection Process ...................................................................... 2853.2. Disinfection Conditions .................................................................................................................. 2863.3. Disinfection Control with Biological Tests ................................................................................... 2873.4. Disinfectant Concentration ............................................................................................................. 288

4. Chlorine and Chlorination ........................................................................................................................ 2884.1. Chlorine Compounds and Elemental Chlorine .............................................................................. 2894.2. Chlorine Feeders ............................................................................................................................. 2904.3. Chlorine Handling Equipment ........................................................................................................ 2914.4. Measurement of Chlorine Residuals .............................................................................................. 2914.5. Chlorine Dosages ............................................................................................................................ 2924.6. Chlorination By-Products ............................................................................................................... 293

5. Chlorine Dioxide Disinfection ................................................................................................................. 2946. Bromine and Bromination ........................................................................................................................ 2947. Iodine and Iodination ............................................................................................................................... 2958. Ozone and Ozonation ............................................................................................................................... 2959. Cost Data ................................................................................................................................................... 295

10. Recent Developments in Halogenation Technology .............................................................................. 29610.1. Recent Environmental Concerns and Regulations ........................................................................ 29610.2. Chlorine Dioxide ............................................................................................................................. 29710.3. Chloramines .................................................................................................................................... 29810.4. Coagulant ......................................................................................................................................... 29810.5. Ozone ............................................................................................................................................... 29910.6. Organic Disinfectants ..................................................................................................................... 29910.7. Ultraviolet (UV) .............................................................................................................................. 300

11. Disinfection System Design ..................................................................................................................... 30011.1. Design Considerations Summary ................................................................................................... 30011.2. Wastewater Disinfection ................................................................................................................. 30111.3. Potable Water Disinfection ............................................................................................................ 303

12. Design and Application Examples .......................................................................................................... 30512.1. Example 1 (Wastewater Disinfection) ........................................................................................... 30512.2. Example 2 (Potable Water Disinfection) ....................................................................................... 30812.3. Example 3 (Glossary of Halogenation, Chlorination, Oxidation, and Disinfection) .................. 308

Nomenclature .................................................................................................................................................. 311References ....................................................................................................................................................... 311

Contents xiii

9 OzonationNazih K. Shammas and Lawrence K. Wang................................................. 3151. Introduction ............................................................................................................................................... 315

1.1. General ............................................................................................................................................ 3151.2. Alternative Disinfectants ................................................................................................................ 316

2. Properties and Chemistry of Ozone ......................................................................................................... 3162.1. General ............................................................................................................................................ 3162.2. Physical Properties .......................................................................................................................... 3162.3. Chemical Properties ........................................................................................................................ 3172.4. Advantages and Disadvantages ...................................................................................................... 319

3. Applications of Ozone .............................................................................................................................. 3193.1. Disinfection Against Pathogens ..................................................................................................... 3193.2. Zebra Mussel Abatement ................................................................................................................ 3203.3. Iron and Manganese Removal ........................................................................................................ 3203.4. Color Removal ................................................................................................................................ 3203.5. Control of Taste and Odor .............................................................................................................. 3213.6. Elimination of Organic Chemicals ................................................................................................. 3213.7. Control of Algae .............................................................................................................................. 3213.8. Aid in Coagulation and Destabilization of Turbidity ................................................................... 321

4. Process and Design Considerations ......................................................................................................... 3214.1. Oxygen and Ozone .......................................................................................................................... 3214.2. Disinfection of Water by Ozone .................................................................................................... 3224.3. Disinfection of Wastewater by Ozone ........................................................................................... 3244.4. Disinfection By-Products ............................................................................................................... 3334.5. Oxygenation by Ozone ................................................................................................................... 3344.6. Advanced Oxidation Processes ...................................................................................................... 337

5. Ozonation System ..................................................................................................................................... 3405.1. Air Preparation ................................................................................................................................ 3415.2. Electrical Power Supply ................................................................................................................. 3445.3. Ozone Generation ........................................................................................................................... 3445.4. Ozone Contacting ............................................................................................................................ 3455.5. Destruction of Ozone Contactor Exhaust Gas ............................................................................... 3485.6. Monitors and Controllers ................................................................................................................ 349

6. Costs of Ozonation Systems .................................................................................................................... 3496.1. Equipment Costs ............................................................................................................................. 3496.2. Installation Costs ............................................................................................................................. 3526.3. Housing Costs ................................................................................................................................. 3536.4. Operating and Maintenance Costs ................................................................................................. 353

7. Safety ........................................................................................................................................................ 353Nomenclature .................................................................................................................................................. 354References ....................................................................................................................................................... 355

10 ElectrolysisJ. Paul Chen, Shoou-Yuh Chang, and Yung-Tse Hung ............................. 3591. Introduction ............................................................................................................................................... 3592. Mechanisms of Electrolysis ..................................................................................................................... 3623. Organic and Suspended Solids Removal ................................................................................................ 363

3.1. Organic and Suspended Solids Removal by Regular Electrolysis ............................................... 3633.2. Organic and Suspended Solids Removal by Electrocoagulation ................................................. 364

4. Disinfection ............................................................................................................................................... 3665. Phosphate Removal .................................................................................................................................. 3686. Ammonium Removal ............................................................................................................................... 3697. Cyanide Destruction ................................................................................................................................. 3698. Metal Removal .......................................................................................................................................... 3709. Remediation of Nitroaromatic Explosives-Contaminated Groundwater ............................................... 372

10. Electrolysis-Stimulated Biological Treatment ........................................................................................ 37410.1. Nitrogen Removal ........................................................................................................................... 37510.2. Electrolytic Oxygen Generation ..................................................................................................... 374

References ....................................................................................................................................................... 376

xiv Contents

11 SedimentationNazih K. Shammas, Inder Jit Kumar, Shoou-Yuh Chang,

and Yung-Tse Hung................................................................................... 3791. Introduction ............................................................................................................................................... 379

