EPA-625/1-77-009
PROCESS DESIGN MANUAL
WASTEWATER TREATMENT
FACILITIES FOR SEWERED
SMALL COMMUNITIES
U. S. ENVIRONMENTAL PROTECTION AGENCY
Environmental Research Information Center
Technology Transfer
fr.S, Kr
a. >jo, 1L 6060-4
October 1977
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ACKNOWLEDGMENTS
This manual was prepared for the U.S. Environmental Protection Agency, Office of
Technology Transfer, by the firm of Camp, Dresser & McKee, Boston, Mass., under the
direction of R. E. Leffel. Major EPA contributors and reviewers were John M. Smith and
James F. Kreissl of the U.S. EPA Municipal Environmental Research Laboratory, Cincinnati,
Ohio, and Denis J. Lussier of the Office of Technology Transfer, Cincinnati, Ohio.
NOTICE
The mention of trade names or commercial products in this publication is for illustration
purposes and does not constitute endorsement of recommendation for use by the U.S.
Environmental Protection Agency.
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CONTENTS
Chapter Page
ACKNOWLEDGMENTS ii
FOREWORD xv
1 INTRODUCTION 1-1
2 FUNDAMENTAL DESIGN CONSIDERATIONS 2-1
2.1 Introduction 2-1
2.2 Area Served 2-5
2.3 Population Served 2-6
2.4 Wastewater Flow 2-7
2.5 Wastewater Characteristics 2-23
2.6 Odors and Other Airborne Pollutants 2-41
2.7 Noise 2-43
2.8 Trucked and Marine Industry Wastes 2-46
2.9 Treatment Requirements and Effluent Disposal 2-48
2.10 Upgrading and Enlarging Existing Plants 2-66
2.11 Pilot- and Laboratory-Scale Testing 2-72
2.12 Reliability Considerations 2-73
2.13 Process Selection 2-73
2.14 Operation and Maintenance Design Requirements 2-79
2.15 References 2-81
3 HYDRAULIC CONSIDERATIONS 3-1
3.1 Introduction 3-1
3.2 Flow Considerations 3-1
3.3 Pumping 3-12
3.4 Re feren ces 3-15
4 FLOW EQUALIZATION 4-1
4.1 Introduction 4-1
4.2 Variations in Wastewater Flow 4-1
4.3 Benefits of Dry Weather Flow Equalization 4-2
4.4 Disadvantages of Dry Weather Flow Equalization 4-7
4.5 Methods of Equalization 4-7
4.6 Equalization Design 4-11
in
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CONTENTS - Continued
Chapter Page
4.7 Examples 4-15
4.8 Cost of Equalization 4-21
4.9 Case Studies 4-22
4.10 References 4-28
5 HEADWORKS COMPONENTS 5-1
5.1 Introduction 5-1
5.2 Racks, Bar Screens, and Comminutors 5-2
5.3 Grit Removal 5-2
5.4 Oil, Grease, and Floating Solids Removal 5-3
5.5 Preaeration 5-4
5.6 Physical Screening 5-5
5.7 Pumping 5-5
5.8 Flow Measuring and Sampling 5-5
5.9 Equalization Tanks 5-8
5.10 Chemical Additives 5-8
5.11 References 5-11
6 CLARIFICATION OF RAW WASTEWATER 6-1
6.1 General 6-1
6.2 Coagulation and Flocculation 6-1
6.3 Solids-Contact Treatment 6-3
6.4 Sedimentation 6-3
6.5 References 6-10
7 ACTIVATED SLUDGE 7-1
7.1 Introduction 7-1
7.2 Description of Basic Processes 7-1
7.3 Modifications for Small Communities 7-5
7.4 Applicable Design Guidelines 7-13
7.5 Oxygen Requirements 7-19
7.6 Clarification 7-20
7.7 Aeration System Design 7-28
7.8 Nitrification 7-36
7.9 Operation and Maintenance 7-44
7.10 Example Design 7-47
IV
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CONTENTS - Continued
Chapter Page
7.11 Case Studies 7-51
7.12 References 7-54
8 PACKAGE (PREENGINEERED) PLANTS 8-1
8.1 General Design Considerations 8-2
8.2 Extended Aeration Units 8-6
8.3 Contact Stabilization Units 8-9
8.4 Rotating Biological Contactor (RBC) Units 8-10
8.5 Physical-Chemical Package Units 8-13
8.6 Operation and Maintenance of Activated Sludge Package Units 8-15
8.7 Case Studies 8-16
8.8 References 8-24
9 FIXED GROWTH SYSTEMS 9-1
9.1 Trickling Filters 9-1
9.2 Rotating Biological Contactors (RBC) 9-37
9.3 References 9-46
10 WASTEWATER TREATMENT PONDS 10-1
10.1 Background 10-1
10.2 Definitions and Descriptions of Wastewater Treatment Ponds 10-4
10.3 General Design Requirements 10-5
10.4 Facultative Pond Design 10-11
10.5 Aerated Ponds 10-35
10.6 Aerobic Pond Design 10-51
10.7 Polishing Pond Design 10-55
10.8 Microbial Cell and SS Removal from Pond Effluent 10-55
10.9 Pathogen Removal 10-58
10.10 Construction and Maintenance Costs 10-59
10.11 References 10-60
11 FILTRATION AND MICROSCREENING 11-1
11.1 Introduction 11-1
11.2 General Types of Granular Media Filters and their Operation 11-1
11.3 Design of Granular Media Filter Installations 11-8
11.4 Sand Beds (Intermittent Filtration) 11-16
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CONTENTS - Continued
Chapter Page
11.5 Microscreening 11-16
11.6 References 11-19
12 PHYSICAL-CHEMICAL TREATMENT 12-1
12.1 Design Considerations 12-2
12.2 Chemicals 12-3
12.3 Unit Operations 12-7
12.4 Costs 12-11
12.5 References 12-11
13 NUTRIENT REMOVAL 13-1
13.1 General Considerations 13-1
13.2 Phosphorus Removal 13-2
13.3 Nitrogen Removal 13-2
13.4 Removal of Soluble Organics 13-4
13.5 Removal of Soluble Inorganics 13-4
13.6 References 13-6
14 SLUDGE AND PROCESS SIDESTREAM HANDLING 14-1
14.1 Background 14-1
14.2 Thickening 14-4
14.3 Stabilization 14-13
14.4 Dewatering 14-22
14.5 Sidestreams Produced 14-26
14.6 Septage Handling 14-27
14.7 Sludge Disposal 14-28
14.8 References 14-30
15 DISINFECTION AND POST AERATION 15-1
15.1 Introduction 15-1
15.2 Chlorination 15-2
15.3 Chlorine Mixing and Contacting 15-9
15.4 Ozonation 15-12
15.5 Small Plant Disinfection Practice 15-18
15.6 Postaeration 15-18
15.7 References 15-19
VI
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CONTENTS - Continued
Chapter Page
16 OPERATION AND MAINTENANCE 16-1
16.1 Management and Organization 16-2
16.2 Factors Affecting Operation 16-3
16.3 Personnel 16-5
16.4 Operation and Maintenance (O & M) Manuals 16-12
16.5 Monitoring 16-18
16.6 Laboratory Facilities 16-19
16.7 Workshop Facilities 16-24
16.8 Safety 16-25
16.9 References 16-25
17 COST EFFECTIVENESS 17-1
17.1 Background 17-1
17.2 Cost-Effectiveness Analysis Regulations 17-1
17.3 Energy Conservation 17-4
17.4 Methodology 17-4
17.5 Cost-Effectiveness of Infiltration/Inflow Reduction 17-6
17.6 Costs 17-6
17.7 References 17-15
18 GLOSSARY G-l
Vll
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LIST OF FIGURES
Figure No. Page
2-1 Effects of Population on Wastewater Flows in Texas 2-8
2-2 Average Domestic Wastewater Flow Rate Versus Population As a
Function of Average Assessed Valuation of Property in 1960 2-9
2-3 Upper and Lower 2.5% Prediction Limits for Average Daily
Wastewater Flow Rates Versus Population As a Function of
Average Assessed Valuation of Property in 1960 2-10
2-4 Daily Household Water Use 2-21
2-5 Relation of Small Plant Design to Collection System 2-22
2-6 Variations in Daily Wastewater Flow 2-24
2-7 Estimated Curves for Design Purposes Showing Relation of
Discharge to Fixture Units 2-25
2-8 Schematic Diagram of Wastewater Characteristics 2-28
2-9 Hourly COD Profile 2-38
2-10 Secondary Treatment Configurations 2-75
2-11 Example Plant Flow Diagram 2-80
3-1 Methods of Wastewater Flow Division 3-3
3-2 Hydraulic Gradient-Wastewater Treatment Plant With Provision
for Future Upgrading 3-7
3-3 Hydraulic Gradient—Wastewater Treatment Plant 3-8
3-4A Hydraulic Gradient—Wastewater Treatment Plant 3-9
3-4B Hydraulic Gradient-Wastewater Treatment Plant 3-10
3-5 Hydraulic Gradient-Wastewater Treatment Plant 3-11
4-1 Typical Dry Weather Flow and BOD Variation of Municipal
Wastewater Before Equalization 4-3
4-2 Sideline Equalization 4-8
4-3 In-Line Equalization 4-9
4-4 Orbal Equalization Unit 4-10
4-5 Hydrograph of a Typical Diurnal Flow 4-15
4-6 Inplant Equalization 4-17
4-7 Side-Line Equalization 4-20
4-8 Walled Lake-Novi Wastewater Treatment Plant 4-23
4-9 Variable Depth Oxidation Ditch, Dawson, Minnesota 4-26
5-1 Typical Metering and Sampling Installations 5-7
6-1 Turbine-Type Flocculators 6-2
6-2 Solids-Contact Treatment Unit 6-4
6-3 Schematic Representation of Settling Zones 6-6
6-4 Rectangular Clarifier With Chain and Flight Sludge Collector 6-8
6-5 Circular Clarifier With Skimmer 6-9
6-6 Circular Clarifier With Inner Flocculation Compartment 6-9
vm
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LIST OF FIGURES - Continued
Figure No. Page
7-1 Conventional Activated Sludge System 7-4
7-2 Extended Aeration Activated Sludge System • 7-6
7-3 Oxidation Ditch Activated Sludge System 7-6
7-4 Carousel Oxidation Ditch at Oosterwolde, The Netherlands 7-8
7-5 Contact-Stabilization Activated Sludge System 7-10
7-6 Completely-Mixed Activated Sludge System 7-10
7-7 Sludge Production in Activated Sludge Systems Treating Domestic
Wastewater 7-17
7-8 Activated Sludge Settling Data for Domestic Wastewater (20° C) 7-22
7-9 Saturation Concentration for Atmospheric Oxygen 7-31
7-10 Schematic of Installation for Submerged Turbine Aerator 7-35
7-11 Mechanical Surface Aerator 7-37
7-12 Nitrification in Completely Mixed Activated Sludge Process 7-38
7-13 Rate of Nitrification at Different Temperatures 7-40
7-14 Two Stage System for Nitrification 7-42
7-15 Single Stage Nitrification System 7-43
7-16 Temperature Effects on Denitrification 7-45
7-17 Schematic Flow Diagram, Woodstock, N.H. 7-52
8-1 Extended Aeration Treatment Plant With Air Diffusers 8-7
8-2 Extended Aeration Treatment Plant With Mechanical Aerator 8-8
8-3 Contact-Stabilization Plant With Aerobic Digester 8-11
8-4 Bio-Disc Treatment Plant 8-12
8-5 Flow Diagram for Physical-Chemical Treatment Plant 8-14
8-6 Met-Pro Physical-Chemical Package Treatment Plant 8-18
8-7 Extended Filtration Package Plant 8-21
8-8 Can-Tex Package 2-Stage (Nitrification) Activated Sludge Plant 8-23
9-1 Multiple Step Systems Using Trickling Filters 9-5
9-2 Trickling Filter Media 9-8
9-3 Percent BOD Removed vs Hydraulic Load Modular Type Media 9-12
9-4 Pounds BOF> Removed vs Hydraulic Load Modular Type Media 9-12
9-5 Flow Diagrams for Small Trickling Filter Plants 9-14
9-6 Applicability of Trickling Filter Formulas 9-21
9-7 Rock MediaTrickling Filter 9-33
9-8 Plastic Media Bio-Oxidation Tower 9-34
9-9 Northbridge, Mass., Schematic Flow Diagram 9-39
10-1 Activity in Facultative Ponds 10-2
10-2 Performance of Polishing Ponds Following Secondary Treatment 10-6
IX
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LIST OF FIGURES - Continued
Figure No.
10-3
10-4
10-5
10-6A
10-6B
10-7
10-8
10-9
10-10
10-11
10-12
10-13
10-14
11-1
11-2
11-3
11-4
11-5
11-6
11-7
11-8
14-1
14-2
14-3
14-4
14-5
14-6
15-1
15-2
15-3
15-4
15-5
Facultative Pond System Configurations
Inlets, Outlets and Baffle Arrangements
Dike and Outlet Design Details
Example Facultative Pond System
Example Facultative Pond System
Aerated Pond Water Temperature Prediction Nomograph for a
Pond Depth of 10 Feet (For Various Loadings and Retention
Times)
Power Level for Oxygen Transfer
Schematic View of Various Types of Aerators
Charleswood Demonstration Ponds, Winnipeg, Canada
System Layout for Blacksburg, Virginia, Diffused Air Aerated Pond
System
BOD Removal for Blacksburg, Virginia, Diffused Air Aerated Pond
System
Relationship Between Oxygenation Factor and BOD Removal in
Waste Ponds
Submerged Rock Filter, California, Missouri
Filter Configurations
Gravity Filter With Float Control
Pressure Filter
Upflow Filter
Run Length vs Influent SS Concentration at Various Flow Rates
Automatic Granular Media Filter
Automatic Backwash Filter
Schematic Diagram of Microscreen Unit
Gravity Thickener
Schematic of an Air Flotation Thickener
Schematic of Aerobic Digester System
Typical Circular Aerobic Digester
Digester With Mechanical Mixer
Digester With Gas Mixing
Chloramine Contact Time Requirements
Free Chlorine Contact Time Requirements
Contact Time for 99% Kill of E. Coli at 2°-6° C in Pure Water
Relative Resistance to HOC1 at 0°-6° in Pure Water
Distribution of HOC1 and OC1" in Water With Variations in pH
Page
10-12
10-22
10-24
10-30
10-31
10-38
10-42
10-44
10-50
10-52
10-52
10-54
10-57
11-2
11-3
11-4
11-6
11-10
11-13
11-15
11-17
14-11
14-12
14-14
14-15
14-20
14-21
15-4
15-4
15-5
15-5
15-6
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LIST OF FIGURES - Continued
Figure No. Page
15-6 Gaseous Chlorination Systems 15-8
15-7 Hypochlorite Generation 15-10
15-8 Chlorine Mixing Methods 15-11
15-9 Contact Chamber With Longitudinal Baffling 15-13
15-10 Impact of Chlorine Tank Baffle Design on Actual Detention Time 15-14
15-11 Ozone System 15-16
15-12 Ozone System 15-17
15-13 Typical Postaeration Devices 15-20
16-1 Average Operation Time 16-7
16-2 Secondary Sludge and Scum Piping System-Normal WAS and
Scum Pump Operations 16-13
16-3 Injector Water System 16-14
16-4 Process Flow Path 16-15
17-1 Initial Costs of Wastewater System Components 17-7
17-2 Annual Costs of Wastewater System Components 17-8
XI
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LIST OF TABLES
Table No. Page
2-1 Quantities of Wastewater 2-11
2-2 Package Treatment Plant Sizing Data 2-13
2-3 Typical Daily Flows and BOD Considerations 2-16
2-4 Fixture Units Per Fixture or Group 2-26
2-5 Wastewater Characteristics 2-27
2-6 Organisms Found in Wastewater Treatment 2-35
2-7 Average Composition of Domestic Wastewater, mg/1 2-37
2-8 Some Chemical Substances in Industrial Wastes 2-39
2-9 Average Single Number Sound Pressure Levels 2-44
2-10 Summary of Existing Municipal Wastewater Reclamation Projects 2-64
2-11 Possible Trickling Filter Plant Modifications 2-68
2-12 Possible Activated Sludge Plant Modifications 2-70
2-13 Application of Treatment Plants for Small Communities 2-77
2-14 Operational Characteristics of Various Treatment Processes 2-78
3-1 Pumping Alternatives For Various Treatment Plant Needs 3-14
4-1 Effect of Flow Equalization on Primary Settling: Newark, New
York 44
4-2 Principle Design Criteria, Dawson, Minnesota, Wastewater
Treatment Plant 4-27
4-3 Performance Data, Dawson, Minnesota 4-28
5-1 Headworks Units 5-1
5-2 Theoretical Maximum Overflow Rates For Grit Chambers 5-4
7-1 Design Criteria of Modified Activated Sludge Processes 7-12
7-2 Commonly Used Values for Synthesis Constant a' and
Autooxidation Constant b' 7-20
7-3 Final Clarifier Criteria 7-23
7-4 Method for Solving a Bulking Problem 7-26
7-5 Return Sludge Rate Required To Maintain MLSS at 2,000 MG/L
for Various SVI Values 7-27
7-6 Oxygen Transfer Capabilities of Various Aeration Systems 7-34
7-7 Summary of Conditions Advantageous for Nitrifier Growth 7-41
7-8 Woodstock, N.H., Oxidation Ditches 7-53
8-1 Commercial Biological Package Plants 8-13
8-2 Data for a Physical-Chemical Plant 8-17
8-3 Performance Evaluation of Aquatair Model P-3 8-20
8-4 Characteristics of Aquatair Package Systems 8-20
Xll
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LIST OF TABLES - Continued
Table No. Page
8-5 Package 2-Stage Nitrification Activated Sludge Plant 8-22
9-1 Trickling Filter Classifications 9-2
9-2 Comparative Physical Properties of Trickling Filter Media 9-9
9-3 Summary of Example Designs 9-22
9-4 Northbridge, MA, Rock Trickling Filters 9-38
10-1 BOD5 :BODU Ratio Effects of k Values 10-8
10-2 Evaluation of k Values for the Field Lagoons at Fayette, Missouri 10-8
10-3 Recommended Design Criteria Facultative Wastewater Treatment
Ponds 10-13
10-4 Biological Activity Data for Ponds 10-14
10-5 Approximate Values of Solar Energy 10-16
10-6 Belding, Michigan, Intermittent Discharge Pond System 10-26
10-7 Types of Aeration Equipment for Aerated Ponds 10-43
10-8 Effluent Concentrations for Charleswood Demonstration Ponds,
Winnipeg, Canada, 21 Month Average 10-48
10-9 Design Criteria: Charleswood Demonstration Ponds 10-49
10-10 Design Criteria for Blacksburg, Virginia, Diffused Air Aerated Pond
System 10-51
10-11 Operating and Maintenance Relationships in the Form Y = aX*> 10-59
14-1 Average Quantities of Sludge 14-5
14-2 Characteristics of Various Sidestreams Primary and Activated
Sludge Treatment Plants 14-7
14-3 Gravity Thickener Loadings 14-9
14-4 Characteristics of Supernatant From Aerobic Digesters 14-17
14-5 Characteristics of Anaerobic Digester Supernatants 14-22
14-6 Criteria for the Design of Sandbeds 14-24
14-7 Average Characteristics of Septage 14-28
16-1 Common Factors Affecting Operations 16-4
16-2 Typical Employee Performance Matrix for Extended Aeration
Activated Sludge Plant 16-8
16-3 Partial Listing of Schools and Training Programs for Plant
Operators 16-10
16-4 Possible References for an Operator's Library 16-16
16-5 Desired Detail on Each Process System in O&M Manual 16-17
16-6 Typical Sampling and Testing Program 16-20
16-7 Minimum Laboratory Equipment Needs for Typical 1 MGD or
Smaller Wastewater Treatment Plants Where Backup Laboratory
Facilities Are Not Easily Available 16-21
Xlll
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LIST OF TABLES - Continued
Table No. Page
16-8 Laboratory Equipment List for Monitoring 16-22
17-1 6.125% Compound Interest Factors 17-2
17-2 Cost Indices (Average Per Year) 17-9
17-3 Estimated Capital Costs for Alternate Treatment Processes With a
Design Flow of 1 mgd 17-11
17-4 Estimated Total Annual Costs for Alternative Treatment Processes
With a Design Flow of 1 mgd 17-12
17-5 Construction Costs for Unit Processes for Wastewater Treatment
Plants 17-13
17-6 Cost Functions of Municipal Waste Treatment 17-14
xiv
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FOREWORD
The formation of the United States Environmental Protection Agency marked a new era of
environmental awareness in America. This Agency's goals are national in scope and
encompass broad responsibility in the area of air and water pollution, solid wastes,
pesticides, and radiation. A vital part of EPA's national water pollution control effort is the
constant development and dissemination of new technology for wastewater treatment.
It is now clear that only the most effective design and operation of wastewater treatment
facilities, using the latest available techniques, will be adequate to meet the future water
quality objectives and to ensure continued protection of the nation's waters. It is essential
that this new technology be incorporated into the contemporary design of waste treatment
facilities to achieve maximum benefit of our pollution control expenditures.
The purpose of this manual is to provide the engineering community and related industry a
new source of information to be used in the planning, design and operation of present and
future wastewater treatment facilities for sewered small communities. It is recognized that
there are a number 01 design manuals, manuals of standard practice, and design guidelines
currently available in the field that adequately describe and interpret current engineering
practices as related to traditional plant design. It is the intent of this manual to supplement
this existing body of knowledge by describing new treatment methods, and by discussing
the application of new techniques for more effectively removing a broad spectrum of con-
taminants from wastewater.
Much of the information presented is based on the evaluation and operation of pilot,
demonstration and full-scale plants. The design criteria thus generated represent typical
values. These values should be used as a guide and should be tempered with sound
engineering judgement based on a complete analysis of the specific application.
This manual is one of several available through the Technology Transfer Office of EPA to
describe recent technological advances and new information. Future editions will be issued
as warranted by advancing state-of-the-art to include new data as they become available, and
to revise design criteria as additional full-scale operational information is generated.
Companion publications describing treatment alternatives for non-sewered communities are
available in the form of Technology Transfer Seminar Handouts. They may be obtained
by writing:
U.S. EPA
ERIC
26 W. St. Clair
Cincinnati, Ohio 45268
xv
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CHAPTER 1
INTRODUCTION
1.1 Background of Wastewater Treatment
Historically, treatment of wastewater in the United States has been a catchup phenomenon
employed only when required by existing legislation to protect the public health.
Communities have generally been reluctant to bear the high capital and operating cost for
wastewater treatment in excess of the minimum requirements.
During the 19th century public concern for the effects of raw wastewater discharge on the
health and well-being of expanding population increased, and communities began to plan for
and construct sewage collection and treatment systems. These earlier treatment processes
included screens, grit chambers, settling tanks, anaerobic digestion, Imhoff tanks, trickling
filters, and activated sludge. Disinfection, when employed, was largely by chlorination.
In the 1940's and 1950's, existing processes were refined and improved and additional
processes were developed to the extent that they could be put into general use. Included in
the latter were facultative stabilization ponds, oxidation ditches, ponds, extended aeration,
contact stabilization, high-rate filters, and slow sand filters. Preengineered "package"
treatment units came into relatively common use for the less than 1-mgd wastewater
treatment plant. New processes during these years for sludge treatment included vacuum
filtration, centrifugation, and incineration.
In the 1960's other new processes emerged, including aerated stabilization ponds, rotating
biological disks, aerobic digestion of sludge, microscreens, physical-chemical treatment,
phosphorus and nitrogen removal, and wet oxidation of sludge. However, some of these are
not yet used extensively for smaller treatment plants. For many years, the disposal of
wastewater (treated or untreated) into surface waters was considered a legal use of such
waters. Also, disposal of sludge and wastewater on the land, irrespective of its effects on the
soils or underlying ground water, was considered acceptable. As these practices increased and
the self-purification capacities of waters were exceeded, more and more nuisance conditions in
surface waters became evident.
Water pollution control laws were passed (Water Quality Act of 1965) to maintain water
quality standards that permitted accepted water uses for the receiving waters. Under these
laws, the States established the best uses for the different receiving waters within their
boundaries, and required only treatment of wastewater sufficient to meet the specific
receiving water quality standards. The Federal Government then established guidelines and
reviewed plans of facilities that were to be partially financed by Federal funds. This method
of pollution control proved inadequate, since the condition of surface and ground water in
many locations continued to deteriorate. Eutrophication, fish kills, oil spills, ocean waste
dumping, and damage to ecosystems were becoming excessive, and public groups became
more insistent on better control of wastewater discharges.
1-1
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Under the 1972 Federal Water Pollution Control Act Amendments, all publicly owned
treatment facilities must meet, as a minimum, Federal secondary treatment effluent
standards. In addition, where State receiving water quality standards require even better
effluents, advanced treatment must be given for further reduction of such pollutants as
phosphorus, ammonia, nitrates, compounds of chlorine, toxic substances, excessive oxygen
demands, or excessive solids. The reuse of wastewater, after treatment, for industrial uses
and irrigation is becoming more common. Other water conservation measures to reduce the
amount of wastewater receiving costly treatment are also beginning to appear. Some surface
waters that receive treated wastewater are now showing improvement and beginning to
return to their natural quality.
1.2 Small Plants
Treatment plants smaller than 1-mgd capacity and their discharges have not received as
much attention by community, State, or Federal governments as the larger, metropolitan
plants. Small community budgets for operation and maintenance of their treatment plants
have often been given a low priority, resulting in part-time operation by inadequately
trained personnel. The result has been the proliferation of many small unreliable treatment
systems that too often do not meet effluent requirements.
Smaller plants are often located on smaller streams or lakes, or serve recreational areas
where receiving water quality is very high. In these situations, the discharge of inadequately
treated wastewater from even the smallest plants for a relatively short period can cause
severe damage. Small plants must therefore be designed, constructed, operated, and
maintained as efficiently and reliably as the larger plants.
1.3 Goals
The design goal for small wastewater treatment plants is to meet Federal and State effluent
and water quality standards reliably while:
1. Utilizing processes that require a minimum of operator time
2. Providing equipment that is relatively maintenance free
3. Operating efficiently through a wide range of hydraulic and organic loadings
4. Using a minimum of energy
5. Meeting emergencies, such as biological process upsets and equipment failures,
without damage to the receiving waters, or to the soil if disposal is to the land
6. Conserving and improving environmental factors and natural resources
1-2
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CHAPTER 2
FUNDAMENTAL DESIGN CONSIDERATIONS
2.1 Introduction
The purpose of a wastewater treatment works is to remove impurities to such an extent
that the treated effluent will meet State and Federal requirements and be suitable for dis-
posal or reuse. Both the process of removing objectionable constituents from the waste-
water and the final disposal must be accomplished in an environmentally acceptable manner.
This chapter describes the more important factors relevant to meeting these goals at smaller
treatment works and how they are considered in the overall design.
2.1.1 Fundamental Approach
The design of a wastewater treatment plant is based on the expected volume and kind of
raw wastewater and on the required effluent limitations. The initial design step, after
probable influent characteristics and effluent requirements have been determined, is tenta-
tively to select alternative processes to be used at the available treatment plant sites. This
part of the design is perhaps the most critical, because it necessitates a complete analysis of
the collected data, as well as an understanding of each available alternative and how each
will fit into the overall design.
2.1.2 Design Differences Between Small and Large Plants
Small wastewater treatment plants are used in localities where it is not possible or eco-
nomically feasible to connect to a larger wastewater treatment system. Climate, topography,
distances between communities, and political boundaries are also considerations in choosing
an independent small treatment plant. Sometimes a community must pretreat its waste
before the waste can be discharged to a centralized system. Localities where small plants are
used would include farms, isolated rural homes, fishing and hunting cabins, resort areas,
schools, suburban housing developments, trailer parks, highway rest areas, tourist parks,
work camps, hospitals, and ports. This manual is primarily concerned with the domestic
wastewater from small communities, although many of the same principles apply to treat-
ment works for any type of inhabited locality.
The fluctuation in volume and the characteristics of wastewater discharges cause more diffi-
culty for a small than for a large community. Wastewater flow from small communities will
generally have greatly accentuated peaks and minimums. Small sources sometimes exhibit
a much stronger waste in terms of suspended solids, organic matter, nitrogen, phosphorus,
and/or grease, particularly if septage is added to the wastewater being treated. Toxic mate-
rials may be present in septage but can often, if pretreated, be handled by a properly de-
signed small plant. Seasonal variations in flow because of high groundwater level (which
could more than double the flow for several months) can make multiple treatment units
necessary at small plants. Some processes are highly sensitive to temperature or precipita-
tion and thus cannot be used in some localities. A more detailed discussion of these
2-1
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variations, including causes and effects on a plant, is given later in this chapter.
The operation and maintenance requirements of treatment plants also vary with plant size.
Usually, the smaller the plant the smaller the budget for operation and maintenance. Larger
plants can more easily justify employment of a full-time crew to operate and maintain the
plant; however, in some smaller plants only a part-time operator may be economically
feasible. Possible solutions to this problem will be discussed in Chapter 16.
Small wastewater treatment plants often must meet specific effluent requirements, espe-
cially if 1) the effluent is discharged to a small watercourse, therefore requiring a higher
than secondary degree of treatment; 2) the effluent will be directly or indirectly reused; or
3) the effluent-receiving stream requires the removal of a specific constituent.
In summary, effluent requirements, variations in the flow and the characteristics of the
wastewater, availability of well-trained operation and maintenance personnel, plant location,
climate, and availability of funds are the major factors to be considered in the design of a
small plant.
2.1.3 Design Periods and Stages
It is general engineering practice to select a design period of 10 to 20 years from the initia-
tion of the design phase for wastewater treatment works (1). Sometimes the design period
for small plants is only 4 to 5 years. (An example of this would be the use of small plants in
suburban areas to meet effluent limitations until interceptors and centralized metropolitan
treatment facilities are constructed and placed in operation.) In selecting a design period,
the design engineer must weigh factors such as the type and degree of expected community
development, current and projected interest rates, the economic level of the community in-
volved-and its willingness to pay for wastewater treatment.
An overextended period would result in excessive expense which could have been deferred
for several years for unused capacity. Consequently, the plant would operate at less than
peak efficiency because of underloading. On the other hand, too short a period would result
in a plant that might be overloaded in a relatively short time, and a design that could not
take advantage of any possible "economies of scale." A slightly underloaded plant is better
than an overloaded plant, particularly if overloading may cause degradation of the environ-
ment; therefore, care must be taken to insure that the plant is safely adequate.
An efficient approach to this problem is the use of stepped development, to increase the
range of possible designs and still allow rural or underdeveloped communities to construct
the initial facilities. In this way, the financial burden on the existing population is reduced,
and the plant can develop to meet community needs. Many of the available packaged plants
(preengineered, factory-fabricated plants) have been designed with this in mind—they can be
expanded by adding similar units, by using initially unused compartments, and by adding
different units that improve the degree of treatment. Some of these packaged plants can be
used temporarily in one location and then moved to another if designed for such use in
multiple locations.
2-2
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Another reason for not designing for too long a time is that many of the treatment units
used in smaller plants are standard items that are constantly being improved. Such improve-
ments mean many of the units become inefficient (compared to newer units) with time, and
must be phased out of production, and can result in problems in replacing or repairing worn
or broken equipment.
2.1.4 Site Planning and Other Environmental Considerations
Traditionally, site and route selection for wastewater collection and treatment facilities has
been concerned primarily with sites that would permit the flow of wastewater by gravity.
It has often been possible to locate treatment plants in isolated areas where occasional plant
nuisances would not seriously affect adjacent neighborhoods. Spreading urbanization, how-
ever, and the increasing need to provide wastewater treatment within existing urban areas
would require the use of sites in, or rights-of-way through, developed neighborhoods and
commercial areas.
The objective of an environmental assessment, as now required by Public Law 92-500, is to
minimize any possible adverse effects of the facility on the environment. An environmental
assessment is not only concerned with the land use planning and zoning aspects of siting but
also with odor, air pollution, noise, lighting, architectural design, landscaping, and other
environmental factors.
An environmental analysis of a site, which includes evaluation of the physical, ecological,
esthetic, and social aspects of the environment existing at the site, provides the basis for the
design of alternative solutions. In addition, requirements for existing and proposed land use,
zoning, and planning must be considered in developing the alternatives (2) (3).
The environmental evaluation process should begin early in the planning stage. First, it takes
a substantial period of time to do an adequate environmental evaluation, even in cases in
which the total amount of effort required is small. (The lengthy period results from delays
in assembling pertinent data, interviewing people, observing environmental conditions
several times during a year, etc.). Second, the environmental evaluation should have some
impact on the initial assumptions and on the development and screening of alternatives
before facilities design is begun.
Alternatives may involve various feasible sites, flow reduction measures (including the cor-
rection of excessive infiltration or inflow), the treatment of overflows, alternative system
configurations, land treatment or reuse of wastewater, phased development of facilities, or
improvements in operation and maintenance. The U.S. EPA requires that such alternatives
be considered for any project in order for it to be eligible for funding (3).
The alternatives must be judged in terms of their net environmental effect. Treatment of
pollutants should benefit the local water quality problem. Abatement practices to restore
surface water should not shift the environmental problem to other media such as ground-
water, which is more difficult to treat (3). Emphasis should be focused on preventive
measures as well as on abatement or corrective measures.
2-3
-------
To be considered are multiple use of facility sites and rights-of-way, such as:
Pumping stations
Pedestrian and bicycle paths
Municipal incinerators
Ecosystems and conservation areas
Police or fire protection units
Public works garages
Power generation stations
Transportation centers
Training centers
Emergency public medical facilities
Recreational open space
Waterfront access
The possible impacts, both beneficial and detrimental, of the interceptor and sewer systems
on land use development must be considered in the design of these systems. Growth in an
area also may be stimulated beyond the existing rate of growth after wastewater collection
and treatment facilities are constructed. On the other hand, if wastewater facilities are not
adequate, or are not easily accessible, growth will most likely be stifled. Effects of proposed
construction on both short- and long-range development should be evaluated in the siting
analysis.
Conservation of energy is as important as conservation of money. Some processes, such as
ponds, use little energy. Designing structures to minimize heat loss is important. Under-
ground structures can be kept at a uniform temperature more easily than structures above
ground. Maximum advantage should be taken of the sun, topography, and of windscreens
to reduce freezing and chilling.
Federal effluent requirements and State receiving-water quality standards can only be met
by sizing wastewater facilities to meet the demands of the community reliably, at least until
the next stage of construction is expected to be completed. Costs of unused standby
capacity to be paid for with inflated currency must be considered in determining the design
period.
Flow predictions must be based on existing and probable future zoning and land use, and
should consider local, regional, and State land use, transportation, utility, and business or
development plans. Differences between existing land use and proposed plans should be
reconciled with the appropriate public planning and engineering agencies and interested
citizen groups. Prediction of growth patterns and regional configurations, with their
attendant wastewater flows, requires reasonable estimates of both low and high options,
verified by local regional officials and private developers.
Some of the more important environmental factors that may be affected if a wastewater
facility is not satisfactorily designed, constructed, maintained, or operated are listed below.
A discussion of these factors is presented in reference (1).
1. Locating the facilities in a location compatible with such works.
2. Arranging the landscaping and architecture so as not to disturb neighborhood
esthetics.
3. Designing the facilities so as not to interfere with flood plain storage or with
natural drainage.
2-4
-------
4. Utilizing construction methods that do not affect the native ecological systems;
cause air pollution, erosion, or flooding; excessively exceed the ambient noise
level; or cause damage during blasting and pile driving operations.
5. Scheduling trucking or truck routes compatible with existing conditions.
6. Preventing the entrance of polluted water overflows or leakages into surface or
subsurface waters.
7. Siting construction and operation lighting so as not to cause a nuisance.
8. Designing facilities to prevent the escape of odors or air pollutants, and providing
means for control of potential odors or of air pollutant emissions.
9. Designing facilities to prevent noise levels detrimental to the neighborhood or to
the workers.
10. Providing for efficient operation and maintenance of the facilities and grounds in
a manner that will not have an adverse impact on the environment.
For more information on site planning, see references (2) through (13).
2.2 Area Served
The area served by a small treatment plant plays a significant part in the determination of
flow and wastewater characteristics. The limits of this area may be defined by natural
drainage basins, political boundaries, specific wastewater treatment requirements, or any
combination of these. Within this area, the existing and planned future land use and accom-
panying resident population are the prime determinants in estimating the expected volume
and characteristics of the wastewater to be treated.
First, the existing land usage must be assessed and any available data collected. In portions
of the area that have reached saturation population or are completely developed with com-
mercial or industrial sites, only existing data may be needed to determine expected waste-
water flow and its probable characteristics, unless the area is expected to be redeveloped.
If redevelopment occurs, the changes in land use may change the flow and characteristics of
the wastewater.
Most small wastewater treatment facilities are located in rural or suburban areas where large
portions of the service area are underdeveloped or being developed. Both local and regional
trends in development of the area, as well as possible future expansion of the plant, must be
examined. Local trends should include the economic development and change in tax base
relative to construction costs and taxes. Regional trends should include the location of
various economic activities such as shopping areas or industrial parks, the rate of growth for
suburban areas, changes in political boundaries, possible annexation or service agreements
with other communities, status of transportation systems, and possible use of other regional
utilities. Other sources of this information include regional plans, zoning maps and reports,
engineering investigations, reports by regulatory agencies, and actual investigations and
examinations of the area.
Much of this information on area development should be available in the comprehensive
State, region, and community wastewater planning reports required under Public Law 92-500.
2-5
-------
2.3 Population Served
Population is a major factor affecting wastewater flow and characteristics, and the density
of the population living or working in an area has probably the most significant immediate
effect. Other factors related to population, such as socioeconomic status, however, can also
affect the wastewater quality and flow.
2.3.1 Population Data
Population data are available from many sources:
1. U.S. Census Bureau reports.
2. Planning agencies (including municipal, State, regional, or county agencies).
3. Utility records (installation records from telephone, electricity, gas, water, and
wastewater utilities).
4. Building permits.
5. School records.
6. Chamber of Commerce.
7. Voter registration lists.
8. Post Office records.
9. Newspaper records.
2.3.2 Changes in Population
There are three primary causes of change in population: 1) the birth rate, which is affected
by cultural norms, sociopsychological factors, and socioeconomic status; 2) the death rate,
which is affected by living conditions and available medical technology; and 3) migration,
which is affected by people trying to improve their living conditions by moving into or out
of the community.
Some of the factors involved in population migration are the desire for better economic
opportunities, the availability of land for development, and the desire for benefits available
in a new area (such as transportation for workers, materials, and products; education; and
public utilities). The initiative shown by a municipality for planning housing developments
and for attracting new industry is also an important factor in population migration.
2.3.3 Population Projections
Population projections are the basis for all major planning decisions, and adequate time
should be allowed for this important work. For wastewater treatment plant design, projec-
tions of future populations are used to calculate expected quantities of wastewater and its
constituents. The methods commonly used in population projections include:
1. Graphical.
2. Mathematical (arithmetic or geometric).
3. Ratio and correlation.
2-6
-------
4. Component.
5. Employment forecast.
The selection of a particular method will depend on the amount and type of data available
and the characteristics of the area itself. Regardless of the method used, most experts agree
that the accuracy of projections is an inverse function of length of period, area, size, and
rate of population change. The area served by a small plant, although normally small, may
change at a high rate. Great care should be taken in forecasting future trends. To aid in the
selection of the best method for use in a given area, the designer should consult references
(8), (9), (14), (15), and (16).
2.4 Wastewater Flow
The standard base reference in sizing wastewater treatment units is the average daily flow—
the total quantity of liquid tributary to a plant in 1 year divided by the number of days in a
year. Design average flow is the annual average daily flow during the last year of a given
design period. Maximum daily flow is the largest volume expected during a 24-hour period
of the design year; peak flow is the maximum expected flow during that period. Minimum
daily flow is the smallest volume expected during a 24-hour period of the year; extreme
minimum flow is the minimum expected flow during that period. Peak flow is important in
the hydraulic sizing of conduits and other units to eliminate flooding. The maximum pump-
ing capacity will normally be established by the peak flow. Minimum flows are important in
sizing conduits to prevent settling of solids at low flow. Average flows are used for sizing
sludge handling facilities or determining chemical quantities. A description of the various
flows, along with their determination and use, is given in subsection 2.4.7.
An analysis of wastewater flows from smaller communities in Texas was completed in 1970
(17). These averages are plotted in Figure 2-1. Figure 2-2 shows the average daily domestic
wastewater flow rates as a function of average assessed property value in 1960 and popula-
tion size (18). Figure 2-3 shows peak and minimum flow rates as a function of population
size and average assessed valuation of property in 1960 (18).
The first of two basic methods of estimating wastewater flows is to gage the flows in an
existing system and project these flows for the expected development of the area. The
second method is to estimate the total flow from each of the various components of the
served area, such as domestic, commercial, institutional, industrial, stormwater, and ground-
water contributions. Wastewater flows for the various components are often estimated using
the average values from one or more of many references, as shown in Tables 2-1, 2-2, and
2-3 (19). Other similar tables can be found in references (20) and (21). These values have
been developed only as a guide. The actual values can vary greatly, depending on factors
that will be discussed below. In all cases, any values used in the design should be checked
with the regulatory agencies.
2-7
-------
160
140
120
Q
O 100
Q.
O
**
o
-I 80
tr
UJ
lit
60
40
20
10 20 30 40
POPULATION (1,000'S)
50
*AVERAGE GEOMETRIC DEVIATION WAS
APPROXIMATELY 1.4 TO 1.6.
FIGURE 2-1
EFFECTS OF POPULATION ON
WASTEWATER FLOWS IN TEXAS (17)
2-8
-------
1000
UJ
I-
:D
z
(T
UJ
Q.
to
Z
o
(9
•b
O
_l
u.
100
100 1000
POPULATION, PERSONS
10,000
FIGURE 2-2
AVERAGE DOMESTIC WASTEWATER FLOW RATE VERSUS POPULATION
AS A FUNCTION OF AVERAGE ASSESSED VALUATION OF PROPERTY IN 1960 (18)
2-9
-------
loop
UJ
I-
3
z
3E
cr
UJ
Q.
(O
Z
o
CD
*
O
100
FLOW WHICH WILL BE EXCEEDED
975% OF ALL DAYS
FLOW WHICH WILL BE EXCEEDED
2.5% OF ALL DAYS
I 25,000
! 22,500
— $20,000
" 17,500
15,000
—$ 12,500
$10,000
$7,500
$5,000
100 1000
POPULATION, PERSONS
10,000
FIGURE 2-3
UPPER AND LOWER 2.5% PREDICTION LIMITS FOR AVERAGE DAILY
WASTEWATER FLOW RATES VERSUS POPULATION AS A FUNCTION OF
AVERAGE ASSESSED VALUATION OF PROPERTY IN 1960 (18)
2-10
-------
TABLE 2-1
QUANTITIES OF WASTEWATER (19)
Gallons Per Person Per Day1
Type of Establishment (Unless Otherwise Noted)
Airports (Per Passenger) 5
Apartments, Multiple Family (Per Resident) 60
Bathhouses and Swimming Pools 10
Camps
Campground With Central Comfort Stations 35
With Flush Toilets, No Showers 25
Construction Camps (Semipermanent) 50
Day Camps (No Meals Served) 15
Resort Camps (Night and Day) With Limited Plumbing 50
Luxury Camps 100
Cottages and Small Dwellings With Seasonal Occupancy 50
Country Clubs (Per Resident Member) 100
Country Clubs (Per Nonresident Member Present) 25
Dwellings
Boarding Houses 50
Additional for Nonresident Boarders 10
Luxury Residences and Estates 150
Multiple Family Dwellings (Apartments) 60
Rooming Houses 40
Single Family Dwellings 75
Factories (Gallons Per Person, Per Shift, Exclusive of
Industrial Wastes) 35
Hospitals (Per Bed Space) 250+
Hotels With Private Baths (Two Persons Per Room) 60
Hotels With Private Baths 50
Institutions Other Than Hospitals (Per Bed Space) 125
Laundries, Self-service (Gallons Per Wash, i.e., Per Customer) 50
Mobile Homes Parks (Per Space) 250
Motels With Bath, Toilet, and Kitchen Wastes
(Per Bed Space) 50
Motels (Per Bed Space) 40
Picnic Parks (Toilet Wastes only) (Per Picnicker) 5
Picnic Parks With Bathhouses, Showers, and Flush Toilets 10
Restaurants (Toilet and Kitchen Wastes Per Patron) 10
Restaurants (Kitchen Wastes Per Meal Served) 3
Restaurants Additional For Bars and Cocktail Lounges 2
Schools
Boarding 100
Day, Without Gyms, Cafeterias, or Showers 15
2-11
-------
TABLE 2-1 (continued)
QUANTITIES OF WASTEWATER (19)
Gallons Per Person Per Day1
Type of Establishment (Unless Otherwise Noted)
Schools (continued)
Day, With Gyms, Cafeterias and Showers 25
Day, With Cafeteria, But Without Gyms, or Showers 20
Service Stations (Per Vehicle Served) 10
Swimming Pools and Bathhouses 10
Theaters
Movie (Per Auditorium Seat) 5
Drive-in (Per Car Space) 5
Travel Trailer Parks Without Individual Water and Sewer
Hookups (Per Space) 50
Travel Trailer Parks With Individual Water and Sewer
Hookups (Per Space) 100
Workers
Construction (at Semipermanent Camps) 50
Day, at Schools and Offices (Per Shift) 15
11 gallon/person/day = 3.785 liters/person/day.
2-12
—T
-------
TABLE 2-2
PACKAGE TREATMENT PLANT SIZING DATA (21)
Type of Facility
Airports (Per Passenger)
Airports (Per Employee)
Apartments—Multiple Family
Boarding Houses
Bowling Alleys—Per Lane (No Food)
Campgrounds—Per Tent or Travel Trailer Site-
Central Bathhouse
Camps—Construction (Semipermanent)
Camps—Day (No Meals Served)
Camps—Luxury
Camps-Resort (Night and Day) With
Limited Plumbing
Churches—Per Seat
Clubs-Country (Per Resident Member)
Clubs—Country (Per Nonresident Member Present)
Courts—Tourist or Mobile Home Parks With
Individual Bath Units
Dwellings—Single-Family
Swellings—Small, and Cottages With
Seasonal Occupancy
Factories-(Gallons, Per Person, Per Shift,
Exclusive of Industrial Wastes), No Showers
Add for Showers
Hospitals
Flow Rate
gpcd1
50
25
10
2503
BOD5
Ib/cap/day2
5
15
75
50
75
50
50
15
100
50
5
100
25
50
75
0.020
0.050
0.175
0.140
0.150
0.130
0.140
0.031
0.208
0.140
0.020
0.208
0.052
0.140
0.170
0.140
0.073
0.010
0.518
Runoff
hr
Shock Load
Factor3
16
16
16
16
8
16
16
16
16
16
4
16
16
16
16
16
8
16
low
low
medium
medium
medium
medium
medium
medium
medium
medium
high
medium
medium
low
medium
medium
high
medium
-------
TABLE 2-2
PACKAGE TREATMENT PLANT SIZING DATA (21)
Type of Facility
Hotels-With Private Baths (Two Persons Per Room)
Institutions—Other Than Hospitals (Nursing Homes)
Laundromat
Motels-(Per Bed Space)
Motels-With Bath, Toilet, and Kitchen Wastes
Offices—No Food
Parks-Picnic (Toilet Wastes Only)
(Gallons Per Picnicker)
Parks—Picnic, With Bathhouses, Showers, and
Flush Toilets
Restaurants—(Kitchen Wastes Per Meal Served)
Restaurants—(Toilet and Kitchen Wastes Per Patron)
Restaurants-Additional for Bars and Cocktail Lounges
Schools—Boarding
Schools—Day, Without Cafeterias, Gyms, or Showers
Schools—Day, With Cafeterias, But No Gyms or Showers
Schools—Day, With Cafeterias, Gyms and Showers
Service Stations—(Per Vehicle Served)
Shopping Centers—(No Food—Per Sq Ft)
Shopping Centers—(Per Employee)
Stores-(Per Toilet Room)
Swimming Pools and Bathhouses
Sports Stadiums
Flow Rate
gpcd1
60
125
400
40
50
15
BODS
lb/cap/day2
0.125
0.260
varies
0.083
0.140
0.050
0.010
Runoff
hr
16
16
12
16
16
8
8
Shock Load
Factor3
medium
medium
high
medium
medium
high
high
10
7
10
3
100
15
20
25
12
0.1
15
400
10
5
0.021
0.015
0.021
0.006
0.208
0.031
0.042
0.052
0.021
0.050
0.832
0.024
0.020
8
8-12
8-12
8-12
16
8
8
8
8
16
16
16
8
4-8
high
high
high
high
medium
high
high
high
high
medium
medium
medium
high
very high
-------
TABLE 2-2
PACKAGE TREATMENT PLANT SIZING DATA (21)
Shock Load
Type of Facility Flow Rate BOD5 Runoff Factor3
gpcd1 Ib/cap/day2 hr
Theatres-Drive-in (Per Car Space) 5 0.010 6 high
Theatres-Movie (Per Auditorium Seat) 5 0.010 6 high
Trailer Parks-Per Trailer 150 0.350 16 medium
11 gal/capita/day = 1 liter/capita/day
21 Ib/capita/day = 0.4536 kg/capita/day
-*The shock load factor would be difficult to quantify, but is included as an indication of possible conditions causing high flow or
strong concentrations.
-------
to
o\
Class
Subdivisions, Better
Subdivisions, Average
Subdivisions, Low Cost
Motels, Hotels, Trailer Parks
Apartment Houses
Resorts, Camps, Cottages
Hospitals
Factories or Offices
Factories, Including Showers
Restaurants
Schools, Elementary
Schools, High
Schools, Boarding
Swimming Pools
Theatres, Drive-In
Theatres, Indoor
Airports, Employees
Airports, Passengers
TABLE 2-3
TYPICAL DAILY FLOWS AND BOD CONSIDERATIONS1 (23)
Persons
per Unit
Daily Flow
gal/person2
BOD
Ib/person
With Garbage
Average Grinder
Average
Wastewater BOD
mg/13
3.5
3.5
3.5
2.5
2.5
2.5
Per Bed
Per Person
Per Person
Per Meal
Per Student
Per Student
Per Student
Per Swimmer
Per Stall
Per Seat
Per Employee
Per Passenger
100
90
70
50
75
50
200
20
25
5
15
20
100
10
5
5
15
5
0.17
0.17
0.17
0.17
0.17
0.17
0.30
0.06
0.07
0.02
0.04
0.05
0.17
0.03
0.02
0.01
0.05
0.02
0.25
0.23
0.20
0.20
0.25
0.20
0.35
—
—
0.06
0.05
0.06
0.20
—
—
—
—
—
205
220
290
400
225
400
200
360
340
450
320
360
205
360
450
250
450
480
-------
TABLE 2-3
TYPICAL DAILY FLOWS AND BOD CONSIDERATIONS1 (23)
NJ
!•"•
o
Class
Bars, Employees
Bars, Customers
Dairy Plants
Public Picnic Parks
Country Clubs, Residents
Country Clubs, Members
Public Institutions
(nonhospital)
Persons
per Unit
Per Employee
Per Customer
Per 1,000 Ib Milk
Per Picnicker
Per Resident
Per Member
Per Resident
Daily Flow
BOD
Ib/person
With Garbage
Average Grinder
Average
Wastewater BOD
gal/person^
15
2
100-250
5-10
100
50
0.05
0.01
0.56
0.01
0.17
0.17
—
—
to 1.66
—
0.25
0.20
mg/13
450
800
650-2000
250
205
400
100
0.17
0.23
205
* Consult with your State and local health department or pollution control agency for specific data.
21 gal = 3.785 liters.
31 Ib BOD = 0.454 kg BOD.
-------
2.4.1 Domestic Component
The average wastewater flow from small residential areas is normally derived from the
population density times the estimated per capita wastewater flow rate. If available, water
records may be used to determine per capita consumption, and from this, per capita waste-
water flow can be estimated. The proportion of municipal water supply that reaches the
sewer is normally about 60 to 80 percent of consumption, depending on climate (20). How-
ever, the percentage can vary from as low as 40 to greater than 100 and is affected by such
factors as leakage, lawn sprinkling, swimming pools, firefighting uses, consumers not con-
nected to sewers, infiltration, and water from private sources discharged to the sewer.
2.4.2 Commercial Component
In small communities with only a small amount of commercial development, the com-
mercial contribution can be assumed to be included in the per capita domestic wastewater
flow. In areas where commercial development is significant, the wastewater quantities can
be estimated in terms of gpd/acre (1/d-ha), based on comparative data from existing devel-
opments. Commercial wastewater flow can vary greatly, and thus a careful comparison of
commercial areas is important. Factors such as the amount and type of water-cooled air
conditioning utilized can greatly influence the estimated wastewater flow.
2.4.3 Institutional and Recreational Components
Institutions such as hospitals, schools, or prisons, and facilities such as campgrounds, resorts,
motels, hotels, trailer villages, contribute flows that are primarily domestic. The best
method of determining flows from these facilities is by gaging existing flows or comparing
these flows with flows from similar facilities. Sources such as schools, campgrounds, and
resorts will have large seasonal and weekend variations in flow.
2.4.4 Industrial Component
The industrial component can vary greatly, depending on the type of industry, its size and
supervision, and the control of various processes within the operation.
Industrial wastewater quantities are normally determined by gaging existing flows and com-
paring them with similar industries. In all cases the flow determination, whether from exist-
ing industry or new industry, should be accompanied by thorough investigations conducted
with the industrial representatives. Some of the important questions to be asked are:
1. What quantities of water can be reused within the industry for process cooling,
lawn irrigation, or other possible functions?
2. What control is required to prevent dumping of large quantities of wastewater?
3. What effect do waste constituents and their concentration have on the allowable
flow rate to the sewer?
4. Which pollutants can be eliminated or reduced by changes in manufacturing pro-
cesses or by in-plant treatment and recycling?
2-18
-------
5. Will detention tanks, equalization tanks, or pretreatment reduce problems of
wastewater discharges?
6. Can nonpolluted water sources such as air conditioning or industrial cooling water
be separated from the wastewater system?
7. Is pretreatment of industrial wastes desirable—how much, type, etc.?
2.4.5 Stormwater and Groundwater Components
Stormwater and groundwater can enter a collection system from many sources and in quan-
tities which can significantly affect a small treatment facility. The quantity of extraneous
water can be greatly reduced by proper inspection, repair, and construction of a collection
system.
Public Law 92-500, passed in 1972 to amend the Federal Water Pollution Control Act, re-
quires that sewer systems be free of any excessive extraneous water. In addition, grants for
treatment works made available through this act will not be approved without sufficient
evidence that the sewer system discharging to the treatment works is not subjected to ex-
cessive infiltration. The law does authorize funds for the required studies of the systems.
The sources of extraneous water to be investigated in this study can be classified as infiltra-
tion and inflow. Infiltration is normally groundwater that enters a sewer through openings
such as defective sewer pipe, inadequate pipe joint connections, and cracks in manhole
walls. Groundwater infiltration will normally increase with rises in groundwater levels,
porous subsoil conditions, defective material and poor workmanship. Poorly laid house
connections, especially, can have a great effect on the amount of infiltration, and therefore
the installation of such connections should be thoroughly checked for leakage. Additional
information on infiltration and allowances for design can be found in references (24) and
(20).
Stormwater inflow is considered to be water that enters a sewer system from sources such as
roof or floor drains, footing drains, cooling water discharges, manhole covers, or connec-
tions with storm sewers. This water normally does not require as much treatment as would
be available in a wastewater treatment works and should be eliminated from the sanitary
system. A more detailed discussion can be found in the ASCE/WPCF Sewer Design Manual,
MOP-9 (24).
2.4.6 Wastewater Flow Variations
Wastewater flow varies by day, week, and year with variations in water consumption, infil-
tration, and inflow. The amount of variation tends to increase with a decrease in sewer
system size, because of the lack of damping effects from longer flow times found in larger
systems. The ratio of peak flow to extreme minimum will vary from about 10 to 1 for
systems serving populations of about 10,000, to greater than 20 to 1 for some small systems
in which domestic wastewater is the major component of the total flow. This large variation
is particularly true of flow from sources such as large regional schools or a complex of apart-
ment buildings, where there may not be any flow during part of the night. The extreme low
2-19
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flow usually occurs between 2 a.m. and 6 a.m., with two peaks occurring during the daylight
hours around 9 a.m. and 6 p.m. The amplitude and time of peaks or depressions are directly
related to the lifestyle of the population served. An example of daily household water use
variation is shown in Figure 2-4 for several selected homes in Colorado, which are represen-
tative of families residing in suburban residential developments (25).
The daily variation caused by commercial or institutional sources are often more uniform
and will tend to damp the household variations. Exceptions to this are hotels, motels, or
similar establishments, which usually have variations coinciding with households.
The effect of industrial flow on daily variations will depend on the type of processes used
and the waste discharges involved. In many instances, an industrial discharge can be con-
trolled so that it can have an equalizing effect on total waste flow.
Weekly variations are most often caused by commercial, industrial, or recreational sources.
Household waste flow is generally not varied throughout a week, with the exception of an
activity such as washday. Waste flow from commercial or industrial sources tends to be uni-
form during daytime of the 5 working days, with heaviest discharges from recreational
sources occurring during weekends. If both sources are located on the same system, one may
tend to offset the other.
Yearly variations are most often caused by industries, recreational activities, or institutional
sources. Some industries are seasonal and have discharges which change throughout the year.
(For instance, food-processing industries operate at their peak when crops are being har-
vested.) Other variations in industrial waste can be caused by the changes in production re-
quired by demand. Recreational activities such as those in beach resorts, camps, or ski
areas will cause changes in waste flow during the year. Significant changes in flow can occur
where there are schools, such as regional schools or colleges in small towns where the
students contribute a large part of the wastewater load on the treatment plant.
Infiltration and inflow will also cause important variations during wet seasons. Infiltration
can affect variations in flow by raising or lowering the base flow level. Although infiltration
is relatively steady, it can change during the year and even be absent for several months,
depending on the location and quality of the system. During the wet season of the year
when groundwater is high, infiltration can account for a large percentage of the flow.
Variations in wastewater flow depend greatly on the size of the collection system and
population density (see Figure 2-5). If a large system has a low population density, the varia-
tions will be reduced because of higher infiltration, which also reduces the wastewater
strength. If the density increases, the ratio of wastewater to infiltration will increase, causing
larger variations and stronger wastewater. Short-term flow variations affected by distance
and long-term variations affected by infiltration and population density must be considered.
A final and very important source of damping, either at the source of industrial discharges,
at a pumping station, or at the treatment works, is flow equalization. The subject of flow
equalization is covered in more detail in Chapter 4.
2-20
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HOUSEHOLD WATER USE G.P.H./HOME
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m
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D
1st STAGE PLANT
DESIGNED TO SERVE
SATURATED
POPULATION
ODD DODOO
A. SMALL COLLECTION SYSTEM-HIGH POPULATION DENSITY
1st STAGE PLANT
B. LARGE NEW COLLECTION SYSTEM-LOW POPULATION DENSITY
FIGURE 2-5
RELATION OF SMALL PLANT DESIGN TO COLLECTION SYSTEM
2-22
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2.4.7 Design Flow Rates
In general, it has been found that smaller communities (fewer than 50,000 people) generate
smaller wastewater flow per capita than larger communities. The smaller the community,
the less the infiltration, ordinarily, and the larger the percentage of household wastewater in
relation to commercial or industrial wastewater. Williamson (17), in Texas (1971), developed
a characteristic wastewater flow curve (shown in Figure 2-1) in which the geometric standard
deviation was from about 1.4 to 1.6.
In designing wastewater treatment works, there are five flows that should be considered in
process selection, equipment sizing, and analysis of operation: peak, maximum 24-hour,
average daily, minimum 24-hour, and extreme minimum. The processes selected should be
able to operate efficiently over the entire range. (A more detailed discussion of how flow
variation affects design and operation is given in the relevant process chapters.) Each of
these flows should be determined for 1) the initial conditions of operation, 2) the future
conditions at the end of the design period, and 3) if staged construction is used, the various
periods of development.
The maximum and minimum flows can be found by two common methods. The first, using
graphs that have been developed from existing flow records (available from a number of
sources), compares the average flow with the other important rates. Figure 2-6 shows the
ratio of extreme flows to average daily flow in New England based on dry weather maxi-
mums for domestic wastewaters. Other examples can be found in the ASCE/WPCF MOP-9
(24).
For flows from large buildings such as schools, apartments, hotels, or hospitals, the peak
discharge may be estimated by the fixture-unit method. This method uses load factors for
common plumbing fixtures, which are weighted values related to the flow rate for each type
of fixture. These factors are expressed as fixture units, which can be defined as approxi-
mately 1 cfm of flow. Table 2-4, taken from the National Plumbing Code (26), lists fixture
units per fixture or group. To determine the peak flow, the total number of fixture units
within the system is found, using the load factors and number of each type of fixture. The
total fixture unit value is then used on curves developed by R.F. Hunter (27) to determine
probable peak flow. Figure 2-7 is based on Hunter's curves.
The Hunter method is usually conservative, because it makes minimum allowance for factors
that cause damping. The average single-family house or apartment has about 12 fixture
units and about 4 persons per family. The number of fixture units varies from one location
to another; therefore, care should be taken using any one figure for different communities.
2.5 Wastewater Characteristics
Wastewater is used water containing suspended and dissolved substances from sources such
as residences, commercial buildings, industrial plants, and institutions, along with ground-
water and stormwater. Depending on amount, type, and form, these wastes will impart
various characteristics to the flows. An understanding of these characteristics, which can be
2-23
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IS .2 25 .3 .4 .5 .6 .7 .8 .9 LO 1.5 2
o
E
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0
5
o
Ul
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UJ
o
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<
EXTREME MINIMUM ON MINIMUM DAY
.15 .2 .25 .3 .4 .5 .6 .7 .8 .9 1.0
AVERAGE DAILY DISCHARGE (MGD)
1.5
FIGURE 2-6
VARIATIONS IN DAILYWASTEWATER FLOW
2-24
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450
400
to
to
Q.
3
o
Ul
QL
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CD
O
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350
300
250
200
150
100
I I I I I I I I
NOTE; CURVES SHOW PROBABLE AMOUNT |
OF TIME INDICATED PEAK FLOW WILL
BE EXCEEDED DURING A PERIOD OF
CONCENTRATED FIXTURE USE
_SYSTEM IN WHICH FLUSH
VALVES PREDOMINATE
SYSTEM IN WHICH FLUSH
TANKS PREDOMINATE
68 10 12 14 16 18 20 22 24
FIXTURE UNITS ON SYSTEM (HUNDREDS)
26
28
FIGURE 2-7
ESTIMATE CURVES FOR DESIGN PURPOSES
SHOWING RELATION OF DISCHARGE TO FIXTURE UNITS
-------
TABLE 2-4
FIXTURE UNITS PER FIXTURE OR GROUP (26)
Fixture or Group Fixture Unit Value as Load Factors
One Bathroom Group Consisting of Tank-Operated
Water Closet, Lavatory, and Bathtub or
Shower Stall 6
Bathtub1 (With or Without Overhead Shower) 2
Bidet 3
Combination Sink-and-Tray 3
Combination Sink-and-Tray With Food-Disposal Unit 4
Dental Unit or Cuspidor 1
Dental Lavatory 1
Drinking Fountain 1 /2
Dishwasher, Domestic 2
Floor Drains 1
Kitchen Sink, Domestic 2
Kitchen Sink, Domestic, With Food Waste Grinder 3
Lavatory 1
Lavatory 2
Lavatory, Barber, Beauty Parlor 2
Lavatory, Surgeon's 2
Laundry Tray (One or Two Compartments) 2
Shower Stall, Domestic 2
Showers (Group) Per Head 3
Sinks:
Surgeon's 3
Flushing Rim (With Valve) 8
Service (Trap Standard) 3
Service (P Trap) 2
Pot Scullery, etc. 4
Urinal, Pedestal, Siphon Jet, Blowout 8
Urinal, Wall Lip 4
Urinal Stall, Washout 4
Urinal Trough (Each 2-ft Section) 2
Wash Sink (Circular or Multiple), Each Set of Faucets 2
Water Closet, Tank-Operated 4
Water Closet, Valve-Operated 8
1A shower head over a bathtub does not increase the fixture value.
Note: For a continuous or semicontinuous flow into a drainage system, such as from a
pump, pump ejector, air-conditioning equipment, or similar device, two fixture units shall
be allowed for each gpm of flow.
2-26
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TABLE 2-5
WASTEWATER CHARACTERISTICS (22)
Physical Chemical Biological
Solids Organics Protista
Temperature Proteins Viruses
Color Carbohydrates Plants
Odor Fats, Oils, and Grease Animals
Surfactants Pathogens
Phenols
Pesticides
Inorganics
pH
Chlorides
Alkalinity
Nitrogen
Phosphorus
Sulfur
Toxic Compounds
Heavy Metals
Gases
Oxygen
Hydrogen Sulfide
Methane
divided into biological, chemical, and physical, is essential in the design and operation of a
treatment works. A detailed breakdown of these characteristics is given in Table 2-5 (22)
and Figure 2-8 (22). Descriptions of these characteristics and methods for their analysis can
be found in Standard Methods (28), Proposed Criteria for Water Quality (29) (30), and else-
where (31) (32) (33). In this chapter, only the general characteristics and their effects on
small treatment plants will be discussed. The specific effects of wastewater characteristics on
the processes are described in the relevant process chapters.
2.5.1 Physical Characteristics
The more important physical characteristics of wastewater include the various types of
solids present and the temperature, color, and odor.
2.5.1.1 Solids
Total solids, including floating matter, can be divided into suspended (settleable and nonset-
tleable), colloidal, and dissolved solids. Each of these can then be divided into organic and
mineral solids. Total solids can be defined as all matter that remains as residue upon evapo-
ration at 105° C (or 180° C, to include metallic hydroxides if the pH is greater than 9).
2-27
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N)
OO
GENERAL ANALYSIS
Color
Turbidity
Temperature
Toxicity
Odor
1 Organic
Anoly»i«
1
£2,- ESS
Dwond Concwitration
I
Calculattd ......
ThOD TOC
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j 1 TOD [
1 1
•tJn'da'rd COD
mtthodi ™'w '•"
L ._ J ,
01 UOD
c«ous r» |
fj
out f* 1 '
J ratio
COD/ BOD
TOC/ BOO
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1
Organic
groups
1 1
Total out 1 Total Phmohes | o^owT<
| 1
(|f |
1 FrM 1 1 EmuMwdl 1 „...„, 1 Subttituttd
| Oi II O'l II 1 Pl»™>l«:
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FIGURE 2-8
SCHEMATIC DIAGRAM OF WASTEWATER CHARACTERISTICS (22)
-------
Solids can be classified by filtering a known volume of wastewater and determining the
weight of suspended solids (SS) greater than about 0.5 to 1 micron (micrometer, /urn) in
diameter. Suspended solids can be divided using a cone-shaped container (Imhoff cone) to
determine the solids that will settle in 60 minutes. A settleometer test, using a cylinder 6 in.
(152 mm) high and 5 in. (127 mm) in diameter and measuring the rate of settling and con-
solidation, is coming into common use for operations.
Filterable solids consist of colloidal and dissolved solids. The colloidal solids consist of
particles between about 1 millimicron (nanometer, nm) and about 0.5 to 1 ju. Bacteria,
viruses, phages, and other cellular debris fall generally into the colloidal fraction. Dissolved
solids consist of molecules or ions present in true solution in water. Although the average SS
concentration of municipal wastewater generally ranges from 150 to 300 mg/1, this value
may vary widely. The fraction dissolved may also vary widely, depending on the age and
sources of the wastewaters. The longer the travel time in sewers, for example, the larger the
fraction of dissolved solids because of microbial activity; the shorter the travel time the
smaller the fraction of dissolved solids.
Nonfilterable solids, along with the nonsettleable portion of the SS, are not normally
removed by plain sedimentation, and therefore require biological oxidation or chemical
coagulation, with sedimentation for removal.
Each type of solids can be subdivided into organic (volatile) and inorganic (ash) fractions,
based arbitrarily on volatility at 550° C. (At this temperature, the organic or volatile frac-
tion is driven off as gas, and the inorganic, mineral, or fixed fraction will remain as ash.) The
volatile solids content of sludge indicates the quantity of organic solids in an activated
sludge system or the stability of sludge entering or leaving digesters. Settleable solids indi-
cate the quantity of sludge that can be removed by plain sedimentation. If a wastewater
contains few Settleable solids, the selection of a process including primary sedimentation
may not be justified. Similarly, if the Settleable solids are largely organic and the following
process is extended aeration, the primary clarifier may be omitted.
2.5.1.2 Temperature
Temperature of wastewater varies throughout the year and with location. During most of
the year, the temperature of the wastewater is higher than the ambient temperature. Only
during the hot summer months will the temperature be less than the ambient. The amount
of hot water used in households or received from industries will keep the wastewater
warmer than the ambient environment, and in the case of industrial discharges can raise the
temperature above the normal range. Optimum temperatures for bacterial activity are from
about 25° C to 35° C. Aerobic digestion and nitrification stops when the temperature rises
to 50° C. When the temperature drops to about 15° C, methane-producing bacteria become
quite inactive, and at about 5° C, the autotrophic nitrofying bacteria practically cease func-
tioning. At 2° C, even the heterotrophic bacteria acting on carbonaceous material become
essentially dormant.
2-29
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Wastewater temperature is important because of its effects on aquatic life, chemical and bio-
logical reactions and reaction rates, and suitability for reuse. Changes in temperature be-
cause of effluent discharges can cause changes in the type of aquatic life. In many cases,
warmer water will promote the growth of undesirable water plants and wastewater fungi.
Higher temperatures will tend to increase both biological and chemical reaction rates.
Oxygen is less soluble in warm water, however, and this will decrease the amount of oxygen
transfer. These factors make the design of efficient aeration or ventilation systems critical in
biological treatment, to take full advantage of the increased biological activity if oxygen
transfer is difficult.
In colder climates, freezing is an important consideration in selection and design of units.
Trickling filters can develop ice formation, which may stop rotation of distribution arms.
Spray from mechanical aerators can cause ice to form on the aerators and the platforms
and can cause them to freeze solid if there is a power failure.
2.5.1.3 Color
Color of wastewater is normally used to describe the condition of waste within the treat-
ment and disposal process. Fresh wastewater usually has a gray tint; as the organic material
is broken down and the oxygen is depleted, the color changes to black. Black wastewater is
normally anaerobic or septic. In some locations the color is changed by industrial waste dis-
charges. Changes in color should warn an operator of possible process upsets or failures.
2.5.1.4 Odor
Odor in wastewater is associated with decomposing and putrescent organic matter. If indus-
trial wastes are added, odor may be caused by chemical compounds such as ammonia,
phenol, sulfide, and cyanide. Odor and color can indicate the condition of wastewater.
Fresh wastewater has a distinct, musty odor, which is much less offensive than that of septic
or anaerobic wastewater. More information on odor prevention and control is presented in
section 2.6.
2.5.2 Chemical Characteristics
Chemically, wastewater can be described roughly by its organic solids, inorganic solids, and
dissolved gas constituents, which are closely related and often interact. This interaction can
be both beneficial and detrimental to treatment and disposal. An understanding of the
various chemical characteristics and a complete analysis of the waste constituents are needed
during design to reduce the detrimental effects and to take advantage of the benefits.
2.5.2.1 Organic Matter
Organic matter is present in settleable, nonsettleable, colloidal, and dissolved solids, and
accounts for a large part of the pollutants. In average strength wastewater, the SS are about
75 percent organic matter and the filterable solids significantly lower. The settleable
2-30
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organics can be removed by plain sedimentation. Other solids require biological flocculation,
assimilation, or various forms of chemical or physical treatment.
Organic substances include proteins, carbohydrates, hexane solubles (fats, greases, oils, etc.),
surfactants, phenols, and pesticides. Many of the substances are readily biodegradable; if
discharged unaltered to a receiving water, they would cause a depletion of the available
oxygen and could damage aquatic life and cause a change from aerobic to anaerobic condi-
tions. Biological treatment units take advantage of this biodegradability: microorganisms
growing under proper conditions (adequate oxygen, temperature, pH, etc.) oxidize the
organics to a stable form that can be removed under controlled conditions.
Other organic substances, such as some detergents and chlorinated hydrocarbons, are not
easily biodegradable, or are degradable but toxic to most microorganisms. These substances
can usually be removed by physical-chemical treatment.
Basic tests to determine the organic content of wastewater are the biochemical oxygen
demand (BOD), chemical oxygen demand (COD), and total organic carbon (TOC) tests.
Other methods include determination of the volatile solids fraction of total solids; total,
albuminoid, organic, and ammonia nitrogen; or oxygen consumed.
BOD can be defined as the approximate quantity of oxygen that will be used by micro-
organisms in the biochemical oxidation of organic matter. It indicates the strength of
domestic and industrial waste in terms of the oxygen required if the flow were discharged
into a natural watercourse. In treatment plant design, BOD is one of the parameters used for
the selection and sizing of units. The bottle test for BOD is the one commonly referred to in
the following discussion.
The major reasons for the wide use of the BOD bottle test are, first, the test does not re-
quire expensive equipment, and second, it has been the simplest test to measure the amount
of organic matter that will be biologically degraded under relatively natural conditions.
The disadvantage of this test is that it is essentially a bioassay, which requires time and con-
trolled conditions. The BOD test does not necessarily represent actual field conditions, be-
cause temperatures, concentrations, mixing levels, and seed bacteria in the bottle vary from
actual conditions. The conditions required to allow aerobic living organisms to function
uninhibited include proper temperature, sufficient oxygen, all needed nutrients for bacterial
growth, and no toxic substances. The conditions in a diluted sample of strong wastewater
held in a bottle for several days at a constant temperature may not be representative of
actual conditions in the treatment process or stream.
There are several important measures of the oxygen demand of a wastewater: 5-day BOD
(BOD5), ultimate carbonaceous oxygen demand (UBOD), ultimate oxygen demand (UOD),
and nitrogenous oxygen demand (NOD). In a 20° C BOD bottle test, the bacteria present
break down organic material by metabolic reactions. Carbohydrates and sugars and then
proteins are broken down to simpler compounds. The heterotrophic bacteria that oxidize
carbonaceous material reproduce rapidly. After the nitrogenous material is broken down
2-31
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and hydrolyzed to ammonia, the more slowly reproducing autotrophic (nitrifying) bacteria
react. If they are only present in small numbers, little if any NOD will be satisfied until 6 to
10 days have passed. The BOD5 test, then, usually represents only carbonaceous demand
and averages about two-thirds of the UBOD. By the 25th to 30th day both the UBOD and
the NOD are usually fairly well satisfied. The UBOD curve added to the NOD curve pro-
duces the UOD curve.
A sample taken from a trickling filter or activated sludge unit can contain a significant
amount of nitrifying bacteria; this amount will cause early nitrification in the bottle and
affect the test. If nitrifying bacteria are present in large quantities, they should be inhibited
for more meaningful results of the BOD5 test, or to obtain UBOD. For more detailed dis-
cussion see references (20), (28), and (32).
NOD can be determined by running two series of BOD tests on the same wastewater: one
series with nitrifying bacteria inhibitor and one series without. Readings should be taken
from the 3d day of incubation to the 30th, and the results plotted for each series. The
difference in the final set of curves represents the NOD. This parameter can also be obtained
by multiplying the ammonia-nitrogen (determined by the total Kjeldahl nitrogen test) by
4.6.
Chemical oxygen demand (COD) is a measure of the strength of domestic and industrial
wastes in terms of the total amount of oxygen required to oxidize most organic matter to
carbon dioxide and water. The major limitation of this test is the inability to distinguish be-
tween biologically oxidizable and biologically inert organic matter. In the absence of cata-
lysts, COD results do not include biologically oxidizable acetic acid, aromatic hydrocarbons,
and straight chain aliphatics, but do include nonbiodegradable cellulose. As a result, the
COD of domestic wastewater is normally higher than the BOD5. The BOD5 and COD tests
can be correlated for a particular waste as long as the ratio of biodegradable to nonbiode-
gradable organics does not change. This ratio can vary greatly between units in a treatment
plant. The major advantages of the COD are that the test is less time consuming (3 hours
rather than 5 days) and more reproducible. The test is also useful if the wastewater contains
toxic substances, or for indicating the presence of toxic substances.
Total organic carbon (TOC) is another test that measures organic matter in wastewater. A
small known quantity of sample is injected into a high temperature furnace. The organic
matter is oxidized in the presence of a catalyst to carbon dioxide, and the carbon dioxide is
measured with an infrared analyzer. The test requires relatively expensive equipment, but
results are obtained very rapidly.
2.5.2.2 Inorganic Matter
The most important inorganic substances present in wastewater include acidic and basic
compounds of nitrogen, phosphorus, chlorine, and sulfur; toxic compounds; and heavy
metals. These components alone or by interaction with other wastewater components can
affect the growth of organisms, cause corrosion, or produce odors. Inorganic dissolved solids
can be measured in terms of specific conductance in micromhos (per cm3), using a conduc-
2-32
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tivity meter which correlated for that specific wastewater. This test is very temperature de-
pendent.
The pH can affect both the treatment methods and metal equipment exposed to the waste-
water. Biological, chemical, and physical treatment processes operate optimally in specific,
but often different, ranges of pH. The natural alkalinity of wastewater in many cases will be
a sufficient buffer to keep the pH within the normal, fairly neutral range for best biological
activity. If the pH goes outside this range, biological treatment may not be feasible. A high
or low pH waste could also cause corrosion problems.
Alkalinity is important in chemical treatment such as coagulation, chlorination, or ammonia
removal by stripping (see Chapter 12).
Nitrogen, carbon, phosphorus, and certain trace elements are essential to the growth of
plants and animals. In the design of biological treatment units, the nutrients available may
limit the treatability of the waste. If an adequate amount of the essential nutrients is not
available, it may be necessary to add these nutrients to the influent. For further discussion
see Section 2.5.5.
Nitrogen is present in nature in five principal forms: organic nitrogen, ammonia, nitrite,
nitrate, and gaseous elemental nitrogen. Organic nitrogen is normally contained in plant and
animal protein. Ammonia is produced by decomposition of organic matter, chemical manu-
facture, or bacterial reduction from nitrites. Nitrates are formed by bacterial oxidation of
ammonia to nitrites and then to nitrates. Gaseous nitrogen is produced under anaerobic con-
ditions, if small amounts of carbon are present, by reduction of nitrates to nitrites and then
to nitrogen gas. Nitrates are a necessary constituent of plant fertilizer and are changed to the
organic form by plants. For a detailed discussion of the nitrogen cycle, see references (20)
and (32).
Raw wastewater contains mostly organic and ammonia nitrogen (the fresher the wastewater,
the more organic nitrogen). During aerobic biological treatment, the organic nitrogen is re-
moved or converted to other forms. Depending on the time provided for treatment, any
ammonia-nitrogen present may be oxidized to nitrite and nitrate by two specific forms of
bacteria. If waste is discharged before nitrification occurs, the effluent will contain
ammonia. Ammonia in the effluent can be detrimental for several reasons: first, it is toxic
to some plant and animal life, and second, the oxidation to nitrate can occur in the receiving
water and thus use up large quantities of the free oxygen. In addition, the presence of
ammonia can hinder chlorination or seed germination. Nitrate in the effluent, although it is
a secondary oxygen source, is also detrimental. Because of its nutrient value, it promotes ex-
cessive growth of algae or other organisms. In the case of reclaimed wastewater to be used
for ground water recharge, nitrate in drinking water can have serious and sometimes fatal
effects on infants. The type of treatment required for nitrogen is therefore dependent on
the method of ultimate disposal. For nitrogen removal, see Chapters 7, 9, 10, 12, and 13.
Phosphorus, as stated previously, is required for reproduction and synthesis of new cell
tissue, and, therefore, its presence is necessary for biological treatment. Domestic wastewater
2-33
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is relatively rich in phosphorus because of its high content in human waste and in synthetic
detergents. There is usually adequate phosphorus in wastewater to allow biological treat-
ment. Phosphorus, as a nutrient, can also cause excessive growth of algae in lakes or slow
streams. For phosphorus removal, see Chapter 12.
Chlorides and many other elements, such as sodium, which are dissolved or dissociate in
water, are not removed in ordinary waste treatment processes.
Sulfur can cause corrosion of piping (in its acid forms) and odors (hydrogen sulfide gas and
other sulfur compounds). Sulfates are reduced by bacteria under anaerobic conditions to
sulfides, including hydrogen sulfide (I^S). I^S can be oxidized biologically to sulfuric acid,
which is highly corrosive.
Toxic compounds and heavy metals are also present in wastewater and can have a significant
effect on treatment and disposal. Many of the heavy metals are necessary in trace quantities
for growth of biological life but are toxic in larger concentrations. The presence and amount
of these substances should be determined and treatment processes designed, if necessary, for
their removal, particularly if downstream ecosystems are to be protected. Most toxic com-
pounds and heavy metals are from industrial sources and can be eliminated by pretreatment.
A list of some chemical substances presented in industrial waste is given in Subsection 2.5.4.
For additional information on the toxicity of some components, see reference (33).
Gases sometimes found in raw wastewater that are important in the design of treatment
works include nitrogen (N2), carbon dioxide (CC^), hydrogen sulfide (r^S), ammonia
(NH3), methane (CI-^), and oxygen (C^). Nitrogen and ammonia are important because of
their roles in biological processes. Nitrogen gas released by anaerobic action from sludge will
interfere with settling. Carbon dioxide is present in the atmosphere and is found in waste-
water because of its solubility and its production by living organisms, providing a carbon
source for some microbes. Hydrogen sulfide and methane are derived from the anaerobic de-
composition of organic matter and are toxic to man. Hydrogen sulfide is a colorless, toxic,
inflammable gas with an odor of rotten eggs. Methane is a colorless, odorless, toxic, com-
bustible hydrocarbon that, if available in large quantities, can be used as fuel for generators.
Anaerobic digesters are sometimes designed to collect and store methane as an energy source
at larger treatment plants.
Dissolved oxygen is essential for the continued respiration of aerobic organisms. Oxygen is
only slightly soluble in water and may have to be added if biological units are to function
properly (see Chapter 7). Oxygen solubility decreases with increased temperature and solids
content. In designing aeration units, the oxygen supply should satisfy summer needs, and
the detention time should be sufficient for slower winter activity.
2-34
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2.5.3 Biological Characteristics
The designer of wastewater treatment works must have a basic knowledge of the principal
organisms found in wastewater, surface water, and soil, and should understand the condi-
tions associated with active organisms and the chemicals that may be present. Much of this
information can be found in Chapter 10 and in references (20), (33), and (34).
The organisms present at the point of disposal (to surface water or groundwater) can be
used to indicate the degree of pollution or the toxicity of treated wastewaters. The orga-
nisms in raw wastewater can be removed by treatment processes and with chemical addi-
tions under controlled conditions in a wastewater treatment plant.
Most wastewater contains large, well-mixed populations of microorganisms as well as the
chemical components discussed previously. Under proper conditions, growing populations
of these organisms can assimilate many chemical components into a removable form.
Biological treatment units such as activated sludge plants (Chapter 7), trickling filters
(Chapter 9), and stabilization ponds (Chapter 10) provide these conditions. Organisms can
also be used to convert organic solids (sludge) removed from wastewater to more stable and
less objectionable forms. The conditions for sludge biological conversion are usually pro-
vided in aerobic or anaerobic digesters, or the sludge can be chemically stabilized (chapter
14).
The principal groups of organisms important in wastewater treatment can be classified as
shown in Table 2-6. References (20), (33), (34), (35), and (36) contain more detailed
information on these organisms, and their function in treatment processes is discussed in
Chapters 7, 9, 10, and 14.
TABLE 2-6
ORGANISMS FOUND IN WASTEWATER TREATMENT
Moneran Protistan Plants Animal
Bacteria Fungi Seed Plants Vertebrates
Blue-Green Algae Protozoa Ferns Invertebrates
Algae Mosses
Liverworts
Viruses are also present in wastewater in large quantities and are of concern because many
are pathogenic. In addition to viruses, there are many other organisms that are pathogenic.
Depending on the type and degree of treatment, most can be removed. Some of the patho-
gens entering a treatment plant will die off naturally, given adequate time. Others may find
hosts that can sustain them and pass them on to other life forms. Viruses and some protistan
phyla that form spores or cysts can survive for long periods and may eventually reach a host.
Because of the possibility of pathogens passing through a treatment plant, and because of
the ways in which they may survive and cause disease, it is important to provide reliable dis-
infection facilities.
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To determine the degree of removal (or disinfection) of pathogens in wastewater treatment
plants, total coliform, fecal coliform, and fecal streptococci determinations (20) (28) (34)
are used.
Coliform organisms are present in large numbers in the excretions of warm-blooded animals
and are easily counted and more resistant to adverse conditions than most pathogens. Be-
cause of these facts, the presence of fecal coliform organisms is taken as a valuable indica-
tion of the presence of pathogens. Disinfection is discussed in detail in Chapter 15.
2.5.4 Wastewater Composition
Composition refers to the combined physical, chemical, and biological components found in
wastewater. The components can vary in amount, type, and form, depending on the sources
of the wastewater. In addition to the normal sources of constituents, the background com-
ponents present in water supplies and their effect on the wastewater must be considered.
Water supplies are relatively pure and may dilute the wastewater. There are, however, con-
stituents such as chlorides, sulfates, and carbonates that are not removed by conventional
water or wastewater treatment and that can build up to problem levels. This mineral buildup
can result from minerals dissolved in ground water, salt spray reaching water supplies in areas
near the ocean, and mineral pickup during wastewater reuse cycling. It can also be the result
of such treatment as nitrogen removal by breakpoint chlorination.
Wastewater composition will vary daily, weekly, and yearly for reasons similar to those for
flow variation. Therefore, only major causes of composition variation will be discussed in
this chapter. Domestic wastewater composition can vary greatly, depending on the lifestyle
of the population. Table 2-7 lists normal ranges of the average composition of domestic
wastewater. Garbage disposals require relatively small amounts of water but can greatly
increase the strength of wastewater in terms of BOD, COD, SS, and grease.
Figure 2-9 shows the variations of COD for five sources of household waste. The area under
the curves for each source indicates the amount of COD contributed daily. This profile is an
example of variations for families residing in the suburban mountain residential develop-
ments of Colorado (25).
Commercial, recreational, and institutional wastewaters contain constituents similar to
domestic waste, but vary in strength and quantity, depending on the source. Many equip-
ment manufacturers have developed guides to determine strength of wastewater from
various sources. Tables 2-2 and 2-3 are samples of these guides. The designer will have to use
this information and any available data to determine the waste strengths to be expected.
Local public health or environmental protection authorities should be consulted to check
design criteria requirements for the specific location.
Industries can have a significant effect on wastewater constituents, depending on the size
and type. Some chemical substances found in industrial wastes are shown in Table 2-8, along
with some industries providing these substances. The best method of determining industrial
waste constituents and strength is by survey.
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TABLE 2-7
AVERAGE COMPOSITION OF DOMESTIC WASTEWATER, mg/11
Composition Range
Solids, Total
Dissolved, Total
Mineral
Organic
Suspended, Total
Mineral
Organic
Settleable, Total
Mineral
Organic
Biochemical Oxygen Demand 20° C
5-Day Carbonaceous
Ultimate Carbonaceous
Ultimate Nitrogenous
Total Oxygen Demand (TOD)
Chemical Oxygen Demand (COD)
Total Organic Carbon (TOC)
Nitrogen (Total as N)
Organic
Free Ammonia
Nitrites
Nitrates
Phosphorus (Total as P)
Organic
Inorganic
Chlorides
Alkalinity (as CaCO3)
Grease
700-1,000
400-700
250-450
150-250
180-300
40-70
140-230
150-180
40-50
110-130
160-280
240-420
80-140
400-500
550-700
200-250
40-50
15-20
25-30
10-15
3-4
7-11
50-60
100-125
90-110
1 Assuming 100 gallons of wastewater per capita with a relatively soft potable water supply,
no industrial wastewater, and median use of garbage grinders from a community whose
citizens have a moderate income.
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JC.
^
>§
Q
O
O
46 8 10 N
6 8 10 M
TIME
FIGURE 2-9
HOURLY COD PROFILE (25)
2-38
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TABLE 2-8
SOME CHEMICAL SUBSTANCES IN INDUSTRIAL WASTES (33)
Chemical
Acetic Acid
Alkalies
Ammonia
Arsenic
Cadmium
Chromium
Citric Acid
Copper
Cyanides
Fats, Oils, and Grease
Fluorides
Formaldehyde
Free Chlorine
Hydrocarbons
Hydrogen Peroxide
Lead
Mercaptans
Mineral Acids
Nickel
Nitro Compounds
Organic Acids
Phenols
Silver
Starch
Sugars
Sulfides
Sulfites
Tannic Acid
Tartaric Acid
Zinc
Common Source
Pickle and Beetroot Manufacture, Acetate Rayon
Cotton and Straw Kiering, Wool Scouring, Cotton Mercerizing,
Laundries
Gas and Coke Manufacture, Chemical Manufacture
Sheep Dipping, Fellmongering
Plating
Plating, Aluminum Anodizing, Chrome Tanning
Soft Drinks and Citrous Fruits
Copper Plating, Copper Pickling, Cuprammonium Rayon Manufacture
Gas Manufacture, Plating, Case-Hardening, Metal Cleaning
Wool Scouring, Laundries, Textile Industries, Petroleum Refineries,
Engineering Works
Scrubbing of Flue Gases, Glass Etching, Atomic Energy Plants, Fer-
tilizer Plants, Metal Refineries, Ceramic Plants, Transistor Factories
Synthetic Resin Manufacture, Penicillin Manufacture
Laundries, Paper Mills, Textile Bleaching
Petrochemical and Synthetic Rubber Factories
Peroxide Bleaching of Textiles, Rocket Motor Testing
Battery Manufacture, Lead Mines, Paint Manufacture
Oil Refineries, Pulp Mills
Chemical Manufacture, Mines, Iron and Copper Pickling, DDT Manu-
facture, Brewing Textiles, Battery Manufacture, Photoengraving
Plating
Explosive Factories, Chemical Works
Distilleries, Fermentation Plants
Gas and Coke Manufacture, Synthetic Resin Manufacture, Textile
Industries, Tanneries, Tar Distilleries, Chemical Plants, Dye Manu-
facture, Sheep Dipping
Plating, Photography
Food Processing, Textile Industries, Wallpaper Manufacture
Dairies, Breweries, Preserve Manufacture, Glucose and Beet Sugar
Factories, Chocolate and Sweet Industries, Wood Processing
Sulfide Dyeing of Textiles, Tanneries, Gas Manufacture, Viscose
Rayon Manufacture
Wood Pulp Processing, Viscose Film Manufacture, Bleaching
Tanning, Sawmills
Dyeing, Wine Making, Leather Manufacture, Chemical Works
Galvanizing, Zinc Plating, Viscose-Rayon Manufacture, Rubber Pro-
cessing
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2.5.5 Biological Process Requirements
Biological treatment processes require nutrients for proper operation. Most of the nutrients
required for biological activity are normally present in sufficient quantity in the influent.
The nutrient concentration, however, must be determined if industrial wastes are present.
The normal rule of thumb for determining N and P is that 5 Ib of nitrogen and 1 Ib of phos-
phorus are required for each 100 Ib of BOD removed by microbiological activity. Without
the proper nutrient balance, the efficiency of a unit will decrease, and the unit may become
overloaded with fungi, which can flourish on lower levels of N and P.
Trace elements are also important in the design and selection of biological units. Although
most wastewater will support biological growth, the type of organism is often controlled by
the availability of these elements.
Numerous studies have found that 16 elements and 3 vitamins (growth factors) are needed
for the growth of microorganisms. The elements include N, P, K, Ca, Mg, Na, S, Fe, Mn, Cu,
Zn, Mo, B, Cl, Co, and V. If there is a deficiency of nutrients or trace elements, the orga-
nism population tends to change to filamentous forms with the ability to subsist on minimal
nutrients. This deficiency may occur if the wastewater contains improper ratios of organic
matter to needed nutrients to support a balance of growth of conventional organisms. The
addition of the deficient elements has in many cases improved the degree of treatment ob-
tained.
A deficiency can also be aggravated by precipitation of trace elements reacting with
hydrogen sulfide. This could be true if the plant flow were to contain a large percentage of
septage or if there were a buildup of anaerobic plume in a long collection system. At the pH
values normally encountered in domestic wastewater, iron, zinc, copper, cobalt, and vana-
dium will be precipitated as sulfides. The presence of hydrogen sulfide also may cause in-
activation of the vitamins present (37).
The optimum pH range for operation of a biological process is from 6 to 8. Fungi will begin
to dominate below 6 and will take over almost completely at 4.5. As the pH rises above 9.5,
a toxic effect occurs, and almost no microorganisms will survive at 11.0. Above and below
the optimum range, the efficiency of operation will decrease. A rapidly occurring change in
pH can have a toxic effect on the microorganisms.
The toxicity of elevated concentrations of heavy metals such as chrome, lead, copper, and
mercury, as well as insecticides, cyanides, and high concentrations of phenols, can signifi-
cantly affect efficiency. As with other biological systems, a trickling filter can become
acclimated to many toxic materials if the concentration is low and does not vary greatly. A
surge or shock load will upset a trickling filter, but the recovery is more rapid than with
most other systems.
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2.6 Odors and Other Airborne Pollutants
There are many areas and processes in wastewater facilities that can be potential sources of
airborne pollutants if not prevented from being so by satisfactory design, construction,
operation, and maintenance. Airborne pollutants include odors; noxious, toxic, or asphyxi-
ating gases; particulates from sludge incinerators; and aerosols from trickling filters, aeration
basins, cooling towers, stripping towers, and ventilation systems.
Even "fresh" wastewater and "digested" sludge have odors that may not be acceptable to
the general public. Organic material containing sulfur or nitrogen may, in the absence of
oxygen, be partially oxidized anaerobically and give off such odorous substances as
hydrogen sulfide, mercaptans, skatoles, indoles, and amines. Any location (such as eddies)
in which raw wastewater becomes anaerobic, or organic solids (such as sludge, slime, scum,
or grit) are allowed to accumulate, may become a source of odor.
Odor prevention and control measures include:
1. Passing and enforcing strict sewer ordinances to limit entrance of potentially
odorous substances.
2. Regular and careful cleaning, including frequent removal of slime, scum, and grit
accumulations and regular inspection and maintenance of all plant structures.
3. Preventing anaerobic conditions, or removing odorous conditions, by adding
chlorine, hydrogen peroxide, ozone, potassium permanganate, lime, or sodium
hydroxide.
4. Maintaining adequate levels of dissolved oxygen by aerating; oxygenating; adding
ozone, peroxide, or nitrate; or diluting with aerated wastewater.
5. Preventing sludge accumulating or aging by frequent solids withdrawals, adequate
mixing in tanks, sufficient velocity of flow, or placing smooth transitions in
structures to eliminate "dead" pockets.
6. Placing potentially odorous units such as vacuum filters, sludge thickeners, or
sludge holding tanks in structures with forced ventilation; placing a dome over the
odorous unit with forced ventilation, or placing a floating cover on the unit.
7. Preventing overloading by recirculating, equalizing flows, or providing overflow
units.
8. Pretreating to remove odor-causing substances.
9. Preventing incinerator odors by maintaining temperatures throughout the burning
area above 1,400° F (760° C).
10. Treating vented odorous gases by ozonating; bubbling through chlorine contact
tank; wet scrubbing; combustion at temperatures above 1,400° F (760° C); cata-
lytic oxidation; passing through activated sludge tank; activated carbon adsorp-
tion; treating wood chip adsorption; or filtering through a soil bed.
For more details on odor testing, odor intensity calculations, and solutions to odor prob-
lems, see references (2) and (38) through (54).
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Aerosols are defined as suspensions of approximately microscopic (smaller than 5 to 10
microns) solid or liquid particles dispersed in the atmosphere. Particulates are considered to
be liquid or solid particles of any size dispersed in the air. Aerosols and particulates may be
organic, inorganic, or a combination of both. Foams, mists, dusts, smog, fumes, and smoke
are among the different forms of aerosols and particulates. The liquid aerosols are generated
at wastewater works when bubbles of air from wastewater or liquid sludge burst and dis-
charge tiny droplets into the air. All aeration, ventilation, evaporation (cooling towers),
spray irrigation, and stripping activities involving wastewater or sludge are potential sources
of aerosols. Activated sludge, trickling filter, and aeration units (particularly those handling
raw wastewater) are probably the major potential sources of pollutional aerosols. With the
increasing demands for reuse of wastewater, however, cooling and stripping towers may also
become a source of air pollution. Treated (but not disinfected) wastewater is sometimes
used in these towers for evaporational cooling for stripping out volatile organics and inor-
ganics. Pathogens and toxic or injurious organic and inorganic pollutants in the wastewater
may be aerosolized.
Recently, aerosols have proven to be deserving of attention. Pathogens present in the aero-
sols must be considered a potential source of disease and infection, because samples down-
wind of wastewater treatment works have been found to include significant numbers of
E coli, A aerogenes, and pathogenic enteric organisms. No evidence, however, has been
found to indicate that aerosols affect the health of wastewater treatment facility workers or
others.
Gases that can cause air pollution are emitted from treatment works in wastewater, sludge,
and liquid sidestream processing; wastewater collection, pumping, and transmission; and
disposal operation. Explosive, toxic, asphyxiating, and flammable gases are also hazards.
These are discussed in the U.S. EPA technical report and technical bulletins of safety (55)
(56) (57).
Controls for particulates and gases from incinerators are well established. EPA has developed
criteria and standards for permissible levels of pollutants in stack emissions. These standards
establish limits on particulate discharge and opacity.
Explosive, toxic, noxious, lachrymose, or asphyxiating gases found at wastewater works in-
clude chlorine, methane, ammonia, hydrogen sulfide, carbon monoxide, and oxides of
nitrogen, sulfur, and phosphorus. If there is a possibility that such a gas can escape from the
works or into work areas, in dangerous or nuisance concentrations, the knowledge might
affect the operation of the works and the use and development of adjacent land. Therefore,
it is of the utmost importance that all precautions be taken to insure against the escape of
such gases. They are usually more detrimental within wastewater structures than in adjacent
areas.
Of the gases (including those above) collected from wastewater works structures by ventila-
tion systems, some could be unsafe or could adversely affect the environment if discharged
directly to the atmosphere. Consideration should be given to monitoring such ventilation
discharges for objectionable gases. Consideration should also be given to providing standby
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means for neutralizing, stabilizing, or destroying gases that might significantly affect the
environment.
Additional design, operation, and maintenance criteria for control of aerosols, particulates,
and gases at wastewater works are presented in WPCF Manual No. 1, "Safety in Wastewater
Works" (58). Safety factors and controls are presented in references (55), (56), and (57).
2.7 Noise
Wastewater facilities can be an unacceptable irritant to nearby inhabitants if they are
designed, constructed, operated, or maintained in such a manner that they produce exces-
sive noise. Noise prevention and control measures, noise level standards, noise analysis, and
examples of calculation on noise transmission at wastewater treatment works are discussed
in references (2) and (97).
Evaluation of noise levels in a community is a complex problem involving individual percep-
tion of how an objectionably noisy environment is defined. Definitions of these perceptions
is difficult and involves measurements of ambient noise levels in the area.
Maximum noise levels for working areas are defined by regulations authorized under the
Federal 1970 Occupational Safety and Health Act (OSHA) and by its predecessor, the
Walsh-Healey Act (59) (60). Acceptable levels for adequate speech communication in the
working environment are available (61). Community noise levels have also been researched,
and information exists on desirable levels (62) (63) (64).
Noise control needs are determined by comparing noise source measurements (sound pres-
sure level, source direction, sound direction, sound power, etc.) with a specified design goal
or criterion for acceptable noise levels at a listener's position. Sound pressure is sensed by a
microphone and amplified in a sound level meter for frequency analysis or display on a
decibel meter (65).
Sound power levels (Lw) should not be confused with sound pressure levels (Lp), which are
also expressed in decibels. Sound power level is related logarithmically to the total acoustic
power radiated by a source. Sound pressure level specifies the acoustic "disturbance" pro-
duced at a point. Sound pressure level depends on the distance from the source, losses in the
intervening air, room effects (if indoors), etc. Sound power level is analogous to the heat
production of a furnace, while sound pressure level is analogous to the temperature pro-
duced at a given point in a building. In another example using light bulbs, the wattage is
analogous to 1^ and the brightness analogous to Lp. The noise of a piece of equipment may
be expressed by the phrase "the sound power level is 60 dBA" or "the sound pressure level
is 60 dBA at 3 ft (0.9 m)." An increase of 3 decibels indicates a doubling of sound power,
whereas an increase of approximately 10 decibels indicates a doubling of perceived sound
pressure. Typical sound pressure decibels are logarithmic units for measuring the relative
levels of various acoustical quantities (66) on a scale beginning with 0, for faintest audible
sound, through 130, which is the approximate threshold for pain. Average sound pressure
levels encountered are shown in Table 2-9.
