Physicochemical Treatment Processes
Edited by
Lawrence K. Wang,
PhD,
PE,
DEE Zorex Corporation, Newtonville, NYLenox Institute of Water Technology, Lenox, MA Krofta Engineering Corporation, Lenox, MA
Yung-Tse Hung,
PhD,
PE,
DEEDepartment of Civil and Environmental Engineering Cleveland State University, Cleveland, OH
Nazih K. Shammas,
PhDLenox Institute of Water Technology, Lenox, MA
H
ANDBOOK OFE
NVIRONMENTALE
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eISBN 1-59259-820-x
Printed in the United States of America. 10 9 8 7 6 5 4 3 2 1 Library of Congress Cataloging-in-Publication Data
Physicochemical treatment processes / edited by Lawrence K. Wang, Yung-Tse Hung, Nazih K. Shammas.
p. cm. — (Handbook of environmental engineering) Includes bibliographical references and index.
ISBN 1-58829-165-0 (v. 3 : alk. paper)
1. Water—Purification. 2. Sewerage—Purification. I. Wang, Lawrence K. II. Hung, Yung-Tse. III.
Shammas, Nazih K. IV Series: Handbook of environmental engineering (2004) ; v. 3.
TD170 .H37 2004 vol. 3 [TD430]
628 s—dc22 [628.1/ 2004002102
v
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 prob- lems 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 con- trol.” 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 avail- able 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) cur- rently 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 natu- rally from the fundamental principles and theories of chemistry, physics, and math- ematics. 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 appli- cation, we first present the development of the scientific basis of a particular subject, fol- lowed by exposition of the pertinent design concepts and operations, and 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.
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 ex- hibit greater flexibility and originality in the definition and innovative solution of envi- ronmental 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 com- mented 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 chap- ters 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 pos- sible 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, natu- ral control processes, radioactive waste disposal, thermal pollution control, and physico- chemical treatment processes; and (2) to employ a multithematic approach to environmental pollution control because air, water, land, and energy are all interre- lated. The organization of the series is mainly based on the three basic forms in 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 gradu- ate 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
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 try- ing times.
Lawrence K. Wang Yung-Tse Hung Nazih K. Shammas
ix
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
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
6.7. Cations in Solution ... 114
6.8. Temperature ... 114
7. 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
8. Coagulation Control ... 118
8.1. Jar Test ... 119
8.2. Zetameter ... 120
8.3. Streaming Current Detector ... 121
9. Chemical Feeding ... 121
10. Mixing ... 122
11. Rapid Mix ... 124
12. Flocculation ... 125
13. Design Examples ... 127
Nomenclature ... 137
References ... 138
5 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
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. Introduction ... 199
2. Process Description ... 199
3. Softening and Recarbonation Process Chemistry ... 201
4. Lime/Soda Ash Softening Process ... 203
5. Water Stabilization ... 205
6. 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
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
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
11 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
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
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. Introduction ... 573
2. Process Flow Diagrams for GAC Process ... 576
3. Adsorption Column Models ... 577
4. 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
16 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
E. ROBERT BAUMANN, PhD • Department of Civil Engineering, Iowa State University of Science and Technology, Ames, IA
CHEIN-CHI CHANG, PhD, PE • District of Columbia Water and Sewer Authority, Washington, DC
SHOOU-YUH CHANG, PhD, PE • Department of Civil and Environmental Engineering, North Carolina A&T State University, Greensboro, NC
DURGANANDA SINGH CHAUDHARY, PhD • Faculty of Engineering, University of 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
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 Char-
lotte, 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
Screening and Comminution
Frank Deluise, Lawrence K. Wang, Shoou-Yuh Chang, and Yung-Tse Hung
CONTENTS
FUNCTION OFSCREENS ANDCOMMINUTORS
TYPES OFSCREENS
PHYSICALCHARACTERISTICS ANDHYDRAULICCONSIDERATIONS OFSCREENS
CLEANINGMETHODS FORSCREENS
QUANTITY ANDDISPOSAL OFSCREENINGS
COMMINUTORS
ENGINEERINGSPECIFICATIONS ANDEXPERIENCE
ENGINEERINGDESIGN
DESIGNEXAMPLES
NOMENCLATURE
REFERENCES
1. FUNCTION OF SCREENS AND COMMINUTORS
In order for water and wastewater treatment plants to operate effectively, it is neces- sary to remove or reduce early in the treatment process large suspended solid 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 com- minution (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.
1
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
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 pro- tected 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 (par- ticularly 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 recov- ery of valuable materials that exist as finely divided solids in industrial waste streams.
Most fine screens use a relatively fine mesh screen cloth (openings anywhere 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 horizon- tal 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.
Fig. 1. Location of screens and comminutors in a wastewater treatment plant.
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 there- fore 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. 2. Revolving disc screen: (a) screen front (inlet side) view and (b) screen side view section.
Fig. 3. Revolving drum screen.