1.1. Historical ......................................................................................................................................... 3791.2. Definition and Objective of Sedimentation ................................................................................... 3801.3. Significance of Sedimentation in Water and Wastewater Treatment .......................................... 380

2. Types of Clarification .............................................................................................................................. 3803. Theory of Sedimentation .......................................................................................................................... 381

3.1. Class 1 Clarification ....................................................................................................................... 3823.2. Class 2 Clarification ....................................................................................................................... 3863.3. Zone Settling ................................................................................................................................... 3873.4. Compression Settling ...................................................................................................................... 390

4. Sedimentation Tanks in Water Treatment ............................................................................................... 3904.1. General Consideration .................................................................................................................... 3904.2. Inlet and Outlet Control .................................................................................................................. 3914.3. Tank Geometry ................................................................................................................................ 3924.4. Short Circuiting ............................................................................................................................... 3924.5. Detention Time ................................................................................................................................ 3924.6. Tank Design .................................................................................................................................... 393

5. Sedimentation Tanks in Wastewater Treatment ..................................................................................... 3945.1. General Consideration and Basis of Design .................................................................................. 3945.2. Regulatory Standards ...................................................................................................................... 3955.3. Tank Types ...................................................................................................................................... 395

6. Grit Chamber ............................................................................................................................................ 3986.1. General ............................................................................................................................................ 3986.2. Types of Grit Chambers ................................................................................................................. 3996.3. Velocity Control Devices ............................................................................................................... 4006.4. Design of Grit Chamber ................................................................................................................. 402

7. Gravity Thickening in Sludge Treatment ................................................................................................ 4037.1. Design of Sludge Thickeners ......................................................................................................... 405

8. Recent Developments ............................................................................................................................... 4068.1. Theory of Shallow Depth Settling ................................................................................................. 4078.2. Tube Settlers .................................................................................................................................... 4098.3. Lamella Separator ........................................................................................................................... 4108.4. Other Improvements ....................................................................................................................... 411

9. Sedimentation in Air Streams .................................................................................................................. 4129.1. General ............................................................................................................................................ 4129.2. Gravity Settlers ............................................................................................................................... 413

10. Costs .......................................................................................................................................................... 41410.1. General ............................................................................................................................................ 41410.2. Sedimentation Tanks ....................................................................................................................... 41410.3. Gravity Thickeners .......................................................................................................................... 41610.4. Tube Settlers .................................................................................................................................... 416

11. Design Examples ...................................................................................................................................... 418Nomenclature .................................................................................................................................................. 426References ....................................................................................................................................................... 427Appendix: US Yearly Average Cost Index for Utilities ............................................................................... 429

12 Dissolved Air FlotationLawrence K. Wang, Edward M. Fahey, and Zucheng Wu ......................... 4311. Introduction ............................................................................................................................................... 431

1.1. Adsorptive Bubble Separation Processes ...................................................................................... 4311.2. Content and Objectives ................................................................................................................... 434

2. Historical Development of Clarification Processes ................................................................................ 4352.1. Conventional Sedimentation Clarifiers .......................................................................................... 4352.2. Innovative Flotation Clarifiers ....................................................................................................... 437

3. Dissolved Air Flotation Process .............................................................................................................. 4403.1. Process Description ......................................................................................................................... 440

Contents xv

3.2. Process Configurations ................................................................................................................... 4413.3. Factors Affecting Dissolved Air Flotation .................................................................................... 443

4. Dissolved Air Flotation Theory ............................................................................................................... 4444.1. Gas-to-Solids Ratio of Full Flow Pressurization System ............................................................. 4444.2. Gas-to-Solids Ratio of Partial Flow Pressurization System ......................................................... 4464.3. Gas-to-Solids Ratio of Recycle Flow Pressurization .................................................................... 4474.4. Air Solubility in Water at 1 Atm .................................................................................................... 4484.5. Pressure Calculations ...................................................................................................................... 4494.6. Hydraulic Loading Rate .................................................................................................................. 4494.7. Solids Loading Rate ........................................................................................................................ 451

5. Design, Operation, and Performance ....................................................................................................... 4535.1. Operational Parameters ................................................................................................................... 4555.2. Performance and Reliability ........................................................................................................... 455

6. Chemical Treatment ................................................................................................................................. 4557. Sampling, Tests, and Monitoring ............................................................................................................ 457

7.1. Sampling .......................................................................................................................................... 4577.2. Laboratory and Field Tests ............................................................................................................. 457

8. Procedures and Apparatus for Chemical Coagulation Experiments ...................................................... 4579. Procedures and Apparatus for Laboratory Dissolved Air Flotation Experiments ................................ 459

9.1. Full Flow Pressurization System .................................................................................................... 4599.2. Partial Flow Pressurization System ............................................................................................... 4609.3. Recycle Flow Pressurization System ............................................................................................. 461

10. Normal Operating Procedures ................................................................................................................. 46210.1. Physical Control .............................................................................................................................. 46210.2. Startup .............................................................................................................................................. 46310.3. Routine Operations ......................................................................................................................... 46410.4. Shutdown ......................................................................................................................................... 464

11. Emergency Operating Procedures ........................................................................................................... 46411.1. Loss of Power .................................................................................................................................. 46411.2. Loss of Other Treatment Units ....................................................................................................... 465

12. Operation and Maintenance ..................................................................................................................... 46512.1. Troubleshooting .............................................................................................................................. 46512.2. Labor Requirements ........................................................................................................................ 46512.3. Construction and O&M Costs ........................................................................................................ 46512.4. Energy Consumption ...................................................................................................................... 46512.5. Maintenance Considerations .......................................................................................................... 46612.6. Environmental Impact and Safety Considerations ........................................................................ 468

