Physicochemical treatment processes volume 3 (handbook of environmental engineering) lawrence k wang, yung tse hung, nazih k shammas

Physicochemical Treatment Processes


VOLUME 3
HANDBOOK OF ENVIRONMENTAL ENGINEERING

Physicochemical
Treatment Processes
Edited by
Lawrence K. Wang, PhD, PE, DEE
Zorex Corporation, Newtonville, NY
Lenox Institute of Water Technology, Lenox, MA
Krofta Engineering Corporation, Lenox, MA

Yung-Tse Hung, PhD, PE, DEE
Department of Civil and Environmental Engineering
Cleveland State University, Cleveland, OH

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


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Physicochemical treatment processes / edited by Lawrence K. Wang, Yung-Tse Hung, Nazih K. Shammas.
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Preface
The past 30 years have seen the emergence of a growing desire worldwide to take
positive actions to restore and protect the environment from the degrading effects of all
forms of pollution: air, noise, solid waste, and water. Because pollution is a direct or
indirect consequence of waste, the seemingly idealistic demand for “zero discharge”
can be construed as an unrealistic demand for zero waste. However, as long as waste
exists, we can only attempt to abate the subsequent pollution by converting it to a less
noxious form. Three major questions usually arise when a particular type of pollution
has been identified: (1) How serious is the pollution? (2) Is the technology to abate it
available? and (3) Do the costs of abatement justify the degree of abatement achieved?
The principal intention of the Handbook of Environmental Engineering series is to
help readers formulate answers to the last two questions.
The traditional approach of applying tried-and-true solutions to specific pollution problems has been a major contributing factor to the success of environmental engineering, and
has accounted in large measure for the establishment of a “methodology of pollution control.” However, realization of the ever-increasing complexity and interrelated nature of
current environmental problems makes it imperative that intelligent planning of pollution
abatement systems be undertaken. Prerequisite to such planning is an understanding of the
performance, potential, and limitations of the various methods of pollution abatement available for environmental engineering. In this series of handbooks, we will review at a tutorial
level a broad spectrum of engineering systems (processes, operations, and methods) currently being utilized, or of potential utility, for pollution abatement. We believe that the
unified interdisciplinary approach in these handbooks is a logical step in the evolution of
environmental engineering.
The treatment of the various engineering systems presented in Physicochemical
Treatment Process shows how an engineering formulation of the subject flows naturally from the fundamental principles and theories of chemistry, physics, and mathematics. This emphasis on fundamental science recognizes that engineering practice
has in recent years become more firmly based on scientific principles rather than its
earlier dependency on empirical accumulation of facts. It is not intended, though, to
neglect empiricism when such data lead quickly to the most economic design; certain
engineering systems are not readily amenable to fundamental scientific analysis, and in
these 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 application, we first present the development of the scientific basis of a particular subject, followed by exposition of the pertinent design concepts and operations, and detailed
explanations of their applications to environmental quality control or improvement.
Throughout this series, methods of practical design calculation are illustrated by numerical

examples. These examples clearly demonstrate how organized, analytical reasoning leads
to the most direct and clear solutions. Wherever possible, pertinent cost data have been
provided.
v


vi

Preface

Our treatment of pollution-abatement engineering is offered in the belief that the
trained engineer should more firmly understand fundamental principles, be more aware
of the similarities and/or differences among many of the engineering systems, and exhibit greater flexibility and originality in the definition and innovative solution of environmental pollution problems. In short, environmental engineers should by conviction
and practice be more readily adaptable to change and progress.
Coverage of the unusually broad field of environmental engineering has demanded
an expertise that could only be provided through multiple authorships. Each author (or
group of authors) was permitted to employ, within reasonable limits, the customary
personal 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 treated
in great detail, and many less important topics had to be merely mentioned or commented on briefly. All of the authors have provided an excellent list of references at the
end of each chapter for the benefit of the interested reader. Because each of the chapters is meant to be self-contained, some mild repetition among the various texts was
unavoidable. In each case, all errors of omission or repetition are the responsibility of
the 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 possible the authors have used the British system along with the metric equivalent or vice
versa. The authors sincerely hope that this doubled system of unit notation will prove
helpful rather than disruptive to the readers.
The goals of the Handbook of Environmental Engineering series are: (1) to cover the
entire range of environmental fields, including air and noise pollution control, solid waste
processing and resource recovery, biological treatment processes, water resources, natural control processes, radioactive waste disposal, thermal pollution control, and physicochemical treatment processes; and (2) to employ a multithematic approach to

environmental pollution control because air, water, land, and energy are all interrelated. The organization of the series is mainly based on the three basic forms in which
pollutants and waste are manifested: gas, solid, and liquid. In addition, noise pollution
control is included in one of the handbooks in the series.
This volume, Physicochemical Treatment Processes, has been designed to serve as a
basic physicochemical treatment text as well as a comprehensive reference book. We
hope and expect it will prove to be of high value to advanced undergraduate or graduate students, to designers of water and wastewater treatment systems, and to research
workers. The editors welcome comments from readers in all these categories. It is our
hope that this book will not only provide information on the physical, chemical, and
mechanical treatment technologies, but will also serve as a basis for advanced study or
specialized investigation of the theory and practice of the individual physicochemical
systems covered.
The editors are pleased to acknowledge the encouragement and support received
from 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


Preface

vii

patiently borne our reviews and numerous queries and comments. We are very grateful
to our respective families for their patience and understanding during some rather trying times.
Lawrence K. Wang
Yung-Tse Hung
Nazih K. Shammas


Contents
Preface ............................................................................................................................ v
Contributors ................................................................................................................. xix

1
Screening and Comminution
Frank J. DeLuise, Lawrence K. Wang, Shoou-Yuh Chang,
and Yung-Tse Hung ....................................................................................... 1
1. Function of Screens and Comminutors ....................................................................................................... 1
2. Types of Screens ........................................................................................................................................... 2
2.1. Coarse Screens .................................................................................................................................... 2
2.2. Fine Screens ........................................................................................................................................ 2
3. Physical Characteristics and Hydraulic Considerations of Screens .......................................................... 3
4. Cleaning Methods for Screens ..................................................................................................................... 5
5. Quality and Disposal for Screens ................................................................................................................ 6
6. Comminutors ................................................................................................................................................ 7
7. Engineering Specifications and Experience ................................................................................................ 8
7.1. Professional Association Specifications ............................................................................................ 8
7.2. Engineering Experience .................................................................................................................... 11
8. Engineering Design .................................................................................................................................... 12
8.1. Summary of Screening Design Considerations ............................................................................... 12
8.2. Summary of Comminution Design Considerations ........................................................................ 14
9. Design Examples ........................................................................................................................................ 15
9.1. Example 1: Bar Screen Design ......................................................................................................... 15
9.2. Example 2: Bar Screen Head Loss ................................................................................................... 16
9.3. Example 3: Plugged Bar Screen Head Loss .................................................................................... 17
9.4. Example 4: Screen System Design .................................................................................................. 17
Nomenclature .................................................................................................................................................... 18
References ......................................................................................................................................................... 18