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TABLE 2-9
AVERAGE SINGLE NUMBER SOUND PRESSURE LEVELS (2)
Interior Noises dBA
Bedroom At Night
Quiet Residence
Residence With Radio
Small Office or Store
Large Store
Large Office
Electric Typewriter At 10 ft
Factory Office
Automobiles
Factories
School Cafeterias
Railroad Cars
Garbage Disposals
Airplane Cabins
Noises At 3 Ft From Source
Whispering
Quiet Ventilating Outlet
Quiet Talking
Noisy Ventilating Outlet
Business Machines
Lathes
Shouting
Power Saws
Power Mower
Farm Tractors
Power Wood Planers
Pneumatic Riveter
Outside Noises
Leaves Rustling
Bird Calls
Quiet Residential Street
150 to 200 ft From Dense Traffic
Edge of Highway With Dense Traffic
Car At 65 mph At 25 ft
Propeller Plane At 1,000 ft
Pneumatic Drill At 50 ft
Noisy Street
Under Elevated Train
Jet Plane At 1,000 ft
Jet Takeoff At 200 ft
50-hpSirenAt 100ft
30-40
39-48
47-59
47-59
51-63
57-68
62-67
60-73
64-78
65-93
76-85
77-88
78-83
88-98
30-35
4147
59-66
60-75
71-86
73-83
74-80
93-101
94-102
94-103
97-108
100-120
10-15
40-45
40-52
55-70
70-85
75-80
75-84
80-85
84-94
88-97
100-105
120-125
130-135
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Usually, noise can be most efficiently controlled at the source. A designer can control
machinery noise by specifying the least noisy equipment consistent with performance. To
meet acceptable acoustical levels, the designer should 1) specify allowable sound power
levels (or sound pressure levels at a specified distance) for the equipment; 2) submit labora-
tory or field measurements for approval; and 3) perform field conformance tests.
Designers of wastewater treatment works should consider including noise control measures
(such as source control of machinery noise, special architectural treatment to absorb sound
and to isolate noisy equipment, buffer zones within the plant between noisy and quiet areas,
and site planning and buffer zones) to minimize impact on community noise levels. These
measures should take into account both interior noise and community noise criteria dis-
cussed above.
The following techniques, singly or in combination (67) (68), may be used to reduce
machinery noise:
1. Segregating noisiest elements in groups.
2. Vibration damping (using materials like lead sheet in foundation).
3. Isolating vibration (mounting on springs).
4. Sound absorbing enclosures (with hard outer shell and sound absorbent liner).
5. Sound attenuating at exhaust or intakes of fans or compressors.
6. Providing full personnel enclosure observation booths in auditory damage risk
areas.
7. Providing partial protective booths (open in rear).
8. Plenum treatments (devices admitting low-velocity air, to prevent escape of exces-
sive noise).
9. Pipe lagging (lining or covering that absorbs radiated noise).
10. Providing partial barriers.
11. Lining ducts.
12. Using silencers, mufflers, or mutes (attenuating noise from high velocity flow of
gases by multiple reaction of sound waves from acoustically absorbent surfaces;
eliminating turbulent flow; and reducing flow velocity).
13. Providing ear plugs and muffs.
14. Reducing motor speed (to lowest practical requirements in combination with size
and pressure to produce required power).
15. Selecting valves not normally noisy (pilot-operated or compound valves rather
than direct-acting or single stage).
16. Keeping air out of hydraulic systems.
17. Preventing development of cavitation in pumps (by keeping suction line velocities
to less than 5 ft/s (1.5 m/s, keeping inlet lines short and with a minimum of bends
and joints, etc.).
18. Reducing turbulent flow next to flat metal plates.
19. Using rubberlike flexible connections in drive shafts.
20. Reducing gear noise by maintaining equipment, controlling alignment, and using
enclosures.
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The following techniques may be used to reduce construction and operation noise:
1. Replacing individual operation and construction techniques by less noisy ones; for
example, using welding instead of riveting, mixing concrete offsite instead of
onsite, and employing prefabricated structures instead of building them onsite.
2. Selecting the quietest alternative items of equipment; for example, electric instead
of diesel-powered equipment, hydraulic tools instead of pneumatic-impact tools.
3. Scheduling equipment operations to keep average noise levels low; for example,
scheduling the noisiest operation to coincide with times of highest ambient levels,
keeping noise levels relatively uniform in time, turning off idling equipment, and
restricting working hours.
4. Increasing the number of machines at work at any one time (this will reduce the
duration of noise exposure, although it will increase the noise level during that
particular time of operation).
5. Making use of speed limits to control noise from vehicles.
6. Keeping noisy equipment operations as far as possible from site boundaries.
7. Providing enclosures for stationary items of equipment and barriers around partic-
ularly noisy areas on the site or around the site itself.
8. Locating haul roads behind natural earth berms or embankments.
9. Maintaining noise control devices.
10. Replacing mufflers before breakdown.
11. Replacing warped, bent, or damaged engine enclosures and ineffective insulation.
Noise control measures will be effective only as long as control devices are properly main-
tained.
The recently adopted Federal construction guide on noise control is a good source for
general contract specifications for any new wastewater facility or for extensive alterations to
existing facilities (69).
Further information and details on noise and its control can be found in references (2),
(70), and (71).
2.8 Trucked and Marine Industry Wastes
Many small wastewater treatment plants will be located in rural areas or near small harbors
or ports. At these locations the plants may be required to treat waste from isolated sources
(which may be trucked to the plant) or waste from vessels (which may be conveyed to the
plant by several methods). In many cases the ratio of these wastes to be treated to the
normal load on the plant will be high, and therefore the design must take this factor into
consideration.
2.8.1 Trucked Wastes
Trucked wastes can include septic tank sludge and chemical toilet wastewater as well as
almost any chemical that is discharged by an industry or a marine vessel. Extreme caution
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should be taken to prevent upset of a plant treating trucked wastes, to avoid shock loading
of a treatment process.
Trucked human wastes can be classified as septic tank sludge (septage), privy vault wastes,
and recirculating and chemical toilet wastes. The solids content, organic strength, and toxic
chemical concentration of these wastes are the most important factors in developing dilu-
tion criteria for their acceptance at the treatment works. The type of treatment available
will dictate the dilution required. If the facility consists of a biological process with no pri-
mary clarification, the biological process must be designed to handle the additional load or
separate treatment must be provided.
The increase of solids from synthesis of soluble organic matter in trucked wastes can upset
the equilibrium of a biological process. A practical limit (without pretreatment for the
allowable increase in solids) is up to 10 to 15 percent of the concentration of the mixed
liquid solids, to prevent a substantial upset in a biological system (72).
The high oxygen requirements for stabilization of these wastes may overtax the plant's
oxygenation capacity, if sufficient dilution is not provided or if the aeration facilities are
not adequately sized to take trucked wastes.
The addition of chemical toilet wastes to biological treatment processes should be avoided,
if possible, because of the accumulation of toxic chemicals. Processes other than biological
should be used. The chemicals used in these toilets vary but primarily consist of zinc or
formaldehyde compounds. Therefore, periodic checks should be made to prevent these
toxic chemicals from retarding or completely stopping the biological process.
Some of the primary requirements of a transfer station for handling trucked waste and con-
trolling flow to the facility include:
1. Adequate storage capacity to equalize flows.
2. Ease of truck unloading without spillage.
3. Comminution and screening.
4. Odor control.
5. Pumping flexibility and reliability.
6. Possibly aeration, or mixing to provide oxygen or to keep solids in suspension.
Additional information on the characteristics of trucked wastes and the design of facilities
to handle them may be found in Chapter 14 and in references (72) and (73).
2.8.2 Marine Industry Wastes
Marine industry wastes would include waste from vessels and dock facilities. The waste from
vessels in some ways is similar to trucked wastes. Vessel wastewater would include domestic
waste bilge water and nonoily ballast water. Domestic wastewater could include water from
toilet, galley, sink, shower, and laundry. Many ships are equipped with holding tanks, recir-
culating toilets, evaporating systems, or incineration devices, which can alter the quantity
and character of the wastewater.
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The disposal of wastewater from vessels requires considering both collection and treatment.
To collect vessel wastewater, a central pump-out facility in the harbor or a sewer system at
active docks, or mobile collection by truck, barge, or railroad car may be provided. The
necessary treatment may be located at a municipal treatment plant or in a harbor-based
vessel treatment system. Factors to be considered in designing facilities to treat vessel waste-
water include:
1. Storage for large seasonal and daily variations.
2. Potential agricultural pest infestation from foreign wastes.
3. Access to dock facilities.
4. Remoteness of municipal services.
5. Large volumes of bilge and ballast water.
6. Time constraints on disposal.
7. Excessive waste handling under current practice.
Bilge water is water collected in the lowest part of the ship, resulting from leaks or spills.
It may be contaminated with oil, solvents, rust, scale, and many other materials. Treatment
of bilge water would involve oil separation and disposal operations such as skimming,
chemical treatment, flotation, adsorption, and incineration. Other processes may also be re-
quired to eliminate physical or chemical contaminants that may be present. Treatment may
be possible in a municipal system with proper pretreatment, or an independent treatment
system may be required.
Ballast water is commonly used to compensate for underloading of vessels. This ballast may
be taken from polluted harbors and eventually discharged in cleaner harbors. Ballast water
can have many of the physical, chemical, and biological characteristics of bilge water and
must be handled with similar caution.
Currently, there is not much information available on the characteristics of marine industry
wastes other than sanitary wastes. There are many studies underway that will provide more
data on vessel wastewater treatment systems and the characteristics of vessel wastewater.
For more detailed discussion of the information presented in this subsection, see references
(74) and (75).
2.9 Effluent Disposal
Pollutants can be removed from wastewater to any degree desired, depending on the treat-
ment processes used. The goal of sound engineering in wastewater treatment is to provide a
degree of treatment consistent with the requirements for disposal or reuse at a minimum
cost. The disposal may be by dilution in lakes, rivers, estuaries, or oceans or discharge on
land by agricultural use, recreational use, groundwater recharge, or evaporation. Reuse
overlaps the disposal methods by including irrigation, groundwater recharge, and im-
poundment for recreation. In addition, reuse includes use as industrial water (which, along
with agricultural use, has great potential in the United States) and municipal use (which
now occurs indirectly, but may occur directly in the future).
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Water pollution control, by setting receiving water or stream quality standards, led to
removing a minimum of the pollutants and letting nature complete the job by self-purifica-
tion. The amount of self-purification or assimilative capacity depends on many factors, in-
cluding volume of flow, available oxygen, and capacity for reoxygenation. Problems of over-
taxing the assimilative capacity of the receiving waters and the unfair use of this capacity by
upstream users have prompted development of the "effluent standards" for wastewater dis-
posal, now part of Federal law. States and regions still enforce stream standards which are
more stringent than Federal effluent standards in some locations.
The following is a brief discussion of effluent disposal alternatives. More detailed discussions
of standards, water quality criteria, and effluent disposal conditions are contained in
references (20), (76), (77), (78), and (79).
2.9.1 Disposal to Surface Waters
Wastewater disposal to surface water is the most common method used to date. This
method includes disposal to streams, lakes, estuaries, and oceans. After water quality stan-
dards (based on use and permissible thresholds of pollutants and on best use of the receiving
water) have been determined, the treatment requirement for a specific discharge can be
determined.
2.9.1.1 Rivers or Flowing Streams
During wet seasons, a river's flow and velocity increases and carries soil and organic material
stripped from the surface of the earth and the river bed. When precipitation stops, the river
loses carrying capacity and drops some of this material into pools and ponds along its route.
With each cycle the ecological equilibrium is upset as water flow increases or decreases, and
solids are scoured up and deposited.
The oxygen resource of a stream is another factor that is important in the assimilative
capacity of a stream. Oxygen in a river can be obtained from its tributaries, surface drainage,
groundwater inflow, reaeration from the atmosphere, and photosynthesis of aquatic plants
and algae. For a detailed discussion of these factors, see references (20), (33), and (78).
Human activities will have varying effects on a stream. The most important factors in waste-
water discharges include excessive organic loading, suspended matter, nutrients, and the
various ions or compounds that change the quality of the water.
Organic loading is a most significant factor in wastewater disposal to a stream, because of
the limited assimilative capacity of any stream. Depending on the flow and the oxygen
resources in the stream, the addition of organic matter to a stream can have effects ranging
from no noticeable effect to septic conditions with noxious odors, floating sludge, and die-
off of all higher life forms. The majority of the organic matter is associated with the SS of a
treatment plant effluent.
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Nutrients added to a stream may increase the aquatic life in the stream, which in turn could
cause an oxygen demand when the additional organisms die. In addition to the reduction of
dissolved oxygen in the stream, the increased aquatic life can produce taste and odor not
associated with septic conditions.
Suspended matter in a treated wastewater discharge is normally present in small quantities
yet can significantly affect a river. An excess of fine material can hasten the filling of the
riverbed and damage much of the littoral environment in which fish spawn and their food
chain flourishes.
Various ions and compounds in the effluent may have different effects on different streams,
depending on the stream's characteristics. Toxic compounds can reduce aquatic life. Other
ions and material may be precipitated in a stream or adsorbed on particles that settle to the
stream bed. Many ions and compounds will be degraded or converted to other forms that
will be harmless to the environment. Some materials such as DDT, lead, or mercury can be
concentrated in the stream food chain and may eventually harm many higher life forms,
including humans.
In determining the effluent limitations for disposal to a river, all of the factors mentioned
previously must be considered. After the water quality criteria have been determined, the
available dilution and DO can be used to determine effluent requirements. These require-
ments can then be compared with the appropriate effluent standards. Each discharge will
have to be considered for its specific limitations. The most critical time for most river
disposal will usually be when the flow is at a minimum, reducing dilution, and when water
temperature is high, reducing oxygen transfer.
2.9.1.2 Lakes or Stored Water
Wastewater discharged into lakes will normally reduce the concentration of pollutants, de-
pending on the size of lake and characteristics such as stratification and vertical mixing.
Wave action, mixing, precipitation, aquatic plants, and inflow can provide reoxygenation
and increase the DO. The bacterial content of stored water can decrease because of lack of
proper food, sedimentation, disinfection by sunlight, and depredation by other organisms.
More important than the effect of lakes or reservoirs-on waste is the effect of wastewater
on the lake or reservoir. Lakes, depending on size and amount of inflow and outflow, can be
seriously affected by many wastewater components. In areas where evaporation is signifi-
cant, the concentration of salts and total solids in a lake can be increased by evaporation.
The most serious effects of wastewater are 1) reducing DO resulting from the BOD of the
waste, 2) nutrients affecting eutrophication, and 3) concentrating pollutants in the food
chain.
Wastewater disposal to a lake will depend on size and depth of the individual lake. Small
shallow lakes will normally be considered completely mixed and the total volume available
for dilution. Lakes, ponds, and reservoirs having depths greater than 10 to 15 ft (3.28 to
4.92 m) will be subjected to season-related cycles of stratification and vertical mixing.
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Stratification results from increased water density with depth, resulting from decreasing
temperature. The stratification of a lake can be divided into three zones. The top is a zone
of circulation known as the epilimnion. This zone is subject to mixing because of wind
action or diurnal factors, caused by the sinking of a thin layer of surface water that is cooled
at night. Within this layer, the dissolved oxygen, light, and carbon dioxide are plentiful for
aquatic life. The middle zone is the thermocline, which is characterized by a rapid decline in
temperature and dissolved oxygen. This zone is extremely resistant to mixing and is often
the location of the highest quality water in the lake. The bottom zone is the zone of stagna-
tion or hypolimnion. Within this zone, dead organic matter is deposited by sedimentation,
and the water is devoid of oxygen and at a relatively low temperature.
Vertical mixing of deep lakes usually will occur once or twice a year, at least when the
temperature of the lake becomes uniform at approximately 39° F (4° C), the temperature at
which water density approaches a maximum. When this condition occurs (normally in spring
and fall), the lake water becomes unstable, and wind disturbance can cause vertical circula-
tion in the entire lake. During this upset period, which lasts a few weeks, the lake may
become completely mixed.
Eutrophication is a term used to describe the process of maturation of a lake from a nu-
trient-poor (oligotrophic) to a nutrient-rich (eutrophic) body of water. Most lakes in their
early stages of development were nutrient poor, with a small amount of nutrients derived
from weathered soil or degradable organic matter. As a lake matures, the concentration of
nutrients will build up, depending primarily on inflow and outflow conditions.
Human activities have caused artificial enrichment to occur, changing the condition of many
lakes in the United States from nutrient-poor to nutrient-rich. This change has occurred over
a very short period of time and in most cases has been caused by the discharge of waste-
water into lakes and other stored water, along with the runoff from agricultural land, farm-
land, and other areas where commercial fertilizers may have been used.
Because of the sensitivity of lakes to nutrient addition, it is very important to consider very
carefully the discharge of nutrient-containing wastewater. The effects of eutrophication can
be severe, and its development should be retarded as much as possible. Once eutrophication
has set in, it is very difficult and in most cases impractical to overcome. A more detailed
discussion on eutrophication can be found in reference (78).
The concentration of toxic pollutants in the lake food chain is another serious problem,
with the occurrence of waste disposal to lakes and other locations where fish and shellfish
live.
Lead, mercury, DDT, and several other substances that can be found in wastewater have
been shown to become concentrated in the food chain. These pollutants, which may not be
harmful in low concentrations, can be concentrated to such an extent that they seriously
affect higher life forms.
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2.9.1.3 Tidal Estuaries and Oceans
In tidal estuaries, dilution is complicated by tidal action, which can carry portions of the
waste back and forth in the same region for many cycles. This is caused by differences in
density of fresh water, wastewater, and sea water; wind action and density currents that
work against vertical mixing; coagulating and flocculating effects of saline water; and shore
and bottom configurations. Reference (20) discusses the dilution of wastewater in estuaries
and oceans.
Dilution, dispersion, and movement of wastewater discharged into the ocean or into large
lakes offer special design problems. The initial dilution is normally provided by the design
of diffusers to cause mixing by jet action and density differences. Dispersion by diffusion
cannot be controlled by engineering design, but selection of an ocean outfall location,
where normal currents will assist in dispersion, is important.
2.9.2 Land Application of Wastewater
Land application of wastewater has been practiced worldwide for over 135 years. In Mel-
bourne, Australia, approximately 14,000 acres (57 km2) of pasture have been irrigated with
wastewater for over 80 years. In 1850, in Berlin, Germany, a wastewater farm was started,
in which 21,000 acres (85 km2) were irrigated by 1905. Since 1935, 57,000 acres (231
km2) of pasture and vegetables have been irrigated in Leipzig, Germany. A great deal of
information on this practice in North America is contained in references (80), (81), (82),
and (83).
The most common reasons for use of land application would be to provide supplemental
irrigation water to augment groundwater supplies, distance and cost limitations of transport
to other suitable disposal locations, and cost advantages over other forms of treatment.
Land application uses include 1) irrigating crops (such as grasses, alfalfa, corn, sorghum,
citrus trees, grapes, and cotton), and 2) irrigating areas for recreational purposes (such as
parks, golf courses, sports grounds, ornamental fountains, and artificial lakes). Municipal
uses include landscaping streets, highway media strips, and school grounds. In addition,
irrigation of cemeteries, college grounds, airports, green belts, and forest preserves can be
provided. Augmenting of groundwater supplies by recharging aquifers with treated waste-
water is being done to prevent salt intrusion.
Methods of land application can vary, depending on the conditions at each location. The
methods can be classified as irrigation, overland flow, and infiltration-percolation systems.
Irrigation is the application to the land of wastewater, by spray or ridge and furrow, to en-
hance the growth of plants. Overland flow is the application of wastewater to grassed slopes.
The vegetation acts as a fixed film contactor. Infiltration-percolation is the application of
large amounts of water to a porous soil, which infiltrate the soil surface.
Although land disposal has been used for many years, there are a large number of unknown
factors concerning the effects of the treated wastewater on the environment, and vice versa.
Many research projects have been undertaken or are underway to provide the needed infor-
mation. The research has shown that, with good management and proper monitoring and
control of the systems, successful use of land application can be obtained. In designing a
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successful system, the following factors must be considered :
1. Availability and type of land.
2. Aesthetics.
3. Economics.
4. Topography.
5. Underlying geologic formations.
6. Ground water level and quality.
7. Soil type and drainability.
8. Wastewater characteristics and degree of pretreatment.
9. Purification effects of soil, soil bacteria, and plants.
10. Public health.
11. Possible buildup of toxic substances.
A thorough consideration of these factors requires the combined efforts of geologists,
environmental engineers, agronomists, soil scientists, social and behavioral scientists, and
medical-health personnel. Because of the complexity of land application systems and the
number of disciplines involved, great care should be taken in designing a land application
system for a small plant. To point out the complexity in such a system, some of the more
important factors are discussed below.
The degree of treatment before land application will depend on the method of application;
the rate of application; odor problems; possible ponding; type of vegetation to be irrigated;
physical, chemical, or biochemical properties of the soil; and public health concerns. These
factors, important in determining the required pretreatment, are interrelated with each
other and with the other design considerations mentioned previously. For example, in con-
sidering public health concerns, the possibility of inhaling pathogenic aerosols from a spray
irrigation system should be evaluated. Mosquito breeding can be a problem resulting from
ponding, if high rates of application are used. The presence of minerals such as sodium or
nitrogen, which can build up in a groundwater supply, may be a problem. The minerals
present in wastewater will be affected in different ways by the purification effects of the
soil, soil bacteria, and plants.
The natural purification processes depend on the interaction of many physical, chemical, or
biochemical factors. The effects of these factors will vary, depending on the conditions at a
land application site. Some of the important factors would include:
1. Oxidation or reduction.
2. Adsorption or desorption.
3. Ion exchange.
4. Precipitation or dissolution.
5. Aerobic or anaerobic decomposition.
6. Antibiosis or symbiosis.
7. Filtration.
8. Plant uptake of minerals.
More detailed discussion of these factors is included in references (80), (81), (82), (84), and
(85). Several publications are also available covering results of land application projects.
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Major projects include the Muskegon County wastewater management system (86) (87), and
the Penn State studies (88) (89), both of which use spray irrigation, and the Flushing
Meadows project (90) in Arizona, which uses infiltration for groundwater recharge.
2.9.3 Reuse of Wastewater
Reuse of wastewater can be divided into four categories: municipal, industrial, recreational,
and irrigational reuse. Many projects have reclaimed water for different reuse applications.
Lawrence (91) tabulated some of the existing wastewater reclamation projects (Table 2-10),
and discussed systems to satisfy effluent quality requirements for reuse of wastewater. Irri-
gational reuse has been discussed in Section 2.93. Factors involved in other reuse
applications and some of the water quality criteria are discussed below.
Recreational reuse is normally the impoundment of wastewater for recreational purposes
and is often combined with irrigational reuse. Indian Lake Reservoir near Lake Tahoe serves
as a reservoir for irrigation water as well as a recreational lake. The water quality criteria
must be stringent, because dilution is not available or adequate in most instances. The waste-
water quality parameters affecting recreational use the most are SS, oxygen-demanding
organics, bacteriological and virological quality, nutrients, and toxicants. Reduction and
control of the pollutants defined by these parameters would allow reuse of treated waste-
water for this application. The best documented example is the recreation project at Santee,
California (91).
Industrial reuse accounts for the largest quantity of wastewater use in the United States.
Most of this water is used for cooling purposes, with smaller amounts for boiler-feed water
and process waters. Most industries are accustomed to drawing available water and treating
it to a degree suitable for a particular use. Water quality requirements for various industrial
activities would include the following:
1. Electronics—high-quality water, often approaching completely demineralized
water.
2. Food Processing—municipal water quality or better, for specific processing needs.
3. Manufacturing (including chemicals)—municipal water quality or lower, depend-
ing on the product to be manufactured.
4. Pulp and Paper Mills—specific requirements on dissolved inorganic substances—
particularly chlorides and iron—and hardness (low color and turbidity also re-
quired for some operations).
5. Steel and Metals—generally used low-quality water; particularly concerned about
corrosion, hence restrictions on chlorides and pH and temperature in cooling
operations.
6. Boiler Feed Water—required quality of feed water dependent on operating pressure
range of boiler; specific requirements approaching complete demineralization,
including removal of ammonia, phosphates, organics, dissolved inorganics, and
deoxygenation (even municipal-quality water is treated before becoming accept-
able boiler-feed water).
7. Cooling Water—quality requirements low; biologically treated municipal waste-
water often used directly as cooling water (low-quality waters may require chemi-
cal additions for prevention of mineral scale or biological slime formation).
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TABLE 2-10
SUMMARY OF EXISTING MUNICIPAL WASTEWATER RECLAMATION PROJECTS (91)
Reclaimed Water Use
MUNICIPAL
Groundwater Recharge
Introduction to Water Distribution System
Groundwater Recharge
INDUSTRIAL
Power Plant Cooling Water
Chemical Plant Cooling Water
Power Plant Cooling Water
Steel Mill Cooling and Process
Refinery Cooling, Boiler Feed, and Cooling
Refinery and Petrochemical Cooling and Boiler Feed
Industrial Water
RECREATIONAL
Recreational Lakes, Swimming Pool
Recreational Lake
IRR1GATIONAL
Alfalfa Irrigation
Citrus Grove Irrigation
Pasture, Corn, and Rice Irrigation
Pasture, Corn Irrigation
Cotton and Milo Irrigation
Landscape Irrigation at CCCSD
Irrigation of Golden Gate Park
Golf Course Irrigation
Location
Whittier Narrows, California
Windhoek, South Africa
Nassau County, New York
Burbank, California
Midland, Michigan
Los Alamos, New Mexico
Baltimore, Maryland
Amarillo, Texas
Odessa, Texas
Denver, Colorado
Santee, California
Camp Pendleton, California
Gait, California
Pomona, California
Woodland, California
South Lake Tahoe, California
Fresno, California
Pacheco, California
San Francisco, California
Ventura, California
Reclaimed
Water
Production
mgd
10
1.4
18.5
1
6
2
95
6
2.5
102
0.4
0.7
0.3
6
1.9
3.5
18
0.5
1
0.5
Basic Type of Treatment
Secondary (Biological)
Secondary (Biological) + Pond + Phys.-Chem.1
Secondary (Biological) + Phys.-Chem.
Secondary (Biological)
Secondary (Biological)
Secondary (Biological)
Secondary (Biological)
Secondary (Biological)
Secondary (Biological)
Secondary (Biological) + Phys.-Chem.
Secondary (Biological) + Natural Media Filtration
Secondary (Biological) + Pond
Primary + Ponds
Primary + Ponds
Primary + Ponds
Secondary (Biological) + Phys.-Chem.
Primary
Secondary (Biological)
Secondary (Biological)
Secondary (Biological)
1 Phys.-Chem. usually means coagulation-sedimentation, activated carbon adsorption, filtration, nitrogen control.
2Plans call for capacity expansion to 100 mgd by 1986 for general municipal use as well as industrial use.
-------
Municipal reuse of wastewater has been practiced for about 50 years in nonpotable applica-
tions. Such uses include flushing water in water-short resort areas, where dual distribution
systems have been provided.
Indirect reuse of river water (as potable water) occurs if upstream communities discharge
wastewater into the river. A similar condition exists in groundwater supplies, in areas where
groundwater recharge is practiced. In both situations, dilution water is an important factor.
Direct reuse is now receiving added attention because of water shortages in many areas. The
main experience in direct reuse has been in Windhoek, South Africa, where about one-third
of the total water supply consists of treated wastewater. In this case, the treatment consists
of trickling filtration, maturation ponds, alum flotation, foam fractionation, filtration,
carbon treatment, and breakpoint chlorination.
The accepted quality criteria for municipal water supplies are the USPHS drinking water
standards of 1962. These standards were developed to judge the quality of treated water
drawn from relatively pure sources. These standards are not adequate in the areas of trace
metals, trace organics, and virological quality, which would be extremely important in
evaluating renovated wastewater for direct municipal reuse. There is also concern about the
concentration of sodium and nitrogen compounds in reclaimed water and the reliability of
reclamation treatment systems.
There are many unanswered questions related to human health about water reuse for munic-
ipal water purposes. Currently, reuse for drinking water purposes should be allowed only if
all other reasonable water sources become unavailable. If reuse is considered, it should be
approached cautiously, with the maximum amount of researching and monitoring.
2.10 Upgrading or Enlarging Existing Plants
Upgrading or enlarging existing treatment plants may be required because of:
1. Inadequate initial design.
2. Increase or change in loading patterns.
3. Deterioration of facilities.
4. Changes in effluent or receiving water quality standards.
The goals listed in Chapter 3 must be kept in mind in the upgrading or enlarging of existing
small wastewater treatment plants. Duplicating an existing treatment process, equipment, or
plant is not always the most cost-effective method of upgrading one or more characteristics
of a plant effluent. The EPA publication, Process Design Manual for Upgrading Existing
Wastewater Treatment Plants (92), directs itself to the upgrading of treatment facilities and
should be consulted before any plant upgrading is undertaken. This subject is also discussed
in references (93) and (94).
Representative simpler methods of upgrading or enlarging treatment plants are presented
below. Refer to the design criteria presented in later chapters for specific information.
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2.10.1 Clarifiers
Generally, the efficiency of a clarifier is measured by the amount of solids separated from
wastewater and retained in the clarifier. Inefficient clarifiers are usually hydraulically or
organically overloaded, poorly operated, or inadequately designed. Upgrading alternatives
include:
1. Adding clarifier area.
2. Adding flocculation or chemicals ahead of the clarification process.
3. Adding parallel centrifugal wastewater concentrators.
4. Increasing the settleability of the SS by reducing anaerobic conditions (in which
methane, nitrogen, or other gas releases buoy up the solids), preaerating, pre-
chlorinating, decreasing sludge age, or shortening sludge retention time in clarifier
to control gas formation.
5. Improving oil and grease removal.
6. Improving screening and grit removal.
7. Improving skimming.
8. Equalizing flows into clarifier by using an equalizing tank.
9. Eliminating surges in flow by using smaller or variable speed pumps.
10. Improving inlet and outlet design.
11. Improving sludge withdrawal system.
12. Improving scum removal system.
13. Improving cleaning and general maintenance.
14. Returning activated sludge to the primary influent.
15. Adding activated carbon to the primary influent.
2.10.2 Trickling Filters
The efficiency of a biological filter depends on maintaining an active population of aerobic
microbes in a thin layer of zoogleal film on the filter media surfaces. Some of the variables
affecting the performance of trickling filters are:
1. Wastewater Characteristics—large variations in the type or quantity of oxygen-de-
manding biodegradables reduce the efficiency of a filter.
2. Filter Media—a larger surface area per unit of volume, along with a higher per-
centage of void space per unit of volume in a packed media, normally allows
higher organic and hydraulic loadings without loss of efficiency.
3. Filter Depth—deeper filters with lower loadings promote nitrification (depth
needed for best treatment efficiency varies with type of media and loading).
4. Recirculation—media must be kept wet; good biological activity requires relatively
uniform organic feeding (sometimes recirculation of up to 400 percent is neces-
sary for improving efficiency, if feed flow otherwise is intermittent).
5. Hydraulic and Organic Loading—large variations or surges in either hydraulic or
organic loading (particularly in hydraulic loading) will have a marked effect on
efficiency.
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6. Ventilation—essential, because aerobic conditions are required for efficient micro-
bial activity.
7. Wastewater Temperature—performance of filters (particularly nitrification) dete-
riorates if temperature is lowered.
Upgrading possibilities for trickling filters are listed in Table 2-11.
2.10.3 Activated Sludge
The efficiency of an activated sludge unit depends on maintaining over 1 to 2 mg/1 of dis-
solved oxygen and an active population of aerobic microbes in a thoroughly mixed aeration
tank. Table 2-12 lists the upgrading possibilities for activated sludge plants.
2.10.4 Wastewater Treatment Ponds
The efficiency of ponds depends on 1) maintaining an optimum environment for an active
population of the essential microbes in contact with all the biodegradable wastewater con-
stituents for a sufficient length of time, and 2) adequate removal of bacterial, algal, and
other microbial cells from the pond effluent before discharge.
Techniques now available for upgrading this type of treatment are discussed in several publi-
cations (95) (96). Successful process modifications include:
1. Improving the pond outlet system to reduce escape microbial cells from each
pond.
2. Decreasing pond loading.
3. Increasing the number of pond cells in the system.
4. Adding pond recirculation.
5. Adding baffles to unaerated ponds to improve plug flow characteristics.
6. Improving the methods for distributing influent uniformly across the pond cell.
7. Improving dike construction and maintenance.
8. Adding storage cells with sufficient capacity for twice-a-year-only discharge.
9. Adding supplemental aeration or mixing.
10. Adding polishing units such as rock filters, intermittent sand filters, land applica-
tion, chemical addition, microstrainers, and chlorination-clarification.
Design details for these modifications are presented in Chapter 10.
The primary purposes of such modifications to ponds are to:
1. Prevent short circuiting of wastewater in unaerated ponds.
2. Prevent escape of unstabilized organic material in the effluent.
3. Provide supplemental oxygen.
4. Provide better conditions for removal of algal and bacterial cells.
5. Provide storage adequate to use intermittent discharge, if effluent quality is
periodically below requirements.
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TABLE 2-11
POSSIBLE TRICKLING FILTER PLANT MODIFICATIONS
Modification
Add positive ventilation
Change media
Increase recirculation
Add sludge recirculation
Add activated sludge
After trickling filter
Ahead of trickling filter
Add nitrifying trickling filter
When To Use
For strong wastes and deep filters in
which natural draft is not adequate to
provide oxygen needed
In rock filters in which increase in media
surface (and biomass) will provide needed
performance improvement
For lightly loaded filters in which
initial removal capacity is not fully
utilized in one-pass contact time, and
to keep media wet
Where more biomass is needed to
increase treatment
If polishing is required after an over-
loaded filter
If the trickling filter can be used for
nitrification
After settling tanks of existing
secondary system, to provide polishing
and nitrification
Remarks
Check with media manufacturers
Synthetic media are available with from
two to four times the surface-to-volume
ratio of stone; fine rock media cannot be
used for high loadings, because of
clogging
May also be needed for dilution to main-
tain aerobic conditions in heavily loaded
filters
Requires thorough pilot study
Advantageous if first unit can be con-
verted to "high rate" (high loading)
operation
Effluent polishing such as filtration can
greatly improve treatment
Because of low sludge yield, effluent
probably can be applied directly to
granular-media filters for solids removal;
advantageous if effluent filters are to be
included in upgrading
-------
TABLE 2-11 (continued)
POSSIBLE TRICKLING FILTER PLANT MODIFICATIONS
to
ON
o
Modification
Add roughing filter ahead of
trickling filter
Add recirculation and convert low
rate filter to high rate filter
Add aerated equalization tank
Change dosing tank or change to
smaller pump or variable speed pumps
Improve distributor arm
Improve drainage system
Add septage in off seasons to
aerated equalization tank
Improve clarification by adding
clarifier, by chemical addition, by
adding flocculator, and/or by
upgrading existing primary clarifier
Change from one-stage to two-stage
by adding clarifier and high rate
filter
Add polishing units such as maturation
ponds, filters, activated carbon, or
post-aeration
When To Use
To reduce loading on overloaded
filter
To increase capacity of low-rate filter
To equalize hydraulic loading rates
To reduce time when not dosing
To change dosing rate or make
distribution uniform
To improve natural ventilation
To equalize organic loading at plant
with large seasonal fluctuations
To reduce organic loading on filter
To increase capacity of organically
overloaded plant
If effluent needs additional treatment
before discharge and to reduce peak
discharges of BOD, SS, or E coli
Remarks
Intermediate settling not needed;
roughing filters are quite temperature
sensitive
Only if final clarifier has required
capacity
Will also reduce odors and improve
settling at final clarifier
Some small plants almost cease func-
tioning during school vacations, resort
off seasons, etc.
If not overloaded hydraulically
If not overloaded hydraulically
-------
TABLE 2-12
POSSIBLE ACTIVATED SLUDGE PLANT MODIFICATIONS
to
Modification
Tighten process control
Add final clarifier capacity
Increase aerator solids level
Increase air supply or improve its
distribution
Change contact time to 15 to 30 min
Divide reactor into multiple
compartments in series
Change to contact stabilization
When To Use
To minimize soluble BOD in aerator
effluent and improve settling
characteristics of sludge
If solids loading on final clarifier
limits the solids level that can be
maintained in aerator
To increase removals of waste
constituents assimilated by slow-
growing microorganisms
If DO cannot be maintained above
1 to 3 ppm at all loads and locations
In contact stabilization system in
which tests indicate rapid initial
removal is below level desired
If short circuiting has significant
effect on BOD levels in aerator effluent
For upgrading overloaded plug flow of
multicompartmented tanks in which
initial removal is adequate
Remarks
Requires flexibility in return waste
sludge and air rates and requires control
instrumentation
Check buildup of sludge in clarifiers at
high flow rates
Consider effect on final tanks and
adequacy of oxygen supply
Consider oxygen aeration
Check effects on settling characteristics
of sludge; requires small variation in flow
Avoid in small plants with wide diurnal
flow variations
-------
TABLE 2-12 (continued)
POSSIBLE ACTIVATED SLUDGE PLANT MODIFICATIONS
to
ON
to
Modification
Change to step aeration
Change to completely mixed
activated sludge
Improve existing clarification
Change to completely mixed and
add polishing units
Add polishing units such as
maturation ponds, filters, activated
carbon or postaeration
Change from diffused air system to
mechanical aerators
Add a second stage aeration and
clarifier unit
Add super-rate roughing filter
ahead of aeration unit
When To Use
For upgrading overloaded plug flow of
multicompartmented tanks to higher
removals than possible with contact
stabilization
For upgrading any conventional
activated sludge system and for
conversion of small contact
stabilization plants
To decrease solids loading on aeration
units
For increasing flow capacity in extended
aeration units
To improve effluent quality and to
reduce peak discharge of BOD, SS, or
E coli
To increase dissolved oxygen in aeration
tank without increases in energy or
operating cost
To obtain nitrification while increasing
capacity
To decrease solids loading on aeration
units
Remarks
Piping changes will be very costly for
long plug-flow tanks; step aeration
preferable in such cases
-------
6. Provide adequate storage for complete containment, if evaporation and infiltra-
tion equal wastewater inflow (care must be taken to prevent such nuisances as
flies, mosquitoes, or odors if complete containment is used).
2.10.5 Sludge and Process Sidestreams
The effectiveness of sludge treatment and resulting process sidestream treatment is measured
by whether the treated sludge can be satisfactorily disposed of without nuisance and
whether the process sidestream, during handling and treatment, causes nuisance conditions,
interferes with other plant treatment processes, or degrades the plant effluent.
Designs for upgrading the treatment and handling of sludge and process sidestreams are
described in references (93), (98), and (99). Design details are also summarized in Chapter
14.
2.11 Pilot- and Laboratory-Scale Testing
Pilot- or laboratory-scale testing of a treatment process or of equipment is sometimes em-
ployed as a design aid. However, a pilot plant for treatment process testing is seldom em-
ployed for domestic wastewater for small municipalities, unless a large proportion of the
flow is from industry. Some laboratory-scale testing is advisable to determine biological rate
coefficients for specific wastewaters, if a pilot plant is not available. If feasible, however,
pilot-scale testing of treatment systems is the best method of design.
2.12 Reliability Considerations
It is very important that small wastewater treatment facilities be designed and constructed
in such a manner that they can reliably and consistently produce a treated wastewater meet-
ing effluent and receiving-water quality standards, when operated and maintained properly.
EPA reliability guidelines for wastewater treatment plants are presented in Design Criteria
for Mechanical, Electric, and Fluid System and Component Reliability (100). These guide-
lines should be considered by designers of treatment facilities for small communities.
2.13 Process Selection
2.13.1 Variations to Meet Growth
The ratio of extreme minimum flow on the minimum day in a year to peak flow on the
maximum day in the same year may be from 10-20 to 1 at small wastewater works. Even
the ratio of the maximum day flow to average day flow in a year is usually from 5 to 10 to
1. Infiltration of groundwater and stormwater into the collection system can double the
average wastewater flow from a community. If a major amount of the flow to the plant is
seasonal (schools, resorts, etc.), the variations can be even greater. Engineers must take into
account these large variations in the design of the plant.
2-63
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Large variations in flow to clarifiers, contact-stabilization systems, and certain advanced
wastewater treatment units will adversely affect the efficiency of treatment. Equalization
tanks can be used to reduce the daily flow variations. For further information on equaliza-
tion of flows, see Chapter 4. Treatment ponds and extended aeration units are less affected
by large flow variations than are other types of treatment commonly used in small plants.
Some types of structures are more amenable to upgrading than others. For instance, 5- to
6-ft (1.5- to 1.8-m) deep low-rate filters can be modified to high-rate filters by recirculating
a portion of the filter effluent to the filter feed and changing the distributor arm if the
clarifiers, piping, and drains are designed for the higher loading. Similarly, extended aeration
units can be converted to step aeration, then to completely mixed units, and later to contact
stabilization units, with forethought on the part of a designer. Another method of meeting
variations of flow in activated sludge units is to compartmentalize the tanks and use suffi-
cient portions to make volumes roughly proportional to the flow.
2.13.2 Wastewater Treatment Processes
The wastewater treatment processes that should be considered for use in plants serving
municipalities of 100 to 10,000 persons are those requiring a plant that can meet the goals
stated in Chapter 1. The effluents from most plants of less than 1 mgd (0.044m3/s) will be
required to meet secondary effluent requirements.
Typical plant configurations that, if properly designed, could meet secondary treatment
standards and also the other goals of Chapter 1, are shown in Figure 2-10. If the hydraulic
and organic loadings do not vary greatly, the equalization tank might not be needed. If
adequate operation and maintenance are available, the polishing process may not be needed.
Conditions under which these processes should be considered are listed in Table 2-13 (37).
A comparison of operational characteristics of some systems is presented in Table 2-14.
2.13.3 Sludge Treatment Processes
For small treatment plants, most types of sludge processing are too complicated and require
a higher level of experience than is usually available. The types most commonly employed
are aerobic digestion and anaerobic digestion. Stabilized sludge is usually dried on drying
beds, although other processes are sometimes used in larger or more sophisticated plants.
The various sludge treatment processes available are described in Chapter 14. Detailed
information on the treatment of sludge and process sidestreams is found in references (98)
and (99).
Only sludge dewatering is required for sludges from well-designed stabilization ponds and
sometimes, under certain circumstances, for oxidation ditches, for extended aeration acti-
vated sludge, and for extended filtration tower systems. Sludge digestion may not be
needed, if the sludge aging in the last three processes is sufficient for adequate stabilization
and if the plant is sufficiently isolated. Lime treatment may allow satisfactory drying of an
2-64
r
-------
undigested sludge that has undergone a longer biological treatment. The stabilized sludge
should be withdrawn for dewatering and disposal before solids accumulation might begin to
affect SS removal in the final clarifier.
2.13.4 Integration of Processes
Once the information on present and projected flows and organic loadings (with expected
variations and characteristics) has been determined, a preliminary study should be made of
the various combinations of processes that can efficiently meet effluent requirements. Then,
three to five of the most feasible processes should be selected for more detailed cost-effec-
tiveness studies (see Chapter 18).
The need for some commonly included process units in treatment systems depends on loca-
tion climate, hydraulic loading, organic loading, characteristics of the organic loading,
effluent requirements, type of operation and maintenance personnel available, and funding
possibilities. Such units include screens, comminutors, equalization tanks, primary clarifiers,
sludge stabilization processes, and polishing units. All secondary treatment systems require
disinfection and sludge-handling capabilities. For operation and maintenance, all plants will
require a laboratory and a workshop, although these may be limited to bare necessities,
depending on availability of backup services and facilities.
Primary clarifiers may not be required, or the size may be limited to that needed to remove
only the coarsest settleable solids and floatable greases and oils (if the biological process can
effectively handle the increased load at a cost less than that of adding clarifier capacity).
A flow diagram (such as that shown in Figure 2-11) should be developed for most of the
alternative systems—those that include recirculation of recycled process sidestreams—to
insure that all inputs into each unit are considered in the design. Recirculated sludge and re-
cycled supernatant, centrate, or filtrate usually carry high concentrations of solids, even
though the flow rate may be small. The drainage from sludge-drying beds shortly after the
beds are loaded can also be a temporary problem. Some solids are gasified during treatment
processes, and thus a careful balancing is required.
2.14 Operation and Maintenance Design Requirements
The optimum space and facilities for operating and maintaining wastewater treatment pro-
cesses vary for each type of equipment, each layout pattern, and each specific plant site.
The equipment manufacturers' manuals indicate recommended operation and maintenance
space and facilities. (They are not, however, necessarily optimum under all conditions.)
Some design manuals list a 3- to 4-ft (0.9 to 1.2-m) clear space around pumps, while the
optimum on different sides might be 0, 3, 8, or 20 ft (0, 0.9, 2.4, or 6.1 m).
Individual attention must be given on how to best install, replace, repair, operate, and main-
tain each piece of equipment in conjunction with all adjacent equipment, taking into con-
sideration site and structural limitations. Questions that should be considered by designers
in optimizing operations and maintenance space and facilities include:
2-65
-------
PRIMARY SOLIDS
SEPARATION
EQUALIZATION
TANK
BIOLOGICAL CONTACT SURFACE
UNIT (FILTER OR BIO-DISKS)
FINAL SOLIDS REMOVAL
[-POLISHING UNIT
IF NECESSARY
SCREENING AND I *. \
GRIT REMOVAL | >•.. --------- < — '
f / "••.... _y^S_LUDGE DRYING
~ 1 __
SLUDGE
PROCESSING
DISINFECTION
DISPOSAL
i —r—
L J
DISPOSAL—1
BIO-DISK OR TRICKLING FILTER PLANT
WASTEWATER
SLUDGE
PROCESS SIDE-
STREAM
CHEMICAL MIXING
COAGULATION AND
FLOCCULATION
RECARBONATION
(IF LI WE USED)~7
i )
FILTRATION
ACTIVATED
CARBON
v
PHYSICAL-CHEMICAL TREATMENT PLANT
CONTACT BASIN
-REAERATION TANK
•s"^ i 1
I i—H I -*-
\_y j. i
•
•
• ••••»» »4^^ • • •*
CONTACT STABILIZATION ACTIVATED SLUDGE PLANT
FIGURE 2-10
SECONDARY TREATMENT CONFIGURATIONS
2-66
-------
AERATION TANK
I
>*\ J-—i
',J---UJ—
COMPLETE MIX ACTIVATED SLUDGE PLANT
EXTENDED
FILTRATION TOWER
"EFV
U1
!\
^ ,
)
EXTENDED AERATION OR EXTENDED FILTRATION PLANT
•STABILIZATION POND OR AERATED
LAGOON SYSTEM
L J
AERATED LAGOON OR STABILIZATION POND FACILITY
L_J
OXIDATION DITCH FACILITY
FIGURE 2-10 (continued)
SECONDARY TREATMENT CONFIGURATIONS
2-67
-------
TABLE 2-13
APPLICATION OF TREATMENT PLANTS FOR SMALL COMMUNITIES
Process
Rotating Biodisk
Trickling Filters
Low Rate
Intermediate Rate
High Rate
Super Rate
Roughing
Extended (Tower)
Activated Sludge
Complete Mix
Contact Stabilization
Extended Aeration
Oxidation Ditch
Stabilization Ponds
Type Wastewater and Locations Available
Small communities. Cover against freezing, heavy precipita-
tion, or to control odors. Can be used if filamentous orga-
nisms would trouble other biological processes, because they
are less susceptible to such activity.
Cold weather affects efficiency, so covering may be required.
Nitrification possible with deep filters and low loadings. Can
be used if filamentous organisms would trouble other bio-
logical processes. Low contact surface loading rates necessary
to achieve high removal efficiencies.
General application. Resistant to shock organic or toxic load-
ings.
Good if BOD is largely colloidal or suspended. Used in pack-
age plants, if large fluctuations in flow are not expected.
Good for smaller communities. Good for package plants in
housing developments and recreational areas.
General application. Good for smaller communities. Gen-
erally reliable with minimum operation.
Good if large land areas are available. Should process for
microbial cell removal before discharge or provide storage for
semiannual discharge. Completely contained type can be used
if infiltration and evaporation exceed inflow and precipita-
tion.
Physical-Chemical
Can be used if intermittent organic loading makes biological
treatment unreliable, if space is limited (e.g., cold climates
require indoor housing), and if high-quality effluent, includ-
ing phosphorus removal, is required.
2-68
r
-------
TABLE 2-14
OPERATIONAL CHARACTERISTICS OF VARIOUS TREATMENT PROCESSES
PROCESS
Item
Process Characteristics
Reliability with respect to:
Basic Process
Influent Flow Variations
Influent Load Variations
Presence of Industrial Waste
Industrial Shock Loadings
Low Temperatures (<20° C)
Expandability to Meet:
Increased Plant Loadings
More Stringent Discharge Requirements
with Respect to:
SS
BOD
Nitrogen
Operational Complexity
Ease of Operation and Maintenance
Power Requirements
Waste Products
Potential Environmental Impacts
Site Considerations
Land Area Requirements
Topography
Rotating Disk
Good
Fair
Fair
Good
Fair
Sensitive
Good, must add disk
modules
Good, add filtration or
polishing lagoons
Improved by filtration
Good, denitrification
must be added
Some, to simple
Very Good
Low
Sludges
Odors
Trickling Filters
f
Good
Fair
Fair
Good
Fair
Sensitive
Good
Good, add filtration or
polishing lagoons
Improved by filtration
Good, denitrification
must be added
Some, to simple
Very Good
Relatively High
Sludges
Odors
Activated Sludge*
Fair
Fair
Fair
Good
Fair
Good
Fair to good if designed
conservatively
Good, add filtration or
polishing lagoons
Improved by filtration
Good, nitrification-
denitrification must be
added
Moderately Complex
Fair
High
Sludges
_
Activated Sludge^
Very Good
Good
Good
Good
Good
Good
Good, ultimately more
volume will be required
Good, add filtration or
polishing lagoons
Improved by filtration
Good, denitrification
must be added
Some
Very Good
Relatively High
Sludges
_
Facultative Lagoons
Good
Good
Good
Good
Fair
Very Sensitive
Fair, additional
required
Add additional
and filtration
ponds
lagoons
Improved by filtration
Fair
Simple
Very Good
Low
Sludges
Odors
Moderate plus buffer
zone
Relatively Level
Moderate plus buffer
zone
Relatively Level
Moderate plus buffer
zone
Relatively Level
Large plus buffer zone
Relatively Level
Large plus buffer zone
Relatively Level
Complete mix and contact stabilization.
Extended aeration and oxidation ditch.
-------
BIOLOGICAL AND ALUM SOLIDS
PROD
1180
522
1702
JCTION
fi478l LB/DAY BOD
REMOVED:
LB/DAY BIOLOGICAL SOLIDS
803 LB/DAY P REMOVED
2 LB/DAY IN SLUDGE
14.8 LB/DAY BIOL SYNTH
653LB/DAYPRECIP WITH
||4I2| LB/DAY ALUM
GENERATING
LB/DAY ALUM SOLIDS
LB/
DAY
IS
l«>
o
21
LB,
'DAY
0 007 MGD VACUUM FILTRATE
ITS
30OO
106
3000
5
150
18
300
LB/
/DAY
mV|
0 004 MGD SUPERNATANT
IZ50
.,10"
6ZS
s.io4
0 2
10
0 4
20
L!W
m,/,
0 0025 MGD PRIMARY SLUDGE
40
40
0 2
4 S
LB,
'DAY
*V|
0 02 MGD THICKENENCR OVERFLOW
0.03 MGD INTERMEDIATE
CLARIFICR SLUDGE
290
35
250
35
• 2IJ L%Ay
.0 25 0 "»S/|
1004 MGD INTERMEDIATE
CLARIFIER OVERFLOW
REMOVED
E MOVED
ICAL
'DAY
/
„„(
SOLIDS TO
1 ILB/
1 1 'DAY
GAS a WATER
3040
4.I04
LB/D«
0 01 MGD THICKENED SLUDGE
I8O
I0«
-
1
-
1
LIU
""Vi
0.00 MGD FINAL CLARIFIER
SLUDGE
2500
4»I04
<
ISO
500
LB/
'DAY
m«/i
0008 MGD DIGESTED SLUDGE
450
LB'DAY
CHEMICALS:
FeClj 123 LB/ DAY
CoO . 250 L8/OAY
NITRIFYING!
(ACTIVATED/
vSLUDSE/
FINAL
ICLARIFIERl
~"l 1
'DAY
O.OO08MGD WATER
9750 LB/DAY CAKE
TO LANDFILL
167
20
167
20
8
1 0
8
1 0
U8/i»Y
m*l
0999 MGD EFFLUENT
LEGEND
ss
ss
BOO
BOD
COD
COD
P
P
NH 5 LB/DAY
NH, mg/l
VOLUME FLOW, MOD-STREAM NAME
FIGURE 2-11
EXAMPLE PLANT FLOW DIAGRAM
2-70
-------
1. What minimum space is required on each side, top, and underneath, for installa-
tion, replacement, inspection, repair, operation, and maintenance (IRIROM) of
each piece of equipment?
2. Will scaffolding be required for any IRIROM procedure? If so, which, if any,
parts, including wall and floor attachments, of the scaffolding should be made
permanent? Should space be provided for temporary scaffolding?
3. Should space be provided under the equipment in the form of pits or tunnels or
by placing the equipment on platforms to make all required IRIROM procedures
feasible? Can such spaces underneath be used for placing other facilities also? Have
all underfloor spaces been provided with adequate drains and been made safe
against toxic, flammable, or explosive gases? Have methods been included to
ventilate and to prevent or control possible odor emissions?
4. Are facilities and space available to remove any and all parts and to bring in and
install any replacements? Are overhead, traveling cranes, winches, or chain hoists
needed for any IRIROM procedures? Will there be enough use for these to make
some permanent provisions in the structure? Have provisions been made to store
the temporary equipment or otherwise make it readily available by leasing or rent-
ing?
5. Are adequate lighting and ventilation facilities available for all IRIROM proce-
dures?
6. If pieces of equipment must be replaced, are wall or ceiling openings sufficient to
do so without enlargement?
7. Are service outlets for all utilities, such as electricity, gas, and water, required to
complete all IRIROM procedures readily available near each piece of equipment
or facility? For instance, are hose bibs for flushing purposes located close to all
areas that require regular or periodic cleaning?
8. Are drains, or drainage channels, or low curbs available around tanks, equipment,
and facilities to intercept all leaks of water, wastewater, or chemical solution that
might otherwise flow over working areas or walkways? Are such channels below
floor level and grate covered?
9. Have monitoring, sampling, and flow-gaging locations been designed into the
facility? Can the monitoring, sampling, and flow-gaging equipment be easily re-
moved for cleaning or repair? Are they easily accessible?
10. Are all piping, valves, and fittings below floor level, on walls, or more than 6.6 ft
(2.0 m) above the floor? Are all valves on the work space side of all piping? Is
there sufficient space to loosen stuck valves? Can all valves be removed for re-
pairs? Can all sludge-carrying lines be cleaned after each use?
11. Have all equipment and machinery been designed to keep noise levels below the
EPA standard levels?
12. Does the chemical storage area^have sufficient room to maneuver both the pallets
and the pallet-handling equipment?
13. Are all heating and ventilating ducts located where they will not interfere with
placement of piping?
14. Have mechanical or pneumatic means been furnished to remove screenings or grit
from pits so it will not be carried by hand up stairways?
15. Will all work areas be sufficiently well lit and roomy for efficient use of personnel
2-71
-------
time? Is any space sufficiently difficult to work in to cause operators to avoid
working there or to cause supervisors and inspectors to avoid inspecting it?
2.15 References
1. Wastewater Treatment Plant Design. WPCF Manual of Practice No. 8, Washington, D.C.
2. Direct Environmental Factors at Municipal Wastewater Treatment Works. Camp Dres-
ser & McKee, technical report, U.S. EPA, Office of Water Program Operations, Wash-
ington, D.C. (in press).
3. Facilities Planning Summary, U.S. EPA (January 1971).
4. Odum, E.P., Fundamentals of Ecology, 3rd ed. Philadelphia: W.B. Saunders Co.
(1971).
5. Brooks, M., Planning for Urban Trails. ASPO, Chicago (December 1969).
6. McHarg, I.L., Design With Nature. New York: Natural History Press (1969).
7. Leopold, L.B., et al, A Procedure for Evaluating Environmental Impact. Geological
Survey Circular 645, U.S. Geological Survey Department, Washington, D.C.
8. Goodman, W.I., and Freund, E.G., ed., Principles and Practices of Urban Planning.
ICMA, Washington, D.C. (1968).
9. Chapin, F.S., Jr., Urban Land Use Planning. Urbana, 111.: University of Illinois Press
(1965).
10. Claire, W.H., Urban Planning Guide. ASCE, New York (1969).
11. Lynch, K., Site Planning, 2nd ed. Cambridge: MIT Press (1971).
12. Standard Land Use Coding Manual. U.S. Department of Transportation, U.S. Govern-
ment Printing Office (1969).
13. Guidelines to Water Quality Management. U.S. EPA, Washington, D.C.
14. McJunkin, F.E., "Population Forecasting by Sanitary Engineers." Journal of the Sani-
tary Engineering Division, ASCE, vol. 90, no. SA4 (1964).
15. The Methods and Materials of Demography, two volumes. U.S. Bureau of Census
(Washington, D.C.).
16. Obers Projections, Concepts, Methodology, and Summary Data, vol. 1. U.S. Water Re-
sources Council, Washington, D.C. (1972).
2-72
-------
17. Williamson, P.E., Wastewater Treatment Facilities in Small Texas Communities.
Master's thesis, Austin: University of Texas (1971).
18. Geyer, J.C., and Lentz, J.J., Evaluation of Sanitary Sewer System Designs. Johns
Hopkins University School of Engineering (1962).
19. Manual of Septic-Tank Practice, rev. 1967. HEW no. (HSM) 72-10020 (formerly PHS
no. 526), U.S. Government Printing Office, Washington, D.C. (1967).
20. Wastewater Engineering. Metcalf & Eddy, New York: McGraw-Hill (1972).
21. Goldstein, S.N., and Moberg, W.J., Jr., Wastewater Treatment Systems for Rural Com-
munities. Commission on Rural Water, Washington, D.C. (1973).
22. Handbook for Monitoring Industrial Wastewater. U.S. EPA Office of Technology
Transfer, EPA-625/6-73-002 (August 1973).
23. Suburbia Systems, Inc., as referenced by Goldstein, S.N., and Mobey, W.J., Wastewater
Treatment Systems for Rural Communities, Commission on Rural Water, Washington,
D.C. (1973).
24. "Design and Construction of Sanitary and Storm Sewers." WPCF Manual of Practice
No. 9, ASCE Manual of Engineering Practice No. 37 (1969).
25. Bennett, E.R., Linstedt, K.D., and Felton, J., Comparison of Septic Tank and Aerobic
Treatment Units: The Impact of Wastewater Variations on These Systems. Presented at
the Rural Environmental Engineering Conference, Warren, Vt. (September 1973).
26. National Plumbing Code. United States of America Standards Institute, USASI A40.8
(1955).
27. Hunter, R.B., Methods of Estimating Loads on Plumbing Systems, report BMS 65,
National Bureau of Standards, Washington, D.C. (1940).
28. Standard Methods for the Examination of Water and Wastewater, 13th ed. APHA
(1971).
29. Proposed Criteria for Water Quality, vol. 1. U.S. EPA, Washington, D.C. (October
1973).
30. Proposed Water Quality Information, vol. 2. U.S. EPA, Washington, D.C. (October
1973).
31. Methods for Chemical Analysis of Water and Wastes. U.S. EPA Office of Technology
Transfer, EPA-625/6-76-003a (1974).
2-73
-------
32. Sawyer, C.N., and McCarty, P.L., Chemistry for Sanitary Engineers, 2nd ed. New
York: McGraw-Hill (1967).
33. Klein, L., River Pollution II. Causes and Effects. London: Butterworth & Co. (1962).
34. McKinney, R.E., Microbiology for Sanitary Engineers. New York: McGraw-Hill (1962).
35. Palmer, C.M., Algae in Water Supplies. U.S. Public Health Service, publication 657,
Washington, D.C. (1962).
36. Hawkes, H.A., The Ecology of Wastewater Treatment. New York: Pergamon Press
(1963).
37. Tchobanoglous, G., Water Treatment for Small Communities. Prepared for the Con-
ference on Rural Environmental Engineering, Warren, Vt. (September 1973).
38. Process Design Manual for Sulfide Control in Sanitary Sewerage Systems. U.S. EPA,
Office of Technology Transfer, EPA-625/1-74-005 (October 1974).
39. Adams, A., and Spendlove, J.C., "Coliform Aerosols Emmited by Sewage Treatment
Plants." Science, vol. 169, pp. 1218 to 1220 (18 September 1970).
40. Ledbetter, J.O., "Air Pollution from Wastewater Treatment," Water and Sewage
Works, vol. 113 (2), pp. 43 to 45 (February 1966).
41. Albrecht, C.R., Bacterial Air Pollution Associated with Sewage Treatment Process.
Master's thesis, Gainsville: Florida University, Gainsville College of Engineering
(August 1968).
42. Air Pollution. Stern, A.C., ed., vols. 1, 2, 3, 2nd ed. New York: Academic Press (1968).
43. Jones, F.W., "What Are Offensive Odors About a Sewage Works." Sewage Works
Journal, vol. 4, No. 1, pp. 60 to 71 (January 1932).
44. Kremer, J.G., Odor Control Methods, Experimentation and Application. County Sani-
tation Districts of Los Angeles County, Calif, (no date).
45. Cohn, M.M., "Sewer Works Odors and Their Control." Sewage Works Journal, vol. 1,
no. 5, pp. 568-577 (October 1929).
46. Turner, D.B., Workbook of Atmospheric Dispersion Estimates. National Technical In-
formation service, U.S. Department of Commerce (revised 1970).
47. Measurement of Odor in Atmospheres. ASTM D 1391-57 (reapproved 1967).
2-74
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48. Sawyer, C.N., and Kahn, P.A., "Temperature Requirements for Odor Destruction in
Sludge Incineration," Journal Water Pollution Control Federation, vol. 32, no. 12,
pp. 1274 to 1278 (December 1960).
49. Dague, R., "Fundamentals of Odor Control." Journal Water Pollution Control Federa-
tion, vol. 44, no. 4, pp. 583 to 594 (April 1972).
50. Post, N., "Counteraction of Sewage Odors." Journal of Sewage and Industrial Wastes,
vol. 28, no. 2, pp. 221 to 225 (February 1956).
51. Unangst, P.C., and Nebel, C.A., "Ozone Treatment of Sewage Plant Odors." Water and
Sewage Works, reference no. 1971, vol. 118, RN, pp. R42, 43 (August 1971).
52. Carlson, D.A., and Leiser, C.P., "Soil Beds for the Control of Sewage Odors." Journal
Water pollution Control Federation, vol. 38, no. 5, pp. 829 to 840 (May 1966).
53. Santry, I.W., Jr., "Hydrogen Sulfide Odor Control Measures. "Journal Water Pollution
Control Federation, vol. 39, no. 3, pp. 459 to 463 (March 1966).
54. Ledbetter, J.O., and Bandall, Clifford, W., "Bacterial Emissions From Activated Sludge
Units." Industrial Medicine and Surgery, pp. 130 to 133 (February 1965).
55. Safety in the Design, Operation, and Maintenance of Wastewater Treatment Works.
Technical report, U.S. EPA, Office of Water Program Operations, Washington, D.C.
(1975).
56. Safety in the Operation and Maintenance of Wastewater Treatment Works. Technical
Bulletin, U.S. EPA, Office of Water Program Operations, Washington, D.C. (1975).
57. Safety in the Design of Wastewater Treatment Works. Technical Bulletin, U.S. EPA,
Office of Water Program Operations, Washington, D.C. (1975).
58. "Safety in Wastewater Works," WPCF Manual of Practice No. 1, Washington, D.C.
(1959).
59. "Occupational Noise Exposure," Federal Registrar, vol. 36, no. 105, 1910.95, OSHA
(29 May 1971).
60. Errata published in Federal Registrar, vol. 34, no. 96, Walsh-Healey Public Contract
Act, Federal Registrar, vol. 34, No. 96, rule 50-204.10 (20 May 1969).
61. Beranek, L.L., Noise and Vibration Control. New York: McGraw-Hill (1971).
62. Information on Levels of Environmental Noise Requisite to Protect Public Health and
Welfare With an Adequate Margin of Safety. EPA 550/9-74-004 (March 1974).
2-75
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63. Community Noise. U.S. EPA, NTID 300.3, Government Printing Office, 5500-0041,
Washington, D.C. (1971).
64. Noise Assessment Guidelines. U.S. HUD, Government Printing Office, 2300-1194,
Washington, D.C. (1971).
65. Gatley, W.S., "Industrial Noise Control." Mechanical Engineering (April 1971).
66. Fundamentals of Noise: Measurement, Rating Schemes and Standards. National Bureau
of Standards, NTID 300-15, U.S. Government Printing Office, Washington, D.C.
(1972).
67. Beranek, L.L., "Industrial Noise Control." Chemical Engineering (27 April 1970).
68. Seebold, J.G., "Process Plant Noise Control at the Design Engineering Stage." Trans-
actions of the ASME, paper no. 70-pet-ll (1970).
69. Environmental Impact Analysis: An Introduction to Analytical and Presentation
Methods. California Council of Civil Engineers and Land Surveyors, Sacramento:
Central Printing Co. (July 1972).
70. Cheremisinoff, P.N., and Young, R.A., "Materials and Methods of Noise Control."
Pollution Engineering (October 1974).
71. Becker, R.J., and Skistis, S.J., "Hydraulically Operated Machine Noises." Environ-
mental Science and Technology (November 1974).
72. Smith, S.A., and Wilson, J.C., "Trucked Wastes: More Uniform Approach Needed."
Water and Wastes Engineering (March 1973).
73. Kolega, J.J., "Design Curves for Septage." Water & Sewage Works (May 1971).
74. Weinberger, L.W., Waller, R., and Gumtz, G.D., "Vessel Waste Control At Duluth-
Superior Harbor." Pollution Control in the Marine Industries, T.F.P. Sullivan, ed.,
International Association for Pollution Control, pp. 187 to 220, Washington, D.C.
(1973).
75. Bruderly, D.E., and Piskura, J.R., "Solid Waste, Liquid Waste, Air Pollution and Noise
Pollution Management Planning for Ports." Pollution Control in the Marine Industries,
pp. 235 to 265.
76. McKee, J.E., and Wolf, H.W., Water Quality Criteria, 2nd ed. Publication no. 3-A, State
Water Quality Control Board, Sacramento (1963).
77. Water Quality Criteria. Report of the National Technical Advisory Committee to the
Secretary of the Interior, FWPCA, Washington, D.C. (April 1968).
2-76
-------
78. McGauhey, P.M., Engineering Management of Water Quality. New York: McGraw-Hill
(1968).
79. Proposed Criteria for Water Quality. U.S. EPA, Washington, D.C. (October 1973).
80. Wastewater Treatment and Reuse by Land Application, vol. 1, U.S. EPA, Office of
Research and Development, EPA-660/2-73-006a, Washington, D.C. (1973).
81. Ibid., vol. 2.
82. Evaluation of Land Application Systems. Technical Bulletin, U.S. EPA, Office of Water
Program Operations, Washington, D.C. (1974).
83. Survey of Facilities Using Land Application of Wastewater. U.S. EPA, Office of Water
Program Operations, Washington, D.C. (1973).
84. Fogg, C.E., Land Application of Sewage Effluents. Presented at the 28th annual meet-
ing, Soil Conservation Society of America, Hot Springs, Ark. (October 1973).
85. Water Pollution Control in Low Density Areas. Proceedings of Rural Environmental
Engineering Conference, W.J. Jewell and R. Swan, eds., Hanover, N.H.: University
Press of New England (1975).
86. Cowlishaw, W.A., "Update on Muskegon County, Michigan Land Treatment System."
Presented at ASCE annual and national Environmental Engineering Convention, Kansas
City, Mo. (October 1974).
87. Chiken, E.I., Polonisik, S., and Wilson, C.D., "Muskegon Sprays Sewage Effluents on
Land." Civil Engineering (May 1973).
88. Parizel, R.R., Kardos, L.T., Sopper, W.E., Myers, E.A., Davis, D.E., Parrel, M.A., and
Nesbitt, J.B., Wastewater Renovation and Conservation. Administrative Committee on
Research, Pennsylvania: Penn State University (1967).
89. Kardoz, L.T., et al., Renovation of Secondary Effluent for Reuse as a Water Resource.
Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.
(1974).
90. Bouwer, H., Rice, R.C., Escaruga, E.D., and Riggo, M.S., Renovating Secondary Sew-
age by Ground Water Recharge with Infiltration Basins. U.S.1 EPA, WPC research series
16060 DRV 03/72, Washington, D.C. (March 1972).
91. Lawrence, A.W., Design of Wastewater Treatment Systems to Satisfy Effluent Quality
Requirements based on Intended Use. Ithaca: Cornell University (September 1971).
2-77
-------
92. Process Design Manual for Upgrading Existing Wastewater Treatment Plants. U.S. EPA,
Office of Technology Transfer, EPA-625/l-71-004a (1974).
93. Upgrading Existing Wastewater Treatment Plants, Case Histories. U.S. EPA Technology
Transfer Seminar Publication, EPA-625/4-73-005a (August 1973).
94. Upgrading Existing Wastewater Treatment Plants. Presented at U.S. EPA Technology
Transfer Design Seminar (August 1972).
95. Lewis, R.F., and Smith, J.M., Upgrading Existing Lagoons. U.S. EPA, Office of Re-
search and Development (October 1973).
96. Upgrading Lagoons. U.S. EPA Technology Transfer Seminar Publication, EPA-625/4-
73-00la (August 1973).
97. Thumann, A., and Miller, R.K., Secrets of Noise Control. Atlanta: Fairmont Press
(1974).
98. Sludge and Process Liquid Sidestreams at Wastewater Treatment Works. Camp Dresser
& McKee, U.S. EPA background report (1974).
99. Municipal Wastewater Treatment Plant Sludge and Liquid Sidestreams. Camp Dresser
& McKee, U.S. EPA Technical Bulletin (1974).
100. Design Criteria for Mechanical, Electric, and Fluid System and Component Reliability.
U.S. EPA Technical Bulletin, EPA-430-99-74-001 (1973).
2-78
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CHAPTER 3
HYDRAULIC CONSIDERATIONS
3.1 Introduction
Hydraulic theories must be properly applied during the design phase of a wastewater plant,
to insure that the plant functions satisfactorily. Hydraulics must be considered in designing
pumps, valves, flow measuring devices, and equipment controls, as well as pipes, channels,
and basins. Hydraulic flow characteristics must be determined for all critical combinations
of flows into conduits, channels, and basins. This chapter discusses hydraulic criteria rele-
vant to the design of small wastewater treatment plants.
3.2 Flow Considerations
3.2.1 Limiting Velocities
Velocity of flow and turbulence affect the efficiency and reliability of hydraulic controls,
pumping, valving, fluid measuring devices, and various processes. Some considerations in-
fluencing the hydraulic design of these items are discussed below.
Conduits carrying wastewater with grit and settleable organic solids should be designed, if
possible, to have a scouring velocity at least once a day. Peak velocities, at the beginning and
end of the design period, should be considered in selecting the design scouring velocity.
Scouring velocity depends on the density and other characteristics of the wastewater and
the grit. Scouring velocities vary from 1.5 to 4 ft/sec (0.45 to 1.20 m/s); 3 ft/sec (0.9 m/s) is
usually satisfactory. If horizontal velocities are less than about 2.0 ft/sec (0.6 m/s), grit will
settle.
If floe is to be kept in suspension, the horizontal velocity at the bottom of the basin or tank
should exceed 20 ft/min (0.10 m/s) (1). Therefore, a design bottom velocity of about
30 ft/min (0.15 m/s) should be used to keep organic solids in suspension. In the initial stages
of plant operation, when minimum velocities may become less than 30 ft/min, scouring
velocities of 1.5 to 2.0 ft/sec (about 0.45 to 0.60 m/s) must be scheduled for about 1 hr
daily to scour sludge deposits.
Head losses from friction and obstructions in a conduit are approximately proportional to
the square of the velocity. If pumping is required, the maximum head loss used for design
should be determined by cost-effectiveness studies, considering energy, equipment, and
effects on processes. Upper limiting velocities should be established in designing specific
plant piping, to prevent possible erosion of, or damage to, conduits or their linings. This
maximum velocity (which can vary for specific conditions) should range from about 5 to 12
ft/sec (1.5 to 3.6 m/s). Supercritical velocities should be avoided.
3-1
-------
3.2.2 Flow Division
Equal hydraulic division of flow does not necessarily divide the organic load equally. Unless
the SS are homogeneously dispersed throughout the liquid and the relative momentum of all
particles is approximately equal at the point of flow division, the SS will usually divide
unequally. Some turbulence is desirable at each point of diversion, to achieve sufficient
homogeneity.
In general, flow will split equally if 1) there is a sufficiently large head loss through the con-
trol, compared to the differences in head loss available, and 2) there is an equality of head
loss across the weir, if weirs are used. If the head loss constraint cannot be met, symmetry
of layout tends to achieve the same purpose.
Figure 3-1 shows several methods of obtaining equal flow division. The velocity through an
orifice or other pressure control device varies as the fractional power of the head. A small
percentage of difference in head losses through the device still results in relatively equalized
velocities of fluids entering several channels (or chambers) from one channel (or chamber).
The required head loss for a stipulated deviation in flow—such as where the flow varies as
the square root of the head (e.g., through orifices)—can be determined from the following
equation (2):
h0 = (2Ah)/(l-m2)
where
hg = head loss through the outlets in which the available head is greatest (the head
loss is usually based on equal flow split)
SAh = greatest difference in head (piezometric head or water surface elevation)
available for dividing the specified flow
m = minimum permissible ratio of discharge rates through the outlets, deter-
mined from the ratio of minimum to maximum divided flow
In considering the flow dividing structure shown in Figure 3-1 (III), assume that friction
losses in the influent distribution channel upstream from the four tanks are negligible, com-
pared to the velocity head. Assuming that the plant flow is to be equally split, with a devia-
tion of no more than 5 percent, the value of "m" is (100 — 5)/100, or 0.95. Flow is
assumed to enter each tank through two submerged ports with a discharge coefficient of
0.6. If the velocity in the distribution channel is to be about 1 ft/sec (0.3 m/s), Ah would
then be the last velocity head when the water changed direction (90° x Ah = v2/2 g = 1/64.4
= 0.016 ft). The head loss (ho) that must occur at each port would be ho = Ah/(l - m2),
or ho = 0.016/(1 - 0.952) = 0.164 ft. The size of the port can thus be determined from the
equation for head loss through an orifice:
3-2
-------
OVERFLOW WEIR
BAFFLE-
~\
\
r v
^COMPLETELY
H MIXED
WASTEWATERx
J
SPLITTING PARTITIONS-
/
/
_
I PLAN USING COMPLETELY
MIXED SPLITTING BOX
BARS TO CAUSE
TURBULENCE
II PLAN USING TEES
III PLAN USING ADJUSTABLE LEVEL
ORIFICE INLETS AND OUTLETS
IY PLAN USING PARSHALL
OR CUTTHROAT FLUMES
PROFILE
OVERFLOW
WEIRS
PLAN
V UPFLOW INLET
FIGURE 3-1
METHODS OF WASTEWATER FLOW DIVISION
3-3
-------
h0 = (Q/0.6A)2/2g
A = [Q2/(h0)(2g)(0.6)2]0.5
A = [0.12/(0.164) (64.4) (0.36)]°-5
A = 0.0513ft2
D = (4A/7T)0-5 (12 in./ft) = 3.068 in.
where
A = area
D = diameter
Five of the more common methods for uniformly dividing flows and organic loadings are
shown in Figure 3-1. Short discussions of these methods follow. (It is assumed in each case
that grit separation has taken place before flow division.)
Plan I. If the fluid is completely mixed when divided, the suspended solids load in
each part of the flow will be proportional to the volume. Several methods are used to
obtain complete mixing (i.e., to attain a minimum velocity of about 0.5 ft/sec [0.15
m/s]): mechanical mixers, diffused aeration, jets of recirculated effluent, and various
combinations of these methods. V-notch weirs, rectangular weirs, or orifices may be
used to separate the flows uniformly from a completely mixed body of water.
Plan II. When a flow is divided, there will normally be more suspended solids in the
portions having the more distinct change of direction. For example, if flows are split
from one channel into three parallel channels, the two outer channels will contain
more solids (if the mixing was not complete). If flows are divided using a series of tees,
the upstream bends will create centrifugal forces that affect the path the solids take
downstream. To create a sufficient increase in velocity to obtain the needed turbulence
and mixing and thus insure maximum homogeneity at the point of division, barracks
or posts can be placed in the channel upstream of the 90° bends. If pipes, tees, or wyes
are used for the division of settled flows, bars can be placed at the upstream joint of
the tee or wye, to cause the required mixing before division. If a straight section of
conduit 6 to 8 diameters (or channel widths) long can be located upstream of the
divider, effects of centrifugal forces will be well damped and will allow a division of
solids more nearly proportional to the subdivided flows.
Plan III. If flows are split from a channel at points not equidistant from the entrance
flow, the hydraulic head at successive points along the channel will be different, de-
pending on the velocity heads and friction losses. Reference (2) contains detailed
analyses of this problem. To obtain a relatively even split of flows, the equation ho =
(£Ah)/(l — m2) applies. Here, also, an equal split of suspended material requires suffi-
cient mixing in the dispersion conduit, to obtain a relatively homogeneous solids dis-
persion.
3-4
-------
Plan IV. If a relatively uniform head is available across a channel (i.e., no nearby
upstream changes in direction), with approach velocities of less than about 2 to 3 ft/sec
(0.6 to 0.9 m/s), Parshall or cut-throat flumes may be used to divide the flows equally.
The design of such flumes is given in references (3), (4), (5), (6), (7), (8), and (9).
Every effort should be made to insure a relatively homogeneous mixture before divi-
sion.
Plan V. A simple method of obtaining good mixing (ideal for small plants) is use of a
bottom entrance to the splitting box. The flow enters the box in the middle and can be
split most accurately there, with flows at 90° to the center line of the inverted siphon,
or at the sides of the box. The entrance velocities should be kept relatively low with
respect to the depth of the splitting box, to prevent excessive surface turbulence and
resultant variations of head at the flow control device. Weirs, or orifices with sufficient
head loss, can be used to divide the flows satisfactorily. For works built in two stages,
flows from A and B can be used for one stage and from C and D for the other stage.
With an odd number of divisions, a round splitting box is simpler.
3.2.3 Plant Hydraulic Gradient
Hydraulic gradient, diagrams of the main stream and all sidestream flows at the treatment
plant should be prepared, to insure that adequate provision is made for all head losses.
Because cost and availability of energy are significant factors in the operation of a plant, the
design allowances for the head loss of each unit, as well as through the entire plant, from the
entrance through the outfall, must be carefully considered, particularly if pumping is re-
quired. Unnecessary head allowances increase the cost of pumping; insufficient allowances
make operation difficult and expansion costly.
Bernoulli's theorem (basic to hydraulic system design) states that, under conditions of
steady flow, the sum of the velocity head, pressure head, and head from elevation at any
point along a conduit or channel is equal to the sum of these same heads at any other down-
stream point plus the losses in head between two points resulting from friction or turbu-
lence. A line representing only the sum of the pressure head and the elevation head at a
number of points in a system represents the hydraulic grade line or equivalent free water
surface. In wastewater treatment plant designs, the hydraulic grade line (or free water sur-
face only) is usually shown. Although the total energy grade line is frequently not shown, it
should always be calculated to obtain a more accurate representation of the changes in
water surface elevation. References (3), (5), (6), (10), (11), and (12) discuss this in more
detail.
Energy changes usually requiring consideration in developing hydraulic profiles include the
following:
1. Head losses from wall friction in conduits.
2. Head losses caused by sudden enlargement of flow, such as flows into tanks and
larger conduits; sudden contractions, such as at entrances, orifices, inlets, weirs,
3-5
-------
and flumes; sudden changes in direction, such as bends, elbows, and tees; sudden
changes in slope or drops, such as after weirs and flumes; and possible obstruc-
tions because of deposits in a conduit.
3. Heads required to allow discharges over weirs and through flumes, orifices, and
other measuring, controlling, or flow division devices.
4. Head gains or losses from momentum changes.
5. Head allowances for expansion of facilities, to meet future requirements.
6. Head allowances for maximum water levels in the receiving waters, to prevent
backup of flow in the outfall, if that might cause difficulty.
7. Head allowances for unusual restrictions in the downstream flow, which could
back up the wastewater stream and submerge measuring or control devices.
8. Head gained by pumping.
9. Head requirements for flow through comminutors, bar screens, fine screens, mix-
ing tanks, equalizations tanks, flocculation tanks, clarifiers, aeration tanks, filters,
carbon contactors, ion exchangers, chlorine contactors, ozone contactors, and all
other treatment processes.
10. Head losses that may result if air causes bulking, or escapes from the wastewater
to form air pockets, thus restricting flow.
11. Combined momentum and side-wall energy losses in a conduit, if the flow is split
along the side of the conduit (two-thirds of the head required may be needed to
provide the changes in momentum).
In addition to the average design flow grade line, the designer should prepare grade lines for
minimum flows at the time of startup and peak flows at the end of the design period.
Hydraulic profiles serve as a very useful tool for insuring that all pertinent head losses are
accounted for and that the plant can satisfactorily function hydraulically, both now and in
the future. Hydraulic profiles should also be prepared for all feasible alternative designs.
Figures 3-2, 3-3, 3-4, and 3-5 show typical hydraulic profiles for small wastewater treatment
plants. Figure 3-2 illustrates units that may be required in a later stage of construction in
developing a hydraulic gradient to upgrade an existing primary plant. Figure 3-3 shows the
hydraulic gradient of a small plant on a site where the flow can be maintained by gravity.
This plant is designed to meet variations in flow while reliably producing a properly treated
effluent. Figure 3-4 shows structures that cross the principal stream of flow and create a
need for bends, causing additional head losses.
This plant was expanded once and is currently undergoing a second expansion and upgrad-
ing. Note that provisions are made for energy dissipation and reaeration when the stream
flow is average. Because two pumping stations are involved, there are significant head losses
to consider in addition to the static head, to obtain the dynamic head requirements for the
pumps. Figure 3-5 shows a typical small secondary treatment plant located in a flat area
near a stream. Pumping required before treatment provides sufficient head for gravity flow
through the plant.
References (5) and (12) contain detailed discussions of the various types of head losses and
how they are calculated. References (3), (4), (6), (9), (10), (11), (13), and (14) also contain
detailed information on head losses.
3-6
-------
590
580
570
CHANNEL
PIPES WITH
PROVISION FOR
FUTURE
AERATION
BASINS PIPE
HYDRAULIC GRADIENT
S90
560
FIGURE 3-2
HYDRAULIC GRADIENT-WASTEWATER TREATMENT PLANT
WITH PROVISION FOR FUTURE UPGRADING
-------
CHLORINATION
AND CONTROL
HYDRAULIC GRADIENT
25
25
FIGURE 3-3
HYDRAULIC GRADIENT-WASTEWATER TREATMENT PLANT
-------
BAR RACK GRIT
CHAMBER
I-COLLECTING
MANHOLE
•o
HYDRAULIC GRADIENT
1020
1010
1000
990
980
1020
1010
1000
990
980
970
960
950
940
J935
FIGURE 3-4
HYDRAULIC GRADIENT-WASTEWATER TREATMENT PLANT
-------
r PARS HALL
FLUME AND
DISTRIBUTION
STRUCTURE
IOOO
STEPPED
SPILLWAY HO 2
WITH FOAM
CONTROL
DISTRIBUTION BOX
INCLUDING RECYCLE
TO TRICKLING FILTER
STEPPED
SPILLWAY NO i
WITH FOAM
CONTROL
FINAL
SEDIMENTATION
TANK NO.3
CHLORINE
CONTACT
TANKS
INTERMEDIATE
SEDIMENTATION TANKS
HYDRAULIC GRADIENT
EL 983.08
EL.981.87
EL 979.50
EL .978.39
ESTIMATED HWL BY CORPS
OF ENGINEERS FOR
INTERMEDIATE REGIONAL
FLOOD (100-YEAR )
EL »«3.0±
CONTINUED FROM
FIGURE 3-4
/•EL 968 91
EL.968.59
APPROX W S
EL 950 8
950
FIGURE 3-4 (continued)
HYDRAULIC G RAD IE NT-WASTE WATER TREATMENT PLANT
-------
520
PUMP.
PROCESS
AND CONTROL
PIPE BUILDING
SCREEN r PAR SHALL FLUME
-PARSHALL
FLUME
-PIPE
r- SCREEN r
\ GRIT \
\ ICHAMBERA
PIPE AERATION TANK PIPE SECONDARY CLARIFEf PIPE
CHLORINE OIFFUSER [-MANHOLE
CHLORINE
CONTACT TAHK^L L I (JUTFALL
PIPE
MILLERS RIVER
520
EL 50748
510
500
490
480
-HYDRAULIC GRADIENT
510
500
490
480
470
470
FIGURE 3-5
HYDRAULIC GRADIENT-WASTEWATER TREATMENT PLANT
-------
3.2.4 Sludge Flow Hydraulics
The flow characteristics of sludges vary according to the types and concentrations of organic
solids and chemicals, Some sludges have flow characteristics similar to clear water; others
have pseudoplastic flow characteristics. Many sludges are thixotropic in nature; that is, their
viscosity increases with time if in a static state, but on agitation returns to its original value.
The flow of sludges with lower solids contents and viscosities close to that of water may be
calculated, using turbulent-flow friction losses in common hydraulic formulas. Return acti-
vated sludge and sludge from intermediate or final clarifiers generally have waterlike flow
characteristics. Sludges from primary clarifiers, scum thickeners, holding tanks, and digesters
have pseudoplastic or thixotropic characteristics.
If pseudoplastic type sludges are to be pumped long distances, velocities must be kept in the
laminar flow ranges, to keep the head losses down. Reference (15) contains design details on
this type of flow.
In wastewater treatment plants, sludge flows are normally intermittent. Thus, to scour any
solids that separate during periods of quiescence, sludge velocities of 3 to 5 ft/sec (0.9 to
1.5 m/s), or higher for more waterlike sludges, should be kept. For heavier sludges and
grease, velocities of 5 to 8 ft/sec (1.5 to 2.4 m/s) are needed. Because all sludge piping
must pass 3-in. (75 mm) solids and must be at least 4 in. (100 mm) in diameter to pass the
solids and permit cleaning, the flow invariably will be in the turbulent rather than the
laminar range (provided the drawoff rate is sufficient). Therefore, once sludge flow has
started, formulas for turbulent flow of fluids can be used. If the Hazen-Williams formula is
used, C factors (dependent on conduit roughness) from 60 to 90 may be used (20 to 40
percent less than for wastewaters).
To prevent disruption of treatment processes, provisions must be made in all sludge conduits
for cleanouts, vent pipes, taps for steam or hot water, and access for mechanical cleaning.
Pumps, if at all possible, should have a positive suction head of at lest 3 ft (0.9 m) or higher
(particularly for reciprocating pumps), plus head losses in the suction line. Standby sludge
pumps must be provided (15).
3.3 Pumping
References (3), (5), (6), (10), (11), (13), (14), and (16) and the catalogs and data issued by
pump manufacturers contain a wealth of detail for design procedures. Current technical
publications regularly contain discussions of some aspect of pumping.
Because wastewater flowing into a plant continues—although pumps may stop—spare
pumps, spare power sources, screening, overflow pond storage, etc., which will automat-
ically begin functioning when a unit stops, must be provided. Reliable, efficient pumping
with standby equipment should be incorporated in the design of the system (15).
3-12
-------
3.3.1 Types of Wastewater and Sludge Pumps
Types of pumps available for use at wastewater facilities for various needs are listed in Table
3-1. The pumping system selected must be able to meet the varying head conditions caused
by differences in free water level in the wet well and the receiving body of water, plus all the
head losses in the conduit. Head losses in the conduit, which depend on flow variations
throughout the life of the pumping system, include wall friction and losses at entrances, out-
lets, valves, measuring devices, elbows, bends, tees, reducers, and any other location or cross-
sectional area where the flow changes direction. Whenever two or more pumps will be trans-
ferring liquid from a single source into a common conduit, head curves for all combinations
of pumps in the pumping system must be developed to determine a program that will meet
all head flow combinations,
A process for selecting the best pump for handling raw wastewater is described in references
(12) and (17).
3.3.2 Pump Drive Equipment
Directly connected electric motors are commonly used to drive pumps at small wastewater
treatment plants. If two independent sources of electrical power are unavailable (as is
usually the case), internal combustion engines that burn diesel oil, gasoline, propane, or
methane may be used to provide standby electrical generation capacity or direct power for
pumps (15).
Squirrel cage motors are most commonly used in small plants, because of their low cost,
reliability, and ruggedness. They can be used if the pumping operation is continuous and the
loads uniform. Auxiliary control or special windings are advised if operation is intermittent
or the load fluctuates, to provide better starting conditions. Wound rotor motors are gen-
erally used for variable speed operation. High-speed motors with reduction gears are used to
operate valves, gates, screens, mixers, flocculators, and other slow-speed equipment.
Factors to be considered in comparing alternative drives include energy usage, reliability,
simplicity of operation and maintenance, ruggedness, and cost of each type of drive meeting
the pertinent conditions (variability of flow, head, and available power). Characteristics to be
determined by the engineer for each drive are type of motor or engine; horsepower (kW);
electric current, voltage frequency and phases; starting torque, voltage, and current; speed;
speed reducer requirements; alternative drive requirements; overload, underload, overvolt-
age, undervoltage, and other required motor protection devices; type of bearings, insulation,
and ventilation; noise reduction; mounting or setting; fire-proof and safe fuel supply; and
means of simple replacement or overhaul on site. The details on this subject can be found in
references (3), (5), (10), (11), and (12) and from manufacturers of drive equipment.
3-13
-------
TABLE 3-1
PUMPING ALTERNATIVES FOR VARIOUS TREATMENT PLANT NEEDS
Pump Type
Air Lift
Centrifugal
Diaphragm
Plunger
Progressive
Capacity
Screw
Torque-flow
Vertical
Turbine
Description
Used for lifting liquid from wet wells or basins. Air under pressure is introduced
into the liquid to reduce apparent specific gravity of the air/liquid mixture and
thus cause it to rise to the discharge outlet.
Consists of an impeller fixed on a rotating shaft and enclosed in a casing with
an inlet and discharge connection. The rotating impeller creates pressure in
the liquid by the velocity created by the centrifugal force. Pump is
commonly classified according to impeller as radial, mixed flow, or axial.
Uses a flexible diaphragm or disk, generally made of rubber, metal, or
composition material, fastened at the edges of a cylinder. When the disk is
moved at its center by a piston in one direction, suction is created and
liquid enters from the suction line. When the disk is moved in the other
direction, pressure forces the liquid out the discharge line. Check valves
are required on both suction and discharge lines.
A reciprocating pump with a plunger which does not contact the cylinder
walls, but enters the cylinder and withdraws through packing glands, thus
alternating suction and pressure.
Positive displacement, screw type pump that uses a single-threaded rotor,
operating with a minimum of clearance in a double-threaded helix of
soft or hard rubber or other material.
Uses a spiral screw operating in an open inclined trough.
Uses rotating-inducing element, which is entirely out of the flow path of
the liquid through the pump.
A centrifugal pump, which uses a pump shaft in a vertical position.
May Be Used For
Secondary Sludge Recirculation
and Wasting
Raw Wastewater
Secondary Sludge Recirculation
and Wasting
Settled Primary and Secondary
Effluent
Chemical Solution
Primary Sludge
Skimmings
Secondary Sludge Recirculation
and Wasting
Thickened Sludge
Chemical Solution
Skimmings
Thickened Sludge
Chemical Solution
Raw Wastewater
Settled Primary and Secondary
Effluent
Grit
Raw Wastewater
Primary Sludge
Secondary Sludge Recirculation
and Wasting
Thickened Sludge
Settled Primary and Secondary
Effluent
-------
3.4 References
1. Camp, T.R., "Outstanding Papers." Civil Engineering Classics, ASCE (1973).
2. Camp, T.R., and Graber, S.D., "Dispersion Conduits." Journal of the Sanitary Engi-
neering Division, ASCE (February 1968).
3. Babitt, H.E., and Baumann, E.R., Sewerage and Sewage Treatment. New York: John
Wiley & Sons (1967).
4. Chow, V.T., Open Channel Hydraulics. New York: McGraw-Hill (1959).
5. Davis, C.V., and Sorensen, K.E., Handbook of Applied Hydraulics. New York:
McGraw-Hill (1969).
6. King, H.W. (O. Wisler Co.), and Woodburn, J.G., Hydraulics. New York: John Wiley &
Sons (1948).
7. Fluid Meters: Their Theory and Application. ASME Research Committee on Fluid
Meters (1971).
8. Handbook for Monitoring Industrial Wastewater. U.S. EPA, Office of Technology
Transfer, EPA-625/6-71-002 (1971).
9. Water Measurement Manual, 2nd ed. U.S. Bureau of Reclamation, Denver (1967).
10. "Wastewater Treatment Plant Design." ASCE Manual of Practice No. 36 and WPCF
Manual of Practice No. 38 (1959).
11. Waste-water Engineering. Metcalf & Eddy, New York: McGraw-Hill (1972).
12. "Design and Construction of Sanitary Storm Sewers." ASCE Manual of Practice No. 37
and WPCF Manual of Practice No. 9 (1960).
13. Rouse, H., Elementary Mechanics of Fluids. New York: John Wiley & Sons (1946).
14. , Advanced Mechanics of Fluids. New York: John Wiley & Sons (1959).
15. Design Criteria on Mechanical, Electric, and Fluid System and Component Reliability.
U.S. EPA 430-99-74-001, Office of Water Program Operations (1974).
16. Recommended Standards for Sewage Works. Great Lakes-Upper Mississippi River
Board of State Sanitary Engineers (1973).
17. Hydraulic Institute Standards. Hydraulic Institute, New York (1964).
3-15
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CHAPTER 4
FLOW EQUALIZATION
4.1 Introduction
Equalization of fluctuating wastewater flows will make hydraulic pollutant loadings more
uniform and may improve the effectiveness or reliability of essential treatment processes.
Equalization can, therefore, be a valuable technique in the upgrading of existing facilities
and should be considered a feasible design alternative for new treatment facilities. If the
system experiences large, erratic discharges of industrial wastes, equalization may be easily
justified.
Storing above-average flow in a basin, pond, tank, or conduit (a less desirable alternative) for
uniform release to treatment processes during periods of below-average flow will achieve
equalization. A detailed discussion of flow equalization for large municipal facilities can be
found in the Process Design Manual for Upgrading Existing Wastewater Treatment Plants
(1). This chapter discusses the criteria for the design of equalization facilities for small
wastewater treatment plants.
4.2 Variations in Wastewater Flow
The daily wastewater flow from a small community usually varies more in quality and
quantity than does the flow from larger communities. (Details of variations are presented in
Chapter 2). Hydraulically, the major portion of flow from a small community occurs during
the daylight hours, with minimal domestic wastewater flow from 2 a.m. to 6 a.m. Two or
three peak flows during the day and one minimum flow at night may occur. For some treat-
ment processes to operate and use space most efficiently, the influent flow should be kept
relatively consistent in rate and concentration of constitutents.
In smaller communities, quality control during the construction of household piping, service
connections, and sewer installations is often inadequate, resulting in infiltration and storm-
water inflow. Ground water may infiltrate sewers if the water table rises above the pipes at
any time, thereby doubling or tripling the average flow during that period (as long as several
months in some locations). Although stormwater inflow may be for a shorter period, it may
keep the collection system full for up to a week at a time. Stormwater inflow results largely
from 1) roof, cellar, or footing drains connected to the sanitary sewer; 2) stormwater
quickly reaching gravel pipe bedding and then entering the sewers through leaky joints; and
3) inflow through faulty manhole construction. Stormwater inflows may temporarily dilute
and modify the concentrations of the pollutants in the raw wastewater. It may, under some
circumstances, be feasible and cost-effective to provide some capacity in the equalization
unit, to modify some of the effects of stormwater inflows. It is seldom (if ever) worthwhile,
however, to provide sufficient capacity for equalization of large flows resulting from sea-
sonal ground water infiltration.
4-1
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Irregular wastewater discharges can be expected from schools and small industries (dairies,
canneries, tanneries, etc.), which often have high flows for limited periods during weekdays
and minimal flows during nights, weekends, holidays, or vacations. Resort or tourist busi-
nesses usually discharge wastes more heavily on weekends, holidays, and during vacation
periods.
In smaller communities, the concentration of the various polluting constituents can vary
considerably by the hour, day, season, and year. As shown on Figure 4-1, the BOD of
domestic wastewater usually is somewhat proportional to flow. If the flow contains signifi-
cant portions of nonresidential wastewater, these variations in concentration will seldom
follow the patterns of the hydraulic flow variations.
Small durations of high concentrations of toxic material, caused by a spill, will "poison" a
biological process. Normally biodegradable substances, if highly concentrated, may inhibit a
biological process. Equalization lessens the adverse impact of such shock loadings.
A common problem at small treatment plants is the intermittent dumping of septic tank
pumpage (septage) collected from nearby unsewered homes and communities. It is good
design practice to provide a specially designed receiving facility to equalize or treat septage
prior to its introduction to the treatment facility. Small industries commonly discharge
certain process wastewaters on an intermittent basis in batches that are often trucked to the
treatment plant for disposal. If these concentrated discharges are compatible with the treat-
ment processes, they may be mixed in an equalization tank with the domestic wastes and
fed to the treatment processes uniformly. However, these discharges, along with chemical
toilet collections, are often quite toxic to the treatment facility and require separate treat-
ment by chemicals, etc., to prevent serious upsets in treatment plant operations.
The large number of factors causing variability of flow and strength of wastewater makes it
particularly important, in analyzing the feasibility of equalization, to gage and sample exist-
ing wastewater flows. The gaging and sampling should take place during periods of dry
weather when the ground water surface is below the sewers and during periods of high
ground water and rainstorm runoff, to determine the effects of each on the flow rates and
wastewater constituents.
A reasonable 24-hr wastewater flow curve must be developed as a basis for the design. This
diurnal flow curve should consider the existing variations in daily, seasonal, and annual
24-hr flow and the probable increases in flow throughout the design period. In general, the
24-hr curve showing the largest variation ratios should be used for the expected design
flows. A typical diurnal flow curve is shown in Figure 4-1.
4.3 Benefits of Dry-Weather Flow Equalization
Flow equalization has a positive impact on all treatment processes, from primary treatment
to advanced waste treatment.
4-2
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BOD5MASS LOADING
PEAK; AVERAGE= 3.14
MINIMUM! AVERAGE^ 0.05
PEAK: MINIMUM=43
AVERAGE FLOW: 67,300 GPD
AVERAGE BOD5 MASS
LOADING: 4 6 Ib/hr
12 r
-i 300
BOD5 CONCENTRATION
i-IO
-9
- 8
- 7
-6
5
CO
z
Q
4 9
V)
V)
- 3
Q
O
CD
- I
12
MIDNIGHT
12
MIDNIGHT
TIME
FIGURE 4-1
TYPICAL DRY WEATHER FLOW AND BOD VARIATION
OF MUNICIPAL WASTEWATER BEFORE EQUALIZATION
4-3
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4.3.1 Impact on Primary Settling
The most beneficial impact on primary settling is the reduction of peak overflow rates, re-
sulting in improved performance and a more uniform primary effluent quality. Flow equali-
zation permits the sizing of new clarifiers on the basis of equalized flow rates rather than
peak rates. In an existing primary clarifier that is hydraulically overloaded during periods of
peak diurnal flow, equalization can reduce the maximum overflow rate to an acceptable
level. A constant influent feed rate also prevents hydraulic disruptions created by sudden
flow changes in the clarifier—particularly those caused by additional wastewater lift pumps
suddenly coming online.
LaGrega and Keenan (2) investigated the effect of flow equalization at the 1.8-mgd waste-
water treatment plant in Newark, New York. An existing aeration tank was temporarily con-
verted to an equalization basin, and the performances of primary settling under marginal
operating conditions, with and without equalization, were compared. The results are shown
in Table 4-1.
TABLE 4-1
EFFECT OF FLOW EQUALIZATION ON PRIMARY SETTLING
NEWARK, NEW YORK
Normal Flow Equalized Flow
Primary Influent SS, mg/1 136.7 128.0
Primary Effluent SS, mg/1 105.4 68.0
SS Removal in Primaries, percent 23 47
Note: Average flow slightly higher in unequalized portion of study.
It has been demonstrated (3) (4) that preaeration can significantly improve primary settling,
as discussed in Chapter 8. Roe (3) concluded that preaeration preflocculates suspended
solids, thereby improving settling characteristics. This benefit may be realized by aerated
equalization basins; however, it may be diminished if the equalized flow is centrifugally
pumped to the primary clarifier, because of the shearing of the floe.
If recirculation is limited or nonexistent, trickling filter media may dry out and the biota
activity may be reduced, thus reducing treatment efficiency. Equalization may be beneficial
if recirculation ratios above 4 to 1 are needed.
The detention time in the contact tank of a contact-stabilization type of activated sludge
plant should be between 25 and 40 minutes for the process to achieve its best efficiency.
Flow equalization may be necessary in maintaining control of this detention period in small
plants, if flow variations are excessive.