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 0.635×5.08cm (1/4×2in.), while bars up to 3.66 m (12 ft) long might be 0.952×6.35cm (3/8×2.5 in.). Longer bars or bars used for operating conditions caus- ing unusual stress might be as large as 1.59×7.62 cm (5/8×3 in.). The bars must 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
Fig. 4. Elements of a mechanical bar screen and grit collector.
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 maxi- mum 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
(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 dis- charge 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
(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
h=0 0222.
(
V22−V12)
with English units h=0 0728.(
V22−V12)
with SI unitsh V V CD g
= 2 −
2 1
2
2 h V V
= 2 −g
2 1
2
2 Table 1
Net Area Ratios for Bar Size and Openings
Bar size Opening
cm in. cm in. Net area ratio
0.635 1⁄4 1.27 1⁄2 0.667
0.635 1⁄4 2.54 1 0.800
0.635 1⁄4 3.81 11⁄2 0.856
0.952 3⁄8 1.27 1⁄2 0.572
0.952 3⁄8 2.54 1 0.728
0.952 3⁄8 3.81 11⁄2 0.800
1.270 1⁄2 1.27 1⁄2 0.500
1.270 1⁄2 2.54 1 0.667
1.270 1⁄2 3.81 11⁄2 0.750
screenings with it. At the top of the screen, the screenings are deposited on a grid 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 installa- tions, 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 auto- matic cleaning cycle may be easily established to keep the bars relatively clear at all times.
Mechanically cleaned screens use moving rakes attached to either chains or cables 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 man- ufacturers 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 106gallons (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
screenings vary from 71.1m3/106m3(9.5 ft3/MG) for a 0.952 cm (3/8 in.) opening to 3.74 m3/106m3(0.5 ft3/MG) for a 6.35 cm (2.5 in.) opening. Taking a common open- ing of 2.54 cm (1 in.), the average quantity of screenings expected would be about 22.4 m3/106m3(3 ft3/MG), and the maximum quantity expected would be 37.4 m3/106m3. Fine screens with openings from 0.119 to 0.318 cm (3/64 to 1/8 in.) have typical screenings of 224.4 to 37.4 m3/106m3 (30 to 5 ft3/ MG) of sewage flow.
The density of all screenings from a typical municipal sewage treatment plant is approx 800–960 kg/m3(50–60 lb/ft3).
Screenings may be disposed of by grinding and returning them to the flow, by burial in landfill areas or at the plant site, or by incineration. Incineration usually requires par- tial dewatering of the screenings by some type of pressing and therefore is not usually practical except for large installations with large volumes of screenings.
6. COMMINUTORS
The handling and disposal of screenings is at best a disagreeable and expensive pro- cedure unless the product has some recovery value. To overcome this problem, devices were developed to cut up large screened material into small, relatively uniform size solids, without removal from the line of flow. These devices are generally referred to as comminutors (8–14). Figure 6 shows the essential elements of a comminutor, and Fig. 7 shows a crosssection of a typical comminutor. Various methods are used to accomplish the cutting of the solids.
Fig. 5. Quantity of screenings from wastewater as a function of openings between bars.
(Source: US EPA)
Fig. 6. Essential elements of a comminutor.
One type of comminuting device uses a slotted, rotating drum mounted vertically in the flow channel. Liquid passes through the slots down through the bottom of the drum and into the downstream channel. The solids are retained on the outside of the drum and carried by the drum to stationary comb bars mounted against the main casing of the comminutor. Mounted on the drum are hardened cutting teeth and shear bars (usually removable for sharpening or replacement) that pass through the comb bars, thereby cut- ting the solids. The small particles that result from the cutting operation then pass through the slots of the drum with the liquid flow.
Another type of device uses a stationary vertical semicircular screen grid (installed convex to the flow), with rotating circular discs on whose edges are mounted the cutting teeth. The grid intercepts the larger solids, while smaller solids pass through the clearing space between the grid and cutter discs. The rotating cutter teeth move the intercepted solids around to a stationary cutter comb where the solids are sheared as the teeth pass through the comb.
A third type of comminutor also uses a stationary vertical semicircular screen grid with horizontal slots, but is installed concave rather than convex to the flow. Ahead of the screen, a vertical arm with a cutter bar attached oscillates back and forth so the teeth on the cutting bar pass between the horizontal slots. The oscillating cutter bar carries the trapped solids to a stationary cutter bar mounted on the screen grid where the teeth of the cutters mesh and thereby shear the solids.
Various size comminutors are commercially available. For low flows, units as small as 10.16 cm (4 in.) in diameter are available, while units with 137.16 cm (54 in.) diam- eter can handle flows up to 3.15 m3/s (72 million gallons / d [MGD]). Most of the units use slot widths of either 0.635 cm (1/4 in.) or 0.952 cm (3/8 in.). Power requirements vary from 186 W (1/4 hp) for the smaller units to 1491 W (2 hp) for the larger units.
7. ENGINEERING SPECIFICATIONS AND EXPERIENCE 7.1. Professional Association Specifications
The Water Pollution Control Federation (WPCF) Technical Practice Committee explains the screening process and equipment (1), as well as the types of bar screens and bar racks and the differences between them.