2

Flow Equalization and Neutralization
Ramesh K. Goel, Joseph R. V. Flora, and J. Paul Chen ............................... 21

1. Introduction ................................................................................................................................................. 21
2. Flow Equalization ....................................................................................................................................... 21
2.1. Flow Equalization Basin Calculations ............................................................................................. 23
2.2. Mixing and Aeration Requirements ................................................................................................. 25
2.3. Mixer Unit ......................................................................................................................................... 26
3. Neutralization ............................................................................................................................................. 28
3.1. pH ....................................................................................................................................................... 28
3.2. Acidity and Alkalinity ...................................................................................................................... 29
3.3. Buffer Capacity ................................................................................................................................. 30
3.4. Hardness ............................................................................................................................................ 31
4. Neutralization Practices ............................................................................................................................. 32
4.1. Neutralization of Acidity .................................................................................................................. 32
4.2. Neutralization of Alkalinity ............................................................................................................. 33
4.3. Common Neutralization Treatments ................................................................................................ 34
5. pH Neutralization Practices ....................................................................................................................... 36
5.1. Passive Neutralization ...................................................................................................................... 36
5.2. In-Plant Neutralization ..................................................................................................................... 36
5.3. Influent pH Neutralization ................................................................................................................ 36
5.4. In-Process Neutralization ................................................................................................................. 37
5.5. Effluent Neutralization ..................................................................................................................... 38
5.6. Chemicals for Neutralization ........................................................................................................... 38

ix


x

Contents
5.7. Encapsulated Phosphate Buffers for In Situ Bioremediation ......................................................... 39
6. Design of a Neutralization System ............................................................................................................ 39

7. Design Examples ........................................................................................................................................ 40
Nomenclature .................................................................................................................................................... 43
References ......................................................................................................................................................... 44

3

Mixing
J. Paul Chen, Frederick B. Higgins, Shoou-Yuh Chang,
and Yung-Tse Hung ..................................................................................... 47
1. Introduction ................................................................................................................................................. 47
2. Basic Concepts ........................................................................................................................................... 48
2.1. Criteria for Mixing ............................................................................................................................ 50
2.2. Mixing Efficiency ............................................................................................................................. 52
2.3. Fluid Shear ........................................................................................................................................ 54
3. Mixing Processes and Equipment .............................................................................................................. 55
3.1. Mixing in Turbulent Fields ............................................................................................................... 55
3.2. Mechanical Mixing Equipment ........................................................................................................ 58
3.3. Impeller Discharge ............................................................................................................................ 69
3.4. Motionless Mixers ............................................................................................................................ 71
3.5. Mixing in Batch and Continuous Flow Systems ............................................................................. 73
3.6. Suspension of Solids ......................................................................................................................... 77
3.7. Static Mixer ....................................................................................................................................... 84
4. Design of Facilities ..................................................................................................................................... 86
4.1. Pipes, Ducts, and Channels .............................................................................................................. 86
4.2. Self-Induced and Baffled Basins ...................................................................................................... 89
4.3. Mechanically Mixed Systems .......................................................................................................... 90
Nomenclature .................................................................................................................................................... 99
References ....................................................................................................................................................... 100

4

Coagulation and Flocculation
Nazih K. Shammas ......................................................................................... 103
1. Introduction ............................................................................................................................................... 103
2. Applications of Coagulation .................................................................................................................... 104
2.1. Water Treatment ............................................................................................................................. 104
2.2. Municipal Wastewater Treatment .................................................................................................. 104
2.3. Industrial Waste Treatment ............................................................................................................ 104
2.4. Combined Sewer Overflow ............................................................................................................ 104
2.5. Factors to be Considered in Process Selection .............................................................................. 105
3. Properties of Colloidal Systems .............................................................................................................. 105
3.1. Electrokinetic Properties ................................................................................................................ 105
3.2. Hydration ......................................................................................................................................... 106
3.3. Brownian Movement ...................................................................................................................... 106
3.4. Tyndall Effect .................................................................................................................................. 106
3.5. Filterability ...................................................................................................................................... 107
4. Colloidal Structure and Stability ............................................................................................................. 107
5. Destabilization of Colloids ...................................................................................................................... 109
5.1. Double-Layer Compression ............................................................................................................ 110
5.2. Adsorption and Charge Neutralization .......................................................................................... 110
5.3. Entrapment of Particles in Precipitate ........................................................................................... 111
5.4. Adsorption and Bridging between Particles .................................................................................. 111
6. Influencing Factors ................................................................................................................................... 112
6.1. Colloid Concentration ..................................................................................................................... 112
6.2. Coagulant Dosage ........................................................................................................................... 112
6.3. Zeta Potential .................................................................................................................................. 112
6.4. Affinity of Colloids for Water ........................................................................................................ 113
6.5. pH Value .......................................................................................................................................... 113
6.6. Anions in Solution .......................................................................................................................... 114

Contents

7.

8.

9.
10.
11.
12.
13.