4-4
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4.3.2 Impact on Biological Treatment
In contrast to primary treatment or other mainly physical processes in which concentration
damping is of minor benefit, biological treatment performance can benefit significantly
from concentration fluctuation damping and flow smoothing. Concentration fluctuation
damping can protect biological processes from upset or failure from shock loadings of toxic
or treatment-inhibiting substances. Inline equalization basins are, therefore, preferred to
sideline basins for biological treatment applications.
Improvement in effluent quality because of stabilized mass loading of BOD on biological
systems treating normal domestic wastes has not been adequately demonstrated to date. The
effect is expected to be significant if diurnal fluctuations in organic mass loadings are
extreme; that is, at a wastewater treatment plant receiving a high strength industrial flow of
short duration. Damping of flow and mass loading will also improve aeration tank perfor-
mance, if aeration equipment is marginal or inadequate in satisfying peak diurnal loading
oxygen demands (5).
The optimum pH for bacterial growth is between 6.5 and 7.5. Inline flow equalization can
be effective in maintaining a stabilized pH within this range.
Flow smoothing can be expected to improve final settling more than primary settling. In the
activated sludge process, flow equalization will also stabilize the solids loading on the final
clarifier. For a given size clarifier, this means:
1. The MLSS concentration can be increased, thereby decreasing the F/M and in-
creasing the SRT. This may result in an increased level of nitrification and a de-
crease in biological sludge production. It may also improve the performance of a
system operating at an excessively high daily peak F/M.
2. Diurnal fluctuations in the sludge blanket level will be reduced, thus reducing the
potential for solids being drawn over the weir by the higher velocities in the zone
of the effluent weirs.
In a new design, equalization permits the use of a smaller clarifier, with a better probability
of consistently meeting effluent requirements without loss of efficiency.
4.3.3 Impact on Chemical Treatment
In chemical coagulation and precipitation systems using iron or aluminum salts, the quantity
of chemical coagulant required for phosphorus precipitation is proportional to the mass
flow of phosphorus entering the plant. If lime is used, the amount required is proportional
to the incoming alkalinity of the wastewater and desired reaction pH. Other chemical treat-
ment applications in small plants also require proportional chemical dosing. For denitrifica-
tion, the amount of methanol to be added is proportional to the mass flow of nitrate enter-
ing the denitrification reactor.
4-5
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Because in small plants the mass flow variation of these contaminants is generally much
greater than it is in large plants, the degree of automatic control required and associated
capital costs are proportionally higher. Equalization of the flow and some damping of con-
centration variation will permit utilization of less sophisticated control equipment and will
reduce control equipment cost, resulting in more effective use of chemicals.
4.3.4 Impact on Filtration
Rapid sand filters are usually designed for a constant rate of flow and are more efficient if
operated at an equalized flow rate. A more constant flow rate will reduce the required sur-
face area and produce more uniform filtration cycles. If slow sand filters are operated on an
intermittent dosing-resting cycle, treatment will not be seriously affected by flow variations.
4.3.5 Impact on Activated Carbon Adsorption
If the activated carbon process is sized for peak flows and the capacity of the carbon regen-
eration system is adequate for use during surges or organic loadings, equalization is not
necessary. Smaller (or fewer) carbon contactors may be required, however, if flow equaliza-
tion is used (6).
4.3.6 Impact on Pumping and Piping
Pump and pipe capacities downstream of equalization units can be reduced in new designs.
Equalization tanks upstream of long lines to the treatment plant can reduce the size and
cost of such pipelines, as well as equalize the flows through the treatment processes.
4.3.7 Impact on Plant Operation
Process control may be simplified by equalization. Treatment processes, typically sized for
the average flow of the maximum day, often do not completely meet the peak demands of
the incoming wastes. This short-term overloading has an adverse effect on the treatment
efficiency (2). Equalization tanks can also be used for temporary storage of wastewater
removed from units undergoing repair or maintenance.
4.3.8 Miscellaneous Benefits
The equalization basin provides an excellent point of return for recycled concentrated waste
streams, such as digester supernatant, sludge dewatering filtrate, and polishing filter back-
wash.
Some BOD reduction is likely to occur in an aerated equalization basin (7) (8) (9). A 10- to
20-percent reduction has been suggested (9) for an inline raw wastewater basin. However,
the degree of reduction will depend on the detention time in the basin, the aeration pro-
vided, the wastewater temperature, and other factors.
4-6
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Roe (3) observed that preaeration may improve the treatability of raw wastewater, by creat-
ing a positive oxidation-reduction potential and thereby reducing the degree of oxidation re-
quired in subsequent stages of treatment.
Equalization of flow reduces the probability of excessive chlorine dosage, which in turn
reduces the possibility of producing toxic chlorine compounds.
4.4 Disadvantages of Dry-Weather Flow Equalization
Disadvantages occurring under certain circumstances include:
1. The operation and maintenance costs of the equalization facilities may nullify its
advantages.
2. It may strip out f^S if the raw wastewater is anaerobic, causing an odor problem.
3. If equalization retention times are excessive, the drop in wastewater temperature
in cold weather may affect clarification, disinfection, and some biological pro-
cesses.
4.5 Methods of Equalization
Normal domestic wastewater flows, excluding most groundwater infiltration or stormwater
inflow, may be equalized. In many cases, however, even the dry-weather flow will include
some ground water infiltration. Every feasible effort should be made to reduce both ground
water infiltration and stormwater inflow to a minimum before the treatment facility is con-
structed (1) (10). The treatment plant, however, must be designed both for the existing and
expected future wastewater flows that will reach the plant within the design period. Flow
equalization methods include:
1. A good method for small-flow plants is (after degritting) designing equalization
into a treatment process unit, such as an aerated lagoon, an oxidation ditch, or an
extended aeration tank, by allowing for variable depth operation and a discharge
controlled to near the average 24-hr flow rate. The Dawson, Minnesota, waste-
water treatment facility uses a variable volume oxidation ditch as a steady-state
control device to equalize the flow to subsequent units (11).
2. Sideline equalization tanks may be sized to receive and store flows in excess of
the average daily flow rate and then to return the stored wastewater at a rate that
will raise subaverage plant flow to the average rate (see Figure 4-2). The organic
loading variations on the subsequent processes are partially affected, particularly
during periods of less than average flow. This scheme minimizes pumping require-
ments at the expense of less effective concentration damping.
3. Inline equalization tanks, sized in the same manner as sideline tanks, equalize the
outflow at near the average daily flow rate (see Figure 4-3). This equalization re-
sults in significant concentration and mass flow damping. An orbal inline equaliza-
tion basin (Figure 4-4) has been incorporated into the Duck Creek Wastewater
Treatment Plant at Garland, Texas (12).
4-7
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RAW
WASTEWATER
BAR SCREEN AND/OR
COMMINUTOR-
CONTROLLED FLOW
PUMPING STATION
EQUALIZATION
BASIN
GRIT
REMOVAL
SLUDGE
PROCESSING
RECYCLE
FLOWS
OVERFLOW
STRUCTURE
FLOW METER AND
CONTROL DEVICE
PRIMARY
TREATMENT
I
SECONDARY
TREATMENT
FINAL
EFFLUENT
FIGURE 4-2
SIDELINE EQUALIZATION
4-8
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RAW
WASTEWATER
SLUDGE PROCESSING
RECYCLE FLOWS
BAR SCREEN AND/OR
COMMINUTOR
GRIT
REMOVAL
BYPASS
EQUALIZATION
BASIN
-CONTROLLED FLOW
PUMPING STATION
FLOW METER AND
CONTROL DEVICE
FINAL
EFFLUENT
FIGURE 4-3
INLINE EQUALIZATION
4-9
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EFFLUENT
INFLUENT
VANES
-BRUSH
AERATORS
'-FLOW
DEVELOPER
FIGURE 4-4
ORBAL EQUALIZATION UNIT
4-10
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4. Extra capacity provided in large trunk or interceptor sewers leading to the treat-
ment plant for intermittent stormwater inflow may be used for storage of peak
flows. The flow from the trunk sewer can then be controlled at near the average
flow rate. Such velocities may allow deposition of solids in the sewer, however.
Nightly or semiweekly drawdown, with adequate velocities to flush out deposited
solids, must be designed into such a system, if some method of continuous mixing
in the conduit is not provided. Above average BOD concentrations may occur
during the flushing. This method of equalization is less attractive to smaller com-
munities, however.
Provision of compartmentalized or multiple basins will allow flexibility in dewatering a por-
tion of the facility for maintenance or equipment repair while still providing some flow
equalization. Single basin installations, which may be used for small plants, must maintain
complete treatment during dewatering. This will require a bypass line around the basin, to
allow the downstream portion of the plant to operate if the flow equalization facility is out
of service. *
The equalization unit may be placed at a pumping station site at the edge of the collection
system, to reduce the size of the trunk or interceptor sewer to the treatment plant. This will
reduce the required sizes of treatment process units as well as of the pipeline and may make
cost effective the use of an equalization unit (particularly if the pipeline affected is over 0.5
mile [0.8 km] in length) (13). This location for equalization is particularly good, if odor
problems from septic wastewater in the pipeline occur.
4.6 Equalization Design
Factors requiring evaluation in the selection of the type, size, and mode of operation of an
equalization process are listed below (1) (2) (8) (11).
1. Degree of flow rate and organic loading equalization required to insure reliable
and efficient process performance.
2. Optimum location in the system.
3. Type of equalization best suited to items 1 and 2.
4. Optimum volume required for equalization.
5. Compartmentalization needed to best handle present and future flows.
6. Type of construction.
7. Aeration and mixing equipment.
8. Pumping and discharge flow rate control.
9. Minimum operation and maintenance requirements under adverse weather and
flow conditions.
10. Feasible alternative treatment components sized for peak flows.
Treatment processes sized for peak flows at minimum water temperatures, to eliminate the
need for equalization for consistent reliability, should be compared with sizes reduced to
meet equalized needs of processes operated at equalized flows and characteristics, to ascer-
tain the cost effectiveness for each. At some smaller treatment plants, some degree of
4-11
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equalization may be essential if consistently acceptable plant effluent is to be obtained,
particularly if the quality of the effluent must meet very strict standards.
Some variation in the feed to any process is normally permissible, because the design of
most processes includes some allowance for such variations. For example, final clarifiers for
activated sludge units are designed for the rate of flow expected on the average day of the
design year, but the overflow rate at maximum flow is used to control the design. After
studying the settling characteristics expected of the suspended solids and the provisions in-
cluded in the design to optimize settling (such as the spacing, location, and size of the
effluent launders), the clarifier overflow rate is selected, taking into consideration the ratio
of peak to average flow and associated solids loadings on the day of maximum flow in the
design period. If the day's flow is equalized, the peak overflow rate will equal the average
and allow for reduced capacity. The designer must evaluate the proposed design and 1)
determine the amount of fluctuation that can be satisfactorily handled without impairing
performance, 2) provide equalization sufficient to insure fluctuations no more than that
amount, and 3) compare that scheme to others requiring greater or lesser degrees of equali-
zation, to determine the optimum design.
4.6.1 Determining Necessary Volume for Equalization
To determine the required equalization storage capacity, the maximum variation in 24-hr
flow expected in the design should first be established. Figure 4-5 illustrates a typical ex-
ample of a set of flow and BOD curves for the maximum day at the end of a design period.
The mass diagram, hydrograph, or 24-hr maximum day flow can be developed from Figure
4-1 and plotted as on Figure 4-5. To obtain the volume required to equalize the 24-hr flow:
1. Draw a line between the points representing the accumulated volume at the begin-
ning and end of the 24-hr period. The slope of this line represents the average rate
of flow.
2. Draw parallel lines to the first line through the points on the curve farthest from
the first line. These lines are shown as A and B on Figure 4-5.
3. Draw a vertical line between the lines drawn in No. 2. The length of this line
represents the minimum required volume.
The equalization volume needed to balance the diurnal flow is a larger percentage of the
day's flow for smaller than for larger wastewater treatment plants. The volume needed for
equalization of flows will usually vary from about 20 to 40 percent of the 24-hr flow for
smaller plants and from about 10 to 20 percent of the average daily dry-weather flow for
larger plants.
To meet the expected diurnal flow variations over the entire design period without excessive
use of energy, the equalization basin should be divided into two or more independent cells.
In addition, extra storage volume is needed to (4):
1. Balance unexpected changes in diurnal flows.
4-12
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CO
z
o
UJ
INFLOW MASS
DIAGRAM-
AVERAGE RATE
OF FLOW
67,300 GAL/DAY
12
MIDNIGHT
12
NOON
TIME OF DAY
REQUIRED
EQUALIZATION
VOLUME,
16,000 GALLONS
12
MIDNIGHT
FIGURE 4-5
HYDROGRAPH OF A TYPICAL DIURNAL FLOW
4-13
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2. Provide continuous operation of mixing and aeration equipment.
3. Provide some volume for (at least) partial equalization of intermittent organic
loadings from, for example, recycling of sludge scraped from bottom of equaliza-
tion tank, shock discharges of septage, or discharges of process sidestreams.
4. Provide freeboard above maximum water level.
It should be noted that, although space outside the operating volume cannot be used to
compute flow equalization, it does enter into concentration equalization.
4.6.2 Basin Construction
Equalization basins may be constructed of earth, concrete, or steel. Earthen basins are gen-
erally the least expensive. They can normally be constructed with side slopes varying be-
tween 3:1 and 2:1 horizontal to vertical, depending on the type of lining used. To prevent
embankment failure in areas of high groundwater, drainage facilities should be provided for
groundwater control. If aerator action and wind forces cause the formation of large waves,
precautions should be taken in design to prevent erosion. Provision of a concrete pad
directly under the equalization basin aerator or mixer is customary. The top of the dikes
should be wide enough to insure a stable embankment and, for economy of construction,
sufficient to accommodate mechanical compaction equipment.
Inline basins should be designed to achieve complete mixing to optimize concentration
damping. Elongated tank design encourages plug flow and should be avoided. Inlet and out-
let configurations should be designed to prevent short circuiting. Designs that discharge
influent flow as close as possible to the basin mixers are preferred.
4.6.3 Air and Mixing Requirements
The mixing and aeration equipment for equalization basins must be selected carefully. In a
typical equalization unit, the level of wastewater in each cell in use will fluctuate about 40 to
90 percent of the design depth. As the cells fill and empty, the aeration and mixing power
requirements vary. In freezing climates, floating aerators may become "frozen in" during a
power outage.
Mixing equipment should be designed to blend the contents of the tank and to prevent
deposition of solids in the basin. To minimize mixing requirements, grit removal facilities
should precede equalization basins if possible. Aeration is required to prevent the wastewater
from becoming septic. Mixing requirements for blending a municipal wastewater having an
SS concentration of approximately 200 mg/1 range from 0.02 to 0.04 hp/1,000 gal (0.004
to 0.008 kW/m3) of storage. To maintain aerobic conditions, air should be supplied at a rate
of 1.25 to 2.0 cfm/1,000 gal (0.009 to 0.0015 m3/m3 -min) of storage (13).
Mechanical aerators will provide both mixing and aeration. The oxygen transfer capabilities
of mechanical aerators operating in tap water under standard conditions vary from 3 to 4 Ib
O2/hp-hr (1.8 to 2.4 kg/kW-hr). Baffling may be necessary to insure proper mixing, particu-
larly with a circular tank configuration. Minimum operating levels for floating aerators gin-
erally exceed 5 ft, and vary with the horsepower and design of the unit. The horsepower
4-14
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requirements to prevent deposition of solids in the basin may greatly exceed the horse-
power needed for blending and oxygen transfer. In such cases, it may be more economical
to install mixing equipment, to keep the solids in suspension and furnish the air require-
ments through a diffused air system, or to mount a surface aerator blade on the mixer.
Note that other factors, including maximum operating depth and basin configuration, affect
the size, type, quantity, and placement of the aeration equipment. In all cases, the manufac-
turer should be consulted.
Additional information on mixing and aeration design is presented in Chapters 7 and 10.
4.6.4 Selecting Pumping Equipment
Because of the large variation in hydraulic head necessary for the operation of equalization
tanks, pumping is normally required. If a pumping station is required in the headworks, the
pumps can be designed for the additional head needed for equalization basin operation.
Gravity discharge from equalization will require an automatically controlled, flow-regulated
device.
Flow-measuring devices are required downstream of equalization units, to monitor the
equalized flow. Control of preselected equalization rates can be either automatic (with
manual standby) or manual.
4.6.5 Miscellaneous Design Considerations
The following features, based on recommendations presented in Process Design Manual for
Upgrading Existing Wastewater Treatment Plants (1), should be considered in designing
equalization units:
1. Providing a means of measuring, monitoring, and controlling the flow from the
equalization basin.
2. Providing a means of varying the mixing as the depth varies, to conserve energy.
3. Screening and degritting raw wastewater before equalization, to prevent grit and
rag problems.
4. Requiring adjustable legs and low water cutoff for protection of floating aerators.
5. Providing means for cleaning grease and solids accumulation from equalization
unit walls.
6. Providing means for removing scum, foam, and floating material.
7. Providing an emergency high-water overflow.
4.7 Examples
Two comparative examples illustrating inplant and sideline equalization follow. In both
cases, the data shown on Figures 4-1 and 4-5 will form the basis for the computations. The
plant to be studied is an extended aeration, activated sludge plant, consisting of bar screens,
4-15
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wedge-wire screens, pumping, aeration tank, clarifier, and chlorination facility. The compu-
tations will deal only with 1) additional facilities required to equalize the flow to the clari-
fier and chlorination units, and 2) resulting changes in the sizes of the aeration tank, clari-
fier, and chlorinator facilities.
4.7.1 Inplant Equalization
The inplant equalization will provide the needed equalization volume (16,000 gal, or
60.6 m3 [Figure 4-5]) by adding that amount to the size of the aeration tank. An example
of the detailed design of an extended aeration, activated sludge system is shown in Section
7-9. For this example (Figure 4-6) the volume is assumed to equal the 24-hr average flow
(67,000 gpd, from Figure 4-5).
The aeration tank size, without equalization volume, will be
V = 67,300/7.48 = 9,000 ft3
Assuming a depth of 15 ft, the area will be
A = 9,000/15 = 600 ft2
The diameter will be
D = (4A/7r)°-5
D = 27.6 ft
The equalization tank volume to be added will be
V= 16,000/7.48 = 2,139 ft3
The additional depth will be
Ad = 2,139/600 = 3.6 ft
To incorporate the equalization volume in the aeration tank, the depth below high water
will be 18.6 ft (5.7 m). This additional depth requires the amount of reinforced concrete in
the walls to be increased by 11.8 cu yd (9.02 m3).
The steel baffle in front of the effluent port will be 4 ft by 4 ft (1.2 m by 1.2 m). The baffle
will open on the sides, 1 ft (0.3 m) from the wall, and centered on the discharge port.
A proportional controller with a hydraulically controlled butterfly valve, placed on a metal
platform on the outside of the aeration tank, will be used to control the daily flow. The
daily flow will be adjustable, between 20 and 60 gpm (1.3 and 3.8 1/s). A venturi-type meter
and transmitter will be used to measure flow and to actuate both the butterfly valve control
and the chlorine dosage control. A 3-in. (76.2-mm) butterfly valve will be operated and
4-16
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TREATMENT PLANT CONFIGURATION
COMPRESSOR
PUMPED
INFLUENT
SUBMERGED
TURBINE
C!
VENTURI TYPE METER
HYORAULICALLY OPERATED
BUTTERFLY VALVE
EFFLUENT TO CLARIFIER
AERATION-EQUALIZATION TANK
FIGURE 4-6
INPLANT EQUALIZATION
4-17
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controlled hydraulically. The plant pressurized potable water system will provide water for
the hydraulic system. Head loss through the meter and valve will be 1 ft (0.3 m) or more.
The additional average requirements for pumping will be about:
hp = QWH/550 (efficiency)
hp = t(67,300)(8.33)(3.5)]/[(2)(550)(0.6)(86,400)]
hp = 0.034
where
Q = flow, mgd
W = density, lb/ft3
H = head, ft
By equalizing the flows, the clarifier volume and the chlorination facility capacity can be
halved, because the peak flow (144,000 gpd, or 545 m3/d [from Figure 4-11]) will be re-
duced to 67,300 gpd (254.7 m3/d).
Because the clarifier overflow rate at maximum flow should be no more than 800 gpd/ft2
(32.6 m3/m2 -d), the clarifier area required for unequalized flow will be
A = (144,000 gpd)/(800 gpd/ft2)
A = 180ft2
To obtain a minimum detention time of 120 minutes, the side wall depth at unequalized
flow will be
d = (144,000)/[(12)(7.48)(180)]
d = 8.9 ft (use 9 ft + 1.5 ft of freeboard)
The diameter of the tank will therefore be
D = (4A/;r)0-5 = 15.1 ft
With equalized flow, the clarifier area may be reduced to
A = (67,300 gpd)/(800 gpd/ft2)
A = 84 ft2
4-18
-------
The depth will remain 9 ft (2.7 m).
The diameter will then be
D = (4A/rr)°-5 = 10.3ft
The required quantity of reinforced concrete will be reduced by 10.4 cu yd.
The chlorine contact pipe needed for 30 minutes' detention for unequalized flow will re-
quire a volume of
V= 144,000/[(24X2)(7.48)] =400 ft3
Assuming a 3-ft (0.9-m) diameter pipe, the length needed will be
L = V/A = 400/[(0.7854)(3)2] = 56.6 ft (use 60 ft)
The chlorine contact pipe for equalized flow will require a volume of
V = 67,300/1(24X2X7.48)] = 188 ft3
Assuming a 2-ft- (0.6-m) diameter pipe, the length then will be
L = V/A = 188/[(0.7854)(2)2] = 60 ft
The length-to-diameter ratios would be 20:1 and 30:1, respectively, insuring a relatively
plug-type flow.
4.7.2 Sideline Equalization
The change in plant configuration from that shown on Figure 4-6, to add sideline equaliza-
tion, is shown on Figure 4-7. The volume needed for equalization remains 2,139 ft3 (59.9
m3) for the 24-hr design flow. Assuming an operating depth of 8 ft (2.4 m), the area
of the sideline equalization tank will be
A = 2,139/8 = 267.4 ft2
The diameter will be
D = (4A/7r)°-5 = 18.5ft
An additional depth of 4 ft (1.2 m) below low water level will be needed for operation of
the turbine; 1.5 ft (0.45 m) above high water level will be required for freeboard, making
the total tank depth 13.5 ft (4.05 m).
4-19
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a—
TREATMENT PLANT SCHEMATIC
DRIVE COMHPRESSOR
6"OVERFLOW
EQUALIZATION TANK
FIGURE 4-7
SIDELINE EQUALIZATION
4-20
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A submerged turbine aerator, similar to that used in the areation tanks, should be used for
aeration and mixing in the equalization basin. The turbine should be large enough to prevent
anaerobic conditions. The horsepower needed for the submerged turbine will be
hp = (0.04 hp)( 16,000 gal)/1,000 gal
= 0.64 hp (use a 3/4-hp drive)
The compressor should be able to provide air against a head varying between 6 ft and 15 ft
(1.8m and 4.5m).
Air required = (1.5 ft3/min)( 16,000 gal)/( 1,000 gal)
= 24 ft3/min
Three additional pumps will be required, each with a capacity of 25 gpm (1.6 1/s) against
about a 25-ft (7.5-m) head; one perhaps should be variable speed. These will require addi-
tional space in the pumping station, plus additional wet well volume, piping, and valves.
It is possible to substitute a contact-stabilization, activated sludge system for the extended
aeration system, if sideline equalization is used. However, because of the added sludge to be
wasted and its drying capacity, this system would require an aerobic digestion tank of 15 to
20 days' retention for waste sludge for further stabilization, before it is placed on the sludge-
drying beds. This alternative should also be included in the cost-effectiveness study for a
treatment plant for a specific location.
4.8 Costs of Equalization
The costs of adding inplant or sideline equalization to a 67,300-gpd (254.7 m3/d) waste-
water treatment plant, using the examples in Section 4.7 and a U.S. EPA Treatment Cost
Index of 225, are presented in the following subsection.
4.8.1 Inplant Equalization
Capital Cost Changes
Added aeration tank volume (11.8 yd3
reinforced concrete) $ 3,290
Proportional controller and butterfly valve installation 2,500
Clarifier (reduced size) (10.4 yd3 reinforced concrete) 1,005
Chlorine contact pipe (reduced size) 620
Subtotal $ 7,415
Annual Cost Changes
Added electrical power use (at $0.04/kWh) $ 10
Added O & M labor 500
Subtotal $ 510
4-21
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Intangible Benefits
Better chance that effluent will consistently meet standards; minimization of
chlorine dosage reduces chance of toxic chlorine compounds in effluent.
4.8.2 Sideline Equalization
Capital Cost Changes
Equalization tank (48.1 yd-* reinforced concrete) $11,230
Equalization pump installation 17,380
Flow meters (two) 3,600
Extra piping and valves 2,570
Submerged turbine and compressor 12,120
Reduction in clarifier size (10.4 yd3 reinforced concrete) 1,005
Reduction in chlorine pipe contractor size 620
Subtotal $48,525
Annual Cost Changes
Added electric power use (at $0.04/kWh) $ 1,450
Added O & M labor 1,350
Subtotal $ 2,800
Intangible Benefits
Better chance that effluent will consistently meet standards; minimization of
chlorine dosage reduces chance of toxic chlorine compounds in effluent.
4.9 Case Studies
4.9.1 Walled Lake-Novi Wastewater Treatment Plant
The Walled Lake-Novi wastewater treatment plant is a new (1971), 2.1-mgd facility employ-
ing" sideline flow equalization. It was designed to meet stringent effluent quality standards,
including 1) a summer monthly average BOD2Q of 8 mg/1, 2) a winter monthly average
BOD2Q of 15 mg/1, and 3) 10 mg/1 of SS. The facility used the activated sludge process, fol-
lowed by multimedia tertiary filters. Ferrous chloride and lime are added ahead of aeration
for phosphorus removal. Sludge is processed by aerobic digestion and dewatered on sludge
drying beds. A schematic diagram of this facility is shown in Figure 4-8.
A major factor in the decision to employ flow equalization was the desire to load the ter-
tiary filters at a constant rate. The equalization facility consists of a 315,000-gal (1.190-m3)
concrete tank, equivalent in volume to 15 percent of the design flow. The tank is 15 ft
(4.5 m) deep and 60 ft (18 m) in diameter. Aeration and mixing are provided by a diffused
air system with a capacity of 2 cfm/1,000 gal (0.015 m3/m3 -min) of storage. Chlorination
is provided for odor control. A sludge scraper is used to prevent consolidation of the sludge.
4-22
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EQUALIZATION
PUMPS
FLOW
EQUALIZATION
BASIN
PROCESS
PUMPS
FLOW METERS
COMMIIMUTOR AND AERATED
GRIT CHAMBER
LIME & POLYELECTROLYTE
CHLORINE CONTACT
CHAMBERS
FIGURE 4-8
WALLED LAKE-NOVI WASTEWATER TREATMENT PLANT
4-23
-------
Operation of the equalization facility is described below (14). The process pumping rate is
preset on the pump controller, to deliver the estimated average flow to the treatment pro-
cesses. Flow delivered by these pumps is monitored by a flow meter that automatically
adjusts the speed of the pumps to maintain the average flow rate. When the raw wastewater
flow to the wet well exceeds the preset average, the wet well level rises, thereby actuating
variable speed equalization pumps, which deliver the excess flow to the equalization basin.
When the inflow to the wet well is less than the average, the wet well level falls and an auto-
matic equalization basin effluent control valve opens. The valve releases enough wastewater
to the wet well to reestablish the average flow rate through the plant. Because this is a new
plant (as opposed to an upgraded plant), no comparative data exist. However, the treatment
facility is generally producing a highly treated effluent, with BOD and SS less than 4 mg/1
and 5 mg/1, respectively (13).
4.9.2 Novi Interceptor Retention Basin
The Novi interceptor retention basin (15) illustrates the utilization of an equalization basin
within the wastewater collection system.
A portion of the wastewater collection system for the city of Novi, Michigan, discharges
to the existing Wayne County Rouge Valley interceptor system. Because of the existing
connected load on the Wayne County system, Novi's wastewater discharge to the inter-
ceptor system is limited to a maximum flow rate of 4 cfs (0.11 m^/s). This rate was
matched by the existing maximum diurnal flow form the city. To serve additional popula-
tion, it was decided to equalize wastewater flows to the interceptor system. By continuously
discharging to the interceptor at an average rate of flow, total wastewater flows from Novi
to the Wayne County Rouge Valley interceptor system could be increased by a factor of
2.6.
An 87,000-ft3 (2,436-m^) concrete basin was constructed for equalizing flows. The tank has
a diameter of 92 ft (28 m) and a depth of 10.5 ft (3.2 m). Aeration and mixing are provided
by a diffused air system capable of delivering 2 cfm/1,000 gal (0.015 m^/m^ -min) of stor-
age.
A manhole located upstream of the equalization basin intercepts flow in the existing Novi
trunk sewer. This intercepted wastewater flows into a weir structure, which allows a maxi-
mum of 4 cfs (0.11 m3/s) to discharge into the Wayne County system. Wastewater in excess
of the preset average overflows into a wet well and is pumped to the equalization basin. If
flows in the interceptor fall below the preset average, a flow control meter generates a signal
opening an automatic valve on the effluent line of the basin, allowing stored wastewater to
augment the flow.
4.9.3 Dawson, Minnesota
To improve operation and reduce the costs of biosolids clarification, chemical treatment,
final clarification, and chlorination, equalization was considered necessary at the Dawson,
Minnesota, wastewater treatment plant. Rather than build a separate equalization chamber,
4-24
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the aeration unit was designed as a variable depth oxidation ditch, to obtain both hydraulic
and solids loading equalization (16).
The variable depth oxidation ditch shown in Figure 4-9 illustrates this process. Design cri-
teria for this plant are contained in Table 4-2; performance data are included in Table 4-3.
4-25
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-GRIT FROM REMOVAL
CHAMBER
.,:.?y?>-£v
to
PLAN VIEW
5Z
18
18
SECTION A-A
40.2
n
"—38
FIGURE 4-9
VARIABLE DEPTH OXIDATION DITCH
DAWSON, MINNESOTA
-------
TABLE 4-2
PRINCIPLE DESIGN CRITERIA, DAWSON, MINNESOTA,
WASTEWATER TREATMENT PLANT
Area Served
1970 Population
2020 Design Population
2020 Design Population Equivalent
1970 Flow, mgd
2020 Design Flow, mgd
Influent BOD, mg/1
Effluent BOD required, mg/1
Estimated Industrial Flow (first 5 years)
Receiving Water
Aeration Channel
Overall Length, ft
Overall Width, ft
Minimum Operating Depth, ft
Maximum Operating Depth, ft
Volume (minimum depth), ft3
Volume (maximum depth, ft3
BOD Loading (minimum volume), lb/1,000 ft3 day
BOD Loading (maximum volume), lb/1,000 ft3 day
Detention Time (minimum depth), hr
Detention Time (maximum depth), hr
City of Dawson, Minnesota
1,800
2,200
2,400
0.20
0.26
230
5
Negligible
West Branch of the Lac Qui Parle River
244
84
3.0
4.1
25,675
36,730
19.5
13.5
17.7
25.4
Bio-Solids Separation Unit
Volume, gal
Volume, ft3
Detention Time, hr
Surface Area, ft2
Designed Hydraulic Loading Rate, gpd/ft2
Chemical Quick Mix Tank
Volume, ft^
Detention Time, sec
Chemical Slow Mix Tank
Volume, ft 3
Detention Time, minutes
Biochemical Solids Separation Unit
Volume, ft3
Detention Time, hr
Surface Area, ft2
Designed Hydraulic Loading Rate, gpd/ft2
32,000
4,275
3
450
580
9.5
24
360
15
42,400
5,670
4
648
400
4-27
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TABLE 4-3
PERFORMANCE DATA, DAWSON, MINNESOTA
Average Concentration
Period No. of Samples Characteristic Influent Effluent
mg/1 mg/1
9/7/73 to 12/13/73 9 Nitrogen 46.1 21.45
9/7/73 to 12/13/73 8 Phosphorus 9.8 3.9
9/7173 to 12/13/73 11 BOD5 245 3.7
9/17/73 to 12/13/73 11 SS 267.5 7.2
9/7/73 to 12/13/73 13 COD 908 45
Note: Without chemical addition.
1. Analyses were based on grab samples or composites collected between 11 a.m. and
3 p.m.
2. Average flow from 9/7/73 to 12/12/73 was 185,000 gpd.
3. MLSS varied between 3,000 and 9,000 mg/1, with the higher MLSS value in colder
weather.
4. F/M varied between 0.3 and 0.6 Ib BOD/lb MLSS.
5. Sludge age was about 30 days.
4.10 References
1. Process Design Manual for Upgrading Existing Wastewater Treatment Plants. U.S. EPA,
Office of Technology Transfer, EPA-625/l-74-004a (October 1974).
2. LaGrega, M.D., and Keenan, J.D., "Effects of Equalizing Wastewater Flows." Journal
Water Pollution Control Federation, vol. 46, No. 1 (January 1974).
3. Roe, R.C., "Preaeration and Air Flocculation." Sewage Works Journal, vol. 23, No. 2,
pp. 127-140(1951).
4. Seidel, H.F., and Baumann, E.R., "Effect of Preaeration on the Primary Treatment of
Sewage." Journal Water Pollution Control Federation, vol. 33, No. 4, pp. 339-355
(1961).
5. Boon, A.G., and Burgess, D.R., "Effects of Diurnal Variations in Flow of Settled Sew-
age on the Performance of High Rate Activated Sludge Plants." Water Pollution Con-
trol, pp. 493-522(1972).
6. Process Design Manual for Carbon Adsorption. U.S. EPA, Office of Technology Trans-
fer, EPA-625/l-73-002a (October 1973).
4-28
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7. Unpublished data, Duck Creek Wastewater Treatment Plant, Garland, Texas, WPC-
TEX-1076(1973).
8. Wallace, A.T., "Analysis of Equalization Basins." Journal Sanitary Engineering Divi-
sion, ASCE (December 1968).
9. Private communication with Dr. E. Robert Baumann, Iowa State University, Ames
(11 December 1973).
10. Sewer System Evaluation. U.S. EPA, Office of Water Programs Operations (October
1973).
11. Private communication with Mrs. Ella Lindseth, city clerk, Dawson, Minnesota
(November 1974).
12. "Process Design Summary," Garland, Texas (1974).
13. Smith, J.M., Masse, A.N., and Feige, W.A., "Upgrading Existing Wastewater Treatment
Plants." presented at Vanderbilt University (18 September 1972).
14. Operation and Maintenance Manual for Wastewater Treatment Plant, Walled Lake Arm,
Huron-Rouge Sewage Disposal System. Johnson & Anderson, Inc., Oakland County,
Michigan: Department of Public Works (June 1973).
15. Operation and Maintenance Manual for Sewage Retention Reservoir, Novi Trunk Ex-
tension No. 1, Huron-Rouge Sewage Disposal System. Johnson & Anderson, Inc., Oak-
land County, Michigan: Department of Public Works (September 1973).
16. Grounds, H.C., and Mayerson, R.C., "Variable Volume Activated Sludge." Presented at
4th Annual Environmental Engineering and Science Conference, Louisville (4-5 March
1975).
4-29
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CHAPTER 5
HEADWORKS COMPONENTS
5.1 Introduction
The components of a treatment plant upstream of, and providing pretreatment for, primary
clarifiers, flocculators, equalization tanks, or biological units, are considered part of the
headworks. Typical headworks components are wet wells and units for screening and com-
minuting, grit removal, grease and oil removal, and pumping.
These components provide preliminary treatment for wastewater to optimize the operation
and performance of subsequent treatment processes. Headworks components discussed in
this chapter relate to treatment of wastewater that is substantially domestic in origin. In-
dustrial wastewater, it can be assumed, has been pretreated to such an extent that it can be
treated as domestic wastewater without loss of plant efficiency. Federal reliability require-
ments (1) must be considered in selecting unit processes and equipment for wastewater
pretreatment in the headworks.
Design criteria and calculation methodology for headworks components are given in several
publications (1) (2) (3) (4) (5) (6) (7). Table 5-1 lists the units or processes commonly
found in headworks and their functions. Under special circumstances, some functions may
be combined in one unit.
TABLE 5-1
HEADWORKS UNITS
Units or Processes
Racks and bar screens
Communitors and grinders
Grit removal
Skimming (aerated or unaerated)
Preaeration
Fine screens
Pumping
Measuring devices
Sampling wells
Mixing tanks
Functions
Strain out coarse wastewater solids
Macerate and grind wastewater solids into smaller
particles
Intercept and remove sand and grit
Remove lighter-than-water particles (such as grease,
oil, soap, wood, and garbage)
Add oxygen to wastewater, initiate natural floccula-
tion, and control odors
Strain out smaller suspended organic matter
Add sufficient head to wastewater for gravity flow
through plant
Determine influent flows
Provide location to sample plant influent
Mix influent wastewater, recycled solids, effluents, or
sidestreams and chemicals to achieve homogeneity
throughout the wastewater
5-1
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5.2 Racks, Bar Screens, and Comminutors
One process common to most treatment plant headworks is screening out larger solids (rags,
pieces of wood, dead animals, etc.) that would be unsightly or cause difficulty in down-
stream processes. For small plants, the screening is usually accomplished by a hand-cleaned
bar screen or two bar screens in parallel channels. Sometimes the bar screen is followed by
comminution. If the comminutor is down for repair, or if peak flows exceed the commi-
nutor's capacity, the bar screen may constitute the entire pretreatment.
In a bar screen 3/8 in. (10 mm) wide by 2 in. (50 mm) deep or larger, bars are welded to
crossbars on the downstream side with a clear opening between bars of 1 to 1.75 in. (25 to
45 mm). The bars, raked by hand, should not be so long that they are inconvenient to clean.
An easily drained floor (with closely placed drain holes) for temporarily storing the rakings
should be placed downstream from the top of the screen. The average velocity of flow
through the bar screen should be 1 ft/sec (0.3 m/s) or greater.
Mechanically cleaned racks are usually considered ill-suited for use in small plants. Their
design is covered in references (4), (5), and (6).
Screenings from the racks or bar screens can be disposed of by hauling them to approved
landfill areas that are designed and operated to protect groundwater from leachate pollu-
tion.
Comminution devices cut up the solids in the raw wastewater to prevent an adverse effect
on the efficiency of later pumping and treatment processes. If grit removal is included, the
comminution devices should follow grit removal, to protect their cutting surfaces. However,
they should usually precede any pumping units, to protect the pumps from being clogged by
rags and large objects.
Bar racks, channels, and screenings accumulations are often sources of odor problems, if
the channel is not designed for easy and thorough cleaning. Daily "housekeeping" is usually
required.
5.3 Grit Removal
Grit is the heavier suspended mineral matter in wastewater, consisting essentially of sand,
gravel, and cinders. Grit usually contains eggshells, bone chips, seeds, coffee grounds, and
larger organic particles, such as food particles that have passed through garbage disposals.
Grit contains substances with specific gravities much greater than those of the normal
putrescible and oxidizable organic material in wastewater.
Grit removal units should be included in the design of small wastewater treatment plants.
These units are particularly important if the wastewater contains enough grit to cause faster
deterioration and subsequent replacement of equipment such as pumps, centrifuges, and
comminutors; to increase the frequency of cleaning of digesters; or to result in excessive
5-2
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deposits in pipelines, channels, and tanks. Grit is usually removed in controlled velocity
chambers, detritus tanks, or aerated grit chambers. Design criteria for these units are in
references (1), (2), (4), (5), (6), and (7).
In smaller treatment plants, when grit removal has been necessary, manually cleaned parallel
grit channels have been used in combination with a downstream control to maintain a uni-
form velocity of close to 1 ft/sec (0.3 m/s) through the grit channel. The velocity must be
kept within a range that permits the heavier inorganic grit to settle while lighter organic
solids are kept in suspension.
Care must be taken in selecting a downstream control for grit removal channels, to insure
that the velocities are not excessive, particularly along the bottom of the channel during
higher flows. Parshall flumes or Camp weirs provide such control. The Parshall flume is also
commonly used to measure the influent flow. Proportional or Sutro weirs control the aver-
age approach velocity, but tend to cause excessive bottom velocities.
A detritus tank is a grit chamber in which the velocities permit an appreciable amount of
organic matter to settle out with the grit. An aerated detritus tank, or aerated grit chamber,
is a tank in which the organic matter, that would otherwise settle out, is maintained in
suspension by rising air bubbles or some other form of agitation.
Aerated grit chambers have the following advantages:
1. Grit removed is clean enough for disposal without further treatment.
2. Variations in flow have little effect on the efficiency of grit removal.
3. The removal of grease, or other floatables, by flotation and skimming can be com-
bined in one chamber with grit removal.
4. The chamber, because of its mixing capabilities, may provide a good location for
chemical additions to improve plant solids and phosphorus removal and for odor
control and prechlorination.
5. Preaeration adds DO to incoming wastewater, normally devoid of oxygen, before
it is discharged to the next process.
The major disadvantage of aerated grit chambers for small treatment plants is that they
require more operation and maintenance than do manually cleaned grit channels.
In general, grit removal efficiency depends primarily on surface area. The areas commonly
used are listed in Table 5-2 (6).
5.4 Oil, Grease, and Floating Solids Removal
Oil, grease, resin, glue, and floating solids such as soap, vegetable debris, plastics, fruit skins,
and pieces of cork and wood interfere with some processes and should be removed in the
headworks, if large quantities are present in the raw wastewater. Aerated skimming tanks
with detention times of about 3 minutes are commonly used for oil, grease, and floating
solids removal. Air requirements are about 0.03 ft3/gal (0.22 m^/m3) of wastewater for
these units (3). References (3), (4), (5), (6), and (7) contain design criteria for these units.
5-3
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TABLE 5-2
THEORETICAL MAXIMUM OVERFLOW RATES FOR GRIT CHAMBERS (6)
Size of Grit Particle
Approximate
Screen Mesh
No.
48
60
65
80
100
Diameter
mm
0.30
0.25
0.21
0.18
0.15
Overflow Rates1
gpd/ft2
Specific Gravity
2.65
65,500
58,000
46,300
40,900
32,300
2.0
39,600
35,200
28,000
24,800
19,600
1
Liquid temperature about 15° C.
Skimmings from these units are normally putrescent and may cause odor nuisances. Bio-
degradable solids of vegetable or animal origin may be discharged to sludge digestion units.
Other types of skimmings (such as those containing mineral oils) may be buried with screen-
ings. Skimming volumes usually vary from 0.1 to 6.0 ft^/mil gal (6).
5.5 Preaeration
Preaeration of wastewater has been used throughout the United States for over 50 years to
control odor in downstream units (aeration may cause odors by releasing ^S gas) and to
improve treatability of the wastewater. Short aeration periods, up to 15 minutes, have been
found adequate for these purposes. For longer aeration periods, the additional benefits of
grease separation and improved flocculation of solids have also been observed (8).
Although the use of aerated grit chambers is becoming increasingly popular as a pretreat-
ment unit in wastewater treatment plants, they should not be expected to increase substan-
tially BOD or SS removal during primary clarification, because of the relatively short deten-
tion times normally employed. Aerated grit chambers can be combined with preaeration if
grit removal is limited to the upstream portion of the tank.
Preaeration is sometimes located in the distribution channels. With aeration in the channels,
there are added benefits: absence of settled solids, even at reduced velocities, and uniform
distribution of solids to multiple units.
The major parameters to be considered in the design of preaeration facilities are rate of air
application and detention time. To maintain proper agitation, the air supply system should
provide a range of 1.0 to 4.0 cfm/linear foot (0.0015 to 0.006 m3/nvs) of tank or
channel. This range will insure adequate performance for nearly all physical tank layouts
and types of aeration equipment used, if there is more than 0. 1 ft 3 of air supplied per gallon
of wastewater (0.01
5-4
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Effective preaeration has been achieved at detention times of 45 minutes and less (8) (9).
The Ten States Standards (2) recommend a detention time of 30 minutes for effective solids
flocculation if inorganic chemicals are used in conjunction with preaeration. For appre-
ciable BOD reduction, a minimum of 45 minutes is recommended. The use of polyelectro-
lytes may also affect these detention times.
5.6 Physical Screening
Physical screening can be defined as the removal of solids from the wastewater flow by a
screening medium (such units have no appreciable thickness in the direction of flow).
Screening units are also described in Chapters 6 and 11. Different types of coarse and fine
screens are used to remove coarse material at the head of the plant or to remove SS as a part
of, or in lieu of, primary treatment. Design criteria for screening units are presented in
references (2), (3), (4), (5), (6), and (10).
In the past, fine screens used in wastewater plants were mechanically cleaned devices with
openings 1/8 in. (3 mm) or less. Wedge wire screens are becoming accepted as efficient and
economical devices for small treatment plants (10).
5.7 Pumping
Quite frequently, wastewater must be pumped from its point of entry to the treatment pro-
cesses. Pumping facilities often form part of the headworks. The wet well is sometimes used
as the point at which to recycle some plant influent or to add chemicals for odor control.
Subsection 3.3 contains descriptions of design criteria and other aspects of pumping at small
wastewater treatment plants.
5.8 Flow Measuring and Sampling
Means for efficient flow measuring and sampling must be considered by designers of small
wastewater treatment facilities. The flow and time of day should be noted for all samples of
wastewater taken. Locations in a treatment facility where flow measuring and sampling
should be considered are plant influent, equalization tank effluent, recirculation streams,
process sidestreams, sludge withdrawals from the wastewater treatment stream, and plant
effluent.
Measuring devices used in small wastewater treatment plants include Parshall flumes, Palmer-
Bo wlus flumes, and weirs in open channels; venturi tubes, Dall tubes, orifices, nozzles, mag-
netic meters, and pipe bends in closed pipelines; and parabolic flumes, California pipes, and
Kennison nozzles in pipes discharging freely to the atmosphere. Gravimetric and volumetric
containers are used to calibrate flow-measuring devices and sometimes for regularly sched-
uled measuring on a fill-and-draw basis. Positive displacement pumps can be used to obtain
relatively accurate measurements of sludge flow if they are calibrated on a regularly sched-
uled basis. Descriptions and design criteria for these devices can be found in references (4),
(5), (6), and (11) and in manufacturers' catalogs and bulletins.
5-5
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Factors interfering with one or more of the commonly used measuring devices are grease,
floating solids, grit, SS, excessive variation in flow, and inadequate available heads. Flow
velocities, through measuring devices, must be large enough to cause periodic scouring of
any solids that have settled during time of low flow, but should never reach supercritical
levels. Upstream conditions may influence flow measurement if flumes or weirs are used.
The approach flow velocities must be less than critical under all conditions. Drops in the in-
vert should be located far enough upstream from the open channel measuring device for the
hydraulic jump and resulting turbulence to have been dissipated before the flow reaches the
measuring device. Small pipelines under pressure, which might enter open channels ahead of
flumes or weirs at supercritical velocity, can also cause erroneous measurement, if not
located a sufficient distance upstream from the flume or weir. A straight reach should be
provided far enough upstream for the incoming velocity distribution to become uniform
across the entire rectangular cross-section at the entrance to the measuring device.
The difference between liquid levels upstream of a control and control elevation indicate the
flow through a flume or weir. Liquid-level indicators commonly used are floats, pressure
cells, electrical probes, and pneumatic tube bubblers. Conditions downstream of open
channel measuring devices must be designed to avoid any backup of flow that will flood
out the measuring device. With a separate float chamber, the float can be maintained satis-
factorily by most operators. Floatless, liquid-level-indicating devices sometimes require
servicing by manufacturers' representatives when they malfunction.
To measure raw wastewater and other open channel flows, prefabricated Parshall flume
liners, with integral float chambers installed in concrete channels, are relatively trouble free.
Palmer-Bowlus flumes are also prefabricated for installation in pipes or manholes (12). One
type of flume carries an imbedded sensor-element, electronically providing information to
relate water depth to flow rate. Installation of such a Palmer-Bowlus flume is shown in
Figure 5-1. This monitor can also be used to provide an output for automatic control of
treatment processes or sampling.
Adjustable-level V-notched weirs make good overflow and measuring devices from activated
sludge-recirculating and flow-splitting boxes. Constant heads on these boxes can be main-
tained, using progressive cavity pumps or telescoping valves. The liquid-level indicator used
here can be relatively trouble free if a separate float box is used.
Magnetic flow meters are expensive and difficult to maintain, but can measure accurately
over a 10:1 flow range (larger than most) without interference from organic solids, if grease
has been removed from the wastewater. To function most efficiently, however, they must
be installed vertically with straight approaches. They are being used satisfactorily to measure
return sludge at some small activated plants, but not for primary sludge, because of grease
content.
There are many ways of measuring flows and sampling in a manhole with little, if any, loss
of head. Stephenson and Gates (13) describe an arrangement using a V-notch weir, a float in
a small stilling well, a recording meter, and a 24-hour composite sampler, all of which are
shown in Figure 5-1.
5-6
-------
FLOAT CABLE
COUNTERWEIGHT
FLOAT
STILLING WELL
CONC. FILLET
CONC. BASE
TAPERED CONE SECTION
CABLE AND LOCK
RECORDING METER
STRAIGHT BARREL SECTIONS
CONCRETE FASTENER (TYP.)
MARINE PLYWOOD
WEIR PLATE*
2 x4 BRACING
SEALED WITH ROOFING CEMENT,
SAND BAGS AND NEWSPAPER
WEIR AND FLOW METER INSTALLATION (9)
IMBEDDED DEPTH
SENSOR-
FLOW RECORDER
PALMER-BOWLUS FLUME
PALMER-BOWLUS FLUME AND FLOW METER INSTALLATION (13)
FIGURE 5-1
TYPICAL METERING AND SAMPLING INSTALLATIONS
5-7
-------
Manhole installation of the Palmer-Bowlus flume is simple and efficient (5). This device is
especially useful if flow measuring devices are to be added to existing facilities.
Many automatic sampling devices have proved to be satisfactory and have been developed
and patented. The Handbook for Monitoring Industrial Wasfewater (11) points out that ob-
taining good samples and the resulting analytical data depends on:
1. Insuring that the sample taken is truly representative of the liquid.
2. Using proper sampling techniques.
3. Protecting and preserving the samples until they are analyzed.
Sampling procedures are described in references (8) and (11).
5.9 Equalization Tanks
Equalization tanks are sometimes used for small treatment plants and can be included in the
general category of headworks components. Methods used to equalize hydraulic and organic
loadings on various processes are discussed in detail in Chapter 4.
5.10 Chemical Additives
Chemicals for disinfection, pH control, and odor control are sometimes added to the waste-
water in the headworks. Chemicals may be dispersed by adding them to aerated grit
chambers, aerated channels, hydraulic jumps, and pump suction lines. Design for chemical
addition is discussed in Chapter 6.
5.11 References
1. Design Criteria for Mechanical, Electric and Fluid System and Component Reliability.
U.S. EPA Technical Bulletin No. 430-99-74-001 (1973).
2. Recommended Standards for Sewage Works. Great Lakes-Upper Mississippi River
Board of Sanitary Engineers (April 1973).
3. Fair, G.M., Geyer, J.C., and Okun, D.A., Water and Wastewater Engineering. New
York: John Wiley & Sons (1968).
4. Babbitt, H.E., and Baumann, E.R., Sewerage and Sewage Treatment. New York: John
Wiley & Sons (1958).
5. Wastewater Engineering. Metcalf and Eddy, New York: McGraw-Hill (1972).
6. Wastewater Treatment Plant Design. ASCE and WPCF Manual of Practice Nos. 36 and
8, respectively (1958).
5-8
-------
7. Seminar Papers on Wastewater Treatment and Disposal. Boston SCE, sanitary section
(1961).
8. Roe, F., "Preaeration and Air Flocculation." Journal Water Pollution Control Federa-
tion, 23, No. 2, pp. 127 to 140(1951).
9. Seidel, H., and Baumann, E., "Effect of Preaeration on the Primary Treatment of
Sewage." Journal Water Pollution Control Federation, 33, No. 4, pp. 339 to 355
(1961).
10. Process Design Manual for Suspended Solids Removal. U.S. EPA, Office of Technology
Transfer, EPA-625/l-75-003a (January 1975).
11. Handbook for Monitoring Industrial Wastewater. U.S. EPA, Office of Technology
Transfer, EPA-625/6-73-002 (August 1973).
12. Manufacturer's data, Universal Engineered Systems, Pleasanton, Calif. (1974).
13. Stephenson, R.L., and Oates, W.E., "Installation of Sampling Equipment in Manholes,"
WPCF Deeds and Data (January 1974).
5-9
-------
CHAPTER 6
CLARIFICATION OF RAW WASTEWATER
6.1 General
A large portion of the BOD found in raw domestic wastewaters is in the form of suspended
solids, part of which can be removed by plain sedimentation or, to a greater extent, by
chemical coagulation, flocculation, and sedimentation. General design details and case
studies of facilities best suited to accomplish this can be found in the Process Design Manual
for Suspended Solids Removal (1) and the Process Design Manual for Phosphorus Removal
(2). For additional information on the theory and design of chemical coagulation, floccula-
tion, and clarification, refer to Chapters 12 and 14 and references (3) through (11).
6.2 Coagulation and Flocculation
Coagulation, as the term is used here, is the destabilization and initial agglomeration of col-
loidal or fine suspended matter by adding a floe-forming chemical. The jar test is the normal
way to determine both the coagulant dosage required for optimum SS removal and the
characteristics of the floe. This test attempts to simulate the full-scale coagulation-floccula-
tion process in the laboratory. Test procedures may vary, depending on the type of equip-
ment in the plant, but there are common elements. If the wastewater temperature varies
appreciably, jar tests conducted at the lowest expected temperature should represent the
most exacting conditions, because coagulation and settling are retarded by lower tempera-
tures. If coagulants are used, they must be thoroughly dispersed by adequate mixing in the
wastewater. To insure complete mixing, a rapid mix basin, equipped with mechanical
mixers, is normally required. Adequate mixing can be accomplished in 10 to 30 seconds.
Flocculation is the agglomeration of colloidal particles after coagulation, accomplished by
gentle stirring, using either mechanical or hydraulic means. The mixing, provided in a rapid
mixing basin, will promote particle collision but is much too intense to promote floccula-
tion. The design of the flocculation basin and the characteristics of the chemically treated
particles will determine the limiting size and the settling characteristics of the floe. Velocity
gradients in the flocculation basin promote particle collision and growth. These growing
flocculent particles, however, are fragile and increasingly subject to rupture by sheer force
of the velocity gradient. A typical turbine-type flocculator is shown in Figure 6-1.
The peripheral speed of any agitator in a flocculation basin should generally not exceed
2 ft/sec (0.6 m/s), and a variable speed drive should be provided so the speed can be
adjusted to 0.5 ft/sec (0.15 m/s). More detail on the design requirements for efficient
flocculation is presented in reference (1).
6-1
-------
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TURBINE-TYPE FLOCCULATORS
-------
6.3 Solids-Contact Treatment
As far as complete utilization of added chemicals, such as alum, iron salts, and lime, is con-
cerned, it has been found beneficial to continue chemical addition, mixing, and flocculation
simultaneously, in the presence of previously precipitated solids (3). This procedure is
usually practiced with specially designed systems in which clarification takes place in a
single integral structure (called a solids-contact treatment unit), as shown in Figure 6-2.
Solids-contact treatment can also be simulated by recycling settled sludge from the clarifier
to the rapid mixing basin in a conventional clarification sequence. However, as high a con-
centration of solids cannot be maintained in this manner as in an integral solids-contact unit.
Solids-contact treatment is especially beneficial if lime is used for phosphate precipitation,
because it decreases the tendency for supersaturation and subsequent deposition on equip-
ment and conduit surfaces, which can be quite severe with treatment at high pH.
6.4 Sedimentation
Quiescent conditions must be maintained for sedimentation units to be effective. Gravity
sedimentation is universally used to obtain primary separation of SS from wastewaters and
also to separate biological and chemical floes produced in various treatment processes. Al-
though dissolved air flotation has been employed for primary clarification, the effluent SS,
in general, are higher than that from gravity units. Using these units to separate chemical
floes has shown promise but has not yet been adopted in municipal treatment. Flotation has
proved both technically and economically effective for sludge thickening at larger treatment
plants. Some treatment plant designs have omitted primary settling. This may be advan-
tageous, if one or more of the following conditions apply (12):
1. Sludge from the facility is to be treated away from the facility.
2. Aerobic digestion or extended aeration processes are to be used.
If it is possible to eliminate primary settling, the following benefits may be achieved:
1. The construction, operation, and maintenance costs of primary clarification are
eliminated.
2. The total dry weight of the sludge removed is reduced.
3. The activated sludge solids settle faster.
4. The potential source of odor is removed.
The primary clarifier is usually needed prior to trickling filters and rotating biological con-
tactors (RBC) to avoid' clogging problems from floating objects, oils, greases, and the larger
SS. In some newer installations, wedge wire screens have been successfully employed in lieu
of primary clarifiers. If the primary clarifier is followed by biological treatment and sec-
ondary clarification, it is usually not necessary to remove the finer SS in the primary settling
tank, making increased overflow rates permissible. If the wastewater contains larger-than-
average amounts of grease and oil, an aerated grit chamber, followed by a quiescent grease
removal unit, may be used instead of a primary clarifier equipped to remove floating solids.
Wastewater treatment ponds and oxidation ditches do not require primary clarifiers.
6-3
-------
•DRIVE UNIT
LAUNDER
Os
CHEMICAL
FEEDS
EFFLUENT
fr—INFLUENT
SLUDGE"* 4
FIGURE 6-2
SOLIDS-CONTACT TREATMENT UNIT
-------
6.4.1 Basis of Design
Analysis of the ideal settling basin shows that its area should be equal to:
A = Q/vs
where Q = flow through, ft3/min (m3/s)
vs = settling velocity of particles to be removed, ft/min (m/s)
A = surface area, ft2 (m2)
Thus, the basic parameter controlling the size and performance of a settling basin is the
settling velocity of the individual particles to be removed. In the case of a concentrated
suspension, such as activated sludge mixed liquor, this parameter is the initial settling ve-
locity of the suspension, before the proximity of the particles slow their subsidence (7).
Refer to Chapter 7 for design of secondary clarifiers.
The settling rate of particles depends on their size, shape, and density, and on liquid tem-
perature. The liquid temperature has a very significant influence: settling velocity can be 12
ft/min (62 mm/s) at 25° C or 6.5 ft/min (33 mm/s) at 5° C, because of the significant
changes in the density and viscosity over this temperature range.
Concentrated suspensions, present in activated sludge mixed liquor, or the slurry in a solids-
contact treatment unit, settle as a mass of particles and leave a distinct interface between
the floe and supernatant. The settling occurs as shown in Figure 6-3 and has three zones (6)
(13). During the initial period, the floe settles at a uniform rate under conditions of hind-
ered settling. The magnitude of this velocity depends on the initial solids concentration. The
next zone is a transition zone in which the settling velocity decreases continually. Finally,
there is a compression or thickening zone. The initial settling velocity for any given set of
conditions is important, because it determines what the maximum hydraulic overflow rate
can theoretically be before the sludge blanket in a clarifier might expand and eventually
overflow (6) (13).
6.4.2 Basin Design
In raw domestic wastewater, the range of particle sizes in suspension is very broad. Experi-
ence has indicated that about 50 to 60 percent of the SS-can be removed in reasonably sized
settling basins. It is customary to size such primary basins for an upflow velocity or over-
flow rate. Both the upflow velocity and the overflow rate are equal to Q/A gpd/ft2 (0.04
m3/m2 -d). The peak overflow rate may be 2,500 to 3,000 gpd/ft2 (100 to 120 m3/m2 -d)
for primary clarifiers followed by biological treatment processes (1). If the flow is extremely
variable, as may be the case in small plants, the tank should be designed on the basis of a
peak overflow rate of less than 2,000 to 2,500 gpd/ft2 (80 to 100 m3/m2-d). If waste-
activated sludge is returned to the primary clarifier, the peak overflow rate should be
reduced to about, 1,200 to 1,500 gpd/ft2 (48 to 60m3/m2-d) (1).
6-5
-------
(E
UJ
UJ
o
o
I-
<£
UJ
I
CLEAR WATER ZONE
HINDERED SETTLING
CONSTANT COMPOSITION
TRANSITION ZONE
VARIABLE COMPOSITION
B
COMPRESSION ZONE
TIME
CYLINDER
FIGURE 6-3
SCHEMATIC REPRESENTATION OF SETTLING ZONES
6-6
-------
Although basin depth is not considered in the analysis of the ideal basin, in practice it plays
an important role. A certain depth is needed to store settled solids, because they are not re-
moved as soon as they are deposited on the bottom, and depth must be provided to accom-
plish some thickening. The depth also determines the detention time, which should be at
least 120 minutes, to allow possible flocculation to take place. In addition, the flow-through
velocity must be kept low, to insure that solids settling on the bottom are not scoured up.
The maximum horizontal velocities allowable near the sludge layer in a primary clarifier are
on the order of 4 to 5 fpm (0.02 to 0.025 m/s) to prevent possible resuspension of settled
solids (2) (5). Because the effluent takeoff point is quite localized, high upflow velocities
approaching the overflow weir can transport solids, if sufficient depth is not provided be-
tween the sludge blanket or solids on the basin bottom and the overflow weir. Although
weir loading (expressed as gpd/ft) is frequently specified, it is not an independent param-
eter, because higher loadings can be compensated for by deeper basins.
Clarifiers handling chemical floes, such as from aluminum or iron coagulants, should be de-
signed for peak overflow rates no longer than 600 and 800 gpd/ft2 (24 and 32 m3/m2>d),
respectively. With lime treatment, these peak loadings can be as high as 1,600 gpd/ft2 (64
m3/m2'd), if good coagulation is obtained. For uniform dispersion of the influent to the
sedimentation unit, orifices placed in walls at the inlets should be sized to produce veloci-
ties on the order of 0.5 to 1.0 fps (0.15 to 0.3 m/s). Orifices passing wastewater containing
floe should not be smaller than about 0.3 to 0.5 in. (7.5 to 12.5 mm), to minimize floe
breakup (2).
6.4.3 Basin Types
Settling basins can be characterized by the predominant direction of flow, horizontal or
vertical, from inlet to outlet. Horizontal flow basins can be either rectangular or circular,
as shown in Figure 6-2. However, some clarifiers (similar to those shown in Figures 6-4 and
6-5) may have flows that are both horizontal and vertical, by placing effluent launders at
points other than at the periphery or the end of the basin.
Both rectangular and circular units have been used for primary settling and final settling
basins. Rectangular tank length-to-width ratios should be greater than 4:1, to reduce pos-
sible short circuiting. Choice is frequently based on space, construction, costs of multiple
units, and preferences relating to sludge-thickening and removal arrangements. For treat-
ment works with flows less than 1 mgd (0.044 m^/s), circular primary tanks are generally
more economical than rectangular tanks. A discussion of relative economics of construction
and other design factors can be found in reference (1).
6.4.4 Removal of Sludge and Skimmings
Most of the equipment installed in gravity settling basins is used to remove settled and float-
ing oil, grease, or other debris. Standard equipment for circular basins consists of at least
two revolving radial arms with attached angled scrapers, which move settled solids to a cen-
tral sump (shown in Figure 6-6). For rectangular basins, the usual mechanism consists of a
chain and flight collector (shown in Figure 6-4), which move settled solids to a sump at the
inlet end.
6-7
-------
DRIVE UNIT-
SKIMMER
1 ' I I I
SPROCKET
SLUDGE
WITHDRAWAL
PIPE
SLUDGE HOPPER
FLOW
I I I I I I I II
FLIGHT COLLECTOR CHAIN
I I II i 1 l
. . i -^ it '
.-<^'.:• J A -• 'T .'">t ; f: ''• ; *? •'
FIGURE 6-4
RECTANGULAR CLARIFIER WITH CHAIN AND FLIGHT SLUDGE COLLECTOR
-------
DRIVE UNIT
EFFLUENT WEI
SCUM PIPE
SCRAPER BLADES-
SLUDGE
DRAWOFF PIPE
FIGURE 6-5
CIRCULAR CLARIFIER WITH SKIMMER
EFFLUENT
TURBINE-
DRIVE UNIT
SLUDGE
II
H
I
SLUDGE
_$•—INFLOW
FIGURE 6-6
CIRCULAR CLARIFIER WITH INNER FLOCCULATION COMPARTMENT
6-9
-------
A sludge collector for a circular basin (used to move skimmings toward their discharge point)
consists of a radial arm connected to a submerged plate, with an attached flexible sweeper,
which rides on a sloping bottom pushing the sludge to a sump from which it is pumped to
the sludge processing system.
In circular units, sludge sump solids can be stirred gently by a blade attached to the scraper
mechanism, thus combining the processes of sludge thickening and keeping the thixotropic
sludge fluid enough to be discharged by gravity or pumping. Uusally, this removal is done on
a timed cycle. Similar stirring of sump contents is not normally possible in rectangular units,
and problems with sludge "hang-up" have been encountered in rectangular tanks used as
primary clarifier units.
Sludge collector mechanisms for circular tanks are usually lower in cost and require less
maintenance than chain and flight collectors for comparable rectangular units; hence, travel-
ing bridge (instead of chain and flight) collectors have been developed and used extensively
in Europe for rectangular basins handling flows greater than about 1 mgd (0.044 m3 /s).
Recently, they have come into use in the United States but not for small treatment plants.
Although secondary clarifiers for trickling filter plants can be similar to those used for pri-
mary settling plants, secondary clarifiers for activated sludge plants will require special con-
sideration. Refer to chapters 7 and 9 for the design of secondary clarifiers for small plants.
Thickened sludge concentration obtained from a primary clarifier without chemical treat-
ment will be approximately 3 to 6 percent by weight, while the average volume will be 0.3
to 0.5 percent of the raw wastewater flow.
6.5 References
1. Process Design Manual for Suspended Solids Removal. U.S. EPA, Office of Technology
Transfer (January 1975).
2. Process Design Manual for Phosphorous Removal. U.S. EPA, Office of Technology
Transfer (April 1976).
3. O'Melis, C.R., "Coagulation in Water and Wastewater Treatment." Water Quality Im-
provement by Physical and Chemical Processes. E.F. Gloyna and W.W. Eckenfelder,
University of Texas Press (1968).
4. Camp, T.R., Flocculation and Flocculation Basins. Transactions ASCE (1955).
5. Kalinski, A.A., "Solids-Contact Units for Water Treatment. Civil Engineering (Decem-
ber 1960).
6. Black, A.P., Buswell, A.M., Eidsness, F.A., and Black, A.L., "Review of the Jar Test."
Journal American Wastewater Association, vol. 49, p. 1414 (1957).
6-10
-------
7. Camp, T.R., Sedimentation and Design of Settling Tanks. Transactions ASCE, vol. Ill,
p. 895(1946).
8. Kalinske, A.A., Settling Rate of Suspensions in Solids-Contact Units. Proceedings
ASCE, vol. 79, p. 1 (1953).
9. Gulp, G.L., Hsiung, K., and Conley, W.R., "Tube Clarification Process, Operating Ex-
periences." Journal Sanitary Engineering Division ASCE, p. 829 (October 1969).
10. Kalinske, A.A., "Settling Characteristics of Suspensions in Water Treatment Process."
Journal American Wastewater Association, vol. 40, p. 113 (February 1948).
11. Recommended Standards for Sewage Works. Great Lakes-Upper Mississippi River
Board of Sanitary Engineers (1973).
12. Von Der Emde, W., "To What Extent Are Primary Tanks Required?" Water Research,
vol. 6, pp. 395(1972).
13. Package Sewage Treatment Plants Criteria Development, Part 1: Extended Aeration.
National Sanitation Foundation, FWPCA Grant Project Report No. WPD-74 (Sep-
tember 1966).
6-11
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CHAPTER 7
ACTIVATED SLUDGE
7.1 Introduction
Activated sludge has become the most versatile biological process available to the designer of
wastewater treatment plants. The activated sludge process, designed for large communities
and some preengineered (package) plants in small communities, has been successfully em-
ployed for decades in the United States. EPA's Municipal Waste Facilities Inventory (August
1974) lists over 3,000 activated sludge plants, with design flows of less than 1 mgd, serving
about 6,500,000 people in the United States. Package plants for use in smaller communities
are discussed in Chapter 8.
7.2 Description of Basic Processes
7.2.1 Definitions
Definitions of terms used in describing the activated sludge processes are listed below:
F/MV Food-to-micro-organism ratio, or process loading factor; pounds of fresh BODs
applied to the activated sludge system per day per pound of MLVSS in the aera-
tion basin, Ib BOD/day/lb MLVSS.
MLSS Mixed liquor suspended solids; suspended solids in the aerator mixed liquor, mg/1.
MLVSS Mixed liquor volatile suspended solids; volatile suspended solids in the aerator
mixed liquor, mg/1.
SRT Sludge retention time; total pounds of MLVSS in the aerator per pound of VSS
wasted per day (or net solids produced), days.
SCFM Standard cubic foot of gas, measured at a dry pressure of 1.0 atmosphere (10.13
kPa) and 20° C.
SVI Sludge volume index; volume in millimeters occupied by 1. gram of activated
sludge, after settling the aerated mixed liquor for 30 minutes in a 1,000-ml gradu-
ated cylinder.
SVI = (ml settled sludge X l,000)/(mg/l SS)
ISV Initial settling velocity of aerator-mixed liquor, determined after a few minutes of
settling in the settleometer test (1).
Completely mixed—Solids throughout a fluid, including the effluent, are homogeneous.
Plug flow— Particles in the influent pass through the system as a group or plug in the same
period of time.
7-1
-------
7.2.2 Symbols
Symbols for terms used in this chapter are as follows:
Q Average design (24-hr) flow, gpd.
V Aeration tank volume, ft-3.
LJ Primary effluent (to aerator) BODs, mg/1.
Le Aerator effluent BODs, mg/1
F Total BOD5 applied to the activated sludge process; 8.34 (Q)(Lj - Le)/106, Ib/day.
Fe Clarifier effluent BOD5, Ib/day.
Fs Clarifier sludge BOD5 wasted, Ib/day.
S Primary effluent (to aerator) SS, mg/1.
Sj Primary effluent (to aerator) VSS, mg/1.
Se Clarifier effluent SS, mg/1.
Sv Clarifier effluent VSS, mg/1.
Cc Clarifier settled solids, mg/1.
Cv Clarifier volatile settled solids, mg/1.
M Total aerator MLSS, Ib.
Mv Total aerator MLVSS, Ib.
Me Clarifier effluent SS, Ib/day.
Mw Excess VSS produced, Ib/day (see Figure 7-1).
Ms Excess SS produced, Ib/day (see Figure 7-1).
O Oxygen required, Ib/day/BODs removed, Ib/day.
OR Oxygen required, Ib.
ta Aerator retention time.
7-2
-------
R Recycle ratio, Q/QR.
QR Recycle flow, gpd.
7.2.3 Conventional Activated Sludge
"Activated sludge" describes a continuous flow, biological treatment system characterized
by a suspension of aerobic micro-organisms, maintained in a relatively homogeneous state by
the mixing and turbulence induced in conjunction with the aeration process. These condi-
tions are in contrast to those in processes characterized by fixed growths of micro-organisms
attached to solid surfaces, such as trickling filters (see Chapter 9).
Basically, the activated sludge process uses micro-organisms in suspension to oxidize soluble
and colloidal organics in the presence of molecular oxygen. During the oxidation process, a
portion of the organic material is synthesized into new cells. A part of the synthesized cells
then undergo auto-oxidation (self-oxidation, or endogenous respiration) in the aeration tank.
Oxygen is required to support the synthesis and auto-oxidation reactions. To operate the
process on a continuous basis, the solids generated must be separated in a clarifier; the major
portion is recycled to the aeration tank and the excess sludge is withdrawn from the clarifier
underflow for additional handling and disposal (2). The two basic units in an activated
sludge system are the aerator and the clarifier. In a conventional system (as shown on Figure
7-1) the primary effluent and the return sludge enter one end of a rectangular tank (length-
to-width ratio of 5:1 to 50:1), move turbulently through the aerated chamber in a sub-
stantially plug-type flow, and are discharged as a treated mixture at the other end.
In the activated sludge process, several biological, physical, and chemical subprocesses in-
fluence the total performance of the treatment system. These subprocesses are:
1. Dissolution of oxygen into liquid.
2. Turbulent mixing.
3. Absorption of organic substrate by the activated floe.
4. Molecular diffusion of dissolved oxygen and soluble substrate (nutrients) into the
activated floe.
5. Basic metabolism of micro-organisms (cell synthesis).
6. Bioflocculation, resulting from the production of exocellular polymeric sub-
stances during the endogenous respiration phase.
7. Endogenous respiration of microbial cells.
8. Release of CC<2 from the active cell mass.
9. Lysis, or decomposition of dead microbial cells.
Because of the interaction of the above subprocesses and because more than one of these
processes can be altered by some external change (e.g., the intensity of mixing), the exact
cause-effect relations are frequently obscured. A temperature change can affect all the sub-
processes to some extent.
7-3
-------
RAW
PLUG FLOW
AERATION TANK
RETURN SLUDGE
EXCESS SLUDGE
FIGURE 7-1
CONVENTIONAL ACTIVATED SLUDGE SYSTEM
The heterogeneous nature of the microorganism population in the activated sludge bio-
mass—with different metabolic rates and optimum microenvironmental conditions for the
different organisms—makes the entire process exceedingly complex. Furthermore, if the
process is applied to domestic wastewater treatment, the organic and hydraulic loads on the
system change continually; a true steady state is nonexistent in full-scale plants.
The following factors are essential in the design and use of a conventional activated sludge
plant (1):
1. If the wastewater is mixed with a portion of the secondary clarifier sludge and
aerated for a period of 6 to 8 hr (based on the average design flow), the rate of
sludge returned to the aerator (expressed as a percentage of the average waste-
water design flow) is normally about 25 percent, with minimum and maximum
rates of 15 to 75 percent.
2. The normal organic loading rate for conventional activated sludge (expressed as
F/MV) should be about 0.4 to 0.6 Ib BOD5/day/lb MLVSS in the aerator, or 0.2
to 0.4 Ib BOD5/day/lb MLSS (expressed as F/M).
3. Volumetric BOD5 loadings are usually 20 to 40 lb/day/1,000 ft3 of aerator
volume (320 to 640 kg/1,000 m3 -d).
4. The SRT should be sufficient to complete removal of the organic wastes and to
enable the microbial cells to improve in settleability. SRT's range between 5 and
15 days, normally achieving 85 to 95 percent BODs removal, with proper opera-
tion.
5. Initial oxygen demand in the head end of the aeration tank is high, and diminishes
toward the outlet. This can be matched by tapering the air supply.
6. Because of the extreme variations in hydraulic and organic loadings common with
wastewater from small communities, there may be less operational stability than
in extended aeration units.
7-4
-------
7.3 Modifications for Small Communities
7.3.1 General
The wastewater flow and organic content from the average small community will vary
greatly during each 24-hr period, making some activated sludge treatment processes less
dependable than others.
In a completely mixed system, the entire biomass is subjected to any increase in BOD shock
loading or toxicity. In a plug-flow system, the biomass near the entrance initially receives
the full impact of an increase in BOD or toxicity.
The biomass in activated sludge systems will normally have an oxygen uptake rate of 50 to
100 mg/l/hr, depending on the concentration of MLVSS. If a sudden increase in BOD$ load
increases this uptake rate by 10 mg/l/hr when the DO level in the aeration basin is 6 mg/1,
the DO will drop to 0 in 30 minutes; if the DO is maintained at 2 mg/1, it will drop to 0 in
12 minutes.
A study comparing completely mixed and plug flows was made at Freeport, Illinois (3). This
study was done with full-scale, parallel systems. The report on the study concludes:
An inherent difference in the biological environment in completely mixed and plug-flow systems is
the uniform substrate composition throughout the completely mixed system compared with the vari-
able concentration in the plug-flow system. At the head of the plug-flow system the influent is dis-
persed in only a small part of the tank volume into which return sludge is added. Not only is the F/M
ratio high in the head of the plug-flow system but the return sludge organisms face another shock
situation inasmuch as they are coming from the region of the system in which substrate concentration
is not only the lowest but of different character than that presented by the influent.
On the basis of the data developed it can be concluded that the completely mixed system does pro-
vide for improved treatment during periods of extreme shock loads involving a decreased detention
time and increased organic load.
The (essentially) completely mixed activated sludge systems now commonly used to treat
wastewater from small communities are 1) extended aeration (low loading rate), 2) oxida-
tion ditch (low loading rate), 3) contact stabilization (high loading rate), and 4) completely
mixed (high loading rate).
Information in this chapter is confined to design of activated sludge aeration and clarifier
units for small communities. The general characteristics distinguishing pertinent activated
sludge systems are summarized in sections 7.3.2 through 7.3.6.
7.3.2 Extended Aeration System
This system (illustrated on Figure 7-2) operates in the endogenous respiration phase of the
bacterial growth cycle, which occurs when the BOD loading is so low that organisms are
starved and undergo partial auto-oxidation. The loading (F/MV) is the low rate range,
7-5
-------
SCREENED AND
DEGRITTED RAW
WASTEWATER
FIGURE 7-2
EXTENDED AERATION ACTIVATED SLUDGE SYSTEM
SCREENED AND
DEGRITTED RAW
WASTE WATER
AERATION
ROTOR
'J
RETURN SLUDGE
FINAL \EFFLUENT
CLARIFIES t »•
EXCESS
SLUDGE
FIGURE 7-3
OXIDATION DITCH ACTIVATED SLUDGE SYSTEM
7-6
-------
about 0.05 to 0.15 Ib of BOD/lb of MLVSS/day. Because of the oxidation of more volatile
solids during the long sludge retention time, the waste sludge production is relatively low.
The MLSS ranges from 3,000 to 5,000 mg/1. The hydraulic retention time in the aeration
basin is about 24 hr. In cold climates at such long retention times, the temperature of the
liquid can drop to below 40° F (5° C), with a resultant slowing down of bacterial activity. It
is customary to omit the primary clarifier in these systems, to simplify and reduce the waste
sludge handling arrangements.
7.3.3 Oxidation Ditches
This system (illustrated in Figure 7-3) was originally developed in the Netherlands for the
extended aeration process in small towns. It consists of a continuous channel, usually in the
form of an oval "racetrack" or ring, with an aeration rotor or rotors, revolving on a hori-
zontal shaft, which supply oxygen by intense surface agitation and also impart motion to
the liquid around the channel (4). There are over 100 installations in the United States,
many in the northern part of the country. They are considered to be a low rate system with
a completely mixed flow.
For an oxidation ditch to function satisfactorily, the velocity gradients and DO's in all parts
of the ditch should be relatively constant. Horizontal aerators tend to create more turbu-
lence at the surface than near the bottom. In rectangular (more so than in trapezoidal)
channels, means must be employed to prevent eddies and sludge settling next to the
inner wall immediately after a 180° bend. Using turning vanes on the bends or placing the
horizontal aerators immediately after the bend can prevent the settling. To maintain a rela-
tively uniform DO, the velocity in the channels should insure that the travel time between
aerators is no more than 3 to 4 minutes.
The primary clarifier is usually omitted in these installations; the wastewater is only screened
and degritted before aeration. The MLSS in the ditch is usually about 3,000 to 5,000 mg/1,
and the hydraulic retention time is about 24 hr for domestic wastewater. A final clarifier
follows the ditch, with the sludge recycled to a point ahead of the aerating rotor, where the
raw wastewater also enters.
A system somewhat similar to the oxidation ditch has been recently introduced in the
United States from South Africa. Known as the "orbal system," it consists of several inter-
connected channels in a concentric arrangement. Aeration and liquid movement along the
orbal system channels are obtained by perforated vertical disks, submerged for about 40
percent of their diameter, rotating on a horizontal shaft.
Another alternative oxidation ditch system from the Netherlands is the Carousel system,
shown in Figure 7-4. The volumetric loading of the aeration units in these systems is about
30 Ib BOD5 per 1,000 ft3 (480 kg/1,000 m3). A vertical shaft, surface-type mechanical
aerator is employed for aeration and to impart a spiral flow through the channels. The
depth of the channel should be greater than the diameter of the aerator up to about 9 to 10
ft. About 0.6 Ib/day (0.27 kg/day) of dry solids are produced per Ib of BOD5 removed.
7-7
-------
103m
EFFLUENT
TURNING
VANES
7
INFLUENT
•-. J*
_ ^
//
ii
4
1
/'i/\
»x,V/
VERTICAL
SHAFT
MECHANICAL
AERATORS
RETURN
SLUDGE
END OF BAFFLE-
VERTICAL SHAFT
MECHANICAL AERATOR
SECTION A-A
N.T.S
A
SECTION B-B
N.T.S.
FIGURE 7-4
CAROUSEL OXIDATION DITCH ATOOSTERWOLD, THE NETHERLANDS
7-8
-------
7.3.4 Contact Stabilization System
This system is adapted to wastewaters that have an appreciable amount of BOD in the form
of suspended and colloidal solids. The highly adsorptive properties of activated sludge are
used to physically adsorb the suspended and colloidal solids upon the activated sludge in a
short contact period. (The flow diagram is shown in Figure 7-5.) Primary settling may be
omitted, but an equalization unit may be necessary for reliable performance, if the ratio of
peak to minimum flow is greater than about 3:1 or 4:1, as is expected from smaller com-
munities. The raw wastewater is contacted with aerated sludge in a contact basin and com-
pletely mixed and aerated. Suspended, colloidal, and some dissolved organics are adsorbed
on the activated sludge, in an average hydraulic retention time of 20 to 40 minutes. The
sludge is then settled and returned to a stabilization (reaeration) basin with a retention time
of 4 to 8 hr, based on the sludge flow. For very small or package plants, the retention time
in the stabilization basin has been increased to 24 hr with good results. The adsorbed
organics undergo oxidation and are synthesized into microbial cells in the stabilization basin.
This process can handle shock organic and toxic loads better than can a conventional pro-
cess, because of the buffering capacity of the sludge reaeration tank, which is isolated from
the mainstream of flow.
Generally, the total aeration basin volume (contact plus stabilization basins) is only about
50 percent of that required in the conventional system (5) (6). The total biomass in the
system is calculated by adding the MLSS in the contact and stabilization basins. The MLSS
concentration in the stabilization basin is three to five times the MLSS concentration in the
contact basin.
7.3.5 Completely Mixed System
In a high-rate completely mixed system, as in low-rate completely mixed systems, all por-
tions of the aeration basin are essentially homogeneous, resulting in a uniform oxygen
demand throughout the aeration tank. This condition can be accomplished fairly simply in a
symmetrical (square or circular) basin with a single mechanical aerator or by diffused aera-
tion. The raw wastewater and return sludge enter at a point (e.g., under a mechanical
aerator) where they are quickly dispersed throughout the basin. In rectangular basins with
mechanical aerators or diffused air, the incoming waste and return sludges are distributed
along one side of the basin and the mixed liquor is withdrawn from the opposite side, as
shown in Figure 7-6.
All parts of the basin receive the same organic load, and all organisms are fed uniformly,
permitting higher loadings, resulting in a more stable system, and allowing shock and toxic
loads to be handled without as detrimental an effect on microorganisms as that occurring in
plug flow systems (7) (8). However, the high-rate completely mixed system with F/MV
above 0.75 to 1.0 is slightly less efficient that the three systems described in sections 7.3.2,
7.3.3, and 7.3.4.
7-9
-------
^SCREENED AND DEGRITTED
[RAW WASTEWATER
[POSSIBLE FLOW
EQUALIZATION UNIT
-- CONTACT TANK
ALTERNATE
EXCESS SLUDGE
DRAW-OFF
POINT
STABILIZATION
TANK
RETURN SLUDGE
EXCESS
SLUDGE
FIGURE 7-5
CONTACT-STABILIZATION ACTIVATED SLUDGE SYSTEM
SCREENED AND DEGRITTED
RAW WASTEWATER
COMPLETE MIX
AERATION TANK
RETURN SLUDGE
EXCESS
SLUDGE
FIGURE 7-6
COMPLETELY MIXED ACTIVATED SLUDGE SYSTEM
7-10
-------
7.3.6 Pure Oxygen Systems
Since 1969, pilot and full-scale plants studies have been made, using pure oxygen for acti-
vated sludge systems. The system most extensively studied involves covering and compart-
mentalizing the aeration basins to obtain a high percentage of oxygen utilization. Turbine
aerators with compressors for oxygen recycle are used to increase the oxygen dissolution
efficiency (9) (10).
The oxygen must be produced onsite with an oxygen generator or liquid oxygen must be
purchased and stored in tanks.
Pure oxygen, activated sludge package plants are available and are used for industrial appli-
cations; however, they are not yet used to any extent in small municipal plants, because of
the costs and the operation requirements for small units.
7.3.7 Selection of Specific Modifications of the Activated Sludge Process
Table 7-1 summarizes criteria and characteristics for extended aeration, oxidation ditch,
contact-stabilization, and high-rate complete mix processes. Considerations particularly im-
portant in selecting a process are:
1. The amount of excess sludge is less for sludges from extended aeration and oxida-
tion ditches or for sludge from contact-stabilization units with longer detention
time in the stabilization basins.
2. If low-cost, suitable land is available nearby, the oxidation ditch may be the most
cost-effective system.
3. If equalization of flow and loading for following treatment is desirable, it is pos-
sible to design an extended aeration or an oxidation ditch for a 15- to 20-percent
variation in operating depth, and achieve the needed equalization of both flow
and BOD load within the aeration basin itself.
4. If the peak-to-minimum flow ratio is greater than about 4:1, it will be difficult to
achieve efficiency using a contact stabilization system, without some prior equal-
ization of flow.
5. Influents with grease and oil concentrations between 75 and 200 mg/1 require
grease removal pretreatment for high-rate completely mixed activated sludge or
contact stabilization systems but not for extended aeration or oxidation ditch
systems.
6. A completely mixed low-rate extended aeration or oxidation ditch system can
handle toxic or shock loadings better than can high-rate completely mixed or con-
tact stabilization units.
7. If nitrification to reduce ammonia-nitrogen levels in the effluent to 1 to 3 mg/1 is
required, the extended aeration or oxidation ditch can be designed to accomplish
the required removals.
7-11
-------
TABLE 7-1
DESIGN CRITERIA OF MODIFIED ACTIVATED SLUDGE PROCESSES (11) (12)
to
Item
F/My, Ib BOD5/day/lb MLVSS
Sludge residence time, days
MLSS, mg/1
(contact unit/reaeration unit)
Volumetric loading, Ib BOD/day/ 1,000 ft3
Hydraulic detention, hr
(contact unit/reaeration unit)
Recycle ratio (R)
SCFM air/lb BOD5 removed
Lb O2/lb BOD5 removed
Reduction of NH3 as N, percent
Volatile fraction of MLSS
Extended
Aeration
0.05-0.15
20-30
3,000-6,000
10-25
18-36
0.75-1.5
3,000-4,000
1.5-1.8
90
0.6-0.7
Oxidation
Ditch
0.03-0.10
20-30
3,000-5,000
10-20
12-96
0.25-0.75
—
1.5-1.8
90
0.6-0.7
Contact
Stabilization
0.2-0.6
6-12
1,000-3,000
(4,000-10,000)
30-40
0.3-0.7
(3-6)
0.25-1.0
800-1,200
0.7-1.0
20
0.6-0.8
High-Rate
Complete Mix
0.2-0.4
6-12
2,000-5,000
40-60
2.6
0.25-1.0
800-1,200
0.7-1.0
20
0.7-0.8
-------
7.4 Applicable Design Guidelines
7.4.1 Food to Micro-organism Ratio (F/MV)
The most important basic design parameter for activated sludge systems is the organic load-
ing, expressed as food-to-micro-organism ratio (F/MV). Different loadings are used for various
systems; the value of the ratio will change, depending on effluent quality required. This
loading also influences the waste sludge produced, mixed liquor settling characteristics, and
the oxygen requirements. It is represented by the equation:
- Le)/MLVSS
where
F/My = f°°d to micro-organism ratio, Ib BOD/day/lb MLVSS
Q = 24-hr design flow, gpd
V = aerator tank volume, ft^
Lj = BODs in aerator influent, mg/1
Le = BODs in aerator effluent, mg/1
MLVSS = volatile suspended solids in aerator mixed liquor, mg/1
Before the aeration basin volume can be determined, the engineer must select the BOD load-
ing, or F/MV ratio, at which the treatment plant will operate for the design flow. The F/MV
ratio under which the plant operates will control the final effluent quality (by the settling
characteristics achieved for the sludge), the degree of organic removal, and the amount of
waste sludge produced per pound of BOD5 removed. If the F/MV value is above 0.75 to 1.0
lb/BOD5/day/lb MLVSS, the plant is considered a high-rate system.
The soluble BOD5 in the effluent is a function of the F/MV. The F/MV ratio has a moderate
effect on the amount of soluble BOD5 in the effluent for values of 0.25 to 0.5, with the
BOD5 increasing slightly as the F/MV increases. The effect is more pronounced at higher
F/MV values and is also affected by the flow regime (plug flow or complete mix) in the
aerator. Effluent SS may be higher at very high or very low F/MV values, because of the
poorer flocculating properties of the biomass at very low BOD loadings and at F/MV values
of 1.0 and higher (12). In well-operated plants, effluent SS will be least for F/MV values in
the range of 0.25 to 0.50. The primary reason for designing for a relatively low F/MV value
is to minimize waste sludge production, because the disposal of waste activated sludge is
expensive. For example, for settled wastewater at an F/M value of 0.25, the excess VSS is
about 0.38 Ib/lb of BOD5 removed; for an F/M value of 0.75, VSS is about 0.60 Ib/lb
7-13
-------
BOD5—an increase of 58 percent. Proper evaluation of F/MV requires balancing higher
capital cost and operating costs for disposing of the larger sludge volumes against smaller
aeration basins with higher F/M values.
The BOD5 of the effluent SS varies with the F/MV value. For F/MV values below 0.25, the
BOD5 of the SS is about 50 percent, expressed in milligrams per liter; as F/MV values exceed
0.75, the BODs increases to 75 or 100 percent of the SS, expressed in milligrams per liter
(12).
Before the aeration volume can be calculated, the MLSS must be projected on the basis of
the MLSS that can normally be carried for any particular activated sludge system and the
aeration method. With diffused-air aeration systems, the rate of oxygen input will usually
limit the concentration of MLSS that can be carried in the aeration basin. With well-oper-
ated diffused-air systems, the usual range of MLSS for normally loaded plants is 2,500 to
4,000 mg/1. The higher values are for treatment of unsettled domestic wastewater (12).
With mechanical aerators (including both surface aerators and submerged turbines dispers-
ing compressed air), MLSS values have been carried satisfactorily in the range of 3,000 to
4,500 mg/1 (12). The solids in extended aeration plants have lower oxygen requirements, be-
cause of the lower rates of oxygen uptake during endogenous respiration; therefore, higher
values of MLSS (up to 6,000 mg/1) can be carried (3). For design of extended aeration tanks
and oxidation ditches, an MLSS value of 4,000 mg/1 is average for normal municipal waste-
water. In high-rate systems, because of the high rate of synthesis and the resulting increase
in concentration of volatile solids, the normal MLSS will be in the range of 1,500 to 3,000
mg/1, depending on the aeration method (12). For design of completely mixed tanks and the
contact units of contact stabilization systems, an MLSS value of 2,500 mg/1 is average for
normal municipal wastewater. The MLVSS is about 75 to 80 percent of the MLSS for
settled domestic wastewater and 65 to 70 percent for unsettled, but degritted, wastewater
(12).
These values can vary when industrial wastes enter the system. If inert, but volatile, solids
are present (e.g., paper fibers from a papermill waste), the amount of inert solids that are
part of the MLSS must be estimated in determining the MLVSS. A value of MLSS must be
selected to provide good settling characteristics as well as a good F/M ratio. (See Section 7.5
for additional information on settling.)
The aeration volume can be calculated from the following:
V= [0.133(QXLi- Le)]/[(MLVSS)(F/Mv)]
7-14
-------
The aeration basin can be square, rectangular, or circular.
In design of a treatment plant, determination of the F/MV depends on the waste sludge pro-
duced, the MLSS that can be carried with any specific aeration system, the sizing of the
final clarifier, and, most important, how much of a safety factor should be incorporated in
the plant to account for unusual and unexpected conditions that could adversely affect
settling of the aeration solids.
Although BOD loadings for aeration basins have also been expressed as pounds per 1 ,000
ft^/day, comparisons of aeration basin or process performance should be based on the
loading expressed as F/MV.
Retention time is computed from the following equation:
t = [0.133(Li - Le)]/[(F/Mv)(MLVSS)J
where
t = retention time, day.
7.4.2 Sludge Production
The amount of excess solids produced by an activated sludge system is equivalent to the
amount of nondegradable solids entering plus the net volatile solids synthesized in the treat-
ment process. The net volatile solids are equal to the synthesized solids minus those oxi-
dized by endogenous respiration. For domestic wastewater, the nondegradable volatile solids
plus the net synthesized solids can be calculated from the following equation developed
from basic biokinetics (13):
Mw = a(F) - bMv
where
Mw = excess VSS produced, Ib/day
F = BOD5 removed = 8.34 Q(Lj - Le)/106, Ib/day
Mv = total aerator MLVSS, Ib
a,b = constants
The constants a and b can be obtained empirically or from analyses of similar plants. For
settled domestic wastewater, a = 0.70 and b = 0.075; for unsettled wastewater, a = 0.80 to
1. 10 and b = 0.08 (13).
7-15
-------
The above relation can be transformed to calculate the excess volatile solids produced per
day (Mw), in terms of pounds per pound of BOD5 removed, by the equation:
Mw/F = a-b/(F/Mv)
This relation is illustrated on Figure 7-7 for both settled and nonsettled wastewater.
The total excess SS produced (in pounds per day) can be calculated by dividing the value of
Mw by the ratio of MLVSS to MLSS. Thus, if this ratio is 0.75, the total excess SS produced
is:
MS = MW/0.75
To calculate the net amount of excess SS produced by the system, the amount of SS passing
over the weir from the final clarifier with the effluent should be subtracted from the quan-
tity of SS in the clarifier underflow. For low values of F/M, these effluent solids can be a
relatively substantial amount. Thus, for settled domestic wastewater, if the final effluent SS
concentration is 25 mg/1, for an F/M value of about 0.25, the solids carried out with the
effluent will be equal to about 35 percent of the total solids to be wasted.
7.4.3 Sludge Retention Time
Sludge retention time is a useful parameter, because it indicates the time the solids remain in
the system. Because the amount of solids in the final clarifier of an activated sludge system
depends on the depth of sludge blanket kept, the solids in the clarifier are not used in cal-
culating the retention time of the solids in the system. The sludge retention time is numeri-
cally equal to:
SRT = M/MS
where
M = total aerator MLSS, Ib
Ms = excess SS produced, Ib/day
Sludge retention time can also be calculated from:
SRT = MV/MW = l/[a(F/Mv) - b]
7-16
-------
NON-SETTLED
WASTEWATER
0.2
0.3 04 05 0.6 0.7 08
Ib BOD5 REMOVED/day/Ib M LV SS I N AERATOR
0.9
l.O
FIGURE 7-7
SLUDGE PRODUCTION IN ACTIVATED SLUDGE SYSTEMS TREATING DOMESTIC WASTEWATER
-------
"Sludge age" is the term frequently used to indicate sludge retention time; however, a some-
what different definition than is given above for sludge retention time has sometimes been
implied. In older literature and textbooks, sludge age is often based on the MLSS in the
aeration basin and the SS entering the basin with the wastewater.
7.4.4 Sludge Recirculation
The recirculation rate required to maintain the desired concentration of MLSS in the aera-
tion basin can be calculated on the basis of mass balances:
QR = QSS/(CS - Ss) = RQ
where
QR = recycle flow, gpd
Q = influent flow, gpd
Ss = MLSS entering final clarifier, mg/1
Cs = concentration of settled solids (sludge) in final clarifier, mg/1
R = recycle ratio = SS/(CS - Ss)
It is advantageous to concentrate the mixed liquor solids from the aeration basin as much as
is practical, to reduce the recycle pumping. Recycle pumping capacity should be at least
equal to 100 percent of Q, if the MLSS do not settle well and the Cs is no more than about
twice the value of Ss.
Frequently, plant operators use the sludge volume index (SVI) determined by the procedure
given in Standard Methods (14) as a basis for adjusting the recirculation rate (R). The SVI is
the ratio of the percentage solids settled by volume after 30 minutes of settling to the per-
centage suspended solids by weight in the unsettled aerated liquor. The value of R selected
will depend on the settleability of the sludge. A poorly settling sludge—one that has a rela-
tively high index—will require a higher recycle rate, to maintain the necessary MLSS. For
concentrations of MLSS in the range of 1,500 to 5,000 mg/1, an SVI below 100 indicates
a well-settling sludge. As is discussed in section 7.5, the settling rate is considerably affected
by the actual value of MLSS; for example, it decreases for higher concentrations.
Sludges with SVI's above about 200, generally called "bulking" sludges, are one of the prin-
cipal causes of poor plant performance. However, because the SVI is not a basic parameter,
its value is not necessarily indicative of a bulking sludge. Thus, if the MLSS is 1,000 mg/1
and the sludge settles to 25 percent of its original volume in 30 minutes, the SVI equals 250.
In this example, the sludge is not bulking. Bulking sludges may cause a high loss of solids
with the effluent from the clarifier. Causes of bulking and possible remedies are discussed in
section 7.6.5.
7-18
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7.5 Oxygen Requirements
The oxygen required for carbonaceous oxidation in the activated sludge process can be cal-
culated from a relation derived from basic biokinetics, with empirical coefficients that
depend on the type of wastewater. The relation is:
0RC = [a' (F/MV) + b'] Mv
where
ORC = carbonaceous oxygen required, Ib/day
F/MV = Ib BOD5/day/lb MLVSS in aerator
a' = constant (about 0.55 for domestic wastewater)
b' = constant (about 0.15 for domestic wastewater)
Mv = MLVSS in aerator, Ib.
The value of F/MV affects the oxygen requirement, in that the amount needed for
endogenous respiration becomes relatively high if the BODs load is low, and vice versa, as
shown below:
Activated Sludge System F/MV
High Rate 1.0 0.70
Low Rate 0.1 1.65
Note that the oxygen required is dependent on the amounts of influent BOD and MLVSS in
the aeration basin. Values of a' and b' from a number of sources are shown in Table 7-2. If
industrial. wastes are added to the municipal wastes and the wastewater influent is quite
septic, or a large proportion of residences have garbage grinders, the values in Table 7-2 for
a' and b' do not apply. Depending on the completeness of the oxidation (the amount of
synthesis and endogenous respiration), the BOD5 may not truly represent the organic load-
ing.
If nitrification occurs, oxygen must be supplied in addition to the oxygen required for re-
moval of the carbonaceous BOD. The oxidation of ammonia-nitrogen to nitrate requires
about 4.6 Ib of oxygen per pound of ammonia-nitrogen oxidized, which must be added to
that calculated for removing BODs- The additional oxygen required for nitrification can be
found using the following equation:
0RN = (38.4)(Q)(ANH3)/106
where
7-19
-------
ORN = nitrogenous oxygen required, Ib/day
Q = plant inflow, gpd
ANH3 = [(influent NH3 -N) - (effluent NH3 -N)], mg/1
TABLE 7-2
COMMONLY USED VALUES FOR SYNTHESIS
CONSTANT a' AND AUTO-OXIDATION CONSTANT b'
Method of
©2 Supply
Oxygen
Air
b'
0.635
0.5
0.706
0.48
0.52
0.53
0.5
0.77
0.138
0.055
0.049
0.08
0.09
0.15
0.1
0.075
Reference
EPA, Various Plants (15)
Heukelekian, Orford & Mangelli (16)
Bergman and Borgering (17)
Eckenfelder & O'Connor (18)
Logan &Budd( 19)
Quirk (20)
Emde (MLSS = 3,500-10,000 mg/1) (15)
Smith (Hyperion Plant) (15)
7.6 Clarification
7.6.1 General Information
The two purposes served by a final clarifier are removal of SS from the effluent and thicken-
ing sludge to be returned to the aeration basin. In an activated sludge system, the final clari-
fier must be considered an integral part of the process and designed accordingly. Its area
depends on the settling rate of the MLSS coming from the aeration basin; its depth depends
on the thickening characteristics of the sludge in the clarifier. Because activated sludge solids
form a relatively concentrated suspension, their settling takes place (see Figure 6-3) under
conditions of hindered settling. The initial settling velocity (ISV) remains constant until the
particles in suspension become so concentrated that they "rest" on each other; the process
is then termed thickening.
The ISV is the parameter that must be used in determining the permissible hydraulic loading
on the clarifier. The ISV cannot be accurately determined in a standard 1,000-ml cylinder.
For operations in smaller plants, the 2-liter Mallory type cylinder, 5 in. (127 mm) in
diameter and 6 in. (152 mm) deep, has produced relatively good results. For design pur-
poses, however, procedures similar to those developed by Dick and Ewing (21), which in-
clude gentle stirring, are preferred. These procedures use columns with heights more nearly
that of the sludge depth in the clarifier (i.e., 3 to 6 ft [0.9 to 1.8 m]), diameter of about
3.5 to 4 in. (89 mm), and tip speeds on the stirrer of about 10 in./min (254 mm/min).
7-20
-------
Primary clariflers are discussed in chapter 6. For additional information and details on final
clarifiers, see chapters 9, 12, and 13 and the U.S. EPA. Process Design Manual for Upgrading
Existing Wastewater Treatment Plants (2).