13. Recent Developments in Dissolved Air Flotation Technology ............................................................. 46813.1. General Recent Developments ....................................................................................................... 46813.2. Physicochemical SBR-DAF Process for Industrial and Municipal Applications ....................... 47013.3. Adsorption Flotation Processes ...................................................................................................... 47113.4. Dissolved Gas Flotation .................................................................................................................. 47113.5. Combined Sedimentation and Flotation ........................................................................................ 472

14. Application and Design Examples .......................................................................................................... 472Nomenclature .................................................................................................................................................. 491Acknowledgments .......................................................................................................................................... 492References ....................................................................................................................................................... 493

13 Gravity FiltrationJ. Paul Chen, Shoou-Yuh Chang, Jerry Y. C. Huang,

E. Robert Baumann, and Yung-Tse Hung ............................................... 5011. Introduction ............................................................................................................................................... 5012. Physical Nature of Gravity Filtration ...................................................................................................... 502

2.1. Transport Mechanism ..................................................................................................................... 5022.2. Attachment Mechanisms ................................................................................................................ 5042.3. Detachment Mechanisms ................................................................................................................ 504

3. Mathematical Models ............................................................................................................................... 5043.1. Idealized Models ............................................................................................................................. 5053.2. Empirical Models ............................................................................................................................ 509

xvi Contents

4. Design Considerations of Gravity Filters ................................................................................................ 5104.1. Water Variables ............................................................................................................................... 5104.2. Filter Physical Variables ................................................................................................................. 5114.3. Filter Operating Variables .............................................................................................................. 517

5. Applications .............................................................................................................................................. 5225.1. Potable Water Filtration ................................................................................................................. 5225.2. Reclamation of Wasterwater .......................................................................................................... 522

6. Design Examples ...................................................................................................................................... 527Nomenclature .................................................................................................................................................. 539References ....................................................................................................................................................... 540

14 Polymeric Adsorption and Regenerant DistillationLawrence K. Wang, Chein-Chi Chang, and Nazih K. Shammas ............... 5451. Introduction ............................................................................................................................................... 5452. Polymeric Adsorption Process Description ............................................................................................ 547

2.1. Process System ................................................................................................................................ 5472.2. Process Steps ................................................................................................................................... 5472.3. Regeneration Issues ........................................................................................................................ 547

3. Polymeric Adsorption Applications and Evaluation .............................................................................. 5483.1. Applications .................................................................................................................................... 5483.2. Process Evaluation .......................................................................................................................... 550

4. Polymeric Adsorbents .............................................................................................................................. 5504.1. Chemical Structure .......................................................................................................................... 5504.2. Physical Properties .......................................................................................................................... 5524.3. Adsorption Properties ..................................................................................................................... 552

5. Design Considerations ............................................................................................................................. 5525.1. Adsorption Bed, Adsorbents, and Regenerants ............................................................................. 5525.2. Generated Residuals ....................................................................................................................... 555

6. Distillation ................................................................................................................................................ 5576.1. Distillation Process Description ..................................................................................................... 5576.2. Distillation Types and Modifications ............................................................................................ 5576.3. Distillation Process Evaluation ...................................................................................................... 560

7. Design and Application Examples .......................................................................................................... 560Acknowledgments .......................................................................................................................................... 570References ....................................................................................................................................................... 571

15 Granular Activated Carbon AdsorptionYung-Tse Hung, Howard H. Lo, Lawrence K. Wang,

Jerry R. Taricska, and Kathleen Hung Li ................................................ 5731. Introduction ............................................................................................................................................... 5732. Process Flow Diagrams for GAC Process ............................................................................................... 5763. Adsorption Column Models ..................................................................................................................... 5774. Design of Granular Activated Carbon Columns ..................................................................................... 585

4.1. Design of GAC Columns ................................................................................................................ 5854.2. Pilot Plant and Laboratory Column Tests ...................................................................................... 590

5. Regeneration ............................................................................................................................................. 5916. Factors Affecting GAC Adsorption ......................................................................................................... 592

6.1. Adsorbent Characteristics ............................................................................................................... 5926.2. Adsorbate Characteristics ............................................................................................................... 592

7. Performance and Case Studies ................................................................................................................. 5938. Economics of Granular Activated Carbon System ................................................................................. 5959. Design Examples ...................................................................................................................................... 602

10. Historical and Recent Developments in Granular Activated Carbon Adsorption ................................ 62310.1. Adsorption Technology Milestones ............................................................................................... 62310.2. Downflow Conventional Biological GAC Systems ...................................................................... 62510.3. Upflow Fluidized Bed Biological GAC System ........................................................................... 627

Nomenclature .................................................................................................................................................. 628References ....................................................................................................................................................... 630

Contents xvii

16 Physicochemical Treatment Processes for Water ReuseSaravanamuthu Vigneswaran, Huu Hao Ngo,

Durgananda Singh Chaudhary, and Yung-Tse Hung ............................ 6351. Introduction ............................................................................................................................................... 6352. Conventional Physicochemical Treatment Processes ............................................................................. 636

2.1. Principle ........................................................................................................................................... 6362.2. Application of the Physicochemical Processes in Wastewater Treatment and Reuse ................ 651

3. Membrane Processes ................................................................................................................................ 6583.1. Principle ........................................................................................................................................... 6583.2. Application of Membrane Processes ............................................................................................. 661

References ....................................................................................................................................................... 675

17 Introduction to Sludge TreatmentDuu-Jong Lee, Joo-Hwa Tay, Yung-Tse Hung, and Pin Jing He ............. 6771. The Origin of Sludge ................................................................................................................................ 6772. Conditioning Processes ............................................................................................................................ 678

2.1. Coagulation ..................................................................................................................................... 6782.2. Flocculation ..................................................................................................................................... 6812.3. Conditioner Choice ......................................................................................................................... 6812.4. Optimal Dose ................................................................................................................................... 682

3. Dewatering Processes ............................................................................................................................... 6843.1. Dewatering Processes ..................................................................................................................... 6843.2. Sludge Thickening .......................................................................................................................... 6853.3. Sludge Dewatering .......................................................................................................................... 687