5

xi
6.7. Cations in Solution .......................................................................................................................... 114
6.8. Temperature .................................................................................................................................... 114
Coagulants ................................................................................................................................................ 114
7.1. Aluminum Salts ............................................................................................................................... 115
7.2. Iron Salts .......................................................................................................................................... 116
7.3. Sodium Aluminate .......................................................................................................................... 116
7.4. Polymeric Inorganic Salts ............................................................................................................... 117
7.5. Organic Polymers ............................................................................................................................ 117
7.6. Coagulation Aids ............................................................................................................................. 118
Coagulation Control ................................................................................................................................. 118
8.1. Jar Test ............................................................................................................................................. 119
8.2. Zetameter ......................................................................................................................................... 120
8.3. Streaming Current Detector ............................................................................................................ 121
Chemical Feeding ..................................................................................................................................... 121

Mixing ....................................................................................................................................................... 122
Rapid Mix ................................................................................................................................................. 124
Flocculation .............................................................................................................................................. 125
Design Examples ...................................................................................................................................... 127
Nomenclature ............................................................................................................................................ 137
References ................................................................................................................................................. 138

Chemical Precipitation
Lawrence K. Wang, David A. Vaccari, Yan Li, and Nazih K. Shammas ... 141
1. Introduction ............................................................................................................................................... 141
2. Process Description .................................................................................................................................. 142
3. Process Types ........................................................................................................................................... 142
3.1. Hydroxide Precipitation .................................................................................................................. 142
3.2. Sulfide Precipitation ....................................................................................................................... 144
3.3. Cyanide Precipitation ...................................................................................................................... 145
3.4. Carbonate Precipitation .................................................................................................................. 145
3.5. Coprecipitation ................................................................................................................................ 146
3.6. Technology Status ........................................................................................................................... 146
4. Chemical Precipitation Principles ........................................................................................................... 146
4.1. Reaction Equilibria ......................................................................................................................... 146
4.2. Solubility Equilibria ........................................................................................................................ 147
4.3. Ionic Strength and Activity ............................................................................................................ 148
4.4. Ionic Strength Example .................................................................................................................. 149
4.5. Common Ion Effect ......................................................................................................................... 150
4.6. Common Ion Effect Example ......................................................................................................... 150
4.7. Soluble Complex Formation ........................................................................................................... 151
4.8. pH Effect ......................................................................................................................................... 152
4.9. Solubility Diagrams ........................................................................................................................ 152
5. Chemical Precipitation Kinetics .............................................................................................................. 152
5.1. Nucleation ....................................................................................................................................... 153

5.2. Crystal Growth ................................................................................................................................ 153
5.3. Aging ............................................................................................................................................... 154
5.4. Adsorption and Coprecipitation ..................................................................................................... 154
6. Design Considerations ............................................................................................................................. 155
6.1. General ............................................................................................................................................ 155
6.2. Chemical Handling ......................................................................................................................... 155
6.3. Mixing, Flocculation, and Contact Equipment ............................................................................. 156
6.4. Solids Separation ............................................................................................................................ 157
6.5. Design Criteria Summary ............................................................................................................... 157
7. Process Applications ................................................................................................................................ 158
7.1. Hydroxide Precipitation .................................................................................................................. 158
7.2. Carbonate Precipitation .................................................................................................................. 159
7.3. Sulfide Precipitation ....................................................................................................................... 160
7.4. Cyanide Precipitation ...................................................................................................................... 161
7.5. Magnesium Oxide Precipitation ..................................................................................................... 162


xii

Contents
7.6. Chemical Oxidation–Reduction Precipitation ............................................................................... 162
7.7. Lime/Soda-Ash Softening .............................................................................................................. 162
7.8. Phosphorus Precipitation ................................................................................................................ 162
7.9. Other Chemical Precipitation Processes ........................................................................................ 163
8. Process Evaluation ................................................................................................................................... 163
8.1. Advantages and Limitations ........................................................................................................... 163
8.2. Reliability ........................................................................................................................................ 164
8.3. Chemicals Required ........................................................................................................................ 165
8.4. Residuals Generated ....................................................................................................................... 165
8.5. Process Performance ....................................................................................................................... 165

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

6

Recarbonation and Softening
Lawrence K. Wang, Jy S. Wu, Nazih K. Shammas,
and David A. Vaccari ................................................................................. 199
1.
2.
3.
4.
5.
6.

Introduction ............................................................................................................................................... 199
Process Description .................................................................................................................................. 199
Softening and Recarbonation Process Chemistry ................................................................................... 201
Lime/Soda Ash Softening Process .......................................................................................................... 203
Water Stabilization ................................................................................................................................... 205
Other Related Process Applications ........................................................................................................ 206
6.1. Chemical Coagulation Using Magnesium Carbonate as a Coagulant .......................................... 206
6.2. Recovery of Magnesium as Magnesium Carbonate ...................................................................... 207
6.3. Recovery of Calcium Carbonate as Lime ...................................................................................... 207
6.4. Recarbonation of Chemically Treated Wastewaters ..................................................................... 208
7. Process Design .......................................................................................................................................... 208
7.1. Sources of Carbon Dioxide ............................................................................................................ 208
7.2. Distribution Systems ....................................................................................................................... 210

7.3. Carbon Dioxide Quantities ............................................................................................................. 212
7.4. Step-by-Step Design Approach ...................................................................................................... 212
8. Design and Application Examples .......................................................................................................... 215
Nomenclature .................................................................................................................................................. 226
Acknowledgments .......................................................................................................................................... 227
References ....................................................................................................................................................... 227

7

Chemical Oxidation
Nazih K. Shammas, John Y. Yang, Pao-Chiang Yuan,
and Yung-Tse Hung ................................................................................... 229
1. Introduction ............................................................................................................................................... 229
1.1. Dissolved Oxygen and Concept of Oxidation ............................................................................... 230
1.2. The Definition of Oxidation State .................................................................................................. 231
2. Theory and Principles .............................................................................................................................. 233
2.1. Stoichiometry of Oxidation–Reduction Processes ........................................................................ 234
2.2. Thermodynamics of Chemical Oxidation ...................................................................................... 236
2.3. Kinetic Aspects of Chemical Oxidation ........................................................................................ 240
3. Oxygenated Reagent Systems .................................................................................................................. 243
3.1. Aeration in Water Purification and Waste Treatment ................................................................... 243
3.2. Hydrogen Peroxide and Peroxygen Reagents ............................................................................... 246
3.3. High-Temperature Wet Oxidation ................................................................................................. 248
4. Transition-Metal Ion Oxidation Systems ................................................................................................ 256
4.1. Chromic Acid Oxidation ................................................................................................................ 256
4.2. Permanganate Oxidation ................................................................................................................. 258
5. Recent Developments in Chemical Oxidation ........................................................................................ 261
5.1. Ozone (O3) Processes ..................................................................................................................... 261
5.2. Ultraviolet (UV) Processes ............................................................................................................. 262
5.3. Wet Oxidation ................................................................................................................................. 263

Contents

xiii

5.4. Supercritical Water Oxidation ........................................................................................................ 264
5.5. Biological Oxidation ....................................................................................................................... 264
6. Examples ................................................................................................................................................... 264
Nomenclature .................................................................................................................................................. 268
References ....................................................................................................................................................... 269