The initial settling rate of a suspension of solids will depend on the density and size of the
individual particles, the concentration of the solids, and the water temperature. The density
of the individual particles in the MLSS will depend on the type of organisms, which will
vary with the type of organic matter in a wastewater and the SRT. Also, the amount and
type of inert material entrained in the floe will have a significant influence. For example, an
activated sludge developed from unsettled domestic wastewater will have heavier particles
than that developed from settled wastewater. The relative amount of soluble organic matter
in a wastewater does not necessarily determine the density of the floe particles. Certain
types of soluble organlcs present in industrial wastewaters can produce very dense biological
growths. Activated sludge floe is an agglomeration of heterogeneous micro-organisms held to-
gether by the bioflocculation caused by extracellular polymeric products produced by cer-
tain organisms. In the floe, the insoluble and inert material that was present in the waste-
water is enmeshed. In general, the floe particle is an amorphous mass without consistent or
physically identifiable properties.
Proper bioflocculation in the activated sludge basin is extremely important for the adequate
performance and functioning of the treatment process. This type of flocculation is quite
strong; even after severe handling of the activated sludge mixed liquor by pumps in the
sludge recycle step, the floes reform. Agitation in the aeration basin usually results in an
energy gradient (G) of about 100, higher than normally used for flocculation. With very low
and very high F/M values, activated sludge does not flocculate well. Activated sludge solids
may become somewhat dispersed, because the flocculation is weak; this dispersal prevents
proper clarification. Insertion of a mechanical flocculation step between the aeration basin
and the final clarifier, to assist in agglomerating poorly adhering floe, is beneficial. The
retention time in such a basin need not be over 10 minutes; the value of G should be below
50.
Figure 7-8 contains a conservative approximation for settling data, obtained from some 10
sources reported in the literature. The references, listed at the end of this chapter, are keyed
to the numbers and symbols shown. These data are for good quality, nonbulking sludges,
corrected to a liquid temperature of 20° C (68° F) with SVI values below 100.
The BODs load (F) in pounds per day and the value of MLSS determine the aeration or
reactor basin size. Also, the value of MLSS determines the clarifier size, because the settling
velocity of the solids decreases as the concentration increases. The initial settling velocity
for three values of MLSS with corresponding theoretical peak overflow rates are given be-
low. Because conditions for settling in secondary clarifiers are not ideal, the theoretical peak
overflow rate should be reduced by 15 to 30 percent for actual design.
7-21
-------
15
10
9
8
7
6
t-:
-C
> 5
£/) 4
s
V
N^
A
O
A~
RECOMMENDED
DESIGN CURVE
1.5
LEGEND
(REFERENCES)
0(22) a (27)
0(23) A(28)
• (24)
• (21)
®(25)
X(26)
MLSS, mg/l.xlO
O(29)
A (30)
« WESTGATE 02 PLANT
FAIRFAX COUNTY, VA.
FIGURE 7-8
ACTIVATED SLUDGE SETTLING DATA FOR DOMESTIC WASTEW ATE R (20° C)
7-22
-------
Theoretical Peak
ISV Overflow Rate
ft/hr gpd/ft2
10.0 1,800
7.0 1,260
4.2 760
Because the final clarifier is a critical unit in the activated sludge process, good engineering
practice demands adequate sizing. For small community treatment plants, final clarifiers
design should be based on peak flows. Partial equalization of flow, to reduce peak flow rates,
may be warranted. Because clarification is sensitive to sudden changes in flow rates, only
variable speed pumps should precede the clarifier in the plant. Typical overflow rate and
solids loading criteria are given in Table 7-3.
TABLE 7-3
FINAL CLARIFIER DESIGN CRITERIA (2)
Type of Peak Over- Peak Solids
Treatment flow Rate1 Loading2
gal/ft2/day IbSS/day/ft2
Extended Aeration or Oxidation 800 20-30
Ditch
Completely Mixed or Contact 1,000-1,200 40-50
Stabilization
*The peak overflow rate does not include sludge recycle flow, which
leaves through the bottom of the clarifier. Sludge settling characteris-
tics at the lowest temperatures to be expected should be considered.
2 Loadings are based on peak plant inflow plus the sludge recycle flow.
The possible necessity of reducing loading to meet subnormal settling
characteristics should be considered.
The depth of clarifiers must be sufficient to permit the development of a sludge blanket,
especially if it is possible that sludge may bulk. The side water depth should be no less than
10 ft (3 m) for anMLSSof 2,000 mg/1; this minimum should be increased about 1 ft (0.3 m)
per each MLSS increase of 1,000 mg/1, to 14 ft (4.2 m) for an MLSS of 6,000 mg/1 (28).
The depth of interface between clarified wastewater and the top of the sludge blanket
should be designed to be no less than about 4 to 5 ft (1.2 to 1.5 m) below the effluent
weirs; the sludge blanket should be no thicker than 3 to 4 ft (0.9 to 1.2 m) during normal
operation.
7-23
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The final clarifier in an activated sludge system cannot be used effectively to obtain sludge
as thick as is desired for the sludge to be wasted. The solids in the clarifier are part of the
process and must be kept in proper condition for use in the aeration basin, to treat the in-
coming wastewater. Because of the oxygen uptake rate of the activated solids (from 25 to
100 mg/l/hr), any dissolved oxygen in the liquid from the aeration basin will usually dis-
appear rapidly in the clarifier (generally, in 5 to 10 minutes). Therefore, when the solids
subside in the clarifier, they are out of contact with any dissolved oxygen for a prolonged
period—as long as 1 to 3 hr. This environment is not a desirable one for aerobic organisms;
therefore, an effort should be made to recycle them as quickly as possible to the aeration
basin.
Furthermore, if such solids remain under zero oxygen conditions for over 30 minutes, de-
pending on the liquid temperature, anaerobic conditions will develop with resultant release
of methane, carbon dioxide and other gases, and flotation of the solids. If nitrates have been
formed because of nitrification in the aeration basin, the facultative organisms will break
down the nitrates and release nitrogen gas, with resultant buoying of the solids. A high
degree of thickening should not be attempted, to move the solids rapidly through the clari-
fier. The solids that settle to the basin bottom should not be allowed to remain there for
more than 30 minutes.
To insure proper handling of the activated solids in the clarifier, a rapid sludge removal
system is recommended for use with the scraper mechanism, by placing several suction
drawoff pipes along the scraper arms, so that the settled solids can be drawn off over the
entire basin area without being moved to a central sump. Equipment of this type is available
as standard design mechanisms. Desirable operational features in suction-type mechanisms
include independent flow controls for each -drawoff and visible outflows. An alternative to
suction drawoff pipes is placement of a radial channel from the sludge well, into which the
sludge is scraped on each rotation and then removed to the sludge well by a screw conveyor
in the bottom of the channel. Waste sludge is usually drawn from the central sump, to which
any heavy grit or sand that may have entered the basin is moved by the scrapers (or screw
conveyors).
The velocity of any part of the collecting device through the water, particularly near the
sludge blanket, must be restricted sufficiently, to prevent interference with solids agglomera-
tion and settling. The excess activated sludge to be wasted will not normally be thickened in
the final clarifier to the degree required for further processing. Sludge will leave the clarifier
with about 0.5- to 2.0-percent dry solids, depending on the concentration of the aeration
basin solids and whether the wastewater treated in the activated sludge process has received
primary settling. The volume will be about 1 to 2 percent of raw wastewater flow. Waste
activated sludge may be further thickened to about 4- to 5-percent solids (see chapter 14),
if desired.
An activated sludge can have poor settling characteristics, because of 1) poor biofloccula-
tion, 2) excessive bound water, 3) small gas bubble entrainment in the floe, 4) growth of
types of bacteria or fungi (filamentous organisms) that have a large surface area compared to
their mass, or 5) excessive amounts of hexane soluble oils and greases.
7-24
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7.6.2 Poor Bioflocculation
Too low or too high an F/MV ratio, toxicants in the wastewater, a lack of essential nutrients,
or some unknown reason may cause carryover of solids in the effluent. The flocculation of
the solids from the aeration basin can sometimes be aided by a slow-speed mechanical floc-
culator ahead of, or integral with, the clarifier. If industrial wastes are present in varying
amounts in domestic wastewater, such mechanical flocculation equipment has sometimes
been used, to counteract any effects of poor bioflocculation. The addition of a coagulant,
such as alum, may be very helpful.
7.6.3 Bound Water
Bound water occurs if the bacterial cells composing the floe swell, because of addition of
water, until their density approaches that of the water (2) (31).
7.6.4 Gas Bubble Entrainment
Poor settling is frequently said to be caused by "overaeration." Usually, this term indicates
conditions of high dissolved oxygen in the aeration basin. Bacteriologists have shown that
values of DO above 2 mg/1 have no effect on the bacterial metabolism. For values of liquid
DO above about 2 mg/1, the limit to bacterial growth is the transfer of food to the cell sur-
faces or the inherent maximum growth rate of the organism. Therefore, high DO is neither
deleterious nor beneficial. However, other conditions associated with obtaining a high DO
may cause poor settling. The excessive agitation accompanying high aeration may degrade
bioflocculation; it may also cause fine bubbles of air (most likely nitrogen, with which the
liquid is saturated) to adhere to the floe, keeping it from separating out, thus retarding the
settling rate or even causing flotation.
7.6.5 Bulking
Sludge bulking is generally used to describe MLSS that settle very poorly in the final clari-
fier. However, it should only be used to describe aeration basin solids that are light and
voluminous and that, under a microscope, show a heavy growth of filamentous organisms
(either bacteria or fungi). The term should not be used to describe solids that do not settle
well because of poor bioflocculation. Although filamentous organisms are always present in
the heterogeneous population of organisms in the activated sludge, they are not a dominant
species unless the environment is particularly favorable to their growth or unfavorable to
normal spherical organisms.
For small plants, the SVI is a good guide for the operator's use in determining the rate of
sludge return required, and when sludge must be wasted to lower the MLSS. After identify-
ing a bulking condition with the SVI test, a microscopic examination of the mixed liquor
can be made, to identify the organisms (if they are the cause of the bulking). Bulking will
often be caused by filamentous organisms. Filamentous bacteria found include Sphaerotilus
natans, Bacillus cereus var. mycoides, Thiothrixsupp., andBeggiatoasupp. (32) Occasionally,
7-25
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to
ON
6.
TABLE 7-4
METHOD FOR SOLVING A BULKING PROBLEM (32)
Identify bulking as distinct from inadequate design, poor operation, deflocculated sludge,
foam-forming sludge, rising sludge, septic sludge, etc.
;
Characteristics of bulking: Sludge settles and leaves a clear supernatant but SVI is high
(>150 ml/gm); low solids concentration in return sludge; and high sludge blanket in
final settling tank.
2. Identify if filamentous or nonfilamentous
bulking by microscopic examination of
mixed liquor and return sludge.
4-
Filamentous bulking.
;
3. Determine if filamentous organisms are
bacteria or fungi.
;
Bacterial filamentous bulking; identify
organism, if possible.
;
4. Look for source of massive inoculum of
filamentous bacteria in wastewater or
process return flows.
4-
Filamentous bacteria growing in activated
sludge.
;
5. Establish theory of treatment :
a. Kill sludge and start over.
*~ b. Use bactericide on return sludge.
c. Use flocculant or weighing agent
to decrease SVI.
I
Establish objectives, minimum time, and
control procedures for course of treatment.
7. Carry out course of treatment and collect
data for evaluation of treatment.
8. Evaluate treatment results.
I
Unsuccessful
"Nonfilamentous bulking
•Fungal bulking
-Chlorinate return sludge at 5 to 10 mg/1.
•Look for industrial waste problem.
"Massive inoculum of filamentous
bacteria
•Use bactericide to eliminate inoculum.
•Unsuccessful
-Successful
-Continue as needed.
-------
filamentous fungi species such as Geotrichum, Candida, and Trichoderma, have been iso-
lated from bulking sludge (32). Other factors that may cause poor settling include poor
clarifier design; poor operaton and microbial problems, such as deflocculation; septic sludge;
rising sludge; floating sludge; pinpoint floe; and foam-forming sludge. An organized trial-
and-error approach to solving bulking problems is presented in reference (32) and shown in
Table 7-4. The rate of sludge return should be specifically determined by the concentration
of VSS in the recycled sludge—not by the SVI. The rates of sludge return required to main-
tain an MLSS at 2,000 mg/1 for various SVI values are shown in Table 7-5 (32). However,
for the operation of a specific plant, if one or more specific values of MLSS are desired, the
engineer should develop a table similar to Table 7-5 during plant startup for operator use,
because the SVI must be correlated with the correct MLSS.
TABLE 7-5
RETURN SLUDGE RATE REQUIRED TO MAINTAIN MLSS AT 2,000 mg/1
FOR VARIOUS SVI VALUES (32)
Return Sludge Required Return
SVI1 Solids Sludge Rate
mg/1 % of Influent Rate
50 20,000 11.1
100 10,000 25.0
200 5,000 66.7
250 4,000 100
400 2,500 400
500 2,000 Infinite
!SVI = (ml settled sludge X l,000)/(mg/l SS).
Detailed information on determining the cause of bulking and the use of chemicals to con-
trol it is given in reference (32).
Although high carbohydrate wastes are frequently thought to cause filamentous growths,
this cause-effect relation is not quite valid, because many activated sludge plants treat in-
dustrial wastes having sugars, alcohols, and other carbohydrates. The important criterion is
the carbon (or BOD) to nitrogen (or phosphorus) ratio, because a high ratio is detrimental
to the growth of organisms that are not filamentous.
Wastewater that has relatively high sulfides will cause the growth of a special filamentous
type of sulfur bacteria. If this process occurs, only chlorination of the recycled sludge will
be effective.
Fungus organisms are generally filamentous; a pH below about 6 favors their growth. This
condition can occur if high BOD loadings produce large amounts of carbon dioxide, which
may not be effectively removed by aeration.
7-27
-------
Because of the relatively large surface-to-volume ratio of "filamentous types, compared to
more compact organisms, filamentous types have an advantage if the DO level is very low
(below 0.5 mg/1) or the concentration of essential nutrients is low. These organisms are,
however, aerobic and are destroyed by completely anaerobic conditions. In fact, some plant
operators have found that the growth of these organisms can be controlled, if the sludge is
subjected to several hours of anaerbic conditions; for example, a long detention time in a
final clarifier. They can also be controlled by continuously adding a toxicant, such as
chlorine or hydrogen peroxide, to the recycled sludge for about 24 hr. The chlorine dosage
should be between 10 and 20 mg/1, or about 0.2 to 1.0 Ib of chlorine per 100 Ib of return
sludge SS (2). Because doses of chlorine above 20 mg/1 may cause deflocculation, the dosage
should be determined in the laboratory. Although chlorine is apparently effective against
Sphaerotilus, Thiothrix, and Beggiatoa, it is not effective against Bacillus cereus. The
hydrogen peroxide dosage should be about 200 mg/1, based on plant influent flow (2). The
large surface-to-volume ratio of these organisms makes them more susceptible to such toxi-
cants than more compact organisms.
Raising the pH to over 8.0 with lime to control bulking has a multiple effect, causing
weighting of the floe, acting somewhat as a bactericide, and causing some additional floc-
culation.
7.6.6 Hexane Solubles
Communities with a large number of restaurants and filling stations—such as those in resort
or tourist areas—may have abnormally high concentrations of greases and oils in their waste-
waters. To prevent interference with sludge settling (by lowering the density), grease and oil
in the conventional activated sludge aerator tank influent should be less than 75 mg/1 and
preferably less than 50 mg/1. The hexane solubles loading should be less than about 0.15
Ib/day/lb MLSS (33).
7.7 Aeration System Design
7.7.1 General Design Considerations
After the oxygen requirements have been calculated for an activated sludge system (or any
other process requiring aeration), the type and capacity of the aeration system can be deter-
mined and the cost evaluations made. The three general types of aeration systems in use for
the activated sludge process and other unit processes requiring oxygen input are:
1. Diffused air.
a. fine bubble
b. coarse bubble
2. Submerged turbine with compressed air spargers.
3. Surface-type mechanical entrainment aerators.
7-28
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The first two systems require air compressed to a pressure sufficient to overcome the hydro-
static head above the air inlets or diffusers in the aeration basin, plus the head losses in
1 ) the diffusers or spargers and 2) the piping, fittings, and the piping, fittings, and valves be-
tween the compressor and the aeration basin. The agitator-sparger system has an operational
advantage over the diffused air unit, because, although air may be reduced during low flows,
the necessary mixing may be continued by the action of the turbine. The third type of aera-
tion system uses agitators located near the liquid surface which entrain air and pump large
quantities of liquid through the aeration zone surrounding the surface agitator. To obtain
detailed information on the use and installation of various aeration systems, see WPCF
Manual of Practice 5 (34).
To select the proper capacity for any aeration system for the actual conditions at a treat-
ment plant, it is necessary to consider the rating of the aeration system. To compare all the
various aeration systems from an energy requirement standpoint, the method of rating such
systems on the basis of oxygen input per unit time per unit of applied power (Ib/hr/kW)
should be used.
The principal parameters that control the rate of oxygen dissolution into wastewater for any
aeration system are 1) liquid temperature; 2) partial pressure of the oxygen characteristics
of wastewater, compared to clean water; and 3) dissolved oxygen to be maintained in the
liquid under design conditions. The first two parameters are taken into account by the satu-
ration level for oxygen for conditions at the treatment plant and by temperature correction.
The effect of the wastewater characteristics must be estimated from published information
or measured in the laboratory.
Aeration systems are rated for standard conditions defined as 1) clean water at a temper-
ature of 20° C (68° F), 2) mean sea level atmospheric pressure (10.13 kPa), and 3) zero dis-
solved oxygen in the aerated liquid. Knowing the efficiency or oxygen input rate for
standard conditions permits calculation for actual conditions, using the following:
il£L\( ,T-20
o o Cs
where
E = actual oxygen absorption efficiency
E0 = oxygen absorption efficiency under standard conditions
N = rate of oxygen input into wastewater under actual conditions Ib/hr/kW
N0 = rate of oxygen input under standard conditions, Ib/hr/kW
Cs = oxygen saturation concentration for clean water under standard conditions, mg/1
(9.20 mg/1 for clean water)
7-29
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Csw = oxygen saturation concentration for clean water under actual conditions, mg/1
(Csw values are given on Figure 7-9)
CL - desired oxygen concentration, mg/1
(3 = the ratio of saturation values of wastewater to clean water at wastewater temper-
ature and actual atmospheric pressure (approximately 0.95 for domestic waste-
water
a. = relative air-water interface diffusion rate, about 0.90 for mechanical aerators and
about 0.40 for porous diffusers, for normal domestic wastewater as compared to
clean water; varies primarily with the effects of changing concentrations of sur-
face active agents
T = temperature, ° C
The ability to diffuse oxygen into wastewater from air bubbles or air-water interfaces, as
compared to clean water, depends on the various soluble and suspended substances in the
wastewater and the method of aeration. Soluble surface active agents, such as detergents,
can have a significant effect.
The relative ability to diffuse oxygen through air-water interfaces is measured by the factor
a. Although for most wastewaters this is less than unity, for some wastewaters and aeration
methods it can be greater than unity. For raw domestic wastewater, this factor varies from
0.35 to 1.50, depending on the aeration method (36) (37). The value to use in activated
sludge aeration basins equipped with mechanical aerators is 0.90. For diffused air systems,
the value is much less and must be determined for the specific systems. (It is usually about
0.40 for porous diffusers.) The number 1.024, raised to the power (T —20), measures the
relative effect of liquid temperature on the molecular diffusion of oxygen into water. CL is
the dissolved oxygen concentration required under steady-state conditions. In activated
sludge aeration basins, it is usually 2 mg/1.
In the above relation for relative oxygen input, the liquid temperature affects the oxygen
input rate or efficiency in two ways that almost cancel each other. Thus, the values of E/E0
or N/N0 for liquid temperatures of 10° C (50° F) and 30° C (86° F) are only different by
about 5 percent—the lower value being at the higher temperature. That is, the liquid tem-
perature itself does not have a great influence on the oxygen input rate or absorption effi-
ciency.
7.7.2 Diffused Air Systems
Diffusers commonly used in activated sludge systems include 1) porous plates laid in the
basin bottom, 2) porous ceramic domes or tubes connected to a pipe header and lateral
system, 3) tubes covered with synthetic fabric or wound filaments, and 4) specially designed
spargers with multiple openings. Because clogging of porous diffusers by precipatation of
7-30
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0>
E
Z
UJ
CD
>
X
o
Q
bJ
>
O
CO
CO
13
12
I I
10
ELEVATION, FT
8 10 12 14 16 18 20 22 24 26 28 30
WATER TEMPERATURE (°C)
FIGURE 7-9
SATURATION CONCENTRATION FOR ATMOSPHERIC OXYGEN(35)
7-31
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solids on the exterior, by organic growths, and by dirt carried in with the compressed air has
been a problem, regular cleaning is necessary— although the time interval depends on the
composition of the wastewater and the size of the openings in the diffusers.
The mixing equipment for aeration or oxygen dissolution must be sized to keep the solids
in uniform suspension at all times. Depending on basin shape and depth, 4,000 mg/1 of
MLSS require about 0.75 to 1.0 hp/ 1,000 ft3 (0.02 to 0.03 kW/m3 ) of basin volume to pre-
vent settling, if mechanical aerators are employed; 30 cfm of air per 1,000 ft3 (1.8 std m3/
h-m3) are needed, if a diffused air system is used. Usually, the power required to supply the
oxygen will equal or exceed these values, particularly if oxygen is to be supplied for nitrifi-
cation. Values of MLSS higher than 4,000 mg/1 would require higher power inputs for mix-
ing. Power values will also vary with the type of solids produced from different wastewaters.
Oxygen absorption efficiency varies from 4 to 12 percent, under standard conditions (E0)
for diffusers and spargers in domestic wastewater activated sludge aeration basins. The varia-
tion depends on opening (bubble) size and the general design and arrangement in the basin
(38). The higher efficiency is obtained with diffusers producing finer bubbles. If the value of
E0 is 8 percent, the actual efficiency, E, for sea level conditions, if maintaining 2 mg/1 of
dissolved oxygen in the aeration basin and treating domestic wastewater, will be about 5 per-
cent.
°-°4 °R
where
Qa = air required, scfm
OR = oxygen required, Ib/day
E = oxygen absorption efficiency (as a ratio) under actual conditions
The power required to compress the air at standard conditions will depend on the sub-
mergence of the diffusers or spargers and the pressure losses in the air piping. It can be cal-
culated from:
P =
where
0.168 Qa
P = power required to compress air, kW
Qa = air required, scfm
e = compressor efficiency, usually 0.60 to 0.85
p = compressor outlet pressure, Ib/sq in.
7-32
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The following relation can be used to calculate the pounds of oxygen that can be dissolved
per hour per horsepower input to a compressor:
|"/p+14.7\ 0.
[(-i4?rj
. .283
N=6-23(E)(e)
where
N = oxygen that can be dissolved, Ib/hr/kW
E = oxygen absorption efficiency (as a ratio) under actual conditions
e = compressor efficiency (as a ratio)
p = compressor outlet pressure, Ib/sq in.
For example, using E = 0.05, e = 0.70, and the compressor outlet pressure as 7.5 psi, the
value of N would be 1.74 Ib/hr/kW (0.79 kg/kW-h).
The oxygen absorption efficiency of diffusers of all types is significantly affected by sur-
face-active agents (such as detergents) in wastewater. Long bubble retention times in the
liquid allow the surface-active agent to accumulate in the bubble-water interface and present
a barrier to rapid transfer of oxygen from the bubble to the liquid. It has been shown that a
typical domestic wastewater detergent concentration can cause a reduction in oxygen
absorption efficiency of 40 to 60 percent of that obtained in clean water for porous dif-
fusers (31) (39) (40). Oxygen transfer efficiencies of various aeration systems are shown in
Table 7-6. Because manufacturers rate the oxygen absorption efficiency of their equipment
using clean water, it is important to account for the possible effects on oxygen absorption
of surface-active agents in design calculations.
Oxygen transfer efficiency in wastewater, using mechanical aerators, is higher than in clean
water. The turbulence and rapid restructuring of interface surfaces apparently prevent any
accumulation of surface-active agents in the air-liquid interfaces.
The oxygen absorption efficiency or oxygen input rate per unit power for any aeration
system or device must be corrected for the actual conditions that will occur at the treatment
plant. Any test data used for aerator evaluation must be carefully checked, to insure that
the tests actually simulated installation conditions and were not made under completely dif-
ferent hydraulic and geometric conditions.
7.7.3 Submerged Turbine Aeration Systems
Since 1950, the submerged turbine (used widely in the chemical process industry) has come
into use for activated sludge aeration (41). It has been used primarily in industrial waste
activated sludge treatment plants and is considered the desired aeration system for very deep
basins, for activated sludges having high oxygen uptake rates, and for high concentrations of
MLSS, as in aerobic digesters.
7-33
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TABLE 7-6
OXYGEN TRANSFER CAPABILITIES OF VARIOUS AERATION SYSTEMS (2)
Type of Aeration System
Diffused Air, Fine Bubble
Diffused Air, Coarse Bubble
Mechanical Surface Aeration,
Vertical Shaft
Agitator Sparger System
Standard
Transfer
Rate1
lbO2/hp-hr
2.5
1.5
3.2
2.1
Effective
Transfer
Rate2
lbO2/hp-hr
1.4
0.9
1.8
1.2
transfer rate at standard conditions, i.e., tap water, 20° C, 760-mm baro-
metric pressure, and initial DO = 0 mg/1.
^Transfer rate at following specific field conditions:
a = 0.85
0 = 0.9
T=15°C(8.2°F)
Altitude =500 ft (152m)
Operating DO = 2 mg/1 for air aeration, 6 mg/1 for oxygenation
The system consists of a radial-flow turbine located below the mid-depth of the basin. Com-
pressed air is supplied to the turbine through a sparger (see Figure 7-10). The total power re-
quired for this system is equal to the sum of the air compressor power and that needed to
drive the turbine. The oxygen absorption efficiency depends on the relative air loading of
the turbine and its peripheral velocity.
Studies have shown that, for optimum power consumption, the power should be about
equally divided between that for the air compressor and that for the turbine (41). Under
standard conditions, the oxygen absorption efficiency for minimum power will be in the
range of 15 to 25 percent—20 percent is a reasonable value. Thus, for most conditions, the
required air volume will be less than one-half that for an air diffusion system. The com-
pressor pressure will be less, because the turbine is usually located several feet above the
basin bottom and the air sparger has insignificant head loss. A convenient and relatively eco-
nomical method for upgrading overloaded activated sludge plants is afforded, because by
installing turbines in the existing aeration basins the oxygen input can be doubled (42).
The peripheral speed of the turbines, for least total power consumption, is 10 to 15 ft/sec
(3 to 4.5 m/s). For standard conditions, the oxygen input averages 3.4 to 4.0 lb/hp-hr (2.0
to 2.4 kg/kWh). Generally, the power usage for a given oxygen input will be 30 to 40 per-
cent less than for air diffusion systems (43).
7-34
-------
DRIVE
n
COMPRESSOR
L.L.
TURBINE
V
1
«•• v-*-«-y
•i
•i
V.
If
AIR
•f .-•>-. -,\T«: :~4 ^'.; A." «V » >. •»':• v
FIGURE 7-10
SCHEMATIC OF INSTALLATION FOR SUBMERGED TURBINE AERATOR
-------
7.7.4 Surface-Type Aerators
The most recent improvement in aeration equipment for activated sludge systems is in the
surface entrainment aerator. (A general design is shown in Figure 7-11). These turbine-type
units entrain atmospheric air by producing a region of intense turbulence at the surface
around their periphery. They are designed (if operated at a peripheral velocity of about 15
to 20 ft/sec [4.5 to 6 m/s]) to pump large quantities of liquid, thus dispersing the entrained
air and agitating and mixing the basin contents. These aerators are the most efficient from a
power consumption standpoint, because they will provide 2.8 to 3.5 lb/hp-hr (1.7 to 2.1
kg/kWh) of oxygen under standard conditions.
To attain optimum flexibility of oxygen input, the surface aerator can be combined with
the submerged turbine aerator. Several manufacturers supply such equipment, with both
aerators mounted on the same vertical shaft. Such an arrangement might be advantageous, if
space limitations require the use of deep aeration basins.
7.7.5 Mixing Requirements
In addition to supplying the necessary oxygen, any aeration or oxygenation system must
provide the necessary mixing and agitation of the aeration basin contents. This mixing re-
quirement is usually expressed as the power needed per unit basin volume. For diffuser
systems, this power requirement can be converted to an air requirement, which will amount
to about 20 to 30 scfm of air per hour per 1,000 ft3 (1.2 to 1.8 std m3/h-m3) of basin
volume. For aeration basins preceded by primary settling of domestic wastewater, the power
needed, if using surface turbines whose peripheral velocity is 15 to 20 ft/s (4.5 to 6 m/s), is
about 1/2 to 2/3 hp/1,000 ft3 (0.013 to 0.018 kW/m3). Without primary settling, the
power needed to prevent settling out of wastewater solids is 3/4 to 1 hp/1,000 ft3 (0.020 to
0.026 kW/m3). For submerged turbines, as described in section 7.6.3, the turbine power
should be about half the above figures, to insure proper mixing. Power input is not a suffi-
cient criterion for keeping solids in suspension; manufacturers of such equipment should
guarantee that their units will keep the MLSS at all points within plus or minus 10 percent
of the average.
Another type of surface aerator is the Kessener "brush," which is commonly used in oxida-
tion ditch systems. This aerator has a horizontal shaft spanning the basin, to which are
attached radial prongs with some transverse members that impart, when rotating, intense
agitation of the liquid at the surface and also induce a flow in the ditch of about 1 ft/sec
(0.3 m/s) which keeps the activated sludge solids in suspension.
7.8 Nitrification
Nitrification (biological oxidation of ammonia to nitrates) is accomplished by a special
group of aerobic organisms called nitrifiers. In the presence of oxygen, an inorganic source
of carbon (carbon dioxide, carbonates, and bicarbonates), and ammonia, the bacteria
Nitrosomonas will catalyze inorganic chemical reactions, producing nitrates. In turn, a
second species of bacteria, Nitrobacter, produces nitrates from the nitrites. These autotrophic
7-36
-------
DRIVE
FIGURE 7-11
MECHANICAL SURFACE AERATOR
nitrifiers have a lower rate of population growth than the heterotrophic aerobes, which
oxidize carbonaceous material—about 1 to 2 percent of the E coli growth rate (12). The
nitrifiers are also temperature and pH sensitive, with little resistance to organic and inor-
ganic toxicants.
An environment that will maintain a stable, healthy population of nitrifiers will satisfac-
torily reduce ammonia content for effluents requiring low ammonia. This environment can
be accomplished in activated sludge units and in trickling filters and rotating biological con-
tactors. Aerated facultative ponds with adequate recirculation and retention time can also
maintain a healthy population of nitrifiers.
Because of their slow reproduction rate, the nitrifiers must be retained in adequate numbers
in the nitrification unit for a relatively long period. This requirement means recirculation of
sludge or nitrifying unit effluent containing a sufficient concentration of the nitrifiers to the
influent of the nitrifying unit.
The optimum growth rate of nitrifiers occurs at a temperature of about 30° C and a pH of
about 7.2 to 8.5, although some studies indicate that nitrifiers may become acclimated to a
lower pH and reproduce at near the maximum rate (12). In activated sludge nitrification,
the F/M is about 0.05 to 0.15 day~ *. The SRT should be greater (by a safety factor of about
1.8 to 2.0) than the reciprocal of the Nitrosomonas' growth rate constant, to allow for the
upsets to nitrifier growth which can occur from shock loading in the wastewater influent
(44). Figure 7-12 indicates the effluent ammonia-nitrogen concentration to be expected for
single-stage, completely mixed nitrifier reactors for various 1^ values.
7-37
-------
100
z
UJ
o
z
o
o
UJ
o
o
tr
<
z
o
km = RATE CONSTANT FOR SYNTHESIS
OF NITROSOMONAS, mq/l
0.
2 5 10 20 50 100
SLUDGE RETENTION TIME(SRT), DAYS
FIGURE 7-12
NITRIFICATION IN COMPLETELY MIXED ACTIVATED SLUDGE PROCESS
7-38
-------
The growth rate of Nitrosomonas is described by the equation (13):
where
/x = growth rate of Nitrosomonas, day~l
Mmax = maximum growth rate, day" *
NH4 = concentration of ammonia, mg/1
km = concentration of ammonia, when n = 0.5 ptmax, mg/1
For Nitrosomonas at 20° C,
11^ = 0.33 day-1
km = 1.0 mg/1 of ammonia-nitrogen
For Nitrobacter at 20° C,
Mmax=0.14day-l
A typical variation of nitrification with temperature is shown in Figure 7-13 (44).
Ammonia-nitrogen concentrations of less than 60 mg/1 do not usually inhibit nitrification
(44).
Theoretically, alkalinity is destroyed by nitrification at the rate of 7.2 Ib for each pound of
ammonia-nitrogen oxidized to nitrate (44). If the influent alkalinity is not sufficient to leave
a residual alkalinity of 30 to 50 mg/1 after nitrification is completed, additions of lime, soda
ash, or caustic soda may be required to hold the pH at the optimum level for the nitrifiers.
At pH's below 7.8, the CC>2 resulting from nitrification will be washed out of the liquid by
the aeration process, which will markedly reduce the lime requirements.
A summary of the conditions advantageous to nitrifier growth is given in Table 7-7.
7-39
-------
33
>
33
T]
O
O
-n
m
33
m
en
S
m
PERCENT OF GROWTH RATE AT 30»C
M Of * W 9> ^
O O o O O O
g
O
O
m
x
>
c
3)
°
-------
TABLE 7-7
SUMMARY OF CONDITIONS ADVANTAGEOUS FOR NITRIFIER GROWTH
(13)(44)(45)(46)
Characteristic
Permissible pH Range (95 percent of Nitrification)
Permissible Temperatures (95 percent Nitrification), °C
Optimum Temperature, °C
DO Level at Peak Flow, mg/1
MLVSS, mg/1
Heavy Metals Inhibiting Nitrification
Cu
Zn
Cd
Ni
Pb
Cr
Toxic Organics Inhibiting Nitrification
Halogen-substituted Phenolic Compounds
Halogenated Solvents
Phenol and Cresol
Cyanides and All Compounds From Which Hydrocyanic
Acid Is Liberated on Acidification
Oxygen Requirement (Stoichiometric, Ib O2/lb NH3 *N,
plus Carbonaceous Oxidation Demand)
Maximum Clarifier Overflow Rate, gpd/ft3
Design Value
7.2-8.4
15-35
30° (approximately)
> 1.0
1,200-2,500
< 5 mg/1
< 5 mg/1
< 5 mg/1
< 5 mg/1
< 5 mg/1
< 5 mg/1
< 0 mg/1
< 0 mg/1
< 20 mg/1
< 20 mg/1
4.6
1,000
7-41
-------
7.8.1 Two-Stage Nitrification
Because conditions for the oxidation of carbonaceous matter and nitrogenous matter are
different, particularly during cold weather, two-stage activated sludge may be used for small
plants if shock loadings or abrupt changes in temperature or pH are probable. In this system,
the first stage is designed to oxidize the carbonaceous BODs and the second stage is de-
signed for nitrification. Essentially, this system is two separate activated sludge processes
acting in series.
The first stage can be designed as a high-rate completely mixed unit, to lessen the impact of
variations in loading. For normal NH^ concentrations, the BODs in the first stage clarifier
effluent should be about 50 mg/1, to maintain a satisfactory MLVSS level in the second stage
aerator, because the weight of cells synthesized from ammonia-nitrogen is only about 10
percent of the weight of the ammonia-nitrogen. The BOD^ to NH3-N ratio in the second
stage should be between 2 and 4. Higher BODs values lead to undesirably high solids con-
tent for good nitrification; lower values result in poor flocculation. Sludge return, sludge
wasting, and oxygen control in the first stage are independent of those in the second stage.
The second-stage nitrification unit can then more easily be designed to carry an SRT match-
ing the ammonia-nitrogen load, to prevent a "washout" of nitrifiers. Because the ammonia
to be oxidized is soluble and is not adsorbed by the activated floe, it is not removed in the
clarifier and its oxidation time is the same as the actual detention time. Because the rate of
ammonia oxidation is essentially linear, plug flow is desirable in the second stage. Plug flow
can be achieved in the second stage by dividing the aeration basin into three or more com-
partments in series, as shown in Figure 7-14. In two-stage systems, the first stage should be
completely mixed, to reduce the possibilities of shock loadings on the nitrification unit. Pro-
visions should be made in the design to maintain adequate amounts of alkalinity in the
second stage influent; for example, lime addition to maintain a relatively uniform pH of
about 7.4 to 7.6 during nitrification.
NITRIFICATION
CARBONACEOUS
BOD
REMOVAL
RETURN SLUDGE
EXCESS
SLUDGE
"1 r
I
V.
r
J
^
\^
*
J
I FIN
ICLAR
RETURN SLUDGE
AL \
IFIER) *
EXCESS
SLUDGE
FIGURE 7-14
TWO-STAGE SYSTEM FOR NITRIFICATION
7-42
-------
The extra capital and operating expense of a two-stage system for a small community is
seldom justified; other alternatives should be examined.
7.8.2 Single-Stage Nitrification
If nitrification is not required during cold weather or 1 to 3 mg/1 of ammonia-nitrogen in
the effluent is permissible, single-stage nitrification may be used. Because nitrifiers are easily
killed by shock toxic loadings or abrupt changes in pH or temperature, completely mixed
extended aeration is most commonly used for single-stage nitrification (Figure 7-15).
RETURN SLUDGE
EXCESS
SLUDGE
AERATOR
FIGURE 7-15
SINGLE-STAGE NITRIFICATION SYSTEM
For consistent single-stage (as in two-stage) oxidation of both carbonaceous and nitrogenous
matter, the reactor must be designed and operated under loading conditions and controls
that will insure maintenance of an adequate population of nitrifiers. The minimum SRT's
necessary for nitrification (shown on Figure 7-12) were developed to predict the nitrifica-
tion efficiency of a completely mixed reactor (15). In practice, a safety factor of 1.1 to 1.3
may be used, based on the probability of toxic of hydraulic shock loadings.
The F/MV loadings should average about 0.05 to 0.15 Ib BOD5/day/lb MLVSS in a single-
stage nitrification system and lower for nitrification below 5° C (40° F).
7.8.3 Denitrification
If the ammonia-nitrogen is oxidized to nitrate and the DO is less than about 1 mg/1 (prefer-
ably 0.0 to 0.2 mg/1), heterotrophic bacteria can utilize the oxygen in the nitrates and
nitrites for metabolism, releasing the nitrogen to the atmosphere as a gas. Controlled de-
nitrification can take place in 1) a third-stage aerator-clarifier system, 2) a completely sub-
merged rotating disk contactor system, or 3) a high-rate granular media filter. If a suspended
growth system is used, the MLSS should be in the range of 2,000 to 3,500 mg/1 (44).
7-43
-------
The denitriflcation rate varies considerably with the temperature, as indicated in Figure 7-16
(44). Until more data on the effect of temperature on the denitrification of different
nitrified wastewaters are secured, pilot studies will normally be necessary.
The effluents from nitrifying units are deficient in carbonaceous material. To supply the
essential carbon nutrient, methyl alcohol (methanol), glucose (corn sugar), or a carbona-
ceous waste, such as brewery waste, is added. Each pound of nitrate requires about 2.5 to
3.0 Ib of methanol, or its equivalent, to complete the reduction process. The methanol
required can be estimated from the following equation:
Ib methanol = [2.47 (Ib of NO3-N reduced)] + [1.53 (Ib of NO2'N reduced)] +
[0.87 (Ib of DO consumed)]
The amount of methanol to be added should match the denitrifier needs, to meet effluent
BOD and N requirements; therefore, it may be necessary to equalize the loading, as a pre-
treatment process.
Because of the possible high concentrations of both nitrogen and carbon dioxide in the
effluent of denitrification aerators, which create a supersaturated condition inhibiting
settling, it is recommended that about 5 to 10 minutes of separate aeration be provided
before clarification, to strip out gaseous nitrogen. After separate aeration, the settling
properties of denitrification sludge are similar to conventional activated sludge.
7.9 Operation and Maintenance
The operational procedures for the activated sludge process prepared by the U.S. EPA
National Field Investigation Center provide excellent information (1).
In activated sludge systems, it is customary to provide at least two aeration basins in
parallel, so that if one must be taken out of service, the plant can operate with one basin for
a short period (although it may be overloaded and the treatment deteriorate somewhat).
Small plants (below 0.1 to 0.3 mgd [0.004 to 0.013 m3/s]) would not normally have
parallel aeration basins or dual clarification facilities; however, if reliably meeting effluent
requirements is essential, two small plants should be constructed in parallel. Reliability
guidelines, as established by EPA, may require (depending on where the effluent is dis-
charged) spare motors and drives for any mechanical equipment. It is customary to provide
a standby compressor for diffused air systems.
The most important control parameters in activated sludge systems are the DO in the aera-
tion basin and the MLSS.
It is desirable to maintain a minimum aeration tank DO of about 1 to 2 mg/1. Initially, a
plant will normally be underloaded, and the aeration may provide more DO than is required.
There is no benefit in carrying higher than 3 to 4 mg/1 of DO, if the aeration rate is kept
reasonably adjusted to the oxygen requirements of the biomass in the aerator. If the DO
levels are not controlled, localized regions may become saturated with oxygen or super-
7-44
-------
DENITRIFICATION RATE - LBS OX ID IZED N ITROG EN REMOVED / DAY / LB MLVSS
m
m
33
>
c
3D
O
C/3
O
~Z.-
o
m
O
>
CD
C
33
m
m
2
T3
m
3)
>
H
7)
m
o
O
-------
saturated with nitrogen or carbon dioxide. These conditions lead to adsorption of fine
bubbles on the floe, causing poor settling and possibly flotation. The aeration capacity of
the aerators should be sufficiently flexible, to reasonably match the DO requirements of the
variable wastewater flows and loadings.
If variations in loading cause poor BOD5 or SS removal, some means of (at least) partial
equalization of flows should be investigated.
During the plant startup period, it is important to establish the aerator mixed liquor charac-
teristics that will most reliably meet effluent requirements. Factors characterizing the mixed
liquor (e.g., the initial settling velocity, the size and relative concentration of floe particles,
the clarity of the settled liquid, and the compactability of the settled sludge) can be estab-
lished in a simple settleometer test (1). Such characteristics should be determined for
various combinations of MLVSS in the aerator, F/M ratios, and SRT's when the influent
wastewater has its highest and lowest temperatures and its largest daily variations in load-
ings. To obtain optimum effluent quality for plants employing primary clarification, the
effects of each variation in effluent BOD5 and SS must be monitored and evaluated. In addi-
tion, influent BOD5, SS, VSS, temperature, and pH must be carefully noted, to enable plant
personnel to make accurate judgments regarding operating conditions.
An emergency action that can be taken if the quality and character of the microbial solids
deteriorate is adding powdered carbon to the wastewater entering the aeration basin and to
the basin itself. Such short, periodic additions of carbon can improve the settling of the
solids, adsorb toxicants, and reduce the soluble organic load the organisms must handle,
thus providing rapid corrective action (44) (47).
Bulking sludge is an operational problem that occurs in higher loaded plants, if care is not
taken to maintain 1) sufficient dissolved oxygen in the aeration basin, and 2) a sludge re-
cycle rate high enough to keep the concentration of MLSS adequate and the loading within
design limits. Bulking results from increased growth of filamentous bacteria at the expense
of normal spherical bacteria and with resultant poor settling of the MLSS in the final clari-
fier (see Section 7.6). To correct this condition rapidly, the filamentous organisms, because
of their large surface-area-to-volume ratio, can be selectively destroyed by large doses of
chlorine or hydrogen peroxide. The latter is more effective, because it has a less deleterious
effect on the desirable organisms (48).
If solids do not settle or settle poorly because of buoyancy caused by denitrification in the
final clarifier, the addition of hydrogen peroxide to the secondary clarifier inflow is effec-
tive in suppressing such denitrification.
Most activated sludge treatment plants are affected by large variations in flow and loading
more when the F/M loading is greater than 0.5 Ib BODs/day/lb MLSS under aeration than
they are when the F/M is below this value. Recovery from shock loadings usually takes place,
without affecting the average plant effluent, during night and weekend low flow periods
(13).
7-46
-------
Provisions should be made to bypass a nitrification unit, if there are excessive concentra-
tions of any substance that inhibits nitrifiers (see Table 7-7). If the nitrifier population is
destroyed, it usually takes several weeks for recovery.
7.10 Example Design
7.10.1 Site and Wastewater Characteristics
Principally Domestic Wastewater
Limited Land at Plant Site, Elev. 1,000 ft (304 m)
Influent BOD5, mg/1 200
Influent SS, mg/1 250
Influent VSS, mg/1 175
Influent NH3-N, mg/1 20
Flow, gal/cap/day 100
Peak-to-Average Ratio 4:1
Average-to-Minimum Ratio 4:1
Population 2,000
Minimum Wastewater Temperature, °C +15
Minimum Air Temperature, °C —10
pH Range 7.2-7.8
Hexane Solubles, mg/1 75
Effluent BOD5, mg/1 <30
Effluent SS, mg/1 <30
7.10.2 Design
Assumptions
Extended Aeration System
Two Duplicate Aerator-Clarifier Units
Pretreatment (Screening and Grit Removal Only)
F/MV, lb BOD5 /day/lb MLVSS 0.10
MLSS, mg/1 4,000
MLVSS, mg/1 2,800
BOD5 Loading (p. 7-2)
F = 8.34(QXLi-Le)/106
F = 8.34(200,000X200 - 30)/106
F = 284
7-47
-------
Micro-organism Mass in Aerator
Mv = 284/0. 10
Mv = 2,840 Ib
Volume of Aerator (p. 7-14)
V= [0.133(Q)(Li- Le)]/[(MLVSS)(F/Mv)]
V= [0.133(200,000)(200- 30)] /[(2,800)(0. 10)]
V= 16,150 ft3
Sludge Retention Time (p. 7-16)
SRT=l/[a(F/Mv)-b]
(assume a = 1.1 and b = 0.08)
SRT= 1 /[1. 1(0.10) -0.08]
SRT = 33 days
Net Sludge Production (VSS to be wasted) (p. 7-16)
Mw = (Mv)[a(F/Mv) - b]
Mw = 2,840[1.1(0.1) -0.08]
Mw = (2,840(0.03) = 85 Ib/day (38.5 kg)
Liquid Retention Time (not including recycle flow)
t = V/Q = (15,960)(7.48)(24)/200,000
t= 14.3 hr
Minimum Aerator Liquid Temperature
For the short period liquid is exposed to cold air in the aerator and clarifier, and the many
factors that determine the amount of heat loss, it is assumed in this example that the tem-
perature drop is about 5° C (41° F) under design conditions. Therefore, the liquid tempera-
ture in the aeration chamber may drop to as low as 10° C (50° F).
Nitrification
At 10° C (50° F), with an SRT of 33 days and a liquid retention time of 14.3 hr, it can be
assumed that nitrification will be relatively complete.
Oxygen Requirements
Carbonaceous Oxygen Demand (p. 7-19)
0RC = [a' (F/MV) + b'] Mv
7-48
-------
0RC = [0.55(0.10) + 0.151(2,840)
ORC = 582 Ib/day (264 kg/day)
Where a' = 0.55
Where b'= 0.15
Nitrogenous Oxygen Demand (p. 7-19)
ORN = (4.6)(8.34)(Q)(ANH3)/106
°RN = (4.6)(8.34)(200,000)(20)/106
0RN = 153 Ib/day (70 kg/day)
Total Oxygen Required = ORC + ORN
= 582+ 153
= 73 5 Ib/day (334 kg/day)
Aerator Selection
Since freezing is not likely to be a problem, and surface aerators are more efficient in the
use of power, surface aerators will be selected for this design. If freezing were a problem,
diffused aeration would be more efficient.
Power Requirements if. 7-29)
For operation peripheral velocities of about 15 to 20 fps (4.5 to 6 m/s), use an aerator that
under standard conditions, will provide 5.8 Ib O2/hp'hr (2.0 kg O2/kWh). Under acutal con-
ditions:
N = N0 gCswc CL
!. 024)T-20
where
N0 = 5.8
U = 0.95
Csw = 9.6 (from Figure 7)
CL = 2.0 (assumed)
C = 9 2
\^s ^.z.
a = 0.9
N= 58 [(0.95X9.6)-2.01 (o.9)(1.024)-5
7-49
-------
N = (5.8)(0.77)(0.9)(0.888)
N = 3.61bO2/hp-hr
Power provided = (735)/(3.6)(24) = 8.5/hp-hr, or 4.5/hp-hr for each aerator
Aeration Tank
Provide two 15-ft-deep (4.5 m) square aeration basins with a common wall.
Area of each basin:
A=16,150/(2)(15)
A =538 ft2 (50m2)
Clarifiers
Provide two clarifiers. Wastewater with an MLSS of 4,000 mg/1 and a depth of 12 ft (3.6 m)
at 20° C (68° F) will have an ISV of about 6 ft/hr (1.8 m/hr) and a peak overflow rate of
about l,100gpd/ft2 (44 m3/m2 d). At a liquid temperature of 10° C (50° F), the ISV might
reduce to about 5 ft/hr (1.5 m/hr) and the peak overflow rate to about 800 gpd/ft2
(32 m^/m2 'day). The area required for each clarifier would then be:
A = Q/(800)(2)
A = (200,000)(4)/1600
A =500 ft2 (46m2)
The diameter of each would be:
D= [4(500)/n] °'5
= 25 ft (7.6 m)
The solids organic loading to the clarifier, with 100-percent recirculation would be:
MV/A = 2,800(8.34X200,000)/(2)(500)(24X106)
= 0.19 Ib/ft2/hr (0.92 kg/m2/hr)
or well below the limit of about 1.25.
If the solids concentration in the settled solids is 1 percent and the MLSS is 4,000 mg/1, the re-
cycle flow would be (p. 7-18):
QR = QSS/(CS - Ss)
QR = (200,000)(4,000)/( 10,000 - 4,000)
QR = 133,333 gal/day (503,998 I/day)
where Cs = 10,000 (assumed)
7-50
-------
Emergency Operation
If one aerator-clarifier unit is out of operation, the plant could be operated as a conven-
tional system. The F/MV ratio could be increased to 0.4, which would still meet SS and
BOD5 effluent requirements of 30 mg/1 each; however, the capability to meet shock load-
ings and maintain the MLVSS in the single aerator and clarifier units would be reduced.
VSS wasting would be increased to:
Mw = 2,840 [(1.0(0.40)-0.08]
Mw = 1,022 Ib/day (463 kg/day)
The SRT would drop to:
SRT = !/[(!.0(0.4)-0.08]
SRT = 2.8 days
Therefore, nitrogenous oxygen demand would not be exerted.
If one clarifierInust be removed from service, the MLSS concentration should be reduced to
2,000 or less and the MLVSS to about 1,400.
7.11 Case Study
7.11.1 Woodstock, New Hampshire, Oxidation Ditches
The Pemigewasset River, which flows through the town of Woodstock, has been classified
for C use, requiring that the plant effluent does not 1) cause a reduction of the DO below 5
mg/1; 2) cause a pH outside 6.8 to 8.5; 3) contain chemicals inimical to fish life; 4) cause
unreasonable sludge deposits; 5) cause unreasonable turbidity, slick, or odors; or 6) dis-
charge unreasonable surface-floating solids. In addition, the State of New Hampshire ruled
that a minimum of secondary treatment was necessary. To meet EPA requirements for
secondary treatment effluents, the BODs and the SS both must be reduced to 30 mg/1 or
less and the MPN of coliforms reduced correspondingly.
To meet these requirements, a wastewater treatment plant was constructed in 1971 which
included bar screen, comminutor, wastewater pumps, two grit chambers, two oxidation
ditches, two clarifiers, sludge storage tanks, chlorine contact chamber, sludge pumps, scum
pit, and sludge drying bed. The configuration of this plant is shown on Figure 7-17. Design
and operational data are listed in Table 7-8.
7-51
-------
RETURN SLUDGE
SLUDGE
DRYING
BEDS
GRIT
CHAMBER
RAW
SEWAGE
PUMPS
WET
WELLS
SLUDGE
STORAGE
TANK
CHLORINE
CONTACT
CHAMBER
SCUM PIT
LEGEND
LIQUID
SLUDGE
FIGURE 7-17
SCHEMATIC FLOW DIAGRAM - WOODSTOCK, N.H.
7-52
-------
TABLE 7-8
WOODSTOCK, N.H., OXIDATION DITCHES
Design Data
Design Year
Population To Be Served
24-Hr Flow
Average, mgd
Maximum, mgd
Minimum, mgd
Raw Wastewater
BOD5,mg/l
SS,mg/l
VSS, mg/1
1990
1,100
0.140
0.650
0.040
220
215
140
Aeration Tank
F/M, Ib BOD5/day/lb MLSS 0.05
Sludge Recycle Ratio 1:1
Average Retention Time, hr 26
Surface Aerators, hp 12
Secondary Clarifier
Maximum Overflow Rate, gpd/ft2
Depth at Weir, ft
Retention Time at Max.
Flow.min. 165
550
8
Performance Data, January 1974 to April 1975
Primary
Influent
SS
mg/1
min./max.2
Final
Effluent
SS
mg/1
min./max.2
Aeration
Tank
MLSS
mg/1
min./max.2
Sludge
Volume
Index
min./max.2
Primary*
Influent
BQDS
"mgTT
Final
Effluent
•ODS
~mg7l
Flow
mgd
min./max.2
1974
Jan.
Feb.
Mar.
Apr.
May
June
July
Aug.
Sept.
Oct.
Nov.
Dec.
20/92
10/130
18/92
6/34
2/60
8/28
12/218
10/170
8/72
14/66
58/142
20/108
6/10
6/14
2/10
2/10
2/14
2/8
6/18
2/8
4/14
6/14
6/54
6/14
3080/3590
3330/4000
2320/5180
1980/4670
3000/4410
3680/4520
4100/4900
3610/5150
4220/5630
5030/5640
4430/7460
3400/5360
91/118
89/105
83/136
81/103
74/96
72/86
65/89
81/111
81/90
88/95
89/138
86/102
1975
Jan.
Feb.
Mar.
Apr.
10/94
22/128
14/134
24/100
2/14
2/16
2/12
4/14
4020/5620
3230/6190
4710/6470
3360/5740
84/97
73/106
72/142
72/75
144
96
72
126
93
89
205
298
126
124
165
141
201
132
94
102
5
10
7
3
9
7
11
9
3
5
3
5
.09 1/. 138
.088/.161
.076/.161
.087/.175
.093/.161
.093/.137
.098/.122
.078/.130
.070/.133
.058/.116
.060/.093
.072/.129
10
22
7
7
.070/.108
.077/.102
.066/.112
.067/.140
'One composite sample a month analyzed for BOD.
2Minimum and maximum values in the month.
7-53
-------
7.12 References
1. West, A.W., Operational Control Procedures for the Activated Sludge Process, part 1,
observations, and part 2, control tests. U.S. EPA, National Field Investigation Center,
Cincinnati (April 1973).
2. Process Design Manual for Upgrading Existing Wastewater Treatment Plants. U.S. EPA,
Office of Technology Transfer (October 1974).
3. Wastewater Engineering. Metcalf & Eddy, Inc., New York: McGraw-Hill (1972).
4. Pasveer, A., Developments in Activated Sludge Treatment in the Netherlands. Proceed-
ings, Conference on Biological Waste Treatment, Manhattan College, New York (April
1960).
5. Ullrich, A.H., and Smith, M.W., "The Biosorption Process of Sewage and Waste Treat-
ment." Sewage and Industrial Wastes, p. 1248 (1951).
6. Ullrich, A.H., "Experiences With the Austin, Texas, Biosorption Plant." Water and
Sewage Works (January 1957).
7. Smith, H.S., and Paulson, W.L., "Homogeneous Activated Sludge." Civil Engineering
(May 1966).
8. Full-Scale Parallel Activated Sludge Process Evaluation. EPA Report R2-72-065
(November 1972).
9. Brenner, R.C., Oxygen Aeration. Presented at EPA Technology Transfer seminar
(October 1973).
10. Speece, R.E., and Malina, J.F., Applications of Commercial Oxygen to Water and
Wastewater Systems. Water Resources symposium No. 6, University of Texas, Austin
(1973).
11. Tchobanoglous, G., Wastewater Treatment for Small Communities. Conference on
Rural Environmental Engineering, Warren, Vermont (September 1973).
12. Unpublished data, Camp Dresser, & McKee Inc., Boston, Massachusetts (1974).
13. Design Guides for Biological Wastewater Treatment Processes. EPA Report No. 11010-
ESQ-08/71 (August 1971).
14. Standard Methods for the Examination of Water and Wastewater, 13th ed. American
Public Health Association (1971).
15. Unpublished data, EPA Cincinnati (1974).
7-54
-------
16. Heukelekian, H., Orford, H.E., and Manganelli, R., "Factors Affecting the Quantity of
Sludge Production in the Activated Sludge Process." Sewage and Industrial Wastes,
vol. 23, pp. 945-957(1951).
17. Bargman, R.D., and Borgerding, J., Characterization of the Activated Sludge Process.
EPA Report EPA-R2-73-224 (April 1973).
18. Eckenfelder, W.W., and O'Connor, D.J., Proceedings 9th Industrial Waste Conference,
Purdue University (1954).
19. Logan, R.P., and Budd, W.E., Biological Treatment of Sewage and Industrial Wastes,
vol. 1. New York: Reinhold, p. 271 (1959).
20. Quirk, T.P., Sewage and Industrial Wastes, p. 1288 (1959).
21. Dick, R.I., and Ewing, B.B., "Evaluation of Sludge Thickening Theories," Journal
Sanitary Engineering Division. ASCE, p. 9-29 (August 1967).
22. Javaheri, A.R., and Dick, R.I., "Aggregate Size Variations During Thickening of Acti-
vated Sludge." Journal Water Pollution Control Federation, vol. 41, part 2, R-197
(1969).
23. Ford, D.L., General Sludge Characteristics, Advances in Water Quality Improvement,
Physical and Chemical Processes. Austin: University of Texas Press, p. 391 (1970).
24. Reed, S.C., and Murphy, R.S., "Low Temperature Activated Sludge Settling," Journal
Sanitary Engineering Division, ASCE, vol. 95, SA4, p. 747 (1969).
25. Dick, R.I., Thickening, Advances in Water Quality Improvement, Physical and
Chemical Processes. Austin: University of Texas Press, p. 358 (1970).
26. Eckenfelder, W.W., and Melbinger, N., "Settling and Compaction Characteristics of
Biological Sludges." Sewage and Industrial Wastes, p. 1114 (1957).
27. Fischerstrom, C.N.H., et al., "Settling of Activated Sludge in Horizontal Tanks."
Journal Sanitary Engineering Division, ASCE, SA3, p. 73 (June 1967).
28. Rudolfs, W., and Lacy, I.O., "Settling and Compacting of Activated Sludge." Sewage
Works Journal, p. 647 (July 1934).
29. Geinopolos, A., and Katz, W.J., U.S. Practice in Sedimentation of Sewage and Waste
Solids, Advances in Water Quality and Chemical Processes. Austin: University of Texas
Press, p. 83(1970).
7-55
-------
30. Vesilind, P.A., "Discussion of Ref. 4." Journal Sanitary Engineering Division, ASCE,
p. 185 (February 1968).
31. Downing, A.L., "Aeration in the Activated Sludge Process." Journal of the Institute of
Public Health Engineers, Great Britain (April 1960).
32. Pipes, W.O., "Control Bulking with Chemicals." Water and Wastes Engineering
(November 1974).
33. The Impact of Oily Materials on Activated Sludge Systems. EPA Report No. 12050-
DSH-03/71 (March 1971).
34. "Aeration in Wastewater Treatment." WPCF Manual of Practice No. 5 (1971).
35. Dixon, N.P., Nomograph-Dissolved Oxygen Degree of Saturation.. Utah State Univer-
sity, Logan, Utah (1968).
36. King, H.R., "Mechanics of Oxygen Adsorption in Spiral Flow Aeration Tanks."
Journal Water Pollution Control Federation, vol. 27, p. 1007 (1955).
37. Downing, A.L., and Boon, A.G., Oxygen Transfer in the Activated Sludge Process. Pro-
ceedings, Conference on Biological Waste Treatment, Manhattan College, New York
(April 1960).
38. Eckenfelder, W.W., and McCabe, J., "Diffused Air Oxygen Transfer Efficiencies. "Ad-
vances in Biological Waste Treatment, New York: Pergamon Press (1963).
39. Downing, A.L., and Boon, A.G., Oxygen Transfer in the Activated Sludge Process. Pro-
ceedings, 3rd Conference on Biological Waste Treatment, Manhattan College, New
York: Pergamon Press (1963).
40. Downing, A.L., Bayley, R.W., and Boon, A.G., The Performance of Mechanical
Aerators. Institute of Sewage Purification, Great Britain (December 1969).
41. Kalinske, A.A., "Pilot Plant Tests on High-Rate Biological Oxidation Sewage." Water
and Sewage Works (April 1950).
42. Eckenfelder, W.W., and Ford, D.L., "New Concepts in Oxygen Transfer and Aeration."
Advances in Water Quality Improvement, Austin: University of Texas Press (1968).
43. Hughes, L.N., and Meister, J.F., "Turbine Aeration in Activated Sludge Processes."
Journal Water Pollution Control Federation, vol. 44, p. 158, (1972).
44. Nitrification and Denitriflcation Facilities, U.S. EPA Technology Transfer Seminar
Publication (August 1973).
7-56
-------
45. Loveless, J.G., and Painter, A.A., The Influence of Metal Ion Concentrations and pH
Value on the Growth of a Nitrosomonas Strain Isolated From Activated Sludge. Water
Pollution Research Laboratory, Stevenage, Great Britain (October 1967).
46. Tomlinson, T.G., Boon, A.G., and Trotman, G.N.A., "Inhibition of Nitrification in the
Activated Sludge Process of Sewage Disposal" Journal of Applied Bacteriology, vol.
29, No. 2, Great Britain (August 1966).
47. Adams, C.E., "Removing Nitrogen from Wastewater." Environmental Science and
Technology, vol. 7 (August 1973).
48. Cole, C.A., et al., "Hydrogen Peroxide Cures Filamentous Growth in Activated Sludge."
Journal Water Pollution Control Federation, p. 829 (May 1973).
7-57
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CHAPTER 8
PACKAGE (PREENGINEERED) PLANTS
Commercially available wastewater treatment plants, commonly known as "package plants,"
are sold as prefabricated or in easily assembled standard components. Prefabricated plants,
although available with capacities up to 1 mgd (3,780 m3/d), are most commonly used for
flows of less than 50,000 gpd (190 m3/d). This chapter will discuss only plants 50,000 gpd
in size or smaller. Most of these plants are biological facilities, although physical-chemical
and fixed-growth system (trickling filters and rotating biological contactors) package plants
have recently become available (1). The information in this chapter should be coordinated
with the basic design considerations in chapters 7,9, and 11.
The most common preassembled units employ some type of activated sludge process. These
plants, since they were first used in the latter part of the 1940's, have been called "aerobic
digestion" plants, "total oxidation" plants, and "extended aeration" plants. Extended aera-
tion has been accepted as properly descriptive of most of these plants. Based on the average
flow, the detention time in the aeration compartment is usually between 18 and 30 hr, if
domestic wastewater is treated. Contact-stabilization type activated sludge package plants
are also commonly used.
A detailed history of the development and performance of extended aeration and contact-
stabilization plants is given in two reports of the National Sanitation Foundation (2) (3).
In 1950, there were only about six plants, all in Ohio; at present, there are many thousands
in all parts of the country. If properly designed, operated, and maintained, these plants can
provide a good quality secondary effluent. Unfortunately, the majority of these plants do
not reach this degree of treatment, because of unwise economies in design and installation
and inadequate operation and maintenance. When these plants first came into use, the
unfortunate and totally erroneous conclusion was made that there would be no excess
sludge to be wasted. Consequently, the waste sludge went out with the effluent, resulting in
the periodic appearance of excessive SS in the effluent.
Many of the smaller package plants can serve emergency or temporary treatment needs with
a minimum of permanent installation costs. Under such circumstances, the area required
may be a standard lot size, or less. However, it is usually best to maintain some space
(preferably 50 ft [15 m] or more) between the property boundary and the treatment
works, unless the facility is completely enclosed and designed to prevent noise and odor
nuisance. Package treatment plants, like package pumping stations, can be built under-
ground, to minimize adverse environmental impacts in built-up areas.
If properly designed, operated, and maintained, these plants can usually provide satisfactory
treatment for small wastewater flows from housing developments outside metropolitan areas
and businesses and other institutions in outlying areas that generate normal domestic waste-
waters with reasonably consistent flow patterns. The inherently conservative design of the
plants results in easier operation than conventional activated sludge systems but does not
eliminate the need for proper operation.
8-1
-------
A partial list of available biological package plants is shown in Table 8-1.
8.1 General Design Considerations
An important consideration in selecting and sizing these package plants is the average 24-hr
flow and its diurnal variations. As stated previously, flows from small populations can
exhibit extreme variability, both hourly and daily. Biological processes are not noted for
their ability to take wide and sudden variations in organic load or hydraulic flow. Conserva-
tive design is, therefore, required, because the flow during several hours of the day could be
many times the average 24-hr flow. By using extended aeration times and low BOD loading,
the activated sludge process generally can tolerate wide variations in load during a 24-hr
period.
To properly design the clarifier portion of a package activated sludge plant, several of the
following considerations must be made:
1. The average MLSS to be used for design purposes (about 2,500 mg/1 to 5,000
mg/1) will usually achieve better settling characteristics (3).
2. The peak overflow rate, including the flows used to control foam and scum
return as well as sludge return, must determine sizing. As a general rule, the
clarification area should be sized so that an overflow rate of 200 to 400 gpd/ft^
(8 to 16 m^/m^-d) is not exceeded during peak flows. If in doubt, the smaller
figure should be used.
3. Scum removal and return provisions are required, if a primary settling unit does
not precede the aeration units. All package plants should be designed to retain
floatables within the system. Clarifier skimmers should permit the operator to
adjust the rate and frequency of skimming, to maintain scum removal most
efficiently and to least interfere with solids settling. The amount of foaming will
depend on the wastewater and the biodegradability of any surfactants or
detergents present.
4. There should be at least 3 to 4 ft (0.9 to 1.2 m) of clear water between the water
surface and the top of the sludge blanket, to prevent SS carryover into the
effluent. To maintain this clear water depth, a total depth of at least 10 ft (3 m),
and preferably 12, 13, or 14 ft (3.6, 3.9, or 4.2 m), should be provided if the
MLSS is to be 4,000, 5,000, or 6,000 mg/1, respectively.
An evaluation of the package plant as an alternative treatment facility indicates both
advantages and disadvantages to be considered before selection (4).
1. Advantages usually include:
Smaller land area requirement
Smaller hydraulic head loss
Fast and low-cost field installations
Reduced excess activated sludge
Generally little or no odor
Low capital cost
8-2
-------
TABLE 8-1
COMMERCIAL BIOLOGICAL PACKAGE PLANTS
Manufacturer
Model
Remarks
EXTENDED AERA TION
Bio-Pure, Inc.
Can-Tex Industries
Clow (Aer-O-Flow)
Davco
Defiance of Arizona
Dravo Corp.
Eimco Corp.
Extended Aeration Co.
FMC Corp., Environmental
Equipment Div.
Keene Corp.
Lakeside Equipment Corp.
Mack Industries
Marolf Hygienic Equipment
Co.
Permutit-Sybron
Polcon Corp.
Pollution Control, Inc.
Pollutrol Technology, Inc.
Purestream Industries, Inc.
Puretronics
Purification Science, Inc.
Richards of Rockford, Inc.
Smith & Loveless
Suburbia Systems, Inc.
Stang Hydronics, Inc.
Sydnor-Hydrodynamics
Model BP
Tex-A-Robic
Model S, SO, C
6DA2-12DA40
1.65EA-40EA
Mobilpack E
Aeropack E
ADC
Model SS
Extended Aeration
Stepaire
Oxy-Pak
EA Aerator Plant
Spirojet EA and EA R
Model MV
Stress - Key
Amcodyne E.A. Plants
Polcon Package Plant
Activator S
Puritrol
Model P
STP-600
Ecolog Systems
Rich-Pack A
ModelB
Model D
Model CY
Model RE
DCSC-50-DCSC-1000
A-D
Centri-Swirl
600 - 10,000
5,000 - 25,000
50,000-1,250,000
1,000- 100,000
2,000 - 40,000
50,000 - 500,000
1,650 - 40,000
2,500- 35,000
30,000 - 2,000,000
2,000-1,000,000
500 - 46,000
15,000 - 35,000
17,000 - 35,000
2,000 - 22,500
72,000 - 360,000
50,000-1,000,000
1,500 - 50,000
2,000 - 50,000
Includes two-stage batch
clarifier and batch chlori-
nation
Field erected; circular tank
The larger plants must be
field erected
Field erected
Field erected
Field erected for sizes greater
than 15,000 gpd
1,000- 5,000
7,500 - 15,000
35,000- 175,000
1,000 - 20,000
160,000
2,000 - 12,500
1,500- 150,000
1,500-1,000,000
4,000 - 25,000
30,000 - 225,000
5,000 - 40,000
1,000 - 100,000
1,500- 25,000
3,000 - 100,000
1,000 - 25,000
30,000 -
30,000-1,000,000
10,000 - 500,000
Field erected
Uses cage rotors for aeration
Field erected
Modular, field erected
Field erected
Batch operation
Field erected
Field erected; requires con-
struction of lined, earthen
aeration basins
Field erected
8-3
-------
TABLE 8-1
(continued)
Manufacturer
Model
Capacity
Remarks
Texas Tank, Inc.
Topco Co.
Water & Sewage, Inc.
Water Pollution Control
Corp.
A-D
Aero-Fuse
Model EA
Sanitaire Mark I, Mark II
Sanitaire Mark IV
1,500 - 50,000
3,000 - 20,000
1,000- 35,000
30,000 - 2,500,000
Field erected; can also be
operated as conventional,
contact stabilization or
step aeration process
CONTA CT STABILIZA TION
Can-Tex Industries
Clow (Aer-O-Flow)
Davco
Dravo Corp.
FMC Corp., Environmental
Equipment Div.
Gulfsten Bio-Con
Lakeside Equipment Corp.
Marolf Hygienic Equipment
Co.
Permutit-Sybron
Pollution Control, Inc.
Purification Science, Inc.
Smith & Loveless
Walker Process Equipment
Water & Sewage, Inc.
Westinghouse
Tex-A-Robic
Model CS
11DAC20-12DAC70
Mobilpack C
Aeropack C
Stepaire
Stabilaire SL-150
BC20P - BC80P
Spirojet CS
Amcodyne C.S. Plant
Activator CS
Contact Stabilization System
Model B
Model D
Model CY
Model RE
Model V
Sparjair
Model A
RCS
30,000 - 50,000
50,000 -1,250,000
50,000 - 500,000
20,000 - 70,000
50,000 - 500,000
10,000 - 20,000
30,000 - 2,000,000
100,000 - 500,000
20,000 - 50,000
20,000 - 80,000
2,500 - 3,000,000
50,000 - 3,000,000
40,000 •
10,000 •
30,000 •
15,000-
17,000-
2,000 •
72,000 •
2,000 •
20,000
15,000
10,000
30,000
1,000,000
120,000
1,000,000
35,000
35,000
22,500
360,000
90,000
500,000
50,000
50,000
1,000,000
Factory assembled; rectangu-
lar design
Circular design
Can be operated as extended
aeration plant at reduced
capacity
Factory assembled
Factory assembled; rectangu-
lar design
Factory assembled; circular
design
Can be operated as extended
aeration or contact stabi-
lization
Factory assembled; rectangu-
lar design
Custom designed; rectangu-
lar design
Rectangular design
Field erected
Field erected
Circular design
Rectangular units
Circular units
8-4
-------
TABLE 8-1
(continued)
Manufacturer
STEP - AERATION
FMC Corp., Environmental
Equipment Div.
CONVENTIONAL
ACTIVATED SLUDGE
FMC Corp., Environmental
Equipment Div.
Smith & Loveless
Walker Process Equipment
Water Pollution Control Co.
Model
Stepaire
Completaire
Model V
Swirlmix
Sanitaiie Mark IV
Capacity
gpd
100,000 - 500,000
Remarks
15,000- 25,000
2,000 - 90,000
100,000 - 2,000,000
30,000 - 2,500,000
Can be operated as extended
aeration or contact stabi-
lization
Field erected
Field erected; can also be
operated as conventional,
contact stabilization or
step aeration process
COMPLETE MIX
Dorr-Oliver, Inc.
100-500
100,000 - 500,000
8-5
-------
2. Disadvantages include possibilities of:
High power costs
High operation costs
Noise pollution
8.2 Extended Aeration Units
Because these systems do not employ primary clarification, the treatment plant normally
consists of a screen or comminutor, aeration basin, clarification compartment, and disinfec-
tion facilities. Because primary settling is not provided, the aeration basin should have
sufficient agitation to keep in suspension the heavier solids not normally present in the
MLSS of activated sludge plants. To insure sufficient agitation to prevent settling (excluding
oxygen input requirements), the aeration compartment should have a power input of about
3/4 to 1 hp/1,000 ft3 (2 to 2.5 kW/100 m3), if mechanical aerators are used. For a diffused
air system, there should be at least 30 cfm of air supplied per 1,000 ft3 (1.8 m3/m3 -h)
of aeration volume.
The BOD5 loading should insure that the maximum food-to-micro-organism (F/M) ratio is
about 0.05 to 0.15 Ib BOD5/day/lb MLVSS. The MLSS is usually maintained in the range of
3,000 to 6,000 mg/1. The above loading for normal domestic wastewater is equivalent to a
hydraulic detention time of about 1 day (24 hr) in the aeration basin at average flow. The
average detention time may vary from 18 to 36 hr. At this BOD loading, the solids produc-
tion is minimal. The excess solids, including the volatiles entering with the raw wastewater,
which are not degraded, amount to about 0.3 to 0.5 Ib/lb BOD removed, or about 400 to
1,200 Ib of dry solids per million gallons (0.05 to 0.14 kg/m3) of normal domestic waste-
water.
The excess solids produced must be removed and disposed of. A separate sludge storage
basin or compartment, which will hold the waste solids for at least 1 week and preferably 2
to 3 weeks, is desirable. This compartment can also serve as an aerobic digester, because it
should be aerated to prevent septicity and the separation of solids. Aerobic digestion for
about 7 to 10 days, if the liquid temperature is above 20° C, can result in a further reduc-
tion in volatile solids of up to about 30 percent, depending on the amount of endogenous
respiration that took place in the activated sludge process (5).
Stabilized solids can be dewatered on a sand bed for eventual land disposal. In larger plants,
mechanical sludge dewatering with vacuum filter or belt filter may be considered. Figures
8-1 and 8-2 illustrate two designs of extended aeration, activated sludge type package plants
in which the flow is completely mixed. A positive means for sludge recycle is generally
required for the proper performance of these plants.
The areation requirements for biological oxidation should be based on an oxygen input of
about 2 Ib/lb BOD5 applied. This is equal to the ultimate carbonaceous BOD plus the
oxygen requirements for oxidation of ammonia-nitrogen to nitrates (nitrification), which
will generally occur because of the long sludge age (about 20 to 40 days). Nitrification
should occur in well-designed and operated extended aeration plants, unless the liquid
8-6
-------
EFFLUENT
EFFLUENT
rSCUM BAFFLE
FINAL
TANK
1f
AERATION TANK
L
SLUDGE RETURN
TROUGH
rTANK
INLET
INFLUENT
PLAN
-BLOWER
SCUM RETURN
EFFLUENT AIR LIFT-v
WEIR y \
V
EFFLUENT ^
r
FINAL
\
TANK
/
AIR LINE-v J
I — — W
t
=3 t
1 TANK INLET-7
/
=a=i 1 1 ^=*t
\^ ij L7^
AERATION TANK
^DIFF USERS
y ,,
Jr*
* INFLUENT
SECTIONAL ELEVATION
FIGURE 8-1
EXTENDED AERATION TREATMENT PLANT
WITH AIR DIFFUSERS
8-7
-------
BYPASS -
TO
SETTLING
TANK
BAR-SCREEN
CHAMBE
AERATION TANK
B
INFLUENT
PLAN
1
B
J
-SLUDGE RETURN
-J
SLUDGE
RETURN PUMP
PUMP
PIT
LINE
r
EFFLUENT
I—WEIR TROUGH
ECHANICAL
AERATOR
rSLUDGE
RETURN
PIPE
SECTION A-A
SLUDGE HOPPER-
SECTION B-B
FIGURE 8-2
EXTENDED AERATION TREATMENT PLANT
WITH MECHANICAL AERATOR
8-8
-------
temperature drops below 5° C (40° F). Nitrification may cause several problems in extended
aeration system operation, including the following:
1. Sludge rising, if denitrification takes place (because of anaerobic conditions) and
produces nitrogen gas, which tends to buoy up the solids, thus interfering with
settling and sometimes causing flotation.
2. The oxidation of ammonia-nitrogen, producing nitric acid, may reduce the pH
and affect process efficiency, if wastewater alkalinity is not sufficient to buffer
the system.
3. Interference in the BODs bottle test, if nitrification occurs, will indicate higher
BOD5 results than possible from first-stage carbonaceous oxidation.
Well-established extended aeration package plants will decrease ammonia-nitrogen to around
1 mg/1, if the aerator temperature is above about 55° F (13° C) (2). For more information
on extended aeration systems, see chapter 7.
8.3 Contact Stabilization Units
The contact stabilization process has been incorporated into package plants (particularly
for larger capacities), because it allows the total aeration basin volume to be reduced from
that used in the single basin extended aeration process. The F/M ratio is comparable to that
of the extended aeration systems, considering the total solids in the system outside the
clarifier. However, F/M ratios are much higher, if based on the solids in the contact basin
alone. Because the concentration of solids in the stabilization basin is two to three times the
concentration in the contact basin, an aeration basin volume reduction is possible, if a
contact-stabilization plant is designed in accordance with original criteria. However, there is
evidence that, incorporated into package plants, this process lacks stability and does not
provide the BOD and SS removal efficiencies expected (6). Contact-stabilization is most
valuable for wastewater in which most of the BOD is suspended or colloidal and the flow
is quite uniform.
If time in the contact chamber becomes extended, basic design criteria are violated in terms
of BOD loadings. The characteristics of the solids in the contact chamber depend on the
organic and biological loading and the extent of biological activity in that chamber. The
contact-stabilization process, as originally designed and tested, had a detention time of 20 to
40 minutes in the contact chamber. Suspended and colloidal organic solids and some of the
soluble organic solids are adsorbed by the well-oxidized sludge coming from the stabilization
chamber. However, if the detention time in the contact chamber reaches 1.5 to 2 hr,
biological activity may start and result in a high rate activated sludge process, with a reduc-
tion in settleability and more SS in the clarifier effluent. Unfortunately, the detention can-
not be kept at 20 to 40 minutes over the wide range of flows coming to small treatment
plants, without prior flow equalization. To maintain the detention time at 20 to 40 minutes
in the contact chamber at the time of peak flow during the day, the contact time for the
24-hr average flow would be 1 to 2 hr or more. Some State and other regulatory agencies
require a contact chamber detention time of 2 to 3 hr at average flow. These longer contact
times make the contact-stabilization process less efficient and less effective (6).
8-9
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Contact stabilization should be used only for larger, more uniform flows, or if flows to the
plant have been equalized. The size of the contact chamber should be sufficient to maintain
a detention time of about 20 to 40 minutes most of the time. It is unlikely that effluent
standards can be met using contact stabilization in plants smaller than 50,000 gpd (200
m3/day), without some prior flow equalization. In plants sized for future capacity, flexibili-
ty should be incorporated in the design of the contact chamber, so its size can be reduced
for the low flows during the initial period of plant operation. Flexibility can be attained by
dividing the contact chamber into two or three compartments and using only one or two
initially. The compartments not in use can be connected to the stabilization section. Stabili-
zation can be increased to nearly 25 hr (from the normal 3 to 6 hr), to make the unit less
sensitive to shock or toxic loadings. An integral aerobic sludge digester can also be incor-
porated in the plant for the waste activated sludge, as shown in Figure 8-3.
The MLSS is usually 1,000 to 3,000 mg/1 in the contact basin and 4,000 to 10,000 mg/1 in
the stabilization, or reaeration, chamber. About 0.7 to 1.0 Ib (0.32 to 0.45 kg) of O2 is
required for each Ib of BOD5 removed, or 800 to 1,200 cfm (24 to 36 m3) of air per pound
of C<2 removed. The sludge generated varies from about 2,500 to 10,000 gal/million gal
of plant influent.
Nitrification cannot be expected to occur in a contact-stabilization plant, because 1) the
ammonia in the liquid is poorly adsorbed by the solids in the contact chamber, and 2) the
time is not sufficient. Some nitrification will, of course, occur in the stabilization basin,
particularly if a long detention period is provided, but the ammonia present in the major
portion of the raw wastewater will pass out with the effluent from the contact basin.
8.4 Rotating Biological Contactor (RBC) Units
These units are widely used in Europe in small prefabricated plants—there are over 700
installations in West Germany, France, and Switzerland. They afford stable operation,if
conservatively sized; with hydraulic loadings of 0.25 to 1.55 gpd/ft2 (0.01 to 0.06
m3/m2 -d), they will produce 85 percent BODs removal to liquid temperatures of 40° F (5°
C). For low temperature operation, the loading should be below 1 gpd/ft2 of disk area (0.04
m3/m2 -d) (7). They are now being designed and constructed in the United States.
Because the flow velocity and turbulence in the compartment containing the disks are not
high enough to keep heavy primary wastewater solids in suspension, a primary settling unit
must precede the disks. Solids would, then, be wasted from both primary and final clarifiers
for treatment and disposal. Recirculation of solids or liquid has not normally been practiced
with RBC's.
The advantages of these units are low maintenance, low power, minimal odor and fly
nuisance, and low noise levels. However, the units should be housed to prevent damage to
the disks by high winds and vandalism, to keep heavy rains from washing the growth off the
disks, and to prevent freezing problems. Figure 8-4 illustrates a package plant of this type.
In rotating biological contactors at hydraulic loadings of under 1 gpd/ft2 (0.04 m3/m2 -d) of
disk area, nitrification may occur on the disks toward the end of the flow-through chamber
8-10
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COMMINUTOR
INFLUENT
PLANT EFFLUENT
RETURN WASTE
SLUDGE AIRLIFT
^SLUDGE DRAW-OFF
AND PLANT DRAIN
FIGURE 8-3
CONTACT STABILIZATION PLANT
WITH AEROBIC DIGESTER
8-11
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^.
-------
at liquid temperatures down to about 40° F (about 5° C). The solids production from these
units, at the low loadings, will be about 0.3 to 0.7 Ib/lb BOD removed; they may concen-
trate to 2 to 3 percent in the final clarifier (7). Loadings of 2 to 6 Ib BODs applied per day
per 1,000 ft^ (0.01 to 0.03 kg/m^) of disk surface have been recommended as the
maximum (8). Power requirements are reported to be about 0.2 hp-hr/lb BODs removed.
For more information on RBC's, see chapter 9.
8.5 Physical-Chemical Package Units
The typical flow diagram for a commercial physical-chemical (P-C) wastewater treatment
plant is shown on Figure 8-5 (9). The plant can treat wastewater that has been screened or
comminuted and degritted. The first step of the system is chemical coagulation followed by
clarification. After clarification, the flow enters a downflow carbon compartment for
filtration and some removal of soluble organics. Additional removal of the soluble organics
occurs in a following upflow carbon compartment.
For domestic wastewater, an effluent with about 25 mg/1 COD and 10 mg/1 of BOD, with
SS below 5 mg/1, can be obtained. Phosphorus as P can be reduced to less than 0.5 mg/1;
color and turbidity can be minimized. The activated carbon is normally disposed of as it
is exhausted, because regeneration for small plants is not cost effective.
Different chemicals tend to generate different amounts of sludge (see section 13.2):
1. Lime generates 6,000 to 14,000 gal/mgd of plant influent, with 6 to 10 percent
dry solids.
2. Alum generates 10,000 to 30,000 gal/mgd of plant influent, with 0.5 to 1.5
percent dry solids.
3. Iron salts generate 10,000 to 25,000 gal/mgd of plant influent, with 1.0 to 2.5
percent dry solids.
Both the operating costs and capital costs of this type of plant, in contrast to activated
sludge package plants, are relatively high. The plant can be started and stopped without
large adverse effects on treatment; it can produce a high quality effluent with a low BOD
and phosphorus. However, ammonia cannot be removed, unless additional processing is
added to the basic sequence. For more information on physical-chemical units, see chapters
12 and 13.
The use of physical-chemical package plants has been largely restricted to cold climates,
where their small size, "on-off' operation, and high reliability are requisite. However,
additional application will undoubtedly occur, because of their inherent resistance to toxic
compounds in wastewaters and ability to remove heavy metals and refractory compounds.
Physical-chemical package plants are primarily available in two generalized processing
schemes: granular carbon and powdered carbon systems.
The granular carbon systems are generally similar to the schematic diagram shown in Figure
8-5. Powdered carbon systems employ simultaneous chemical coagulation and powdered
carbon contacting in a single step. Final solids and powdered carbon removal may be
accomplished by sedimentation and/or filtration.
8-13
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INFLUENT-
EFFLUENT-*
/ SLUDGE \
BLANKET }-
y CLARIFIER
ARIFIER I
—- -^^
DOWN FLOW \
ACTIVATED
CARBON
/ UPFLOW \
ACTIVATED
V CARBON /
CHEMICAL
WASTE
SLUDGE
BACKWASH
TO WASTE
BACKWASH
WATER
FIGURE 8-5
FLOW DIAGRAM FOR PHYSICAL-CHEMICAL
TREATMENT PLANT (9)
8-14
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8.6 Operation and Maintenance of Activated Sludge Package Units
The major design problems that affect field operation of the package activated sludge units
include (10):
1. Hydraulic shock loads-the large variations in flow from small communities,
accentuated by the use of oversize pumps where wastewater is pumped
2. Very large fluctuations in both flow and BOD loading
3. Very small flows that make difficult the design of self-cleansing conduits and
channels difficult
4. Inadequate or nonpositive sludge return, requiring provisions for a recirculation
rate of up to 3:1, for extended aeration systems to meet all normal conditions
5. Inadequate provision for scum and grease removal from final clarifier
6. Dentrification in final clarifier, with resultant solids carryover
7. Inadequate removal and improper provision for handling and disposing of waste
sludge
8. Inadequate control of MLSS in the aeration tank
9. Inadequate antifoaming measures
10. Large and rapid temperature changes
11. Inadequate control of air supply
Possibly, the major factor causing poor performance is directly related to the quality and
amount of operator attention. Unless such plants receive at least some attention and main-
tenance daily from a qualified operator, they should not be installed, because the effluent
quality will invariably be quite poor. For more information on staffing requirements for
operation and maintenance, see chapter 16.
O & M requirements for physical-chemical plants, established by manufacturers to necessi-
tate from 2 to 4 hr daily, involve chemical makeup, sludge handling, and preventive main-
tenance procedures common to other plants.
In cold climates, small plants may experience operating problems. In such climates, it is
preferable to install one plant in the ground and/or to house it, to conserve heat. A long
aeration period dissipates heat, particularly if mechanical surface type aerators are used.
Such aerators should be designed to prevent freezing from liquid splashed on metal parts
and the platform. If such plants are installed in northern climates, diffuser systems, with
compressed air, should be used for the oxygen supply.
If it is known that operation and maintenance may be minimal, one or more of the follow-
ing should be considered for inclusion in the specifications:
1. Flow recorders for influent, return sludge, and effluent
2. Sampling connections, or ports, with easy access
3. Automatic air regulation
4. Automated variations in submergence of surface aerators
5. Continuous DO recorders
6. Automatic adjustable skimming mechanisms
8-15
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7. Automatic defoaming sprays
8. Automatic timing of periodic operations, such as skimmers, return sludge, sludge
wasting, and air blower
Some suggestions to improve and maintain the efficiency of package plants are:
1. All pumps, laboratory equipment, and supplies necessary for good performance
should be placed in a control building onsite.
2. A schedule showing all regular and intermittent maintenance procedures and
emergency procedures should be posted.
3. A minimum schedule of required daily and intermittent tests and process observa-
tions should be established.
4. A simple but complete operation and maintenance manual, keyed to the
capability of the probable standby operator should be required.
5. Adequate training of operators, assistant operators, and replacement operators
should be provided.
6. An adequate supply of the equipment and material required to maintain, monitor,
and control the safe, efficient, and simple operation of the facilities should be
specified.
7. The plant should be designed for good public relations, by providing for odor,
noise, and landscaping control.
8. Regular wasting of digested sludge to a dewatering facility (such as a sludge drying
bed) before final disposal should be provided.
9. Pumps or blowers should be placed beside, and not above, aeration tanks or
clarifiers as a safeguard against dripping oil and dropped tools entering the units.
10. Tank covering guards and high, locked fences of good quality should be provided.
11. Adequate, handy sources of water for cleaning purposes should be provided.
Additional information on the operation of package treatment plants can be found in
references (2) (6) and (10).
8.7 Case Studies
8.7.1 Physical-Chemical Package Plant
A Met-Pro physical-chemical package plant was installed at Indian Hills Housing Develop-
ment, Lower Salford Township, Pennsylvania, and put into operation in the first part of
May 1974. Engineering data and test results of samples taken on 8 May 1974 and 6 June
1974 are shown in Table 8-2. A schematic plan showing the movement of wastewater,
sludge, and chemicals in the plant is shown on Figure 8-6 (11).
Comminuted wastewater is pumped from an equalizing tank (not shown) by raw waste
pump (a) to flash mix tank (b). Coagulant feeder (c) delivers a proportional amount of
chemical solution to the flash mix tank, where intimate contacting is accomplished by
means of a high speed agitator. From the flash mixer, the wastewater flows by gravity into
the flocculating section of the clarifier (d), where gentle agitation promotes floe formation.
8-16
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TABLE 8-2
DATA FOR A PHYSICAL-CHEMICAL PLANT
DESIGN CRITERIA
Capacity
7,000 gpd
Shipping Wt,lb
5,700
Overall Size
8 ft X 7 ft X 8.7 ft
Hp_
2.75
OPERATING DATA
Sample Source
COD
mg/1
BOD5
mg/1
SS
mg/1
Total P
mg/1
May8, 1974
Raw Influent 360
Clarifier Effluent 40
Adsorber Effluent 20
Filter Effluent 10
107
18
6
10
262
4
12
2.31
0.14
0.80
0.66
June 6, 1974
Raw Influent 487 212 316
Clarifier Effluent 112 61 62
Adsorber Effluent 28 8 16
Filter Effluent 18 10 8
12
0.38
0.42
0.13
As sedimentation takes place in the clarifier, the settleable solids are collected and are
pumped (e) to disposal. A disposable media filter (f) is an option for sludge concentrating
prior to ultimate disposal (11).
A controlled amount of chlorine solution is pumped into the surge and disinfectant contact
tank (j) for disinfection. The clarifier effluent flows up through the granular carbon
adsorber (h), for removal of dissolved organic materials, and into the surge tank (j)- Air is
fed into the bottom of the adsorber aerator (i), maintaining the fluidized carbon bed in an
aerobic condition. Filter pump (k) pushes the disinfected wastewater through the pressure
filter (m) for final polishing.
8.7.2 Extended Filtration Package Plant
The national Sanitation Foundation (NSF) evaluated the performance of the Aquatair
Model P-3 package treatment plant in June 1974 and reported that the system incorporates
8-17
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FILTRATE
SLUDGE TO
DISPOSAL
©
LEGEND
(a) RAW WASTEWATER PUMP
(b) FLASH MIX TANK
© COAGULANT FEEDER
(d) FLOCCULATOR - CLARIFIER
(e) SLUDGE PUMP
(T) DISPOSABLE MEDIA FILTER
(9) DISINFECTANT FEEDER
(h) UPFLOW GRANULAR CARBON ADSORBER
(T) ADSORBER AERATOR
(T) SURGE a DISINFECTANT CONTACT TANK
(k) FILTER PUMP
(rn) PRESSURE FILTER
FIGURE 8-6
MET-PRO PHYSICAL-CHEMICAL
PACKAGE TREATMENT PLANT (11)
8-18
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biological treatment of organic matter in a high rate bio-oxidation tower and an air-injected
recirculation/aeration chamber. The extended filtration system is a superrate trickling filter,
with recycling of sludge only from the final clarifier to the filter influent (12). The unit
tested was designed to treat 3,000 gpd (11.4 m^/d), with about 5 Ib/day (2.29 kg/day) of
BOD5, in a 16-hr interval, effective at a maximum of 187.5 gpd (0.71 m^/d). Characteristics
of Aquatair systems and typical NSF performance data are presented in Tables 8-3 and 8-4.
Raw wastewater enters a sealed primary settling tank, which overflows into an aerated
recirculation chamber (see Figure 8-7). Floating and settled solids, including grit and grease,
are trapped and stored for 6 to 12 months and undergo anaerobic action. Excess sludge
from the final clarifier is also stored here for anaerobic treatment, while the major portion
of the clarifier sludge is returned to the recirculation/aeration basin. After the raw overflow
mixes with the aeration tank contents, a set amount is pumped to the top of the bio-
oxidation tower, which has 35 ft2 (3.2 m2) of surface area; another portion of the pumped
flow is returned to the recirculation/aeration basin through a jet ejector, to mix and aerate
the water/sludge mixture. The clarifiers are designed for a maximum overflow rate of 250
gpd/ft2 (10 m3/m2'd). The wetting rate on the filters is maintained between 0.8 and 1.5
gpm/ft2 (47 and 88 m3/m2 -d).
It should be noted that the National Sanitation Foundation standards were used for testing
this unit and the data are representative of different specified flow regimes. Because of
better than normal operation, the data represent a measure of performance capability under
the conditions of testing.
8.7.3 Nitrification in Extended Aeration Plant
A study was conducted by CAN-TEX on an extended aeration package plant at Weatherford,
Texas, to determine the characteristics needed to obtain nitrification (13). Because efforts
with single stage treatment were not successful at lower temperatures, the plant was split
into two separate aerator-clarifier subunits. The first-stage unit was operated from February
1973 through June 1973; the second stage from September 1973 through March 1974,
obtained only limited data. During the first period, the water temperature varied from about
18° to 28° C; in the second period, it varied from about 28° to 80° C. During the first
period the DO in the aeration unit dropped below 2 rng/1 several times, greatly reducing
NH3 removal. Nitrification recovered in several days when the DO returned to over 2 mg/1.
The O2 requirement to produce nitrification in the first-stage period was about 170 percent
of 6005; in the second stage, it was reduced to 150 percent of the BOD5- In the second
stage, it was found that the DO could fall to 1 mg/1 without lowering the removal in the
first stage; however, more than 2 mg/1 were required in the second stage.
Design and operation data are shown in Table 8-5. A plan and elevation of a CAN-TEX
packaged two-stage nitrification activated sludge plant are shown in Figure 8-8.
8-19
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TABLE 8-3
PERFORMANCE EVALUATION OF AQUATAIR MODEL P-3
Min. Max. Avg.
Flow, gpd 2,800 3,300 3,100
DO,mg/l 2.9 6.5 4.6
Temperature, °C 14 24 20
pH 6.9 7.4 7.2
SVI 119 300 181
MLVSS,mg/l 1,000 2,250 1,708
Influent Effluent
BOD5,mg/l 175- 702 5- 16
SS, mg/1 192- 692 11- 58
VSS,mg/l 166-590 9- 36
COD, mg/1 402-1,784 16- 112
Alkalinity as CaCO3, mg/1 191- 236 66-102
NH3-N, mg/1 18.9- 28.1 2.1-10.3
NO3-N,mg/l 0.1- 1.0 11-19.7
Phosphate, mg/1 21.4- 31.1 16.1-28.8
TABLE 8-4
CHARACTERISTICS OF AQUATAIR PACKAGE SYSTEMS
Design Capacity, gpd
Item 10,000 20,000 30,000 40,000 50,000
Component Volume, gal
Sludge Holding Tank 2,250 4,500 6,750 9,000 11,250
Recirculation Tank 3,750 7,500 11,250 15,000 18,750
Clarifier 1,667 3,166 5,000 6,670 8,333
C12 Contact Tank 217 396 625 834 1,042
Bio-Oxidation Tower, ft3 240 448 640 896 1,152
Weight, Ib 11,300 18,050 24,800 31,400 35,000
8-20
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OBSERVATION
PORT-
VENT
LINE
AIR INJECTOR
FEED LINE
00
to
SLUDGE PUMP
PREWIRED CONTROL
PANEL IN WEATHER
PROOF HOUSING
CHLORINATOR
HOUSING
SLUDGE RETURN 8
SKIMMER SUMP
^==Q J
SECTION (T)
VENT LINE
CLEANOUT
-RECIRCULATION
CHAMBER INLET
PORT
AIR INJECTOR
DISCHARGE LINE
FIGURES-?
EXTENDED FILTRATION PACKAGE PLANT
-------
TABLE 8-5
PACKAGE 2-STAGE NITRIFICATION ACTIVATED SLUDGE PLANT
Design Criteria
Stage 1 Stage 2
Average 24-hour flow, mgd 15,000 15,000
Aeration retention time, hr 10 12
Clarifier overflow rate, gpd/ft2 420
SRT,day 12
Operating Data
Raw Aerator #1 Clarifier #1 Aerator #2 Clarifier #2
Wastewater1 Effluent Effluent Effluent Effluent
BODSsmg/l 229 36 17
SS,mg/l 240 47 17
MLSS, Ib 1900 2900
NH3-N, mg/1 28 11
N02-N03,mg/l 0 15
Total Kjeldahl N, mg/1 26 11 Q
DO, mg/1 4.2 5.1
Temperature, °C 16 16
raw wastewater is not pretreated before entering the first aeration compartment.
8-22
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- EXCESS SLUDGE
AIRLIFT
AERATION
ZONE NO. I
AERATION
ZONE NO Z
-CLARIFIEH NO. 2
-NOTCH WEIR
EFFLUENT
GROUT BOX
FLOW
BAFFLE
-CONCRETE SLAB WITH
REINFORCING BARS
ELEVATION
NOTES :
•PRETREATMENT CONSISTED OF SCREENING AND COMMINUTION.
•POST TREATMENT CONSISTS OF GRANULAR FILTRATION AND
CHLORINATION.
FIGURE 8-8
CAN-TEX PACKAGE 2-STAGE (NITRIFICATION)
ACTIVATED SLUDGE PLANT
8-23
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8.8 References
1. Snoeyink, V.L., and Mahoney, J.A., Summary of Commercially Available Wastewater
Treatment Plants. University of Illinois, Technical Report No. AFWL-TR-72-45, NTIS
distribution No. AD-747-032, Urbana, Illinois (July 1972).
2. Package Sewage Treatment Plants Criteria Development, Part I: Extended Aeration.
National Sanitation Foundation, FWPCA Grant Project Report No. WPD-74
(September 1966).
3. Package Sewage Treatment Plants Criteria Development, Part II: Contact Stabilization.
National Sanitation Foundation, FWPCA Grant Project Report No. WPD-74 (June
1968).
4. Ludzack, F.J., Biological Treatment Technology. EPA-430/1/73-017, NTIS PB 228
148 (December 1973).
5. Municipal Wastewater Treatment Plant Sludge and Process Sidestreams, U.S. EPA,
Office of Water Programs (1975).
6. Dague, R.R., Elbert, G.F., and Rockwell, M.D., "Contact Stabilization in Small
Package Plants.' 'Journal Water Pollution Control Federation, No. 44 (1972).
7. Bio-Surf Process Package Plants for Secondary Wastewater Treatment. Autrol
Corporation Bio-Systems Division, Milwaukee (1974).
8. Tchobanoglous, G., "Wastewater Treatment for Small Communities." Public Works
(July 1974).
9. Kreissl, J.F., and Cohen, J.M., "Treatment Capability of a Physical-Chemical Package
Plant." Water Research, No. 7 (1973).
10. Seymour, G.G., "Operation and Performance of Package Treatment Plants." Journal
Water Pollution Control Federation, No. 44 (1972).
11. Private communications, Met Pro Systems, Division 5th and Mitchel Avenue, 19446
(Lansdale, Pennsylvania: June 1974).
12. Trickling Filter/Activated Sludge Package Plant brochures, Aquatair (1975).
13. Study bulletins, CAN-TEX Research Division, Weatherford, Texas (1975).
8-24
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CHAPTER 9
FIXED FILM SYSTEMS
Biological processes used for treatment of wastewater can be classified as suspended growth
systems or fixed film systems. Suspended growth systems are discussed in Chapter 7. Fixed
film systems provide surface area for the growth of a zoogleal slime. This slime or film con-
tains the major portion of microorganisms that provide treatment. The fixed film systems
can be further divided into units with stationary media (trickling filters) and units with
moving media (rotating biological contactors).
9.1 Trickling Filters
9.1.1 General Description
A trickling filter contains a stationary medium providing surface area and void space. The
zoogleal film develops on the surfaces and the void space allows air and wastewater to pass
through the medium and come in contact with the microorganisms in the film. The orga-
nisms utilize the oxygen and material in the wastewater for their metabolism.