4. Stabilization Processes ............................................................................................................................. 6914.1. Hydrolysis Processes ...................................................................................................................... 6914.2. Digestion Processes ........................................................................................................................ 695

5. Thermal Processes .................................................................................................................................... 6995.1. Sludge Incineration ......................................................................................................................... 6995.2. Sludge Drying ................................................................................................................................. 7015.3. Other Thermal Processes ................................................................................................................ 702

References ....................................................................................................................................................... 703

Index ........................................................................................................................... 705

xviii Contents

Contributors

E. ROBERT BAUMANN, PhD • Department of Civil Engineering, Iowa State University ofScience and Technology, Ames, IA

CHEIN-CHI CHANG, PhD, PE • District of Columbia Water and Sewer Authority,Washington, DC

SHOOU-YUH CHANG, PhD, PE • Department of Civil and Environmental Engineering,North Carolina A&T State University, Greensboro, NC

DURGANANDA SINGH CHAUDHARY, PhD • Faculty of Engineering, University of TechnologySydney (UTS), New South Wales, Australia

J. PAUL CHEN, PhD • Department of Chemical and Biomolecular Engineering, NationalUniversity of Singapore, Singapore

FRANK DELUISE, ME, PE • Emeritus Professor, Department of Mechanical Engineering,University of Rhode Island, Kingston, RI

EDWARD M. FAHEY, ME • DAF Environmental, LLC, Hinsdale, MAJOSEPH R. V. FLORA, PhD • Department of Civil & Environmental Engineering, University

of South Carolina, Columbia, SCRAMESH K. GOEL, PhD • Department of Civil and Environmental Engineering, University

of Wisconsin, Madison, WIPIN JING HE, PhD • School of Environmental Science and Engineering, Tongji University,

Shanghai, ChinaFREDERICK B. HIGGINS, PhD • Civil and Environmental Engineering Department, Temple

University, Philadelphia, PAYUNG-TSE HUNG, PhD, PE, DEE • Department of Civil and Environmental Engineering,

Cleveland State University, Cleveland, OHJERRY Y. C. HUANG, PhD • Department of Civil Engineering, University of Wisconsin–

Milwaukee, Milwaukee, WIINDER JIT KUMAR, PhD • Eustance & Horowitz, P.C., Consulting Engineers, Circleville, NYDUU-JONG LEE, PhD • Department of Chemical Engineering, National Taiwan University,

Taipei, TaiwanKATHLEEN HUNG LI, MS • NEC Business Network Solutions, Irving, TXYAN LI, PE, MS • Department of Environmental Management, State of Rhode Island,

Providence, RIHOWARD LO, PhD • Department of Biological, Geological and Environmental Sciences,

Cleveland State University, Cleveland, OHHUU HAO NGO, PhD • Faculty of Engineering, University of Technology Sydney (UTS),

New South Wales, AustraliaNAZIH K. SHAMMAS, PhD • Graduate Environmental Engineering Program, Lenox Institute

of Water Technology, Lenox, MAJERRY R. TARICSKA, PhD, PE • Hole Montes Inc., Naples, FL

xix

JOO-HWA TAY, PhD, PE • Division of Environmental and Water Resource Engineering,Nanyang Technological University, Singapore

DAVID A. VACCARI, PhD, PE, DEE • Department of Civil, Environmental and Ocean Engineering,Stevens Institute of Technology, Hoboken, NJ

SARAVANAMUTHU VIGNESWARAN, PhD, DSc, CPEng • Faculty of Engineering, University ofTechnology Sydney (UTS), New South Wales, Australia

LAWRENCE K. WANG, PhD, PE, DEE • Zorex Corporation, Newtonville, NY; Lenox Instituteof Water Technology, Lenox, MA; and Krofta Engineering Corporation, Lenox, MA

JY S. WU, PhD • Department of Civil Engineering, University of North Carolina at Char-lotte, Charlotte, NC

ZUCHENG WU, PhD • Department of Environmental Science and Engineering, ZhejiangUniversity, Hangzhou, People’s Republic of China

JOHN Y. YANG, PhD • Niagara Technology Inc., Williamsville, NYPAO-CHIANG YUAN, PhD • Technology Department, Jackson State University, Jackson, MS

xx Contributors

1Screening and Comminution

Frank Deluise, Lawrence K. Wang, Shoou-Yuh Chang, and Yung-Tse Hung

CONTENTS

FUNCTION OF SCREENS AND COMMINUTORS

TYPES OF SCREENS

PHYSICAL CHARACTERISTICS AND HYDRAULIC CONSIDERATIONS OF SCREENS

CLEANING METHODS FOR SCREENS

QUANTITY AND DISPOSAL OF SCREENINGS

COMMINUTORS

ENGINEERING SPECIFICATIONS AND EXPERIENCE

ENGINEERING DESIGN

DESIGN EXAMPLES

NOMENCLATURE

REFERENCES

1. FUNCTION OF SCREENS AND COMMINUTORS

In order for water and wastewater treatment plants to operate effectively, it is neces-sary to remove or reduce early in the treatment process large suspended solid materialthat might interfere with operations or damage equipment. Removal of solids may beaccomplished through the use of various size screens placed in the flow channel. Anymaterial removed may then be ground to a smaller size and returned to the processstream or disposed of in an appropriate manner such as burying or incineration. Analternative to actual removal of the solids by screening is to reduce the size of the solidsby grinding them while still in the waste stream; this grinding process is called com-minution (1–8). Coarse screens (bar racks) and comminutors are usually located at thevery beginning of a treatment process, immediately preceding the grit chambers (Fig. 1).To ensure continuous operation in a flow process, it is desirable to have the screens orcomminutors installed in parallel in the event of a breakdown or to provide for overhaulof a unit. With this arrangement, flow is primarily through the comminutor and divertedto the coarse (bar) screens only when necessary to shut down the comminutor. Finescreens are usually placed after the coarse (bar) screens.