8

Halogenation and Disinfection
Lawrence K. Wang, Pao-Chiang Yuan, and Yung-Tse Hung .................... 271
1. Introduction ............................................................................................................................................... 271
2. Chemistry of Halogenation ...................................................................................................................... 274
2.1. Chlorine Hydrolysis ........................................................................................................................ 274
2.2. Chlorine Dissociation ..................................................................................................................... 275
2.3. Chlorine Reactions with Nitrogenous Matter ................................................................................ 275
2.4. Chlorine Reactions with Other Inorganics .................................................................................... 279
2.5. Chlorine Dioxide (ClO2) Applications .......................................................................................... 281
2.6. Chlorine Dioxide Generation ......................................................................................................... 281
2.7. Chlorine Dioxide Reaction with Nitrogenous Matter ................................................................... 282
2.8. Chlorine Dioxide Reactions with Phenolic Compounds and Other Substances .......................... 283
2.9. Bromine Hydrolysis ........................................................................................................................ 283
2.10. Bromine Dissociation ..................................................................................................................... 283
2.11. Bromine Reactions with Nitrogenous Matter ................................................................................ 284
2.12. Iodine Hydrolysis ............................................................................................................................ 284

2.13. Iodine Dissociation ......................................................................................................................... 284
2.14. Iodine Reactions with Nitrogenous Matter .................................................................................... 285
3. Disinfection with Halogens ...................................................................................................................... 285
3.1. Modes and Rate of Killing in Disinfection Process ...................................................................... 285
3.2. Disinfection Conditions .................................................................................................................. 286
3.3. Disinfection Control with Biological Tests ................................................................................... 287
3.4. Disinfectant Concentration ............................................................................................................. 288
4. Chlorine and Chlorination ........................................................................................................................ 288
4.1. Chlorine Compounds and Elemental Chlorine .............................................................................. 289
4.2. Chlorine Feeders ............................................................................................................................. 290
4.3. Chlorine Handling Equipment ........................................................................................................ 291
4.4. Measurement of Chlorine Residuals .............................................................................................. 291
4.5. Chlorine Dosages ............................................................................................................................ 292
4.6. Chlorination By-Products ............................................................................................................... 293
5. Chlorine Dioxide Disinfection ................................................................................................................. 294
6. Bromine and Bromination ........................................................................................................................ 294
7. Iodine and Iodination ............................................................................................................................... 295
8. Ozone and Ozonation ............................................................................................................................... 295
9. Cost Data ................................................................................................................................................... 295
10. Recent Developments in Halogenation Technology .............................................................................. 296
10.1. Recent Environmental Concerns and Regulations ........................................................................ 296
10.2. Chlorine Dioxide ............................................................................................................................. 297
10.3. Chloramines .................................................................................................................................... 298
10.4. Coagulant ......................................................................................................................................... 298
10.5. Ozone ............................................................................................................................................... 299
10.6. Organic Disinfectants ..................................................................................................................... 299
10.7. Ultraviolet (UV) .............................................................................................................................. 300
11. Disinfection System Design ..................................................................................................................... 300
11.1. Design Considerations Summary ................................................................................................... 300
11.2. Wastewater Disinfection ................................................................................................................. 301

11.3. Potable Water Disinfection ............................................................................................................ 303
12. Design and Application Examples .......................................................................................................... 305
12.1. Example 1 (Wastewater Disinfection) ........................................................................................... 305
12.2. Example 2 (Potable Water Disinfection) ....................................................................................... 308
12.3. Example 3 (Glossary of Halogenation, Chlorination, Oxidation, and Disinfection) .................. 308
Nomenclature .................................................................................................................................................. 311
References ....................................................................................................................................................... 311


xiv

Contents

9

Ozonation
Nazih K. Shammas and Lawrence K. Wang................................................. 315
1. Introduction ............................................................................................................................................... 315
1.1. General ............................................................................................................................................ 315
1.2. Alternative Disinfectants ................................................................................................................ 316
2. Properties and Chemistry of Ozone ......................................................................................................... 316
2.1. General ............................................................................................................................................ 316
2.2. Physical Properties .......................................................................................................................... 316
2.3. Chemical Properties ........................................................................................................................ 317
2.4. Advantages and Disadvantages ...................................................................................................... 319
3. Applications of Ozone .............................................................................................................................. 319
3.1. Disinfection Against Pathogens ..................................................................................................... 319
3.2. Zebra Mussel Abatement ................................................................................................................ 320
3.3. Iron and Manganese Removal ........................................................................................................ 320
3.4. Color Removal ................................................................................................................................ 320

3.5. Control of Taste and Odor .............................................................................................................. 321
3.6. Elimination of Organic Chemicals ................................................................................................. 321
3.7. Control of Algae .............................................................................................................................. 321
3.8. Aid in Coagulation and Destabilization of Turbidity ................................................................... 321
4. Process and Design Considerations ......................................................................................................... 321
4.1. Oxygen and Ozone .......................................................................................................................... 321
4.2. Disinfection of Water by Ozone .................................................................................................... 322
4.3. Disinfection of Wastewater by Ozone ........................................................................................... 324
4.4. Disinfection By-Products ............................................................................................................... 333
4.5. Oxygenation by Ozone ................................................................................................................... 334
4.6. Advanced Oxidation Processes ...................................................................................................... 337
5. Ozonation System ..................................................................................................................................... 340
5.1. Air Preparation ................................................................................................................................ 341
5.2. Electrical Power Supply ................................................................................................................. 344
5.3. Ozone Generation ........................................................................................................................... 344
5.4. Ozone Contacting ............................................................................................................................ 345
5.5. Destruction of Ozone Contactor Exhaust Gas ............................................................................... 348
5.6. Monitors and Controllers ................................................................................................................ 349
6. Costs of Ozonation Systems .................................................................................................................... 349
6.1. Equipment Costs ............................................................................................................................. 349
6.2. Installation Costs ............................................................................................................................. 352
6.3. Housing Costs ................................................................................................................................. 353
6.4. Operating and Maintenance Costs ................................................................................................. 353
7. Safety ........................................................................................................................................................ 353
Nomenclature .................................................................................................................................................. 354
References ....................................................................................................................................................... 355