Many variations of trickling filter systems have been developed and used successfully. The
EPA Municipal Waste Facilities Inventory of August 1974 indicates that there are approxi-
mately 2,700 trickling filter plants with design flows of less than 1 mgd, serving about
9,400,000 people in the United States. In the past, the trickling filter has been considered
ideal for plants serving populations of 2,500 to 10,000.
Trickling filters historically have been popular for use in small plants, because of their
ability to recover from shock loads and to perform well with a minimum of skilled technical
supervision, and because of their economy in capital and operating costs.
9.1.2 Process Description
The trickling filter process depends on biological activity to oxidize the complex organic
matter in wastewater. For operating efficiency, the proper ecological environment must be
maintained: a continuous supply of food, water to keep the organisms in the zoogleal slime
moist, and oxygen to keep conditions aerobic. A distribution system is provided to insure
uniform application of wastewater on the medium, along with an underdrain system to
collect the wastewater that has passed through the medium and to provide spaces for proper
ventilation.
If a trickling filter is operating correctly, the medium becomes coated with a zoogleal film,
which is a viscous, jellylike substance containing bacteria and other biota. The zooglea pro-
duce exoenzymes, which catalyze chemical reactions between the suspended, colloidal, and
dissolved organic solids adsorbed onto, or slowly moving over, the film. Activity on the sur-
face of the film is normally aerobic, provided there is adequate ventilation. Some anaerobic
decomposition near the medium surface may occur, because the diffusion of oxygen through
9-1
-------
the film is largely molecular and, thus, quite slow. Depending on factors such as pollutant
loading, type of organic matter, type of medium, temperature, presence of essential nutri-
ents, and hydraulic loading, the film builds up until excess solids separate from the medium
("slough off) and are carried in the wastewater to a clarifier, in which settleable solids are
removed for further treatment and disposal.
9.1.3 Classifications of Trickling Filters
Developments in the design and operation of trickling filters have resulted in four major
classifications: low-rate, intermediate-rate, high-rate, and super-rate filtration, which are
differentiated primarily by their hydraulic and organic loads. Recirculation and medium
configuration also play a part in filter classification. Low-rate filters do not include recir-
culation; super-rate filters, in most cases, only recirculate to maintain a minimum wetting
rate. Table 9-1 shows the four common classifications, with ranges of hydraulic and organic
loading.
TABLE 9-1
TRICKLING FILTER CLASSIFICATIONS
Parameter
Hydraulic Loading,
million gallons per
acre per day (mgad)1
Organic Loading,2
lb(BOD)/day/1,000
Low-Rate Intermediate High-Rate Super-Rate
Filter Filter Filter Filter
1-4
5-20
4-10 10-30
15-30 30-60
30-50
50-100
1 Includes recirculation (1 mgad = 0.935 m3/m2'd)
2Does not include organic load resulting from recirculation (1 lb/d/1,000 ft3 =
0.016kg/m3-d)
The low-rate trickling filter is a very dependable unit, providing consistent effluent quality
over a wide range of organic loading. This system, with associated primary and final settling
tanks, will normally provide 85 percent BOD removal. The operation is very simple, because
dosing is intermittent (at not more than 5-minute intervals) with no recirculation. The
depth is normally between 6 and 10 ft (1.8 to 3.0 m), which, along with a low loading rate,
may allow the unit to provide a high degree of nitrification. Two major problems with low-
rate filters are odor and filter flies (Psychoda).
The intermediate-rate filter is similar in design to the high-rate units and is operated with
recirculation. The major problem with the intermediate-rate filter is that hydraulic loadings
within this range apparently lead to a stimulation of organic growth, which clogs filters. This
clogging has been solved in some instances by using a large medium: 3 to 4 in. (75 to 100
mm) in size; however, some filters have operated well with smaller rock.
9-2
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High-rate trickling filters are normally 3 to 6 ft (0.9 to 1 .8 m) deep and require some opera-
tional skill to control high loadings and recirculation (which could be one to four times the
influent flow). Because of high loadings and the relatively shallow depth, sloughing is nor-
mally continuous, and filter fly larvae are washed away, eliminating problems with flies and
clogging (ponding). High loadings on a high-rate filter prevent the development of nitrifying
bacteria, thus eliminating nitrification. The BOD removal efficiency of these units normally
ranges between 65 to 75 percent, although they can stabilize large amounts of organic
matter per unit volume ( 1 ).
Super-rate trickling filters have become available with the use of synthetic media having
large void space and high surface area per unit volume (high specific surface). The super-rate
filter, which consists of towers 10 to 40 ft (3 to 12 m) deep has accommodated hydraulic
loadings of 2.4 gpm/ft2 (140 m^/m2 -d) and higher, although normal loadings are between
0.5 and 1.5 gpm/ft2 (29 to 88 m3/m2 'd). These towers are often referred to as bio-oxida-
tion towers. Shallow trickling filters less than 1 0 ft (3 m) high, using manufactured media of
the bulk packing type or other open media suitable for use in shallow units, could also be
super rate.
Extended filtration is usually a subclassification of super-rate filtration. This process com-
bines the super-rate trickling filter with controlled sludge recycle, similar to an activated
sludge process. The sludge recycle is provided to maintain a high solids concentration in the
trickling filter influent. These solids act in a way similar to the mixed liquor SS in an acti-
vated sludge process. This combination of suspended and fixed growths is intended to
achieve a high degree of oxidation and stabilization of sludge solids. This process is also dis-
cussed in Section 8.8.2.
9. 1 .4 Application of Process at Small Plants
Trickling filters have been used to provide secondary treatment for wastewaters that are
amenable to aerobic biological processes. These filters are capable of providing adequate
treatment of domestic waste, if effluent quality of 20 to 30 mg/1 of BOD is acceptable (2).
The effluent quality from a trickling plant requires special consideration. If proper condi-
tions exist, nitrification may occur in a trickling filter. These conditions, which include
temperature, pH, presence of nitrification inhibitors, and solids retention time, will be dis-
cussed further in Chapter 13 and Section 9.1.8. The presence of ammonia, nitrogen, and
nitrifying bacteria in the effluent and in the BOD bottle will cause a high BOD determina-
tion. If the possibility of nitrification exists, the samples should be nitrifier-inhibited to
obtain a carbonaceous
Although a single-step trickling filter can achieve secondary treatment if designed and
operated properly, the use of one step is not recommended for small plants. A single filter in
a small plant would not meet reliability Class I or Class II conditions set forth in the U.S.
EPA technical bulletin, Design Criteria for Mechanical, Electric, and Fluid System and Com-
ponent Reliability (3). Trickling filters in parallel would increase reliability, but such units
would have to be designed conservatively, to provide reliability over the entire range of load-
ings with a minimum of operation. New designs of small plants can provide reliability and
flexibility by using trickling filters as roughing filters or in multiple-step systems.
9-3
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Roughing filters are normally high-rate or super-rate units, designed to remove from 40 to
65 percent of the BOD5. They are commonly used if only partial treatment is desired, to re-
duce the loadings on subsequent biological processes.
Using roughing filters provides two benefits: 1) as the required BOD removal decreases, the
required loading rates increase rapidly, which reduces the volume of medium required and
also the cost per pound of BOD removed; 2) the units may be upset by severe shock load-
ing or toxic components in the waste and still regain original capacity rapidly. The units,
therefore, may help to protect any subsequent biological treatment.
Some of the reasons for multiple-step systems include 1) better stage construction over a
period of years, 2) protecting and improving following processes (roughing treatment), 3)
reducing dissolved solids and BOD, and 4) minimizing costs of processes designed for nitrifi-
cation. Multiple trickling filters can obtain better quality and more consistent effluent with
less medium than required for a single filter plant (4).
Several possible process combinations are illustrated in Figure 9-1. Within these basic com-
binations the types and numbers of units can vary. Figure 9-1 has been included as a guide
to selecting a process. Final selection of process combinations and types of units will depend
on the required treatment, local economics, and specific loading conditions of each situa-
tion.
Selecting the classification for a trickling filter in these combinations will depend heavily on
the capital and operating costs, and on the type and degree of treatment required. Factors
important in this selection will be discussed in the following sections. In general, as loading
rates are decreased, the volume of medium required increases and the amount of recircula-
tion and the flexibility decrease.
Design of other components, such as aeration tanks, sedimentation tanks, fine screening
devices, and effluent polishing methods, are discussed in other chapters. The use of fine
screening in place of primary settling (as indicated in Figure 9-1) is discussed in sections
9.1.5 and 9.1.7. Recirculation arrangements are discussed in section 9.1.5.
Flexibility can also be provided to allow operation over a wide range of conditions. The
recirculation system can be designed to provide a minimum wetting rate at low flows, opera-
tion of filters in series or parallel, and internal recirculation during no-flow periods.
Combination A in Figure 9-1 is a simple multiple-step system, which is excellent for use in
small plants. The system can be automated to function with a minimum of operation man-
power time, using simple equipment that requires little maintenance. Combination B can
provide a higher degree of treatment than A, including nitrification, increased BODs and SS
removal, and possibly denitrification, depending on the method of effluent polishing.
9-4
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PRIMARY SETTLING OR ,NTERMEDIATE-
FINE SCREENING * RAT"
HIGH-RATE OR
SUPER-RATE
LOW-RATE,
INTERMEDIATE-
RATE.
HIGH-RATE OR
SUPER-RATE
FINAL SETTLING
PRIMARY SETTLING OR |NTERMED|ATE .
FINE SCREENING * RATE,
HIGH-RATE OR
SUPER- RATE
INTERMEDIATE
SETTLING
LOW-RATE,
INTERMEDIATE-
RATE .
HIGH-RATE OR
SUPER-RATE
EFFLUENT
POLISHING
PRIMARY SETTLING OR
FINE SCREENING M
INTERMEDIATE-
RATE,
HIGH- RATE OR
SUPER-RATE
AERATION
TANK
FINAL SETTLING
PRIMARY SETTLING,
COMMINUTION OR
FINE SCREENING *
AERATION
TANK
INTERMEDIATE
SETTLING
LOW-RATE,
INTERMEDIATE-
RATE OR
HIGH- RATE
EFFLUENT
POLISHING
NOTE:
LISTS BELOW EACH UNIT INDICATE TYPES OF
UNITS WHICH CAN BE USED. NOT INCLUDED
BUT NECESSARY TO COMPLETE PROCESSES'
COARSE SCREENING, GRIT REMOVAL AND
DISINFECTION.
* FINE SCREENING MAY REPLACE PRIMARY SETTLING
BEFORE TRICKLING FILTER DEPENDING ON WASTE-
WATER AND MEDIA USED (SEE TEXT)
FIGURE 9-1
MULTIPLE-STEP SYSTEMS USING TRICKLING FILTERS
9-5
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Combination C, using a trickling filter as a roughing filter, can be employed if shock loads of
high strength waste or toxic materials are likely to occur. This system would require more
operation and maintenance than Combination A, to provide reliable consistent treatment
over the loading range. Nitrification also can be provided with Combination C, by sizing the
trickling filter as a first step unit to remove BODS to about 50 mg/1, and by designing the
activated sludge process to provide the nitrification as well as BODs and SS polishing.
Combination D is similar to combination B but adds the control flexibility of an activated
sludge process. This process allows control of the first step, which could improve operating
reliability of the second step, but requires more operation and maintenance.
Combinations B and D provide a nitrifying filter, followed by effluent polishing. Because
sludge production from a nitrifying filter is small, the need for secondary clarification is
marginal. Some type of effluent polishing, however, should be provided to catch any solids
that may be produced.
9.1 .5 Basic Design Concepts
Although the design of a trickling filter appears simple, there are a number of variables that
affect performance. Some of these variables have been studied, and definite patterns have
been established. However, conclusions are often difficult, because of the number of param-
eters involved, the range of variation of each parameter, and the number of combinations
used. The major parameters affecting performance include the following:
1 . Wastewater characteristics
2. Media type
3. Pretreatment
4. Hydraulic and organic loading
5. Recirculation
6. Depth of filter bed
7. Ventilation
8. Temperature
In an operating filter, these factors interact. These interactions and the variables within each
factor are discussed below.
9.1.5.1 Wastewater Characteristics
Treatability of a waste is dependent on dissolved BOD, presence of essential nutrients, pH,
and toxicity. During the trickling filter process, the BOD removal from a domestic waste-
water that is low in dissolved BOD will exceed the removal from an industrial wastewater
with a high percentage of dissolved BOD.
9-6
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Wastewater treatment in trickling filters, as in any biological treatment process, requires
nutrients and trace elements in the wastewater for proper operation. (Domestic wastewater
normally will contain a sufficient amount of these nutrients and trace elements.) If a
deficiency occurs, the growth of organisms stabilizing the organic matter can be reduced,
allowing filamentous forms to develop. Fixed growth reactors, such as trickling filters (or
rotating disk units), will usually have less trouble from filamentous biota than will activated
sludge types of treatment. Caution must be exercised in using trickling filters, because, in
some cases, additional growth can cause clogging of the filter.
If the possibility of a shock load of toxic waste exists, a trickling filter can be used to
protect subsequent processes. A surge of shock load will upset a trickling filter, but because
of the basic process and operating characteristics, recovery is much more rapid (without
requiring changes in operation) than it is in other systems.
9.1.5.2 Media Types
Materials used as trickling filter media include crushed traprock, granite, limestone, hard
coal, coke, cinders, blast-furnace slag, wood (resistant to rotting), ceramic material, and
plastics. Redwood (Figure 9-2a) is available in racks approximately 4 ft (1.2 m) by 3 ft
(0.9 m), fabricated of lath spaced 0.7 in (17.8 mm) apart on a horizontal plane. The racks
are stacked vertically, 2 in. (5 cm) apart. Plastic media are available from several manufac-
turers in two major types: bulk-packed (Figure 9-2c), consisting of small plastic shapes
similar to short pieces of tubing with internal fins, and modular (Figure 9-2b), consisting of
corrugated plastic sheets welded together to form units approximately 2 ft (0.6 m) by 2 ft
(0.6 m) by 4 ft (1.2 m). Some comparative physical properties of trickling filter media are
included in Table 9-2.
Medium selection depends on a number of interrelated considerations described below.
1. Specific Surface Area. This is the amount of medium surface per unit volume
available for biological growth. Greater surface area permits a larger mass of
biological slime within the filter and a higher organic loading rate per unit volume.
2. Void Space. This is air space within the medium through which wastewater and
air pass, thereby coming in contact with the fixed slime growths. For rock or slag
medium, a decrease in size will tend to increase specific surface area and decrease
void space. The higher the organic loading rate, the more air space per unit volume
is required.
3. Unit Weight. Media with high unit weight, requiring heavier bases and walls, may
limit the configuration of a filter, affecting installation cost.
4. Media Configuration. Randomly packed media such as granite, blast furnace slag,
and bulk-packing plastic are able to disperse the hydraulic loading rapidly, before
great penetration occurs. Rapid dispersion allows this type of media to be used at
low hydraulic loadings and shallow depths. Modular-type plastic media do not
provide the same dispersion; therefore, the required depth is greater, permitting
greater organic loading rates. The wooden-rack media are designed to provide
9-7
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£
°> £
u/ ^
or -y
O
uT
Q:
-------
TABLE 9-2
COMPARATIVE PHYSICAL PROPERTIES OF TRICKLING FILTER MEDIA (5)
Specific
Nominal Unit Surface Void
Medium Size Weight Area Space
in. lb/ft3 ft2/ft3 percentage
Plastic (Bulk Packing) - 4 35 to 60 93 to 96
Plastic (Modular) 20 by 48 2 to 6 25 to 30 94 to 97
Del-Pak Redwood 48 by 35 10.3 14
Granite 1 to 3 90 19 46
Granite 4 67 13 60
Blast Furnace Slag 2 to 3 68 20 49
dispersion similar to that obtained in randomly placed bulk-packing materials and
can, therefore, be used in shallow filters.
Corrugations, fins, and other irregularities, which are provided in manufactured
media and present in natural media (such as rock, slag, etc.) are valuable in
improving oxygen transfer to the biomass (i.e., they cause turbulence in the
wastewater passing through the media in thin layers).
5. Media Material and Size. The most common materials used for randomly packed
media are crushed slag, stone, gravel, and uncrushable gravel. These materials
should be sound, durable, nearly equidimensional, and resistant to freezing and
thawing, as determined by the sodium soundness test. Normal size range for
these materials is between 1 and 3.5 in. (25 to 90 mm). Uniformity is important,
to insure adequate pore space. Gradation is normally restricted to 1 in. (25 mm)
or 1.5 in (40 mm) between upper and lower sizes; e.g., 2.5 to 3.5 in. (65 to 90
mm) or 1.5 to 3 in. (40 to 75 mm). For more details on randomly packed media,
see references (6) and (7).
Plastic trickling filter media are normally constructed of polyvinyl chloride
(PVC) for modular media, or polyethylene (PE) for bulk-packing media. PVC and
PE are plastics that are highly resistant to chemical or biological degradation.
Although there are some chemicals that will affect the physical properties of these
plastics, there is little chance that any of these materials would be present in
municipal wastewater in sufficient concentrations to have a noticeable effect on
them at temperatures below 140° F (60° C). PVC is used for modular-type media,
because of its rigidity and because it can be made in thin sheets or foils. PE can
be formed in the shapes used in bulk-packing media but lacks the rigidity required
9-9
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for modular media. Other factors influential in selecting these plastics are low cost
and reduced flammability. Care must be exercised in their design and specifica-
tions, to prevent damage from exposure to the sun.
6. Availability. The relative availability of media may have a major effect on the
filter design. In many areas of the country, sound durable rock is unavailable, and
a compromise may be required.
7. Cost. The cost of trickling filters primarily depends on the medium used. Care
must be taken to compare medium cost with the work the medium will accom-
plish and with the effect it will have on both the construction required and the
operation of the unit.
9.1.5.3 Pretreatment
Pretreatment may refer to the use of trickling filters for treatment of wastewater before
discharge to a municipal system, or it may refer to the preceding treatment process.
Use of trickling filters prior to discharge of wastewater into a municipal system can be
implemented by industries to meet sewer ordinance rquirements, reduce sewer charges based
on strength, and treat difficult wastes. Design criteria for using trickling filters for pre-
treatment of industrial wastes are, however, beyond the scope of this manual.
Pretreatment, provided ahead of trickling filters in a small plant, might include the
following:
1. Chemical treatment, to remove or control heavy metals or toxic substances or
phosphorus
2. Neutralization, to keep the pH in the proper range for biological activity
3. Pretreatment with chlorine or hydrogen peroxide, to control septicity and odor
4. Preaeration, to control odor and septicity; increase BODs and SS removal and
efficiency in primary sedimentation; and aid in grease removal
5. Equalization, to reduce variations in flow or characteristics of wastewater
6. Bar racks, grit removal, comminution, fine screening, or primary sedimentation,
to reduce organic loadings and solids, which may clog the trickling filter nozzle,
the media, or the underdrains
The effects of pretreatment should be apparent and require no further discussion. When and
how these steps are used are covered in more detail in other chapters of this manual.
Primary sedimentation deserves further consideration. In the past, it was necessary to
precede trickling filters by primary sedimentation, because of clogging problems. However,
studies have shown that degritted, comminuted wastewater can be applied directly to
modular-type plastic medium filters, without primary clarification. Tests have shown that
solids in trickling filter effluent settled better than those in comminuted wastewater (8)
(9). Fine screening has been found suitable for replacing grit removal, comminution, and
9-10
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primary settling, if waste characteristics permit. Fine screening is discussed further in Section
9.1.8.
9.1.5.4 Hydraulic and Organic Loading
Hydraulic loading is the total volume of wastewater, including recirculation, applied to a
filter per day, per unit surface area. Ranges of hydraulic loading for the various filter classi-
fications have been included in Table 9-1. In the past, both hydraulic and organic loading
were related to the performance of trickling filters. Various design formulas related to the
performance of filters have been developed and used with varying degrees of success. The
effect of hydraulic loading on filter performance is closely related to dispersion of flow
within the medium and to contact time, which are dependent on depth, specific surface, and
configuration of the medium. Hydraulic loading variations, in combination with other
factors, may have caused some of the variations in design formulas. In rock trickling filters,
the limited range of sizes and configurations, the use of depths within a small range, and
the relatively small variations in organic concentration for domestic waste, tend to keep
variations low. With the development of new media and the use of greater depths, hydraulic
loading becomes more important.
In bulk-type media, such as rock or loose plastic packing, the dispersion within the medium
is good and will occur at shallow depths; therefore, low hydraulic loadings can be applied.
Contact time within a filter with a bulk medium is high, because this dispersion causes
complete wetting of the surface available and the medium configuration tends to slow
passage through the filter. In modular media, because of the configuration, dispersion is
poor, and flow through the medium is rapid. Because of poor dispersion, modular media
require a higher hydraulic loading and greater depths to allow uniform wetting of the
surface. Several manufacturers require a minimum of 0.5 gpm/ft2 (29.3 m3/m2 -d), with the
normal range between 0.5 and 1.5 gpm/ft2 (29 to 88 m3/m2-d). Flows as high as 6 to 8
gpm/ft2 (352 to 469 m3/m2 -d) have been used, but normally, rates above 3.5 gpm/ft2 (205
m3/m2-d) are not recommended (10). The effects of hydraulic loading of modular-type
media on BOD removal and pounds of BOD removed are shown in Figures 9-3 and 9-4 (11).
Depending on the type of distribution system and flow conditions, the application rate to
the filter may be continuous, intermittent, or varying. A trickling filter requires flow to
keep biological growth moist, but rest periods of short duration can help control filter flies.
In high-rate and super-rate filters, flies are not a problem, because high flows provide con-
tinuous flushing of the zoogleal films, keeping them thin and highly active. Methods of
applying hydraulic load will be discussed further in section 9.1.10.
Organic loading is the amount of soluble organic material to be treated by the filter per day,
per cubic unit of filter. If flow is recycled, part of the organics not removed in the filter are
placed back on the medium, adding to the organic load, to make up the applied organic
loading. Some investigators have omitted recycled organics; others have included them in
the organic loading rate.
9-11
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90
UJ
Q
UJ
70 —
ui
(T
00
60-
50
DOMESTIC WASTE WITH
200-400 ing/I BOD5
WATER FILM
TRANSPORT
LIMITING
INCOMPLETE—I
WETTING '
I I
0.5 1.0
HYDRAULIC LOAD Q, gpm/ft2
1.5
FIGURE 9-3
PERCENT BOD REMOVED vs HYDRAULIC LOAD (11)
MODULAR TYPE MEDIA
I48
U-
O
O
o
\
-------
9.1.5.5 Recirculation
Recirculation returns a portion of the trickling filter effluent to the filters. This recircula-
tion may include clarified liquid, settled sludge, or both. The amount of recycling is
normally expressed as the recirculation ratio: the ratio of recirculated flow to incoming
flow. The selection of what is recycled, amount of recycle, and arrangement of recycle flow
will depend on economics and the designer's judgement as to which benefits are most
desirable. To aid in discussing recirculation, possible flow diagrams for small trickling filter
plants are shown in Figure 9-5. These diagrams and the benefits of recirculation for use in
small plants are discussed below.
Originally, trickling filters were low-rate units which did not use recirculation. As the
trickling filter process was developed, recirculation was found to increase the removal of
BOD and was thus provided in the design of intermediate and high-rate units. If the
hydraulic loading reached those of the super-rate units, it was found that a further increase
in loading by recirculation was not beneficial. Recirculation is provided for many super-
rate units but is used primarily for maintenance of a minimum wetting rate.
Recirculation has provided many benefits in the use and operation of trickling filters,
depending on the class of filter and the conditions of installation. Some factors requiring
consideration include the following:
1. Dampening of variations in loading. If recirculated flow passes through a settling
tank, the variations of daily loadings can be somewhat dampened. The major
problem with recycling through settling tanks is that the tanks must be designed
and constructed to accommodate the increased flows. Consequently, the initial
cost is increased.
2. Maintaining minimum flow rates. In many small plants, the flow rates approach
zero; in some cases, incoming flow stops for short periods at night. Recirculation
will allow the unit to continue operation without starting and stopping the rotary
distributor (requiring a dosing system) and will provide continuous moisture to
keep the zoogleal film active. In super-rate filters, recycling is used to maintain
the minimum wetting rate required for uniform wetting of the medium surface.
3. Reducing sludge handling equipment. If sludge from a final sedimentation tank is
recirculated to the primary tank, requirements for sludge handling are reduced.
4. Dilution of waste-water characteristics. Recycle of effluent tends to buffer pH and
to dilute strong or toxic waste.
5. Increasing, contact of organics with active microbes. Wastewater is continuously
inoculated with active biological material. Increased contact time of organics with
organisms will improve removal of dissolved and suspended solids by biofloccula-
tion and will reduce sludge volume (by aerobic digestion within a tower trickling
filter). Removal of soluble organics will also increase. Reaeration will occur in the
filters, keeping organisms more active. These factors increase the amount of BOD
removed per unit volume of medium, thus reducing the size of filter required.
9-13
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(a)
(b)
(c )
(e
1st
2nd
RS
RS ;
RS
( f )
R-t- RS
R + RS
LEGEND
R RECIRCULATION
RS RETURN SLUDGE
SEDIMENTATION TANK
TRICKLING FILTERS
FIGURE 9-5
FLOW DIAGRAMS FOR SMALL
TRICKLING FILTER PLANTS
9-14
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6. Maintaining biological film. Sludge recycling may help sustain a viable bacterial
colony on media under shock loads or periods of no organic flow. If sludge is
recycled from a clarifier to its preceding filter in a multiple-step system, colonies
can be kept separate.
7. Reducing tendency to clog. Recycling can increase the hydraulic loading and keep
the flow continuous, resulting in a more uniform and continuous sloughing of
solids.
8. Minimizing fly problems. Increased flow will wash fly eggs and larvae through the
filter.
9. Reducing odor. Recirculation will provide reaeration, which will help keep the
filter aerobic and raise the dissolved oxygen (DO) concentration in the filter
influent.
Many recycle arrangements have been used in small trickling filter plants. In the design of
a small plant, the flow diagram providing the simplest and most economical process possible
should be used, if the required treatment can be obtained. Recirculation arrangements
shown in Figure 9-5 can be used in the suggested systems in Figure 9-1.
Recirculation arrangements (a), (b), and (c) (Figure 9-5) could be used for System A (Figure
9-1). Arrangement (a) is the simplest, providing recycle back to the head of the first
trickling filter, without involving a sedimentation tank. This arrangement provides minimal
flexibility and may not be economical if the range of loadings is wide.
Arrangement (b) does not involve sedimentation tanks with the recycle flow and provides
extreme operational flexibility over varying loading conditions. Pump, valves, and other
control devices can be provided to operate the trickling filters in series or parallel and to
recycle flow around each filter. Additional flexibility can also be provided to allow alterna-
tion of the filter positions when operating in series. This diagram shows how flexibility and
reliability can be provided, depending on the economics of each condition. In any cost-
effectiveness analyses, flexibility must be carefully evaluated and provision should be made
for as much flexibility as possible.
Arrangement (c) is similar to (b) and provides the same flexibility. The recirculation passes
through the sedimentation tank, requiring the tanks to be sized for process flow plus
recycle. This arrangement would increase the cost of the tanks but may be justified if
shock organic or toxic loadings may occur.
Diagrams (d), (e), (0, and (g) show flow arrangements that could be used in Systems B, C,
and D (Figure 9-1). Arrangement (d) provides direct recirculation (similar to (a) and b)) and
can be used for a unit in any position of Systems B, C, or D (Figure 9-1). Direct recycle
would be simple to provide, and therefore, cost effective, unless dampening of loads is
required. Gulp (12) compared the performance of arrangements (d) and (e) and concluded
that the effluent quality of (d) was at least comparable with that of (e). Gulp also found
that there was less tendency toward filter ponding with direct recirculation.
9-15
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Arrangement (e) could be used in System B (Figure 9-1) for the first-step filter. Systems B,
C, and D could use arrangement for first and/or second steps. Arrangement (g) could be
used in System B for the first-step units, combining sludge return to the primary
sedimentation tank with recycle.
The arrangements discussed are suggested for use in designing small plants. Many possible
arrangements (not all included here) can be used to provide control, flexibility, and
reliability. Final selection will depend on the considerations mentioned above.
The amount of recycled flow can be more important than the arrangement, and a number of
pumping combinations have been used to provide the required amount. The recirculation
ratio is generally kept between 0.5 and 4.0, although ratios of 10 and above have been used
(2). Caller and Gotaas have determined that ratios greater than 4 are uneconomical and do
not increase efficiency (13).
Depending on individual requirements, various pumping arrangements have been used to
provide the amount of recirculation required. Pumping can be provided to recirculate:
1. At low flows
2. At a constant rate at all times
3. At a constant percentage of wastewater flow
4. At a rate inversely proportional to wastewater flow
5. At several constant rates
The control of pumping can be automatic (according to preset values) or manual (by the
plant operator). Automation will depend on plant size, staffing, etc.
9.1.5.6 Filter Depth
The depth of trickling filter media is important in design, because of the relations of depth
to contact time, flow distribution, ventilation, and loading (both hydraulic and organic).
With some deep low-rate filters, a degree of nitrification has occurred as an added benefit
to the treatment.
The relative importance of depth depends on the type of filter. If bulk-type media are used,
depth of 6 to 10 ft (1.8 to 3.0 m) for low rate and 3 to 6 ft (0.9 to 1.8 m) for intermediate
and high rate are adequate. The selection of depths within these ranges can be based on
balancing with the area, to keep the volume constant.
If modular-type media are used, depth becomes a major design factor. Most plastic media
manufacturers have maximum and minimum depth recommendations. A minimum of 10
ft (3.0 m) is required, to insure a reasonable detention time and allow for the dispersion of
wastewater within the media. Depths approaching the upper limit of 40 ft (12.2 m) are
usually controlled by the cost of pumping and wall construction.
9-16
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9.1.5.7 Ventilation
As an aerobic biological process, a trickling filter system requires air to operate, making
ventilation an important design consideration. Ventilation provides oxygen for the aerobic
organisms and purges the system of waste gases.
Natural ventilation in a trickling filter is caused by the difference in temperature between
the ambient air and the wastewater. Heat exchange between the wastewater and the air
within the medium changes the air temperature, resulting in a density change which sets
up a convection current within the filter. During warm weather, the air is cooled by the
colder wastewater, raising the density and thereby causing a downward flow of air. During
cold weather, when the wastewater is warmer than the air, air flow is upward.
In designing a unit, natural ventilation can be achieved by (14):
1. Designing underdrains and channels to flow no more than half full at maximum
day flow
2. Providing ventilation manholes at both ends of the main collection channels
3. Providing an open area of slots in underdrain blocks not less than 15 percent of
the filter area
4. Providing 1 ft2 (0.1 m2) or more of open ventilation, including manholes or vent
stacks, for each 250 ft2 (23 m2) of filter area
Some filters that are extremely deep or heavily overloaded will require forced ventilation.
In such cases, a system using reversible fans will provide ventilation, supplementing any
available natural ventilation. The design of such a system should provide 1 cfm/ft2 (0.3 m3/
m2 -min) of filter area (14).
If trickling filters are operated in extremely low air temperatures, the airflow may have to
be controlled to prevent freezing of the unit. The airflow required by the filter could be
reduced to about 0.1 cfm/ft2 (0.03 m3/m2 -min) of filter area. This condition is very impor-
tant in modular-type, plastic medium towers, which are extremely open and allow high
airflow conditions within the unit.
Manufacturers of modular-plastic media have different recommendations on the amount of
ventilation area required. These recommendations include 5 to 10 percent of the tower
surface area, 1 to 2 ft2/!,000 ft3 (0.3 to 0.6 m2/100 m3) or 1 ft2 (0.1 m2) for each 10 to
15 lin ft (3.1 to 4.6 m) of filter perimeter. These figures are useful as guides to the amount
of ventilation required. The actual design should provide adequate ventilation area, which
can be adjustable for use as required.
9.1.5.8 Temperature
Low temperatures will affect any biological wastewater treatment process. The effects may
be physical (freezing), biochemical (slowing reactions), or biological (lowering biological
activity). The trickling filter process may be subjected to all of these conditions. Reduced
9-17
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wastewater temperature will slow down biological activity, reduce settleability because of
a change! in viscosity of the wastewater, and reduce the gas transfer efficiency. Low ambient
air temperature will tend to lower wastewater temperature and can cause ice formation on
the units.
A study of trickling filter plants in Michigan (15) compared the efficiencies of the units at
mean air temperatures of 67° to 73° F (19 to 23° C) and 25° to 31° F (-3 to -1° C), for a
period of 3 years. Some conclusions are:
1. There is a significant difference in efficiencies between summer and winter
months.
2. Recirculation of wastewater has a marked cooling effect during winter operation,
with a decrease in efficiency.
3. Lower air temperature has more effect on plants that recirculate than those that
do not.
4. In plants that recirculate, the efficiency'changed 21 percent between winter and
summer operation.
5. Filter efficiency was affected by reduced natural ventilation in plants not recir-
culating, if the air and wastewater temperatures were the same.
A study of cold weather operation of modular-type plastic media in Canada (8) concluded
that:
1. Although ambient temperatures varying from -12.1 to 86.9° F (-24.5 to 30.5° C)
were encountered, a variation of only 52.7° F (11.5° C) was observed in the
influent wastewater temperature.
2. Operation of a full-scale, modular-type trickling filter should not present any
special problems over and above those normally encountered in any biological
waste treatment process.
3. A modular-type plastic packed trickling filter can be shut down for several days
during subzero weather; within 24 hours of startup, the reactivated biota should
attain a level of efficiency greater than 90 percent of their original value.
4. Heat loss at the fixed film and liquid interface was negligible in once-through
applications. Cooling did occur after discharge from the column of packing. Thus,
if recycle is provided, drastic decreases in tower influent temperatures may result.
If units are designed for use in cold areas, a number of special considerations are required,
particularly with regard to the drop in filter efficiency during the winter period. In some
trickling filter systems, the change in efficiency may be conpensated for by the following:
1. Small filters may be enclosed in or placed next to a structure in which heat is
available, to prevent problems during extremely cold periods.
2. Recirculation can be provided with controls that would allow reduction or shut-
off during cold weather.
3. Continuous flow systems with fixed nozzles can be designed to reduce icing
problems.
9-18
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4. Clearance can be provided between the rotary distribution arms and filter walls
and medium, to reduce the chance of stoppage caused by ice buildup.
5. Drains can be provided to allow draining of the distribution system, when it is
shut down in cold weather.
6. Filters can be placed in areas protected from winds with high sidewalls providing
wind protection.
7. If multiple units are used in parallel, control can be provided to shut down some
units (which can be restarted quickly) during cold periods, providing greater flows
for units that would continue to operate.
8. Covers can be provided to protect the filters from wind and to control ventilation.
9. Ventilation ports can be provided with controls to regulate air flow during cold
periods.
10. Filters can be designed to compensate to some extent for the effect of
temperature.
The effect of temperature on efficiency of a filter can be estimated by the following relation
(5) (16):
F _p fl (T-20)
bT - b200 '
where
Ef = filter efficiency at temperature T
£20 = filter efficiency at 20° C
T = wastewater temperature, °C
0 = constant varying from 1.035 to 1.041
9.1.5.9 Miscellaneous Concepts
In addition to the factors affecting performance discussed previously, factors affecting over-
all design and process selection are listed as follows:
1. Noise. Trickling filters operate with little noise, because compressors or aerators
are not used.
2. Antifoam Requirements. Antifoam spray systems normally are not required with-
in the trickling filter process.
3. Plant Odors. Due to inadequate design and operation, aerosols and odors may be
generated from a trickling filter. Anaerobic primary clarifier effluent, inadequate
ventilation, drainage, or excessive organic loading can lead to anaerobic conditions
and cause odor pollution. The odors or aerosols may become windborne from
distributor arm discharge.
9.1.6 Design Formulas and Criteria
Although trickling filters are relatively simple to operate and maintain, sizing of the units
(medium, volume, and depth) is often difficult. There have been many attempts to develop
9-19
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methods for sizing these units, with varying degrees of success. The difficulties are the
number of variables to consider, the amount of variation in each parameter, and the inter-
action of each. Analyses of operating data have been used to establish equations and curves
that best fit the available data. Results of these analyses have led to the development of the
following formulas or design methods:
1. Ten States Standards
2. National Research Council formula
3. Velz
4. Schulze
5. Echkenfelder
6. Galler-Gotaas
These formulas are presented and discussed in several other publications in detail (5) (17)
(18) (19) (20) (21) (22) (23); therefore, a duplicate discussion will not be presented here.
Although these formulas represent attempts to include many of the variables that can affect
trickling filter operations, the use of any one of these formulas does not universally reflect
the actual performance of filters.
In using these formulas, the engineer should take care to use the one most suited to the
specific design conditions. None of them is generally applicable to all conditions. Figure 9-6
is intended to provide a guide for selecting the proper formula. Some of the formulas have
been developed for specific conditions (e.g., the Schulze formula for synthetic media with-
out recirculation). Other formulas may be more generally applicable but may not have been
developed sufficiently in some of the areas. For these formulas, only the most suitable uses
are indicated on the chart.
Design results using different formulas for the same conditions are summarized in Table 9-3.
The summary shows the wide variation in volume found, using these formulas for equal
design conditions.
The examples used for the table were not for optimum designs. Using an iterative process,
varying some design parameters, an optimum volume and configuration can be established.
Within this iterative process, care must be taken to include economics in selecting the best
design. It will be up to the designer to make the final decision on volume and configura-
tion. Wherever possible, flexibility in parameters such as recirculation or ventilation should
be incorporated in the design, to insure the capability of the unit to operate adequately
under actual conditions.
9-20
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DOMESTIC WASTEWATER
INDUSTRIAL WASTEWATER
TEN STATES STANDARDS r — — J ROCK MEDIA
NATIONAL RESEARCH
COUNCIL FORMULA
ECKENFELDER FORMULA
GALLER-GOTAAS FORMULA
V J VELZ FORMULA
SCHULZE FORMULA
SYNTHETIC MEDIA
WITH RECIRCULATION
WITHOUT RECIRCULATION
FIGURE 9-6
APPLICABILITY OF TRICKLING FILTER FORMULAS
-------
TABLE 9-3
SUMMARY OF EXAMPLE DESIGNS
Conventional NRC Schulze Eckenfelder Galler-Gotaas
Design One Step Two Step (Plastic Media) One Step Two Step One Step Two Step
BOD Removed, Percent
First Step
BOD Removed, Percent
Second Step
BOD Removed, Percent
Total
Recirculation Ratio,
First Step
Recirculation Ratio,
Second Step
Filter Depth, ft
Volume, ft3
One Step
Volume, ft3
First of Two Steps
Volume, ft3
Second of Two Steps
Volume, ft3
Total of Two Steps
83 83 60 83 83 60 83
57.5 - - 57.5
83 83 83 83 83 83 83
2.26 1.0 1.0 0.0 (Design) 1.0 1.0 1.0
2.0 (At Min.)
1.0 - - 1.0
5 - - 18.5 6.0 6.0 6.0
8,090 26,500 - 7,550 11,500 - 48,000
2,510 - - 1,080
5,100 - - 5,500
8,090 26,500 7,610 7,550 11,500 6,580 48,000
60
57.5
83
1.0
1.0
6.0
_
3,168
5,000
8,168
Notes: 1. All filters of rock medium, except Schulze.
2. Wastewater flow, 0.5 mgd.
3. Primary effluent BOD, 140 mg/1.
4. Final effluent BOD, 24 mg/1.
-------
9. 1 .7 Example Designs
9.1.7.1 Site and Wastewater Characteristics
Principally domestic wastewater.
Land available for purchase as required.
Site is flat, located next to a large river.
Influent BOD5 , mg/1 200
Influent SS, mg/1 250
Influent NH3-N, mg/1 20
Flow, gal/cap/day 100
Peak to average ratio 4:1
Average to minimum ratio 4 : 1
Population 2,000
Minimum wastewater temperature, 45° F (7° C)
Minimum air temperature, 14° F (-10°C)
pH range 7.2 to 7.8
Hexane Solubles, mg/1 75
Effluent BOD , mg/ 1 < 30
Effluent SS, mg/1 <30
9. 1 .7.2 Design of Rock Trickling Filter Plant
Additional Design Conditions
1. Plant reliability, Class II [from EPA reliability guidelines (3)]
2. Pretreatment: coarse screening, grit removal, and primary sedimentation (30 percent
removal)
A minimum of two trickling filters must be provided, designed so that, with one unit out of
operation, the average daily flow can be handled by the remaining unit.
Sizing of Filter
Use 1 : 1 recirculation ratio to allow flexibility and control of filter operation. For a rock
medium filter, the NRC formula can be used. The basic NRC formula for the design of
single step trickling filters is:
W I 0-5
1+K — '
VF
9-23
-------
where
E = fraction of BODs removed in a single filter
W = organic load influent to the filter (lb/dayBOD5) = (8.34) QLj
V = filter volume in 1,000 ft3 or acre-ft
K = constant 0.0561 for V in 1,000 ft3 and 0.0085 for V in acre-ft
1 +r
F = recirculation factor =r~rm — n\ o
[i + (i — V)T\*
P = fraction of BOD5 available (0.85 < P < 0.95); generally, use P = 0.90
Q = flow rate (mgd)
L- = filter influent BOD5 (mg/1)
r = recirculation ratio, or ratio of recycled flow (R), to total plant influent
flow (Q)
Rearranging terms and solving for rock medium volume in 1 ,000 ft3 for the first step filter,
the NRC formula becomes:
v. 0.263 QL,
1-1-1
L1 ~ E J
BOD5 in primary effluent = (200) (1 - 0.3) = 140 mg/1.
Efficiency of a single filter (E)
140-30
E= -0.79
140
Volume using NRC formula
V = (0.0263)(0.2 mgd)(140 mg/1)
0.79
V = (0.0263)(0.2)(140)(0.605)(14.15)
V = 6.30 or 6,300 ft3
Given a depth of 6 ft (1.8 m), determine area and loading rates.
6,300 -
Area= 1,050 ft2
6
9-24
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^1 4A (4X1.050)
D*. = =
7T 7T
D = 36.56 ft
A 40-ft-diameter (12-m) trickling filter would provide additional area and volume for clear-
ance at end of distributor arms and area at center column.
Actual area of such a filter:
40-ft diameter (overall) 1,257 ft2
Assume 9-in. arm clearance
(periphery loss) -93 ft2
and 3-ft diameter for center
column (center loss) - 7 ft2
Actual area 1,157ft2
Check hydraulic loading.
Each filter designed for 0.2 mgd plus 0.2 mgd recycle or 0.4 mgd.
(0.4X43,560) , r
Hydraulic loading = — = 1 5 mgad
(I , i D / )
Check organic loading
Actual volume = (1,157)(6) = 6,942 ft3
Organic loading -°™°* - 33. 16/1,000 &
Both loading rates are within the high-rate filter ranges (Table 9-1).
To allow operation between a maximum flow of four times the average (0.2 mgd) and a
minimum flow of one-fourth the average, the plant must be designed with great flexibility.
One method of design is to operate two filters in series, up to 1.5 times the average flow of
0.3 mgd, with 0.2 mgd recycle (total 0.5 mgd). Operation in a series maintains the biological
slime in both filters during low flow conditions. If high flow occurs, the units should be
switched to parallel operation. Each unit would be able to handle 0.5 mgd, which exceeds
four times the average or 0.8 mgd maximum flow. At extreme maximum flow, the recycle
would be reduced to zero, with 0.4 mgd to each filter. Operation with recycle and in series
will result in higher costs, but will provide better than average treatment up to two times
the average flow. Above this amount, the treatment will be reduced.
9-25
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If the two units operate in series up to four times the design flow with 0.2 mgd recycle,
maximum flow through each unit would be:
(4)(0.2) + 0.2 = 1.0 mgd or 694 gpm
Minimum flow design may be controlled by minimum plant flow, minimum required flow
for reaction distributor, or minimum flow plus recycle. At low plant flow, recycle may not
be required, except to maintain a minimum wetting rate or rotation of the distributor.
Final selection of maximum and minimum flows to the distributor must be based on operat-
ing conditions and the limits of rotary distributor design.
Minimum plant flow = 0.05 mgd or 35 gpm
With 0.2 mgd recycle = 0.25 mgd or 174 gpm
Using the available recirculation of 0.2 mgd or 138 gpm, the units can operate at minimum
flow plus recycle, requiring a rotary distributor with a 4 to 1 range (from 700 gpm
maximum to 175 gpm minimum).
Recirculation System
Recirculation for this process should be provided around each trickling filter, with a maxi-
mum flow rate of 0.2 mgd. Control of each system should be provided to operate the units
with any recycle rate at design flow and to reduce the rate as total flow reaches the capacity
of the distributor. Below design flow, control of recycle will be required to maintain
minimum flow to the distributors.
Underdrain and A ir Vent Systems
For a rock trickling filter, the use of vitrified clay underdrain blocks will provide adequate
support of the medium and adequate space for air flow. The blocks will be placed on a floor
with a pitch of 1 percent to a center collection channel.
Design the center channel for a minimum velocity of 2 fps (0.6 m/s) and to flow only half
full at 0.4 mgd (0.01 8 m3/s).
Channel cover blocks are available in lengths of 16, 18, 24, 30, 32, and 36 in. (406, 457,
610, 762, 813, and 914 mm) and require 3-in. (76-mm) bearing area on the ends.
9. 1 .7.3 Design of Plastic Media Trickling Filter Plant
With plastic media, the Schulze formula can be used. This formula is:
9-26
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where
Le = BOD5 of unsettled filter effluent, mg/1
L = BOD5 of filter influent, mg/1
fl = 1.035(T-2°)
K = constant (treatability)
D = filter depth, ft
Q = flow rate, gpm/ft2
T = temperature °C
Additional design conditions
1. Design plastic medium towers for conditions described in 9.1.7.1 and 9.1.7.2.
2. Provide nitrification during dry-weather months at maximum dry-weather flow
(assume maximum dry-weather flow is approximately equal to average daily flow).
Sizing of Filter
Determine treatability of wastewater by finding the weighted value of K, given the
load to the filter for each component, as follows:
Component
Domestic
Industry 1
Industry 2
Flow
mgd
0.15
0.04
0.01
BOD5
mg/1
140
119
228
Load
Ib/day
175
40
19
Percent
75
17
8
Totals 0.20 487 234 100
K values based on previous studies:
K at 68° F Fraction K
Domestic 0.08 (0.75) (0.08) 0.0600
Industry 1 0.06 (0.17) (0.06) 0.0102
Industry 2 0.02 (0.08) (0.02) 0.0016
0.0718
Determine required depth:
Assume n = 0.5 for medium used and Q = 0.85 gpm/ft2
8 = 1.035(7"20) =0.639
L /L = j*£ - (0.639)(0.0718) D/(0.85)°-5
e * 140
9-27
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0.214 = e -0-046D/0.92 = g -0.05D
Ln (0.214) = -0.05D
Two 16-ft towers would provide the required treatment at 7° C.
Determine surface area:
At 0.85 gpm/ft2 (note: 0.2 mgd = 138.9 gpm)
138.9 -
Area = - = 163.4ft2
0.85
If circular towers are used:
D=
Use 1 4- ft diameter towers.
Check actual surface loading.
Actual area =— — — = 153.9 ft2
4
1389
Surface loading = - - = 0.90 gpm/ft2
Check effluent quality with two towers, 16 ft deep with 0.90 pgm/ft2 surface loading
i = e -(0.639)(0.07 18)(32)/(0.90)0-5
Le =0.2 1(140) = 29.4 mg/1
Nitrification design
Check warm weather operation of towers in series.
Effluent concentration from first tower with minimum wastewater temperature of 13° C
(a normal minimum summer temperature):
e = 1.035 (13-2°) = 0.79
Le/Li - e ' (°-79) (°-0718) (16)/(0.90)°-5 = Q 3g
9-28
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Effluent BOD5 from first tower = (0.38) (140) = 54 mg/1
(16)(14)27T «
Volume of one filter = '' = 2,463 ft3
(4)
Organic loading on second filter.
(54) (8.34) (0.2)
(2.643) 5
This loading does not meet the five to eight range desired (as discussed in Section 9.19).
Try with effluent from both towers or one tower 32 ft deep with additional filter or filters
for nitrification.
^ = e -(0.79X0.0718X32)/(0.90)°-5 = Q.15
M
Effluent BOD5 = (0.15)(140) = 21 mg/1
BOD5 loading - (21) (8.34) (0.2) - 35 Ib/day
At 8 lb/1,000 ft3/day, volume = 4,375 ft3
At 14-ft diameter, depth = 28 ft
Two 14-ft diameter towers, each 30 ft deep, would provide nitrification in
warm weather and could be designed to operate in parallel during high
flow periods.
Check hydraulic loading rates.
Maximum loading = 555.6 gpm/153.9 ft2 = 3.6 gpm/ft2 (each filter)
Minimum hydraulic loading should be 0.5 gpm/ft2
Minimum flow rate = (0.5) (153.9) = 76.95 gpm
Minimum plant flow = 34.72 gpm
Therefore, 77-35 = 42 gpm recycle is required at minimum flow.
Recirculation System
Recirculation for this system could be provided by a pump arrangement that would return
50 gpm of filter effluent if plant flow dropped below 80 gpm and would stop if 80 gpm
were exceeded.
9-29
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Rotary Distributor Selection
A rotary distributor can be selected as in the previous example, using 280 gpm maximum,
80 gpm minimum, and 3.5:1 range for a 14-ft-diameter trickling filter.
9.1.8 Clarification Requirements
Design of clarifiers for a trickling filter plant requires consideration of the total combination
of unit processes, type and location of recycle flow(s), filter loading rates, and type of treat-
ment provided. Clarification would normally be provided as primary treatment,
intermediate sedimentation, or final sedimentation. In small plants, the elimination or
simplification of a process is desirable. With proper design of the complete process, some
sedimentation steps can be simplified or eliminated.
In Ohio and Maine, plants have been designed and operated using fine raw wastewater
(wedgewire) screens in place of primary sedimentation before plastic medium trickling
filters. The Ohio installation (2.3 mgd), as discussed by Wittenmyer (24) (25), found that
raw wastewater screens can replace comminutors and primary clarifiers preceding a trickling
filter and allow operation of the filter with rotary distributor, without clogging.
A 0.15-mgd (50,000 gal/8-hr day [557 m3/day]) plant in Gorham, Maine, has been
operating on wastewater from a school, using fine screens preceding a plastic medium tower
with a fixed nozzle distribution system. Clogging has occurred only about once a month,
and only in the last nozzle at the end of the distribution pipes. Clogging there can be easily
cleaned and is not considered a serious problem (26). This example illustrates that primary
sedimentation can be replaced with fine screens on small plants treating domestic waste-
water, without clogging the trickling filters.
Intermediate sedimentation has been eliminated between trickling filters or between a
trickling filter and an activated sludge process in many plants, reducing the sludge handling
steps, and simplifying the process.
In designing a system to eliminate intermediate settling, the following factors may be useful:
1. If a filter without intermediate clarification is used, the percentage BOD removal
in the filter will be 15 (difference between, not percentage difference) below that
with clarification for modular-type plastic media (10).
2. If a trickling filter is used preceding the activated sludge process without inter-
mediate settling, the humus solids would be similar to return activated sludge, and
the unsettled (or soluble) BOD would be considered as load on the activated
sludge process (10).
The design of a clarifier to remove sloughed trickling filter solids (humus or fixed film
solids) is similar to the design of raw wastewater sedimentation tanks (see Chapter 6).
Humus is generally more dense and will settle to higher solids concentration faster than
activated sludge. The amount of solids to be separated would be low compared to separation
9-30
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of the mixed liquor suspended solids; therefore, the design is controlled by hydraulic
loading, not by solids loading.
Production of humus sludge depends on waste characteristics, loading, type of medium,
and other conditions in a biofilter. An average of 20 to 30 percent of the BODs removed is
converted to sludge (domestic wastewater or carbohydrate wastes), if no distinction is made
between soluble and insoluble components (27). The amount of sludge requiring treatment
and disposal would be less for trickling filters than for activated sludge systems.
Humus sludge, which sloughs off a trickling filter, settles more readily and is more easily
dewatered than are other secondary sludges. The anaerobic action occurring at the surface
of the medium and the resultant reduction of volatiles in the growths probably account for
the increased density of sludge. A sludge concentration in the final clarifier of 3 .to 4 percent
solids can be easily obtained, compared to 0.5 to 1.5 percent for activated sludge (27)
(28).
If activated sludge bulking is a problem, as it is with a strong carbohydrate waste, trickling
filters may be a practical solution (28).
In addition to the requirements for design discussed in chapter 6, trickling filter sedimenta-
tion tanks should be designed for a hydraulic loading of 1,000 to 1,200 gpd/ft^ (40 to 48
m^/m^'d) for peak flow conditions (5). In small plants, the peak flow is very important
because of large variations in flow.
In plants receiving large amounts of hexane solubles, primary clarification with efficient
skimmers, or other grease removal devices, are required. Excessive grease can clog nozzles
and reduce filter performance by covering the biomass. Limitation on quantities are similar
to those for activated sludge processes, as discussed in Chapter 7.
9.1.9 Nitrification
Nitrification, as discussed in chapter 13, can be accomplished by biological treatment.
Trickling filters will bring about various degrees of nitrification, depending on depth, size
and type of medium, loading, liquid temperature, carbonaceous matter in wastewater,
pH, and the presence of inhibitors.
Balakrishnan and Eckenfelder (29) studied nitrification in trickling filters, using a 6-ft-deep
(1.8 m) laboratory scale unit, and correlated data from pilot plants and plants reported in
the United States and Great Britain. These studies included trickling filters with bulk-
packing media (e.g., broken rock and blast furnace slag) and some manufactured bulk media
(e.g., Raschig rings and Berl saddles).
Conclusions of the studies are:
1. Hydraulic loading significantly affects the degree of nitrification which can be
obtained. In the laboratory model, nitrification dropped from 72 percent to 52
9-31
-------
percent, when hydraulic loading increased from 10 to 30 mgad (9.35 to 28.05
m3/m2/d).
2. Above a temperature range of 59° to 77° F (15° to 30° C), temperature had a
great influence on nitrification.
3. For a given flow, specific surface, and temperature, the percentage of nitrification
increased as the filter medium depth increased.
Buddies and Richardson (30) performed a detailed research program that demonstrated the
feasibility of utilizing plastic medium (modular-type) in a step treatment system to achieve
biological nitrification. This study concluded the following:
1. Plastic medium trickling filters are capable of achieving consistent, high-level
nitrification (greater than 90 percent conversion), if operating on a low BODs
waste stream (15 to 30 mg/1) containing ammonia-nitrogen in the range of 10 to
20mg/l.
2. Increased recycle provided improved flow stabilization, but had minimal effect
on the overall degree of nitrification.
3. There appears to be a final effluent limitation for removal of ammonia-nitrogen
below the range of 1 to 2 mg/1.
4. Visible slime growth on the medium was thin, tough, and resistant to drying,
and net solids production was low. The SS and BOD5 levels in the effluent were
not significantly different from those in the influent. The tower effluent was
passed directly to a mixed-media filter, without intermediate clarification.
In addition to the above conclusions, the study has shown that, at hydraulic loading rates
between 0.5 to 2.0 gpm/ft2 (29.3 to 117.3 m3/m2 -d), organic loadings should be 5 to 8 Ib
BOD/1,000 ft3/day (0.080 to 0.128 kg/m3-d), for the modular, plastic media tested
(specific surface, about 25 to 30 ft2/ft3).
The hydraulic loading rate should be kept low, and conditions such as temperature, pH, and
the presence of toxicants should be carefully considered.
9.1.10 Equipment and Materials of Construction
Trickling filters of the rock medium type (shown in Figure 9-7) and plastic medium bio-
oxidation towers (shown in Figure 9-8) are composed of the medium, a distribution system,
an underdrain system, and walls. The equipment used for distribution and the materials
used in walls and underdrains are important in filter design. Media have been discussed in
Section 9.1.5.2.
The two types of distribution systems commonly used in the United States are rotary
distributors and fixed nozzle systems. Another type, used in Europe, is a longitudinally
traveling distributor. These systems are designed to provide uniform distribution of waste-
water over the filter surface, with continuous or intermittent dosage. The choice of system,
size, and configuration depends on available hydraulic head, variations in applied flow, filter
configuration (round, square, or hexagonal), and method of flow application.
9-32
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OJ
OJ
CONCRETE
WALL
BLOCKS
FOR DRAINAGE-
FILTER
MEDIA-
ISTRIBUTOR
ARM
SUPPORT
COLUMN
BLOCKS
FOR DRAINAGE
FIGURE 9-7
ROCK MEDIA TRICKLING FILTER
-------
A'
-------
The rotary distributor has been most popular because it is reliable, providing smooth and
uniform intermittent dosing with minimal maintenance. The distributors normally consist
of two or more arms that rotate around a center support. These arms are hollow and have
nozzles that distribute the wastewater on the filter surface. The distributor is driven by the
wastewater discharging from the nozzles on the side of the arms. If the flow rate varies
greatly or a minimum head is available, motor-driven distributors can be used. For reaction-
driven units, the speed of revolution normally varies with flow rate. The design should
provide a speed of one revolution every 10 minutes, or less for a two-arm distributor at
design flow.
The distributor arms should be designed with adequate adjustments, to set alignment and
insure proper balance. Minimum clearance between the bottom of the arms and the
medium, or end of arms and walls, should be 6 in. (150 mm). In colder regions, where icing
may be a problem, 12 in. (0.3 m) of clearance should be provided. The arm may have a
constant cross section or may be tapered to provide minimum transport velocities at low
flows. At maximum flow, the peripheral speed of the arm should not exceed 4 fps (1.2
m/s). If flow variation is great, additional arms can be provided with weir arrangements, to
allow use at higher flow conditions. The ends of the arms should be provided with quick-
opening gates for easy flushing.
Distributors can be obtained for filters of 20-ft (6 m) diameter or greater. In selecting a
distributor, a unit that can be easily maintained and that allows removal of the bearings
without complete disassembly should be provided. The bearings may be grease or oil
lubricated, with seals similar to the neoprene gasket type. Mercury seals have been banned
by the U.S. EPA, because of possible contamination of the effluent (31).
The fixed nozzle system is not normally used in rock filters designed in recent years.
However, in plastic medium towers, these systems are being increasingly utilized. The shape
of the medium lends itself to placement in rectangular units, which can then be provided
with fixed nozzle systems for continuous or intermittent dosage. Distribution piping and
nozzles can be made of many materials. Several manufacturers provide plastic nozzles,
which work well with polyvinyl chloride (PVC) pipe, will not clog easily, can be cleaned
easily when clogged, and are economical to install. This type of system can be designed
using the dispersion conduit approach, recommended by Camp and Graber (32), to provide
uniform distribution (illustrated in Chapter 3).
The underdrain system is another important part of a trickling filter. This system supports
the filter medium, provides space for passage of wastewater and sloughed solids through the
filter, enables collection of wastewater and solids, and provides space for ventilation of the
filter.
Rock trickling filters or other bulk-packaged units are normally designed using special
vitrified clay blocks with slotted tops and drain channels with curved inverts. These blocks
may be obtained from members of the Trickling Filter Institute (33).
9-35
-------
The open area of the slots in the top of the underdrain blocks should be a minimum of 15
percent of the filter area. The blocks are normally placed on a floor that is pitched at a 1
to 2 percent slope toward a main collection channel.
All channels should be provided with adequate slope to insure a minimum velocity of
2 fps (0.6 m/s) at average daily flow. The underdrain system should be designed so that no
more than 50 percent of the cross section area is submerged at maximum daily flow. The
ends of the main collection channels should be provided with open grating manholes, to
provide ventilation and access for inspection and cleaning. The overall area for the open
grating and vent stacks (placed around the filter) should provide 1 ft2/250 ft2 of filter area
(14).
Underdrains for synthetic media filter are designed like those for bulk-packed filters, with
different requirements for ventilation and amount of support. Manufacturers of these media
have recommendations for both support and amount of ventilation required (10) (11).
Some of the systems that have been used are vitrified clay blocks, corrosion-resistant
grating, or concrete beams, some examples of these systems are shown in references (10)
and (11).
Walls for trickling filters contain the medium, protect the filter from wind, and allow for
flooding the filter to eliminate problems with filter flies and ponding. Although some rock
filters have been constructed without walls (allowing the medium to slope off at its angle
of repose), normal construction consists of concrete block, vitrified clay block, or concrete.
These walls are designed to contain the medium, restrain earth loads from backfill, and
allow flooding of the filter. Rock filters are normally shallow, because of practical con-
struction limits and ventilation considerations.
The modular-type medium is usually self-supporting; therefore, walls are not required for
support. These units do not have problems with flies or clogging and hence, do not require
flooding. The main reasons for constructing walls around modular filters are protection
from wind, containment of water trickling through the medium, and general appearance of
the tower (normally constructed aboveground). Materials used for wall construction, there-
fore, are numerous. Some walls used for this type of filter have been constructed of
concrete block, poured concrete, steel plate, redwood, or structural frame and panel con-
struction. Selection of wall material and construction should depend on life of the filter,
availability of material, appearance, and economics. If sectional materials or blocks are used,
the wall should be sealed to prevent seepage of wastewater through the wall at joints.
Seepage can stain the outside of the unit and detract from its appearance in a short period
of time.
9.1.11 Northbridge, Massachusetts, Rock Trickling Filters—Case Study
Northbridge is located in central Massachusetts, about 15 miles south of the city of
Worcester. The Northbridge wastewater treatment facility is located on the Blackstone
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River, which is considered class C, based on the State of Massachusetts "Waste Quality
Standards." This classification requires that the plant produce an effluent that will not
reduce the river's dissolved oxygen (DO) below 5 mg/1; cause a pH outside a range of 6.0
to 8.5; contain chemicals harmful to fish life; or cause unreasonable sludge deposits, turbid-
ity, odors on discharge, or floating solids. The plant is also required by the EPA to provide
secondary treatment (30 mg/1 or less BODs and SS).
To meet these requirements, the plant uses comminution, primary sedimentation, high-rate
rock trickling filters, secondary sedimentation, sand filter beds (during low river flow
periods), and hypochlorination. Sludge processing equipment includes a degritter for
primary sludge, sludge holding tanks, chemical conditioning units, and vacuum filter (sludge
and grit are hauled to landfill). Table 9-4 shows design and operational data, and Figure 9-9
is a schematic flow diagram of the facility.
9.2 Rotating Biological Contactors
9.2.1 General Description
Rotating biological contactors (RBC) have been used to a great extent in Germany, France,
and Switzerland, and are now becoming popular in the United States. They have been used
for almost 20 years in Europe, in plants ranging in size to serve populations of 12,000 to
100,000, treating both domestic and industrial wastes. In the United States, the process has
been developed in package plants for flows between 10,000 and 120,000 gpd (0.44 to 5.25
m^/s x 10'3). It has also been found suitable for plants up to 0.5 mgd and may be used for
largerplants. (For further discussion of package plants see Section 8.4.) Figure 8-4 shows the
basic arrangement of the units and the general configuration that can be used for larger
installations.
9.2.2 Process Description
Basically, the process consists of a series of plastic disks mounted on a horizontal shaft and
placed in a tank with a contoured bottom. The disks rotate slowly in the wastewater, with
about 40 percent of the surface area submerged. During the rotation, the disks pick up a
thin layer of wastewater, which flows over the disk surface and absorbs oxygen from the air.
The fixed biomass film on the disk surface removes both DO and organic material from the
wastewater. As the biomass becomes submerged in the wastewater, additional organic
material is removed.
Excess micro-organisms and other solids are continuously removed, from the film fixed to
the disks, by the shearing forces created by the rotation of the disks in the wastewater. This
rotation also causes mixing, which keeps the sloughed solids in suspension so they can be
carried through each step to a final clarifier.
The rotation disk process is similar to the trickling filter process, because both use fixed
growth reactors. Some of the advantages of trickling filters also apply to the rotating disk
process. These advantages include economics, simple operation and maintenance, suitability
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TABLE 9-4
NORTHBRIDGE ROCK TRICKLING FILTERS
Data
Operation Design
1972 1990
Population Served 8,500 12,500
Flow, 24-Hour
Average, mgd 1.1 1.8
Maximum, mgd 2.4 3.8
Raw Wastewater
BOD5,mg/l 158 193
SS, mg/1 185 206
Primary Sedimentation
Overflow Rate,1 gpd/ft2 489 800
Depth at Weir, ft 10 10
Retention Time,1 hours 3.6 2.2
BODs Removal, percent 35 35
SS Removal, percent 59 60
Trickling Filters
Hydraulic Loading (With Recirculation), mgad 13 28
Organic Loading (No Recirculation),
lb/day/1,000 ft3 28.4 57.0
Recirculation Ratio 0.5 1.0
Final Sedimentation
Overflow Rate,1 gpd/ft2 489 800
Depth at Weir, ft 7.25 7.25
Retention Time,1 hours 2.9 1.8
BODs Removal, percent 82 76
(Trickling Filter And Sedimentation)
Final Effluent
BOD5,mg/l 18 30
Overall Removal, percent 89 85
SS, mg/1 14 16
Overall Removal, percent 92 92
1 Average Daily Flow
9-38
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•iiiiiiiiiiiinii in in in in in n in muni
TO LANDFILL ~^
LEGEND
WASTEWATER
DRY WEATHER TERTIARY TREATMENT
AND RECIRCULATED WASTEWATER
SCUM AND SLUDGE PIPING
HYPOCHLORITE, FERRIC CHLORIDE,
OR LIME FEED
Illiliiiiililllllliu FILTRATE
— — — — - OVERFLOW
Cj ft METERS
CX GATE VALVE
M CHECK VALVE
(~ PUMP
© COMMINUTOR
© PRIMARY SEDIMENTATION BASINS
® TRICKLING FILTERS
® CLARIFIERS
® EFFLUENT STRUCTURE
© CHLORINE CONTACT BASIN
© DOSING CHAMBER
© SAND BEDS
® SCUM BOX
® SCUM HOLDING TANK
© DEGRITTER
© SLUDGE HOLDING TANK
© CHEMICAL CONDITIONER
® VACUUM FILTER
® HIGH LEVEL WET WELL
FIGURE 9-9
NORTHBRIDGE, MASS. SCHEMATIC FLOW DIAGRAM
9-39
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for step and stage construction, resistance to organic and hydraulic shock loads, and low
process control requirements. The main disadvantage of the rotating disk process is the need
to cover the disks to prevent freezing and to protect them from wind damage or vandalism.
In the rotating disk process, the fixed biomass film passes through the wastewater, providing
contact of all organisms with the wastewater, preventing clogging of the medium, and
providing continuous wetting, which prevents development of filter flies. Different aeration
and contact time can be provided by varying the rotational speed of the disks.
9.2.3 Application of Process at Small Plants
RBC units are especially suited for use in small plants, because of the advantages mentioned
previously. They can be used in place of other biological treatment processes to meet
secondary effluent requirements. If proper conditions exist, ammonia-nitrogen removal can
be accomplished by proper sizing and arrangement of the units.
RBC units can be supplied as package plants, complete with steel tank and drive equipment,
for capacities up to 200,000 gpd (800 m^/d), depending on required surface loading rate.
If the units are combined in a package with standard clarifiers and chlorine contact tanks,
the capacity range becomes 5,000 to 90,000 gpd (20 to 360 m-^/d), depending on required
surface loading rate. Plants with greater than 90,000 gpd (360 rn^/d) flow would use
separate clarifier and chlorine contact tanks.
Disks can also be supplied on shafts, which can be mounted in concrete tanks. These units
can treat flows up to 250,000 gpd (1,000 m3/d), depending on surface loading rate. If flows
exceed 250,000 gpd (1,000 m^/d), shafts with varying disk arrangements can be combined
in series and parallel configurations, to provide the type and degree of treatment required.
There are many combinations that can be used for arrangement of multiple RBC units or
RBC units with other process equipment. Design manuals are available which show
suggested RBC combinations, including their use with primary sedimentation tanks, septic
tanks, and equalization tanks. Side-by-side and over-and-under configurations are also shown.
9.2.4 Basic Design Concepts
Much of the development of design criteria in this country has been done in Pewaukee,
Wisconsin (34) (35) (36) (37) (38). The information included in this section (taken from
these references) has been developed on domestic wastewater and is presented here as a
guide. Changes in wastewater characteristics and configuration differences of biological
contactor units may require different approaches to design. Manufacturers of RBC systems
have developed design curves and procedures for use in designing wastewater treatment
facilities. Some important considerations in the design of RBC units include:
1. Loading conditions
2. Retention time, multiple step systems, and rotational speed
3. Wastewater temperature
4. Pretreatment requirements
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5. Clarification requirements
6. Sludge handling
7. Nitrification
9.2.4.1 Loading Conditions
Hydraulic loading is considered as the primary design criterion for rotating disk systems,
stated as gpd/ft2 (m^/m^d) of disk surface area (34). This criterion is based on domestic
wastewater with a relatively small BOD$ concentration range and may not be entirely valid
for wastes with different BODs concentrations. A unit studied in Wisconsin (34) obtained
effluent BODs concentrations of 12, to 20 mg/1 for hydraulic loadings of 0.25 to 1.55
gpd/ft2 (0.01 to 0.06 m3/m2d), with temperatures between 39° and 67° F (4° and 19° C),
and disk speeds of 2 to 5 rpm. In general, as the hydraulic loading decreases, the effluent
decreases, which in turn is related to ammonia-nitrogen removal.
Recirculation of wastewater flow is not normally provided in RBC systems. The RBC
systems are designed in three or more steps, which work similarly to the increased depth of
a super-rate bio-oxidation tower to provide increased retention time. Unlike the bio-oxidation
tower, the RBC unit is wetted in the tank as it rotates, eliminating the need for recycle to
provide a minimum wetting rate.
Organic loading (although not the primary design criterion) is very important in small plants
and will affect the total process design. The reason for this importance is that the food-to-
microorganism ratio is self-regulating within certain limits. If organic loading is constant,
the biomass on the disks will develop to varying thicknesses, depending on the disk position
within the process. A change in organic loading will effect a change in film thickness. In
addition to thickness, the type of organisms in the film will adjust to the conditions.
Organisms that can utilize large quantities of organic matter will locate and develop in the
first step of the process. If organic loading is reduced so that the final steps are lightly
loaded, nitrifying organisms will develop.
Changes in organic loading that occur slowly can be accommodated by adjustments in the
biomass. If an RBC system is operated below design loading, the unit can provide treatment
consistently better than required. As loading increases because of system development, the
RBC unit will adjust accordingly.
Diurnal changes in organic loading, which may occur in larger plants and some small plants
with balanced loading, can be handled by the adjustment of biomass. In small plants with
large, rapid diurnal variations, equalization is desirable. Equalization (discussed in Chapter
4) will help provide efficient BOD5 removal and reduce the size of process equipment.
9.2.4.2 Multiple Step Systems, Retention Time, and Rotational Speed
The rotating disk process is quite suitable for multiple step systems and stage construction.
The number of disks per shaft appears to have some practical limits. Because of the limits,
it is easy to arrange units of disks parallel or in series, to provide steps or stages. Study has
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indicated that three-step systems may be desirable; if nitrification is required, a four-step
system may be needed (37).
Retention time which also affects performance, is controlled in these systems by disk
spacing and tank size. Increasing the spacing or tank size for a given hydraulic load will tend
to increase the treatment capacity. The disk spacing and tank volume have been combined,
for study purposes, into a single parameter, the volume-to-surface ratio. Antonie (37) found
that the upper limit was approximately 0.12 gal of tank volume/ft2 of disk area (0.005
m3/m2) above which the capacity did not increase.
The rotational speed can be varied to control the aeration and the contact time of the organ-
isms with the wastewater. Peripheral disk velocities will range from 30 to 60 fpm (9 to 18
m/s) (34). At these speeds, the biomass is uniformly stripped of excess organisms. At lower
values, aeration becomes limited; above this range, shear forces become great.
9.2.4.3 Wastewater Temperature
Temperature is an important factor in the design of any biological process and should be
considered very carefully.
The Wisconsin tests were normally run at a wastewater temperature of 55° F (13° C) or
above. At this temperature, the rotating disk process provided good BOD removal and
nitrification. If the temperature was lowered, the treatment decreased rapidly.
Data for the lower temperature operation indicated that the degree of treatment at lower
temperatures could be improved by increasing the volume-to-surface ratio (increasing disk
spacing) or by decreasing the hydraulic loading rate (increasing the retention time). For
example, a plant with a volume-to-area ratio of 0.12 may obtain 86 percent BOD removal
at 4 gpd/ft2 (0.16 m3/m2/d) at a temperature above 55° F (13° C). The same unit may
have to be loaded at 2 gpd/ft2 (0.07 m3/m2/d) to obtain the same treatment at lower
temperature.
If the volume-to-area ratio is raised to 0.32, the loading required would be 3 gpd/ft2 (0.12
m3/m2/d).
9.2.4.4 Pretreatment Requirements
The main pretreatment requirements for small plants treating primarily domestic waste
would include primary sedimentation or fine screening and equalization. Primary sedimenta-
tion or fine screening would remove large, dense solids, which might settle out in the RBC
process units. If large amounts of hexane soluble material are expected, a primary settling
step would be preferred, or other grease separation methods would be required. In many
smaller plants, a simple septic tank has been found to provide adequate treatment.
9.2.4.5 Clarification Requirements
Clarification requirements for totaling disk systems are similar to those for trickling filter
systems. Primary settling is required, because the heavier solids cannot be carried through
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the process. Intermediate sedimentation is not required, unless sludge removal is desired
before a final step, such as nitrification. Final sedimentation is required and may be Used
before or after nitrification.
Design of final clarifiers for RBC systems should be similar to design of primary settling
tanks, with the exceptions of hydraulic loading rates and side wall depth. The hydraulic
loadings rates should be 1,000 to 1,200 gpd/ft2 (40 to 48 m3/m2/d) for peak flow (5).
9.2.4.6 Sludge Handling
Sludge produced by the RBC unit is similar to humus sludge from a trickling filter. The
amount of sludge produced will depend on waste characteristics and loading rates. An RBC
unit designed for 80 percent BODs removal would produce about 0.7 Ib of sludge per
pound of BODs removed; 95 percent removal would produce about 0.3 Ib of sludge (39).
Volume of sludge to be handled will be low compared to the volume from an activated
sludge plant. The sludge produced is dense and will settle rapidly. If slow, continuous with-
drawal is provided, sludge concentrations can be maintained at 3 to 4 percent solids (39).
For design, 2 to 3 percent solids are considered reasonable. If secondary sludge is recycled
to the primary clarifier, sludge with 4 to 6 percent solids can be obtained.
A few tests have been conducted (34) to determine the value of sludge recycle. At recycle
rates of 1 to 2 percent of flow, there was no apparent effect on the system parameters.
(Further study at higher rates is desirable.) In general, recycle of sludge or flow is not
practiced.
9.2.4.7 Nitrification
Nitrification can be achieved using a properly designed 4- to 10-step rotating disk process,
provided temperature, pH, and presence of toxic substances are considered. Nitrification is
discussed further in Chapter 13.
As the BOD of the wastewater in the process is reduced in the first steps to 30 mg/1, the
nitrifying organisms begin to establish themselves. As the BOD decreases, the ammonia-
nitrogen removal increases. Nitrification will decrease, if the carbonaceous BOD is increased
above the threshold for a specific wastewater (40). At a hydraulic loading of 1.0 gpd/ft2
(0.04 m3/m2/d), ammonia-nitrogen removal of more than 95 percent and effluent concen-
trations of less than 1.0 mg/1 were attained with wastewater temperature above 55° F
(13° C). Dentrification, using totally submerged disks and methanol addition, is being con-
sidered for rotating disk systems (36).
9.2.5 Example Design
Design procedures have been developed by manufacturers of rotating biological contactor
systems. This information (39) (41) is available through the manufacturers and their rep-
resentatives. The following example design uses the design procedure and criteria of one
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manufacturer. This example shows only a typical design; the reader should not infer that
any one design procedure is the best. The example design will use the site and wastewater
characteristics given in Section 9.1.7.1.
Additional design conditions include:
1 . Plant reliability, Class II
2. Pretreatment : coarse screening, grit removal, and primary sedimentation (30
percent removal)
3. Nitrification of 90 percent required during dry weather months at maximum dry-
weather flow
4. Maximum dry-weather flow approximately equal to average daily flow
Determine Process Surface Area
BOD5 of primary effluent = (200 mg/1) (1 - 0.3) = 140 mg/1.
For final effluent BODs < 30 mg/1, RBC unit must provide 78.6 percent removal.
(140 -30) (100)
=78.6 percent
(Because this is less than 85 percent, the RBC unit would have had to be enlarged if nitrifi-
cation were not included.)
From design curves for 79 percent removal with 140 mg/1 primary effluent, the required
hydraulic loading is 6.9 gpd/ft^.
Surface area required = ——r — , ... -. = 28,986 ft^
6.9 gdp/ft2
Wastewater Temperature Correction
For 45° F (7° C) and 79 percent BOD removal, the temperature correction factor (from
design curves) = 3.0.
The corrected loading rate is—— = 2.3 gpd/ft^.
200,000 gpd ~
Surface area required = — - . .. 0 = 86,957 ft*
2.3 gpd/ft2
Nitrification Design
From design curve for 90 percent ammonia-nitrogen removal, secondary effluent
would be 13 mg/1.
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removal through RBC system would be:
(140 -13) (100) = 9 j t remoyal
(140)
From design curve for 91 percent BODs removal, surface loading rate is 2.3 gpd/ft2.
Surface area required = 200,000gpd = 86 957 ft2
2.3 gpd/ft2
During warm, dry weather months, wastewater temperature would not drop below 55° F
13° C), and a temperature correction for nitrification would not be required.
Equipment Selection
Selection of RBC equipment will depend on site conditions (e.g., available space, contours,
and head loss through the treatment facility). These conditions must be balanced with
the number of standard units available, type and degree of treatment required, reliability,
required steps, and stage construction.
The conditions and standard units indicate that two four-stage RBC shaft assemblies should
be used, providing a total surface area greater than 90,000 ft2 (8,370 m2).
9.2.6 Equipment and Materials of Construction
The basic RBC unit consists of rotating medium and shaft assembly, drive system, shaft
bearings, tankage, and enclosures. In smaller units this equipment can be provided in a
completely assembled package. Larger units require construction of concrete tanks and
enclosures (unless factory-made covers are used). Units supplied for concrete tanks are
shipped in two parts, including the shaft and drive assemblies.
The shaft assembly consists of high density polyethylene or polystyrene disks mounted on
a steel shaft. The shaft is fitted with drive sprocket and shaft bearing, to allow immediate
installation on preset anchor bolts.
The drive assembly consists of the drive motor and a drive system, which could include
belt drive, speed reducer or chain drive units. The drive motor could vary between 0.25 and
7.5 hp (0.19 and 5.6 kW), depending on the shaft size and the area of the contactors.
Construction of concrete tanks will depend on the RBC units used, overall arrangement, and
flow distribution or control requirements. Suggested arrangements are included in design
manuals (39) (40). Flow distribution or control methods can include piping or channels
with various weir, baffle, and valve arrangements.
Enclosures can consist of plastic covers (available through RBC system manufacturers) on
housing constructed around the units. Plastic covers have removable panels and parts, which
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allow inspection of the medium and access to the drive assembly and bearings. In extremely
cold climates, these covers can be supplied with urethane foam insulation.
If alternative means of housing are used, the structure should be designed to meet the
following conditions:
1. All materials of construction must be able to withstand high humidity.
2. Ventilation should be provided, with control available to reduce ventilation in
cold weather. Forced ventilation is not required, but could be provided.
3. Heat is not required even in cold weather, but can be used to reduce humidity
problems.
9.2.8 Operation and Maintenance
The rotating disk process is stable under conditions of fluctuating hydraulic and organic
load and, therefore, does not require recycling of sludge or flow recirculation. This stability
simplifies the operation and eliminates the need for great operating flexibility and
instrumentation.
The units should normally be provided with enclosures; operating in cold weather increases
this need. In cold weather, the addition of a small amount of heat (a few degrees above
wastewater temperature), although it will increase the operating cost slightly, will improve
operation.
Maintenance of these units is also simple. Because the main mechanical components are
those required to rotate the disks, and because separate drive units are provided for each
set of disks, the operation can be shut down, a unit at a time, for servicing. The minimum
number of moving components makes the servicing very easy; only weekly greasing of
bearings and checking of lubricant levels in the chain guards are required. On a quarterly
or semiannual basis, changes of lubricant will be necessary.
9.3 References
1. McKinney, R.E., Microbiology for Sanitary Engineers. New York: McGraw-Hill (1962).
2. "Wastewater Treatment Plant Design." WPCF Manual of Practice No. 8 (1975).
3. Design Criteria for Mechanical, Electric and Fluid System and Component Reliability,
Supplement to Federal Guidelines: Design, Operation and Maintenance of Wastewater
Treatment Facilities. U.S. EPA, Office of Water Programs (1974).
4. Chipperfield, P.N.J., Askew, M.W., and Benton, J.H., "Multiple-Stage, Plastic-Media
Treatment plants". Journal Water Pollution Control Federation, vol. 44, No. 10
(October 1972).
5. Process Design Manual for Upgrading Existing Wastewater Treatment Plants. U.S. EPA,
Office of Technology Transfer (October 1974).
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6. Slag and Stone, Crushed; Gravel, Crushed and Uncrushed (for sewage trickling media).
Federal Supply Service, Federal specification SS-S-448, General Services Administra-
tion (12 August 1966).
7. "Filtering Materials for Sewage Treatment Plants." ASCE Manual of Engineering
Practice No. 13, New York (1961).
8. Jank, B.E., Drynan, W.R., and Sylveston, P.L., The Operation and Performance of
Flocan-Packed Biological Filters Under Canadian Climatic Conditions. Waterloo,
Ontario: University of Waterloo (October 1969).
9. Chipperfield, P.N.J., "Performance of Plastic Filter Media in Industrial and Domestic
Waste Treatment." Journal Water Pollution Control Federation, 39, 11, p. 1,860
(November 1967).
10. Design Manual for Surfpac Biological Oxidation Media. Dow Chemical Co., Midland,
Mich., form No. 175-1216-71.
11. Vinyl Care Biological Oxidation Media. B.F. Goodrich General Products Co., Akron,
Ohio.
12. Gulp, G.L., "Direct Recirculation of High-Rate Trickling Filter Effluent." Journal
Water Pollution Control Federation, vol. 35, No. 6 (June 1963).
13. Caller, W.S., and Gotaas, H.B., "Analysis of Biological Filter Variables." Journal of
the Sanitary Engineering Division, ASCE, vol. 90, No. 6, pp. 59 to 79 (1964).
14. Metcalf & Eddy, Wastewater Engineering. New York: McGraw-Hill (1972).
15. Benzie, W.J., Larking, H.O., and Moore, A.F., Effects of Climatic and Loading Factors
on Trickling Filter Performance. WPCF, vol. 35, No. 4 (April 1963).
16. Howland, W.E., "Effect of Temperature on Sewage Treatment Processes." Sewage
and Industrial Wastes, vol. 5, No. 2, p. 161 (1953).
17. Recommended Standards for Sewage Works. Great Lakes-Upper Mississippi River
Board of State Sanitary Engineers (1971).
18. "Sewage Treatment at Military Installations," National Research Council. Sewage
Works Journal, vol. 18, No. 5, pp. 787 (1946).
19. Velz, C.H., "A Basic Law for the Performance of Biological Beds." Sewage Works
Journal, vol. 20, p. 607 (1948).
20. Schulze, K.L., "Load and Efficiency of Trickling Filters." Journal Water Pollution
Control Federation, No. 3, pp. 245 to 261 (1960).
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21. Germain, J., Economic Treatment of Domestic Waste by Plastic-Medium Trickling
Filters. 38th annual conference, WPCF, Atlantic City, N.J. (October 1965).
22. Eckenfelder, W.W., Trickling Filter Design and Performance. Transactions ASCE, 128,
part HI, pp. 371 to 398 (1963).
23. Eckenfelder, W.W., and Barnhart, W., "Performance of a High-Rate Trickling Filter
Using Selected Media." Journal Water Pollution Control Federation, vol. 35, No. 12,
pp. 1,535 to 1,551 (1963).
24. Wittenmyer, J.D., A Look At the Future Now. Ohio Water Pollution Control
Conference, Dayton, Ohio (20 June 1969).
25. Wittenmyer, J.D., Operating Experiences. Ohio Water Pollution Control Conference,
Dayton, Ohio (16 June 1972).
26. Morton, D., "Personal Communication." Allied Engineering Gorham, Maine (1975).
27. Askew, M.W., "High Rate Biofiltration; Past and Future," Water and Pollution Control,
vol. 69, No. 4, p. 445 (1970), as referenced to by Landine, R.C., in "Plastic Media
Bio filters, Their Application in Canada," Water and Pollution Control (November
1972).
28. Downing, A.L., Water Pollution Control Engineering, British Board of Trade Central
Office of Information (London, 1969), chapter 5, part 2, as referenced to by Landine,
R.C., "Plastic Media Biofilters, Their Application in Canada," Water and Pollution
Control (November 1972).
29. Balakrishnan, S., and Eckenfelder, W.W., "Nitrogen Relationships in Biological Treat-
ment Processes - II. Nitrification Trickling Filters." Water Research, vol. 3, pp. 167
to 174(1969).
30. Duddles, G.A., and Richardson, S.E., "Application of Plastic Media Trickling Filters
for Biological Nitrification Systems." Environmental Protection Technology Series,
EPA-R2-73-199 (June 1973).
31. Technical Bulletin for Use of Mercury in Waste-water Treatment Plant Equipment.
supplement to Federal Guidelines: Design, Operation & Maintenance of Wastewater
Treatment Facilities, U.S. EPA, Office of Water Programs (1971).
32. Camp, T.R., and Graber, S.D., "Dispersion Conduits." Journal of the Sanitary
Engineering Division, ASCE, SA1, pp. 31 to 39 (February 1968).
33. Handbook of Trickling Filter Design. Ridgewood, N.J.: Public Works Journal Corp.
(1970).
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34. "Application of Rotating Disc Process to Municipal Wastewater Treatment." U.S. EPA
Water Pollution Research Series, 17050 DAM (November 1970).
35. Antonie, R.L., Application of the Bio-Disk Process to Treatment of Domestic Waste-
Water. Paper presented in part, 43rd annual conference, WPCF, Boston, Mass. (4 to
9 October 1970).
36. Three Step Biological Treatment with the Bio-Disc Process. Paper, New York WPCA,
Montauk, N.Y. (12 to 14 June 1972).
37. Factors Affecting BOD Removal and Nitrification in the Bio-Disc Process. Paper,
Central States WPCA, Milwaukee, Wis. (14 to 16 June 1972).
38. Kluge, D.L., and Mielke, J.H., Evaluation of a 0.5-MGD Bio-Surf Municipal Waste-
water Treatment Plant. Paper, 45th annual conference, WPCF, Atlanta, Ga. (8 to 13
October 1972).
39. Bio-Surf Design Manual. Autotrol Corporation Bio-Systems Division, Milwaukee, Wis.
(1972).
40. Stover, E.L., and Kincannon, D.F., "One-Step Nitrification and Carbon Removal."
Water & Sewage Works (June 1975).
41. Bio-Surf Process Package Plants for Secondary Wastewater Treatment. Autotrol Corp-
oration Bio-Systems Division, Milwaukee, Wis. (1974).
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CHAPTER 10
WASTEWATER TREATMENT PONDS
10.1 Background
Wastewater treatment (stabilization) ponds are earthen basins, open to the sun and air. They
depend on natural biological, chemical, and physical processes to stabilize wastewater. These
processes, which may take place simultaneously, in elude sedimentation, digestion, oxidation,
synthesis, photosynthesis, endogenous respiration, gas exchange, aeration, evaporation,
thermal currents, and seepage.
For centuries, sewers have carried wastewater into natural ponds and other water bodies,
sometimes, without nuisance conditions resulting. It was not until the 1920'sthat artificial
ponds were designed and constructed to receive and stabilize wastewater. By 1950, the use
of ponds had become recognized as an economical wastewater treatment method for small
municipalities in rural areas.
Wastewater treatment ponds are now commonly employed as secondary treatment systems.
The U.S. EPA Municipal Waste Facilities Inventory of 20 August 1974 indicated that these
ponds constituted 31 percent of the 16,133 secondary treatment systems operating in the
United States and served about 7.3 percent of the 104 million people served by secondary
treatment plants (1). About 66 percent of these ponds serve small communities having flows
of 0.2 mgd (0.01 m3/s)orless(l).
Because of increasingly stringent effluent requirements, ponds, like many other wastewater
treatment processes, will usually require modification of design and operation to meet all
objectives. Pond problems fall into three general areas: 1) unsatisfactory effluent quality, 2)
odors and other environmentally incompatible factors, and 3) water loss. These problems
usually relate to a need in the design for better pond siting, microbial cell removal, and dis-
infection; lack of consideration of temperature effects and odor control; and no minimiza-
tion of short circuiting by better hydraulic design.
The major advantages of ponds are 1) they can handle considerable variations in organic and
hydraulic loading with little adverse effect on effluent quality and 2) they require minimum
control by relatively unskilled operators. Low capital costs and low operation and mainte-
nance costs are also advantages. The major disadvantages are 1) the large land area required,
2) the localized odor problems that occur when conditions become anaerobic (more diffi-
cult to prevent if icing occurs), and 3) excessive accumulation of algal and bacterial cells in
the effluent, which creates a significant BOD and SS load on the receiving waters. It should
be pointed out that the U.S. EPA has proposed raising the SS limitations for ponds on a
case-by-case basis (Federal Register, Vol. 41, No. 172, September 2, 1976).
10.1.1 Natural Stabilization Processes
With careful planning, design, and operation, natural processes can be efficiently utilized in
stabilization ponds (see Figure 10-1).
10-1
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WIND
OX Y G EN
CH,
ALGAE AND AEROBIC BACTERIA
WASTEWATER WITH
DISSOLVED AND
COLLOIDAL ORGANICS
\ SETTLE-X
\ABLE \
\SOLIDS V
<-7
LIQUID
LAYER
WITH
OXYGEN
LIQUID LAYER
WITH LITTLE
OR NO OXYGEN
N2
A
1 1
SLUDGE OR
BENTH AL
LAYER
WITHOUT
OXYGEN
'
PHOTOSYNTHESIS
SUNSHINE + C02-h
H20 + NH3 + P04
»-ALGAE r
02+ H20
SYNTHESIS
ORGANICS + 02
*-BACTERIA +
C02 + H20 +
NH4+ P04
ENDOGENOUS RESPIRATIC
ORGANICS + 02 — *• C02
CELLS*+H20 + NH4 + PC
FACULTATIVE
BACTERIA
NITRIFICATION
NH4 + 02 »-
N02 + 02 *-
SULFUR OXIDATION
H2S + PHOTOSYNTHETIC
BACTERIA *- S
H2S + 02 — -H++ S04=
H2S + 02 — ~H20 + S
/
hel POND
N 1^1 EFFLUENT
-h / °/ WITH
! 1 Si CELLS*
* - / /
— JiL~
...
1^1 DENITRIFICATION
Ivl ORGANIC C+H + NOj— »-
C02 NH5 CH4 H2S 1^1 C02+ N2 + H20+CELLS*
o-iV
-------
The organic materials, suspended or dissolved in the raw wastewater or scoured from the
bottom by intrapond mixing (as a result of wind or thermal turnovers), are biochemically
stabilized, usually aerobically, by bacteria, algae, and, to a smaller extent, other biota. Bac-
teria release enzymes into the water that catalyze chemical reactions, producing simpler
chemicals used to synthesize new cells and aid cell metabolism. The organic carbon is
broken down by heterotrophic bacterial action, which results in the formation of carbon
dioxide (CC^), t^O, and residues. The organic nitrogen, largely in the form of urea or pro-
tein, is broken down by enzyme-catalyzed reactions—initiated by heterotrophic faculative
and anaerobic bacteria—to form ammonia (NH3), new bacterial cells, CC>2, and residues.
Algae grow symbiotically with bacteria: algae utilize CC>2 in photosynthesis to produce new
cells and release oxygen (C^) as one of the byproducts; bacteria utilize this O^ and produce
CC>2 as a byproduct. However, at night and during endogenous respiration algae also use C>2
and oxidize some of the compounds they have produced and stored during the period of
photosynthesis.
The settleable solids, including dead microbial cells, settle to the bottom of the aerobic zone
into an anaerobic zone, reducing the requirements for dissolved oxygen (DO). Such solids
undergo acid and methane (Qfy) fermentation, hydrolysis, and other biochemical changes.
These changes result in the formation of new bacterial cells, provide energy for the cells, and
release Crfy, CO2, hydrogen sulfide (f^S), NH3, various organic acids, and residues. The
NH3 and sulfurous gases that evolve in the anaerobic decomposition rise into the aerobic
zone, where nitrification and sulfide-oxidizing bacteria convert the NH3 to nitrite and
nitrate and the sulfides to sulfates and sulfur. If liquid containing nitrate enters an anaerobic
zone, denitrifying bacteria reduce the nitrate to gaseous nitrogen, which escapes to the
atmosphere. In some cases, nitrogen-fixing bacteria convert gaseous nitrogen to nitrates in
stabilization ponds.
Dead algal and bacterial cells undergo lysis, or disintegration, in the same manner as other
dead organic matter and, in the process, create an oxygen demand. On the other hand, the
products of disintegration, Q^ anc* CO2, provide nutrients for the synthesis of new algal
cells, and the synthesis continues until the synthesis-decay-sedimentation cycle leads to re-
moval of most of the BODs and SS from the wastewater. Provision must be made for the
consistent removal of cells from the liquid zones of the pond system, if a uniformly satis-
factory effluent is to be produced. The optimum temperature for efficient functioning of
treatment ponds is 68° to 77° F (20° C to 25° C). When the water temperature reaches
34° F (1° C) or lower, general bacterial action becomes quite reduced. If an ice cover forms,
there is little available oxygen, and metabolism continues only at a very slow rate. At water
temperatures below about 57° F (14° C), there is little anaerobic methane production or re-
duction of sludge volume.
Although the equations in Figure 10-1 oversimplify the tranformations, they do show the
recycling of carbon in ponds. The net effects of this carbon recycling mechanism are 1) con-
siderable decomposition of the solids originally in the raw wastewater, 2) some loss of the
carbon load of the raw wastewater and bottom sludge (as C©2 or CH4) to the atmosphere,
and 3) conversion of much of the soluble and organic material into bacterial and algal cells.
Unless the microbial cells are removed by settling into the anaerobic bottom sediment, or by
10-3
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a long period of aeration (which allows time for lysing and more complete oxidation), or by
some solids removal process after the pond treatment, little carbon reduction may occur.
Cells that escape into the effluent carry potentially oxidizable organics and nutrients, such
as nitrogen and phosphorus. There are many good references covering the various stages of
pond processes (2) (3) (4) (5).
In other words, half or more of the original organic constituents exert an oxygen demand
for a long time after the initial synthesization process.
10.2 Definitions and Descriptions of Wastewater Treatment Ponds
There is general confusion in the terminology used in the classification of ponds. In this
manual, ponds are classified as follows:
1. Facultative Ponds
2. Aerated Ponds (which can be further classified as to the degree of mixing)
3. Aerobic Ponds
4. Polishing Ponds
5. Anaerobic Ponds
In the U.S. EPA Bulletin Wastewater Treatment Ponds (6), ponds are divided into photo-
synthetic, aerated, and complete retention ponds. The photosynthetic ponds (i.e., faculta-
tive, aerobic, polishing) are subdivided into flow-through and controlled-discharge types.
The aerated ponds are subdivided into complete-mix and partial-mix types.
10.2.1 Facultative Ponds
Facultative ponds are medium depth ponds, with an aerobic zone overlying an anaerobic
zone (with some sludge deposits) and a zone between the two, where facultative bacteria
primarily function. Solids in the anaerobic zone undergo fermentation and hydrolysis, and
the soluble organics and NH3 rise into the aerobic zone to be oxidized. Facultative ponds
are sometimes called oxidation ponds or aerobic-anaerobic ponds. Most wastewater treat-
ment ponds in the United States are facultative ponds and are used to treat domestic waste-
water, industrial wastewater, or a combination of both. Facultative ponds are often used as
the final cells, if anaerobic ponds are used as the initial cells. The design of facultative ponds
is described in Section 10.4.
10.2.2 Aerated Ponds
Ponds using mechanical devices as the principal sources of DO are called aerated ponds.
Completely mixed aerated ponds (also called aerated aerobic ponds) keep all of the solids in
suspension, and C<2 is provided by air diffusers or mechanical aerators. In partially mixed
aerated ponds (also called aerated facultative ponds), only the upper zone is aerated by
diffusers or mechanical aerators; the lower facultative and/or anaerobic zones are relatively
undisturbed. The partially mixed aerated pond is particularly adaptable for northern areas,
because it permits the continuation of aerobic oxidation, under the ice, to prevent spring
10-4
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odor problems. Aerated pond effluents, although they may not contain large amounts of
algae, may contain other suspended microbial cells and biological solids, resulting from the
conversion of the dissolved BOD and the SS from the raw wastewater. Aerated pond design
is described in Section 10.5.
10.2.3 Aerobic Ponds
Aerobic ponds are shallow ponds that contain DO throughout their liquid volume at all
times (i.e., there are no anaerobic zones). Aerobic bacterial oxidation and algal photo-
synthesis are the principal biological processes. Aerobic ponds are best suited to treating
soluble wastes in wastewater relatively free of SS. Thus, they are often used to provide
additional treatment of effluents from primary wastewater treatment plants, anaerobic
ponds, and other partial treatment processes. The design of aerobic ponds is described in
Section 10.6.
10.2.4 Polishing Ponds
Polishing ponds are lightly loaded, aerobic or facultative ponds that polish the effluent
from conventional treatment plants by further reducing the settleable solids, BOD, fecal
bacteria, and NH3. Algal photosynthesis and surface reaeration provide the ©2 for stabiliza-
tion. Polishing ponds should be designed with detention times insufficient to support algal
development—60 hours or less—unless phosphorus (P) removal is a prime concern. Figure
10-2 depicts the effect of a polishing pond on the removal of SS, P, and nitrate-nitrogen,
and on the growth of algae and coliform organisms (7), Polishing pond design is described in
Section 10.7.
10.2.5 Anaerobic Ponds
Ponds that are so heavily loaded organically that they do not have an aerobic zone (except,
possibly, at the surface) are called anaerobic ponds. Only partial stabilization takes place.
Anaerobic ponds must be followed by facultative or aerobic ponds, or other additional
treatment, to complete stabilization of the organic material and provide additional solids
removal.
Anaerobic ponds are typically used as the first step for treatment of strong organic wastes,
such as those from industries processing vegetables and fruits, meats, milk, or other foods.
These ponds are not ordinarily used for treatment of domestic wastewater, although they
may be applied as a first treatment step if the wastewater is abnormally strong because of in-
dustrial discharge. The design of anaerobic ponds is not discussed in this manual, which is
intended for use in the design of facilities treating normal domestic wastewater.
10.3 General Design Requirements
10.3.1 Common Design Considerations
The performance of all wastewater treatment ponds is particularly affected by 1) organic
loading per unit area, 2) temperature and wind patterns, 3) actual detention time, dispersion,
10-5
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o
<
H
Z
LJ
O
z
o
o
LJ
O
o
LJ
UJ
en
(b ) 4 CELLS IN SERIES
COLIFORM \
ORGANISMS \
NITRATE
LOSS
PHOSPHATE
LOSS
3 4
RETENTION TIME (DAYS)
FIGURE 10-2
PERFORMANCE OF POLISHING PONDS
FOLLOWING SECONDARY TREATMENT (7)
10-6
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and mixing characteristics, 4) sunlight energy, 5) characteristics of the solids in the influent,
and 6) amounts of essential microbial nutrients present. Pond efficiency, measured as the
degree of stabilization of the incoming waste particles, is dependent on both biological
process kinetics and pond hydraulic characteristics.
The time required to synthesize cells from waste organic solids (suspended or dissolved) can
be determined by kinetic computations. For kinetic computations, the value of the reaction
rate coefficient (k) for the specific wastewater should be determined as accurately as pos-
sible, taking into account the BOD loading per unit area, quantity and types of pollutants,
temperature, available solar energy, probable types of biota, hydraulic characteristics,
nutrient deficiencies, toxic wastes, source of O2, effluent requirements, and other biological
parameters (8) (9).
BOD bottle determinations of rate coefficients do not give good results, unless the sample is
undiluted and is stirred to the same degree as would be expected in the ponds. Oxygen
uptake tests determine the rate coefficient better than do BOD bottle tests. The types of
respirometers available for obtaining O2 uptake range from simple to complex. The simplest
fits on a BOD bottle and includes a stirrer and DO probe.
The reaction rate coefficient for carbonaceous oxidation is greater than the coefficients for
oxidation of NH3 and nitrites to nitrates. A completely mixed pond, or one with recircula-
tion, will undergo nitrification in less time than will a plug-flow pond without recirculation,
because of the larger initial concentration of nitrifying bacteria. With recirculation to pro-
vide nitrifiers, plug flow is the more efficient. To select the reaction rate coefficient if nitri-
fication is required, the initial concentration of nitrifiers, organic carbon, DO, and waste-
water temperature profile must be considered.
The organic carbon compounds in raw wastewater are more easily oxidized during initial
stabilization (synthesis into microbial cells) than in the endogenous respiration stage. This
difference will affect the values of the reaction rate constant. The rate constant will decrease
as the samples tested are from points further downstream.