1

From: Handbook of Environmental Engineering, Volume 3: Physicochemical Treatment ProcessesEdited by: L. K. Wang, Y.-T. Hung, and N. K. Shammas © The Humana Press Inc., Totowa, NJ

2 Frank Deluise et al.

2. TYPES OF SCREENS

2.1. Coarse Screens

Screens may be classified as coarse or fine. Coarse screens are usually called barscreens or racks and are used where the wastewater contains large quantities of coarsesolids that might disrupt plant operations. These bar screens consist of parallel barsspaced anywhere from 1.27 cm (1/2 in.) to 10.16 cm (4 in.) apart with no cross-membersother than those required for support. The size of the spacing depends on the type ofwaste being treated (size and quantity of solids) and the type of equipment being pro-tected downstream in the plant. These screens are placed either vertically or at anangle in the flow channel. Installing screens at an angle allows easier cleaning (par-ticularly if by hand) and more screen area per channel depth, but obviously requiresmore space.

2.2. Fine Screens

Fine screens have openings of less than 0.25 in. and are used to remove solidssmaller than those retained on bar racks. They are used primarily in water or wastewatercontaining little or no coarse solids. In many instances, fine screens are used for the recov-ery of valuable materials that exist as finely divided solids in industrial waste streams.Most fine screens use a relatively fine mesh screen cloth (openings anywhere from0.005 to 0.126 in.) rather than bars to intercept the solids. A screen cloth covers discs ordrums, which rotate through the wastewater. The disc-type screen (Fig. 2) is a verticalhoop with a screen cloth covering the area within the hoop, and mounted on a horizon-tal shaft that is positioned slightly above the surface of the water. Water flows throughthe screen parallel to the horizontal shaft and the solids are retained on the screen, whichcarries them out of the water as it rotates. Solids may then be removed from the upperpart of the screen by water sprays or mechanical brushing.

The drum-type screen (Fig. 3) consists of a cylinder covered by a screen cloth withthe drum rotating on a horizontal axis, slightly less than half submerged. Wastewaterenters the inside of the drum at one end and flows outward through the screen cloth.Solids collect inside the drum on the screen cloth and are carried out of the water as thedrum rotates. Once out of the water, the solids may be removed by backwater sprays,forcing the solids off the screen into collecting troughs.

Fig. 1. Location of screens and comminutors in a wastewater treatment plant.

Screening and Comminution 3

3. PHYSICAL CHARACTERISTICS AND HYDRAULICCONSIDERATIONS OF SCREENS

The physical characteristics of bar racks and screens depend on the use for whichthe unit is intended. Coarse bar racks, sometimes called trash racks, with 7.62 or10.16 cm (3 or 4 in.) spacing are used to intercept unusually large solids and there-fore must be of rugged construction to withstand possible large impacts. Bar screenswith smaller spacing may be of less rugged construction. As previously mentioned,the spacing between bars depends on the size and quantity of solids being intercepted.Although a screen’s primary purpose is to protect equipment in a sewage-treatmentplant, spacings smaller than 2.54 cm (1 in.) are usually not necessary because today’ssewage sludge pumps can handle solids passing through the screen. Typical barscreens are shown in Fig. 4.

Fig. 2. Revolving disc screen: (a) screen front (inlet side) view and (b) screen side view section.

Fig. 3. Revolving drum screen.

4 Frank Deluise et al.

The screen bars are usually rectangular in cross-section and their size depends on thesize (width and depth) of the screen channel as well as the conditions under whichthe screen will be operating. The longer the unsupported length of the bar, the larger isthe required cross-section. Bars up to 1.83 m (6 ft) in length are usually no smaller than0.635 × 5.08 cm (1/4 × 2 in.), while bars up to 3.66 m (12 ft) long might be0.952 × 6.35 cm (3/8 × 2.5 in.). Longer bars or bars used for operating conditions caus-ing unusual stress might be as large as 1.59 × 7.62 cm (5/8 × 3 in.). The bars must bedesigned to withstand bending as well as impact stresses due to the accumulation ofsolids on the screen.

Many screens, particularly those that are hand-cleaned, are installed with bars at anangle between 60º and 90º with the horizontal. With the bars placed at an angle, thescreenings will tend to accumulate near the top of the screen. In addition, the velocitythrough the screen will be low enough to prevent objects from being forced through thescreen. Optimum horizontal velocity through the bars is approx 0.610 m/s (2 ft/s). Ifvelocities get too low, sedimentation will take place in the screen channel. In the designof the screen channel, it is desirable to have the flow evenly distributed across thescreen by having several feet of straight channel preceding the screen. Flow entering atan angle to the screen would tend to create uneven distribution of solids across thescreen and prevent the proper operation of the equipment.

The required size of the screen channel depends on the volume flow rate and the freespace available between the bars. If a net area ratio is defined as the free area betweenbars divided by the total area occupied by the screen, then a table such as Table 1 maybe set up showing the net area ratio for various combinations of bar size openings.

The bar spacing should be kept as large as practical and the bar thickness as small aspractical in order to obtain the highest net area ratio possible. Once the volume flowrates are known and the net area ratio is determined, the screen channel size may bedetermined. The maximum volume flow rate in cubic meters per second divided by theoptimum velocity of 0.610 m/s will yield the net area required. This net area divided by

Fig. 4. Elements of a mechanical bar screen and grit collector.

Screening and Comminution 5

the net area ratio selected will give the total wet area required for the channel. With thisknown area, the width and depth of the channel may be determined. Usually the maxi-mum width or depth of the channel is limited by considerations other than the actualscreening process. Too wide a screen could present problems in cleaning, and thereforethe maximum practical width for a channel is about 4.27 m (14 ft); the minimum width isabout 0.610 m (2 ft). The depth of liquid in the channel is usually kept as shallow aspossible so that the head loss through the plant will be a minimum. The wet area dividedby the known limiting width or depth will thus provide the dimensions of the channel.