10

Electrolysis

J. Paul Chen, Shoou-Yuh Chang, and Yung-Tse Hung ............................. 359
1. Introduction ............................................................................................................................................... 359
2. Mechanisms of Electrolysis ..................................................................................................................... 362
3. Organic and Suspended Solids Removal ................................................................................................ 363
3.1. Organic and Suspended Solids Removal by Regular Electrolysis ............................................... 363
3.2. Organic and Suspended Solids Removal by Electrocoagulation ................................................. 364
4. Disinfection ............................................................................................................................................... 366
5. Phosphate Removal .................................................................................................................................. 368
6. Ammonium Removal ............................................................................................................................... 369
7. Cyanide Destruction ................................................................................................................................. 369
8. Metal Removal .......................................................................................................................................... 370
9. Remediation of Nitroaromatic Explosives-Contaminated Groundwater ............................................... 372
10. Electrolysis-Stimulated Biological Treatment ........................................................................................ 374
10.1. Nitrogen Removal ........................................................................................................................... 375
10.2. Electrolytic Oxygen Generation ..................................................................................................... 374
References ....................................................................................................................................................... 376


Contents
11

xv

Sedimentation
Nazih K. Shammas, Inder Jit Kumar, Shoou-Yuh Chang,
and Yung-Tse Hung ................................................................................... 379
1. Introduction ............................................................................................................................................... 379
1.1. Historical ......................................................................................................................................... 379
1.2. Definition and Objective of Sedimentation ................................................................................... 380
1.3. Significance of Sedimentation in Water and Wastewater Treatment .......................................... 380

2. Types of Clarification .............................................................................................................................. 380
3. Theory of Sedimentation .......................................................................................................................... 381
3.1. Class 1 Clarification ....................................................................................................................... 382
3.2. Class 2 Clarification ....................................................................................................................... 386
3.3. Zone Settling ................................................................................................................................... 387
3.4. Compression Settling ...................................................................................................................... 390
4. Sedimentation Tanks in Water Treatment ............................................................................................... 390
4.1. General Consideration .................................................................................................................... 390
4.2. Inlet and Outlet Control .................................................................................................................. 391
4.3. Tank Geometry ................................................................................................................................ 392
4.4. Short Circuiting ............................................................................................................................... 392
4.5. Detention Time ................................................................................................................................ 392
4.6. Tank Design .................................................................................................................................... 393
5. Sedimentation Tanks in Wastewater Treatment ..................................................................................... 394
5.1. General Consideration and Basis of Design .................................................................................. 394
5.2. Regulatory Standards ...................................................................................................................... 395
5.3. Tank Types ...................................................................................................................................... 395
6. Grit Chamber ............................................................................................................................................ 398
6.1. General ............................................................................................................................................ 398
6.2. Types of Grit Chambers ................................................................................................................. 399
6.3. Velocity Control Devices ............................................................................................................... 400
6.4. Design of Grit Chamber ................................................................................................................. 402
7. Gravity Thickening in Sludge Treatment ................................................................................................ 403
7.1. Design of Sludge Thickeners ......................................................................................................... 405
8. Recent Developments ............................................................................................................................... 406
8.1. Theory of Shallow Depth Settling ................................................................................................. 407
8.2. Tube Settlers .................................................................................................................................... 409
8.3. Lamella Separator ........................................................................................................................... 410
8.4. Other Improvements ....................................................................................................................... 411
9. Sedimentation in Air Streams .................................................................................................................. 412

9.1. General ............................................................................................................................................ 412
9.2. Gravity Settlers ............................................................................................................................... 413
10. Costs .......................................................................................................................................................... 414
10.1. General ............................................................................................................................................ 414
10.2. Sedimentation Tanks ....................................................................................................................... 414
10.3. Gravity Thickeners .......................................................................................................................... 416
10.4. Tube Settlers .................................................................................................................................... 416
11. Design Examples ...................................................................................................................................... 418
Nomenclature .................................................................................................................................................. 426
References ....................................................................................................................................................... 427
Appendix: US Yearly Average Cost Index for Utilities ............................................................................... 429

12

Dissolved Air Flotation
Lawrence K. Wang, Edward M. Fahey, and Zucheng Wu ......................... 431
1. Introduction ............................................................................................................................................... 431
1.1. Adsorptive Bubble Separation Processes ...................................................................................... 431
1.2. Content and Objectives ................................................................................................................... 434
2. Historical Development of Clarification Processes ................................................................................ 435
2.1. Conventional Sedimentation Clarifiers .......................................................................................... 435
2.2. Innovative Flotation Clarifiers ....................................................................................................... 437
3. Dissolved Air Flotation Process .............................................................................................................. 440
3.1. Process Description ......................................................................................................................... 440


xvi

Contents
3.2. Process Configurations ................................................................................................................... 441

3.3. Factors Affecting Dissolved Air Flotation .................................................................................... 443
4. Dissolved Air Flotation Theory ............................................................................................................... 444
4.1. Gas-to-Solids Ratio of Full Flow Pressurization System ............................................................. 444
4.2. Gas-to-Solids Ratio of Partial Flow Pressurization System ......................................................... 446
4.3. Gas-to-Solids Ratio of Recycle Flow Pressurization .................................................................... 447
4.4. Air Solubility in Water at 1 Atm .................................................................................................... 448
4.5. Pressure Calculations ...................................................................................................................... 449
4.6. Hydraulic Loading Rate .................................................................................................................. 449
4.7. Solids Loading Rate ........................................................................................................................ 451
5. Design, Operation, and Performance ....................................................................................................... 453
5.1. Operational Parameters ................................................................................................................... 455
5.2. Performance and Reliability ........................................................................................................... 455
6. Chemical Treatment ................................................................................................................................. 455
7. Sampling, Tests, and Monitoring ............................................................................................................ 457
7.1. Sampling .......................................................................................................................................... 457
7.2. Laboratory and Field Tests ............................................................................................................. 457
8. Procedures and Apparatus for Chemical Coagulation Experiments ...................................................... 457
9. Procedures and Apparatus for Laboratory Dissolved Air Flotation Experiments ................................ 459
9.1. Full Flow Pressurization System .................................................................................................... 459
9.2. Partial Flow Pressurization System ............................................................................................... 460
9.3. Recycle Flow Pressurization System ............................................................................................. 461
10. Normal Operating Procedures ................................................................................................................. 462
10.1. Physical Control .............................................................................................................................. 462
10.2. Startup .............................................................................................................................................. 463
10.3. Routine Operations ......................................................................................................................... 464
10.4. Shutdown ......................................................................................................................................... 464
11. Emergency Operating Procedures ........................................................................................................... 464
11.1. Loss of Power .................................................................................................................................. 464
11.2. Loss of Other Treatment Units ....................................................................................................... 465
12. Operation and Maintenance ..................................................................................................................... 465