In a plug flow situation, with water temperature at 68° F (20° C) the k value increases as
the ratio of BODs to BODU increases in the wastewater, as shown in Table 10-1 (10). Raw
domestic wastewater usually will have a BOD5 to BODU ratio of about 0.5.
The first order reaction rate coefficient, k, varies with temperature, approximately in accor-
dance with the following equation:
where
kT = reaction rate coefficient at T° C, days"
k2Q = reaction rate coefficient at 20° C, days"
10-7
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6 = a constant, with values of about 1.08 for facultative ponds and
about 1.04 for aerobic ponds (11)
T = pond water temperature, °C
The k values also change appreciably with the value of the BOD loading per unit area. Waste-
water pond studies, indicating the degree of variation in k with BOD loading, are shown in
Table 10-2 (8) (12).
TABLE 10-1
BOD5 TO BODU RATIO EFFECTS ON k VALUES
Raw Wastewater
k BOD5 to BODU
-1
day
0.05 0.43
0.10 0.68
0.15 0.83
0.20 0.90
0.30 0.97
TABLE 10-2
EVALUATION OF k VALUES FOR THE FIELD LAGOONS AT
FAYETTE, MISSOURI (8) (12)
Lagoon1 Influent BOD Effluent BOD Detention Time BOD Loading
No. mg/1 rng/1 day Ib/acre/day
1 267 35 87 20 0.045
2 267 37 44 40 0.071
3 267 48 29 60 0.083
4 267 49 22 80 0.096
5 267 47 17 100 0.129
1 All lagoons are 2.5 ft deep and 0.75 acre in area.
A common value for k at 68° F (20° C) for domestic wastewater is about 0.1 day ^ when
the loading is about 60 Ib/acre/day (67 kg/ha • d) (6,725 kg/km2/day).
10-8
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The hydraulic flow characteristics must provide the conditions assumed for the particular
kinetic theory utilized. Plug flow is achieved, if each particle travels through the pond at the
same rate. This condition requires a design that provides perfect vertical and transverse mix-
ing of the "plug" as it flows through the pond; that is, dead spaces and short circuiting
must, to a large extent, be eliminated. With plug flow, the first order reaction idealized
equation is (8):
1^/1^=1/6^ or t = In (Lj/LeVk
where
Le = effluent BOD, mg/1
Lj = influent BOD, mg/1
e = 2.71828
k = reaction rate coefficient, day" *
t = actual detention time, day
In = natural log or log to base e
Completely mixed flow, the opposite of pure plug flow, occurs if mixing is sufficient to
create uniform characteristics throughout the pond. The idealized first order reaction equa-
tion for completely mixed flow is (8):
Le/Li = 1/(1 + kt) or t = (LJ - Le)/Lek
Actually, the flow through most existing wastewater treatment ponds falls somewhere be-
tween ideal plug flow and ideal completely mixed flow, depending on pond configuration
and hydraulic characteristics.
In many locations, where controlled intermittent discharge or complete containment is not
practiced, additional treatment, after stabilization, for removal of algal and bacterial cells is
necessary to meet consistently BOD, SS, or other effluent requirements before discharge
(6). In the winter, algal activity diminishes, and low winter temperatures, particularly from
the ice cover, have adverse effects on stabilization pond efficiency. Other biological activity
will also diminish, and Crfy fermentation is facultative or anaerobic ponds will practically
cease. Sedimentation becomes the major treatment process remaining, when ponds are iced
over. Even if the detention period is very long, bacterial metabolism, using dissolved BOD
during periods of ice cover, is incomplete.
Controlled, intermittent discharge of well-clarified facultative pond effluents in Michigan
and North Dakota indicates that secondary wastewater treatment standards can be met, if
ponds are adequately constructed and operated (6). Sufficient capacity should be planned
10-9
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for holding wastes without discharge for at least 6 months. The organic loadings generally
are held to 20 to 25 Ib BOD5/acre/day (22 to 28 kg/ha-d) or less, in two or more cells,
with a liquid depth of not more than 6 ft (1.8 m) in the primary cell(s) and 8 ft (2.4 m) in
subsequent cells (6). During the late spring and again in the fall, when the algal growth rate
is slow, pond contents are stabilized, and distinct thermal layers prevent mixing. Ponds may
then be drawn down as long as the SS content meets effluent requirements (13). It has been
found that 2- to 4-week discharge periods during April to May and October to November are
satisfactory (13). The SS content will normally be higher than the BOD5 and, therefore,
can be used as the control (14). This controlled intermittent discharge system is less practi-
cal in the south, where algal growth is prevalent more than 6 months per year, and in areas
where there is a large amount of rainfall and infiltration into sewer systems (14). For more
detail on intermittent discharge pond systems, see references (6), (13), and (14).
Complete containment (retention) may be practiced if the combined evaporation and perco-
lation outflow rates are equal to, or more than, the rainfall and wastewater inflow. The com-
plete containment system is usually feasible only in the drier parts of the western plains and
desert regions. These systems should be located well away and downwind from habitations,
and the pond designed, constructed, and operated to prevent conditions that might lead to
fly, mosquito, and odor nuisances.
10.3.2 Design Data Requirements
Factors to be considered in determining whether a pond system might be a feasible part of
the treatment system for a specific wastewater include:
1. Availability and value of suitable land at the potential treatment plant site
2. Environmental compatibility of a pond with neighboring land uses
3. Effluent quality requirements
4. Wastewater characteristics
To verify whether these four factors can be met, the following should be obtained:
1. Topographical maps, with contours adequately defining the topography of
possible pond sites (latitude and elevation above sea level should be included)
2. Locations of all residents, commercial developments, and water supplies within
0.5 mile
3. Meteorological statistics (daily and monthly variations in temperature, wind direc-
tion, and wind velocity, and monthly values of precipitation, evaporation, and
solar radiation)
4. Surface and subsurface soil characteristics (including percolation rates, load bear-
ing capability, and ground water elevations)
5. Analyses of the wastewater to be treated, including DO, BODs (total and
filtered), BODU, COD, TS, TSS, TKN, total P, pH, alkalinity, sulfates and sulfides,
and essential growth factors, such as iron and potassium
6. Data from at least 1 year's performance of a similar pond in a comparable envir-
onment, a pilot pond using the wastewater to be treated, or a portion of the
10-10
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initial pond construction (refer to Section 10.4.5), to assist in developing design
criteria, including pertinent reaction rate coefficients
10.4 Facultative Pond Design
10.4.1 General Considerations
Facultative pond system configurations vary (see Figure 10-3). However, in general, it has
been shown that series configurations B, D, or E are most efficient. The initial or primary
cell is designed to retain the more easily settleable SS, and the series cells following are de-
signed to decrease both the BODs and the SS to less than 30 mg/1 and the fecal coliforms
to less than 200 per 100 ml, while keeping the pH between 6 and 9.
Empirical methods have been developed for designing facultative ponds, but predicted efflu-
ent quality often differs from actual effluent quality, sometimes substantially. Comprehen-
sive and uniform collection of data from well-designed and operated facilities across the
United States is needed to develop better and more reliable design procedures. Various State
standards for stabilization pond design have often been made quite conservative to compen-
sate for the many relatively unknown design and operational variables that often result in
unsatisfactory operation.
Most of the SS settle out rapidly near the inlet of a primary cell, reducing the actual BOD
loading on the pond by 20 to 30 percent (5). However, some of the settled BOD is reim-
posed on the pond by the ©2 demand in the gases (largely methane, ammonia, and sulfide)
rising from the anaerobically digesting settled solids, which offsets to some extent any
settling of coagulated dissolved solids in the primary cell. Additional depth should be pro-
vided in primary cell(s) for settled solids, both for anaerobic digestion and for storage. This
additional depth normally should be no greater than about 6 ft (1.8 m). It has been recom-
mended that concentric baffles be placed around the inlet, in the middle of the sludge
storage area. These baffles should extend from 1 ft (0.3 m) above the cell bottom to about 4
in. (0.1 m) from the minimum operating water level (15), to keep wind action from mixing
©2 into the anaerobic layer and stopping CH4 fermentation.
In stabilizing the sludge, gases are given off that return some BOD to the overlying waters
during warmer weather, and also help to keep pond water mixed.
In hot weather, facultative pond water depths (exclusive of sludge storage) should be main-
tained between 3 and 5 ft (0.9 and 1.5 m) to control weed growth and improve odor con-
trol. In areas where icing occurs, additional depth must be allowed for wastewater storage
for periods in which ice cover, ice breakup, or thermal overturn prevents the effluent (in the
absence of polishing processes) from meeting quality requirements. Operating depths, in
general, can vary, depending on local conditions, as indicated in Table 10-3.
The primary biological reactions occurring in a facultative pond are bacterial synthesis of
new cells (symbiotically with algal photosynthesis), followed by bacterial and algal endo-
genous respiration. Bacteria utilize reduced organic compounds as substrates; algae utilize
10-11
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1 —
1
* J
— —
1
Oi
! .IT
A. SINGLE
r~
i
i
2
1
3
k>1
I ,
S
B. SERIES
1
1
4
1
1
•r **
C. PARALLEL
3
i
i
-i*.
4
D. PARALLEL-SERIES
1
1
r
1
1
1
* :
I
t
?
*
t
1
I
1
r c
E. PARALLEL - SERIES
LEGEND
1,2,3,4 NO OF UNIT IN SERIES
Q CELL IN POND SYSTEM
^- NORMAL FLOW
*» POSSIBLE RECIRCULATION
O RECIRCULATIDN PUMP
FTI SOLIDS REMOVAL UNIT
FIGURE 10-3
FACULTATIVE POND SYSTEM CONFIGURATIONS
10-12
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TABLE 10-3
RECOMMENDED DESIGN CRITERIA
FACULTATIVE WASTEWATER TREATMENT PONDS
Average winter air temperature
Above 59° F 33° F to 59° F Below 33° F
(15° C) (0° Cto 15° C) (0°C)
Total System
Influent BOD5, Ib/acre/day 40 to 80 20 to 40 10 to 20
Operating Depth, ft 3 to 5 4 to 6 5 to 7
Total Retention Time, days1 25 to 40 40 to 60 80 to 180
Initial Cell of Multi-cell System
Sludge Storage Section, Extra
Depth, ft 2 to 4 3 to 5 4 to 6
Sludge Detention, years 5 to 10 5 to 10 5 to 10
Depth Above Sludge Storage, ft 3 to 5 4 to 6 5 to 7
Retention Time, days 5 to 15 15 to 30 30 to 80
BOD5,lb/acre/day 120 to 180 60 to 120 30 to 60
locations where ice forms, consideration should be given to making detention time in
the ponds 150 to 240 days, or sufficient for the period of ice cover plus 60 days, unless
other means are provided to prevent odor and to polish the pond effluent. If strong winds
(which prevent good sedimentation) frequently occur, the orientation of the long dimen-
sions of the pond should be about 90° to the prevailing strong wind direction, wind breaks
should be provided, and/or retention times increased.
inorganic compounds for synthesis of new cells. A secondary biological action is the con-
sumption of microscopic bacteria and algae by macroscopic protozoa, rotifers, and crusta-
ceans.
The micro-organism growth rate is also dependent on the food-to-micro-organism ratio
(F/M). The byproducts of biochemical actions caused by the exoenzymes of some
micro-organisms are food for other biota. All essential nutrients must be balanced, which
sometimes requires additions of one or more nutrients to insure the most efficient
treatment. Mixing in the aerobic zone and in the anaerobic zone of the primary cell(s) tends
to maintain the balanced population of organisms that most efficiently and rapidly
stabilizes the waste organic material.
For construction grant participation, the U.S. EPA currently requires that a series of at least
three cells be provided for flow-through ponds, with the initial cell sized to prevent anaer-
obic conditions near the surface. The applicant must also agree to provide positive disinfec-
tion, if natural pathogen removal does not meet effluent requirements (6).
10-13
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In ponds receiving settled wastewater, those with flows approximating plug flow have
proved to be more efficient than those with completely mixed or nonideal flow in the re-
moval of BOD, SS, and pathogens (4) (8). To achieve plug flow, transverse end-around
baffles, creating length-to-width ratios of up to 25 to 1 (from inlet to outlet), depending on
the depth and width, have proved to be highly efficient (9). The optimum length-to-width
ratio can be obtained using dye-diffusion tests in pilot plants or existing installations (8) (9).
General criteria currently in use for the design of facultative wastewater treatment ponds (if
more specific data for the wastewater to be treated are unavailable) are given in Table 10-3.
To insure less than 30 mg/1 of SS in continuous discharge pond effluent, it may be necessary
to increase the detention times or to add further solids removal, using processes such as
intermittent sand filters (16), rock filters (17), or chemical coagulation with sedimentation
or flotation and/or filtration (18).
Influent BOD can be synthesized into new cells in 1 to 3 days in the summer and 8 to 10
days in the winter, if there is continuous mixing of the pond's upper layers and adequate
DO for metabolism (15). Endogenous respiration proceeds very slowly; thus, relatively com-
plete stabilization of the organic matter (including lysed dead cells) in well-mixed, aerated
wastewater will take more than 20 days, if the water temperatures are above 68° F (20° C)
and up to 80 days, if the water temperature is near 33° F (0° C), and then only if adequate
mixing and DO are present (15).
Studies indicate that the data in Table 10-4 apply, in general, to facultative wastewater
treatment ponds treating domestic wastewater with pond water at 68° F (20° C) and influ-
ent containing 200 mg/1 BOD5 (19):
TABLE 10-4
BIOLOGICAL ACTIVITY DATA FOR PONDS
Bacteria Algae
Cell Endogenous Cell Endogenous
Synthesis Respiration Synthesis Respiration
Retention Time, days 1 to 2 90 to 1601
Oxygen Required, mg/1 112 150 183
Net Oxygen Produced, mg/1 229
VSS Produced, mg/1 154 187
Inert Organic Solids Produced, mg/1 30 37
Inorganic SS, mg/1 15
NH3 -N Released, mg/1 13 16
CO2 Released, mg/1 108 206
Empirical Organic Formula C5H9O3N
Oxygen Released From Alkalinity By
Algae During Synthesis, if 300 mg/1
of Alkalinity Present, mg/1 96
1 Depending on the rate of sedimentation of cells.
10-14
-------
10.4.2 Process Design of Facultative Ponds
Because the process and hydraulic designs of facultative ponds are interdependent, the first
step is to determine whether the pond will be flow-through, controlled discharge, or com-
plete retention. This selection can be based on the discussions in Section 10.3.1, which
recommend complete retention ponds only for drier, desertlike regions and controlled dis-
charge for those regions with definite seasonal change in climate accompanied by a cold
winter, which suspends algal growth. The second step in process design is to determine
optimum pond configuration (including possible baffles and recirculation) and operating
depths, exclusive of freeboard, sludge storage, and ice storage.
It is not necessary to complete the synthesis and endogenous respiration processes in the
initial cell(s) of a facultative pond system, as long as most of the settleable solids are re-
moved.
Desirable objectives for the design of the initial cell(s) of a multicell series facultative pond
system are:
1. Removal of the maximum amount of SS
2. Establishment of 5- to 10-year sludge storage capacity
3. Maintenance of at least 1.0 mg/1 of DO in the upper 2 to 3 ft of the pond
Most of the settleable solids in the raw wastewater will be removed in the first few hours.
Biochemical action will cause precipitation of additional solids which, with the cellular
material synthesized from the organic material in raw wastewater, will also form settleable
floe with SS and colloidal solids.
For design purposes, the amount of solids settled can be approximated by the equivalent of
the SS in the influent. These settled solids may include 20 to 40 percent of the initial BOD5.
To obtain a 5- to 10-year sludge storage capacity, the designer may assume that the sludge
compacts to about 6 percent dry solids. The maximum sludge storage depth should be no
more than about 6 ft (1.8 m). The sludge storage requirement establishes the minimum area
in the primary cells(s) and, in turn, the minimum hydraulic retention time.
The major oxygen source available to the initial cell(s) to satisfy BOD is algal photosynthesis,
with small amounts of DO resulting from air dissolving into the water at the air-water
interface.
The oxygen available from solar radiation can be approximated by the following equation
(15)(20):
Y0 * 0.5 Smin
10-15
-------
where
Y0 = oxygen yield, Ib 02/acre/day (Kg/ha/day)
Sm|n = solar radiation for the month with the least solar radiation,
cal/cm2/day (see Table 10-5).
Latitude
TABLE 10-5
APPROXIMATE VALUES1 OF SOLAR ENERGY (21)
Month
Degree
N or S Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec.
0 max
min
10 max
min
20 max
min
30 max
min
40 max
min
50 max
min
60 max
min
255
210
223
179
183
134
136
76
80
30
28
10
7
2
266
219
244
184
213
140
176
96
130
53
70
19
32
4
271
206
264
193
246
168
218
134
181
95
141
58
107
33
266
188
271
183
271
170
261
151
181
125
210
97
176
79
249
182
270
192
284
194
290
184
286
162
271
144
249
132
236
103
262
129
284
148
296
163
298
173
297
176
294
174
238
137
265
158
282
172
289
178
288
172
180
155
268
144
252
167
266
176
272
177
271
166
258
147
236
125
205
100
269
207
266
196
252
176
231
147
203
112
166
73
126
38
265
203
248
181
224
150
192
113
152
72
100
40
43
26
256
202
228
176
190
138
148
90
95
42
40
15
10
3
253
195
225
162
182
120
126
70
66
24
26
7
5
1
1 Solar radiation (S), cal/cm2/day (x 0.049 = W/m2)
For design purposes, the BODs loading on the initial cell(s) should be restricted to the oxy-
gen yield, as expressed in the above formula.
The oxygen available from surface transfer varies with the thickness of the liquid film at the
air-liquid interface. Surfactants, greases and oils, and wind action (which causes turbulence
at the surface) have a significant effect on the rate at which O2 diffuses through the film
into the water. The maximum rate in a year may be on the order of 1,000 times the mini-
mum rate. Therefore, unless there is going to be an appreciable breeze every day, the oxygen
transferred through the surface is ignored at this time.
10-16
-------
The initial cell(s) of a multicell pond system should be designed as completely (relatively)
mixed unit(s), depending on inlet velocities, gaseous products from anaerobic decomposi-
tion, surface winds, and thermal currents for mixing in the aerobic zones. The mixed flow in
the primary cell(s) may be approximated by the following idealized, previously stated, first-
order reaction equation:
t=(Lj-Le)Lk
The initial cell(s) should be designed so that they can be loaded individually, in series, or in
parallel. Thus, one unit can perform alone at startup or during sludge removal, and later, as
the load increases, be used in either series or parallel operation. Primary cells should be
nearly square in configuration to insure an even BOD loading and deposit of sludge.
To prevent possible odor problems (which may occur after spring thaws, overloading the
summer oxygen resources, or during thermal turnovers), recirculation of well-aerated efflu-
ent from subsequent cells to the primary cells may be provided to add DO and reduce the
BOD load on the primary cell(s) in a series of cells. Such recirculation of seed alga also re-
duces the possibility of major reductions in algal synthesis and oxygen production by toxic
loadings or by maco-organisms that prey on the more desirable algae.
The effluent from the primary cell(s) should pass through a minimum of two, and prefer-
ably more, additional cell(s) in series, with length-to-width ratios in each cell as large as
possible (up to about 25, when using baffles), keeping the width no less than twice the max-
imum operating depth (8).
If the quality of the effluent is limited only to requirements for removal of BOD, SS, and
pathogens (and not nitrification), the k value can be determined in a pilot plant, using
BODs in the determinations, particularly if the pond system effluent is to be polished be-
fore discharge. If tertiary SS removal is planned, ponds subsequent to the initial pond(s)
only require detention time sufficient to complete the synthesis of the raw wastewater BOD
organics to microbial cells. Conversely, endogenous respiration should be relatively com-
plete, if the pond system effluent is to be discharged without further treatment. Time re-
quired for this more complete stabilization, as stated in Section 10.4.1, will be over 20 days
under the best conditions (when the water temperature is over 20° C), and up to 80 days
when the temperature is near 0° C—and only if proper conditions exist for the removal of
algal and bacterial cells throughout the system.
The necessary retention time can be estimated for the remaining ponds arranged in series,
utilizing the previously stated idealized plug flow equation:
t = In (Lj/Le)/k
10.4.3 Hydraulic and Physical Design of Facultative Ponds
A well-designed facultative pond should minimize upsets, maintenance, and nuisances and
maximize operational flexibility, stability, and BOD and SS removal. Hydraulic and physical
10-17
-------
design features that should be considered include hydraulic retention time, configuration,
recirculation, feed and withdrawal variations, pond transfer inlets and outlets, dike construc-
tion, baffles, bank protection, and algae removal.
10.4.3.1 Hydraulic Retention Time
The actual minimum hydraulic retention time in overflowing ponds varies with the degree of
mixing (or short circuiting) and the rates of rainfall, evaporation, and percolation. Winds,
causing intense mixing and short circuiting, in conjunction with high rates of rainfall or
ground water inflow, tend to decrease the actual detention time, while a high rate of evapor-
ation and percolation tends to increase the actual minimum hydraulic detention time in the
pond. Wind effects can be controlled, to some extent, by orienting the ponds so that the
longer dimension is in the direction of the prevailing wind (with the flow direction opposite
that of the wind if mixing is desirable) and at 90° if mixing is not desired. Wind barriers,
either artificial (snow fence or rigid baffle types) or natural (evergreens), may also be used
to modify the effects of the wind.
A high water table will generally reduce percolation (if wastewater will not adversely affect
the ground water), or impermeable barriers on the pond bottoms may be used to decrease
percolation. In sandy soils, the percolation rate must often be reduced to insure protection
of ground water. The percolation test for ponds is different from that for septic tank drain-
age fields, because a constant hydraulic head is available at the ponds. For percolation test
procedures, see reference (22).
10.4.3.2 Configurations
Basic configurations of pond cells are shown in Figure 10-3. The actual shapes will, of
course, vary with the site, the quality and quantity of wastewater, and the climatic condi-
tions expected.
Single-cell ponds are not as efficient as multicell series ponds in reducing algal and bacterial
concentrations, color, and turbidity (15) (23). At least three, and preferably four to six,
series cells should be used (15). The pond system should be designed to allow any one cell
to be taken out of operation for cleaning and to allow parallel flow through the first cells
(where sludge deposits must be occasionally removed), followed by series flow through the
remaining cells.
The parallel configuration more effectively reduces pond loadings and is less likely to pro-
duce odors in the initial cells. The first cell of the series configuration is more likely to be
overloaded and to produce odors. On the other hand, the series configuration is more likely
to have a lower concentration of SS, BOD, and coliforms in the effluent of the last cell in
the series. The primary cell(s) of a series system must be designed for provision of sufficient
oxygen to satisfy the nonsettleable BOD requirements and the benthos BOD requirements.
10-18
-------
10.4.3.3 Recirculation
Recirculation refers to intercell and intracell recirculation, rather than to mechanical mixing
in the pond cell. Intercell recirculation occurs when the effluent from a downstream cell is
mixed with the influent to an upstream cell. Intracell recirculation occurs when part of the
effluent from a pond cell is mixed with the influent to the same cell. Both methods return
active algal cells to the feed area, to provide additional photosynthetic oxygen production
capacity to help satisfy the organic load. Intracell recirculation provides some of the advan-
tages of a completely mixed environment. Intercell recirculation dilutes the influent to a cell
with an aerated, lower BOD effluent and, therefore, helps prevent odors and anaerobic
conditions in the feed zone of that cell.
Both intercell and intracell recirculation can reduce stratification and thus produce some of
the benefits ascribed to pond mixing. Pond recirculation is not generally as efficient as are
mechanical or diffusion aeration systems in mixing facultative ponds. However, increased
mixing by aeration can also increase the rate of heat loss (which reduces the water tempera-
ture and, in turn, the rate of biological activity) and may interfere with anaerobic methane
fermentation by introducing oxygen into the bottom sediment.
Some of the more common intrapond and interpond recirculation patterns are illustrated in
Figure 10-3.
An advantage of intercell recirculation is that the BOD concentration in the mixture enter-
ing the pond system can be reduced as follows:
0+r)
where
Lm = BODs of mixture, mg/ 1
Le = effluent BODs from last cell, mg/1
Lj = influent BOD5 , mg/1
recycle flow rate
r = R/q = — - - - - = recycle ratio
influent flow rate
By use of recirculation, the organic load can be applied more evenly throughout the cells,
and organic loading and odor generation near the feed points are minimal.
One disadvantage of intercell recirculation is that the retention time of the liquid in each
cell is reduced. The hydraulic retention time of the influent and recycled liquid in the first,
most heavily loaded cell in the series system is (24):
10-19
-------
(l+r)q
where
t = retention time, days
v = volume in primary cell, ft-*
q = influent flow rate, ft* /day
r = recycle ratio
Recirculation may be accomplished with high volume, low head propeller pumps; nonclog,
self-priming centrifugal pumps; Archimedes screw-type pumps; or air lift pumps. Reference
(24) contains a simplified cross section of a propeller pump installation.
Interpond recirculation systems may provide for recycle rates of up to eight times the aver-
age design flow rate (25). Because of the higher recycle rates, a means for thorough mixing
before introduction to the cell is necessary. This mixing can take place in a manhole. Inter-
pond recirculation should ordinarily be from the effluent of the next-to-last series cell to the
influent of the primary cell.
The cost and maintenance problems associated with large discharge flap gates can be elimi-
nated by a siphon discharge. An auxiliary pump with an air eductor can be used to maintain
the siphon. Siphon breaks should be provided to insure positive backflow protection.
Pumping stations should be designed to maintain full capacity with minimal increase in
horsepower, even if the inlet and discharge surface levels fluctuate over a 3- to 4-ft (0.9 to
1.2 m) range. Multiple and/or variable speed pumps can be used to adjust the recirculation
rate to seasonal load changes.
10.4.3.4 Inlets, Outlets, and Short Circuiting
In a California study of short circuiting (26), it was found that some particle detention times
were as low as 1 percent of the calculated time and mean detention times could be 10 per-
cent of the calculated time. Other studies using dyes in stabilization lagoons in South
Africa confirmed that actual particle detention times, using customary designs, often were
less than 1 percent of design time (26). Short circuiting was found to be as much the result
of thermal stratification as of inadequate inlet-outlet design.
Pond configuration should allow full use of the wetted pond area. Transfer inlets and outlets
should be located to eliminate dead spots where scum can accumulate and any short circuit-
ing that may be detrimental to photosynthetic processes. Pond size need not be limited, as
long as proper distribution is maintained by baffling or dikes.
10-20
-------
A single feed pipe and inlet near the center of the pond and a single exit have been com-
monly used since the 1920's, particularly for raw wastewater ponds. However, this usually
results in short circuiting and inconsistent removals of BOD and SS.
The locations and types of inlets and baffles used within each pond can have a major effect
in controlling short circuiting. Typical arrangements are shown in Figure 10-4. To prevent
clogging, single inlets should only be used in the primary cell(s) of a multicell series pond
system treating raw wastewater. They should be located over the deepest part of the sludge
storage area, at a point 8 to 12 in. (0.2 to 0.3 m) above the expected final sludge storage
level and directed vertically upward. In larger primary cells, several inlets, instead of one,
will assist in uniformly dispersing the sludge deposits and the flow. Inlets in primary cells
should discharge vertically, at a point near the top of the sludge storage area. Neither the
inlet pipe nor the nozzles should be less than 4 in. (0.1 m) in diameter, to prevent plugging,
and should be provided with flushing connections.
Multiple inlets can be achieved by placing an inlet pipe that has multiple ports or nozzles
pointing at an angle slightly above the horizontal on one side of a pond cell. The nozzle
head losses should be about 1.0 ft (0,3 m) each, to obtain good mixing velocity and to re-
duce thermal current short circuiting.
If intermittent discharge is desired, it is necessary to design the pond cell outlets so that the
cell water levels can rise several feet while storing wastewaters (as shown in Figure 10-4).
This design requires one or more of the following:
1. Placing one or more manholes, with adjustable height weirs, in the intercell
transfer pipe or transfer channel, to control water levels in the preceding cell.
2. Placing a transverse pipe header, with multiple risers 1.0 to 2.0 ft (0.3 to 0.6 m)
above the bottom, to eliminate the need for a sludge baffle and to reduce the
need for a scum baffle. The risers should be in the form of reducers, to create a
small velocity of about 0.5 ft/sec (0.15 m/s) within the neck of the riser as the
flow reaches the header, to insure an evenly distributed flow. An alternative
method of creating a multiple outlet, which can be used independently or in con-
junction with the first method, is a baffle wall with orifices. The velocity through
the orifices in the baffles or into the mouth of the risers should not exceed about
0.1 ft/sec (0.03 m/s), to prevent resuspension of settled solids or the displacement
of algae from the water surface.
3. If icing is not a problem, using a combination of scum (or floating algae) baffles
extending from above high water to 6 in. (0.15 m) below low water, a bottom
sediment barrier from the bottom up 1.0 to 2.0 ft (0.3 to 0.6 m), and an outlet
or series of outlets also located from 1.0 to 2.0 ft (0.3 to 0.6 m) above the
bottom. If ice is a problem, such scum baffles should be removable for the period
of ice cover. Such scum baffles can be floating and anchored in place.
To obtain maximum removal of microbial cells by settling, the quiescent area near an out-
let must be designed to attain a surface overflow rate during peak flows of less than about
800 gpd/ft2 (32 m^/m^-d). The velocities over the area surrounding the outlet should be
10-21
-------
/
\/
-»-
•*-
SINGLE INLET-
SINGLE OUTLET
MULTIPLE INLET
DOUBLE OUTLETS
WITH UNDER AND
OVER BAFFLES
MULTIPLE INLET-
MULTIPLE OUTLET
PIPE INLET WITH NOZZLES
OUTLET WITH SCUMAND
OVERFLOW BAFFLES
WOODEN OUTLET
WITH ORIFICES-j
\/
/\
1 1 '
I*
^_ — - — •
//
i
t
—
t
\
-MULTIPLE
INLET
— EARTH DIKE
SINGLE INLET
DIKE BAFFLE AND VANE
SINGLE BAFFLED OUTLET
BAFFLED
DOUBLE
OUTLET
MULTIPLE BAFFLES
AND VANES
IN SERIES POND
FIGURE 10-4
INLETS, OUTLETS, AND BAFFLE ARRANGEMENTS
10-22
-------
less than about 4 to 5 ft/min (0.02 to 0.025 m/s), to prevent scour of settled algal and
bacterial cells, which have about the same density as water. Because winds cause most of the
turbulence, the outlet of the scum baffles should be relatively rigid, to absorb the shocks of
waves and not pass them on. It may be necessary to place one or more wind baffles, in addi-
tion to the scum baffle, to obtain sufficient quiescence for the algae to either float or settle
to the bottom. This latter can be achieved by sludge baffles, such as shown on Figure 10-5,
placed around the outlet at a distance sufficient to keep the flow over the baffle at reduced
velocity.
To reduce short circuiting in large cells, dikes or baffles, as shown in the bottom plans of
Figure 10-4, may be constructed to channel the flow. Extensive research has indicated that,
where feasible, placing parallel baffles in cells for end-around flow will decrease short cir-
cuiting and improve efficiency (8).
If the pond system is quite small, barrier walls with a few orifices (creating enough head loss
to insure uniform distribution), instead of dikes, can be used to divide a single-cell pond into
three or more separate cells for better BOD, SS, and coliform removal. Barrier walls in
northern areas must be designed to accommodate rising and falling ice cover without dam-
age (or be removable) and to permit storage for intermittent discharge, if that is an advan-
tage.
Pond and channel dikes usually can be constructed with side slopes between 6 horizontal to
1 vertical and 2 horizontal to 1 vertical. The final slope selected will depend on the dike
material and the bank erosion protection to be provided. All soils, regardless of slope, will
require some type of protection in zones subject to wave action, hydraulic turbulence, or
aerator agitation (for example, around the discharge areas at the recirculation pumping sta-
tion and areas around the influent and effluent connections). If the wind is primarily in one
direction, wave protection can usually be limited to those areas receiving the full force of
the waves. For small pond cells, protection should always extend vertically from at least 1 ft
(0.3 m) below the minimum water surface to at least 2 ft (0.6 m) above the maximum water
surface. Protection against hydraulic turbulence should extend several feet beyond the area
subject to such turbulence. Protective material should not impede the control of aquatic
plant growth. Under all circumstances, dikes should be a minimum of 1 ft (0.3 m) vertically
above maximum wave-induced high water.
Typical design details currently in use are presented in references (8), (9), (15), (23), (24),
(26), (27), and (28). Only designs that insure that the effluent will consistently meet State
and Federal requirements should be chosen, taking into consideration the quality of
control, operation, and maintenance that can be expected.
10.4.3.5 Designing for Good Maintenance
All types of slope protection, such as grass, asphalt, or crushed rock, require regular mainte-
nance and at least semiannual rehabilitation. Odor problems result when scum and sludge
build up at the edge of ponds in weeds, tall grass, or crushed rock. Grass sod and asphalt
provide the easiest-to-clean slope protection and are odor free when well maintained. The
10-23
-------
-ADJUSTABLE HEIGHT
OVERFLOW WEIR
~ INTERCELL
VTRANSFER PIPE
0.3m RIPRAP AT EACH END OF PIPE-
SIMPLE DIKE CROSS SECTION AT TRANSFER PIPE WHERE INTERMITTENT DISCHARGE REQUIRED
f—
-WOOD WALL BARRIER
AND WIND BAFFLES
High Water Level
-Multiple Orifices
Between Posts
HI
MULTIPLE
RISER OUTLET
i i
TRANSVERSE
HEADER PIPE
INTERCELL
TRANSFER PIPE
POST
3'
REVETTMENT
BAFFLED MULTIPLE OUTLET USING HEADER PIPE
-TREATED WOOD
SCUM a WIND
BAFFLES-
Max. Water Level
INTERCELL
TRANSFER
PIPE(S)
TREATED WOODEN POSTS
SCUM AND SLUDGE BAFFLES IN FRONT OF MULTIPLE TRANSFER PIPE OUTLETS
FIGURE 10-5
DIKE AND OUTLET DESIGN DETAILS
10-24
-------
tops of the dikes should be at least wide enough for a 10-ft (3 m) all weather gravel road.
Such a road is essential for pond inspection and for the control of insects, erosion, and plant
growth on the dike surfaces. Because a boat is essential to maintenance, a boat ramp with
paved surfacing at one corner of each pond is helpful. The boat ramp(s) should be placed
downwind, where algae and floating debris might collect, to assist in the removal of the
floating solids. Proper levee maintenance can be an important aid in controlling shoreline
problems.
Without maintenance and good design, aquatic growths may develop in ponds. Pond depths
greater than 3 ft (0.9 m) will discourage rooted growths. If not suitably controlled, plants
can choke off hydraulic operation and create large accumulations of floatable debris. Such
debris usually becomes septic and creates odors and conditions detrimental to photosyn-
thetic activity.
In addition to regular outlets, provisions must be made for overflows as an .alternative
method of drainage, in the event outlets become plugged, and for maintenance. The over-
flow unit can be simply an intercell connecting pipe flowing through a manhole divided by
an adjustable weir. Submerged overflow units must be periodically operated to insure that
they are clean.
Provisions for storm water and high ground water must also be included. All streams and ex-
pected runoff must be diverted around or piped under the pond system. Pond dikes must be
above any expected flood levels.
10.4.4 Belding, Michigan, Intermittent Discharge Pond System
Studies on the performance of the five-cell waste stabilization pond system at Belding,
Michigan, indicate that intermittent discharge can produce an effluent meeting secondary
treatment standards (13). Belding, with a population of 4,000 and several light industries,
has a collection system subject to infiltration. The primary cell is small, only large enough to
remove settleable solids, and functions as an anaerobic pond. The remaining four ponds,
which operate in series, have areas of 20, 25, 7.5, and 7.5 acres (8, 6, 3 and 3 ha), respec-
tively. The cells of this multicell system were designed to meet the Great Lakes-Upper
Mississippi River Recommended Standards for Sewage Works (29). Table 10-6 lists the char-
acteristics of the cell influents and effluents.
10.4.5 Example Computations
An example of the design computations for a facultative pond system that will consistently
meet secondary treatment standards, given that the minimum average daily water tempera-
ture is about 5° C (35° F) and ice cover is not a problem, is presented below:
Site Data:
Latitude 35° N.
Influent BOD, mg/1 200 (Lj)
Influent SS, mg/1 250 (Ss)
10-25
-------
TABLE 10-6
BELDING, MICHIGAN INTERMITTENT DISCHARGE POND SYSTEM (13)
Quality of Contents of Five Lagoons Operated in Series at Belding, Michigan
Analysis
DO,mg/l
NH3-N, mg/1
N03-N, mg/1
PH
Total P, mg/1
OrthoP, mg/1
SS, mg/1
Influent
Raw
Wastewater
0.0
34.6
0.16
7.1
12.5
10
121
Eff. from
Pond
No. 1
0.0
27.3
0.2
7.3
9.9
7.8
76
Effluent quality-Pond No.
Date
1973
11-5
11-7
11-13
11-20
11-22
DO
mg/1
10.5
10.7
10.8
9.7
10.0
BOD5
mg/1
3.0
8.9
10.3
9.4
8.7
Suspended
Solids
mg/1
52
60
102
78
52
Eff. from
Pond
No. 2
27.8
2.7
0.44
8.6
3.0
1.6
146
5, Late Fall and
NH3-N
mg/1
2.4
5.58
5.58
5.82
5.22
Eff. from
Pond
No. 3
11.2
0.4
0.5
8.6
2.5
2.0
31
Eff. from
Pond
No. 4
5.0
0.6
0.19
8.0
4.4
3.4
17
Eff. from
Pond
No.5
10.8
0.5
0.08
8.6
2.9
2.3
22
Whiter Discharge
N03-N
mg/1
0.35
0.33
0.41
1.1
0.97
Total
P
mg/1
2.7
3.9
3.9
3.9
3.5
Total Coli
No./ 100 ml
—
—
—
—
—
Total Discharge-57.6 Million Gallons
1974
1-15
1-18
1-22
1-25
1-29
9.7
10.9
8.2
5.0
10.5
7.2
1.4
5.4
1.2
2.4
Total Discharge— Approximately
4-26
4-29
5-1
5-6
5-7
5-8
5-13
17.5
12.0
10.7
9.4
10.0
10.3
9.6
6.7
10.5
7.8
8.9
9.8
7.0
9.1
9.5
11
13.5
12.5
30
5.7
5.96
7.4
9.0
10.8
0.82
0.66
0.22
0.16
0.15
3.4
3.6
4.0
4.4
5.1
—
—
—
—
—
38 Million Gallons
53
30
16
23
12
16
27
3.3
3.5
2.6
1.0
0.75
0.8
0.1
1.1
1.0
1.3
1.4
1.5
1.4
1.0
2.8
2.9
3.0
3.2
2.8
2.7
2.5
—
<100
<100
1,800
—
5,500
<100
Normal Daily Flow—No Retention
10-26
-------
Flow (including infiltration), gpcd 100 (q)
Population 2,000 (N)
Precipitation, in./yr 35 (900 mm/yr)
Evaporation, in./yr 35 (900 mm/yr)
Minimum Water Temperature, °C 5 (Tm)
Effluent BOD5,mg/l <30 (Le)
Effluent SS, mg/1 <30 (Se)
Because BOD5 does not include about one-third of the carbonaceous BOD, nor a significant
portion of the nitrogenous BOD, it is assumed that this additional BOD can be absorbed by
the receiving waters without damage.
Assumptions
1. Flow-through system
2. Configuration: two parallel primary cells, followed by three cells in series (see
Figure 10-6A)
3. Recirculation: up to 400 percent recirculation of second-series cell effluent to
primary effluent
4. Additional treatment: chlorination facilities only for initial design, pending results
of 1 year of operation of pilot plant, placed in one of the primary cells
5. Reaction rate coefficient: pending results of 1 year of operation, assume k2o
might vary from 0.15 to 0.10
1. Primary Cell Design
Two primary cells in parallel. The volume of daily flow is:
Q = (2,000 persons) (100 gpcd) = 200,000 gpd (800m3/day)
V = (200,000 gpd)/(7.48 gal/ft3) = 26,800 ft3/day (784 m3/day)
Assuming that after 5 years the equivalent of 100 percent of the SS compacted to 6 percent
dry solids, the sludge storage volume needed would be:
v = (250 mg/l)(8.331b/106gal/mg/lK0.2mgdK 1,825 days)
(63 Ib/ft3)(0.06)
= 201,088ft3 (5,630.5m2)
Assuming 3 ft (0.9 m) average depth, the area required is:
A = 201,088 -f 3 = 67,029 ft2 (6,166.7 m2) or 1.5 acres (0.6 ha)
10-27
-------
Using two storage areas, the dimensions would be 185 ft X 185 ft (57.4 m X 57.4 m), with
the bottom of the sludge storage sloped from 2 ft deep (0.6 m) at the edges to 5 ft deep (1.5
m) at the middle for each primary pond.
Assume that 1.5 times this sludge storage area is required in the primary units, with a 3-ft
depth of waste water above sludge storage. The detention time will be:
t= (201,088X1. 5)/26,800
= 1 1 days
The BOD5 loading per acre on the primary ponds should be restricted to one-half the mini-
mum available solar energy. Half the minimum solar energy, at 35° N latitude, from Table
10-5, is equal to approximately one-half the average of the minimum solar energies at 30° N.
and 40° N.:
Lmax = (70 + 24)/(2)(2) - 23.5 Ib BOD5/acre (26.3 kg/ha)
The maximum probable BODs reduction would be obtained by using the plug flow formula.
Assume 1) 20 percent of BODs is removed within a few hours by settling, 2) t = 11 days,
3)6 = 1.08, and4)k20 = 0.10.
kT =
k5 = 0.10/1.08C20-5)
= 0.10/3.17
= 0.0315
L = L-/ekt
6 = (0.8)(200)/2.78(°-0315)(11)
= 160/2.780-35
= 160/1.43
= 112
Maximum BOD5 removal = (160 - 1 12)(8.33)(0.2 mgd)
= 80 Ib/day (36.32 kg/day)
The minimum probable BODs reduction would be obtained by using the complete mix
formula. Assume 1) Lj = 160 mg/1, 2) t = 1 1, 3) 6 = 1.08, and 4) k20 = 0.10. Therefore:
k5 - 0.0315
Le = Lj/d+kt)
= 160/1.35
= 119
10-28
-------
Minimum BOD5 removal = (160 - 1 19)(8.33)(0.2)
= 68 Ib/day (30.87 kg/day)
Pending the results of the pilot plant study, assume the BOD5 requirement is the average of
the probable maximum and minimum
Average BOD5 removed = (80 + 66)/2
= 731b/day(33.14kg/day)
From Table 10-2, it can be assumed that the k values can be reduced about one-third by 100
percent recirculation, during emergencies. Thus, the area required to provide the needed
oxygen is:
A = (73)(0.67)/23.5
= 2.1 acres (0.85 ha)
= (2.1)(43,560 ft2/acre) = 91,500 ft2 (8,509.5 m2)
Two cells, 210 ft by 220 ft (63 m X 66 m) in parallel, each containing sludge storage areas
varying in depth from 2 ft (0.6 m) at the edges to 5 ft (1.5 m) at the middle, will meet pre-
liminary design requirements.
Sludge Baffles
Place two sets of baffles parallel to the sides and 35 ft (10.5 m) and 70 ft (21 m) from the
inlet (see Figure 10-6A). These baffles are to extend from 1 ft (0.3 m) above the bottom to
2 ft (0.6 m) above the sludge storage surface, to reduce velocities near the top of the sludge.
Most of the sludge accumulation will be inside these baffles.
Inlet
Locate three in the center of sludge storage areas, discharging vertically at 6 ft (1 .8 m) above
bottom and 1 ft (0.3 m) above maximum sludge elevation.
Outlets
Length of sludge baffle needed to reduce velocities to less than 5 fpm (1.5 m/min) is:
L = 26,800/(60)(24)(2) = 9.3ft(2.8m)
With a diameter of:
D - (9.3)(4)/ir= 12 ft (3.6m)
Use two concentric surface baffles, with radii of 16 and 8 ft (4.8 and 2.4 m) in the corners
around the outlet, and a 6-ft-radius sludge baffle. Outlet will be vertical with a metal cap
(see Figure 10-6B).
10-29
-------
u-i CD m z
o m -n o
z =! c: £ z
m o -H z -i
o) 2 m m
o i
3J .
m
X
-o
|—
m
Z O
o £
- 35
~ "3
<
w
o
E POND SYSTEM
O SCALE)
GUR
SECTION A-A
c
r~
r
CO
o
CO
-I
m
o
o
rr-
1
1
1
L
T
1
1
1
POSSIBLE
REQUIRES
POSSIBLE
REQUIRES
STORAGE POND IF ICE COVER
INTERMITTENT DISCHARGE
STORAGE POND IF ICE COVER
INTERMITTENT DISCHARGE
1
1
1
1
J
^
1
1
1
^
-------
INFLUENT
CHLORINATION
OUTLETttyp.;
PILOT POND SYSTEM CONFIGURATION
REMOVABLE
SURFACE
BAFFLES
REMOVABLE
SLUDGE BAFFLE
DIKE
2 REMOVABLE SURFACE
BAFFLES^
B
" EFFLUENT
REMOVABLE
METAL CAP
REMOVABLE SLUDGE
BAFFLE
REMOVABLE RISER
OUTLET DETAIL
SECTION B-B
FIGURE 10-6B
EXAMPLE FACULTATIVE POND SYSTEM
(NOT TO SCALE)
10-31
-------
2. Series Ponds Design
Three in series will follow the two parallel primary ponds. The total BODs in the primary
pond influent will have been reduced by the BODs removed in the settled solids and by cell
synthesis. Under the worst circumstances, the minimum BODs reduction would be about
30 percent of the nonsettleable BODs . Without good pilot studies, this should be assumed.
Influent BOD5 to the first cell of the series, assuming no recirculation:
^ = (0.7X1 60 mg/!BOD5)
Lj=112mg/l
(This is confirmed by the probable BODs removal above.)
Effluent BOD5 must be less than 30 mg/1.
Required detention time may be approximated, using plug flow formula. The value of k
may be approximated from:
assume
k2Q = 0.1
k5 = 0.1/1.08(20-5)
ks = 0.03
and
tT = ln(Li/Le)/kT
tT = In (112/30)/0.03 = 1.317/0.03
tj - 44 days
The area required in the series ponds is
A = 106(.2mgd)(44days)/(7.48gal/ft3)(3 ft deep)
A = 392,000 ft2 (36,064m2)
Assume the equivalent of a division dike at 2 ft above the bottom of a 7-ft-high dike, with 5
ft maximum water depth, 10 ft top width and 1 :3 side slopes.
10-32
-------
w= 10 + (2)(3)(5) = 40ft(12m)
The two primary ponds then will be
w = 2 (210) + 40 = 450 ft (135 m)
Using the configuration shown on Figure 10-6A, the width of each series cell will be
(450-80)/3= 123 ft (37m)
and the length of each cell will be
L = 392,000/(123)(3)= 1060 ft (318m)
Baffles
To decrease the short circuiting, the length-to-width ratio in each of the three series cells
may be increased from about 8:1 to about 32:1 by placing a 1,000-ft-long, 6-ft-high baffle
in each pond. This baffle addition would require relocation of the inlets and outlets. Pilot
studies will indicate whether such baffling is cost effective under the specific site conditions.
Inlets
In each of the three series cells where baffles are installed, place a 50-ft-long multiple-nozzle
pipe inlet on the bottom, with the nozzles pointed up and to the rear, to achieve good initial
mixing. If baffles are not installed, the pipe should be installed in a corner parallel to a side
pointing up and toward the opposite corner.
Outlets
The outlets of the series cells are to be the same as those of the primary ponds.
Chlorine Contactor
The simplest chlorine contactor is an effluent pipe, designed to flow full at all times with a
minimum detention time of 30 minutes at peak flow.
Emergency Storage
If the normal operational depth is kept to between 3 and 4 ft (0.9 to 1.2 m) a space above
the normal operational depth (up to 5 ft) will always be available for emergency storage.
This will provide between 20 and 30 days storage.
10-33
-------
3. Pilot Plant Design
A simple method for conducting comprehensive pilot studies would be to construct only
one primary cell initially. Before system flow reaches its design level, using temporary mov-
able baffles, transform it into a multicell series pilot plant. Sludge storage does not need to
be provided for the pilot study. This one primary cell unit, based on detention times, could
handle (1 l/2)/(l 1 + 44), or about 10 percent of the design flow and the wastewater gener-
ated by about (0.1)(2000), or 200 persons (the amount that reasonably might be connected
initially).
Configuration
Divide the one primary cell into one primary and three series cells, as shown in Figure 10-6B.
The area of asingle primary pilot cell would be 11/55 of the primary cell or (11/55)(46,200)
= 9,240 ft2 (850 m2) (about 95 X 100 ft). The widths of the series pilot cells would be
about 35 ft (10.5 m), making the combined length of the three cells 925 ft (278 m). This re-
sults in a total length-to-width ratio of about 26:1, the same ratio as in the design cells.
The configuration of this pilot-system series cells would be near enough to that of the pro-
jected, full-scale system to reasonably simulate the expected performance of the larger sys-
tem for further design purposes.
4. Modifications to the Basic Design (if significant ice cover)
Configuration
The same as in 1 and 2 (except any baffles that might be damaged by ice should be remov-
able, plus two nonbaffled additional cells in series parallel to one side of the first configura-
tion) (see Figure 10-6A).
Storage
To be achieved by making primary cells 1 ft (0.3 m) deeper (6 ft total, excluding sludge
storage) and subsequent cells 3 ft (1 m) deeper (8 ft total) and adding two cells 8 ft deep, to
provide 6 months of storage.
Additional volume required for 180 days' storage
V - (180 days) (26,800 ft3/day) = 4,830,000 ft3 (13,524 m3)
Volume available for storage by deepening primary ponds to 6 ft (2 m) and the series ponds
to 8 ft (2.4 m) is:
Primary ponds: (201,088 X 1.5) = 300,000 ft3 (84,000 m3) (Note: 3 ft storage available
above L.W.L.)
10-34
-------
Series ponds: (385,000 X 5) = 1,925,000 ft3 (Note: 5 ft storage available above L.W.L.)
Total 2,225,000 ft3 (62,300 m3)
Volume required in two storage ponds is:
V = 4,830,000 - 2,225,000 = 2,605,000 ft3
With a depth of 8 ft, the area required is:
A = 2,605,000/8 or 325,625 ft2 (30,283 m2)
Each storage cell could thus be 125 ft x 1,215 ft (37.5 m x 394.5 m).
10.5 Aerated Pond Design
If space is economically available, aerated ponds may be the most cost-effective system in
comparison to other treatment alternatives. Also, if existing facultative or aerobic ponds are
overloaded, aeration facilities may be added to meet pond oxygen requirements. Ponds de-
pendent only on algae and air-water surface transfer for oxygen require 3 to 10 times as
much volume as aerated ponds. BODc; loadings of up to 400 to 500 Ib/acre/day (450 to 550
kg/ha-d) can be used for aerated ponds subject to icing, in contrast to 40 to 80 Ib/acre/day
(45 to 67 kg/ha-d) for unaerated facultative ponds not subject to icing. Aerated ponds can
produce a more consistently satisfactory effluent through the winter and spring than can
facultative ponds. At least three cells are usually required to produce adequate separation of
the bacterial, algal, and other microbial cells produced during the metabolism of organic
waste matter. If the ponds are in porous ground, the cost of sealing is less for aerated ponds
and, in general, the costs of excavation and diking are much less.
Aerated ponds can be either aerobic or facultative. Aerated aerobic ponds are designed to
maintain complete mixing, which requires bottom velocities of about 0.5 fps (0.1 m/s)(2).
Aerated facultative (partially mixed) ponds are designed to maintain a minimum of 2 to 3
mg/1 of DO in the upper zone of the liquid. However, the slower mixing afforded in aerated
facultative ponds allows some settling of SS. The aeration system should be able to transfer
up to 2.0 Ib 02/lb BODs applied uniformly throughout an aerated facultative pond when
the water temperature is 20° C. Intermittently aerated ponds are designed only to provide
sufficient oxygen to prevent anaerobic conditions, usually during periods of ice breakup,
spring and fall turnover, icing, and at night during warm periods when daytime photosyn-
thesis alone would not provide sufficient oxygen. The oxygen requirements under ice cover
can be reduced to about 0.5 Ib/lb BOD5 applied, enough to sustain the much retarded bio-
logical decomposition processes at these temperatures.
10.5.1 Aerated Facultative (Partial Mix) Ponds
The facultative type of aerated pond is more commonly used than the aerobic for several
reasons:
10-35
-------
1. Separate sludge handling facilities, other than drying beds, are not required for
aerated facultative ponds.
2. Aeration equipment is much smaller because complete-mix scouring velocities are
not required in aerated facultative ponds.
3. Less operational control is required in aerated facultative ponds.
4. Less oxygen is required because some of the BOD5 is satisfied anaerobically in
aerated facultative ponds.
5. Extended-aeration, activated sludge or oxidation ditch systems are usually more
cost effective than the aerobic aerated system, which requires clarification and
sludge handling facilities.
In colder climates, aerated ponds should be at least 10 ft (3 m) and up to 20 ft (6.1 m)
deep, to minimize through-the-surface heat losses. Decreasing the area by 50 percent pro-
vides roughly a 7° F (4° C) higher wastewater temperature and results in a 50-percent in-
crease in microbial activity (23).
If detention time is less than 8 to 10 days, SS in the aerated pond effluent generally settle
well. Algal growth becomes increasingly predominant at detention times of 20 days and
above in aerated ponds. Ponds operated at a detention time of more than 20 days in warm
weather, therefore, are apt to have effluents with poor SS settling—similar to those of stabi-
lization ponds (30).
To accommodate mixing inefficiencies, surges, toxicity, seasonal nitrification, and other
factors leading directly or indirectly to peak oxygen demands, a safety factor of up to two
should be considered in designing oxygen-supply equipment based on BOD5 loading. Be-
cause of fluctuations in loading and temperature, simplification of operation, and other
factors, the oxygen requirement determination for sizing the aeration equipment should be
based on peak 24-hr summer loadings. However, the detention time in the aerated ponds
depends on the rate of metabolism during the coldest period of the year, when the oxygen
demand rate is at its lowest. Including the above recommended safety factor of two, the
soluble BOD5 at 20° C (78° F) should be synthesized into cellular material in 2 days (30).
Therefore, the configuration of ponds should be such that the detention time is kept be-
tween about 3 and 10 days in warm weather and between about 8 and 20 days in cold
weather, to reduce the possibility of growth of single-cell green algae.
Algae will be produced in aerobic facultative ponds, unless the velocity gradients are great
enough to prevent algae growth during the detention period. Usually, conditions will be
such that some algae will be produced. To prevent their escape from a cell into the efflu-
ent, a quiescent area should be designed adjacent to each cell outlet, with an overflow rate
of 800 gpd/ft2 (32 m3/m2'd) or less. Baffled outlets, similar to those described in subsec-
tion 10.4.3.4, should be included in the design. In addition, some means to dampen turbu-
lence, such as a fence of vertical slats, should be placed outside the settling zone, but inside
the outer wind baffle. Even so, the concentration of SS in the pond system effluent may not
meet secondary effluent requirements at all times of the year without further solids removal
before discharge.
10-36
-------
Some 1 0 to 20 percent of the oxygen demand in aerated pond systems can be satisfied by
surface aeration under average conditions. In aerated facultative ponds, the equivalent of
about 20 to 30 percent of any settled BODs will rise again into the liquid, while 70 to 80
percent of the settled BOD5 remains in the bottom deposits, depending on the degree of
mixing and anaerobic digestion. In aerated facultative ponds, the combination of surface
aeration and the satisfaction of 70 to 80 percent of the BODs in the settled solids by an-
aerobic processes will reduce the oxygen that must be supplied by artificial means by 20 to
40 percent. These factors, plus the additional energy needed for complete mixing, account
for the 2:1 to 4:1 difference in energy requirements for aerated aerobic and aerated facul-
tative ponds (30).
However, periodic upsets by shock discharges of organic loads may cause increased oxygen
demands. With well-monitored operation, these fluctuations in demand can be relieved by
variations in the rate of recirculation of aerated effluent. During early stages of operation,
an aerated pond is usually underloaded, and detention time is long enough for some nitrifi-
cation to take place, thus requiring more oxygen than the BODs determinations alone
would indicate.
Under normal conditions, the ultimate carbonaceous oxygen demand of the raw wastewater
is about 1.5 times the BODs; tne nitrogenous oxygen demand is about 4.6 times the am-
monia nitrogen, or equal to about 0.5 the 6005. Thus, the ultimate oxygen demand (Lu or
UOD) of domestic wastewater is roughly twice the
Because there is little difference in the rate of oxygen utilization above about 2 mg/1 of DO
and more energy is required to maintain higher levels, the aeration facilities should be de-
signed to permit variation in the rate of aeration.
It is important to remember that the hydraulic characteristics of aerated ponds follow
neither an idealized "completely mixed" pattern nor an idealized "plug flow" pattern (31).
The overall performance of aerated lagoons may be evaluated using unequally sized, square
CSTR's (continuously stirred tank reactors) in series model (31).
The required detention time can be determined on a preliminary basis using the following
equation (32):
where
tj = detention time at T, days
T = water temperature, °C (to obtain water temperature [T] from air
temperature, see Figure 10-7)
Le = effluent BOD5 , mg/1
10-37
-------
2.5(GAL/SQ FT/DAY)
(t=30 DAYS)
0 0.2 O.4 O.6 0.8 U>
(Ta-2)Tj RATIO OF WEEKLY AVERAGE AIR TEMPERATURE
TO INFLUENT WATER TEMPERATURE(°F)
FIGURE 10-7
AERATED POND WATER TEMPERATURE
PREDICTION NOMOGRAPH FOR A POND DEPTH OF 10 FEET (30)
(FOR VARIOUS LOADINGS AND RETENTION TIMES)
10-38
-------
1^ = influent BOD5 , mg/1
kT = BODU removal rate constant at T, days' * = k350(35-T)
where
= BOD5 removal rate at 35° C, days" *(an approximate value
is 1.2) (7)
9 = For domestic wastewaters, lacking the desirable specific pilot
study determinations, 0 can be assumed to be 1.08 for aerated
ponds.
BODU, instead of BODs, should be used to satisfy the ultimate carbonaceous oxygen de-
mand. If SS removal facilities follow an aerated pond, reaction rate values for determina-
tions of design retention time should only include synthesis reactions and not the additional
time required for endogenous respiration reactions.
10.5.2 Aerated Aerobic (Complete Mix) Ponds
Because no settling takes place in an aerated aerobic pond, its primary function is the con-
version of raw organic matter to dissolved solids and cell tissue. Quiescent settling areas ad-
jacent to cell outlets and/or an added SS removal process (such as a clarifier or slow sand
filtration), must follow aerated aerobic treatment before discharge to insure compliance
with SS removal requirements. Unlike algae in facultative ponds, the microbial cells (mostly
bacteria), resulting from aeration of only a few days, usually will agglomerate and settle sat-
isfactorily. Solids are often returned to aerated aerobic ponds to improve performance dur-
ing cold periods. Under these latter circumstances, the pond system becomes a moderately
efficient, modified activated sludge process.
In an aerated aerobic pond, the amount of oxygen required (exclusive of that needed for
nitrification) can be approximated from (10):
Or = aLr + (b) (MLVSS)
where
Or = oxygen required, mg/1
a = ratio of oxygen used to carbonacous BODU removed, which is
usually 0.35 to 0.65 and averages 0.50 for domestic wastewater.
Lr = BOD removed in pond, mg/1
10-39
-------
b = ratio of carbonaceous BODU (mg/1) to MLVSS (mg/1), which is
usually 0.08 to 0.14 and can be approximated as 0.12 for domes-
tic wastewater
MLVSS = mixed liquor volatile suspended solids, mg/1
The second term in this equation can be dropped, if the detention time is small enough to
prevent endogenous respiration. The aerated aerobic pond, with a 24-hr aeration period
(under ideal conditions), represents an economic design for municipal wastewater ponds, if
the minimum water temperature remains above 20° C (43° F), because it requires the
least time and land requirements to reduce the soluble BODs to 4 to 8 mg/1. Aerated ponds
can be considered completely mixed when the power level is equal to or greater than about
30 hp/106 gal (5.9 kW/106 1) of maximum storage volume.
10.5.3 Oxygen Requirements
The rate at which oxygen must be supplied to satisfy oxygen requirements (exclusive of
nitrification) can be determined, pending the pilot plant results, from the following equa-
tions (30). The oxygen requirements in each cell will vary with temperature.
0R = 4.17 X 10-3 (Lu)(l/t+ !>!)(<:)
where
= oxygen required, lb/1,000 gal/day
Lu - ultimate carbonaceous oxygen demand (BODU) of influent, mg/1
t = detention time, days
bj = endogenous oxygen uptake rate, day (about 0.15 for municipal
wastewater)
c = ratio of mixed liquid BODU to influent BODU, which can be as-
sumed (pending pilot plant results) to be about 1.05 in the winter
and 1.20 in the summer.
The oxygen transfer rate must be greater than the oxygen uptake rate. The power level re-
quired to satisfy the oxygen transfer rate, using surface mechanical aerators, can be deter-
mined from (30):
Pv = 1.73 X
10-40
-------
where
Pv = delivered power, hp/10*> gal
OST = oxygen saturation at T, mg/1 (water T can be obtained from Figure
10-7)
OL = required oxygen concentration in pond, mg/1
Lu = ultimate carbonaceous oxygen demand of influent, mg/1
No = oxygen transfer efficiency, Ib O2/hp*hr
t = detention time, days
Figure 10-8 illustrates the possible use of this equation, given an oxygen transfer efficiency
of 1.7 Ib O2/hp-hr, a summer saturation DO of 7.0 mg/1, and a minimum DO to be main-
tained of 2.0 mg/1, at different levels of BOD5 in the influent (30). The curves in Figure
10-8 must be altered to reflect the specific oxygen transfer efficiency of the mechanical
aerator being used and the DO conditions which exist, if they differ from the example.
Aerated facultative ponds can be designed to conserve energy, if aeration supply is reduced
in steps from the entrance to the outlet of the pond. Reduction in steps can be accomplished
by 1) several increases in spacing between diffusers or mechanical aerators or 2) reductions
in power to aerators to more nearly match decreasing oxygen requirements of the organic
matter in the stabilized wastewater. Peak influent BODs concentrations can be reduced by
recirculating treated effluent to the pond influent. There should always be some recircula-
tion (5 to 10 percent) of final aeration cell effluent to the influent, to maintain a satisfac-
tory mix of active microbes.
Other less widely used methods of aeration, such as air guns and helical units, should be con-
sidered. This type of equipment allows greater depths, up to 20 ft (6 m); like diffused
aerators, it can be used effectively, even in freezing temperatures. Care should be taken
when using manufacturers' design criteria, because no standardized rating procedure pres-
ently exists. Table 10-7 presents a comparison of alternate aeration equipment for aerated
ponds. Illustrations of this equipment are given in Figure 10-9.
Diffused aeration through plastic tubing or vertical tubes produces slower mixing than
mechanical aeration, which is more conducive to growth of algae and more uniform distri-
bution of the oxygen resources throughout a facultative aerated pond. However, mechanical
aerators use much less power in transferring oxygen to water than do diffusers. With ade-
quate separation of bacteria and algae from the effluent, a 90-percent BOD5 reduction is
possible. The critical effluent quality parameter from aerated lagoons is the SS concen-
tration.
10-41
-------
ou
40
*_^
"o
p^
^^
\s
IK
^^
\ \
100
^^
^x.
^
^
b\
k^^fc
^
AEROBIC PONDS
»«MB^^ ^
AEROBIC-
ANAEROBIC
""^^
•• — -
mmmmmmmmmt
PONDS
I 2 3
DETENTION TIME (DAYS)
*WHEN:
N0=l.7 Lb Og/hp/hr
0,T*7.0mg/l t = I DAY
0L=2.0mg/l L= 100-150 mg / I
FIGURE 10-8
POWER LEVEL FOR OXYGEN TRANSFER (30)
10-42
-------
TABLE 10-7
TYPES OF AERATION EQUIPMENT FOR AERATED PONDS
o
OJ
O2,lb
Oxygen
Production
(Standard Condition) Ib O2/hp
Floating Mechanical Aerator 1.8- 4.5/hp
ffigh Speed 1.5
Low Speed 2.5 to 3.5
Rotor Aeration Unit
(brush type)
Plastic Tubing Diffuser
Diffused Aeration
Air-Gun
Helical Diffuser
3.5
0.2-0.7/100 ft
0.8 - 1.6/unit
1.2-4.2/unit
0.5 to 1.2
12-20
Power Common
Requirements Depth
Advantages
Disadvantages
hp/106 gal
35
25
100
ft
10-15
Good mixing and aeration Ice problems during freez-
capabilities; easily removed ing weather; ragging
for maintenance. problem without clogless
impeller.
3-10
3-10
12-20
Probably unaffected by
freezing; not affected by
sludge deposits, good for
oxidation ditches.
Not affected by floating
debris or ice; no ragging
problem, uniform mixing
& oxygen distribution.
Not affected by ice; good
mixing.
8-15
Not affected by ice; rela-
tively good mixing.
Requires regular cleaning
of air diffusion holes, en-
ergy conversion effi-
ciency is lower.
Requires regular cleaning
of air diffusion holes, en-
ergy conversion effi-
ciency is lower.
Calcium carbonate build-
up blocks air holes; po-
tential ragging problem
affected by sludge de-
posits.
Potential ragging prob-
lem; affected by sludge
deposits.
-------
DIFFUSER HEAD
PROPELLER-'L__J^^-INTAKE VOLUTE
FLOATING SURFACE AERATOR
-WATER SURFACE
ROTOR
MOUNT
ROTOR BLADES
-FLOAT
FLOATING ROTOR AERATOR
AIR SUPPLY
TUBING
CONCRETE
BASE
HELICAL AERATOR
WATER
FLOW
AIR
SUPPLY
AIR GUN AERATOR
AIR HOLE
POND BOTTOM
PLASTIC TUBE AERATOR
FIGURE 10-9
SCHEMATIC VIEW OF VARIOUS
TYPES OF AERATORS
10-44
-------
Mechanical aerators, used in aerated ponds are generally divided into two types: 1) rotor
and horizontal shaft aerators (Kessener brush) and 2) the more common turbine or propeller
vertical-shaft aerators. In all types, oxygen transfer occurs through a vortexing action and/or
from the interfacial exposure of large volumes of liquid sprayed over the surface.
Rotor aerators, relatively new in the United States, are particularly adaptable to use in
shallow ponds (such as aerobic ponds) that are less than 5 ft (1.5 m) deep. Although no pre-
cise comparison has been made, rotor aerators appear to have a much greater pumping
capacity than the propeller aerators (24).
Propellers aerators require a minimum depth, depending on the horsepower of the unit. For
shallow ponds, a large number of low horsepower units are required, and the cost per horse-
power rises. The propeller aerators tend to recycle much of the volume pumped, especially
in shallow ponds (24).
Floating propeller or turbine aerators are mounted out in the pond, far enough apart to
minimize interference with one another or with other pond features. If used for shallow
ponds, they require minimum depth pits, lined with erosion-resistant surfaces. These
surfaces are usually some form of paving, such as rock, asphalt, or concrete. Power access is
usually via underwater cable; maintenance access is almost always by boat (24). To optimize
aeration and mixing and to avoid interference between units, aerator manufacturers have de-
veloped criteria for minimum areas and depths, depending on tjie horsepower of the aerator
and the configuration of the impeller.
Floating rotor aerators may be mounted in the pond or directly off the dike slopes. The en-
tire dike slope in the immediate vicinity should be provided with erosion protection. Units
mounted on the slope offer easy access for maintenance and repair and the extra reliability
of an above-water power supply (24).
Aeration systems are designed on the basis of their oxygen-transfer rate at standard condi-
tions. Standard conditions are defined as 1 .0 atmosphere dry pressure at 20° C (40° F) for
tap water containing 0.0 mg/1 DO. The required rate at which oxygen must be transferred to
the wastewater to raise the DO the desired amount under actual operating conditions is
given by the following (33):
= 8.33Q(C-q)
where:
AOR = actual oxygen-transfer rate, Ib 02 /day
Q = influent flow, mgd
C = required final DO concentration, mg/1
Cj = DO concentration of the influent, mg/1
10-45
-------
The actual oxygen-transfer rate may be adjusted to standard conditions by applying correc-
tion factors, according to the following equation (33):
SOR-.-^- A°R
1.024T-20a
where
SOR = standard oxygen-transfer rate, Ib 02/day
GS = DO saturation concentration of tap water at temperature T, mg/1
C-20 = ^^ saturati°n concentration of tap water at 20° C, mg/1
T = design temperature of the wastewater, °C
a = C>2 transfer coefficient of the wastewater
0 = 02 saturation coefficient of the wastewater
Diffused air systems are designed to provide firm blower capacity, which is the capacity re-
maining with the largest blower out of service. The maximum air rate required to provide
firm capacity may be computed from the standard oxygen-transfer rate as follows (33):
SOR
1440 et -ya ?o
where
Am = firm blower capacity, cfm
et = diffusivity of ©2 in air
-ya = specific weight of air at design temperature and relative humidity,
pcf
P0 = 02 content of dry air, proportion by weight
Mechanical aeration systems are designed on the basis of the horsepower required to pro-
duce the needed standard oxygen transfer rate (SOR). One aeration design equation pro-
posed by Kormanik for an aeration basin is (33):
P = SOR
24N0FgT?
10-46
-------
where
P = horsepower required
SOR = standard oxygen-transfer rate, Ib 62/day
N0 = C>2 transfer efficiency under standard conditions in tap water, Ib
O2/hp-hr
Fg = correction factor related to basin geometry
77 = aerator efficiency correction
For more information on aerator and aeration design refer to Chapter 7.
10.5.4 Aerated Facultative Pond Design Procedure
A procedure for the design of facultative aerated ponds is as follows:
1. Detention time can be estimated from the following (previously stated) formula:
tT =
Le-kT
2. After establishing the best depth to be used for a specific type of equipment and
location, establish the pond volume from:
V = qt
where
V = volume, ft*
q = influent flow, ft 3 /day
t = detention time, days (from step 1, above)
3. Divide the volume into three or more cells, with the first cell the largest. For
aerated facultative ponds, all cells after the first should have diminished aeration,
thus permitting settling.
4. Determine the daily oxygen requirement for the warmest period to satisfy the
heaviest expected biological oxygen demand for each cell:
OR = 4.17 X 10-3
10-47
-------
(If a pond cell is to be completely mixed, the power level should be equal to or greater than
30 hp/106 gal (5.9 kW/m3) of maximum storage volume, which is more than needed to
satisfy oxygen needs. At this power level, the detention time should be kept to a minimum
to conserve energy.)
5. Determine the power requirements for different types of mechanical aeration
equipment (or air requirements for diffusers) for each cell. If icing could occur,
determine the measures required to prevent capsizing aerators or a power that
failure that would allow ice to inactivate the aerator.
6. Determine if measures must be taken to insure efficient SS removal from the
effluent of each cell, to satisfy effluent requirements. Outlets from aerated
aerobic or aerated facultative ponds should minimize passage of SS in the effluent
by using multiple, well-baffled outlets designed to withdraw wastewater at low ve-
locities from the middle depths of the pond. One method is to use a circle of
stakes (with tops below the L.W.L.) outside the sludge and scum (wind) baffles,
designed to reduce turbulence and to provide sufficient quiescent area, to attain
an overflow rate between the stakes and the outlet baffles of less than 800 gpd/
ft 2 (32 m^/m^-d) at peak flow. Care must be taken to prevent anaerobic condi-
tions from developing in this quiescent area by designing for the minimum deten-
tion time necessary and providing for periodic sludge removal.
7. Determine the type of polishing process needed, if any, to insure satisfactory SS
removal from the pond effluent.
More detailed descriptions of both aerobic and facultative aerated pond designs are pre-
sented in reference (23).
Performance data and design criteria for single- and two-cell aerated demonstration pond
systems at Winnipeg, Canada, are summarized in Tables 10-8 and 10-9, respectively (14).
Flow diagrams of the demonstration pond systems are shown in Figure 10-10. As indicated
in Table 10-8, effluent BOD5 and SS concentrations from the demonstration pond systems
are relatively uniform and, on an average basis, slightly higher than the allowable effluent
concentrations stipulated in the U.S. EPA Secondary Treatment Standards. Effluent quality
could undoubtedly be improved by the provision of separate SS removal facilities, either
within or outside the cell basins.
TABLE 10-8
EFFLUENT CONCENTRATIONS FOR CHARLESWOOD DEMONSTRATION PONDS,
WINNIPEG, CANADA: 21-MONTH AVERAGE (14)
Pond System1 Effluent BOD Effluent SS
mg/1 mg/1
Air-aqua (two cell) 37 34
Surface Aerator (one cell) 38 39
Air-gun (one cell) 34 34
1 (See Figure 10-10)
10-48
-------
TABLE 10-9
DESIGN CRITERIA: CHARLESWOOD DEMONSTRATION PONDS
Parameter Value
Average Design Flow (each pond), mgd 0-6
Influent 5-day BOD, 20° C, mg/1 250
Influent Suspended Solids, 20° C, mg/1 180
Oxygen Utilization Factor, a
(Ib oxygen required per Ib 5-day BOD removed) 1.50
Operating DO, mg/1 2.00
Effluent Temperature, °C
Winter °
Summer 24
Influent Temperature, °C
Winter 9
Summer 18
Mean Ambient Temperature, °C
Winter -27
Summer 24
Treatment Efficiency Required, percent 90
Retention Time, days
Tube Diffusers 30
Surface Aerator 20
Air-gun 20
Operating Depth, ft
Tube Diffusers 10
Surface Aerator 11
Air-gun 17
Pond Volume, gal
Tube Diffusers, Tapered Spacing 15 X 106
Surface Aerators, 8 at 105-ft cc 10 X 106
Air-guns, Tapered Spacings 10 X 106
Mixing Requirements for Surface Aerators 0.013 hp/1,000 gal
Process Loading
Tube Diffusers 15.9 Ib/acre/day 0.52 Ib BOD5/1,000 ft3/day
Surface Aerator 24.7 Ib/acre/day 0.78 Ib BOD5/1,000 ft3/day
Air-gun 37.2 Ib/acre/day 0.78 Ib BOD5/1,000 ft3/day
10-49
-------
EXISTING EFFLUENT
CONTROL CHAMBER—•
OUTFALL
EFFLUENT TO.
AS8INIBOINE RIVER
FROM CONVENTIONAL
CELL f
RECIRCULATION
PIPING
f
• EFFLUENT CONTROL CHAMBER
1 AND FLOW MCASURIN8)
CELL No. 2
SURFACE AREA
• a « AC-
LIQUID DEPTH
«IO FT.
« AIR •}
AQUA 1
CELL No. 1
f
, SURFACE AREA
1
' 3.3 AC.
1 LIQUID DC »fH
1
l_~3
«IOFT.
_
X ' J
"""*
r "x.
-"1
SURFACE
AERATOR
SURFACE
AREA* 4.3 AC
LIOAMD
DEPTH«MFT.
d
*<9
OX
(\~?J
r ^^^^^ ~^^
AIR GUN
SFC. AREA<2.9AC.
LIO. DEPTH* 17 FT.
V
«/
<*/
*/
to /
ov
f
*
MAIN CONTROL
CHAMBER
(RAW WASTEWATER
SAMPLING)
—INFLUENT CONTROL CHAMBER
(AERATED CELLS)
3.6 M.G.D.
-TO CONVENTIONAL CELLS
PUMPING STATION
NOTE
M.G.D.- USA UNITS
FIGURE 10-10
CHARLESWOOD DEMONSTRATION PONDS
WINNIPEG, CANADA
10-50
-------
A three-cell, step aerated pond system in Blacksburg, Virginia, is meeting effluent
standards using a diffused air system (34). Design criteria for this system are given in Table
10-10. The system layout is illustrated in Figure 10-11; BOD5 removal for 27 weeks is
shown graphically in Figure 10-12. The performance of the cells in this system, particularly
with respect to SS removal, could be improved, if necessary, by inclusion of separate SS re-
moval facilities within and/or outside the cells.
TABLE 10-10
DESIGN CRITERIA FOR BLACKSBURG, VIRGINIA,
DIFFUSED AIR AERATED POND SYSTEM
Design Flow, gpd 40,000
Earthen Dikes side slopes 3:1
Depth, ft 8
Total Bottom Area, ft2 16,000
Total Surface Area, ft2 31,744
Volume of Three Ponds, gal 704,640
Detention Time, days 38
Altitude, ft 2,000
Air Temperatures, ° C -20 to 3 7
Operation Time per Week, hr 5
Electrical Costs per Month, $ 30
10.6 Aerobic Pond Design
Aerobic ponds depend on 1) algal photosynthesis, 2) at least 3 hours daily of mixing,
3) good inlet-outlet design, and 4) a minimum annual air temperature above about 5° C
(41° F), to supply the major portion of the required DO (21). Without any one of these
four requisite conditions, an aerobic pond may develop anaerobic conditions or be ineffec-
tive. Because light penetration decreases rapidly with increasing depth, aerobic pond depths
are restricted to 1.5 to 2.0 ft (0.45 to 0.6 m) to maintain active algae growth from top to
bottom. The allowable loading is dependent on available light energy:
Lu = S'
where
Lu = UOD that can be satisfied, Ib/acre/day (maximum loading is about
200 Ib of UOD/acre/day)
S' = light energy, cal/cm2/day (can be approximated from Table 10-5).
10-51
-------
\
\
40'
1
I
40'
J_
/*
1' "II
AERATED '
i. POBDN°-' /ill
'AERATED '
POND No. 2
i n n1 •
AERATED /
r POND / •
1 , N°'3 I/
/
/
/ '\
/ ]
BLOWER
HOUSING
ll EFFLUENT LAB 8 CHEMICAL
^/CHLOR.NAT'ION BUILDING
FIGURE 10-11
SYSTEM LAYOUT FOR BLACKSBURG, VIRGINIA,
DIFFUSED AIR AERATED POND SYSTEM
240
220
200
ISO
1 60
^ I40
C7>
E 120
cf 100
O
00 80
60
40
ZO
10
0
—
/
v
\
S
S
V
\t
\
J A S 0 N D
1969
./
/
/
•-K;
,---
INFLUENT
^>
^^~
L
N!/"
T
1
"^*»»
•^•^"
\
\
\
\
\
\
^ — *
EFFLUENT
--J
^»—
^ —
JFMAMJJ ASOND
1970
_ — •
^— •
— — .
,-— "
^ — -
JFMAMJJAS
1971
FIGURE 10-12
BOD REMOVAL FOR BLACKSBURG, VIRGINIA,
DIFFUSED AIR AERATED POND SYSTEM
10-52
-------
The first equation in section 10.5 can be used to determine the required detention time (t).
The depth can be found from data developed (21) for the energy balance when the highest
percentage BOD5 removal occurs in the form of algal cell production:
d = 3FS't/Op
where
d = depth, ft (should be less than 1.5 ft)
F = light conversion efficiency, percent (0.8 to 2.8) (see Figure 10-13)
S' - light energy, cal/cm2/day (See Table 10-5.)
t = detention time, days
Op = oxygen production, Ib/acre/day
The required detention time typically falls between 5 and 10 days. Aerobic ponds should be
designed to provide a recirculation rate at least equal to the influent flow rate (q), to pro-
vide influent dilution, microbial seeding, and additional DO. Odor is not a problem with
aerobic ponds, if they are operated and maintained correctly. Hydraulic loadings on aerobic
ponds can be 2 to 10 in./day (50 to 250 mm/day).
Intermittent mixing and/or recirculation can be accomplished using airlift pumps, propeller
pumps, brush aerators, or rotor aerators, among other methods (20) (32). Mixing is best
done between 12 a.m. and 5 a.m. and when the pH is above 9.5 to replenish CO. On the
other hand, it is believed that higher pH effectively reduces coliform densities. Mixing
should create a bottom velocity throughout the pond of at least 0.5 ft/sec (0.15 m/s). If the
addition of aeration equipment is necessary, it may be more efficient to design the aerated
aerobic pond as an oxidation ditch (see Chapter 7).
The maximum size of cells in an aerobic pond system should be no more than about 10
acres. The cells should be arranged in end-around channels up to 50 ft wide to better achieve
plug flow. Otherwise, extensive mixing equipment may be necessary. The cells and outlets
should be designed to 1) prevent withdrawal of SS, 2) function with variable depths in the
ponds, and 3) decant the overflow only when the ponds are quiescent. Even so, some addi-
tional processing usually will be necessary to remove algal and other microbial cells from the
effluent before it will consistently meet effluent requirements.
Aerobic ponds usually need lining to prevent infiltration and scour. Chemicals in solution
adversely affecting biological reactions in aerobic oxidation are chromium (Cr^+) and
ammonium (NH^); chemicals which adversely affect photosynthetic oxygenation are cal-
cium (Ca+), chlorine (C^), and chromium (Cr3+).
10-53
-------
(Jt
•t*.
o
UJ
o
at
UJ 100
a:
o
o
CO
ui
u.
•z
UJ
o
cc
UJ
Q.
80
60
40
20
FAIL
POOR
16
20
FAIR
OXYGENATION
• SEDIMENTATION
25
29
33
SATISFACTORY
36
38
38
37
36
34
MINIMUM 9 AM
DISSOLVED OXYGEN, PPM
MAXIMUM 3 PM
DISSOLVED OXYGEN, PPM
OVERDESIGNED
X
0.4 0.8 1.2 1.6 2.0 2.4
OXYGENATION FACTOR F
2.8
3.2
3.6
4.0
FIGURE 10-13
RELATIONSHIP BETWEEN OXYGENATION FACTOR
AND BOD REMOVAL IN WASTE PONDS (27)
-------
10.7 Polishing Pond Design
Maturation ponds are employed to remove additional BODs and bacteria from treated
wastewater, primarily by sedimentation. To prevent algal growth, there should be at least
three units in series, with a detention time in each of 48 hr or less (7) and depths variable
from 3 to 8 ft. (See Figure 10-2.) For best removal of pathogens, detention times in each of
the three or more ponds in series should be greater than 5 days (45). Inlets and outlets must
be designed to prevent short circuiting; the outlets must be designed with very low exit ve-
locities and baffles to minimize escape of cells in the effluent (32). If the receiving stream
flow is small and its water quality important, the ponds should be designed to equalize ef-
fluent flows and loadings before discharge. Experience (35) has shown that polishing ponds
provide a buffering action, preventing adverse fluctuations in secondary plant effluent qual-
ity from reaching the receiving water. Fish may flourish in polishing ponds where the nu-
trient balance is satisfactory , assisting in the removal of SS and nutrients (35). It has been
found that after 12 years of operation, polishing ponds can produce an increase in DO and a
reduction in BOD; fecal organisms may virtually disappear. Although polishing ponds in-
crease the DO of the effluent, they also generate algae, and may, thus, increase SS in the ef-
fluent if not designed and operated correctly. Polishing pond treatment, if discharge is into
an intermittent stream, has been found to reduce fungal and filamentous bacterial growths
in the stream (4).
10.8 Microbial Cell and SS Removal from Pond Effluent
A major problem in all ponds is occasional discharge of microbial cells, which can exert an
oxygen demand on the receiving water. In recent years, there has been a concentrated effort
to develop simple means of removing cellular material from pond effluent. Discussions of
possible methods can be found in references (15), (16), (17), (24), (26), (28), (36), (37),
(38), (39), (40), and (41). Reference (40) has a bibliography containing an additional 136
references. Chapters 11 and 12 present design criteria for such polishing units. The types of
algae found in wastewater treatment ponds can be divided into four classes: 1) green algae,
2) blue-green algae, 3) diatoms, and 4) pigmented flagellates. Green algae, predominant in
efficient lagoons, are nonmotile and less than 10 /i in size; have a negative charge, preventing
natural flocculation or filtration; have a density near that of water; and are kept in suspen-
sion by a mild fluid motion. Blue-green algae are usually filamentous, may form floating
mats with string coating; may develop pig-pen odor; may hinder light penetration; and may
diminish surface aeration and mixing. Diatoms are nonmotile; have a silica shell structure;
and are large enough to clog sand filters. The pigmented flagellates, generally motile, are
smaller than 15 to 30 /u and have a flexible cell wall, which allows them to deform and pass
through small restrictions (18).
Stabilization ponds are usually selected for wastewater treatment because of their simplicity
of operation. Thus, any additional treatment required for removal of algae should be simple
to operate and should reliably remove suspended matter. Of the proven methods, filtration
currently appears to be the simplest (27) (40). In some cases, it may be necessary to add
chemicals and/or clarifiers (or flotation units) before filtration to decrease the load on filters
and to remove single-cell green algae or phosphorus.
10-55
-------
Total nitrogen levels in facultative pond effluents may be quite low. Much of the nitrogen in
the pond influent may be incorporated into the algal cell. Also, nitrification appears to take
place in the ponds followed by some denitrification in the anaerobic bottom zone. With
proper design and operation of the pond treatment system, the insertion of an algal removal
step can produce an effluent low in both oxygen-demanding materials and nutrients.
Intermittent sand filters have been utilized for treatment of settled wastewater since about
1828 (40). At present, additional work is underway to refine design criteria for the inter-
mittent sand filtration of wastewater stabilization pond effluent. Some viable algal cells tend
to pass through the entire depth of the filters (42). The effective size of the sand should be
about 0.17 mm for best BOD removal (1.6).
Conclusions reached on the use of intermittent sand filters by Utah State University are as
follows (40):
1. Length of filter run is related to the influent SS concentration and the hydraulic
loading rate.
2. Intermittent sand filters can be used to treat wastewater and reduce SS to less
than 10 mg/1, VSS to less than 5 mg/1, and BOD5 to less than 10 mg/1.
3. Winter operation of the filters did not create any serious problems.
Studies in 1973 and 1974 at Eudora, Kansas, where submerged rock filters have been polish-
ing the effluent from the Eudora multicell wastewater treatment pond system, indicate the
following (17) (43):
1. The rock sizes selected should be between 25 mm and 125 mm and the range in
size should be no more than 50 mm.
2. A biological film must be developed on the rock before the filters are effective.
3. Because the biological film will function in an anaerobic environment in the sum-
mer and early fall, some postaeration facility (possibly cascade) is required.
4. If sulfate is present in the carrier water and the alkalinity is insufficient to keep
the pH above about 9, hydrogen sulfide will be formed, if anaerobic conditions
exist in the filter. At a pH of 7, about 52 percent of the sulfides present will be in
the form of hydrogen sulfide (t^S); sulfide concentrations is low as 1.0 mg/1 may
cause an odor problem. If the total alkalinity in the lagoon effluent is greater than
260 mg/1 as calcium carbonate (CaCC^), the pH will remain sufficiently high to
prevent an odor problem.
5. It has been estimated that the effective life of a submerged rock filter can be as
much as 20 to 30 years.
6. For periods of peak efficiency in the summer and fall, the maximum hydraulic
loading rates can be 9 gpd/ft3 (1.2 m3/m3/d). This should be reduced to 3
gpd/ft3 (0.4 m3/m3/d) in the winter and spring.
A submerged rock filter has been designed for use in the tertiary cell of a three-cell series
pond system at California, Missouri (17). The design hydraulic loading (horizontal flow) on
this filter is 3 gpd/ft3 (0.4 m3/m3 -d). This rock filter is shown on Figure 10-14.
10-56
-------
I ^DISINFECTION CHAMBER
EFFLUENT COLLECTION
LINE 304 8 cm. Did.
SINGLE PERFORATED
CORRUGATED METAL
PIPE
AERATION
EFFLUENT DISCHARGE
PLAN
LAGOON
BOTTOM
^-CRUSHED ROCK
EFFLUENT
STRUCTURE-
SECTION A-A
FIGURE 10-14
SUBMERGED ROCK FILTER, CALIFORNIA, MISSOURI (17)
10-57
-------
Sedimentation ponds have been recommended by some State regulatory agencies for en-
couraging algal sedimentation within the pond cells. Sedimentation ponds, however, are
limited in efficiency by such factors as wind mixing and algae growth. The smaller and
deeper the pond, the less influence wind has on mixing. Sedimentation pond efficiency also
depends on species type. Motile algae and crustaceans are not efficiently removed in such
ponds. The last pond, or that part of any pond that is to serve as a sedimentation pond,
should be deep (8 to 12 ft [2.4 to 3.7 m]) and designed for sludge removal at least every
other year because algae develop where nutrients are released from anaerobic fermentation
of a sludge layer.
10.9 Pathogen Removal
Wastewater treatment ponds remove the BOD5, SS, and pathogens. Environmental factors
that may be present in wastewater treatment ponds (32) and that may contribute to a de-
crease in pathogen concentration are listed as follows:
1. Aggregation and attachment to settleable solids
2. Dispersion and dilution
3. Predation by other micro- and macro-organisms
4. Bacteriophage, when present
5. Sunlight, increasing complex algal populations
6. Unavailability of essential nutrients
7. Anaerobic pretreatment
8. Higher temperatures
9. HighpH
To produce an effluent meeting secondary requirements in the removal of coliforms,
reductions on the order of 99.99 to 99.999 percent are necessary. Although such reduc-
tions are not usually possible in single ponds, they are attainable in series pond systems,
particularly if each pond is baffled to more nearly achieve plug flow characteristics (3), and
each cell outlet is baffled to prevent wind mixing and provide a sufficiently sized quiescent
area for maximum separation. The efficiency of fecal bacterial removal is reduced by recir-
culation from the last pond to the first pond of a series. If plug flow conditions exist in
aerobic ponds, the efficiency of fecal bacterial removal between 5° C and 20° C (35° F and
43° F) is given by the following equation (4):
where
Ne = Initial fecal bacterial concentration, MPN
NJ = Effluent fecal bacterial concentration, MPN
e - 2.72
10-58
-------
kT = 2.6- 1.19(T-20)
When the lower depths become anaerobic in the summer or the temperature is near 0° C
(32° F), the efficiency of removal is reduced.
To meet effluent requirements, it is frequently necessary to disinfect pond effluent by treat-
ment with chlorine. Excessive chlorine results in degradation of any algal cells present, thus
increasing the BOD. Chlorine doses in excess of 2.0 mg/1 significantly increase the BOD (18)
(43). Therefore, it is usually better to increase the actual (not the theoretical) detention
time and hold the chlorine concentrations to below 2.0 mg/1, if microbial cells have not
been removed prior to chlorination. For further information on disinfection, see Chapter 15.
10.10 Construction and Maintenance Costs
Only general efficiency data are available, based on performances of stabilization ponds de-
signed to meet State standards. Average data for ponds are sparse and do not differentiate
among conventional, single-, or dual-celled ponds and the better designed ponds. These
latter systems may have multiple units in series, with each unit designed to minimize short
circuiting, outlets designed to withdraw pond effluent relatively free of microbial cells, and
sufficient operational storage to discharge only when reasonably cell-free effluent is avail-
able. Because the inefficient pond records are included, average reported effluent quality is
generally worse than it should be.
Relative construction costs for different types of treatment facilities in the United States are
shown in Figures 17-1, 17-2, and 17-4. Relative operation and maintenance costs are listed
in Table 10-11. Other experience indicates that the operation and maintenance costs of fac-
ultative ponds are about one-half that of aerated facultative ponds and about one-fifth that
of extended aeration systems.
TABLE 10-11
OPERATION AND MAINTENANCE RELATIONS IN THE FORM Y = aX^ (33)
Type of Treatment Facility Value for a Value for b
Waste Stabilization Ponds 17.38 -0.4172
Primary Sedimentation Plant 24.95 -0.2634
Activated Sludge Plant 30.10 -0.2460
Trickling Filter Plant 54.99 -0.3569
Note: Y = Operating and Maintenance cost, $/cap/yr (1968), X = Design
Population, persons; a and b = constants.
10-59
-------
10.11 References
1. Unpublished data, U.S. EPA (September 1973).
2. Amin, P.M., and Ganapati, S.V., "Biochemical Changes in Oxidation Ponds." Journal
Water Pollution Control Federation, vol. 44, p. 183 (1972).
3. Klock, J.W., "Sequential Processing in Wastewater Lagoons." Journal Water Pollution
Control Federation, vol. 44, p. 241 (1972).
4. Marais, G.V.R., "Faecal Bacteria Kinectics in Stabilization Ponds." Journal Sanitary
Engineering Division, ASCE, pp. 119-1'39 (February 1974).
5. Hendricks, D.W., and Pote, W.D., "Thermodynamic Analysis of a Primary Oxidation
Pond." Journal Water Pollution Control Federation, vol. 48, no. 2, pp. 333-351 (Feb-
ruary 1974).
6. Wastewater Treatment Ponds. U.S. EPA Technical Bulletin, EPA 430/3-74-011 (March
1974).
7. "Treatment of Secondary Sewage Effluent in Lagoons." Notes on Water Pollution, no.
63, Water Pollution Research Laboratory, Stevenage, Herts, England (December 1973).
8. Thirumurthi, D., "Design Principles of Waste Stabilization Ponds." Journal Sanitary
Engineering Division, ASCE, vol. 95, no. SA2 (1969).
9. Mangelson, K.A., and Walters, G.Z., "Treatment Efficiency of Waste Stabilization
Ponds." Journal Sanitary Engineering Division, ASCE, pp. 407-425 (April 1972).
10. Thimsen, Donald J., Biological Treatment in Aerated Lagoons. 12th Annual Waste Eng-
ineering Conference, University of Minnesota (December 1965).
11. Eckenfelder, W.W., "Comparative Biological Waste Treatment Design." Journal Sani-
tary Engineering Division, ASCE (December 1967).
12. Neel, J.K., McDermott, J.H., and Monday, C.A., "Experimental Lagooning of Raw
Sewage." Journal Water Pollution Control Federation, vol. 33, pp. 603-641 (June
1961).
13. Pierce, D.M., Performance of Raw Waste Stabilization Lagoons in Michigan With Long
Period Storage Before Discharge. Symposium on Upgrading Stabilization Ponds, Utah
State University.
14. Lewis, R.F., and Smith, J.M., Upgrading Existing Lagoons. Prepared for U.S. EPA
Technology Transfer Seminar (October 1973).
10-60
-------
15. Missouri Basin Engineering Health Council, Waste Treatment Lagoons—State of the Art.
U.S. EPA Project No. 17079 EHX (July 1971).
16. Middlebrooks, E.J., and Marshall, G.R., Stabilization Pond Upgrading With Inter-
mittent Sand Filters. Symposium on Upgrading Stabilization Ponds, Utah State Uni-
versity (November 1974).
17. O'Brien, W.J., Polishing Lagoon Effluents With Submerged Rock Filters. Symposium
on Upgrading Stabilization Ponds, Utah State University.
18. Parker, D.S., Performance of Alternative Algae Removal Systems. Seminar on Ponds as
a Wastewater Treatment Alternative, University of Texas, Austin (1975).
19. McKinney, R.E., State of the Art of Lagoon Wastewater Treatment. Symposium on
Upgrading Stabilization Ponds, Utah State University.
20. McGauhey, P.H., Engineering Management of Water Quality. New York: McGraw-Hill
(1968).
21. Oswald, W.J., and Gotaas, H.B., Photosynthesis in Sewage Treatment. Transactions
ASCE, vol. 122(1967).
22. Biological Treatment Technology 162. U.S. EPA, Municipal Permits and Operations
Branch, EPA-430/1-1-73-017 (December 1973).
23. Wastewater Engineering Collection, Treatment, Disposal. Metcalf & Eddy, New York:
McGraw-Hill (1972).
24. Brown and Caldwell, Upgrading Lagoons. U.S. EPA, Office of Technology Transfer
Seminar Publication. (August 1973).
25. Uhte, W.R., Construction Procedures and Review of Plans and Grant Applications.
Symposium on Upgrading Stabilization Ponds, Utah State University.
26. Barsom, G., Lagoon Performance and the State of Lagoon Technology. EPA-R-2-73-
144 (June 1973).
27. Standar, G.J., Meiring, P.G.J., Drews, R.J.L.C., and VanEck, H., A Guide to Pond Sys-
tems for Wastewater Purification. Presented at Jerusalem International Conference on
Water Quality (June 1968).
28. Papers from 2nd International Symposium for Waste Treatment Lagoons, Kansas City,
Missouri (1970).
29. Recommended Standards for Sewage Works. Great Lakes-Upper Mississippi River.
Board of State Sanitary Engineers (1971).
10-61
-------
30. Design Guides for Biological Wastewater Treatment Processes. U.S. EPA Water Pollu-
tion Control Research Series, Project No. 11010 ESQ (August 1971).
31. Murphy, K.L., and Wilson, A.W., "Characterization of Mixing in Aerated Lagoons."
Journal Environmental Engineering Division, ASCE (October 1974).
32. Gloyna, Ernest F., Wastewater Stabilization Ponds. World Health Organization (1971).
33. Process Design Manual for Upgrading Existing Wastewater Treatment Plants. U.S. EPA,
Office of Technology Transfer (October 1974).
34. Gearhart, R.M., "Aeration Proves Efficient." Water and Wastes Engineering (June
1972).
35. Potter, A.H., "Maturation Ponds." Water Research, vol. 6, Great Britain: Permagon
Press, pp. 781-795(1972).
36. O'Brien, W.J., McKinney, R.E., Turvey, M.D., and Martin, D.M., "Two Methods for
Algae Removal." Water and Sewage Works (March 1973).
37. Kothandaraman, V., and Evans, R.L., "Removal of Algae From Waste Stabilization
Pond Effluent." Illinois State Water Survey Circular 108 (1972).
38. Golueke, C.G., and Oswald, W.J., "Harvesting and Processing Sewage Grown Algae."
Journal Water Pollution Control Federation, vol. 37 (4), pp. 471-498 (1965).
39. Middlebrooks, G.J., et al., Evaluation of Techniques for Algal Removal From Waste-
water Stabilization Ponds. Review paper PR JEW 115-1, Utah Water Research Labora-
tory (January 1974).
40. Marshall G.R., and Middlebrooks, E.J., Intermittent Sand Filtration to Upgrade Exist-
ing Wastewater Treatment Facilities. Utah Water Research Laboratory, PR JEW 115-2
(February 1974).
41. Process Design Manual for Suspended Solids Removal. U.S. EPA, Office of Technology
Transfer (January 1975).
42. Horn, L.W., "Kinetics of Chlorine Disinfection in an Ecosystem." Journal Sanitary
Engineering Division, ASCE (February 1972).
43. Ramani, R., Design Criteria for Polishing Ponds. Seminar on Ponds as a Wastewater
Treatment Alternative, University of Texas, Austin (1975).
10-62
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CHAPTER 11
FILTRATION AND MICROSCREENING
11.1 Introduction
Granular media filtration and microscreening are used as effluent polishing techniques in
treatment plants to increase BOD and SS removal. Direct granular media filtration of a good
quality secondary effluent (BOD5 and SS less than 25 mg/1) will produce an effluent having
BOD5 and SS in the range of 5 to 10 mg/1. With chemical treatment (e.g., phosphorus re-
moval) followed by sedimentation and granular media filtration, a secondary effluent having
a BOD of about 5 mg/1 or less and SS of 1 to 2 mg/1 can be obtained. Suspended solids in
effluents can also be reduced by means of mechanical strainers, such as the rotating drum
microscreen. Reference is made to EPA Process Design Manual(s) Suspended Solids Removal
and Upgrading Wastewater Treatment Plants (1) (2).
11.2 Types of Granular Media Filters and Their Operation
Different filter configurations most commonly in use now are shown on Figure 11.1.
11.2.1 Downflow Type
The most common type of granular media filter is the downflow filter, with either single-,
dual-, or multimedia, represented as (a), (c), and (d) in Figure 11-1. Older designs of these
filters use a single medium, usually sand. During backwashing of the filters, the medium be-
comes graded; fine grains are on top and coarse grains at the bottom. Filtration, therefore, is
from fine to coarse.
With two different density media, such as coal and sand, the filtration can range from coarse
to fine. This method of wastewater filtration is the preferred one, and is frequently called
"in-depth" filtration. Because of the large amount of SS and the presence of organic floes,
single-medium filters tend to clog rapidly as a result of the accumulation of solids in the top
layer of the filter.
These filters can be designed to operate with a gravity head, in which case the tank or basin
containing the filter media is usually open (as shown in Figure'11-2). However, pressure
filters are frequently used for the smaller capacities (as shown in Figure 11-3), because they
may be more economical. Pressure tanks are usually vertical for plant capacities up to 1 mgd
and horizontal for larger capacities. Because pressure filters can operate at higher head losses
(frequently up to 20 ft) than gravity filters, the length of filter runs between backwashings
can be substantially increased. In addition, if another process (e.g., adsorption in activated
carbon columns) follows filtration, the backwash water from pressure filters can be dis-
charged to a point above the filter elevation without repumping.
11-1
-------
OVERFLOW TROUGH
MEDIA
DEPTH
30 "-40"
UNDERDRAIN —
CHAMBER
1 MPI 1 1FK1T
1
"- H
£^rsANp7#i
-+• \
1
EFFLUENT*
6-
1
~H__
10'
*^
i n n n n rfr
V*:V:' FINE '!••••'.