From Bernoulli’s equation, the theoretical head loss for frictionless, adiabatic flowthrough the bar screen is

(1)

where h = head loss, m (ft), V2 = velocity through bar screen, m/s (ft/s), V1 = velocityahead of bar screen, m/s (ft/s), and g = 9.806 m/s2 (32.17 ft/s2).

To determine the actual head loss, the above expression may be modified by a dis-charge coefficient, CD, to account for deviation from theoretical conditions. Values of CDshould be determined experimentally, but a typical average value is 0.7. The equationthen becomes

(2)

(2a)

(2b)

4. CLEANING METHODS FOR SCREENS

Bar screens or racks may be cleaned by hand or by machine. Hand-cleaning limitsthe length of screen that may be used to that which may be conveniently raked by hand.The cleaning is accomplished using a specially designed rake with teeth that fit betweenthe bars of the rack. The rake is pulled up toward the top of the screen carrying the

h V V= −( )0 0222 22

12. with English units

h V V= −( )0 0728 22

12. with SI units

hV V

C gD

= −22

12

2

hV V

g= −2

212

2

Table 1Net Area Ratios for Bar Size and Openings

Bar size Opening

cm in. cm in. Net area ratio

0.635 1⁄ 4 1.27 1⁄ 2 0.6670.635 1⁄ 4 2.54 1 0.8000.635 1⁄ 4 3.81 11⁄ 2 0.8560.952 3⁄ 8 1.27 1⁄ 2 0.5720.952 3⁄ 8 2.54 1 0.7280.952 3⁄ 8 3.81 11⁄ 2 0.8001.270 1⁄ 2 1.27 1⁄ 2 0.5001.270 1⁄ 2 2.54 1 0.6671.270 1⁄ 2 3.81 11⁄ 2 0.750

screenings with it. At the top of the screen, the screenings are deposited on a grid orperforated plate for drainage and then removed for shredding and return to the channelor for incineration or burial. Hand-cleaning requires a great deal of manual labor and isan unpleasant job. Because hand-cleaning is not continuous, plant operations may bematerially affected by undue plugging of the screens before cleaning as well as by largesurges of flow when the screens are finally cleaned. Plugging of the screens could causetroublesome deposits in the lines leading to the bar screens, and surges after cleaningcould disrupt the normally smooth operations of units following the screens.

Mechanical cleaning overcomes many of the problems associated with hand-cleaning.Although the initial cost of a mechanically cleaned screen will be much greater than for ahand-cleaned screen, the improvement in plant efficiency, particularly in large installa-tions, usually justifies the higher cost. The ability to operate the cleaning mechanism onan automatically controlled schedule avoids the flooding and surging through the plantassociated with plugging and unplugging of the screens. After a short while, a preset auto-matic cleaning cycle may be easily established to keep the bars relatively clear at all times.

Mechanically cleaned screens use moving rakes attached to either chains or cables tocarry the screenings to the top of the screen. At the top of the screen, rake wiper bladessweep the screenings into containers or onto conveyor belts for disposal. The teeth onthe rakes project between the screen bars either from the front or the back of the rack.Both methods have their advantages and disadvantages. The front-cleaned models havethe rakes passing down through the wastewater in front of the rack and then up the faceof the rack. This method provides excellent cleaning efficiency, but the rakes maypotentially become jammed as they pass through any accumulation of solids at the baseof the screen on the downward travel. A modification of the front-cleaned model has therakes traveling down behind the screen and through a boot under the screen, and thenmoving up the front of the screen. The back-cleaned models eliminate the jammingproblem by having the rakes travel down through the water behind the screen and thentravel up behind the screen with teeth projecting through the bars far enough to pick upsolids deposited on the front of the screen. In models where the rake travels up the backof the screen, the bars are fixed only at the bottom of the screen because the rake mustproject all the way through the bars. It is thus possible for the bars to move as they aresupported only by the traveling rake teeth. With movement of the bars, it is possible forsolids substantially larger than those designed for to pass through the screen. Anotherdrawback of the back-cleaned screen is that any solids not removed from the rakesbecause of faulty wiper blades are returned to the flow behind the screen. Several man-ufacturers have modified both the front- and back-cleaned screens to help reduce someof these problems.

5. QUANTITY AND DISPOSAL OF SCREENINGS

The quantity of screenings is obviously greatly affected by the type and size of screenopenings and the nature of the waste stream being screened. The curves in Fig. 5 showthe average and maximum quantities of screenings in cubic feet per 106 gallons(ft3/MG) that might be obtained from sewage for different sized openings betweenbars. Data for these curves were obtained from 133 installations of hand-cleaned andmechanically cleaned bar screens in the United States. It can be seen that the average

6 Frank Deluise et al.

Screening and Comminution 7

screenings vary from 71.1 m3/106 m3 (9.5 ft3/MG) for a 0.952 cm (3/8 in.) opening to3.74 m3/106 m3 (0.5 ft3/MG) for a 6.35 cm (2.5 in.) opening. Taking a common open-ing of 2.54 cm (1 in.), the average quantity of screenings expected would be about22.4 m3/106 m3 (3 ft3/MG), and the maximum quantity expected would be37.4 m3/106 m3. Fine screens with openings from 0.119 to 0.318 cm (3/64 to 1/8 in.)have typical screenings of 224.4 to 37.4 m3/106 m3 (30 to 5 ft3/MG) of sewage flow.The density of all screenings from a typical municipal sewage treatment plant is approx800–960 kg/m3 (50–60 lb/ft3).

Screenings may be disposed of by grinding and returning them to the flow, by burialin landfill areas or at the plant site, or by incineration. Incineration usually requires par-tial dewatering of the screenings by some type of pressing and therefore is not usuallypractical except for large installations with large volumes of screenings.