12.1. Troubleshooting .............................................................................................................................. 465
12.2. Labor Requirements ........................................................................................................................ 465
12.3. Construction and O&M Costs ........................................................................................................ 465
12.4. Energy Consumption ...................................................................................................................... 465
12.5. Maintenance Considerations .......................................................................................................... 466
12.6. Environmental Impact and Safety Considerations ........................................................................ 468
13. Recent Developments in Dissolved Air Flotation Technology ............................................................. 468
13.1. General Recent Developments ....................................................................................................... 468
13.2. Physicochemical SBR-DAF Process for Industrial and Municipal Applications ....................... 470
13.3. Adsorption Flotation Processes ...................................................................................................... 471
13.4. Dissolved Gas Flotation .................................................................................................................. 471
13.5. Combined Sedimentation and Flotation ........................................................................................ 472
14. Application and Design Examples .......................................................................................................... 472
Nomenclature .................................................................................................................................................. 491
Acknowledgments .......................................................................................................................................... 492
References ....................................................................................................................................................... 493

13

Gravity Filtration
J. Paul Chen, Shoou-Yuh Chang, Jerry Y. C. Huang,
E. Robert Baumann, and Yung-Tse Hung ............................................... 501
1. Introduction ............................................................................................................................................... 501
2. Physical Nature of Gravity Filtration ...................................................................................................... 502
2.1. Transport Mechanism ..................................................................................................................... 502
2.2. Attachment Mechanisms ................................................................................................................ 504
2.3. Detachment Mechanisms ................................................................................................................ 504
3. Mathematical Models ............................................................................................................................... 504
3.1. Idealized Models ............................................................................................................................. 505
3.2. Empirical Models ............................................................................................................................ 509

Contents

xvii

4. Design Considerations of Gravity Filters ................................................................................................ 510
4.1. Water Variables ............................................................................................................................... 510
4.2. Filter Physical Variables ................................................................................................................. 511
4.3. Filter Operating Variables .............................................................................................................. 517
5. Applications .............................................................................................................................................. 522
5.1. Potable Water Filtration ................................................................................................................. 522
5.2. Reclamation of Wasterwater .......................................................................................................... 522
6. Design Examples ...................................................................................................................................... 527
Nomenclature .................................................................................................................................................. 539
References ....................................................................................................................................................... 540

14

Polymeric Adsorption and Regenerant Distillation
Lawrence K. Wang, Chein-Chi Chang, and Nazih K. Shammas ............... 545
1. Introduction ............................................................................................................................................... 545
2. Polymeric Adsorption Process Description ............................................................................................ 547
2.1. Process System ................................................................................................................................ 547
2.2. Process Steps ................................................................................................................................... 547
2.3. Regeneration Issues ........................................................................................................................ 547
3. Polymeric Adsorption Applications and Evaluation .............................................................................. 548
3.1. Applications .................................................................................................................................... 548
3.2. Process Evaluation .......................................................................................................................... 550
4. Polymeric Adsorbents .............................................................................................................................. 550

4.1. Chemical Structure .......................................................................................................................... 550
4.2. Physical Properties .......................................................................................................................... 552
4.3. Adsorption Properties ..................................................................................................................... 552
5. Design Considerations ............................................................................................................................. 552
5.1. Adsorption Bed, Adsorbents, and Regenerants ............................................................................. 552
5.2. Generated Residuals ....................................................................................................................... 555
6. Distillation ................................................................................................................................................ 557
6.1. Distillation Process Description ..................................................................................................... 557
6.2. Distillation Types and Modifications ............................................................................................ 557
6.3. Distillation Process Evaluation ...................................................................................................... 560
7. Design and Application Examples .......................................................................................................... 560
Acknowledgments .......................................................................................................................................... 570
References ....................................................................................................................................................... 571

15

Granular Activated Carbon Adsorption
Yung-Tse Hung, Howard H. Lo, Lawrence K. Wang,
Jerry R. Taricska, and Kathleen Hung Li ................................................ 573
1.
2.
3.
4.

Introduction ............................................................................................................................................... 573
Process Flow Diagrams for GAC Process ............................................................................................... 576
Adsorption Column Models ..................................................................................................................... 577
Design of Granular Activated Carbon Columns ..................................................................................... 585
4.1. Design of GAC Columns ................................................................................................................ 585
4.2. Pilot Plant and Laboratory Column Tests ...................................................................................... 590

5. Regeneration ............................................................................................................................................. 591
6. Factors Affecting GAC Adsorption ......................................................................................................... 592
6.1. Adsorbent Characteristics ............................................................................................................... 592
6.2. Adsorbate Characteristics ............................................................................................................... 592
7. Performance and Case Studies ................................................................................................................. 593
8. Economics of Granular Activated Carbon System ................................................................................. 595
9. Design Examples ...................................................................................................................................... 602
10. Historical and Recent Developments in Granular Activated Carbon Adsorption ................................ 623
10.1. Adsorption Technology Milestones ............................................................................................... 623
10.2. Downflow Conventional Biological GAC Systems ...................................................................... 625
10.3. Upflow Fluidized Bed Biological GAC System ........................................................................... 627
Nomenclature .................................................................................................................................................. 628
References ....................................................................................................................................................... 630


xviii
16

Contents
Physicochemical Treatment Processes for Water Reuse
Saravanamuthu Vigneswaran, Huu Hao Ngo,
Durgananda Singh Chaudhary, and Yung-Tse Hung ............................ 635
1. Introduction ............................................................................................................................................... 635
2. Conventional Physicochemical Treatment Processes ............................................................................. 636
2.1. Principle ........................................................................................................................................... 636
2.2. Application of the Physicochemical Processes in Wastewater Treatment and Reuse ................ 651
3. Membrane Processes ................................................................................................................................ 658
3.1. Principle ........................................................................................................................................... 658
3.2. Application of Membrane Processes ............................................................................................. 661
References ....................................................................................................................................................... 675