:•*#;.. ......j^i-'
•••;• ";?-i." ••':•• 'i'-.'-j'
'^ 'SAN '"^:-
t •*""
1
--GRID TO
RFTAI N
SAND
-UNDERDRAIN
CHAMBER
EFFLUENT
INFLUENT
(a) CONVENTIONAL DOWNFLOW
FILTER
(b) UPFLOW FILTER
>&" SILICA .
Jo SAND
GARNET
SAND
UNDERDRAIN
~ CHAMBER
(c) DUAL MEDIA DOWNFLOW FILTER
(d) MULTI-MEDIA DOWNFLOW FILTER
FIGURE 11-1
FILTER CONFIGURATIONS
11-2
-------
FLOAT-
CONTROL
VALVE
UNDERDRAIN
SYSTEM
I I
FIGURE 11-2
GRAVITY FILTER WITH FLOAT CONTROL
11-3
-------
BACKWASH-
4NFLUENT
FILTERED
EFFLUENT _
BACKWASH
FIGURE 11-3
PRESSURE FILTER
11-4
-------
11.2.2 Upflow Filters
Upflow filters were first used in Europe to obtain in-depth filtration. The filtration is from
coarse to fine media, using a single density material. (A typical design is shown in Figure
11-4.) The filter has been used primarily for removing above-average concentrations of SS,
usually in the range of 20 to 100 mg/1. Studies have shown that a filter of this design, having
a depth of 6 ft (1.8 m) and a medium effective size of 1.8 to 0.95 mm, could produce an
effluent having a turbidity of 2 to 40 mg/1 (given an influent having a turbidity of 10 to 280
mg/1) (3). If a polyelectrolyte is added to the raw water, the turbidity is reduced to 1 mg/1
or less. The filtration rate is 6 gpm/ft^.
To prevent fluidization of the sand bed, a grid is installed at the top of the bed. For back-
washing, the resistance of the grid to fluidization is destroyed by an initial agitation of the
bed with air.
The upflow filter has been used in several installations in England for tertiary treatment of
effluent from biological treatment plants (4). Additional studies on upflow filtration have
been reported by McKinney and Hamann (5).
11.2.3 Media Type and Configurations
Until recently, the granular media filter, as used for potable water filtration, had a single
medium (usually sand) with an effective size (ES) of about 0.50 to 0.74 mm and a uni-
formity coefficient (UC) of 1.5 to 1.8. The ES of a filter medium is defined as the size (mea-
sured in millimeters) for which 10 percent by weight of the material is finer and 90 percent
is larger. The UC is the ratio of the size for which 60 percent by weight is smaller to the ES.
As the UC approaches 1.0, particle sizes become more uniform. If a single-medium filter is
backwashed and the bed fluidized, the finest particles collect at the top of the bed. In such
filters, the solids removed from the water are usually retained in the top few inches of the
bed. In recent years in the United States, and for many years in Europe, the filter media
utilized promote solids accumulation throughout a large portion of the filter depth, thus
permitting the removal of more solids from the water and the use of higher filtration rates.
Coarse media (1 to 2 mm ES) and deeper beds are frequently used. To prevent rapid clog-
ging of the top layer of the bed, the filter bed is arranged so that coarse media are above (or
first in the direction of filtration) and finer media below (or last in the direction of filtra-
tion). This is made possible by use of dual-, tri-, or multimedia filters, with the top layer
being the lowest density material (e.g., anthracite coal on top of sand). The upflow filter has
filtration from coarse to fine with a single medium (normally silica sand).
By proper selection of the media sizes and their UC, it is possible to obtain any desired
amount of intermixing of the different media in dual- and multimedia filters. There is some
evidence (inconclusive) that intermixing is beneficial (6) (7).
11-5
-------
SAND HOLDING GRID-,
1.1.. . JA... .11.
FINE
SAND
_•."•.'•,•••.'_•• '_ .._. • . ..-t .--i.v
'"'.' J - . .
.••."-.•.' ' ' .••••'•• -.'.•••'JK
•«•} . • • :••: '•>-SK;
.;-.r'.••{'- • • • * :•• :« .. . .-.sv
-COARSE
SAND
GRAVEL
UNDERDRAIN
NOZZLES
FILTERED
EFFLUENT
UNDERDRAIN
COMPARTMENT
INFLOW
AND
BACKWASH
FIGURE 114
UPFLOW FILTER
AIR FOR
BACKWASH
11-6
-------
The normal filter media depths are 24 to 36 in. (0.6 to 0.9 m). However, for filtering waste-
waters having SS in the range of 25 to 100 mg/1, deeper beds are used with coarser media, to
provide the necessary volume for solids accumulation and thus insure filter runs of reason-
able length (e.g., 8 to 24 hr) (8).
For wastewater filtration, and with downflow filters, the dual- or multimedia filter should
be used, because of the inherent advantages for removal of a larger amount of suspended
solids.
11.2.4 Backwashing
Granular media filters have been traditionally cleaned by an upward flow of water, which
fluidizes the filter bed. In European potable water treatment plants, it has been standard
practice to use a combination air and water wash; such designs have been adopted in many
newer water supply treatment installations in the United States.
Wastewater filters usually receive higher solids loads, which adhere more tenaciously to the
filter media. It is, therefore, generally accepted that some sort of auxiliary scouring methods
or devices are essential to obtain adequate cleaning of wastewater filters.
The other auxiliary cleaning method is surface wash, using either fixed or rotating high pres-
sure water nozzles. The nozzles are located about 1 to 2 in. above the top of the bed. Nor-
mally, while the surface wash is on the upflow, one backwash rate is set lower after the sur-
face wash is terminated. With the deeper penetration of solids into the dual- and multi-
media filters, surface wash has been found inadequate to thoroughly clean the lower por-
tions of the bed.
Air wash, therefore, is now being adopted for most wastewater filters. Studies (9) have
shown that the most powerful filter cleaning method is a concurrent wash with air and
water above fluidization velocity, followed by a normal air scour and subsequent water
wash.
11.2.5 Flow Control
Potable water filters usually have elaborate flow-control systems, to maintain a constant
flow through a filter as it becomes clogged. It was believed that good solids removal necessi-
tated constant rate operation of a filter; however, it has been shown that gradual changes in
filtration rate are not detrimental to filter effluent quality. In fact, sudden changes in flow
rate, such as can occur with automatic flow controllers on filter effluent lines, cause solids
breakthrough and poorer effluent quality (10). The use of filter effluent flow controllers,
which are responsive to the flow differential produced by a venturi or orifice and are
manually or automatically set for a certain constant flow, is not normally required or prac-
tical for wastewater filters. Such control systems require considerable maintenance and serve
no useful purpose regarding effluent quality.
11-7
-------
If two or more filters are installed (as they usually are), the total flow may be equally split
between the operating filters. Without flow controllers, the water level above the filter
media in each filter will depend on the cleanliness of the filter; this level is highest in the
filter having the largest head loss. To prevent the water level from dropping below the top of
the filter bed, the filters discharge into a storage basin through an effluent box, where the
water level is maintained constant by an overflow weir.
Another arrangement that can be used if the flow is equally split among several filters is that
shown on Figure 11-2. In this case, the water level above the filter is maintained essentially
constant by a valve in the filter effluent line actuated by the water level above the filter. The
valve opening for any filter in operation will depend on the degree of filter clogging.
Another filter flow arrangement particularly applicable to wastewater filtration if wide
variations in daily flow are encountered (e.g., in smaller plants) is "variable declining rate
filtration," described in reference (10). In this arrangement, no control valves are used and
the filtration rate through any filter depends on how dirty it is; that is, on loss of head.
The various filters are connected to a large influent flume or conduit having negligible head
loss. There is, of course, an influent valve to each filter, so it can be taken out of service for
washing. The filters discharge into a common storage basin over a weir, located to prevent
the water level from dropping below the filter bed when the filters are in operation. Suffi-
cient height is provided in the filter box above the media to obtain a reasonable filter run
before a filter must be taken out of service for washing. Distribution of the inflow depends
on the condition of any individual filter; for example, the cleanest filter has the highest rate.
The water level is the same in all filters in operation. When a filter is taken out of service,
the flow is redistributed among the other filters, with a gradual increase in their filter rate.
Despite rate of inflow changes, there are no abrupt changes in filtration rates.
The above flow-control schemes are applicable to gravity filters. Usually, for pressure filters,
the filter influent is centrifugally pumped and the flow to individual filters is controlled by
flow control devices. A flow and head loss indicator on each filter shows the operator the
condition of each filter. Manufacturers of pressure filters can supply all the needed appurte-
nances and equipment for automating such installations to a high degree, if desired.
11.3 Design of Granular Media Filter Installations
11.3.1 Pretreatment
Direct filtration of secondary plant effluent can produce a final effluent having SS of 5 to
10 mg/1. Direct filtration generally will reduce SS by about 70 percent, with influent SS
below 35 mg/1 (11). Coagulation with an aluminum or iron salt and/or a polymer, followed
by settling, can reduce the load on filters and produce effluents with SS below 5 mg/1. Pre-
treatment ahead of filters is the principal influence on filter performance, because it affects
the characteristics of the solids applied to a filter.
11-8
-------
To avoid the cost of a clarification basin, coagulation with alum or an iron salt is sometimes
employed directly ahead of filtration without any settling. The chemical is mixed with the
secondary effluent, flocculated in a 10- to 15-minute basin, and filtered. Such coagulation
can also be done with organic coagulants (polymers). If coagulation is practiced ahead of
filters without any settling, the filters should have coarser media at the top (for downflow
filters); also, the filter should be deeper to accommodate the increased amount of solids
removed.
Chlorination ahead of wastewater filters is recommended, to control the growth of slimes on
the filter walls, in the media, and in the underdrain system. In some cases, it has been found
that continuous Chlorination is not needed and that high dosages on an intermittent, short-
term basis are effective. About 30 to 50 mg/1 of chlorine for a short period (2 to 3 hr as
needed) are adequate.
11.3.2 Filtration Rate and Head Loss
Filtration rates for the various types of filters will range from 2 to 8 gpm/ft^ (81.4 to 325.6
l/m^-min), depending on the SS in the wastewater, the size of media, and the desired quality
of filtered effluent. The length of the filter run before washing will depend on the above
factors and the terminal head loss. For gravity filters, it is customary to design for an avail-
able head loss of about 6 to 10 ft (1.8 to 3 m). With pressure filters, higher head losses are
generally used.
Filtration rate and head loss control the length of run. Usually, the filter run is terminated
when the head loss reaches some predetermined value. However, it has become usual prac-
tice in recent years, especially when for in-depth filtration, to control the length of a run by
monitoring the effluent SS with a calibrated turbidimeter. With coarse media, deep filters,
the head loss may not be a good criterion for filter washing. Solids breakthrough is, of
course, a direct indication of when to terminate a filter run.
It is not possible to predict the proper filter rate, effluent quality, length of run, and head
loss development for a given filter media, depth, and wastewater SS. Pilot plant studies are
necessary to establish proper design data. Some typical pilot plant data are shown in Figure
11-5. Such data can be obtained for a pilot filter over the range of parameters shown, if the
filter is producing an effluent having SS below the desired limit.
For small treatment plants, pilot plant studies may not be practical or justifiable, particu-
larly if the wastewater is from solely domestic sources. In that case, designs will be based on
previous experience; they must, however, be conservative.
The inflow rate for which filters should be designed is the maximum 24-hr flow, if 24-hr
filter runs are planned. Hourly variations in flow will balance over the day. However, if 8-hr
filter runs are planned, the peak 8-hr flow should be used in sizing filters for the filter rate
selected.
11-9
-------
12 in. of 1.84 mm ANTHRACITE
12 in. of 0.55 mm SAND
10 20 30 40
INFLUENT SOLIDS CONCENTRATION, mg/l
FIGURE 11-5
RUN LENGTH VS. INFLUENT SS CONCENTRATION AT VARIOUS FLOW RATES
11-10
-------
11.3.3 Media Size and Configuration
The granular media used in water and wastewater filtration are silica sand, anthracite coal,
and garnet sand. Their specific gravities are as follows:
Silica Sand 2.65
Anthracite Coal 1.35-1.75
Garnet Sand 4.0-4.2
When media are used with the above specific gravities in filters, and when proper selection
of particle sizes is made, after backwashing with water, the coarse, lighter coal will be on
top, the heavy, fine garnet at the bottom, and the silica sand between the two.
The use of coarse-to-fine media filtration is important for handling the type of solids present
in wastewaters—particularly those of organic nature, which tend to be removed in the top
layers because of their particle strength and adhering characteristics. The coarse medium on
top allows the solids to be captured in the entire depth of, for example, a coal layer on top
of sand in a dual-media filter. The fine sand layer serves as a polishing filter bed. Such in-
depth filtration permits removal of a larger total amount of solids before backwashing is re-
quired.
Normal domestic wastewater filtration of secondary effluents or after coagulation and
settling can be best accomplished in coal-sand filters. The coal size should be between 1 and
2 mm, with an ES of 1.0 to 1.2 mm. To eliminate excessive intermixing of the coal and
sand, the effective size of the sand should be about one-half the coal, or 0.55 mm. The UC
should be less than 1.65. With media of these sizes, the entire bed will be fluidized at the
same backwash rate.
Although the depth of the filter must be established, the only reasonable method for deter-
mining the optimum value is pilot plant operation. In normal domestic wastewater filtration
(as indicated previously), for dual-media filters the coal layer should be 15 to 20 in. (380 to
50(5 mm) and the sand layer 12 to 15 in. (305 to 380 mm).
Deep beds of a single coarse medium (usually sand) have been used for obtaining large solids
storage, to facilitate handling high concentrations (50 to 100 mg/1) of SS with filtration
only.
The medium size ranges from 2 to 3 mm; the bed depth from 4 to 6 ft. Frequently, the
filtration rate for such filters ranges from 10 to 30 gpm/ft2 (408 to 1,220 l/m2-min). In a
recent investigation of SS removal from an activated sludge treatment plant effluent (12),
it was found that SS of up to 60 mg/1 were reduced to less than 10 mg/1 by such filters.
11-11
-------
11.3.4 Backwash Systems
Backwashing wastewater filters is more difficult than backwashing the usual potable water
filters, because the solids removed are greater in quantity and they tend to adhere more to
the filter media. It is generally agreed that auxiliary scouring devices or other methods are
necessary to adequately clean wastewater filters. Such auxiliary scouring is accomplished by
surface wash arrangements or by use of air wash in conjunction with water wash. Air-wash
systems in wastewater filters are strongly recommended because, in addition to providing
the necessary scouring action, the air provides aeration and limits anaerobic conditions
(which can develop in wastewater filters).
The backwash water rate should fluidize the filter bed completely and expand it by at least
10 percent. Excessive bed expansion has not been found to be beneficial for proper cleans-
ing of the filter media. The required backwash rate will depend on the size and type of media
and the water temperature. For water at about 20° C, the dual-media filter described in
section 11.3.3 will require a backwash rate of about 22 gpm/ft2 (895.4 1/m^ -min).
Filter underdrain systems are designed to provide as uniform a distribution of wash water
over the filter area as possible.
One system used extensively in potable water filtration consists of a central manifold pipe,
or conduit, and lateral piping with orifices directed downward. The manifold and laterals are
covered with 1) coarse gravel, 2) layers of fine gravel on top, and 3) the filter media.
Several proprietary false bottom arrangements are available (Wheeler and Leopold), which
replace the manifold and lateral piping. Gravel is placed on top of the false bottom. Diffi-
culties have been encountered with underdrain systems employing gravel, because higher
backwash rates have caused displacement of the gravel with resultant nonuniform wash
water distribution.
In recent years, an underdrain system that has come into frequent use has a false bottom of
concrete or steel in which specially designed nozzles or strainers are installed on about 6-in.
(152.4-mm) centers. These nozzles, or strainers, are usually made of plastic materials, which
are resistant to high and low pH and to chlorine (Figure 11-6). These strainers eliminate the
need for gravel, because the openings are smaller than the filter media.
This system is easily adapted to air-water wash operation, because, by use of an extension
tube on the strainers, which project into the chamber below the false bottom, it can be used
for distributing only air or air and water simultaneously. The only problem that such
strainers may present for wastewater filtration is that the backwash water may have some
suspended matter resulting from biological growths in the backwash storage tank, which
could cause partial clogging of the small openings in the strainers. Also, biological growths
may occur in the strainers themselves. Adequate chlorination ahead of the filters should
control such growths.
11-12
-------
INFLUENT
EFFLUENT
TRANSFER
PIPE
WASHWATER
STORAGE
COMPARTMENT
FILTER \\ FILTER MEDIA
COMPARTMENT
AIR FOR WASH
SHl
FIGURE 11-6
AUTOMATIC GRANULAR MEDIA FILTER
11-13
-------
Air can also be introduced through a separate manifold-laterals-orifice system located just
above a gravel bed. It is not good practice to introduce the air below the gravel, because air
can easily displace gravel.
The amount of air used in air wash systems is about 3 scfm/ft2 (15.3 1/s/m2) of filter area.
Normal backwashing with air and water consists of 1) draining the filter to the top of the
media, 2) turning on the air for 3 to 5 minutes, 3) turning on the water wash approximately
one-half the fluidizing rate for about 2 minutes, and 4) shutting off the air and completing
the wash at the maximum water wash rate.
The backwash cycle is started either manually or automatically when the head loss across
the filter reaches some predetermined value or the filter effluent turbidity reaches a preset
value, as determined by a continuously monitoring turbidimeter.
To have water available for backwashing, some storage must be provided. The volume
depends on the number of filters that will be washed in sequence. As a minimum, there
should be a volume sufficient to wash two filters consecutively. The backwash water can be
supplied by backwash pumps or by gravity from a storage tank.
Preengineered modular design filters that incorporate stored backwash water (Figure 11-6)
are available. These filters are so designed and automated that, after a preset head loss has
developed, the filter goes into its backwash cycle using the stored water. Air wash can be
incorporated in these units. After the wash cycle, the filter reverts to the filter cycle and the
initial flow fills the storage compartment. These units require minimal operator attention;
however, there is no simple way to prolong the backwashing, if the normal backwash period
does not adequately clean the filter.
Another automatic backwash filter design used extensively in water treatment plants is the
traveling backwash assembly (Figure 11-7). The filter bed in these filters normally has a
depth of about 10 to 12 in. (0.25 to 0.30 m) and is divided into many sections about 8 in.
(0.2 m) wide. The backwash assembly moves from section to section, cleaning each in
sequence. The backwash pump mounted on the assembly takes filtered water from the
effluent flume. The system is highly automated; backwashing is started when the loss of
head is about 1 to 2 in. (25 to 50 mm). Basically, only a small amount of solids is allowed to
accumulate in the filter before the media are cleaned. This filtering system does not require
any separate backwash water storage, pump, piping, or controls. The total power require-
ment for a filter bed area of 1,760 ft2 (164 m2) is that of a 5-hp (3.7 kW) motor. This
system has recently been adapted to an activated carbon bed filter having a bed depth of
4 ft (1.2 m) (13). Such a system would be applicable to polishing wastewater effluent by
removing COD and suspended solids.
The backwash water should be sent to a holding tank for equalization and pumped back to
the biological or chemical treatment facility preceding the filters. To eliminate the need for
separate solids removal facilities, the holding tank should be agitated with diffused air or by
a mechanical mixer to keep the solids in suspension. To insure that the suspended solids in
the wash water are captured, it must be given the same treatment ahead of filtration the
wastewater receives.
11-14
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BACKWASH WATER
MOTOR
FLOW DURING FILTRATION
I—h-1—t
POROUS PLATE . •
j • • •••• ".- -A .--.iV.-
PUMP DRIVE
SHAFT
EFFLUENT TO
CLEARWELL—
BACKWASH
PUMP
-EFFLUENT
CHANNEL
FIGURE 11-7
AUTOMATIC BACKWASH FILTER
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The backwash water will normally average about 2 to 5 percent of the wastewater flow, de-
pending on the SS the filters remove. Deep bed, coarse media filters, removing up to 100
mg/1 SS will have backwash water requirements of 5 to 10 percent of the wastewater being
treated.
11.4 Sand Beds (Intermittent Filtration)
Effluents from small treatment plants, especially package type plants, are frequently applied
to outdoor sand filters, which insures a higher degree of treatment and removal of most of
the SS. Smaller plants may frequently be upset, with a resultant increase in SS discharge.
The design of these filters is similar to the intermittent sand filters frequently used in the
past for biologically treating settled wastewater (14).
Such filters should be drained with open joint or perforated tiles, of at least 4 in., laid on an
impervious layer. The sand depth should be about 30 to 36 in. (0.75 to 0.90 m); sand
should be graded, with an effective size between 0.3 and 0.5 mm. Although the flow to such
filters may be continuous in remote installations, the proper operation for effective treat-
ment is to have two, and preferably three, filter beds. The flow is directed to one filter for
24 hr. That filter is allowed to drain and dry for 1 to 2 days, and the flow goes to an adja-
cent filter. A 3-day cycle produces good operation and treatment.
The loading rate over a 24-hr period of settled secondary effluent can be about 500,000
gpad (490,000 m3/m2-d), or about 10 gpd/ft2 (0.5 m3/m2-d). Filtered effluent will nor-
mally have a BOD of 5 to 10 mg/1 and SS of 5 to 10 mg/1.
The solids that have accumulated on top of the sand are periodically removed, together with
the top sand layer, and disposed of. Operation in below freezing climates can create prob-
lems, especially if operation is continuous flow, instead of intermittent. Plowing the top of
the bed into ridges and furrows about 12 to 18 in. (0.3 to 0.45 m) deep has been found
beneficial in keeping the top layer of sand from freezing and in keeping the bed pervious.
Studies (15) were recently conducted in Logan, Utah, on using intermittent sand filters to
filter the effluent from stabilization lagoons, to remove the suspended matter and upgrade
the effluent to meet the regulatory agency BODs standard of 5 mg/1. This effluent quality
was achieved at hydraulic loadings in the range of 400,000 to 800,000 gpad (392,000 to
784,000 m3/m2-d). It was concluded that, for an average loading of 500,000 gpad (490,000
m3/m2-d), the filters would operate approximately 100 days before cleaning would be re-
quired, if receiving a lagoon effluent with an average SS concentration of 20 mg/1. Clean-
ing consists of removing the top layer of sand; eventually, new sand must be added.
11.5 Microscreening
A microscreen unit consists of a motor driven rotating drum, covered with a microscreen
medium, mounted horizontally in a rectangular channel (Figure 11-8). The wastewater
enters the drum through one end and passes out through the screen, with the SS being
retained on the inner surface of the screen. Pressure jets of plant effluent are directed down
11-16
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CONTINUOUS WASH
COMBINED MAIN SHAFT
a WASTE PIPE
FIGURE 11-8
SCHEMATIC DIAGRAM OF MICROSCREEN UNIT
-------
onto the screen, to remove the deposited solids on the inside of the drum. The washwater
and removed solids are captured in a washwater hopper and conducted to a point outside
the drum. The washwater must be recycled upstream to the point where the main portion of
the solids are removed from the wastewater being treated. In a biological plant, it should be
recycled to a point ahead of the aeration basins or trickling filter (e.g., to the primary
settling basin). If the microscreen follows a chemical treatment process, the washwater
should be returned ahead of the point of chemical addition to the wastewater.
Biological growths on the microscreen are controlled by periodic treatment with a chlorine
solution. In some cases, placing ultraviolet lights above the screen medium has been effective
in controlling growths.
Microscreen units have been used to polish secondary plant effluents. By using a fabric with
25-micron Gum) openings, the suspended solids in activated sludge and trickling filter plant
effluents have been reduced from about 20 to 40 mg/1 to about 5 to 10 mg/1. With activated
sludge plant effluents, this reduction can represent a final effluent BOD of about 10 to 20
mg/1. The washwater will amount to about 5 percent of the flow being treated. The filtra-
tion rate will average about 4 to 8 gpm/ft2 (230 to 460 m3/m2-d) of submerged screen area.
Microscreens usually are not very effective in removing alum flocculated solids in final
effluents. The alum floe does not have much shear resistance and seems to "flow" through
the fabric. However, if alum is fed to an aeration basin for phosphorus removal, the solids
remaining in the effluent from a final clarifier can be removed to the same degree as normal
activated sludge solids.
The reader is referred to the Technology Transfer Design Manuals for Suspended Solids Re-
moval and Upgrading Wastewater Treatment Plants for additional information on micro-
screens (1) (2).
In choosing microscreening or rapid sand filters for SS removal from wastewater treatment
plant effluents, several items must be considered. A granular media filter can produce a high
quality effluent if such is required, because effluent SS below 5 mg/1 can be obtained with
proper design of granular media filters. Such an effluent quality would be difficult to obtain
consistently with a microscreen. Some comparative studies between sand filters and micro-
screens were made by the Chicago Metro Sanitary District (16) and in England (17). Micro-
screens cannot cope with sudden load changes as well as sand filters. Sand filters require
about 8 to 10 ft of head, which frequently means pumping and appreciably increased costs.
Microscreens require only about 12 to 18 in. of total head loss. The space requirements for
microscreens are less than those for granular media filters.
On the basis of total and operating cost comparisons made in England (17), microscreening
was somewhat more expensive than sand filtration, even if pumping was required.
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11.6 References
1. Process Design Manual for Suspended Solids Removal U.S. EPA, Office of Technology
Transfer (January 1975).
2. Process Design Manual for Upgrading Existing Wastewater Treatment Plants. U.S. EPA,
Office of Technology Transfer (October 1974).
3. Haney, B.J., and Steimle, S.E., "Potable Water Supply By Means of Up-Flow Filtra-
tion." Journal American Water Works Association, p. 117 (February 1974).
4. Alpe, G., and Barrett, A.D., "Upflow Filtration." Journal New England Water Pollu-
tion Control Association, p. 111 (December 1973).
5. Hamann, C.L., and McKinney, F.E., "Upflow Filtration Process." Journal American
Water Works Association, p. 1023 (September 1968).
6. "Design and Application of Multimedia Filters." Journal American Water Works Asso-
ciation, p. 97 (February 1969).
7. Morey, E.F., "High Rate Filtration-Media Concepts." Public Works, p. 80 (May 1974).
8. Jung, H., and Savage, E.S., "Deep-Bed Filtration." Journal American Water Works
Association, p. 73 (February 1974).
9. Cleasby, J.L., and Baumann, E.R., "Direct Filtration of Secondary Effluents." Pre-
sented at EPA Technology Transfer Design Seminar (February 1974).
10. Cleasby, J.L., "Filter Rate Control Without Rate Controller." Journal American Water
Works Association, p. 181 (April 1969).
11. Kreissal, J.F., Granular Media Filtration of Wastewater: An Assessment. U.S. EPA,
NERC (January 1973).
12. Ultra High Filtration of Activated Sludge Plant Effluent. U.S. EPA, Office of Research,
No. EPA-R2-73-222 (April 1973).
13. Medlar, S., "A New Approach to Activated Carbon Filtration." Water and Sewage
Works, p. 56 (November 1973).
14. "Sewage Treatment Plant Design." ASCE Manual of Engineering Practice (1959).
15. Marshall, G.R., and Middlebrooks, E.J., Intermittent Sand Filtration to Upgrade Exist-
ing Wastewater Treatment\Facilities. No. PRJEW115-2, Utah Water Research Lab, Utah
State University, Logan, Utah (February 1974).
11-19
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16. Lyman, B., et al., "Tertiary Treatment at Metro Chicago By Means of Rapid Sand
Filters and Microstrainers." Journal Water Pollution Control Federation, p. 247
(February 1969).
17. Isaac, P.G., and Hibbard, R.L., "The Use of Microstrainers and Sand Filters for Ter-
tiary Treatment." Water Research, No. 6, Great Britain, p. 465 (1972).
11-20
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CHAPTER 12
PHYSICAL-CHEMICAL TREATMENT
In this chapter, physical-chemical (PC) treatment of wastewater is defined as treatment com-
bining physical and chemical processes whose effectiveness primarily depends on physical
and chemical reactions. Such processes in elude:
1. Coagulation-flocculation
2. Recarbonation
3. Gas stripping
4. Sedimentation
5. Filtration
6. Flotation
7. Adsorption
8. Oxidation-reduction
9. pH adjustment
10. Reverse osmosis
11. Ion exchange
12. Distillation
PC treatment of wastewater employs some combination of the above unit processes to
remove pollutants and alter water quality. By using these processes, it is possible to obtain
any desired effluent water quality.
However, wastewater treatment plant effluent standards usually only require reduction of
biochemical oxygen demand (BOD) to between 5 and 30 mg/1, chemical oxygen demand
(COD) to between 15 and 60 mg/1, and suspended solids (SS) to between 5 and 30 mg/1 for
small municipalities. These levels of quality may be attained by PC treatment, with the fol-
lowing processes (1):
1. Coagulation-flocculation
2. Sedimentation
3. Filtration
4. Adsorption
PC methods may be necessary, when circumstances dictate the removal of substances not
readily removed by biological treatment methods-substances, such as dissolved inorganic
compounds, biologically refractory organic materials, or toxic substances, both organic and
inorganic. The most common inorganic substance normally removed by PC treatment in
municipal wastewater is phosphorus. If an industrial discharge is a component of the munic-
ipal wastewater, substances such as nitrogen, phenols, dyes, heavy metals, and related mate-
rials may be present in significant quantities, and PC treatment may be required. In general,
PC processes are used in smaller wastewater treatment plants for:
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1. Reduction of phosphorus
2. Reduction of nitrogen
3. Treatment of wastes toxic to biological systems
4. Removal of color
5. Removal of biologically refractive materials
PC methods have not been widely used to treat municipal wastewater, because of cost and
lack of demonstrated experience with these methods. However, in recent years nutrient
removal has become obligatory in some States. Reports now available in the technical litera-
ture (2) (3) (4) (5) permit a designer to evaluate the applicability of the available PC pro-
cesses to the particular design problem. These processes may be used alone or in conjunction
with biological processes to obtain optimum results. They should be considered if:
1. The wastewater contains large concentrations of nonsettleable SS (colloidal)
2. A high degree of treatment is required
3. A high degree of reliability is required
4. Fluctuations in quality and quantity of wastewater are beyond the tolerance of
biological systems (fluctuations due to large summer population, small winter
population, and startup and shutdown of a seasonal industry)
5. Plant size must be minimized
There are situations in which PC methods are generally not favorable. Wastewaters contain-
ing large concentrations of dissolved, biologically degradable organic materials, and those
with BOD concentrations relatively high in comparison to COD, are usually treated more
economically by biological methods. For example, a wastewater containing large concentra-
tions of sugar, soluble starch, acetic acid, etc., would be difficult to treat by PC methods.
12.1 Design Considerations
Basic data and preparatory studies required in designing a PC treatment system include:
1. Sufficient chemical and biological analyses of the wastewater, to ascertain the
fluctuations in concentration of various pollutants over a period of time
2. Hourly, daily, and annual variations in raw wastewater flow
3. Treatability studies, using PC unit processes considered for design
The chemical analyses are essential in selecting unit operations to be employed and chemical
additives to be used and in estimating the probable operating cost of the proposed treatment
works. The raw wastewater flow and flow variations obviously should be estimated, to
establish the size of the various treatment units and the auxiliary equipment, such as
chemical feeders. Flow equalization may be necessary to reduce the required capacity of the
chemical feeders and treatment units, whose design is determined by hydraulic loadings. In-
formation on equalization basic design is given in chapter 4.
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Chemical treatability tests usually take the form of jar tests, in which various dosages of
selected chemicals are added to jars of the wastewater on a multiple-position stirring
machine. Instructions for making jar tests are given in the EPA Process Design Manual for
Suspended Solids Removal (4). After suitable periods of mixing, flocculation, and sedimen-
tation, the results of the chemical addition are observed and measured. These results will
establish the suitability of the selected chemicals, the dosages required, the degree of treat-
ment to be expected, and the flocculation characteristics of the waste, and will provide a
means of estimating the weight of sludge generated by the proposed treatment. Considera-
tion of sludge disposal is particularly important in the design of a PC treatment plant, be-
cause of possible unfamiliar characteristics of the sludge produced. Details on sludge han-
dling and disposal are given in chapter 14.
12.2 Chemicals
12.2.1 Alum
Alum (aluminum sulfate) is widely employed in water treatment as a coagulant for color
and turbidity removal. Alum has also been used, to a limited extent, for municipal waste-
water coagulation. Interest in the use of alum treatment of wastewater has greatly increased,
however, because it has become generally recognized that phosphorus can be precipitated
with alum. The simplified chemical reaction used to represent the coagulation process is:
A12(SO4)3 • 14 H20 + 6 HCO^ -> 2 A1(OH)3 + 6 CO2 t + 14 H2O + 3 SO|
A1(OH>3 is a gelatinous, insoluble compound, which entraps and adsorbs suspended par-
ticulate and colloidal material in wastewater into a floe removable by sedimentation and
filtration. HCO3 represents the alkalinity component of the wastewater. The chemical reac-
tion involved in the removal of phosphorus (in the form of phosphate, POJ ) is:
A12(S04)3 • 14 H2O + 2 POf -> 2 A1PO4 + 3 SO| + 14 H2O
The above reaction indicates that, theoretically, removal of 1 Ib (0.45 kg) of phosphorus
requires 9.6 Ib (4.4 kg) of alum. However, because of the competing reaction with the alka-
linity and the necessity to reduce the pH to 5.5 to 6.5 (the minimum solubility range for
A1PO4), dosages of alum greater than the theoretical dosage are required. Therefore, the
aluminum ion dosage is usually about 1.5 to 2.0 times the phosphorus ion removed, ex-
pressed in weight units. The proper dosage for the degree of removal desired can best be
determined by jar tests.
Sodium aluminate (NA2OA12 O3 -3H2O) may be substituted for alum in certain situations
(e.g., if chemicals are added to a biological process for improved phosphorus removal). In
this case, the dose should provide an equivalent amount of aluminum, compared to the alum
dose.
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Alum may be purchased in lump, ground, rice, powdered, or liquid (49 percent solution)
form. Choice of form depends on the amount used and the location of the wastewater treat-
ment plant in relation to the source of alum. For details about the forms, sources, and
design considerations of alum use, refer to the EPA Process Design Manual for Suspended
Solids Removal (4).
12.2.2 Iron Salts
Iron salts react in much the same manner as alum and are capable of coagulating color, tur-
bidity, suspended and colloidal solids, and their associated BOD and COD.
The chemical equation for precipitation of phosphorus and coagulation of wastewater with
ferric ion is:
Few + POf -» FePO4
In water, the ferric ion reacts with the hydroxide ion as follows:
Fe^ + 3 OH" -»• Fe(OH)3
From these reactions, it may be calculated that 1.8 Ib (0.82 kg) of iron are theoretically re-
quired to remove 1 Ib (0.45 kg) of phosphorus. However, as with other coagulants and
coagulant aids, some amount reacts with the alkalinity and is not effective for phosphate
removal. Therefore, the actual required weight ratio of ferric ion to phosphorus ion is
usually about 2.5 to 3.0. The jar test is generally the best guide to required dosage.
Iron coagulants are available in the following forms:
Ferric chloride (anhydrous), FeCl3 350-lb drums
135-lb drums
Ferric chloride (lump), FeCl3 *6 H2O
Ferric chloride (liquid), 37 to 47 percent FeCl3 4,000-gal lots
Ferrous chloride (liquid), 20 to 25
percent FeCl2 (waste pickle liquor)
Ferric sulfate, Fe2(SO4)3 -3 H2O 50- to 100-lb bags
(400-lb drums)
Fe2(SO4)3 -2 H2O 50 to 100-lb bags
(400-lb drums)
Ferrous sulfate, FeSO4 •! H2O 50-lb bags and bulk
Iron coagulants are more or less hygroscopic, depending on the form used. Some, such as
ferric sulfate, can be dry fed, but it is not recommended for small plants that may be un-
attended for long periods. The preferred dosing method is solution feed, which is generally
more reliable than using small dry feeders. Ferric solutions are very corrosive.
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Details concerning the feeding of iron salts are in the EPA Process Design Manual for
Suspended Solids Removal (4).
12.2.3 Lime
Calcium (Ca) ion in the presence of the hydroxide ion will react with phosphate to form the
slightly soluble mineral, hydroxyapatite. The generalized equation for this process is:
3 HPOl + 5 Ca~ + 4 OH" -»• Ca5(PO4)3(OH) + 3 H2O
Lime (calcium oxide [CaO], or quicklime) or calcium hydroxide [Ca(OH)2, or hydrated
lime] are the most common sources of calcium for wastewater treatment. Lime has the ad-
vantage of providing both the calcium ion and the hydroxide ion needed for the reaction.
However, as with alum and the iron salts, acid neutralizing reactions and softening reactions
divert some of the lime dosage from the phosphate reaction. Lime may also be used to pro-
vide additional alkalinity, if needed for the iron and alum coagulation processes.
If lime is used to remove phosphate in a PC treatment plant, a sufficient quantity must be
added to raise the pH to 10 or higher. A pH of 11 or more may be required to reduce the
phosphorus concentration to below 1 mg/1. Jar tests should be used to determine the lime
dosage required for good clarification and the phosphate removal desired. Dosages will be
higher in hard water areas where the softening reactions will have a high lime demand.
A soft water supply could also reduce the use of phosphate-containing detergents, reducing
lime demand at the wastewater treatment plant. Following lime mixing and a short floccula-
tion period, the precipitated hydroxyapatite and calcium carbonate are separated from the
wastewater in a sedimentation basin.
Following sedimentation, advantage may be taken of the high pH of the wastewater to air
strip the ammonia (6) (7). Whether or not the ammonia is removed, the sedimentation basin
effluent will contain excess lime (which may cause difficulties in subsequent processes),
and will have a pH value higher than acceptable for discharge. The excess lime is usually
removed by adding carbon dioxide (or flue gas containing mostly CC^) until the pH is
reduced to about 9.0 to 9.5. At this pH, calcium carbonate is least soluble and may be re-
moved by additional settling. Sometimes an iron salt is also required for good clarification at
this point. If an extremely high quality effluent is required, the sedimentation basin effluent
is filtered, using in-depth filters. Prior to filtration, the wastewater pH may require further
adjustment to about 7.5 to 8.0, to prevent precipitation of calcium carbonate on the filter
media. At these pH values, the calcium is associated with the bicarbonate ion and is fairly
soluble.
Lime may be purchased as quicklime (CaO) or as hydrated lime [Ca(OH)2]. The chemical
difference between the two forms consists of one molecule of water combined with each
molecule of lime in the hydrated form. However, this small chemical difference results in
physical properties that make hydrated lime much easier to use in a small wastewater treat-
ment plant. Quicklime is not suitable for a plant that would use less than about 1 ton per
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day of this chemical, because of storage and handling problems. Further information is avail-
able in the EPA Process Design Manual for Suspended Solids Removal (4).
In some localities, hydrated lime can be purchased as a slurry containing 20 to 30 percent
lime. These slurries are the byproduct of acetylene manufacture. If available within a reason-
able distance, this form of lime may be the cheapest and easiest to handle. It requires a
storage tank with a slow mixer to keep the slurry in suspension.
All forms of lime tend to cause excrustations on pipes and equipment, and thus piping and
equipment should be designed for easy cleaning. Piping should permit the frequent use of a
"polypig" (a plastic insert which is forced through the piping by hydraulic pressure). Pre-
ventive maintenance is essential for successful operation of a lime treatment system.
12.2.4 Coagulant and Filter Aids
One relatively recent advance for removing suspended contaminants in wastewater treat-
ment has been the development of organic polymers, or polyelectrolytes, having properties
that permit them to act as coagulants or coagulant aids. These synthetic or natural poly-
mers, if introduced into wastewater in relatively low concentrations (from less than 1 mg/1
to 10 mg/1), may bring about a coagulation of the suspended matter into settleable or filter-
able floes. Synthetic polymers may be classified on the basis of the type of charge on the
polymer chain. Polymers having negative charges are called anionic; those carrying positive
charges are cationic. Certain compounds carry no charge and are called nonionic.
Many polymers are available for wastewater treatment. The EPA. Process Design Manual for
Suspended Solids Removal (4) lists several different polymer manufacturers.
Selecting the best polymer is based on experience and testing (jar tests are the test method
of choice). Extensive testing should be done before selection. Dosage is just as important as
polymer selection (an excessive dosage may yield poor results). Polymers may be used alone
or in conjunction with other coagulants, to improve settleability and filterability of the
wastewater. Information about these polymers, their availability as feeding requirements,
etc., can be found in reference (4).
12.2.5 Ozone
Ozone is an oxygen molecule containing three atoms of oxygen. In wastewater treatment
plants, ozone has been used principally for odor control in the exhaust air from wastewater
wet wells, screen chambers, and similar locations. In Europe, and to a limited extent in the
United States, ozone has been used as a disinfecting and color reducing agent for municipal
drinking water. Because ozone is a powerful oxidizing agent, it has been suggested that
ozone may be used as a tertiary treatment step to obtain low effluent COD values. An added
benefit would be disinfection. Ozone is also excellent for removing phenols. Above a pH of
10, ammonia may be oxidized to nitrate-nitrogen. For color and COD reduction, ozone may
in some cases replace activated carbon adsorption. Ozonation is discussed in chapter 15.
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12.3 Unit Operations
12.3.1 Mixing and Flocculation
Excellent mixing and flocculation are essential to the economical operation of any chemical
treatment system. Mixing disperses the added chemical throughout the wastewater being
treated. The most efficient utilization of chemicals is achieved by an extremely rapid and
uniform dispersal. Good mixing requires considerable energy input, and velocity gradients in
excess of 300 fps (90 m/s) should be employed. Velocity gradients of this magnitude when
adding coagulants, however, may cause excessive shearing of floe particles, if contained for
over 2 minutes. It is, therefore, necessary to reduce the energy input and the velocity
gradients as soon as the dispersal of the treating agent is complete. In the flocculation stage,
the flocculating agents and the suspended matter in the wastewaters coalesce into relatively
large floe particles, which have improved settling characteristics. Velocity gradients in the
flocculating section should not exceed 100 fps (30 m/s). Flocculation may require from 5 to
30 minutes. The time required may be estimated from the results of the jar tests. Following
flocculation, the wastewater is normally admitted to a sedimentation basin. Excessive turbu-
lence should be avoided in the flow from the flocculation basin to the sedimentation basin,
to keep floe breakup to a minimum. A more complete discussion of mixing and flocculation
is found in EPA Design Manual for Suspended Solids Removal (4).
12.3.2 Sedimentation
Well-conditioned chemical floe will generally settle more slowly than primary wastewater
sludges and more rapidly than activated sludges (4). Few generalizations can be made, how-
ever, concerning the actual rates of settling to be used for design purposes, because these
rates depend on the particular coagulant used, the response to coagulation aids,.and the
chemistry of the wastewater being treated. Present recommendations (4) for peak overflow
rates for sedimentation basins are 500 to 600 gpd/ft2 (20 to 24 m3/m2-d) for alum systems,
700 to 800 gpd/ft2 (29.33 m3/m2-d) for Fe systems, and 1,400 to 1,600 gpd/ft2 (57 to 65
m3/m2-d) for lime systems. Further experience and actual pilot studies will offer more
accurate design information.
Ample provision should be made in sedimentation basin design for the large quantities of
sludge generally produced by chemical coagulation of wastewaters.
The critical design parameter is the peak hourly surface overflow rate. Gross carryover of
solids can cause a downstream filter or adsorption process to fail because of excess head
loss. Such a failure may in turn result in a total failure of the plant to achieve desired results.
12.3.3 Lime Treatment and Recarbonation
Lime treatment of wastewaters for phosphorus removal often requires raising the pH above
10.0 to 11.0. At these high pH values, precipitated calcium carbonate tends to encrust all
downstream processing equipment. Provisions should be made for reducing the pH to about
9.5 before, and about 7.5 following, lime-promoted settling. The pH may be reduced by
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introducing carbon dioxide gas into the wastewater, which will result in the formation of
calcium carbonate. For a small wastewater treatment plant, the carbon dioxide may be ob-
tained in tank truck lots from a commercial supplier. It is usually added to the water
through a diffuser grid system in a recarbonation basin. Recarbonation basin detention time
is usually between 10 to 15 minutes. The floe formed with or without additional chemical
coagulant is then removed by sedimentation in a second basin. A more complete description
of this process is contained in reference (5).
The high pH from lime treatment may also be reduced by the use of a mineral acid, such as
sulfuric acid or hydrochloric acid. Using mineral acid to reduce the pH results in an increase
in the total dissolved solids of the effluent. The method of pH reduction selected will
depend on the effluent quality desired and the costs of the alternative methods.
12.3.4 Filtration
If the PC treatment system chosen includes filtration for improved SS removal, it is
common to employ granular media designs. Use of these filters for polishing effluents from
secondary biological treatment plants is discussed in chapter 11.
Properly designed dual- or multimedia filters have operated at rates of about 5 gpm/ft2 (0.2
m3/m2-min). Filters should be provided with surface or air/water backwashing. Pressure
filters are often desirable for small plants, because of the higher head losses (up to 20 ft
[6m] of head) that may be employed. If filtration is followed by a granular carbon adsorp-
tion step, the effluent from the pressure filter can pass through the downstream carbon
columns without having to be repumped.
Filter backwash waters must be reprocessed through the wastewater treatment system.
Direct return of the wash waters to the head of the plant would create a substantial
hydraulic surge. Therefore, if no flow equalization facility is available for the incoming
wastewater, the backwash water should be collected in a storage tank and recycled to the
head of the plant at a controlled rate. An excellent treatment of the design considerations
for filtration can be found in chapter 9 of the EPA Process Design Manual for Suspended
Solids Removal (4).
The wastewater is passed through the granular carbon in a column. The. flow may be either
upflow or downflow. A complete description of the designs of granular carbon adsorption
systems is included in the EPA Technology Transfer Process Design Manual for Carbon
Adsorption (3).
The amount of carbon used depends on the amount of COD removed. Carbon adsorbs both
biodegradable and other organics; therefore, amount of COD removal must be used in esti-
mating carbon usage. It has been found that for domestic wastewaters, a pound of carbon
may adsorb 0.5 Ib or more of COD (8).
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If the effluent organic concentration from a carbon column exceeds the desired level, the
adsorptive capacity of the carbon is exhausted, for this particular system. The exhausted
column must then be taken out of service and the used carbon replaced with fresh carbon.
In a small plant, the exhausted carbon may be discarded. In larger plants, it may be eco-
nomically desirable to regenerate the spent carbon. This is generally done by thermal
methods. For smaller plants, the carbon must be sent to an offsite regeneration facility. In
any event, a detailed cost analysis is necessary to determine the most economical methods
of dealing with the carbon. Powdered activated carbon treatment is a possible alternative to
adsorption.
12.3.5 Adsorption
Adsorption has been described as a process of interphase accumulation (2) of ions, atoms,
molecules, or colloidal particles. Adsorption can occur at an interface between any two
phases; for example, a liquid-liquid interface, a gas-liquid interface, or a liquid-solid inter-
face. The material being concentrated is referred to as the "adsorbate," and the adsorbing
phase is called the "adsorbent." Many physical and chemical forces (such as van der Walls
forces, electrokinetic forces, thermal forces, etc.) cause this accumulation, which is similar
to surface tension at the phase interface.
Interest in the current discussion centers on the adsorption of pollutants from a solution on
a solid. For the majority of systems encountered in wastewater treatment, the principal
driving forces are the lyophobic behavior of the solute relative to the solvent (water) and the
high affinity of the solute for the solid. A more complete description of the adsorption pro-
cess and the mathmatics of its application will be found in reference (2).
Adsorption may be used to meet extremely high final effluent standards for BOD and COD
in a small biological treatment plant or as a substitute for biological treatment in a complete
PC treatment plant. In this latter case, the carbon adsorption process is used to remove
soluble organics remaining after coagulation and filtration of raw wastewater, to obtain
BODs of less than 20 mg/1. Currently, the most commonly used form of activated carbon is
the granular form. Because powdered carbon currently costs about $0.15 to $0.20/lb, and
granular carbon costs about $0.45 to $0.50/lb, powdered carbon used on a once-through
basis may be justified for small plants not requiring a high degree of treatment.
Granular carbon columns are also used after chlorination to remove partially oxidized
chlorinated organics and chloramines in conjunction with breakpoint chlorination for
ammonia removal (9) (10).
12.3.6 Stripping
Stripping is a method of removing volatile substances from a solution, by exposing the solu-
tion to an atmosphere in which the concentration of the volatile component in the atmo-
sphere is less than the equilibrium concentration in the liquid, in accordance with Henry's
law. The volatile component then tends to leave the solution and enter the atmosphere,
until equilibrium is established. Liquid gas surface films resist the escape of the volatile
12-9
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component, as does the diffusion through the bulk liquid. Strippers are designed to maxi-
mize the rate of escape of the volatile component, by creating constantly renewed interfaces
between the liquid and the atmosphere; reducing the distance of diffusion within the bulk
liquid, providing greater turbulence; and providing an atmosphere with little or none of the
volatile component.
In wastewater treatment practice, there are three types of strippers:
1. The bubble type diffuses air, in the form of small bubbles, into the wastewater.
The small bubbles present an extremely large interface between air and water,
while, at the same time, the interface, is being renewed by the movement of the
bubbles rising in the liquid. Volatile substances in the liquid cross the interface
into the bubbles and are released to the atmosphere when the bubbles reach the
surface.
2. The spray system sprays wastewater into the atmosphere in small droplets. Here
again, a large liquid-air surface is created, permitting the escape of the volatile
components. Spray systems can utilize either natural wind and draft to provide
a change of air and remove the volatile substances or a mechanical draft, using a
fan.
3. The thin film stripper causes wastewater to flow in a thin film over solid surfaces
in the stripper. These surfaces are usually contained in a tower and may consist of
packing material, wooden slats arranged in a stacked grid, or other designs. The
wastewater enters the top of the tower and flows down over the surfaces in a thin
film. Air is blown in the bottom of the tower and exits at the top or may be
drawn upward through the tower by a negative pressure fan. Here again, the vola-
tiles escape from the liquid surfaces. The released volatiles may be removed from
the air before discharge to the general atmosphere by passing the off-gases
through an appropriate absorber.
Details on the design of strippers can be found in reference (6).
12.3.7 Other Unit Operations
The other unit operations listed on the first page of this chapter are of minor interest to the
designer of small municipal wastewater treatment plants and would only be employed in
special situations. They are, therefore, beyond the scope of this manual. These methods are
mentioned here to bring them to the reader's attention, so they will not be overlooked if
their use is indicated.
In case a waste has a high fiber or oil content, dissolved-air flotation may yield results
superior to sedimentation, at a savings of space and capital cost. Reverse osmosis, ion ex-
change, and distillation would only be considered in cases in which water reuse was con-
templated, although selective ion exchange has been studied in connection with NH3 re-
moval (6). Oxidation-reduction processes may involve the use of chlorine, ozone, potassium
permanganate, and hydrogen peroxide. These chemicals may be used for odor control and
BOD reduction.
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12.4 Costs
The total annual costs of a PC treatment facility for comparable BOD5 removal may be
similar to that of an activated sludge plant, depending on the circumstances of the applica-
tion.
The operating cost of a PC plant will be higher than that of a biological plant, although the
initial cost may be lower. A PC plant may be more cost effective than a biological plant. A
biological system can remove ammonia NH3 by conversion to nitrate, while normal PC
treatment (chemical coagulation, clarification, carbon adsorption designs) does not. As noted
earlier, PC plants normally remove phosphorus. Biological facilities normally do not, but can
be modified to do so. Therefore, the required effluent criteria will greatly affect the choices
of a treatment system and the relative economics of the alternative approaches.
12.5 References
1. Physical-Chemical Treatment of Combined and Municipal Sewage. U.S. EPA, EPA-R2-
73-149(1973).
2. Oxidation and Adsorption in Water Treatment Theory and Application. University of
Michigan, Division of Sanitary and Water Resources Engineering (February 1965).
3. Process Design Manual for Carbon Adsorption. U.S. EPA, Office of Technology
Transfer (October 1973).
4. Process Design Manual for Suspended Solids Removal. U.S. EPA, Office of Technology
Transfer (January 1975).
5. Gulp, G.L., Physical-Chemical Treatment Plant Design. Presented at U.S. EPA Tech-
nology Transfer seminar, Pittsburgh (August 1972).
6. Physical-Chemical Nitrogen Removal U.S. EPA Technology Transfer Seminar Publica-
tion (July 1974).
7. Physical-Chemical Wastewater Treatment Plant Design. U.S. EPA Technology Transfer
Seminar Publication (January 1974).
8. Kreisel, J.F., and Westrick, J.J., "Municipal Waste Treatment by Physical Chemical
Methods." Applications of New Concepts of Physical-Chemical Treatment, Pergamon
Press (1972).
9. Barnes, R.A., Atkins, P.P., and Scherger, D.A., Ammonia Removal in a Physical-
Chemical Waste-water Treatment Process. EPA report No. EPA-R2-72-123 (November
1972).
10. Bauer, R.C., and Snoeyink, V.L., "Reactions of Chloramines with Activated Carbon."
Journal Water Pollution Control Federation, p. 2,290 (1973).
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CHAPTER 13
NUTRIENT REMOVAL
13.1 General Considerations
If the wastewater from a municipality has been treated by the standard methods of primary-
secondary treatment, the effluent will normally contain about 20 to 50 mg/1 of BOD5,
30 to 90 mg/1 of COD, 20 to 40 mg/1 of total nitrogen, and 8 to 15 mg/1 of total
phosphorus (1), as well as minor amounts of other constituents. These materials serve as
nutrients for the natural biota in the receiving waters, sometimes stimulating excessive
growth of particular species. This overgrowth frequently creates undesirable conditions such
as oxygen depletion, fish kills, odors, unsightly concentrations of algae, discolored or turbid
water, and unpleasant tastes. To prevent such biological stimulation, it may be desirable to
limit one or more of the nutrients in a treatment plant effluent. Generally, one of a number
of essential nutrients is in short supply and, therefore, growth-limiting in any-particular
natural environment. If the natural growth-limiting nutrient can be prevented from entering
the receiving water in the treatment plant effluent, excessive biological response resulting
from the discharge can be prevented. Prevention of excessive growth is the objective of
nutrient removal.
Numerous studies have found that, in general, phosphorus is the growth-limiting nutrient
in the natural freshwater environment (2). Nitrogen is generally limiting in the marine
environment (2). Less frequently, it will be found that carbon or some other element is
limiting. As a result of these studies, much attention has been given to methods for
removing nitrogen and phosphorus from municipal wastewater.
13.2 Phosphorus Removal
As noted in chapter 12 of this manual, phosphorus may be removed in a physical-chemical
(PC) treatment system by the addition of iron or aluminum salts, or lime, to raw wastewater
prior to sedimentation and filtration. There are, however, many biological treatment plants
that also require phosphate removal. Some phosphate will be removed by the biological
treatment via the excess sludge. The amount of phosphorus in the excess sludge dry solids
is variable, ranging upward from 1.0 percent (the average percentage found in human waste
dry solids). Methods for maximizing the biological uptake of phosphorus are not well under-
stood and, therefore, cannot be relied upon as a means of nutrient removal at this time.
Hence, it is necessary to fall back on chemical treatment to achieve reliable phosphorus
removal from wastewater.
Using chemicals to obtain good coagulation will usually result in a high degree of phosphate
precipitation (see section 12.2). Such coagulation followed by sedimentation of raw
domestic wastewater will remove 50 to 80 percent of the BOD5, which will reduce the load
on any subsequent biological treatment processes significantly. The aeration requirements
and the excess sludge production will in turn be reduced.
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Chemicals used for phosphate removal in a biological treatment plant may be added: 1)
before primary sedimentation, 2) in the aeration basin of an activated sludge unit, 3) before
secondary sedimentation, 4) after secondary sedimentation, or 5) after second-stage
sedimentation and prior to filtration (if the plant is a two-stage biological plant). Chemical
addition of lime should not be used if the sludge is to be biologically digested. In a two-stage
biological system, coagulants should not be added in the second stage, because the increased
sludge wasting would excessively deplete the nitrogen-oxidizing microorganisms and
inactivate the system.
Aluminum and iron salts will not affect the aerobic organisms in the aeration basin, or the
anaerobic organisms if the sludge is anaerobically digested. However, sometimes dewatering
the sludge is difficult. If these chemicals depress the sludge pH below 6.5, lime or some
other alkali should be used to adjust the pH to 7.9, before the sludge is sent to the digester.
Conservatively speaking, the amount of chemical sludge produced will be equal to about
4 mg/1 for each milligram per liter of aluminum ion added and 2.5 mg/1 for each milligram
per liter of ferric ion added. The amount of chemical solids precipitated with lime treatment
depends on the pH maintained and the alkalinity of the wastewater, but roughly about three
times as much sludge by weight will be produced, compared to ordinary primary settling.
13.3 Nitrogen Removal
In the freshwater environment, because nitrogen is not limiting, ammonia or organic forms
in the effluent may exceed the direct nutrient requirements of the receiving water systems.
This excess nitrogen can serve as an energy source for nitrifying bacteria, which oxidize the
nitrogen to nitrates. If nitrogen is thus utilized, oxygen is extracted from the dissolved
oxygen resources of the receiving waters. About 4.6 parts of oxygen are utilized in
converting each part of ammonia to nitrate. The nitrogen in municipal wastewater can,
therefore, be equivalent to 80 to 150 mg/1 of BOD, if not removed before discharge.
Detailed discussion of nitrogen control is presented in the Process Design Manual for
Nitrogen Control (3).
13.3.1 Nitrogen Transformations in Biological Systems
Almost all the nitrogen in raw wastewater is present either in organic compounds or as
ammonia. The principal sources of ammonia are urea and feces, in which nearly 25 percent
of the dry solids is nitrogen (4). Urea is broken down by the bacteria enzyme urease to
ammonia and carbon dioxide, in sewers, in primary clarifiers, and in biological treatment. If
the wastewater received at a treatment plant is relatively fresh, containing an appreciable
concentration of undecomposed urea, some of the urea may remain in the effluent from a
secondary treatment plant. There is no known PC system for removing urea or converting it
to ammonia. Thus, if wastewaters contain significant quantities of urea, biological treatment
would appear to be necessary to insure conversion of the urea-nitrogen to either ammonia-
nitrogen or nitrate-nitrogen, which can then be removed by either PC or biological processes
(5). A good current review of this subject is given in reference (6).
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13.3.2 Physical-Chemical Nitrogen Removal Methods
A form of stripping adaptable to small plants resulted from studies made in Israel showing
that the ammonia concentration in a secondary effluent, if treated with lime to raise its pH
to 11, could be reduced 90 percent if passed through 10 ponds in series, each with a
residence time of 1.5 days (7). The liquid temperature was 59° F to 68° F (15° to 20° C).
Removal was accomplished by surface desorption into the atmosphere.
Ammonia can be oxidized to nitrogen gas by adding a sufficient amount of chlorine to the
wastewater to render the weight ratio of chlorine to ammonia about 9:1. This reaction must
be carried out at a pH between 6.5 and 8.0 to avoid formation of chloramines (especially
the trichloramine) (8) and of nitrates. The wastewater should receive a high degree of
treatment for removal of organics, to prevent the formation of complex organochlorides
and to keep the chlorine requirements to the theoretical amount needed for ammonia
oxidation. The chlorides will be increased in the effluent stream by the amount of chlorine
used. Also, because the pH is depressed, the addition of an alkali, such as lime or caustic
soda, may be necessary.
It is possible to obtain greater than 95 percent removal of ammonia with this process.
Economics and limits on chloride concentration in the effluent may dictate its applicability,
however. It is desirable to follow the chlorine contact basin with an activated carbon
column. Carbon catalyzes the ammonia-chlorine reaction and minimizes the discharge of
chloramines in the effluent (9) (10).
Ammonia can be removed biologically by conversion to nitrates (the process of nitrification
is described in chapters 7,8, and 9). Such nitrification can be obtained in lightly loaded
activated sludge systems, in a two-stage trickling filter, or in a lightly loaded bio-disk unit.
Denitrification is accomplished biologically in either a suspended growth or fixed growth
unit. By maintaining anaerobic conditions, the facultative organisms present will break down
nitrates as a source of oxygen and release nitrogen gas. A nitrifying-activated sludge process
can be followed by a reactor containing a suspension of organisms, which are kept out of
contact with free oxygen and slowly agitated to release the nitrogen gas bubbles to allow the
solids to settle properly in a gravity clarifier (11). The reactor should have a hydraulic
detention time of about 2 hours. To obtain proper settling, about 30 minutes are necessary
to insure that bubbles of nitrogen gas are detached from the floe. Aeration assists in this
separation.
Denitrification has been studied using either packed towers or rapid sand filters (12) (13).
Several plants using such systems are under design. A plant using large medium packed
towers and also fine medium columns is being operated under an EPA demonstration grant
to study both systems for denitrification. Initial results indicate that both systems can
reduce nitrate-nitrogen (14). The packing in the large medium towers consists of 10 ft (3 m)
of 5/8-in. (15.9 mm) plastic rings; the fine medium columns have 6.5 ft (1.95 m) of 0.12 to
0.16 in. (3 to 4 mm) sand.
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Because the denitrification organisms require a source of organic carbon, it is usually
necessary to feed some methanol (or other organic carbon source) to the denitrification
process in proportion to the incoming nitrate concentration, if the wastewater streams have
a low BOD. About 3 Ib of methanol are required for each pound of nitrate-nitrogen
removed, if there is no other source of readily available carbon in the nitrified wastewater.
Biological systems can reduce ammonia-nitrogen to below 1 mg/1, if they are designed for
the existing liquid temperature conditions. A biological system can also reduce total
nitrogen to less than 3 mg/1.
13.4 Removal of Soluble Organics
Inasmuch as green plants have the ability to convert carbon dioxide or bicarbonates and
water to complex organic substances by photosynthesis, it is seldom practical to attempt to
limit algal growth by controlling the availability of carbon. However, to control the growth
of heterotrophic microorganisms, the removal of organic matter as a nutrient is occasionally
desirable.
In chapters 7,9, and 10, consideration was given to the biological methods used in removing
soluble organic matter from wastewater. In chapter 12, PC methods were discussed.
However, it is often necessary to remove or reduce to trace amounts the dissolved organics
remaining after conventional treatment methods have been applied. Under these
circumstances, the final stage of treatment might employ activated carbon adsorption or
ozonation. These processes are suitable for small treatment systems (see chapter 12). If
ozone is used, the effluent should be checked for the presence of undesirable oxidation
products of certain organic materials such as phenols, detergents, and organic acids (15).
13.5 Removal of Soluble Inorganics
Occasionally, a designer finds it desirable to remove soluble inorganic compounds, which
may occur in excessive concentrations. If land disposal of treated wastewater becomes a
widely accepted practice, it may be necessary to limit the amounts of certain of these
materials in the effluent.
A study of the chemistry of the metals reveals that, in general, most metals can be oxidized
or reduced to a form in which they may be precipitated as the hydroxide of the metal (e.g.,
mercury is precipitated as the sulfide). Most of the metals may be removed from wastewater
by reverse osmosis or ion-exchange techniques. These methods create concentrated solutions
of the metals or regenerate chemicals, which require further treatment prior to disposal. A
more difficult problem is the reduction of the resulting sulfate and chloride concentrations.
Ion exchange and reverse osmosis are again the two most likely processes, but economics
would probably limit their application to only the most critical needs.
Excessive chlorides and sulfates in municipal waste usually occur as a result of ground-water
infiltration, use of salt water for flushing, or a substantial industrial waste discharge. At the
present time, there are only two methods available to a small wastewater treatment plant for
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reducing the chlorides and sulfates in its effluent: reverse osmosis and distillation; reverse
osmosis is the preferred method. Deionization, using ion exchange resins, is employed by
industry, but is considered impractical for municipal use because of the cost and disposal
problems involved with spent regenerants.
13.6 References
1. Process Design Manual for Upgrading Existing Wastewater Treatment Plants. U.S.
EPA, Office of Technology Transfer (October 1974).
2. Eutrophication, Causes, Consequences, Correctives. National Academy of Sciences,
Washington, D.C. (1969).
3. Process Design Manual for Nitrogen Control. U.S. EPA, Office of Technology
Transfer (October 1975).
4. Camp, T.R., Water and Its Impurities. New York: Reinhold Publishing (1963).
5. Bayley, R.W., Thomas, E.V., and Cooper, P.P., "Some Problems Associated With the
Treatment of Sewage by Non-Biological Processes." Proceedings of the Conference on
Applications of New Concepts of PC Wastewater Treatment, Pergamon Press (1972).
6. Adams, C.E., "Removing Nitrogen From Wastewater." Environmental Sciences and
Technology, p. 696 (August 7, 1973).
7. Folkman, Y., and Wachs, A.M., "Nitrogen Removal Through Ammonia Release From
Holding Ponds," Proceedings of the Sixth Annual Conference on Water Pollution
Research, Jerusalem, Pergamon Press (June 1972).
8. Pressley, T.A., Bishop, D.F., Pinto, A.P., and Cassell, A.F., Ammonia-Nitrogen
Removal By Breakpoint Chlorination. EPA Report No. EPA-670/2-73-058.
9. Bauer, R.C., and Snoeyink, V.L., "Reactions of Chloramines with Active Carbon,"
Journal Water Pollution Control Federation, 45, p. 2,290 (1973).
10. Ammonia Removal in a Physical-Chemical Wastewater Treatment Process. EPA
Report No. EPA-R2-72-123 (November 1972).
11. Nitrification and Denitrification Facilities. EPA Technology Transfer Seminar
Publication (August 1973).
12. Control of Nitrogen in Wastewater Effluents. Presented at EPA Technology Transfer
Design Seminar (March 1974).
13. Jeris, J.S., and Flood, F.J., "Plant Gets New Process." Water and Wastes Engineering,
p. 45 (March 11, 1974).
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14. Description of the El Lago, Texas, Advanced Wastewater Treatment Plant. Report of
Harris County Water Control and Improvement District, Seabrook, Texas (March
1974).
15. Manley, T.C., and Niegowski, S.J., Ozone. Encyclopedia of Chemical Technology, vol.
14, New York: John Wiley & Sons (1967).
13-6
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CHAPTER 14
SLUDGE AND PROCESS SIDESTREAM HANDLING
14.1 Background
14.1.1 Introduction
The U.S. EPA recently published Process Design Manuals on sludge treatment and disposal
(1) and upgrading wastewater treatment plants (2); a technical report on municipal waste-
water treatment-plant sludge and liquid sidestreams (3) is at press. Three U.S. EPA
publications on sludge processing and several WPCF Manuals of Practice are directed to the
design of all sizes of treatment facilities. This chapter presents those sludge processing design
factors which are applicable to small domestic wastewater treatment works or are not
covered in detail in references (1), (2), (3), and (4). Recently published books on sludge
include Treatment and Disposal of Wastewater Sludges (5).
General requirements for installation, replacement, repair, operation, and maintenance of
piping, fittings, equipment, and facilities for sludge handling are described in Section 2.14.
For small plants, which normally will use biological processes proceeding well into the
endogenous respiration stage, the volume of sludge produced will usually range from 0.3 to
0.7 percent of the volume of wastewater treated. Using extended aeration, instead of high
rate activated sludge, will reduce the dry solids to be processed in sludge handling facilities
by about 40 to 60 percent.
There are several processes for sludge treatment and disposal utilized at wastewater treat-
ment works larger than 1 mgd in size. These processes are usually not economical, or their
operation is not simple enough, for smaller works. They include filter presses, incineration,
heat conditioning and wet-air oxidation. Only when a plant approaches 1 mgd in size, or
when special conditions exist, are the anaerobic digestion, vacuum or belt filtration, and
centrifugation processes feasible.
Sludge treatment is usually minimized, if sufficient land is available, by using ponds in
which the sludge is treated and only removed every 3 to 10 years. The treatment problem is
also minimized if extended aeration or oxidation ditches are used, because the sludge solids
undergo endogenous respiration, reducing the weight of dry volatile solids to be treated.
Whenever sludge is handled, extra care must be taken to prevent nuisance level emissions of
odors. The allowable level, of course, is generally higher in rural areas than in urban areas.
The nuisance level odor that will cause the operator to avoid the sludge processing area
is relatively low for most operators. Therefore, to insure careful, regular inspection or
monitoring and maintenance of the sludge processing facilities, it is very important that
odor prevention and control be strongly considered in their design. Some types of sludge
processing, such as aerobic digestion (or lime treatment) and wet sludge land disposal, and
14-1
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sand bed dewatering and dry sludge landfill, are less prone to odorous conditions than other
alternatives at smaller plants. Prevention and control of odors are discussed in Section 2.6
and in reference (6).
14.1.2 Descriptions of Terms
Although many of the terms in this chapter are commonly used by experienced engineers,
in some publications the descriptions of certain processes are not consistent with general
usage. A few important descriptions of terms from reference (3) are presented below. A
more complete compilation of definitions can be found in the joint APHA, ASCE, AWWA,
and WPCF Glossary— Water and Wastewater Control Engineering (7).
Centra te
Centrate is the liquid extracted from a sludge in a centrifuge, used either for thickening or
dewatering. Its composition depends on the physical and/or chemical treatment of the
sludge, the centrifugal force used in the unit, and the design of the centrifuge.
Conditioning
To release liquid from sludges that are flocculent or of the hydroxide type, it is usually
necessary to treat them with various chemicals, to subject them to some drastic physical
conditions (such as heat or cold), or to process them biologically. These processes are
referred to as "conditioning," and may be necessary to accomplish the desired thickening or
dewatering.
Decantate
Various sludges are thickened in gravity-type thickeners before processing. The overflow
from such units is sometimes referred to as "decant liquid" or "decantate".
Dewatering
Dewatering is the removal of a sufficient amount of additional liquid, so that the thickened
sludge attains properties of a solid; i.e., it can be shoveled, conveyed on a sloping belt, and
handled by typical solids handling methods. Although there are some methods for final
disposal of thickened sludges, many final disposal methods require that the thickened sludge
be further dewatered. Dewatered sludge is usally in the form of a "cake," such as that
produced by a centrifuge, sand drying bed, or vacuum filter.
Filter Backwash
Filter backwash is the water resulting from backwashing and removing solids retained by a
granular media filter. Various types of granular media filters are used to remove physically
most of the suspended solds (SS) from settled effluents. These filters are periodically
backwashed to remove the accumulated solids. Therefore, the backwash water contains SS
at concentrations that may vary from a few hundred to several thousand milligrams per liter
(mg/1).
Filtrates
If sludges are dewatered on vacuum filters, filter presses, or other devices in which the liquid
is separated from the solids by a differential force across a porous fabric or screen, the
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extracted liquid is referred to as "filtrate." Its characteristics depend on the sludge
conditioning methods used, the chemicals applied, the character of the filtering media, and
the type of sludge being processed.
Final Disposal
Sludge, either thickened, dewatered, biologically or chemically altered, or reduced to ash by
incineration, must be returned to the environment. This final disposal must have minimal
detrimental effect, if any, on the environment. Final disposal will usually utilize the land or,
in some cases, the air or the ocean. Also, in some instances, the end product, resulting from
any of several treatment steps (described later in this report), can be reused by recycling
back to some other treatment process, where it will produce no adverse effects and can
possibly have some economic value.
Liquid Skimmings
Much of the material skimmed from primary clarifiers and final clarifiers is liquid, such as
oil or water, with floating grease and other debris. It must be disposed of properly and with
a minimal amount of nuisance and odors. Typically, it is handled with the waste sludge from
the primary clarifier.
Process Sidestream or Recycle
Processes used to prepare sludge for final disposal generally result in the formation of a
liquid requiring further treatment. Such liquids include supernatant, decantate, centrate,
and filtrate. The solids in these sidestreams sometimes can be recycled directly back to the
main process line of flow. Often, however, these liquids are very concentrated and will upset
normal processes if they are not first given special treatment before being recycled. Some
wastewater treatment processes also produce polluted sidestreams, such as filter backwash
and screen wash water, which may be recycled without further processing.
Sand Bed Drainage
Sand bed drainage is the liquid that drains out of sludge applied to sand beds for dewatering.
These beds usually have underdrain systems that carry the liquid to a point where it can be
properly handled for disposal. This liquid may have a high concentration of soluble organic
matter, nitrogen, phosphorus, and heavy metals.
Screen Wash Water
A variety of screens can be used to remove either small or large SS. Most of these screens are
cleaned by physical or hydraulic means. The wash waters from hydraulic cleaning may
contain fairly high concentrations of SS. These wash waters are usually returned to the main
treatment plant flow, upstream of the screen.
Septage
Sludge from household septic tanks, which has been partially stabilized anaerobically, is
commonly called "septage." Septage is usually delivered to wastewater treatment plants in
1,000-gallon loads in frequencies that may vary seasonally. Normally, septage is added to
the raw wastewater in batches, or in a continuous manner if a holding tank is available. At a
few locations, septage has been added directly to the sludge processing stream, ahead of the
conditioning step.
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Sludge Treatment
Sludge treatment prepares the sludge for final disposal, or return to the environment, with
minimal detrimental effect.
Supernatant
Supernatant is the liquid decanted from an anaerobic or aerobic digester. In domestic waste-
water treatment works, this liquid may have a high concentration of suspended and
dissolved organic matter; inorganics such as ammonium compounds, phosphates, and heavy
metals; and various pathogens.
Thickening
Many sludges produced by treatment processes are extremely diluted. The process of
thickening is employed as a final step in the sludge processing sequence, to reduce the
volume of sludge and the size and cost of the subsequent sludge processing equipment.
14.1.3 Quantities and Characteristics
Two principal characteristics that must be known about any raw sludge are the sludge
volume and the solids concentration. With this information and knowledge of the physical
and chemical characteristics of the solids, a decision can be made as to what type of
treatment processing is required before disposal. Average quantities of sludges produced by
various treatment methods are listed in Table 14-1. Characteristics of various sidestreams
resulting from the treatment of sludge are listed in Table 14-2 (3).
The values given in Table 14-1 are for a domestic wastewater having an average BOD5 and
SS of about 200 mg/1 and 250 mg/1, respectively. The volume of sludge from any treat-
ment process is, of course, related to the solids concentration. The volume of the wet solids
can be significantly reduced by various sludge treatment methods, including thickening and
dewatering.
14.2 Thickening
Sludge solids are either originally suspended in wastewater or are generated by chemical
precipitation or by growth of biological organisms. Removal of such solids, at least in regard
to domestic wastewaters, is normally accomplished in relatively quiescent basins, which
allow the solids to settle out by gravity. Simultaneously with clarification, some thickening
of settled sludge frequently takes place.
Before further processing, frequently more thickening is required than can be attained by
gravity settling (8). This is especially true of the hydroxide-type sludges, generated by some
chemical coagulants, and of waste activated sludge. The settled solids thicken with time
and/or by the aid of some slow-stirring mechanical devices, such as pickets or scraper arms.
The latter devices mechanically break up the agglomerated solid particles and release the
liquid entrained or enmeshed in them. This process is referred to as gravity thickening.
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TABLE 14-1
AVERAGE QUANTITIES OF SLUDGE
SLUDGE VOLUME DRY SOLIDS IN SLUDGE
TREATMENT PROCESS
Primary Sedimentation
Gal/Million
Gal of
Wastewater
Raw 2,000-3,000
Separate Anaerobic Digestion 1,000-1,500
Digested and Dewatered (Sand Beds) 300-450
Digested and Dewatered (Vacuum Filters) 225-350
Trickling Filter1
Low Loading
High Loading
Activated Sludge
High Rate
Normal Loading
Extended Aeration
Extended Aeration (Vacuum Filters)
Primary Sedimentation and Activated
Sludge Mixed
Raw
Raw (Vacuum Filter)
Separate Anaerobic Digestion
Digested (Sand Beds)
Digested (Vacuum Filters)
1,000-3,000
2,000-3,000
14,000-19,000
8,500-13,000
3,300-7,000
300-700
7,000
1,400
2,700
1,350
900
Lb/Million
Gal of
Wastewater Percent
1,000
750
750
750
450
750
900-1,900
700-1,600
400-1,200
2,250
4-6
6
40
27.5
4-6
2-5
0.8-1.2
1.0-1.5
1.5-2.0
20
2,300
2,300
1,400
1,400
1,400
1.5-3.0
20
6
40
20
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TABLE 14-1 (continued)
AVERAGE QUANTITIES OF SLUDGE
SLUDGE VOLUME DRY SOLIDS IN SLUDGE
TREATMENT PROCESS
Chemical Precipitation of Phosphorus2
With Lime
With Alum
With Iron Salt
Gal/Million
Gal of
Wastewater
6,000-14,000
10,000-30,000
10,000-25,000
Lb/Million
Gal of
Wastewater Percent
3,000-10,000 6-10
600-2,500 0.5-1.5
800-3,000 1.0-2.5
1 Lower figures apply if primary settling is provided, and the higher figures are for plants not
using primary settling.
2 Assuming soluble phosphorus present, as P, is 10 mg/1 and is reduced to 1 mg/1; alkalinity
is250mg/l.
NOTE: 1 lb/106 gal = 0.12 mg/1
1 gal/106 gal= 1 ml/m3
14-6
-------
TABLE 14-2
CHARACTERISTICS OF VARIOUS SIDESTREAMS
PRIMARY AND ACTIVATED SLUDGE TREATMENT PLANTS (3)
Type of Sidestream
Aerobic Digester Supernatant
Aerobic Digester Supernatant
Sludge Thickener Overflow
(Gravity)
Sludge Thickener Subnatant
(Flotation)
Raw Sludge Centrate or Filtrate
(Chemical Conditioning)*
Centrate or Filtrate
(Lime + Fe Conditioning)*
Centrate or Filtrate
(Lime + Fe Conditioning) *
BOD5
mg/1
,000-
8,000
50-
500
50-
1,000
25-
500
500-
1,000
COD
mg/1
3,000-
15,000
200-
2,000
100-
2,000
50-
200
1,000-
2,000
SS
mg/1
3,000-
10,000
200-
3,000
100-
1,000
25-
500
500-
1,000
NH3
mg/1
400-
1,000
Very
low
25-
100
Low
25-
100
(Soluble)
mg/1
300-
700
25-
200
20-
50
Low
20-
50
Average Vol.
% Treated
Wastewater
0.50-1.0
0.50-1.0
2-3
2-3
0.50-0.75
Remarks
Odorous. SS not too ;
High in nitrates.
May be septic. Solids
settleable.
Only waste activated
handled. Polymer usei
Includes primary and
activated.
500- 1,000- 500- 400- Low
5,000 10,000 5,000 1,000
50- 100- 100-
200 1,000 1,000
0.50—0.75 Anaerobic digested sludge.
0.50—0.75 Aerobic digested sludge.
-------
TABLE 14-2 (continued)
CHARACTERISTICS OF VARIOUS SIDESTREAMS
PRIMARY AND ACTIVATED SLUDGE TREATMENT PLANTS (3)
Type of Sidestream
do
Centrate or Filtrate
(Polymer Conditioning)^
Centrate or Filtrate
(Polymer Conditioning) 1
Centrate or Filtrate
(Heat Conditioning, Including
Decant Liquor) 1
Filter Backwash
BOD5
mg/1
500-
5,000
50-
200
3,000-
10,000
500-
3,000
500-
1,000
COD
mg/1
1,000-
10,000
100-
1,000
10,000-
25,000
1,000-
7,000
1,000-
3,000
SS
mg/1
500-
5,000
100-
1,000
1 ,000-
3,000
1 ,000-
7,000
300-
1,500
NH3 (Soluble)
mg/1
400-
1,000
500-
700
250-
400
25-
35
mg/1
300-
700
25-
200
150-
200
150-
300
10-
15
Average Vol.
% Treated
Wastewater
0.50-0.7:
0.50-0.7:
1.0-1.5
1.5-2.5
2.0-5.0
Remarks
0.50—0.75 Anaerobic digested sludge
1.0—1.5 Undigested primary and waste
activated sludge.
1.5-2.5 SS are colloidal. Not settleable.
2.0-5.0 Secondary effluent polishing.
Mype of sludge handled indicated under Remarks.
-------
The technical and economic significance of sludge thickening is not always apparent or
appreciated. However, the volume depends on the concentration of solids in the liquid. For
example, a sludge with 98 percent moisture has twice the volume of the same sludge
thickened to 96 percent moisture and, thus, would require twice the capacity in the next
treatment processing unit unless some of the liquid is removed.
14.2.1 Gravity Thickening
Gravity thickening (although the most commonly used method at small plants and the least
expensive thickening method) is often troubled with odor problems, particularly at
temperatures above 77° F (25° C) or overflow rates less than 40 Ib/ft2/day (195 kg/m2 -d),
unless odor prevention and control are designed into the facility. Gravity thickening is
sometimes inefficient for increasing the solids concentration of excess activated sludge (1).
The design basis for sludge thickeners is usually the solids loading, expressed in pounds of
solids per square foot per day. The loadings shown in Table 14-3 can be used for sizing
gravity thickeners at small plants (2) (3) (9).
TABLE 14-3
GRAVITY THICKENER LOADINGS
Type of Sludge
Primary
Trickling Filter
Waste Activated
Primary Plus Activated
Primary With Alum
Primary With Iron
Primary With Lime
1lb/ft2/day = 4.9kg/m2-d
Influent Sludge Thickened Sludge
Loading
percent solids
4-6
2-4
0.8-2.0
1.5-3
0.5-1.0
2-4
6-10
percent solids Ib/ft2/day1
7-10
5-7
2-3.5
4-6
1.5-2.0
6-8
10-18
20-30
10-12
4-6
10-12
5-9
12-18
20-40
Gravity thickening can be separated into four activities, which take place in, roughly, four
zones. These activities are clarification, hindered but constant settling rate, transition with
14-9
-------
decreasing settling rate, and compression. They are described in Section 6.4.1 and references
(2) and (10) and are shown on Figure 6-3.
Although the thickener is sized for the average design loading, it must be capable of
adequately thickening the peak loading, taking into consideration the sludge storage
capabilities in other parts of the solids handling system. Normally, sufficient total storage
should be provided for the maximum, 3-day plant solids loading (2).
The details of designing gravity thickeners are contained in references (1), (2), (3), (4), and
(10).
Thickening of flocculated sludges, such as activated and chemical sludges, is aided
considerably by the installation of pickets on the scraper arms, as shown in Figure 14-1.
These pickets may consist of angle-shaped, structural steel vertical arms, installed so that
the apex of the angle is in the direction of motion. Generally, gravity thickening of all types
of sludges is aided by use of such pickets, because they prevent bridging of floe particles.
Also, if gas formation begins because of septic conditions, they aid in releasing gas bubbles
from the sludge layer.
14.2.2 Flotation
The flotation method (see Figure 14-2) used for thickening sludge is known as dissolved-air
flotation. In dissolved-air flotation, the influent wastes are mixed with a portion of recycled
final effluent, which has been pressurized to 40 to 60 psi (276 to 414 kPa). The pressure is
released at the mixing point, immediately followed by the discharge of combined sludge and
pressurized effluent into the thickener. The air comes out of solution in the form of
bubbles. These bubbles attach themselves to the SS of the influent sludge and float them to
the surface, where they are removed in the form of thick scum (11). This method of
thickening is rarely used at small plants, because continuous operator attention is generally
required.
Detailed information on air flotation can be found in references (1), (2), (3), (4), and (10).
14.2.3 Centrifugation
Two types of centrifuges are used for sludge thickening. The disk nozzle centrifuge is used
for waste activated sludge, if no primary solids are mixed in (i.e., if the activated sludge
process is preceded by primary clarification), producing a thickened sludge composed of
about 4 to 7 percent solids. The basket type is used for thickening both waste activated
sludge and mixtures of activated and primary sludge, producing a thickened sludge
composed of about 9 to 10 percent solids. The basket-type centrifuge is primarily a batch
unit, compared with the disk-nozzle type, which runs on a continuous flow basis. However,
the basket-centrifuge can be highly automated, so that manual attention is not required
between batches. More information on centrifugation is presented in references (1), (2), (3),
(4), and (10).
14-10
-------
EFFLUENT WEIR
EFFLUENT
RAISED POSITJON
OF TRUSS ARM
SCRAPER BLADES
UNDERFLOW
ELEVATION
FIGURE 14-1
GRAVITY THICKENER (8)
-------
SKIMMER MECHANISM
EFFLUENT
to
RECYCLED
EFFLUENT -
fd
BOTTOM COLLECTOR
I
z
5)1
PRESSURE TANK
AIR
RECYCLED
EFFLUENT
JO DEWATERING
UNIT
INFLUENT SLUDGE
FIGURE 14-2
SCHEMATIC OF AN AIR FLOTATION THICKENER
-------
14.3 Stabilization
Stabilization is designed to make the organic or volatile portion of sludge less putrescible
and the treated sludge less odorous for final disposal. The stabilization processes best suited
for use at small plants are aerobic digestion, lime treatment, and anaerobic digestion. These
three processes are described briefly below and in much more detail in references (1), (2),
(3), (4), (10), (12), and (13).
14.3.1 Aerobic Digestion
Aerobic digestion is frequently used at small treatment plants, particularly package-type
activated sludge plants. The process consists of aerating sludge in uncovered and unheated
tanks having a depth of 10 to 20 ft (3 to 6 m). The principal operating cost is in the power
required to supply the necessary oxygen and to keep the basin contents properly mixed.
If the dissolved oxygen content is maintained above 1 mg/1, the process produces no
significant odors, and there are no hazardous gases generated. A schematic diagram of the
aerobic digestion process is shown in Figure 14-3, and in Figure 14^4, a typical digester.
The advantages and disadvantages of using aerobic digestion in lieu of other stabilization
processes (1) are:
Advantages:
1. Relatively simple to operate.
2. Requires a small capital expenditure compared with anaerobic digestion.
3. Does not generate significant odors.
4. Reduces the number of pathogenic organisms to a low level.
5. Reduces the quantity of grease or hexane solubles.
6. Produces a supernatant that, if clarified, is low in BOD, solids, and total
phosphorus (P).
7. Reduces the sludge respiration rate.
Disadvantages:
1. Relatively high operating cost.
2. Relatively large energy user.
3. Uncertain design criteria.
If excess activated sludge is maintained at 68° F (20° C) or above, at a detention time of 10
to 12 days and with a DO concentration of 1 to 2 mg/1, the reduction of volatile solids will
be about 30 to 50 percent. (Add about 5 days detention time if the sludge is primary or
mixed activated and primary.) The amount of volatile solids reduced is not as large as with
anaerobic digestion; however, the sludge is sufficiently stable for relatively odorless dewater-
ing on sand beds and for disposal on land or in a landfill. If the sludge temperature drops to
50° F (10° C), the retention time should be increased to 20 days. If it goes down to 41° F
(5° C), the time should be 30 days, if at least 30 percent reduction of the volatile solids is
sought.
14-13
-------
PRIMARY SLUDGE
EXCESS ACTIVATED
OH TRICKLING
FILTER SLUDGE
CLEAR OXIDIZED
OVERFLOW TO
PLANT
SETTLED SLUDGE RETURNED JO AERODIGESTER
FIGURE 14-3
SCHEMATIC OF AEROBIC DIGESTER SYSTEM (1)
-------
AIR PLUG VALVE
SUPERNATANT
DRAW-OFF
WASTE
SLUDGE
DRAW-OFF
4 RETURN
SLUDGE AIR-
LIFT PUMP
FIGURE 14-4
TYPICAL CIRCULAR AEROBIC DIGESTER (1)
14-15
-------
Sludge sent to an aerobic digester with concentrations higher than 3 percent is difficult to
mix and to obtain proper oxygenation in all portions of the basin, especially with
compressed air and diffusers. Surface-type mechanical aerators can be used if the basin
depth is about 10 to 12 ft (3 to 3.6 m); a draft tube can be used for deeper basins. At least 4
ft (1 .2 m) of freeboard above the maximum operating depth is necessary to contain possible
foaming (4). These types of aerators are not recommended for aerobic digesters located in
freezing climates, because they dissipate an excessive amount of heat from the basin
contents. A better aeration and mixing system for either warm or cold climates is a sub-
merged turbine, located about 2 ft (0.6 m) above the basin bottom, with an air sparger
below it, as shown in Figure 9-10. The turbine produces excellent mixing of the digester
contents and disperses the air so that all portions of the basin have sufficient DO.
Aeration equipment is usually sized to obtain sufficient mixing in the digester. If mechanical
aeration equipment is used, there should be 1.0 to 1.5/hp/ 1,000 ft3 (21 to 32 kW/m3) of
basin volume. If diffused air is used, there should be at least 25 to 35 scfm of air supplied
per 1,000 ft3 (20 to 28 std m3 of air/m3/min) of basin volume. The higher values are for
sludges having a combination of primary basin solids and waste activated sludge.
The oxygen requirements for stabilization depend on the loading rate of volatile solids. This
volatile solids loading rate will vary between 0.05 and 0.15 Ib VSS/ft3/day (0.8 to 2.4
kg/m3-d) at 68° F (20° C) sludge temperature, with lower loading rates for lower
temperatures. Assuming that up to 50 percent of the solids will be oxidized, and that the
nitrogen in the organic matter will be oxidized to nitrates, the oxygen requirements will be
approximately 2 times the rate of activated sludge volatile solids destroyed, and 1.6 to 1.9
times the BODs of primary sludge (10).
The pH in aerobic digesters tends to drop, depending on the alkalinity available in the
liquid. This drop in pH is primarily caused by the loss of alkalinity, caused by nitrification.
There is a loss of about 7.2 mg/1 of alkalinity (expressed as calcium carbonate) for each mg/1
of ammonia-nitrogen oxidized to nitrate. If the pH drops below about 6.0, digestion may be
retarded, and the addition of lime or other alkali may be required to raise the pH to
between 7.0 and 7.5. However, extended aeration or oxidation ditch sludge, which is well
nitrified , will have little effect on either pH or oxygen requirements.
An aerobic digester system can operate on a batch or a continuous flow basis. If digested
sludge is allowed to settle, the supernatant can be recycled to the treatment plant. In a
batch system, the aeration can be shut off for 2 to 5 hours without problems and the solids
allowed to settle. The supernatant should be decanted from several feet below the surface,
before more raw sludge is added and aeration started up again.
If activated sludge alone is being digested, the solids will thicken by gravity to 2 to 3 percent
by weight (if mixed with primary solids, they should thicken to a higher percentage solids).
In a continuous system, the digester should be followed by a settler/holder unit, whose
hydraulic overflow rate should be less than about 100 to 150 gpd/ft2 (4 to 6 m3/m2>d)
(14). Some means of aeration and mixing may be needed, if the sludge is to be held
overnight for further processing.
14-16
-------
The supernatant from average aerobic digesters would have the characteristics shown in
Table 144 if the digestion were continued until the volatile solids were reduced by at least
40 percent and quiescent settling conditions were maintained for several hours before the
supernatant was drawn off. Supernatant from properly operating aerobic digesters at
municipal treatment plants can ordinarily be returned to the aeration chamber without
causing difficulties.
TABLE 144
CHARACTERISTICS OF SUPERNATANT FROM AEROBIC DIGESTERS
pH 6.5-7.51
BOD,mg/l 100-500
COD, mg/1 200-1,500
SS,mg/l 100-500
Nitrogen
Ammonia, mg/1 0-10
Nitrate, mg/1 200-500
Total phosphorus as P, mg/1 50-200
1 For air aeration, if wastewater alkalinity is above 250 mg/1.
The volume of supernatant plus sludge liquor after dewatering will average about 1 to 2
percent of the wastewater flow. Detailed information on the design of aerobic digestion
systems is contained in references (1), (2), (3), (4), and (10).
14.3.2 Lime Treatment
Land disposal of raw sludge that has not been stabilized is objectionable, because the sludge
contains a large quantity of pathogenic mircroorganisms. Adequate anaerobic or aerobic
digestion reduces the number of these organisms significantly. Raw sludge also decomposes
rapidly, because of the presence of readily degradable volatile solids, with resultant
production of odors and other nuisance conditions. Adding lime to raw or digested sludges
and raising the pH to between 11.0 to 11.5 reduces the number of pathogenic organisms
(1) (15) (16). The addition of lime, with the resultant rise in pH, also suppresses rapid
decomposition of the highly volatile solids, preventing odorous conditions if the sludge is
disposed of in sufficiently thin layers on the land or in a sanitary landfill. Lime-stabilized
raw sludge dewaters well on sand beds without odor problems (1). Because lime treatment
does not destroy organic material, the pH eventually falls and bacterial action slowly
develops. However, if the pH is raised to between 12.2 to 12.4 and then kept above 11 for
14 days, the sludge will be stabilized (1).
A U.S. EPA sponsored study (17) indicated that the lime dosage required to keep the pH
above 11 for at least 14 days (in pounds of Ca(OH)2 per ton of dry sludge solids) is between
14-17
-------
200 to 600 for septage and 600 to 1,000 for biological sludge. Some testing should be
performed to establish the dosage required. Reduced lime dosages are possible, if the lime-
stabilized sludge is placed on the land or drying beds in layers thin enough to allow drying
under the prevailing climatic conditions, before bacterial action can be regenerated.
14.3.3 Anaerobic Digestion
In anaerobic digestion, the more readily biodegradable solids are converted to gases such as
methane (CH^), carbon dioxide (CC^), and ammonia (NH3). The latter remains in solution
as the ammonium ion at normal pH. The volatile solids in the digested sludge are generally
reduced by 40 to 60 percent. With the incoming solids averaging 80 percent volatile matter,
the result will be a total solids reduction of 32 to 48 percent. The remaining solids are easier
to dewater and do not undergo rapid putrefaction if disposed of in sanitary landfills.
There are two phases in the anaerobic digestion process. In the first step, a wide variety of
organisms called acid formers breaks down complex organics to volatile organic acids. In the
second step, a special group of bacteria known as methane formers breaks down the organic
acids into CH^ and CC^. These bacteria are strict anaerobes and are inhibited by the
presence of any DO. The reproduction rate of methane forming organisms is very low,
compared with other bacteria. For instance, their doubling time is about 4 days, while for
the acid formers it is only several hours. Thus, anaerobic digestion is controlled and the rate
is limited by the methane formers. These organisms are also very sensitive to pH (optimum
is near 7.0) and to a variety of substances, such as heavy metals, chlorinated hydrocarbons,
and soluble sulfides.
Stabilization of the solids and significant reduction in BOD does not occur until the
methane organisms become active. It may take several weeks after a digester is started up
and optimum conditions become established for this to occur. During this period, the rapid
production of organic acids may cause the pH to drop and thereby inhibit the development
of methane bacteria. If this process occurs, it may be necessary to add lime to adjust the pH.
After equilibrium conditions between the acid formers and the methane bacteria become
established, the pH is maintained near neutral. Alkalinity is produced, because the NH3
reacts with the CO2 and forms ammonium carbonate. Good production of CH4 gas means
that a digester is working properly.
There are two general types of anaerobic digestion systems in use: the older conventional
system (without mixing), known as the low-rate system; and the completely mixed,
high-rate system. In the low-rate system, sludge is added near the top and withdrawn from
the bottom. Stratification develops with a scum layer on top; a supernatant with relatively
low SS next; and active digesting layer; and, finally, a layer of settled and stabilized sludge
on the bottom. Generally, only about one-half of the digester volume can be considered
active. The low-rate system is no longer recommended for sludge processing.
In the currently preferred high-rate system, the entire contents of the digester are mixed.
Thus, the entire volume is actively digesting and conditions are essentially uniform through-
14-18
-------
out the entire volume. There are two types of mixing systems commonly employed. In one
type, a turbine is located in the upper part of the digester, with the turbine drive located on
the roof of the tank. The turbine may be unconfined or in a draft tube (Figure 14-5). The
mixing can also be accomplished by compressing the gas generated during digestion and
diffusing it near the digester bottom, to secure a gas-lift pumping action (Figure 14-6).
The temperature in digesters should be between 85° F (29° C) and 95° F (32° C). They are
heated by circulating sludge through an external heat exchanger, using water heated in a
boiler burning the CIfy gas generated by the digestion process. The boilers are equipped
with burners, which can use either the CIfy gas or, as a standby, natural gas or oil if digester
gas is not available. It is difficult to keep a uniform temperature in a small digester in the
winter, because lower temperatures tend to prevent reliable, continuous CH4 production.
The design basis for mixed (high-rate) digesters includes two criteria. One is based on
retention time and the other on solids loading. Theoretically, the retention time criterion
should be solids retention time; however, it has been customary to use hydraulic retention
time. For completely mixed digesters, the solids retention time is equal to the hydraulic
retention time unless the outflow solids are thickened and recycled, which is not usual
practice. The detention time necessary for high rate mixed units should be about 10 to 20
days. The higher figure is preferable because better separation of the solids from the liquid
in the digester outflow is obtained. Digesters having very short retention times (about 10
days) usually are plagued with supernatants having high SS concentration and sludge that
thickens poorly.
The solids loading criterion is 0.15 to 0.40 Ib VSS/ft3/day (2.4 to 6.4 kg/m3 -d) for mixed
(high-rate) units.
With completely mixed digestion, it is usual (and frequently required by regulatory
agencies) to use two tanks in series. The first is referred to as the primary digester, the
second, of equal size, as the secondary digester. This latter tank permits solids from the
primary unit to separate and settle under quiescent conditions. Provision for gas storage is
made in both tanks and, normally, the primary unit has a floating cover. The liquid
supernatant is drawn off the second tank for treatment or recycling to the plant.
Frequently, the second tank is equipped with mixing equipment and can be heated,
similarly to the primary unit, so it can replace the primary digester when that unit is taken
down for maintenance. It can also be equipped to act as a thickener.
The gas production from digesters is estimated from the volatile solids loading. The value
used is 8 to 12 ft3/lb (0.5 to 0.75 m3/kg) of volatile solids added. Digester gas is about 65
percent CH^ and has about 650 Btu/ft3 (19.5 kJ/m3). In comparison, natural gas, which is
nearly 100 percent CH4, has about 1,000 Btu/ft3 (30 kJ/m3).
Anaerobic digester supernatant has a high concentration of pollutants and must be
separately treated or recycled to the treatment plant. The method of handling should
minimize any degradation of the final plant effluent quality. The usual range of
concentrations of various pollutants for digesters processing mixtures of primary and
14-19
-------
DRIVE
"UNIT
to
o
SLUDGE WITHDRAWAL LINE
'i
4
i
\
'*
5
«;
[f
f
*ft
•f.
t
f •
^
^
^
LIQUID LEVEL
FIGURE 14-5
DIGESTER WITH MECHANICAL MIXER
-------
A
«':
«
'f
«
n
GAS COLLECTION
LINE
PIPE
SUPPORT-
BOTTOM SLUDGE
TRANSFER LINE
COMPRESSED
GAS LINE
SLUDGE
TRANSFER LINE-
RAW SLUDGE a
HEATED SLUDGE INLET ^^
RECIRCULATION LINE
TO HEATER
TO GAS
COMPRESSOR
FIGURE 14-6
DIGESTER WITH GAS MIXING
-------
secondary sludges is given in Table 14-5. The lower figures in the ranges can be expected
with two-stage digestion at well-operated, smaller plants.
TABLE 14-5
CHARACTERISTICS OF ANAEROBIC DIGESTER SUPERNATANTS
Primary and Primary and
Trickling Activated
Filter Plants Sludge Plants
pH 6.9-7.1 6.9-7.1
BOD, mg/1 1,000-5,000 2,000-7,000
COD, mg/1 2,000-10,000 4,000-12,000
SS,mg/l 1,000-5,000 3,000-10,000
Ammonia-Nitrogen, mg/1 400-600 400-1,000
Total Phosphorus as P, mg/1 100-300 300-700
The volume of supernatant and liquor from sludge dewatering, such as the filtrate or cen-
trate, with the above indicated concentration of pollutants, will average about 1.0 to 1.25
percent of the volume of wastewater treated. The supernatant characteristics shown in Table
14-5 are for domestic wastewater.
14.4 Dewatering
Dewatering is the removal of a large portion of the entrained liquid in a sludge to facilitate
its final disposal. Further information on this and other aspects of sludge processing can be
found in reference (13).
14.4.1 Sludge Conditioning
For most dewatering operations, a sludge must be conditioned first. In general, condition-
ing encompasses those processes involving biological, chemical, or physical treatment (or the
combination of these) to make the separation of water from sludge easier. According to
this definition, anaerobic and aerobic digestion are conditioning processes, in addition to
being stabilization processes. However, digestion does not provide sufficient conditioning
to dewater the sludge with mechanical equipment, although such sludges do dewater on
sand beds. Small wastewater treatment plants generally employ stabilization and sand bed
dewatering. Therefore, sludge conditioning is not treated comprehensively in this text.
Raw and digester sludges can be conditioned chemically to facilitate dewatering. Drying
time (and bed area) may be reduced by up to 50 percent (9). Such chemical treatment
breaks down the colloidal-gelatinous nature of wastewater sludge, so that the water can be
separated more readily. The inorganic chemicals most commonly used are ferric chloride or
14-22
-------
alum, combined with lime, with ferric chloride generally preferred. Ferric chloride or alum,
without lime, does not prevent development of odors. A dosage of 1 Ib of commercial alum per
100 gallons (1.2 kg/m3) of digested sludge has been used successfully (11). With raw sludge,
lime should always be used if the sludge is to be disposed of on land or in a landfill, so that the
pH can be raised to at least 11. This addition of lime will accomplish some disinfection, delay
decomposition, and suppress odors. The usual amount of ferric chloride is about 3 percent of
the weight of dry solids being dewatered, and the lime dosage is about 10 percent of the dry
solids. These dosages will, of course, vary with the types and character of sludge. Laboratory
tests, such as the leaf test for vacuum filters, can be used to establish proper chemical dosages.
Some binding of the beds may result from excessive use of inorganic chemicals (9). If chemicals
are used, they should be mixed with the sludge immediately prior to application on the beds.