6. COMMINUTORS

The handling and disposal of screenings is at best a disagreeable and expensive pro-cedure unless the product has some recovery value. To overcome this problem, deviceswere developed to cut up large screened material into small, relatively uniform sizesolids, without removal from the line of flow. These devices are generally referred to ascomminutors (8–14). Figure 6 shows the essential elements of a comminutor, and Fig. 7shows a crosssection of a typical comminutor. Various methods are used to accomplishthe cutting of the solids.

Fig. 5. Quantity of screenings from wastewater as a function of openings between bars.(Source: US EPA)

8 Frank Deluise et al.

Fig. 6. Essential elements of a comminutor.

One type of comminuting device uses a slotted, rotating drum mounted vertically inthe flow channel. Liquid passes through the slots down through the bottom of the drumand into the downstream channel. The solids are retained on the outside of the drum andcarried by the drum to stationary comb bars mounted against the main casing of thecomminutor. Mounted on the drum are hardened cutting teeth and shear bars (usuallyremovable for sharpening or replacement) that pass through the comb bars, thereby cut-ting the solids. The small particles that result from the cutting operation then passthrough the slots of the drum with the liquid flow.

Another type of device uses a stationary vertical semicircular screen grid (installedconvex to the flow), with rotating circular discs on whose edges are mounted the cuttingteeth. The grid intercepts the larger solids, while smaller solids pass through the clearingspace between the grid and cutter discs. The rotating cutter teeth move the interceptedsolids around to a stationary cutter comb where the solids are sheared as the teeth passthrough the comb.

A third type of comminutor also uses a stationary vertical semicircular screen gridwith horizontal slots, but is installed concave rather than convex to the flow. Ahead ofthe screen, a vertical arm with a cutter bar attached oscillates back and forth so the teethon the cutting bar pass between the horizontal slots. The oscillating cutter bar carries thetrapped solids to a stationary cutter bar mounted on the screen grid where the teeth ofthe cutters mesh and thereby shear the solids.

Various size comminutors are commercially available. For low flows, units as smallas 10.16 cm (4 in.) in diameter are available, while units with 137.16 cm (54 in.) diam-eter can handle flows up to 3.15 m3/s (72 million gallons / d [MGD]). Most of the unitsuse slot widths of either 0.635 cm (1/4 in.) or 0.952 cm (3/8 in.). Power requirementsvary from 186 W (1/4 hp) for the smaller units to 1491 W (2 hp) for the larger units.

7. ENGINEERING SPECIFICATIONS AND EXPERIENCE

7.1. Professional Association Specifications

The Water Pollution Control Federation (WPCF) Technical Practice Committeeexplains the screening process and equipment (1), as well as the types of bar screensand bar racks and the differences between them.

Screening and Comminution 9

Detailed information is also given by the WPCF on screening equipment operation.Equipment should be checked frequently to ensure that it runs correctly. Screen over-flow should be prevented and cleanliness maintained in order to prevent or eliminate(a) decay of organic matter, (b) offensive odors, and (c) pathogens. On dry days, dailyremoval of debris is sufficient. However, on rainy days, debris should be removed morefrequently because leaves and other matter from combined sewer overflow (CSO) maybe transported to the plant (1).

Screening equipment may require troubleshooting for several reasons: abnormaloperational circumstances (unexpected loads of debris that clog or jam the screening

Fig. 7. Crosssection of a comminutor.

equipment), equipment failure, and control failure. If a mechanically cleaned screenlacks blubber-control systems, it could suddenly receive huge loads of debris that jamits raking mechanisms.

Proper maintenance of screening equipment includes performing routine checks ofcomponents for obstructions, proper alignment, constant speed, and unusual vibrationsand sounds. Screeches may result from a lack of lubrications, while thumps may meanthe components are loose or broken. Proper lubrication is an important preventive main-tenance procedure. Chain-driven bar screens require frequent replacement of chains,sprockets, and other parts that appear to be badly worn. Periodic removal of a link maybe required to make certain that a chain rides smoothly on the sprockets.

A description of comminutors, grinders, and various bar screens, such as trash racks,manually cleaned screens, and mechanically cleaned screens is provided by the WaterEnvironment Federation (WEF) Manual of Practice (2). The types of mechanicallycleaned screens include chain- or cable-driven screens, reciprocating rake screens,centenary screens, and continuously self-cleaning screens. Trash racks, which are usu-ally used in combined systems that have very large debris, are bar screens with largeopenings of 38 to 150 mm. The oldest mechanical-screening device is the chain- orcable-driven screen, which uses a chain or cable to move the rake teeth through thescreen openings. They are produced as front clean/front return, front clean/rear return,and back clean/rear return. The front clean/front return has proven to be the most effi-cient. The up and down motion of the reciprocating rake screen reduces the risk ofjamming and, because their parts are not submerged, they permit simple inspection andmaintenance. The reciprocating screen is at a disadvantage because the single rake lim-its the ability to handle excessive loads and requires high overhead clearance. Cantenaryscreens have heavy tooth rakes, secured against the screen by the weight of its chain anda curved transition piece at the base that provides for effective removal of solids con-fined at the bottom. Continuous self-cleaning screens are comprised of a belt of plas-tic or stainless- steel elements that are pulled through the wastewater to providescreening along the entire length of the screen and are designed with vertical and hor-izontal limiting devices. The size of openings may range from 1 to more than 76 mm.The continuous screening motion provides effective removal of a large number ofsolids, but has the disadvantage of possible carryover of solids due to its frontclean/back return design.

When designing mechanical bar screens, the following parameters should be consid-ered: (a) bar spacing, construction materials, and dimensions; (b) depth of channel,width, and approach velocity; (c) discharge height; (d) angle of screen; (e) screen coverto obstruct wind and improve appearance; (f) coatings for overall unit; (g) drive unitservice factor; (h) drive motor sized and enclosure; (i) spare parts; (j) stipulation ofunneeded screen or bypass manual screen; and (k) head loss through unit.