17

Introduction to Sludge Treatment
Duu-Jong Lee, Joo-Hwa Tay, Yung-Tse Hung, and Pin Jing He ............. 677
1. The Origin of Sludge ................................................................................................................................ 677
2. Conditioning Processes ............................................................................................................................ 678
2.1. Coagulation ..................................................................................................................................... 678
2.2. Flocculation ..................................................................................................................................... 681
2.3. Conditioner Choice ......................................................................................................................... 681
2.4. Optimal Dose ................................................................................................................................... 682
3. Dewatering Processes ............................................................................................................................... 684
3.1. Dewatering Processes ..................................................................................................................... 684
3.2. Sludge Thickening .......................................................................................................................... 685
3.3. Sludge Dewatering .......................................................................................................................... 687
4. Stabilization Processes ............................................................................................................................. 691
4.1. Hydrolysis Processes ...................................................................................................................... 691
4.2. Digestion Processes ........................................................................................................................ 695
5. Thermal Processes .................................................................................................................................... 699
5.1. Sludge Incineration ......................................................................................................................... 699
5.2. Sludge Drying ................................................................................................................................. 701
5.3. Other Thermal Processes ................................................................................................................ 702
References ....................................................................................................................................................... 703

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


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

C HEIN -C HI C HANG , P h D , 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 Technology
Sydney (UTS), New South Wales, Australia
J. PAUL CHEN, PhD • Department of Chemical and Biomolecular Engineering, National
University 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, MA
JOSEPH R. V. FLORA, PhD • Department of Civil & Environmental Engineering, University
of South Carolina, Columbia, SC
RAMESH K. GOEL, PhD • Department of Civil and Environmental Engineering, University
of Wisconsin, Madison, WI
PIN JING HE, PhD • School of Environmental Science and Engineering, Tongji University,
Shanghai, China
FREDERICK B. HIGGINS, PhD • Civil and Environmental Engineering Department, Temple
University, Philadelphia, PA
YUNG-TSE HUNG, PhD, PE, DEE • Department of Civil and Environmental Engineering,
Cleveland State University, Cleveland, OH
JERRY Y. C. HUANG, PhD • Department of Civil Engineering, University of Wisconsin–
Milwaukee, Milwaukee, WI
INDER JIT KUMAR, PhD • Eustance & Horowitz, P.C., Consulting Engineers, Circleville, NY
DUU-JONG LEE, PhD • Department of Chemical Engineering, National Taiwan University,
Taipei, Taiwan
KATHLEEN HUNG LI, MS • NEC Business Network Solutions, Irving, TX
YAN LI, PE, MS • Department of Environmental Management, State of Rhode Island,
Providence, RI
HOWARD LO, PhD • Department of Biological, Geological and Environmental Sciences,

Cleveland State University, Cleveland, OH
HUU HAO NGO, PhD • Faculty of Engineering, University of Technology Sydney (UTS),
New South Wales, Australia
NAZIH K. SHAMMAS, PhD • Graduate Environmental Engineering Program, Lenox Institute
of Water Technology, Lenox, MA
JERRY R. TARICSKA, PhD, PE • Hole Montes Inc., Naples, FL

xix


xx

Contributors

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 of
Technology Sydney (UTS), New South Wales, Australia
LAWRENCE K. WANG, PhD, PE, DEE • Zorex Corporation, Newtonville, NY; Lenox Institute
of Water Technology, Lenox, MA; and Krofta Engineering Corporation, Lenox, MA
JY S. WU, PhD • Department of Civil Engineering, University of North Carolina at Charlotte, Charlotte, NC
ZUCHENG WU, PhD • Department of Environmental Science and Engineering, Zhejiang
University, Hangzhou, People’s Republic of China
JOHN Y. YANG, PhD • Niagara Technology Inc., Williamsville, NY
PAO-CHIANG YUAN, PhD • Technology Department, Jackson State University, Jackson, MS


1

Screening 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 necessary to remove or reduce early in the treatment process large suspended solid material
that might interfere with operations or damage equipment. Removal of solids may be
accomplished through the use of various size screens placed in the flow channel. Any
material removed may then be ground to a smaller size and returned to the process
stream or disposed of in an appropriate manner such as burying or incineration. An
alternative to actual removal of the solids by screening is to reduce the size of the solids
by grinding them while still in the waste stream; this grinding process is called comminution (1–8). Coarse screens (bar racks) and comminutors are usually located at the
very 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 or
comminutors installed in parallel in the event of a breakdown or to provide for overhaul
of a unit. With this arrangement, flow is primarily through the comminutor and diverted
to the coarse (bar) screens only when necessary to shut down the comminutor. Fine
screens are usually placed after the coarse (bar) screens.

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

1


2

Frank Deluise et al.

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

2. TYPES OF SCREENS
2.1. Coarse Screens
Screens may be classified as coarse or fine. Coarse screens are usually called bar
screens or racks and are used where the wastewater contains large quantities of coarse
solids that might disrupt plant operations. These bar screens consist of parallel bars
spaced anywhere from 1.27 cm (1/2 in.) to 10.16 cm (4 in.) apart with no cross-members
other than those required for support. The size of the spacing depends on the type of
waste being treated (size and quantity of solids) and the type of equipment being protected downstream in the plant. These screens are placed either vertically or at an
angle in the flow channel. Installing screens at an angle allows easier cleaning (particularly if by hand) and more screen area per channel depth, but obviously requires
more space.
2.2. Fine Screens
Fine screens have openings of less than 0.25 in. and are used to remove solids
smaller than those retained on bar racks. They are used primarily in water or wastewater
containing little or no coarse solids. In many instances, fine screens are used for the recovery 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 from
0.005 to 0.126 in.) rather than bars to intercept the solids. A screen cloth covers discs or
drums, which rotate through the wastewater. The disc-type screen (Fig. 2) is a vertical
hoop with a screen cloth covering the area within the hoop, and mounted on a horizontal shaft that is positioned slightly above the surface of the water. Water flows through

the screen parallel to the horizontal shaft and the solids are retained on the screen, which
carries them out of the water as it rotates. Solids may then be removed from the upper
part of the screen by water sprays or mechanical brushing.
The drum-type screen (Fig. 3) consists of a cylinder covered by a screen cloth with
the drum rotating on a horizontal axis, slightly less than half submerged. Wastewater
enters 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 the
drum rotates. Once out of the water, the solids may be removed by backwater sprays,
forcing the solids off the screen into collecting troughs.


Screening and Comminution

3

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

3. PHYSICAL CHARACTERISTICS AND HYDRAULIC
CONSIDERATIONS OF SCREENS
The physical characteristics of bar racks and screens depend on the use for which
the unit is intended. Coarse bar racks, sometimes called trash racks, with 7.62 or
10.16 cm (3 or 4 in.) spacing are used to intercept unusually large solids and therefore must be of rugged construction to withstand possible large impacts. Bar screens
with 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-treatment
plant, spacings smaller than 2.54 cm (1 in.) are usually not necessary because today’s
sewage sludge pumps can handle solids passing through the screen. Typical bar
screens are shown in Fig. 4.