Detailed information is also given by the WPCF on screening equipment operation.
Equipment should be checked frequently to ensure that it runs correctly. Screen over- flow should be prevented and cleanliness maintained in order to prevent or eliminate (a) decay of organic matter, (b) offensive odors, and (c) pathogens. On dry days, daily removal of debris is sufficient. However, on rainy days, debris should be removed more frequently because leaves and other matter from combined sewer overflow (CSO) may be transported to the plant (1).
Screening equipment may require troubleshooting for several reasons: abnormal operational circumstances (unexpected loads of debris that clog or jam the screening
Fig. 7. Crosssection of a comminutor.
equipment), equipment failure, and control failure. If a mechanically cleaned screen lacks blubber-control systems, it could suddenly receive huge loads of debris that jam its raking mechanisms.
Proper maintenance of screening equipment includes performing routine checks of components for obstructions, proper alignment, constant speed, and unusual vibrations and sounds. Screeches may result from a lack of lubrications, while thumps may mean the components are loose or broken. Proper lubrication is an important preventive main- tenance procedure. Chain-driven bar screens require frequent replacement of chains, sprockets, and other parts that appear to be badly worn. Periodic removal of a link may be required to make certain that a chain rides smoothly on the sprockets.
A description of comminutors, grinders, and various bar screens, such as trash racks, manually cleaned screens, and mechanically cleaned screens is provided by the Water Environment Federation (WEF) Manual of Practice (2). The types of mechanically cleaned screens include chain- or cable-driven screens, reciprocating rake screens, centenary screens, and continuously self-cleaning screens. Trash racks, which are usu- ally used in combined systems that have very large debris, are bar screens with large openings of 38 to 150 mm. The oldest mechanical-screening device is the chain- or cable-driven screen, which uses a chain or cable to move the rake teeth through the screen openings. They are produced as front clean/front return, front clean/rear return, and back clean/rear return. The front clean/front return has proven to be the most effi- cient. The up and down motion of the reciprocating rake screen reduces the risk of jamming and, because their parts are not submerged, they permit simple inspection and maintenance. The reciprocating screen is at a disadvantage because the single rake lim- its the ability to handle excessive loads and requires high overhead clearance. Cantenary screens have heavy tooth rakes, secured against the screen by the weight of its chain and a curved transition piece at the base that provides for effective removal of solids con- fined at the bottom. Continuous self-cleaning screens are comprised of a belt of plas- tic or stainless- steel elements that are pulled through the wastewater to provide screening along the entire length of the screen and are designed with vertical and hor- izontal limiting devices. The size of openings may range from 1 to more than 76 mm.
The continuous screening motion provides effective removal of a large number of solids, but has the disadvantage of possible carryover of solids due to its front clean/back return design.
When designing mechanical bar screens, the following parameters should be consid- ered: (a) bar spacing, construction materials, and dimensions; (b) depth of channel, width, and approach velocity; (c) discharge height; (d) angle of screen; (e) screen cover to obstruct wind and improve appearance; (f) coatings for overall unit; (g) drive unit service factor; (h) drive motor sized and enclosure; (i) spare parts; (j) stipulation of unneeded screen or bypass manual screen; and (k) head loss through unit.
The designer must consider the effects of the backwater caused by the head loss through the screen when considering a screen location. Many installations comprise an overflow weir to a bypass channel to avoid upstream surcharging if the screen becomes affected by power failure or mechanical problems.
In the past, most screening devices were placed downstream from grit chambers to prevent grit damage of comminutor teeth and combs. However, screening devices are presently placed upstream because they are more cost effective and cause fewer problems
than downstream placement. A structural enclosure for screening devices is most favor- able under windy and freezing climate conditions. An enclosure also reduces the amount of maintenance required and improves aesthetics.
7.2. Engineering Experience
Liu and Liptak (3) stated that the combined mechanical screen and grit collector can be used for small- and medium-sized plants. It is similar to the front cleaned mechanical screen, but rakes are connected to one or more perforated buckets and a steep hopper to collect the grit precedes the screen. The disadvantage of the system is that screenings and grit are mixed (3).
Some plants use coarse-mesh screens instead of screens and comminutors.
Wastewater travels through a basket of wires or rods with a mesh size 1 in. or more.
Coarse suspended matter is left in the basket.
Revolving drum screens may be characterized as having either outward or inward flow. With outward flow, the wastewater can move toward the drum from a direction parallel to its axis. Solids are captured on the inside of the screen. With inward flow, wastewater travels perpendicular to the drums axis and solids are captured on the out- side of the drum. In both systems, the captured solids are lifted above the water level as the drum slowly rotates. Solids are usually removed by water spray, which is the disad- vantage of these systems because solids are then mixed with great amount of spray water (3).
The revolving vertical disk screen is another screening device that employs the same principles as the revolving drum but uses a slowly revolving disc screen. The screen is positioned in the approach channel totally blocking the flow so that it travels through the screen. Solids are raised above the liquid level and washed by water spray. The screen consists of a 2–60-mesh stainless-steel wire cloth and is not suited for handling very large objects, large a