The designer must consider the effects of the backwater caused by the head lossthrough the screen when considering a screen location. Many installations comprise anoverflow weir to a bypass channel to avoid upstream surcharging if the screen becomesaffected by power failure or mechanical problems.

In the past, most screening devices were placed downstream from grit chambers toprevent grit damage of comminutor teeth and combs. However, screening devices arepresently placed upstream because they are more cost effective and cause fewer problems

10 Frank Deluise et al.

than downstream placement. A structural enclosure for screening devices is most favor-able under windy and freezing climate conditions. An enclosure also reduces the amountof maintenance required and improves aesthetics.

7.2. Engineering Experience

Liu and Liptak (3) stated that the combined mechanical screen and grit collectorcan be used for small- and medium-sized plants. It is similar to the front cleanedmechanical screen, but rakes are connected to one or more perforated buckets and asteep hopper to collect the grit precedes the screen. The disadvantage of the system isthat screenings and grit are mixed (3).

Some plants use coarse-mesh screens instead of screens and comminutors.Wastewater travels through a basket of wires or rods with a mesh size 1 in. or more.Coarse suspended matter is left in the basket.

Revolving drum screens may be characterized as having either outward or inwardflow. With outward flow, the wastewater can move toward the drum from a directionparallel to its axis. Solids are captured on the inside of the screen. With inward flow,wastewater travels perpendicular to the drums axis and solids are captured on the out-side of the drum. In both systems, the captured solids are lifted above the water level asthe drum slowly rotates. Solids are usually removed by water spray, which is the disad-vantage of these systems because solids are then mixed with great amount of spraywater (3).

The revolving vertical disk screen is another screening device that employs the sameprinciples as the revolving drum but uses a slowly revolving disc screen. The screen ispositioned in the approach channel totally blocking the flow so that it travels through thescreen. Solids are raised above the liquid level and washed by water spray. The screenconsists of a 2–60-mesh stainless-steel wire cloth and is not suited for handling verylarge objects, large amounts of suspended objects, or greasy, gummy or sticky solids (3).

The inclined revolving disk screen consists of a round flat plate revolving on an axisinclined 10º to 25º, and the disk is comprised of bronze plates with slots 1/6 to 1/2 in.wide. As the liquid passes through the lower two-thirds of the plates, solids are captured,elevated above the water, and removed by brushes.

The traveling water screen, which has limited use in sewage treatment, consists of sev-eral inclined screen trays on two strands of steel chain. The head wheel is powered by amotor that moves screen trays through the sewage for disposal of solids by jets of water.The trays then return to the wastewater. Vibrating screens are used in the food packingindustry to capture grease and meat particles, remove manure, catch animal hair, removefeathers from poultry, and retain vegetable and fruit particles from canning wastes.Vibration reduces the clogging of screens, which are flat and covered by stainless-steelcloth of 20 to 200 mesh.

Microscreens have openings as small as 20 μm and are used to remove fine suspendedsolids from effluent in tertiary treatment units. Hydrasieves is used for industrial effluent intreatment in plants that require an efficiency of 20–35% suspended solids and biochemicaloxygen demand (BOD) removal. No power is needed to operate except to lift the water tothe headbox of the screen. The microscreens are self-cleaning and require little mainte-nance. Wastewater is supplied by gravity or pumped into the headbox of the microscreenconsisting of three slopes of 25º, 35º, and 45º.

Screening and Comminution 11

12 Frank Deluise et al.

8. ENGINEERING DESIGN

8.1. Summary of Screening Design Considerations

Screening devices are designed to remove large floating objects that may otherwisedamage pumps and other equipment, obstruct pipelines, and interfere with the normaloperation of the treatment facilities. As discussed in previous sections, screens used inwater and wastewater treatment facilities or in pumping stations are generally classifiedas fine screens or bar screens.

Fine screens are those with openings of less than 0.25 in. These screens have been usedas a substitute for sedimentation tanks to remove suspended solids prior to biologicaltreatment. However, few plants today use this concept of solids removal. Fine screensmay be of the disc, drum, or bar type. Bar-type screens are available with openings of0.005 to 0.126 in.

Bar screens are used mainly to protect pumps, valves, pipelines, and other devicesfrom being damaged or clogged by large floating objects. These screens are sometimesused in conjunction with comminuting devices. Bar screens consist of vertical orinclined bars spaced at equal intervals (usually 0.5–4 in.) across the channel wherewater or wastewater flows. These devices may be cleaned manually or mechanically.Bar screens with openings exceeding 2.5 in. are also termed trash racks.

The quantity of screenings removed by bar screens usually depends on the size of thebar spacing. Because handling and disposal of screenings is one of the most disagreeablejobs in wastewater treatment, it is usually recommended that the quantity of screeningsbe kept to a minimum. Amounts of screenings from normal domestic wastes have beenreported from 0.5 to 5 ft3/MG of wastewater treated. Screenings may be disposed of byburial, incineration, grinding, and digestion.

Bar screen designs are based mainly on average and peak wastewater flow. Normaldesign and operating parameters are usually presented in the manufacturer’s specifica-tions. The literature (1–7) presents a thorough discussion of the design, operation, andmaintenance of screening devices. General characteristics of bar and fine screens are presented in Tables 2 and 3, respectively. Figure 4 shows a mechanically cleaned bar rack.

8.1.1. Screen Design Input Data

The following input data are required for the design of screens:

Table 2General Characteristics of Bar Screens

Item Hand cleaned Mechanically cleaned

Bar screen sizeWidth, in. 1⁄ 4 to 5⁄ 8

1⁄ 4 to 5⁄ 8

Depth, in. 1 to 3 1 to 3Spacing, in. 1 to 2 5⁄ 8 to 3

Slope from vertical, deg 30 to 45 0 to 30Approach velocity, fps 1 to 2 2 to 3Allowable head loss, in. 6 6

(Source: US Army).