Fig. 3. Revolving drum screen.

4

Frank Deluise et al.

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

The screen bars are usually rectangular in cross-section and their size depends on the
size (width and depth) of the screen channel as well as the conditions under which
the screen will be operating. The longer the unsupported length of the bar, the larger is
the required cross-section. Bars up to 1.83 m (6 ft) in length are usually no smaller than
0.635 × 5.08 cm (1/4 × 2 in.), while bars up to 3.66 m (12 ft) long might be
0.952 × 6.35 cm (3/8 × 2.5 in.). Longer bars or bars used for operating conditions causing unusual stress might be as large as 1.59 × 7.62 cm (5/8 × 3 in.). The bars must be
designed to withstand bending as well as impact stresses due to the accumulation of
solids on the screen.
Many screens, particularly those that are hand-cleaned, are installed with bars at an
angle between 60º and 90º with the horizontal. With the bars placed at an angle, the
screenings will tend to accumulate near the top of the screen. In addition, the velocity
through the screen will be low enough to prevent objects from being forced through the
screen. Optimum horizontal velocity through the bars is approx 0.610 m/s (2 ft/s). If
velocities get too low, sedimentation will take place in the screen channel. In the design
of the screen channel, it is desirable to have the flow evenly distributed across the
screen by having several feet of straight channel preceding the screen. Flow entering at
an angle to the screen would tend to create uneven distribution of solids across the
screen and prevent the proper operation of the equipment.
The required size of the screen channel depends on the volume flow rate and the free
space available between the bars. If a net area ratio is defined as the free area between
bars divided by the total area occupied by the screen, then a table such as Table 1 may
be 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 as
practical in order to obtain the highest net area ratio possible. Once the volume flow
rates are known and the net area ratio is determined, the screen channel size may be
determined. The maximum volume flow rate in cubic meters per second divided by the
optimum velocity of 0.610 m/s will yield the net area required. This net area divided by


Screening and Comminution

5

Table 1
Net Area Ratios for Bar Size and Openings
Bar size
cm
0.635
0.635
0.635
0.952
0.952
0.952
1.270
1.270
1.270

Opening
in.

cm

1

in.
1

1.27
2.54
3.81
1.27
2.54
3.81
1.27
2.54
3.81

⁄4

1

⁄4

1

⁄4

3

⁄8

3

⁄8

3

⁄8

1

⁄2

1

⁄2

1

⁄2

⁄2

1
11⁄ 2
1

⁄2

1
11⁄ 2
1

⁄2

1
11⁄ 2

Net area ratio
0.667
0.800
0.856
0.572
0.728
0.800
0.500
0.667
0.750

the net area ratio selected will give the total wet area required for the channel. With this
known area, the width and depth of the channel may be determined. Usually the maximum width or depth of the channel is limited by considerations other than the actual
screening process. Too wide a screen could present problems in cleaning, and therefore
the maximum practical width for a channel is about 4.27 m (14 ft); the minimum width is
about 0.610 m (2 ft). The depth of liquid in the channel is usually kept as shallow as
possible so that the head loss through the plant will be a minimum. The wet area divided
by 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 flow
through the bar screen is
h=

V2 2 − V12
2g

(1)

where h = head loss, m (ft), V2 = velocity through bar screen, m/s (ft/s), V1 = velocity
ahead 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 discharge coefficient, CD, to account for deviation from theoretical conditions. Values of CD
should be determined experimentally, but a typical average value is 0.7. The equation
then becomes
h=

(

V2 2 − V12
CD 2 g

)

h = 0.0728 V2 2 − V12 with SI units

(

h = 0.0222 V2

2

− V12

) with English units

(2)

(2a)
(2b)

4. CLEANING METHODS FOR SCREENS
Bar screens or racks may be cleaned by hand or by machine. Hand-cleaning limits
the 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 between
the bars of the rack. The rake is pulled up toward the top of the screen carrying the


6

Frank Deluise et al.

screenings with it. At the top of the screen, the screenings are deposited on a grid or
perforated plate for drainage and then removed for shredding and return to the channel
or for incineration or burial. Hand-cleaning requires a great deal of manual labor and is
an unpleasant job. Because hand-cleaning is not continuous, plant operations may be
materially affected by undue plugging of the screens before cleaning as well as by large
surges of flow when the screens are finally cleaned. Plugging of the screens could cause
troublesome deposits in the lines leading to the bar screens, and surges after cleaning
could 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 a
hand-cleaned screen, the improvement in plant efficiency, particularly in large installations, usually justifies the higher cost. The ability to operate the cleaning mechanism on
an automatically controlled schedule avoids the flooding and surging through the plant
associated with plugging and unplugging of the screens. After a short while, a preset automatic 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 to
carry the screenings to the top of the screen. At the top of the screen, rake wiper blades
sweep the screenings into containers or onto conveyor belts for disposal. The teeth on

the 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 have
the rakes passing down through the wastewater in front of the rack and then up the face
of the rack. This method provides excellent cleaning efficiency, but the rakes may
potentially become jammed as they pass through any accumulation of solids at the base
of the screen on the downward travel. A modification of the front-cleaned model has the
rakes traveling down behind the screen and through a boot under the screen, and then
moving up the front of the screen. The back-cleaned models eliminate the jamming
problem by having the rakes travel down through the water behind the screen and then
travel up behind the screen with teeth projecting through the bars far enough to pick up
solids deposited on the front of the screen. In models where the rake travels up the back
of the screen, the bars are fixed only at the bottom of the screen because the rake must
project all the way through the bars. It is thus possible for the bars to move as they are
supported only by the traveling rake teeth. With movement of the bars, it is possible for
solids substantially larger than those designed for to pass through the screen. Another
drawback of the back-cleaned screen is that any solids not removed from the rakes
because of faulty wiper blades are returned to the flow behind the screen. Several manufacturers have modified both the front- and back-cleaned screens to help reduce some
of these problems.
5. QUANTITY AND DISPOSAL OF SCREENINGS
The quantity of screenings is obviously greatly affected by the type and size of screen
openings and the nature of the waste stream being screened. The curves in Fig. 5 show
the average and maximum quantities of screenings in cubic feet per 106 gallons
(ft3/MG) that might be obtained from sewage for different sized openings between
bars. Data for these curves were obtained from 133 installations of hand-cleaned and
mechanically cleaned bar screens in the United States. It can be seen that the average