Polymers can be used for conditioning, and they have the advantage of simplifying chemical
handling systems. However, they are expensive and their use will not suppress odors or
influence the decomposition rate if sludge is to be applied on land or in landfill. Therefore,
they are not suitable for use with raw sludges unless the sludge is to be incinerated; in which
case, polymers will not add significant quantities of solids to the sludge (in contrast to lime
and ferric chloride conditioning).
Heat treatment of sludge accomplishes conditioning and disinfection (10). This treatment is
carried on in specially designed reactors, in which the temperature of the sludge is raised to
about 350° to 400° F (175° to 205° C) and is held for about 30 minutes at a pressure of
about 250 psi. Because of the complexity of the equipment and its high initial cost, this
process is seldom feasible for small treatment plants. More information on the use of heat to
condition sludge is contained in references (1), (3), and (4).
14.4.2 Sand Beds
At small treatment plants, digested or conditioned raw sludges are usually dewatered on
sand beds. Dewatering occurs by drainage and evaporation. The moisture content is usually
reduced to between about 40 and 75 percent on the beds (9). Normally, the drainage occurs
mostly in the first 2 days, after the sludge is pumped out on the bed. The drainage liquid is
collected in an underdrain system and returned to the treatment plant. The drainage from
sand beds, although it requires further treatment, usually will not require pretreatment
before being returned to the plant influent structure. If the sludge has been digested
aerobically, it may have high concentrations of nitrates or phosphates. If the removal of the
nitrogen or phosphorus from the plant effluent is a discharge permit requirement, the effect
of recycling the sand bed drainage should be carefully considered. After the first 2 days,
most of the dewatering takes place by evaporation, causing shrinkage horizontally and
opening vertical cracks, enhancing the evaporation and allowing some additional drainage.
For details of sand bed design, references (1), (3), and (4) and especially (9) should be
consulted. The nature of the sludge and climatic conditions determine the required area of
sludge drying beds. Lime stabilization and sand bed dewatering, for handling peak sludge
loads during emergency periods, should be considered in the design studies of new or up-
graded facilities.
14-23
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The sand bed areas required for drying domestic wastewater sludges are listed in Table 14-6.
TABLE 14-6
CRITERIA FOR THE DESIGN OF SANDBEDS (1) (3) (9)
Type of Sludge Area1 Sludge Loading Dry Solids
ft2/capita Ib/ft2/yr
Primary 1.0-1.5 27.5
Primary and Standard Trickling Filter 1.3-1.7 22.0
Primary and Activated 2.0-3.0 15.0
Primary and Chemically Precipitated 1.8-2.2 22.0
Digested Raw (Lime Treated) Sludge 2.0-2.5 20.0
!The actual area required should be based on laboratory tests of typical sludge, taking
expected climatic conditions into account.
If the sand beds are covered, the area can be reduced 25 to 35 percent; however, covering is
excessively costly (1). In the southern U.S., smaller areas can be used. Dewatering raw
sludge on sand beds is not recommended unless the sludge has been conditioned with lime at
a pH above 11, because of poor drying conditions.
The beds are constructed by laying drainage tiles about 8 to 10 ft (2.4 to 3.0 m) apart on an
impervious layer of soil or on an artifical material, such as asphalt. A layer of graded gravel
is then placed to a depth of 6 to 12 in. (0.15 to 0.30 m) above the top of the tiles, with the
top 3 in. consisting of 1/8- to 1/4-in. (3 to 6 mm) gravel. On top of the gravel is a 10- to
18-in. (0.25 to 0.45 m) layer of sand, having an effective size between 0.30 and 1.20 mm
and a uniformity coefficient of less than 5.0.
Beds are usually 15 to 25 ft (4.5 to 7.5 m) wide and sufficiently long to provide the area
needed for a digester drawdown or other single bed loading. Usually, one sludge discharge
pipe per sand bed is sufficient. If the sludge is thick, multiple discharge pipes should be
provided, especially on large sand beds, so that the sludge can be distributed properly. Each
discharge pipe should terminate at least 12 in. (0.3 m) above the bed surface. Splash plates
should be provided at the pipe ends to promote even distribution and prevent sand
disruption. A dosing depth of 8 to 12 in. (0.2 to 0.3 m) of sludge is usually employed, with
the optimum depth varying at each location, depending on prevailing weather conditions.
There should be at least three beds, to provide flexibility in operation. The number of bed
applications of sludge per year will, of course, depend on climatic conditions and the sand
bed area. Usually, 6 to 10 applications per year are possible, depending on the dryness of
dewatered sludge cake required for the selected type of disposal, the length of the drying
14-24
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season, and the humidity patterns. The dried sludge cake is usually removed manually in
smaller plants. The sludge can be transferred to trucks by building a pair of concrete runway
strips along the center of the beds.
After the dried sludge is removed by hand (using forks) or by machine, the drying beds
require maintenance. Small sludge particles and weeds should be removed from the sand
surface. Periodically, the bed should be disked and the top layer of sand replaced. Usually,
resanding is advisable when 50 percent of the original sand depth is lost, or when the sand
depth is down to 8 in. (0.2 m). Resurfacing sludge beds is perhaps the major expense in
sludge bed maintenance.
Underdrains occasionally become clogged and have to be cleaned. Valves or sluice gates,
controlling sludge flow to the beds, must be kept watertight to prevent wet sludge from
leaking onto the beds during drying periods. Drainage of lines should be provided. Lines
with sludge in them should not be shut off until they are flushed out. Partitions between
sand beds should be so tight that the sludge will not flow from one compartment to
another, especially if the sand surface is taken down too low. The outer walls or banks
around the beds also should be watertight. Grass and other vegetation on banks should be
kept cut.
14.4.3 Belt Filters
These units appear to be well suited to the smaller plant. A belt filter is a continuously
moving, horizontal, porous belt, onto which the conditioned sludge is discharged. After the
sludge has been distributed, an impervious belt (also continuous) is pressed down on the
sludge layer by rollers, thus squeezing the water out. With primary and activated sludge
having 0.5 to 1.0 percent solids, a cake of 18 to 25 percent solids has been obtained (18).
Several designs of these units are available at present (19) (20) (21) (22) (23).
14.4.4 Vacuum Filters
Mechanical dewatering will usually not be economical for smaller plants, although it should
be considered and evaluated in certain cases for plants about 0.5 to 1.0 mgd in size. The
most common mechanical dewatering device is the rotary vacuum filter, of which two types
are available: the drum type, having a fabric attached completely around the drum
periphery; and the belt type, having stainless steel coils or cloth belts, which leave the drum
periphery at one point in its rotation. Filtrate quality depends on the sludge conditioning
process that precedes the filter.
The solids loading that can be applied to a filter is very much dependent on the solids
concentration in the feed. Digested primary sludge, composed of 7 to 9 percent solids, can
be handled at a loading of 4 to 8 Ib/ft2/hr (20 to 40 kg/m2>h), resulting in a cake that is
20 to 28 percent solids. A mixture of primary and activated sludge, composed of about 4
percent solids, can be handled at a loading of 4 to 5 Ib/ft2/hr (20 to 25 kg/m2 -h), resulting
in a cake that is 18 to 24 percent solids. If the conditioning process uses inorganic chemicals,
the resulting inorganic solids must be included in calculating the loading rates.
14-25
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The important auxiliary equipment needed with a vacuum filter are: sludge conditioning
tank with mixer, sludge cake conveyor, vacuum pump, filtrate receiver, and pump. For
waste activated sludge taken from an extended aeration plant, in which the sludge moisture
content is about 99 percent, a thickener and holding tank would also be needed. A filter
leaf test unit, with the same medium as the actual filter, is essential for determining the
optimum chemical dosage prior to application of the sludge on the filter.
14.4.5 Centrifuges
These dewatering units would not likely be considered for a 1-mgd (43.8 1/s) plant or
smaller. The most commonly used dewatering centrifuge for wastewater sludges is the
horizontal, solid-bowl unit. It operates on a continuous basis, receiving any pumpable sludge.
For waste-activated and primary sludge, after conditioning with a polymer, a cake of 15 to
25 percent solids can be produced. The solids capture of a centrifuge is not usually as good
as that of vacuum or belt filters, but chemical conditioning can improve it significantly.
When primary sludge is dewatered in a centrifuge, it should be degritted to a high degree to
prevent excessive abrasion on the bowl. The chief advantages of the centrifuge over a
vacuum filter are the reduced space requirements and the minimal exposure of the sludge to
the atmosphere (which reduces odor nuisances).
Another type of centrifuge being adapted to wastewater sludge dewatering is the basket
centrifuge, which is essentially a batch unit. The basket centrifuge is able to obtain a drier
cake and a better quality centrate than the solid-bowl centrifuge because of improved solids
capture. However, it is usually more expensive than the solid-bowl unit.
The quantity of SS in the centrate from the various types of centrifuges depends on the
type of unit used. Centrates from solid-bowl units have high SS, up to 5,000 to 10,000
mg/1.
14.5 Sidestreams Produced
Wastewater treatment plants produce various liquid sidestreams, most of which are
generated in the different sludge processing steps. The general characteristics of various
sidestreams are listed in Table 14-2. In sections 14.3.1 and 14.3.3, the supernatants from
aerobic and anaerobic sludge digesters were described. For digested sludges, the
characteristics of the liquor produced (such as filtrate or centrate) will depend on the
chemical conditioning of the sludge before dewatering (refer to section 14.4.1). Thus,
conditioning with lime and ferric chloride will precipitate phosphates in the sludge liquor,
and they will be retained in the sludge cake. However, the NH3 and any soluble BOD
present will not be affected and will remain in the liquid sidestream. Conditioning with
polymers will have no influence on the soluble pollutants in the liquor.
When raw, undigested sludge is processed, the major pollutants are the SS and associated
BOD. A gravity thickener overflow can have SS of from 100 to 1,000 mg/1 and BOD5
in the range of 50 to 1,000 mg/1. The volume of such streams can be calculated from the
difference in the solids content of the inflow and outflow sludge streams. It will normally
average about 2 to 3 percent of the wastewater volume being treated.
14-26
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It is important that all the pollutants that can affect the quality of the final plant effluent
be identified in all recycled sidestreams. Then, the load, in pounds per day (kg/d), should
be calculated and added to the load coming into a process with the raw wastewater. In
making cost-effectiveness analyses, the additional plant capacity required to handle loads
from the recycled streams is chargeable to the process that originates those streams. The
BOD and NH3 loads, for example, from recycled anaerobic digester supernatants can add
20 to 50 percent to the load in the incoming wastewater.
Some sidestreams, such as those produced during tertiary treatment, can be produced from
processes other than those associated with sludge processing. The major sidestream of this
type is the backwash water from granular media filters or carbon columns. The principal
pollutant in this sidestream is the SS. These sidestreams are produced at an instantaneous
rate equal to five to eight times the normal hydraulic loading to the filters or columns.
For small plants, in which there are only two or three such filters or columns in parallel,
this backwash rate exceeds the plant inflow rate. Therefore, it is not practical to recycle
such streams directly to the treatment plant. Such backwash streams can be discharged to
holding basins, with air or mechanical agitation to keep the SS in suspension, and then
recycled back to the head of the plant or to a point where coagulation and settling of the SS
will occur. The average concentration of the SS will be 300 to 1,500 mg/1 and these back-
wash streams usually average 2 to 5 percent of the plant hydraulic inflow.
14.6 Septage Handling
The handling of wastes pumped from septic tanks (septage) can be a significant problem
for small treatment plants. Many smaller communities have surrounding areas where
residences are served by septic tanks, their contents periodically pumped into tank trucks by
private operators. Frequently, the tank trucks discharge their contents haphazardly into
manholes of a sewer system connected to a treatment facility or hauled to the treatment
plant and discharged directly into the plant influent wet well. Such discharges can have
serious and detrimental effects on the performance of treatment plants (particularly small
ones) and, hence, many such plants have terminated this practice.
Larger facilities are not nearly as susceptible to these effects, because they usually have a
smaller ratio of septage to raw wastewater. Smaller facilities should use receiving facilities
and holding tanks for "bleeding" a smaller continuous septage flow into the wet well.
Septage is highly variable, as the characterization data in Table 14-7 indicate. It is odorous,
and can impose shock loads on treatment plants, causing upsets of primary clarifiers,
secondary processes, and anaerobic digesters (24).
The amounts of extra solids, BOD, andNH3 loading should be calculated to determine how
the various processing units will be affected and to determine the necessary design capacity.
The oxygen demand on the plant can, for example, be increased substantially.
If the septage-to-wastewater ratio is large enough to overload the plant facilities,
consideration should be given to providing equalization facilities or an independent septage
14-27
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TABLE 14-7
TYPICAL CHARACTERISTICS OF SEPTAGE
BOD5,mg/l 2,500-20,000
COD,mg/l 10,000-70,000
SS,mg/l 2,500-100,000
Volatile solids, percent 50-80
Ammonia nitrogen, mg/1 100-500
Organic nitrogen, mg/1 100-1,500
treatment facility. The equalization facility may consist of a covered aerated holding tank
with the capacity to hold the maximum septage expected at the plant in 24 hours. Because
the air will strip sulfides, which should be oxidized before release to the atmosphere, some
treatment of the vented air from the holding tank should be provided. Methods of treatment
may include ozonation, bubbling the odorous air through chlorinated water, or discharge
through activated carbon or soil beds (see Chapter 2). If the septage contains large solids, it
should be macerated before entering the holding tank. Enough air should be provided to
keep the holding tank contents aerobic and to keep the solids in suspension.
If the septage haulers indiscriminately include raw wastewater from pit toilets, wastewater
from camping trailers and dockside pump-out stations, waste motor oil from pumping
stations, cutting oil, and other hard-to-treat wastes from local small industries, a permit
system, allowing discharge under specified circumstances into the wastewater collection or
treatment system, must be established and enforced if the effluent requirements are to be
met consistently.
14.7 Sludge Disposal
Final disposal of the sludge will be made directly to agricultural land, to sanitary landfills,
or to an incinerator. Digested sludge can be spread on agricultural land, and many studies
are currently in progress on the various aspects of this method of final disposal (9) (25).
Because digestion does not guarantee destruction of all pathogens, appropriate measures
should be taken to prevent health hazards in applying sludge to land. Other concerns are the
effects of nitrogen compounds (especially ammonia and nitrates) and heavy metals (such as
zinc, copper, and nickel), which are concentrated in sludges from normal domestic
wastewater, even if no industrial waste enters the system.
Nuisance-free disposal of liquid sludge on cropland requires high-quality digestion to reduce
pathogen content and to prevent odorous putrefaction. Storing digested sludge in ponds
permits more complete die-off of pathogens. Also, such storage, if aerated, permits nitrifica-
tion of the high NH3 concentration in the sludge, which otherwise might inhibit seed
germination. Storage is required if the ground is frozen and land application is not practiced.
14-28
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Because the sludge is high in nitrogen, and nitrate-nitrogen is not readily retained in most
soils, crops that have a relatively high demand for nitrogen should be planted. The chance of
polluting the groundwater with nitrates should be minimized by limiting nitrogen loadings
to the probable demand.
Distribution of liquid sludge may be made by furrow irrigation or by sprinkler irrigation.
In the northern United States, application of sludge is limited to periods when the ground is
not frozen. During the winter, the sludge is held in ponds.
In places where sludges from domestic wastewaters (without any industrial wastes) have
been applied to agricultural land for extended periods (such as in England), the
accumulation of heavy metals in the soil usually has not been a problem (26), possibly
because of the relatively low rate of application-not exceeding 5 tons of dry solids per acre
per year. Lime in soils can make the metals complex and keep them insoluble and
unavailable to plants. Usually, soils that have had prolonged application of wastewater
sludge will become acidic and require periodic liming (27).
Incineration of dewatered wastewater sludge from wastewater treatment plants of 1 mgd
in size is generally not economical, unless the incinerator is also used for garbage and refuse.
In that case, the sludge cake can be mixed with the garbage and refuse and (because of the
relatively small weight and volume of the sludge) can be disposed of in this manner.
However, the proportion of sludge to other solids should be kept fairly constant.
Composting dewatered wastewater sludge, both alone and mixed with garbage and refuse,
has been studied (28). In the latter case, the sludges provide moisture and nutrients which
are normally lacking in sufficient amounts in garbage and refuse. Composting such solids
mixtures is being practiced in Europe, but various attempts at composting in this country
have not, as yet, proved economical compared with other methods of disposal.
Dewatered raw wastewater solids, composed of about 75 percent moisture, can be
composted alone in a mechanical composter for 8 to 10 days, using forced air to maintain
proper aerobic conditions. The average temperature in the composting sludge is about
140° F (60° C), which destroys most pathogens. The final product has a moisture content
of about 30 percent, an earthy odor, and a fertilizer value equal to that of cattle manure. It
does not undergo putrefaction if piled outdoors and is free of viable plant seeds and
indicator pathogens.
Design criteria of a unit to handle the dewatered sludge from a primary-secondary, 1-mgd
(44 1/s) plant indicate that the volumetric capacity would be 1,000 ft3 (28.4 m3). This is
about one-tenth the volume required for an aerobic sludge digester handling 2 percent
solids. The power required for mixing the composting mass and supplying the necessary air
is estimated to be comparable to the power required for an aerobic digester.
14-29
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14.8 References
1. Process Design Manual for Sludge Treatment and Disposal U.S. EPA, Office of Tech-
nology Transfer (October 1974).
2. Process Design Manual for Upgrading Existing Waste-water Treatment Plants. U.S. EPA,
Office of Technology Transfer (October 1974).
3. Municipal 'Waste-water Treatment Plant Sludge and Liquid Sidestreams. Camp Dresser &
McKee (in press, 1974).
4. "Sewage Treatment Plant Design," WPCF Manual of Practice No. 8 (at press, 1975).
5. Vesilind, P.A., Treatment and Disposal of Wastewater Sludges. Ann Arbor, Michigan:
Ann Arbor Science Publishers (1974).
6. Direct Environmental Factors at Municipal Wastewater Treatment Works. U.S. EPA,
Office of Water Programs Operation (at press, 1975).
7. Glossary-Water and Wastewater Control Engineering. APHA, ASCE, AWWA, WPCF
(1969).
8. Link Belt Thickeners. Book 2959.
9. "Sludge Dewatering." WPCF Manual of Practice No. 20 (1969).
10. Wastewater Engineering. Metcalf & Eddy Inc., New York: McGraw-Hill (1972).
11. Katz, W.J., and Geinopolos, A., "Sludge Thickening by Dissolved Air-Flotation."
Journal Water Pollution Control Federation, 39, p. 946 (1967).
12. "Anaerobic Digestion." WPCF Manual of Practice No. 16.
13. Burd, R.S., A Study of Sludge Handling and Disposal FWPCA publication WP-20-4
(May 1968).
14. Smith, R., Filers, R.G., and Hall, E.D., A Mathematic Model for Aerobic Digestion.
National Environmental Research Center, Cincinnati (July 1973).
15. Doyle, C.B., "Effectiveness of High pH for Destruction of Pathogens in Raw Filter
Cake."/o«ma/ Water Pollution Control Federation, 39, p. 1403 (1967).
16. Farrell, J.B., Smith, I.E., Hathaway, S.W., and Dean, R.D., "Lime Stabilization of
Primary Sludges," Journal Water Pollution Control Federation, 46, p. 113 (1974).
14-30
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17. Courts, C.A., and Schuckrow, A.J., Design, Development and Evaluation of Lime
Stabilization System to Prepare Municipal Sewage Sludge for Land Disposal. Pacific
Northwest Lab, Battelle Institute (1974).
18. DaVia, P.G., Capillary Action Applied to Sludge De-watering. International Water
Conference, Pittsburgh (October 1972).
19. Imhoff, K.R., Sludge Dewatering Tests with a Belt Press.
20. The A utomatic Belt-Filter Press. Bulletin, Carter Co. (April 1972).
21. Goodman, B.I., and Higgins, R.B., Concentration of Sludges by Gravity and Pressure.
Presented at WPCF Annual Conference, Boston (October 1970).
22. "The DCG Solids Concentrator." Permutit Co., Bulletin No. 5161 (1972).
23. Hedenland, I.D., "District Goes Into Reclamation Business." Water and Wastes
Engineering, 10, p. 30 (March 1973).
24. Kolega, J.J., "Design Curves for Septage." Water and Sewage Works, p. 132 (May
1971).
25. Recycling Treated Municipal Wastewater and Sludge Through Forest and Cropland.
Pennsylvania State University, College of Agriculture.
26. Agricultural Use of Sewage Sludge. Notes on Water Pollution, Department of the
Environment, Great Britain (June 1972).
27. Hinesly, T.D., and Soseqitz, B., "Digested Sludge Disposal on Crop Land." Journal
Water Pollution Control Federation, 41, p. 822 (1969).
28. Hart, S.A., Solid Waste Management: Composting. Report No. SW-2C, U.S. Depart-
ment of HEW, Solid Wastes Program (1968).
14-31
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CHAPTER 15
DISINFECTION AND POSTAERATION
15.1 Introduction
Disinfection is the process of destroying pathogens, including viruses, and other harmful
micro-organisms. Chemical agents are most commonly used to disinfect wastewater, although
this disinfection can also be accomplished using radiation, mechanical means, and physical
agents.
The most common chemical disinfectant used in wastewater treatment is chlorine. Other
possible chemical disinfectants include ozone, iodine, bromine, and alkalies (pH's above 11
are relatively toxic to pathogens). Ozone, ultraviolet light, and bromine chloride are
currently being evaluated by the U.S. EPA to determine their potential use in disinfecting
wastewater treatment plant effluents.
Mechanical removal of micro-organisms from wastewater is accomplished to some extent by
flocculation, settling, screening, or adsorption. Treatment processes exhibiting significant
removal of bacteria include high pH lime treatment, series wastewater treatment ponds,
coagulation with alum or iron salts, carbon adsorption, activated sludge, and trickling filters.
However, all of these processes require provisions for terminal disinfection.
The average person discharges 100 to 400 billion coliform organisms per day (1) in about 45
grams of feces. The ratio of total coliforms to fecal coliforms in domestic wastewater is in
the range of 2:1 to 4:1, and the ratio of fecal coliforms to fecal streptococci is 4:1 to 8:1
(2). Salmonella has been isolated from wastewater with total coliform counts as low as
2,200/100 ml (2). Ratios of coliforms to enteric viruses are about 92,000:1 in wastewater
and about 50,000:1 in polluted surface waters (2) (3).
Ideal disinfection of wastewater kills or inactivates the pathogens present and does not
continue its action beyond the treatment facility. The efficiency of chemical disinfection is
primarily dependent on the adequacy and rate of mixing, contact time, concentration of
disinfectant, temperature of water, efficiency of residual control system (in certain cases),
pH, concentration of interfering substances and quality of operational surveillance. Of these
variables, the mixing, concentration of disinfectant, contact time, and residual control
system are the ones primarily affected by the facility design (4).
Two recent actions taken by the U. S. EPA with regard to municipal wastewater disinfec-
tion should be noted: 1) fecal coliform bacteria limitations have been deleted from the
definition of secondary treatment; 2) disinfection of municipal effluents will be on a case-
by-case basis in accordance with state water quality standards. The Federal Register of
July 26, 1976 (Vol. 41, No. 144) contains further information on these points.
15-1
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15.2 Chlorination
Efficiency of chlorine disinfection may also be affected by the concentration of bacteria
and the concentration and reactivity of interfering or chlorine-demanding substances.
Chlorination of wastewater not only kills pathogens, but usually creates chlorinated
compounds such as chloramines, which can be toxic to receiving-water biota. Insufficient
chlorine dosage, mixing, and contact time result in an incomplete kill of pathogens and
indicator organisms. Highly effective or essentially complete disinfection can best be carried
out in water free from suspended material. If the wastewater is chlorinated when SS are
present, bacteria within the suspended particles may not be killed. Existing data indicate
that free chlorine residuals are required for significant viral inactivation, unless prolonged
contact is provided.
Four conditions usually must be met to insure inactivation of viruses by Chlorination (5):
1. The turbidity of the water should be < 1.0 Jtu (preferably <0.1 Jtu).
2. The pH of the water should be close to 7.5 for waters containing ammonia and
<7.0 for ammonia-free waters.
3. Rapid initial mixing of water and chlorine must be provided.
4. A concentration of 0.5 to 1.0 mg/1 of undissociated hypochlorous acid (HOC1)
must be maintained for an actual contact period of >30 minutes.
If elemental chlorine is added to water, it is hydrolyzed and ionized to two forms:
hypochlorous acid (HOC 1) and the hypo chlorite ion (OC1~). In pure water systems with the
pH below 7, HOC1 constitutes from 75 to 95 percent, or more, of the solution; with a pH
above 8, from 20 to 40 percent, or less, of the solution. The disinfection efficiency of HOC1
is from 45 to 250 times greater than that of OCT. HOC1, also, is a very active oxidizing
agent; it will react with oxidizable metals, hydrogen sulfide and organic matter present in
wastewater, and is short lived in the presence of readily oxidized compounds, such as
ammonia, found in wastewater. Because most wastewater effluents contain ammonia, the
following reactions will occur within minutes upon adding chlorine (2):
NH3 + HOC1 -> NH2C1 + H2O
(monochloramine)
NH2C1 + HOC1 -» NHC12 + H2O
(dichloramine)
NHC12 + HOC1 -> NC13 + H2O
(nitrogen trichloride)
The two species predominating in most cases are monochloramine and dichloramine. They
are commonly referred to as the combined available chlorine. Chloramines are much less
potent than hypochlorous acid as a disinfectant.
15-2
-------
Hypochlorous acid also reacts with other organic matter in wastewater, such as amino acids,
and inorganic matter such as sulfites and nitrites, to produce chlorine compounds having
very little or no disinfecting power (2). Design engineers should be aware of the extent of
such side reactions in determining the optimum chlorine dosages to apply to a wastewater
(6).
Total chlorine residual is the sum of the combined and free chlorine concentrations
remaining after a specified period of time. Chlorine demand is defined as the difference
between the chlorine applied and the total residual chlorine remaining at the end of the
contact period. It provides a measure of all chlorine demanding reactions, including
disinfection (6).
Tests are necessary to determine chlorine demand. The amperometric titration test for
chlorine residual is the most reliable, if the operator is well trained in the use of the
instrument (several blank runs are made to "warm up" the instrument, and the electrodes
are regularly checked (7). Lin et al. (8) recommend the modified starch iodide (MSI)
method because of its simplicity, speed, economy, and dependability; and they note that it
is already preferred by WWTP operators in California. Both the amperometric and the basic
starch iodide tests are detailed in Standard Methods (9). The modified starch iodide test is
detailed elsewhere (10). A field evaluation of 38 wastewater facilities indicated that the
most reliable chlorine dosage control method was either 1) a residual control with an
analyzer to provide control based on both change of flow rate and chlorine demand or 2) a
compound loop control in which the control apparatus receives two separate and
independent signals from a flow-measuring device and from a chlorine residual analyzer (4).
Figures 15-1 and 15-2 illustrate some of the concentration vs. contact time relationships for
chlorine disinfection of wastewater coliforms. Figures 15-3 and 15-4 show the relative
effectiveness of different forms of chlorine and the relative resistance of other organisms in
pure water systems. Figure 15-5 shows the relative amounts of HOC1 and OC1" for 0° and
20° C at various pH values. A detailed discussion of all aspects of chlorination is presented
in reference (12).
Careful design of chlorination facilities is essential, because it usually is relatively difficult
for the operator to adjust the initial mixing, the actual contact (short-circuited) time, the
method of controlling the chlorine dosage, or the pH after construction. Efficient operation
of the prior treatment processes may minimize or eliminate some interfering substances,
such as ammonia, calcium bicarbonates, SS, COD, and turbidity. It is particularly important
in wastewater treatment pond systems that the removal of SS in the form of algal and other
microbial cells be relatively complete before chlorination, if disinfection is to be effective
(13). Turbidities should be kept below 1 Jtu and preferably below 0.1 Jtu, if chlorination is
to inactivate viruses effectively (3).
If chlorination (either with gaseous chlorine or hypochlorite) is used, the chlorination
process and the chlorine contact unit ideally should be designed to keep the chlorine
residual in the effluent at a minimum level, consistent with meeting coliform removal
standards. This condition can best be accomplished by providing an initial rapid mix, for
15-3
-------
d = WASTE WATER
DILUTION
O.I
10 20 50 100 200 500 1000
CONTACT TIME FOR 50% E.COLI KILL (MINUTES)
FIGURE 15-1
CHLORAMINE CONTACT TIME REQUIREMENTS (11)
0.5
O
u>
o O2
OV
E
,., 0 I
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: 005
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OOI
V
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c
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\
t
v.
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pH
d =
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\
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7
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d=WASTEWATER
DILUTION
12 5 10 20 50 100
CONTACT TIME FOR 99% E.COLI KILL (MINUTES)
FIGURE 15-2
FREE CHLORINE CONTACT TIME REQUIREMENTS (11]
15-4
-------
10
1.0
UJ
z
cc
o 10
_i
i
o
UJ
£C
.001
«
5&
V'n
\
\
I 2 4 6 8 10 20 4060 100 200 400600 1000
MINUTES
FIGURE 15-3
CONTACT TIME FOR 99% Kl LL OF E. COLI AT 2° - 6° C IN PURE WATER (2)
o oi
I 2 4 6 810 20 40 60 IOO 200 40060O OOO
MINUTES
FIGURE 15-4
RELATIVE RESISTANCE TO HOCI AT 0° - 6° C IN PURE WATER (2)
15-5
-------
100
90
40 '
o
o
Ul
o
cc
LU
100
10 II 12
FIGURE 15-5
DISTRIBUTION OF HOCI AND OCI IN WATER WITH VARIATIONS IN pH (12)
15-6
-------
making quick and complete contact between the chlorine and the pathogens, and followed
by plug-type flow (i.e., each particle in a cross section of flow moves at the same velocity)
through the contact chamber.
Chlorine-induced toxicity, from a complex of chloramine compounds in wastewater
discharges, is becoming a serious environmental concern. Combined chlorine residuals may
be toxic to the existing aquatic organisms (14), and dechlorination may be necessary.
Dechlorination can be accomplished with reducing chemicals (such as sodium bisulfite,
hydrogen peroxide, sulfur dioxide, sodium sulfite, or sodium thiosulfite) or with beds of
activated carbon. Each dechlorination system requires some additional equipment and
possibly pumping, necessitating cost comparisions. In small plants, the addition of sulfur
dioxide or activated carbon is usually the simplest effective method of dechlorination.
When a chlorinated wastewater containing combined forms of chlorine, including
chloramine, is passed through an activated carbon contactor, the following reaction seems to
take place:
2 NHC12 + H2O surface oxides ->N2+4H+ + 4C1- + oxides of C
releasing free nitrogen to the atmosphere and removing the possibly toxic chloramines, if
contact time is sufficient (15).
Typical chlorination systems using gaseous chlorine are shown in Figure 15-6. Using the
vacuum system rather than the pressure vacuum has several advantages, including (16):
1. Elimination of pressurized gas and solution feed lines
2. Elimination of manifold piping and flexible connectors while isolating auxiliary
valves
3. Elimination of need for heat as gas under vacuum that does not reliquify at
temperatures above about -40° F (-40° C)
Because of its obvious simplicity, the vacuum system is more desirable for small wastewater
chlorination facilities.
Calcium or sodium hypochlorite provide much the same chlorinous solution as does gaseous
chlorine. For example, 1 Ib of active chlorine in solution is produced by 1.05 Ib of NaOCl
(100 percent) or 1.01 Ib of Ca(OCl)2 (70 percent). Calcium hypochlorite comes in tablet
form for very small systems, and granular form for larger systems, requiring a solution tank
and a solution feeder. Sodium hypochlorite solution can be purchased or generated onsite
in amounts as low as 1 Ib/day. The hypochlorite solution also eliminates the dangers of
handling gaseous chlorine, although it has a relatively short storage life in solution. Care
must be taken to insure that an inexpensive salt source is available, and that the extra
dissolved solids from the hypochlorite generation (an increase in total dissolved solids results
from the production of each mg/1 of NaHOCl, depending on the efficiency of the process)
will not create a treatment or disposal problem. The generator requires about 1.7 to 3.5
kWh (6.12 to 12.6 mJ) and about 2.0 to 3.3 Ib of salt per pound of equivalent chlorine in
15-7
-------
REMOTE CHLORINATOR
ON CYLINDER IN
SEPARATE ROOM
SEPARATE CHLORINE
METER AND FEED
CONTROL MOUNTED
WHERE CONVENIENT
VACUUM
LINE
WATEJR_
SUPPLY
EJECTOR UNIT
FEEDS CHORINE
WHERE DESIRED
WATER
MAIN
TYPICAL VACUUM CHLORINE FEED SYSTEM (20)
PRESSURE
LINE
CHLORINE
SOLUTION
TYPICAL PRESSURE-VACUUM CHLORINE FEED SYSTEM (20)
FIGURE 15-6
GASEOUS CHLORINATION SYSTEMS
15-8
-------
the resulting sodium hypochlorite solution. Onsite generation will not usually be cost
effective for small plants, except for isolated locations with a low-cost salt source. For more
information on hypochlorite generation, see references (12) and (16).
Systems for onsite generation of NaOCl for small treatment plants are shown in Figure 15-7.
Some hypochlorite generation units, such as those with membrane cells, are more costly to
maintain, or they require pure salt rather than rock salt or sea water. The capital costs of
different units also vary considerably.
An extensive 18-month bacteriological study of the Trinity River basin indicated that, if
chlorination is not efficient, bacterial populations recover as they move downstream from
the effluent discharge points (20). To achieve satisfactory kill of fecal coliform, fecal
streptococci, and solmonellai, the chlorination facilities must be designed to insure adequate
bacteria-chlorine contact at all flows.
In designing a chlorination system, some essential factors to be considered are:
1. Selecting a reliable chlorine feed control system, keyed, if possible, to meet
chlorine demand variations (usually, this means feed activated by flow-measuring
devices for the smallest plants)
2. Selecting a diffuser that doses the chlorine uniformly throughout the influent
stream
3. Providing excellent mixing (mechanical mixer, hydraulic jump, etc.) at the inlet
end of the contactor to homogenize the chlorine and wastewater within 3 to 5
seconds of dosing (4)
4. Providing longitudinal baffles and turning vanes, if necessary, to achieve plug
flow after the initial complete mixing and actually obtain the required contact
time (actual equals theoretical minus short circuiting)
5. Training operating personnel thoroughly
6. Providing a means of pH control, if necessary
7. Providing dechlorination, if downstream ecology will be significantly affected
by chlorinated byproducts
8. Providing nonsettling velocities in the contact chamber
9. Making adequate provision for easy cleaning of contact basin, because
accumulated solids interfere with disinfection (5)
10. Monitoring the chlorine residual
Steps 5, 6, 7, and 10 are not generally fully incorporated in the design of plants treating
fewer than 100,000 gpd.
15.3 Chlorine Mixing and Contacting
Once a mixing device has been constructed, its efficiency cannot be modified easily, so an
effective design is very important to good chlorination. Three methods to achieve chlorine
mixing are shown in Figure 15-8. The turbulence created by a hydraulic jump has proved
excellent for chlorine mixing and has proved to be most effective if the head loss is about
15-9
-------
BRINE-
WATER-
CELL
STACK
CHLORINE
SEPARATOR
SPENT BRINEl
HYDROGEN
WASTE
VENT
SEPARATOR
CAUSTIC
COOLER
VENT
I INERT
REACTOR
PRODUCT
IONICS GENERATION SYSTEM (18)
MAKEUP
WATER
HYPOCHLORITE
DISCHARGE
T
SALT
RECTIFIER
CIRCULATING
PUMP PEPCON
CELL
(STORAGE) HYPOCHLORITE RECYCLE
\TANK /
CHLORINATION USES
PEPCON GENERATION SYSTEM (19)
FIGURE 15-7
HYPOCHLORITE GENERATION
15-10
-------
OUTFALL
CHANNEL
T
r-POND EFFLUENT
-PUMP
TO CHLORINE RESIDUAL-
RECORDER- CONTROLLER
'—SAMPLE LINE AND INJECTOR
WATER SUPPLY
HYDRAULIC JUMP MIXING
CHLORINE
MIXER
r CHLORINE DIFFUSER
-3D
PIPE
CONTACTOR
MITERED TEES a ELBOWS
FOR SHARP TURNS
MECHANICAL MIXING
MIXING BY EXPANSION IN PIPE
FIGURE 15-8
CHLORINE MIXING METHODS
15-11
-------
2 ft (0.6 m). Another simple, but effective, method to achieve good mixing in small plants is
to place the chlorine feed at a centrifugal pump inlet, if the flow to the contactor is pumped.
The design of the chlorine mixing and contacting units must be considered together. The
contact unit should be designed to provide a detention time of not less than 30 minutes
for each molecule of water passing through. Unless model dye studies have been made to
determine the ratio of actual to theoretical detention time, a value of two-thirds may be
used. For example, if the actual time is to be 30 minutes, the contact unit should have a
theoretical detention time of 45 minutes. The ratio might be as high as 0.9 for an excellent
design.
Such plug flow can be obtained to a major degree by 1) using a pipe designed to flow full to
provide contact time; 2) using a long, narrow, concrete-lined channel; or 3) placing end-
around baffles with guide vanes at turns in the contact chamber. The pipe is the best type of
chlorine contact unit for small plants. Length-to-width ratios of 60 to 70 have proven most
effective (4). One effective design for baffled contact chambers, as shown in Figure 15-9,
has a flow-length to channel-width ratio of 72.
Model tests were made by the Metropolitan Sanitary District of Greater Chicago to evaluate
the impact of different baffle designs on actual detention time. The various baffle designs
evaluated and the test results are shown in Figure 15-10. The data show that the baffle
arrangement used in Scheme IIA, using turning vanes, provided the highest actual contact
times.
15.4 Ozonation
Ozone, used to disinfect wastewater or as a tertiary treatment, does not increase the odor,
color, or salt content of the plant effluent. In relatively pure potable water, ozone has a half
life from about 20 to 80 minutes, depending on the COD remaining in the water (21). At
normal temperatures, ozone residuals rapidly disappear if any COD is present. Although it is
twice as powerful an oxidizing agent as hypochlorite ion, ozone has relatively little
disinfecting power until the initial ozone demand of the wastewater has been satisfied. After
this point has been reached, it reacts very rapidly essentially to complete disinfection of
both bacterial and viral pathogens within 5 minutes (22). If normal amounts of oxidizable
materials are present, as in secondary effluent, the dosage required may be between 5 and
20 mg/1 (23). Ozone treatment does not reduce the total dissolved nitrogen. At pH above
10, ammonia also is oxidized to nitrate. CODs of 30 to 50 mg/1 can be reduced about 50
to 70 percent with ozone in one to two hours of actual (not theoretical) contact time.
For domestic wastewaters, the relations between feedwater COD, product COD, feedwater
pH, reaction time, dissolved ozone concentration, and the quantity of ozone dissolved are
discussed in reference (24). An ozone-generating facility must be designed to meet peak
flow periods and maximum dosage levels (23). If dechlorination is required, the costs of
ozonation may become more competitive with the costs of chlorination, especially if liquid
oxygen is available onsite. It takes about 2.5 to 3.5 kWh (9.0 to 12.6 ml) to produce a
pound of ozone, using oxygen feed, and about 6 to 9 kWh (21.6 to 32.4 mJ), using air feed.
15-12
-------
T
RAPID MIX
UNIT
A
L:W RATIO= I8:|
MODAL TIME = 0.70
%PLUG FLOW=95
FLOW LENGTH
TO W RATIO = 72:|
FIGURE 15-9
CONTACT CHAMBER WITH LONGITUDINAL BAFFLING (4)
15-13
-------
SCHEME I
SCHEME IA
3D
dD
\
FLOW, MGD 82.5
WATER DEPTH, FT. 14.2
CONTACT TIME, WIN.
MINIMUM 21.0
MEAN 29.2
MAXIMUM 36.5
82.5
I 4.2
I 7.6
26.4
37.8
SCHEME IB
82.5
I 4.2
I 5.3
23.9
34.8
SCHEME II
SCHEME MA
FLOW, MGD 82.5
WATER DEPTH, FT. 14.2
CONTACT TIME,MIN
MINIMUM 15.2
MEAN 20.5
MAXIMUM 31.9
82.5
I 4.2
25.0
31.1
39.5
FIGURE 15-10
IMPACT OF CHLORINE TANK BAFFLE DESIGN ON ACTUAL DETENTION TIME (6)
15-14
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The primary objections to ozone use are high cost, possible air pollution from excaping
ozone, and high electrical consumption. The advantages of ozone for disinfection for
secondary effluents are:
1. It eliminates tastes and odors; reduces oxygen demanding matter; removes most
colors, phenolics, and cyanides; reduces turbidity and surfactants; and increases
dissolved oxygen.
2. Ozonation can also be used as a tertiary treatment process for oxidation of
residual carbon compounds and for odor control
3. The Maximum Allowable Concentration (MAC) of ozone in air, as established by
the American Council of Governmental Industrial Hygienists, is 0.1 ppm by
volume for continuous human exposure. The threshold odor of ozone is 0.01
to 0.02 ppm. This1 means that a person working near an ozone-handling area
should be able to detect the presence of ozone at concentrations far below the
MAC (6).
4. Ozone can be generated from air, making its supply dependent only on a source
of power. Production of ozone from oxygen should be considered where onsite
generation of oxygen is practiced in conjunction with biological treatment,
because less energy is used.
The two most efficient types of ozone generators are the water-cooled tube and the air-
cooled, Lowther-plate ozonators (24). The latter are generally more efficient (25). The
Lowther-plate ozonator (see Figure 15-11) with a once-through air feed system will require
about 6.3 to 8.8 kWh/lb (13.0 to 19.4 kWh/kg), while with a recycled-pure-oxygen feed
system, it will require about 2.5 to 3.5 kWh/lb (5.5 to 7.7 kWh/kg). Ozonation systems are
shown in Figures 15-11 and 15-12. A typical ozone contactor is also shown in Figure 15-12.
There are four general systems to apply ozone to wastewater (28):
1. Cooled and dried air is fed to the ozonator and the resultant air-ozone solution
(1.3 percent ozone by weight) is mixed with wastewater in a contactor. This
system is limited to very small wastewater systems (because of its inefficiency)
and for use in odor control.
2. An oxygen-enriched feed replaces the air feed in system one (above). A pressure-
saving separator is used to remove nitrogen.
3. Oxygen feed replaces the air feed in system one, and oxygen-rich off gas is
recycled to the front of the loop. Nitrogen is removed from the wastewater
before Ozonation.
4. Air is enriched to about 40 percent oxygen at starting. In each successive cycle
the recycled gas is cleaned, dried, and enriched in oxygen.
Three general types of contactors are usually used. These are the packed bed, the sparged
column, and the sparged column with mixing. The most efficient contactor design for a
particular wastewater at a particular location is apt to be different from the best for another
wastewater with different local conditions. The design of an ozonation system for a
municipal wastewater treatment facility requires thorough knowledge of the local
conditions and of ozonation to optimize the design.
15-15
-------
HIGH VOLTAGE
STEEL ELECTRODE
ALUMINUM HEAT
DISSIPATOR
CERAMIC
DIELECTRIC
GROUND STEEL
ELECTRODE
DISCHARGE GAP
GLASS
SEPARATOR
t—CERAMIC DIELECTRIC COATED
\ STEEL ELECTRODE
=MbO
SECTION "A -A"
LOWTHER PLATE OZONE GENERATOR UNIT
AIR
•*•
COMPRESSOR
DISINFECTED
EFFLUENT
DRYER
PUMP
AIR FEED OZONE TREATMENT SYSTEM
FIGURE 15-11
OZONE SYSTEM (26) (27)
15-16
-------
OZONE
CONTAINING
GAS
EXHAUST
VENT
WASTEWATER
DISINFECTED
EFFLUENT
DETAIL OF CONTACTOR
COMPRESSOR
PRESSURE SWING
OXYGEN SEPARATOR WAI tK
PUMP
OXYGEN-RICH AIR FEED
OZONE TREATMENT SYSTEM
FIGURE 15-12
OZONE SYSTEM (26) (27)
15-17
-------
15.5 Small Plant Disinfection Practice
Current disinfection practice in the design of small wastewater treatment plants is usually
confined to chlorination. Its efficiency is a function of the adequacy and rate of mixing,
contact time, water temperature, pH, interferring substances present, and quality of plant
operation and maintenance.
The steps to take in designing a chlorination system are outlined at the end of section 15.2.
At smaller, more isolated plants, dry sodium hypochlorite is used, with the solution pre-
pared each day or two to prevent loss of effectiveness because of deterioration. The vacuum
chlorinator usually is the cost-effective choice in which liquid chlorine can be obtained
expeditiously. Standby chlorination facilities are required to meet EPA reliability guidelines.
For small plants, a standby dry NaHOCl feed installation will usually prove to be effective,
if an alternative source of liquid chlorine is not available.
To maintain as low a chlorine residual as possible in the plant effluent, while providing enough
chlorine to meet the variable chlorine demand, is difficult at small plants if the flows are also
quite variable. Preferably the chlorine feed should be paced by signals from a chlorine residual
analyzer. If this process is not possible, feed should at least be paced by a flow measuring device.
Mixing of the chlorine in the treated wastewater must be quick and thorough, requiring
adequate turbulence. The three types of mixing (shown in Figure 15-8), or feeding the
chlorine into a centrifugal pump inlet, are all effective methods for good mixing. The
simplest form of efficient contactor is a pipe with a siphon or weir at the lower end to keep
the pipe flowing full. An open channel may be substituted for the pipe. Either should have a
length-to-width ratio of 50 to 80 and a minimum actual detention time at peak flow of
30 minutes.
Means should be provided for regular sampling to determine the fecal coliform count in the
chlorinated effluent, because the chlorine residual is a less accurate indicator of fecal
bacteria survival.
15.6 Postaeration
Most State receiving water quality standards require a minimum stream DO of 4.0 mg/1.
In addition, States are requiring a minimum DO at least equal to the receiving water quality
standards (6).
Postaeration can be accomplished by adding oxygen to the treated effluent before discharge
in several ways: mechanical aeration, diffused aeration, cascade aeration, U-tube aeration,
and ozonation (used if present for other purposes).
Although postaeration in chlorine contactors does not affect chlorine residual, it does
increase short circuiting and, thus, may reduce the effectiveness of the chlorination.
15-18
-------
Postaeration will increase the dissolved oxygen in treatment plant effluent, thus reducing
ultimate oxygen demand and potential odor problems in the stream.
Secondary effluents from biological treatment plants normally contain 0.5 to 2.0 mg/1 of
DO (6). Saturated DO concentrations in plant effluents will normally be between 8 and 14
mg/1. An unsatisfactory BOD of 35 mg/1 with 0.5 mg/1 of DO could be reduced to below
30 mg/1, by adding 6 mg/1 of DO with postaeration, thereby meeting secondary effluent
requirements.
Typical devices used in postaeration are shown in Figure 15-13 (6). These devices and design
considerations for their use in postaeration are described in the Process Design Manual for
Upgrading Wastewater Treatment Plants (6).
The two postaeration methods found to be most simple and yet effective for small
treatment plants are cascade aeration and U-tube aeration, neither of which incorporate
mechanical equipment nor require outside power (other than some loss of head). If
compressed air is available at the plant, diffused aeration may be used advantageously for
postaeration. Otherwise, the more efficient mechanical aerator should be used if
postaeration is necessary and if sufficient head is not available for U-tube aeration or
cascade aeration.
15.7 References
1. Metcalf and Eddy, Wastewater Engineering. New York: McGraw-Hill (1972).
2. Chambers, C.W., "Chlorination for Control of Bacteria and Viruses in Treatment Plant
Effluents," Journal Water Pollution Control Federation (February 1971).
3. "Viruses in Water." AWWA Committee on Viruses, Journal American Water Works
Association (October 1969).
4. White, G.C., "Disinfecting Wastewater with Chlorination /Dechlorination." Water and
Sewage Works (August, September, and October 1974).
5. Gulp, R.L., "Breakpoint Chlorinations for Virus Disinfection." Journal American
Water Works Association (December 1974).
6. Process Design Manual for Upgrading Existing Wastewater Treatment Plants. U.S. EPA,
Office of Technology Transfer (October 1974).
7. McFarren, E., private communication, U.S. EPA, Cincinnati (September 1974).
8. Lin, S., Hullinger, D.L., and Evans, R.L., "Selection of a Method to Determine
Residual Chlorine in Sewage Effluents," Water and Sewage Works, Vol. 118, 360
(November 1971).
15-19
-------
A. DIFFUSED AERATION
B. MECHANICAL AERATION
OXYGEN SOURCE
Q OR AIR COMPRESSOR
Q.
:o
C. CASCADE AERATION
\\ HEAD
•^i^-, i n
-------
9. Standard Methods. APHA, AWWA, WPCF (1972).
10. Browning, G.E. and McLaren, F.R., "Experiences with Wastewater Disinfection in
iz." Journal Water Pollution Control Federation, 39, P 1351 (August 1967).
11. Fair, G.M.,Geyer, J.C. and Okun, D.A., Water and Wastewater Engineering. New York:
John Wiley & Sons (1968).
12. White, G.C., Handbook of Chlorination. New York: Van Nostrand Reinhold (1972).
13. Majumdar, S.B., Ceckler, W.H., and Sproul, O.J., "Inactivation of Poliovirus in Water
by Ozonation." Journal Water Pollution Control Federation (December 1973).
14. Collins, H.F., and Deaner, D.G., "Sewage Chlorination Versus Toxicity-A Dilemma?"
Journal Environmental Engineering Division, ASCE (December 1973).
15. Bauer, R.C. and Snoeyink, V.L., "Reactions of Chloramines with Active Carbon."
Journal Water Pollution Control Federation (November 1973).
16. Gulp, G.L. and Gulp, R.L., New Concepts in Water Purification. New York: Van
Nostrand Reinhold (1974).
17. Connell, G.F. and Fetch, J.J. "Advances in Handling Gas Chlorine," Journal Water
Pollution Control Federation, p. 1,505 (August 1969).
18. Cloromat. Ionics, brochure.
19. Pep-Chlor Systems. Pacific Engineering and Production Co. of Nevada, brochure.
20. Silvey, J.K.G., Abshire, R.L., and Nanez, W.J., II, "Bacteriology of Chlorinated and
Unchlorinated Wastewater Effluents." Journal Water Pollution Control Federation,
Vol. 46, no. 9 (September 1974).
21. Winn, C.S., Kirk, B.S.,and McNabney, R., Pilot Plant for Tertiary Treatment of Waste-
water with Ozone. U.S. EPA, Office of Research and -Monitoring, EPA-R2-73-146
(January 1973).
22. Echelberger, W.F., Pavoni, J.L., Singer, P.C., and Tenney, M.W., "Disinfection of Algal
Laden Waters." Journal Sanitary Engineering Division, ASCE (October 1971).
23. Nebel, Carl, et al, "Ozone Disinfection of Industrial-Municipal Secondary Effluents."
Journal Water Pollution Control Federation (December 1973).
24. McCarthy, J.J. and Smith, C.H., "A Review of Ozone and Its Application to Domestic
Wastewater Treatment. " Journal American Water Works Association (December 1974).
15-21
-------
25. Rosen, H.M., "Ozone Generation and Its Economical Application in Wastewater Treat-
ment." Water and Sewage Works (September 1974).
26. Rosen, H.M., Lowther, F.E., and Clark, R.G., "Get Ready for Ozone." Water and
Wastes Engineering (July 1974).
27. Rosen, H.M., "Use of Ozone and Oxygen in Advanced Wastewater Treatment. "Journal
Water Pollution Control Federation (December 1973).
15-22
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CHAPTER 16
OPERATION AND MAINTENANCE
Studies of wastewater treatment facilities have shown that any inadequacy in the design,
staff, organization, or operation and maintenance (O&M) has invariably led to a waste of
capital, manpower, and energy. Wise use of personnel according to a well-organized O&M
program can conserve these three essential resources and can result in optimum treatment
efficiency at a minimum total cost.
It is the responsibility of the design engineer of small plants to select processes and equip-
ment that will reduce the amount and the complexity of O&M procedures. Treatment
processes selected should minimize operator time and make laboratory testing consistent
with producing the required effluent quality. There is no point in including instrumentation
or equipment that will not be kept functioning. In fact, the small plant should be designed
to be as maintenance free as possible.
Because of perpetually tight budgets and minimal staffing arrangements, newly
demonstrated processes or equipment should be selected only if reliability has been shown
and a warranty of performance is stipulated. The design engineer should be sure that
adequate backup service and ready sources of spare parts for the equipment selected will
be available for the estimated service life of the equipment. The use of tried and proved
equipment should reduce the annual maintenance and repair bill and improve overall plant
operation.
Federal Guidelines on the Operation and Maintenance of Wastewater Treatment Facilities
(1) contains sections on government inspections, staffing, training, records, laboratory and
process control, safety, emergency operations, maintenance, O&M manuals, and financial
controls. The design engineers should carefully consider these guidelines in any operation or
maintenance planning.
The design engineer should, as a minimum, provide a small plant with sufficient capabilities
to:
1. Meet reliably effluent standards with minimum plant supervision.
2. Allow necessary alternative process adjustments to handle shock (or widely
varying) hydraulic, organic, or toxic loadings.
3. Safely divert the flow around malfunctioning units, while providing the minimum
necessary levels of treatment.
4. Secure the necessary assistance in times of emergency and quickly put back on-line
malfunctioning equipment or processes, such as chlorination or aeration.
5. Keep the facilities clean, neat, and odor free with a minimum expenditure of time.
6. Perform necessary preventive maintenance.
7. Allow performance of necessary analytical measurements for process control and
routine monitoring.
16-1
-------
16.1 Management and Organization
Management planning is required on the State, regional metropolitan, and local levels to
supervise, monitor, assist, and/or control the operation and maintenance of small
wastewater treatment works.
Management must provide: 1) personnel at the required level of education and/or training
for each required task: 2) equipment in functioning condition necessary to carry out the
work; 3) required O&M funds; 4) enforcement of sewer ordinances and pretreatment
requirements; and 5) coordination of these activities. Management must select competent
engineers and contractors to design and build the works and provide reviews and approvals
at each stage of their progress.
Operators of smaller wastewater treatment works usually require management assistance for
preparing and implementing an adequate organization plan. This organization plan should
insure an economy of effort, funds and manpower, while providing necessary manpower
backup services. Backup support services should include: 1) specialized electrical and
mechanical maintenance and repair; 2) needed advanced analytical laboratory services; and
3) consultation on process control. The most efficient management organization will vary
according to the location, size, and type of treatment works. Some States, such as
Massachusetts and Vermont, stipulate the number and function of personnel for different
sizes and types of treatment plants.
Support, first and last, is a management problem rather than a technical one. The manager,
probably working under a board of directors, must supervise the technical and clerical
personnel who actually carry out the policies and procedures that have been established. To
do this, the manager must understand the jobs they do and insure that all the pieces —
policies, procedures, personnel, and costs — fit together (2).
The way the wastewater system is viewed by management is also important. The system
should be administered as a business venture with ongoing responsibilities and opportunities,
rather than as a one-time achievement that moves in an unchanging manner once
established.
Although the full-time presence of personnel at smaller wastewater treatment plants is not
always necessary, daily visits to check plant operation, monitor performance, and perform
required maintenance must be made. In locations in which small wastewater treatment
plants are relatively close, one person usually can visit two or more plants each day to
perform the necessary tasks and collect samples. For safety, a one-person crew should
check into an office by phone on arrival and on departure from each plant. Two-person
crews are necessary for the periodic maintenance that would be unsafe for a single person.
In this situation, several one-person operations can be combined into a regional or metro-
politan arrangement for provision of backup needs. The Oakland County, Michigan, Public
Works Department provides operation services for about 33 smaller treatment plants.
The county is divided into five districts, and each plant is visited at least once each day by
one or more of the 16-person operating staff (3). The plants in the county have been
meeting required phosphorus removal as well as secondary treatment effluent standards (3).
16-2
-------
Backup services in this arrangement include: 1) advice and assistance in process control and
O&M; 2) specialized maintenance and repair of electrical and mechanical equipment; 3)
routine and specialized analyses of wastewater samples; 4) procurement of supplies; and 5)
financial administration. The Metropolitan Sewer District of Greater Cincinnati, Ohio,
performs a similar service in the operation of small wastewater treatment plants in the
district (3) (4).
Some sparsely inhabited states are divided into districts to provide these backup services to
local operators and to monitor performance (5). Because of substantial distances between
plants, these backup services can often be provided at only one or two plants per day. The
backup operation, maintenance, laboratory services, and monitoring may be supplied by
contractual agreements with private organizations, if such an agreement would be more
economical and efficient than municipal staffing. Organizations that might provide one or
more of the backup services include consulting engineers, commercial laboratories,
universities, and other government facilities.
Communication between the operating personnel and the design engineer, both before the
plant is designed and after it is in operation, is essential to effective plant operation.
To provide the basis for manpower planning, task factors or work elements (with their
corresponding personnel requirements) must be developed. Manpower planning, with
district, region, or State management for backup services for small wastewater treatment
plants, must be coordinated with present and future work space designs. Such planning must
include:
1. Identification of personnel capabilities
2. Identification of optimal operation tasks
3. Preparation of manpower specifications
4. Identification of training requirements
5. Development of task performance aids
6. Development of work performance evaluation criteria
7. Development of career development patterns
16.2 Factors Affecting Operation
All possible factors affecting efficient and satisfactory operation must be considered by
management when manpower and organization plans are developed (6). Table 16-1 lists
some of these factors and indicates which treatment processes might be affected by each.
Most small secondary wastewater treatment plants are absolutely dependent on the
functioning of an active population of microorganisms to meet effluent standards. The
design engineer should consider a means of obtaining an inoculum of active microbes for
quick startup of biological units initially or after unexpected shutdowns, particularly for
nitrification systems.
Each treatment facility, particularly advanced wastewater treatment plants, may have
difficulties not included in Table 16-1. Sludge and process sidestream problems are discussed
16-3
-------
TABLE 16-1
COMMON FACTORS AFFECTING OPERATIONS
Design and
Operational Factors
Affecting Effluent
Quality or the
Environment
Wrong Detention Time
Wrong Depth
Wrong Width to Length
Wrong Area
Inadequate Inlet Design
Inadequate Outlet Design
Inadequate Mixing
Short Circuiting
Wrong Sized Pumps
Wrong Sized Compressors
Inadequate Pretreatment
Shock Hydraulic Loads
Shock Organic Loads
Excessive Alkalinity (High pH)
Excessive acidity (Low pH)
Low Temperature
Wrong BOD Loading
Wrong MLVSS Loading
Wrong SVI
Wrong SRT
Septic Wastewater
Septic Sludge
Scum
Foam
Inadequate Oxygen Supply
Malfunctioning Aerators
Malfunctioning Diffusers
Malfunctioning Pumps
Malfunctioning Compressors
Dentrification of Sludge
«*
a so
£-S
^ a
-M e
4) §
£<2
X
X
X
X
X
X
X
X
X
Equalization
Tanks
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
Primary
Clarifier
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
Final
Clarifier
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
Activated
Sludge
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
g3
3 B
•* u
c£
Hfc
X
X
X
X
X
X
X
X
X
X
X
X
X
c-
3E
X
X
X
X
X
X
X
X
Granular
Media Filters
X
X
X
X
X
X
X
X
X
X
X
X
Oxidation
Ditches
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
Aerated
Ponds
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
Stabilization
Ponds
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
Disinfection
X
X
X
X
X
X
X
X
X
16-4
-------
Design and
Operational Factors
Affecting Effluent
Quality or the
Environment
Inadequate Monitoring
Clogging of Pipes and Valves
Excessive Grit
Excessive Variation in Flow
Excessive Variation in BOD
Inadequate Recirculation
Inadequate Backwashing
Inadequate Cleaning
Inadequate Flow Control
Inadequate Inspection
Inadequate Preventative
Maintenance
Sludge Bulking
<$
C/3
IB .c?
»i
**
X
X
X
X
X
X
G
o
03
N
II
WH
X
X
X
X
X
X
X
O^tS
.|'S
OnU
X
X
X
X
X
X
X
X
1— 1
C Cd
EC
X
X
X
X
X
X
X
X
•a
H
-+— >
•|^
§1
OS
X
X
X
X
X
X
X
X
o
^ r^
'S.-s
00
X
X
X
X
X
X
X
X
X
X
T3
2"§
-------
must encompass what each process is and does, how to control it, when to take
corrective action, and where to obtain competent aid in time of stress ....
Average man-hour requirements for the basic operation of secondary treatment plants,
other than stabilization ponds, are shown on Figure 16-1. The limit curves are based
primarily on data presented in reference (7), which indicate that a minimum of 3 hours per
day are required for operating and maintaining package plants. However, most permanent
plants will require at least several hours per day. A typical weekly O&M schedule for an
extended aeration treatment plant is shown in Table 16-2.
A manpower plan must provide for: 1) standby personnel for all types of emergencies and
repairs to essential equipment; 2) laboratory facilities and personnel for special analyses:
and 3) electrical and mechanical workshops where repairs can be made for each type of
equipment. Equipment and processes for small wastewater treatment facilities should be
designed for a work shift of 8 hours a day. A three-shift operation requires a minimum of
five operators, which represents a cost of about $150 to $200 per million gallons for a
1-mgd (3,785 m3/d) plant.
More complete listings of tasks to be accomplished by different personnel at various types
of treatment facilities are presented in references (8) and (9).
Training schools and special sources should be available to all operators and mechanics.
Technical and professional schools offer specialized courses for technicians. Onsite training
at the time of plant startup should be provided by equipment manufacturers' representatives
and design engineers. If the operator is to work part-time with other regular assignments,
on-the-job training is still required to insure that the operator's training is adequate.
Correctional training should be provided onsite by regional or state supervisory, control,
or backup personnel. Criteria for the establishment of training are available (10) (11).
Programs for either voluntary or mandatory certification of operators exist in most states.
The certificate is indicative of the knowledge, experience, and competency of the operator.
Certification programs give the operator improved status, greater flexibility in changing jobs,
and opportunity for higher wages. Certification usually provides benefits to the municipal
employer by increased efficiency in plant operation and maintenance, more reliable reports
and records, and more confidence in the operator's recommendations for repairs and
improvements. A typical certification program is described in reference (12).
Table 16-3 is a list of some nationally recognized schools and training programs for waste-
water treatment plant operators. The proceedings of a national conference on educational
systems for such operators at Clemson University contain thorough analyses of training
problems (13).
Initial and annual budgets must provide adequate funds for training. It must be assumed
that most people wish to advance, as they mature, to higher pay levels and added
responsibilities. The budget must take into consideration the training of replacements as a
regular occurrence.
16-6
-------
1.0
0.8
LEGEND
PRIMARY SETTLING,TRICKLING FILTER,
FINAL SETTLING a SLUDGE DIGESTER
A IMHOFF TANK,TRICKLING FILTERS FINAL SETTLING,^, _, _, A _,
COMPLETELY MIXED ACTIVATED SLUDGE, J^;^Yv:V>-?-rV: ?'
SETTLING a SLUDGE HANDLING
LIMIT OF
UPPER RANGE
LIMIT OF
LOWER RANGE
20 30 40
MAN-HOURS /WEEK/ PLANT
FIGURE 16-1
AVERAGE OPERATION TIME
-------
TABLE 16-2
TYPICAL EMPLOYEE PREFORMANCE MATRIX FOR
EXTENDED AERATION ACTIVATED SLUDGE PLANT (2)
Task
Skill
Time
hr
Weekly
Average
INITIAL OVERALL INSPECTION
a. Quick Visual Inspection
b. Check Maintenance Schedule
c. Record Maintenance Jobs
Operator II
(Most Skilled)
Operator II
Operator I
(Intermediate
Skill)
0.08/Day
0.08/2 Days
0.25/Week
0.56
0.30
0.25
CHECK AND MAINTAIN EQUIPMENT
AND TANKS
a. Maintain Inlet Area
Hand Cleaning of Screens
Removal/Disposal of Debris
Comminutor Cleaning
Comminutor Maintenance
Clean Inlet Area
b. Maintain Blower Equipment
Check Blower and Equipment
Clean Filter
Blower and Pump Maintenance
(Oil Change)
c. Clean Aeration Tank
Check, Scrape and Hose Down
Aeration Tank
Helper
(Least Skilled)
Helper
Operator I
Operator II
Helper
Operator I
Operator I
Operator II
0.75/week
0.50/Week
0.50/Week
0.12/Week
0.08/Day
0.04/Day
0.50/4 Weeks
0.50/8 Weeks
0.75
0.50
0.50
0.12
0.56
0.28
0.12
0.06
Helper
0.30/Week
0.50
d. Maintain Air and Return Equipment
Inspect Equipment Operator II
Clean Air Diffusers Helper
Operate Foam Equipment Operator I
Clean Foam Equipment Helper
Adjust Sludge Return Operator II
Clean Sludge Return Helper
Operate Skimmer Return Operator I
Clean Skimmer Return Helper
0.08/Day
0.50/2 Weeks
0.25/2 Weeks
0.50/4 Weeks
0.16/3 Days
2.00/4 Weeks
0.16/Week
0.50/4 Weeks
0.56
0.25
0.13
0.12
0.38
0.50
0.16
0.12
16-8
-------
Task
Skill
Time
hr
Weekly
Average
hr
CHECK AND MAINTAIN EQUIPMENT
AND TANKS (Continued)
e. Clean Clarifier
Clean Sidewalls, Weirs, and
Still Box
Scrape Clarifier Hopper
Helper
Helper
0.25 /Day
0.16/Day
1.75
1.12
f. Sludge Removal
Sludge Wasting
Disposal of Sludge
Clean Sludge System
Operator II
Opearator I
Helper
g. Chlorinator Maintenance
Inspect and Adjust Chlorinator Operator II
Clean Chlorinator and Feed Line Operator I
Refill Chlorinator System Operator I
1.00/Week
2.00/Week
0.50/Week
0.08/Day
0.50/2 Weeks
0.50/2 Weeks
1.00
2.00
0.50
0.56
0.25
0.25
Clean Decks, Weirs, and Troughs
Clean and Store Maintenance
Equipment
PERFORM TESTS AND MAINTAIN
OPERATIONAL LOG
a. Influent Characteristics
b. Aeration Characteristics
c. Clarifier Characteristics
d. Effluent Characteristics
e. 30-Minute Settleability Test
f. DO Test
g. pH Test
h. Chlorine Residual Test
i. BOD5 Test
j. Suspended Solids Test
k. Daily Flow
1. Other Recordings
m. Maintain Books and Test Site
(Room), Other Test Preparation
MAKE OPERATIONAL ADJUSTMENTS
a. Remedial Measures — Other
Helper
Helper
Operator I
Operator II
Operator II
Operator I
Operator II
Operator II
Operator I
Opearator I
Operator II
Operator II
Operator I
Operator I
Operator II
Operator II
16-9
0.50/Day
0.50/Day
3.50
3.50
0.02/Day
0.08/Day
0.02/Day
0.02/Day
0.16/Day
0.16/Day
0.08/Day
0.08/Day
0.40/Week
1.00/Week
0.08/Day
0.16/Day
0.50/Day
1.00/Week
0.14
0.56
0.14
0.14
1.12
1.12
0.56
0.56
0.40
1.00
0.56
1.12
3.50
1.00
-------
Task
FINAL AND PERIODIC OPERATION
a. Maintain Control System
b. Clean Up Plant Site
c. Outside Contancts and Other
Maintenance
TOTALS
Skill
Operator II
Helper
Operator II
Helper
Operator I
Operator II
Time
hr
1.00/Month
4.00/Week
0.16/Day
17.67hr/Week
7.01 hr/Week
13.75hr/Week
38.43 hr/Week
Weekly
Average
hr
0.25
4.00
1.12
TABLE 16-3
PARTIAL LISTING OF
SCHOOLS AND TRAINING PROGRAMS FOR PLANT OPERATORS
Charles County Community College, Maryland - 2-year Associate's Degree and several
specialized short courses.
Contact: Director
Pollution Abatement Technology Department
Charles County Community College
Box 910
LaPlata, Maryland 20646
Telephone-(301)934-2251
Southern Maine Vocational Technical Institute, Maine - 9-month specialized waste-
water operation and several short courses and mobile training unit.
Contact: Chairman
Wastewater Technology Department
Southern Maine Vocational Technical Institute
One Vocational Drive
South Portland, Maine 04106
Telephone - (207) 799-7303
16-10
-------
3. Syracuse University, New York - several 2-week short courses on specialized waste-
water topics.
Contact: Civil Engineering Dept. Environmental Manpower Div.
Hinds Hall or NYS Dept. of Environmental
Syracuse University Conservation
Syracuse, New York 13210 50 Wolfe Road
Telephone - (315) 432-2311 Albany, New York, 12201
Telephone-(518) 457-6610
4. Lowell Technological Institute, Massachusets - several courses of varying length -
both day and evening classes available.
Contact: Division of Water Pollution Control
Training Section
Lowell Technological Institute
Lowell, Massachusetts 01854
Telephone-(617) 459-7193
5. State University of California - correspondence course on basic wastewater operation.
Contact: Dept. of Civil Engineering
State University of California at Sacramento
Sacramento, California 59819
Telephone - (916) 454-6982
6. Water & Sewerage Technical School, Missouri - several wastewater treatment plant
operator's courses for different types of wastewater treatment.
Contact: Water & Sewerage Technical School
Box 348
Neosho, Missouri 64850
Telephone - (417) 451-2786
7. Clemson University, South Carolina - correspondence course manual for wastewater
plant operators and individual courses for different grades.
Contact: Clemson University
Clemson, South Carolina 29631
Telephone - (803) 656-3311
8. University of Colorado, Colorado — Fundamentals (of Treatment) course, instructor's
papers.
Contact: Water and Wastewater Plant Operator's School
University of Colorado
Boulder, Colorado 80302
16-11
-------
Training should be limited to those operations or tasks that will be expected of the trainee
within a short time. Otherwise, trainees should be given refresher training before being
entrusted with an unfamiliar task. Recent trainees should be frequently checked, because
most of them will need some help to adapt techniques to unfamiliar conditions, until they
have had several months on the task.
16.4 Operation and Maintenance (O&M) Manuals
Title II of PL 92-500 authorizes the award of construction grants for wastewater treatment
works. One additional provision, stipulated in the Federal Register of 11 February 1974,
is that:
(a) The grantee must make adequate provisions satisfactory to the Regional Adminis-
trator for assuring economic, effective, and efficient operation and maintenance of
such works in accordance with a plan of operation approved by the State water
pollution control agency or, as appropriate, the interstate agency, after construction
thereof.
(b) As a minimum, such plan shall include provision for: 1) an operation and mainten-
ance manual for each facility, 2) an emergency operating and response program, 3)
properly trained management, operation and maintenance personnel, 4) adequate
budget for operation and maintenance, 5) operational reports, and 6) provisions for
laboratory testing adequate to determine influent and effluent characteristics and
removal efficiencies.
(c) The Regional Administrator shall not pay 1) more than 50 percent of the Federal
share of any Step 3 (construction) project unless the grantee has furnished a draft of
the operation and maintenance manual for review, or adequate evidence of timely
development of such a draft, or 2) more than 90 percent of the Federal share unless
the grantee has furnished a satisfactory final operation and maintenance manual.
The U.S. EPA has prepared guidelines and other criteria for the preparation of O&M
manuals (14) (15) (16). Within these criteria, an O&M manual must be prepared to meet the
unique requirements of each specific plant as defined by the qualifications of the personnel,
the complexity of the equipment, and the amount of work that can be done inhouse by
plant staff.
O&M manuals should be as brief as possible, with well-illustrated, step-by-step descriptions
of: 1) the normal and emergency operation of each process system and subsystem, and 2)
regular maintenance and possible emergency operation and repair procedures. The prepara-
tion of good O&M manuals requires the input of persons knowledgeable in: the probable
capability of operators; the design and operation of the specific plant; process control;
available backup services (i.e., replacement parts, repair shops, and backup laboratories);
training procedures; plant startup; state and federal requirements; sampling and analytical
procedures; mechanical and electrical maintenance; safety procedures; and record keeping.
Diagrams, charts, tables, and pictures should be used to a maximum. Schematic flow
diagrams and isometric drawings should be used in describing systems and subsystems.
Typical drawings that may be used are represented in Figures 16-2, 16-3, and 16-4. Often
these drawings may be taken directly from construction drawings.
16-12
-------
TO FLOTATION
THICKENER!
rTO AERATION TANKS
ON
6"C.I.(GLASS LINED)
FROM FINAL
CLARIFIER NO.I
I FROM FINAL
CLARIFIER NO. 2
LEGEND
PLUG VALVE
CHECK VALVE
REDUCER
MAGNETIC
FLOW METER
FIGURE 16-2
SECONDARY SLUDGE AND SCUM PIPING SYSTEM
(NORMAL WASH AND SCUM PUMP OPERATIONS)
-------
FIGURE 16-3
CHECK VALVE
GATE VALVE
BALL VALVE
PRESSURE GAUGE
PRESSURE RELIEF
VALVE
VALVE NUMBER
INJECTOR WATER SYSTEM
-------
MUNICIPAL AND INDUSTRIAL
WASTEWATER FLOW
— SLUDGE FLOW
FIGURE 16-4
PROCESS FLOW PATH
-------
A reference library should accompany the O&M manual. Suggested reference books are
listed in Table 16-4. These would vary, depending on specific processes and treatment
systems employed. The manufactuers' manuals should be assembled by systems; i.e.,
chlorination, sludge and scum, aeration, chemical feeds, electrical, etc., bound in separate
volumes, labeled, page numbered, and indexed for easy reference. The author of the O&M
manual can then reference specific pages in the reference books or manufacturers' manuals,
rather than prepare detailed descriptions for insertion in the O&M manual.
TABLE 16-4
POSSIBLE REFERENCES FOR AN OPERATOR'S LIBRARY1
1. Standard Methods for the Examination of Water and Wastewater, APHA-AWWA-
WPCF(1971).
2. Water. Annual ASTM standards, part 31 (1974).
3. Manual of Methods for Chemical Analysis of Water and Wastes. U.S. EPA, Office of
Technology Transfer (1974).
4. Manual of Instruction for Sewage Treatment Plant Operation. New York State Depart-
ment of Health, Health Education Service (Albany, N.Y.)
5. Manual of Wastewater Operations; The Texas Water Utility Assoc., Texas State Depart-
ment of Health, Austin, Tex. (1971).
6. Operation of Wastewater Treatment Plants. Sacramento State College California
WPCA, and U.S. EPA, Office of Water Programs (1970).
7. Correspondence Course Manual for Wastewater Plant Operators. Columbia, S.C. (1969).
8. "Safety in Wastewater Works." WPCF Manual of Practice No. 1. Washington D.C.
(1969).
9. "Chlorination of Sewage and Industrial Wastes." WPCF Manual of Practice No. 4
(1951).
10. "Aeration in Wastewater Treatment." WPCF Manual of Practice No. 5 (1971).
11. "Sewer Maintenance." WPCF Manual of Practice No. 7 (1966).
12. "Uniform System of Accounts for Wastewater Utilities." WPCF Manual of Practice
No. JO (1961).
13. "Operation of Wastewater Treatment Plants." WPCF Manual of Practice No. 11 (1970).
14. "Public Relations for Water Pollution Control." WPCF Manual of Practice No. 12
(1965).
16-16
-------
15. "Wastewater Treatment Plant Operator Training Course One" (without visual aids).
WPCF Manual of Practice No. 13 (1966).
16. "Wastewater Treatment Plant Operator Training Course Two" (without visual aids).
WPCF Manual of Practice No. 14 (1967).
17. "Paints and Protective Coatings for Wastewater Treatment Facilities." WPCF Manual
of Practice No. 17 (1969).
18. Handbook for Monitoring Industrial Wastewater. U.S. EPA, Office of Technology
Transfer (August 1973).
best selections will vary, depending on needs of specific treatment works.
The unit process system and subsystem descriptions should include most, if not all, of the
items shown in Table 16-5. If an item is adequately described in one of the references, only
essential or special information should be included in the O&M manual on that item. The
reference should be identified by volume and page for easy and quick availability to the
operator.
TABLE 16-5
DESIRED DETAIL ON EACH PROCESS SYSTEM IN O&M MANUAL
Purpose:
Simple Statement of Purpose.
Description:
Utilizing a Schematic or Isometric Sketch, Describe the Elements of the Process
System or Subsystem.
Design Criteria :
(1) Unit Sizes and Capacities
(2) Hydraulic and Organic Loadings
(3) Detention Times.
Flow Control (Regulation and Distribution of Wastewater):
(1) Manual/Automatic
(2) Weir Settings
(3) Pump Speed Settings
16-17
-------
(4) Gate Openings
(5) Maximum Water Elevation and Ranges
(6) Bypassing the System or Subsystem.
Process Control and Performance Evaluation:
(1) Equipment Controls: Manual, Automatic, and Special Instrumentation
(2) Performance Evaluation: Expected Performance Range, Laboratory
Evaluation, and Visual Evaluation
(3) Process Troubleshooting Guide
Definition of Problems (Cause)
Effect on Process
Possible Remedies
Remedy Reference.
Normal Operation:
(1) Normally On-line Systems
(2) Normal Control/Instrumentation Settings
Weir Settings
Wet Well Levels
RPM Settings
High-Low Speed, etc.
(3) Startup and Shutdown Procedures.
Emergency Operations and Safety Considerations:
(1) Unusual Conditions Specific to Unit Process.
(2) Emergency Operation Procedures.
16.5 Monitoring
Federal and State effluent and receiving water quality standards require that certain pa-
rameters of small wastewater treatment plant effluents be monitored (17). The specific
regulations stipulate which characteristics are to be monitored, where, and how often. The
U.S. EPA secondary treatment effluent standards require regular BOD5, SS, and coliform
tests as a minimum. More stringent requirements than are in the EPA definition of
secondary treatment are sometimes needed to meet regulatory standards in different areas
(18).
For small wastewater treatment plants with flows less than 1 mgd, the minimum sampling
frequency may be a monthly test of the plant effluent for BODs (mg/1), SS (mg/1),
settleable solids (mg/1), pH, residual chlorine (mg/1), and fecal coliform (MPN per 100 ml)
on a weekday (not Saturday, Sunday, or a holiday) when flows are being measured. The
samples tested should be 8-hour composites, except for the chlorine residual, which should
be a grab sample (19).
16-18
-------
Additional sampling and testing may be required for process control at specific plants, and
to obtain data for future upgrading design (20). A sampling program established for each
wastewater treatment plant must include: 1) location of sample; 2) analyses to be made; 3)
information as to whether the samples to be tested are to be based on grab or 2-, 8-, or 24-
hour composites; and 4) information as to whether the composite is to be based on samples
taken at 15-minute, 30-minute, or 1-hour intervals. Table 16-6 shows a typical sampling and
analysis schedule for an aerated, facultative lagoon treatment plant in which the pond
effluent passes through a slow sand filter, is chlorinated before discharge, and must meet
stringent stream water quality standards.
Manholes, sampling ports, or taps should be provided to secure samples from all process
and plant influents and effluents. The interprocess connections also should be designed so
that all interprocess flows can be sampled and measured for control purposes. In other
words, monitoring requires knowledge of the flow rate at the time each sample is taken.
Monitoring of a wastewater treatment plant might require at least periodic measurement of
several of the following: DO, pH, temperature, TKN, HN3-N and NC^-N, phosphates,
nitrifier-inhibited BOD (for 2-, 3-, 5-, and 7-day incubations), oxidation-reduction potential,
combined chlorine, free chlorine, settleable solids, total SS, turbidity, alkalinity, TOC, COD,
insolation, wind velocities and directions, precipitation and evaporation, in addition to plant
effluent 8005, coliform, and SS levels. Many of these parameters can be monitored auto-
matically. Decisions as to which characteristics are essential for satisfactory plant operation,
or are required for control, should be made during the design stages before the plant is
constructed. The selection of a monitoring plan is discussed in reference (21).
16.6 Laboratory Facilities
Recommended laboratory facilities are generally discussed in references (19) and (22).
Municipal wastewater treatment plant laboratories, however, must be specifically tailored
for each individual installation.
If a backup laboratory is not easily available, the facility should include, as a minimum,
laboratory testing facilities to analyze wastewater samples for BODs, SS, settleable solids,
pH, residual chlorine, and fecal coliforms. Even part-time operators should be taught how to
run these tests and be expected to perform them regularly.
The document Estimating Laboratory Needs for Municipal Wastewater Treatment Facilities
(19) states that the laboratory space required will normally be no less than 150 ft2 (14m2)
for trickling filters and treatment ponds, or 180 ft2 (17m2) minimum for activated sludge,
physical-chemical, or AWT plants. The equipment needed for process and effluent control at
plants smaller than 1 mgd is listed in Table 16-7. This equipment must be compatible with
processes selected and the monitoring program. Additional items, other than those listed in
Table 16-7, may be needed for some plants.
The Enforcement Division of EPA region VII (23), prepared a list of laboratory equipment
required to conduct satisfactorily the analyses of wastewater samples for BOD, pH, SS, and
fecal coliform. This equipment list is presented in Table 16-8.
16-19
-------
TABLE 16-6
TYPICAL SAMPLING AND TESTING PROGRAM (7)
SETTLEABLE SOLIDS
SUSPENDED SOLIDS
VOLATILE SUSPENDED SOLIDS
TOTAL DISSOLVED SOLIDS
TOTAL VOLATILE
DISSOLVED SOLIDS
TOTAL SOLIDS
BOD,
DISSOLVED OXYGEN
GREASE
RESIDUAL CHLORINE
COLIFORM ORGANISMS
FLOW
pH
WASTEWATER TEMPERATURE
LEGEND
TYPE OF SAMPLE
C - COMPOSITE SAMPLE
G - GRAB SAMPLE
CB- 8 HOUR COMPOSITE
OF SAMPLES TAKEN
EACH 2 HOURS
FREQUENCY OF SAMPLE
D - DAILY
W - WEEKLY (every 8 days)
M - MONTHLY (every 32 days)
Q - QUARTERLY (Jan, Apr, Jul.Oct )
16-20
-------
TABLE 16-7
MINIMUM LABORATORY EQUIPMENT NEEDS FOR TYPICAL 1-MGD
OR SMALLER WASTEWATER TREATMENT PLANTS WHERE BACKUP
LABORATORY FACILITIES ARE NOT EASILY AVAILABLE
Analytical Balance
Bookcase
Centrifuge
Chlorine Residual Analyzer and Recorder
Eye Wash
File Cabinet
Flow Meter With Totalizer/Indicator
Fume Hood
Hot Plate
Incubator (BOD)
Incubator (microbiological)
Lab Stool
Microscope
Muffle Furnace
Oven pH Meter
Pump (vacuum-pressure)
Refrigerator
Safety Shower
Sterilizer
Still
Thermometers (registering)
Turbidimeter
16-21
-------
TABLE 16-8
LABORATORY EQUIPMENT LIST FOR MONITORING (23)
Description Quantity
BIOCHEMICAL OXYGEN DEMAND (BOD)
Balance, Analytical, Mettler H31 * 1
Beaker, 250 ml 4
Bottle, BOD, 300 ml 12
Bottle, Polyethylene, 8 oz 4
Bottle, Weighing, 30 ml 2
Buret, 25 ml 2
Cylinder, Graduated, 10 ml 2
Cylinder, Graduated, 100 ml 2
Cylinder, Graduated, 1000 ml 2
Flask, Volumetric, 1000 ml 2
Incubator, BOD 1
Oven, Drying 1
Pipet, Measuring, 1 ml 2
Pipet, Volumetric, 5 ml 2
Stirring Rods, Glass 12
Support, Double Burette 1
Tygon Tubing, 1/4 in. by 1/16 in. 10ft
REAGENTS
Calcium Chloride Solution, 2.75% 32 oz
Dextrose Reagent 1 Ib
Ferric Chloride Solution, 0.025% 32 oz
Glutamic Acid 100 gm
Magnesium Sulfate Solution, 2.25% 32 oz
Phosphate Buffer Solution, pH 7.2 32 oz
Potassium Iodide Solution, 10% 32 oz
Sodium Hydroxide Solution, IN 32 oz
Sodium Sulfite Reagent 1 Ib
Starch Solution 16 oz
Sulfuric Acid Reagent, Concentrated 9 Ib
Sulfuric Acid Solution, IN 32 oz
PH VALUE
pH meter, Corning Mo del 7 * 1
TOTAL SUSPENDED MATTER (nonfilterable residue)
Balance, Analytical, Mettler H31 1
Bottle, Wash, 250ml 1
Cylinder, Graduated, 100 ml 2
16-22
-------
Description
Quantity
TOTAL SUSPENDED MATTER (nonfilterable residue) (contd.)
Desiccator, 250 mm
Filter Disks, Glass Fiber, 55 mm
Filter Pump
Flask, Filtering, 500 ml
Forceps
Funnel, Buechner, Plate Diameter = 56 mm
Oven, Drying
Rubber Stopper, 1 hole, No. 7
Rubber Tubing, 1/4 in. by 3/16 in.
Silica Gel, Indicating
FECAL COLIFORM MEMBRANE FILTER PROCEDURE
Autoclave
Autoclave, Pressure Control
Balance, Triple Bean
Bottle, Water Sample, 125 ml
Burner, Tirrill
Cylinder, Graduated, 100 ml
Cylinder, Graduated, 500 ml
Dishes, Petri, 60 by 15 mm, 500/case
Distillation Apparatus, Glass or Water
Demineralizer with Cartridge
Filter Funnel Assembly
Filter Pump
Flask, Erlenmeyer, 125 ml, with Screw Cap
Flask, Erlenmeyer, 500 ml, with Screw Cap
Flask, Filtering, 1000ml
Forceps
Hot Plate
Membrane Filters, 47 mm Diameter, 0.45
Micron Pore Size
Paper, Weighing
Pipet, Serological, 2 ml
Refrigerator
Rubber Stopper, 1 Hole, No. 8
Rubber Tubing 1/4 in. by 1/16 in.
Rubber Tubing 1/4 in. by 3/16 in.
Spatula, 8 in.
Sterilizer, Hot Air (Optional)
Water Bath (±0.2° C)
Water Bath Gable Cover
M-FC Broth
1
1 box
1
2
1
1
1
4
4ft
1-1/2 lb
1
1
1
8
1
2
2
1 case
1
4
2
2
1
1 box
1 package
2
1
4
4ft
4ft
1
1
1
1
1/4 lb
or equivalent
16-23
-------
16.7 Workshop Facilities
It is important that the downtime for repairs be kept to a minimum. If backup workshop
facilities are not readily available, a larger than minimum workshop should be included at
the plant and provision made for sharing it with other community or government agencies.
Some of the tools and equipment that might be considered for different types of plant
workshops, or made available locally, are:
Hand Tools Yard Tools
Hammer Rake
Hand saw(s) Axe
Pliers Pick
Wrenches Shovels
Screwdrivers Hoe
Wheelbarrow
Lawn mower
Sledge hammer
Shop Equipment Miscellaneous Equipment
Work bench with vise Portable pump
Power drill and bits Folding tripod with chain hoist
Tapping machine Chain saw
Soldering irons Portable heater
Grease guns Sewer rods and other pipe cleaners
Power grinder Lathe
Blow torch Milling machine
Paint gun Welding machine
Micrometer Compressor
Ammeter-voltmeter Sewer rods and other pipe cleaners
The workshop must be a dry place and must be able to store not only commonly used tools
and equipment but also rented equipment or tools used in emergencies. There should be a
labeled place for everything, and the operator should keep everything in its place, when it is
not in active use.
Some locked cabinets should be provided, particularly for smaller tools. Spare parts,
especially those easily broken, that wear out often, or are critical to continuous operation,
should also be kept in the workshop.
Preventing potential vandalism should be considered in the design of all facilities. Chain link
fences should surround the site. If feasible, the fence should be far enough from open tanks
and breakable equipment to prevent damage from thrown rocks. In some areas, unattended
buildings should be windowless and equipment enclosures bulletproof; it has been found,
for instance, that chain link fences may incite trespassers and vandals, so unattended build-
ings should be made of brick without windows and with roofs which are not flammable or
easily damaged.
16-24
-------
16.8 Safety
Much has been written already on safety considerations at wastewater treatment plants.
Two particularly helpful publications are the U.S. EPA background report, Safety in the
Design, Operation, and Maintenance of Wastewater Treatment Works (24) and the WPCF
Manual of Practice No. 1, "Safety in Wastewater Works" (25).
16.9 References
1. Federal Guidelines on Operation and Maintenance of Wastewater Treatment Facilities.
U.S. EPA, Office of Water and Hazardous Materials (August 1974).
2. Guide for the Support of Rural Water-Wastewater Systems. Commission on Rural
Water, Chicago (1974).
3. Personal Communications with Oakland County Public Works (1974).
4. Seymour, G.G., "Operation and Performance of Package Treatment Plants." Journal
Water Pollution Control Federation (February 1972).
5. Lamp, G.E., Bauman, E.R., McRoberts, K.L., and Smith, C.E., Estimating Staffing and
Cost Factors for Small Wastewater Treatment Plants Less Than 1 MGD, Part II. U.S.
EPA, Office of Water Program Operations (June 1973).
6. "Operation of Wastewater Treatment Plants." WPCF Manual ofPracticeNo. 11 (1970).
7. Lampe, G.E., Baumann, E.R., McRoberts, K.L., and Smith, C.E., Staffing Guidelines
for Conventional Municipal Wastewater Treatment Plants Less than 1 MGD. Engineer-
ing Research Institute, Iowa State University (June 1973).
8. Isaacs, P.C.G., The Use of Package Plants for Treatment ofWastewaters (June 1964).
9. Estimating Staffing for Municipal Wastewater Treatment Facilities. U.S. EPA, Office of
Water Program Operations (March 1973).
10. Criteria for the Establishment and Maintenance of Two Year Post High School Waste-
water Technology Training Programs. Clemson University and U.S. EPA, Division of
Manpower and Training, Vols. 1 and 2 (1970).
11. Criteria for the Establishment and Maintenance of Two Year Post High School Waste-
water Technology Training Programs — Trainee Workbooks. Clemson University and
U.S. EPA Division of Manpower and Training (August 1973).
12. Rules and Regulations for Certification of Operators of Wastewater Treatment
Facilities. Massachusetts Board of Certification of Operators of Wastewater Treatment
Facilities, Boston (1973).
16-25
-------
13. Educational Systems for Operators of Water Pollution Control Facilities. Proceedings,
Clemson University (November 1969).
14. Considerations for Preparation of Operation and Maintenance Manuals U.S. EPA,
Office of Water Program Operations (1974).
15. Maintenance Management Systems for Municipal Wastewater Treatment Works. U.S.
EPA, Office of Water Program Operations (1973).
16. Guide to the Preparation of Operational Plans for Sewage Treatment Facilities. EPA-
R2-73-263 (July 1973).
17. Handbook for Monitoring Industrial Wastewater. U.S. EPA, Office of Technology
Transfer (August 1973).
18. Blakely, C.P., and Thompson, I.W., "Pollution Control and Energy Conservation: Are
They Compatible?" WPCF Deeds and Data (December 1974).
19. Estimating Laboratory Needs for Municipal Wastewater Treatment Facilities. U.S.
EPA, Office of Water Program Operations, EPA-430/9-74-002 (June 1973).
20. Ingols, R.S., and Morriss, R.H., "Control Monitoring for the Activated Sludge Process."
WPCF Deeds and Data (August 1973).
21. Roesler, J.F., Factors to Consider in the Selection of a Control Strategy. EPA,
Cincinnati (May 1972).
22. Handbook for Analytical Quality Control. U.S. EPA, Office of Technology Transfer
(June 1972).
23. Information Packet on Monitoring for Discharge Permit Compliance. U.S. EPA, Region
VII, Enforcement Division, Compliance Branch, Kansas City, Mo. (July 1974).
24. Hanlon J., and Saxon, T., Technical Report on Safety in the Design, Operation, and
Maintenance of Wastewater Treatment Works. U.S. EPA, Office of Water Program
Operations (1975).
25. "Safety in Wastewater Works." WPCF Manual of Practice No. 1 (1969).
16-26
-------
CHAPTER 17
COST-EFFECTIVENESS
17.1 Background
The objective of cost-effectiveness planning is the minimization of costs (1). In implement-
ing a plan to achieve approved water quality standards, a community (sometimes assisted
by Federal and State funds) will incur capital costs for construction and ongoing costs for
administration, management, operation, and maintenance. To be cost effective, the plan
must minimize the total cost of pollution control to the public and natural resources. A
cost-effective plan for a community must consider regional development as well as national,
state, and community plans for land use, industry, energy, water supply, and transportation.
Each of these plans will contain basic requirements that must be met by a recommended
facility. Alternative facility plans that will meet the basic requirements must be developed
and compared, to determine which plant best meets the total requirements at the lowest
cost.
Billions of dollars will be needed over the next decade for wastewater treatment, and
wastewater treatment is only part of the total municipal capital works that will be needed in
this period. With spending of this magnitude, all areas of planning and management of such
facilities must be carefully organized, to achieve the highest possible levels of cost effective-
ness.
17.2 Cost-Effectiveness Analysis Regulations
In October 1973, Cost-Effectiveness Analysis Guidelines, authorized under Section 212
(2) (c) of the Federal Water Pollution Control Act, Public Law 92-500, became effective
(2). Important requirements of these guidelines include:
1. Planning Period: 20 Years
2. Cost Elements:
a. Contractors, including overhead and profit
b. Land, including relocation
c. Engineering, including design, field exploration, and services during
construction and plant startup
d. Administrative and legal, including costs of bond sales
e. Startup and operator training
f. Interest during construction
g. Operation and maintenance, divided between fixed and variable with
wastewater flow
3. Prices: those prevailing at time of study without considering inflation, unless all
prices will not rise at same rate
4. Interest (Discount) Rate: 6.125 percent per year until 30 June 1975 (see Table
17-1).
17-1
-------
TABLE 17.1
6.125% COMPOUND INTEREST FACTORS
Single Payment
Uniform Series
Years
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
CAP
F/P
1.0612
1.1262
1.1952
1.2684
1.3461
1.4285
1.5160
1 .6089
1.7074
1.8120
1.9230
2.0408
2.1658
2.2985
2.4393
2.5887
2.7472
2.9155
3.0941
3.2836
3.4847
3.6981
3.9247
4.1650
4.4202
4.6909
4.9782
5.2831
5.6067
5.9501
6.3146
6.7014
PWF
P/F
0.9422
0.8879
0.8366
0.7883
0.7428
0.6999
0.6595
0.6215
0.5856
0.5518
0.5200
0.4899
0.4617
0.4350
0.4099
0.3862
0.3639
0.3429
0.3231
0.3045
0.2869
0.2704
0.2547
0.2400
0.2262
0.2131
0.2008
0.1892
0.1783
0.1680
0.1583
0.1492
SFF
A/F
1.00000
0.48514
0.31372
0.22816
0.17695
0.14291
0.11868
0.10058
0.08657
0.07542
0.06635
0.05884
0.05253
0.04716
0.04255
0.03855
0.03505
0.03197
0.02924
0.02682
0.02465
0.02270
0.02094
0.01935
0.01790
0.01659
0.01539
0.01430
0.01329
0.01237
0.01152
0.01074
CRF
A/P
1.06125
0.54639
0.37497
0.28941
0.23820
0.20416
0.17993
0.16183
0.14782
0.13667
0.12760
0.12009
0.11378
0.10841
0.10380
0.09980
0.09630
0.09322
0.09049
0.08807
0.08590
0.08395
0.08219
0.08060
0.07915
0.07784
0.07664
0.07555
0.07454
0.07362
0.07277
0.07199
CAF
F/A
0.999
2.061
3.187
4.382
5.651
6.997
8.425
9.941
11.550
13.258
15.070
16.993
19.034
21.200
23.498
25.938
28.526
31.274
34.189
37.283
40.567
44.052
47.750
51.675
55.840
60.260
64.951
69.929
75.212
80.819
86.769
93.084
PWF
P/A
0.942
1.830
2.666
3.455
4.198
4.898
5.557
6.179
6.764
7.316
7.836
8.326
8.788
9.223
9.633
10.019
10.383
10.726
11.049
11.354
11.641
11.911
12.166
12.406
12.632
12.846
13.046
13.236
13.414
13.582
13.741
13.890
17-2
-------
TABLE 17.1 (Cont.)
6.125% COMPOUND INTEREST FACTORS
Single Payment
Uniform Series
Years
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
CAP
F/P
7.1118
7.5474
8.0097
8.5003
9.0210
9.5735
10.1599
10.7822
11.4426
12.1434
12.8872
13.6766
14.5143
15.4033
16.3467
17.3480
18.4105
19.5382
PWF
P/A
0.1406
0.1324
0.1248
0.1176
0.1108
0.1044
0.0984
0.0927
0.0873
0.0823
0.0775
0.0731
0.0688
0.0649
0.0611
0.0576
0.0543
0.0511
SFF
A/F
0.01002
0.00935
0.00873
0.00816
0.00763
0.00714
0.00668
0.00626
0.00586
0.00549
0.00515
0.00483
0.00453
0.00425
0.00399
0.00374
0.00351
0.00330
CRF
A/P
0.07127
0.07060
0.06998
0.06941
0.06888
0.06839
0.06793
0.06751
0.06711
0.06674
0.06640
0.06608
0.06578
0.06550
0.06524
0.06499
0.06476
0.66455
CAP
F/A
99.785
106.897
114.445
122.454
130.955.
139.976
149.549
159.709
170.491
181.934
194.078
206.965
220.641
235.156
250.559
266.906
284.254
302.664
PWF
P/A
14.030
14.163
14.288
14.405
14.516
14.621
14.719
14.812
14.899
14.982
15.059
15.132
15.201
15.266
15.327
15.385
15.439
15.490
Where:
CAP = Compound amount factor = (1 + i)n = $1 at compound interest
PWF = Present worth factor = 1/(1 + i)n = Present value of $1 due in future
SFF = Sinking fund factor = 1/(1 + l)n - 1 = Periodic deposit to obtain future $1
CRF = Capital recovery factor = i(l + i)n /(I + i)n - 1 = Period payment on $1 loan
USCAF = Uniform series CAP = (1 + i)n - 1/i = How $1 deposited periodically grows
USPWF = Uniform series PWF = (1 + i)n - 1/i (1+ i)n = Present value of $1 payable
periodically
F = Compound amount or future value
P = Initial investment or single Payment
A = Annual investment or periodic payment
17-3
-------
5. Service life:
a. Land, permanent
b. Structures, 30 to 50 years
c. Process equipment, 15 to 30 years
d. Auxiliary equipment, 10 to 15 years
6. Salvage Value:
a. Land, full value
b. Structures, specific market or reuse value at end of design period, estimated
using straight-line depreciation
17.3 Energy Conservation
Energy conservation must be as carefully considered as cost effectiveness and environmental
impacts in choosing the best alternative. A report entitled Electrical Power Consumption for
Wastewater Treatment (3) contains general information on energy costs for various typical
treatment processes. Additional information on power requirements is presented in chapters
9 and 16 of this manual.
The costs of gasoline, oil, and natural gas are expected to remain relatively high, making
other energy sources more competitive. Therefore, the present worth of alternative equip-
ment employing different energy sources should be evaluated very carefully.
17.4 Methodology
The literature describes many methods of cost-effectiveness analyses; in addition, state or
regional authorities may have cost-effectiveness analyses guidelines. The primary aspects to
be considered in seleecting viable alternatives for a cost-effectiveness study include:
1. The requirements listed in the U.S. EPA Cost-effectiveness Analysis Guidelines (2)
2. Minimum quality criteria for the receiving water or land as established by Federal,
State, and regional governments
3. Effluent quality requirements
4. The possibility of some form of land disposal (usually considered by the EPA an
alternative to be studied)
5. Environmental compatability requirements, as established in the National
Environmental Policy Act of 1969
6. Area and regional master plans
7. Financial Capabilities of the community to meet initial and annual wastewater
costs
8. Available energy resources, both short and long term
9. Local capabilities for operation and maintenance of wastewater facilities
10. The habits, attitudes, and social patterns of the residents of the community
The short- and long-term effects on the community and region of not building or of phased
construction should also be considered. Building facilities in stages is sometimes the only
feasible alternative.
17-4
-------
In studying alternatives, the designer should include vulnerability studies and costs of stand-
by emergency facilities for each feasible alternative. Five steps in reducing system vulner-
ability include:
1. List disastrous events that could strike the facility or locality.
2. Examine the system's vulnerability to each type of possibly disastrous event.
3. Determine critical components and design protective measures.
4. Determine the cost of protection versus the cost of loss of service of the system.
5. Recommend the appropriate actions.
Some of the areas benefitting from clean water include recreation, aesthetics, community
water supplies, fish and other aquatic life, wildlife, agriculture, industry, channel and ship
maintenance, corrosion control, and commercial fishing. Some of the costs of a wastewater
treatment facility to a community include damages caused by odors, noise, aerosols, and
damage to aesthetics. The added cost of new technology to make each alternative partially
or wholly environmentally compatible should be included in any analysis.
If interest rates are high, phased construction to minimize unused capacity must be care-
fully examined in any cost-effectiveness analysis. Public participation in the final evaluation
of the cost-effectiveness analysis is essential, particularly if costs must be increased to
achieve environmental compatibility.
Reference (4) presents methods to be used in preparing estimates of capital and annual
costs; these methods may also be used in the cost estimates of selected alternative solutions.
1. Treatment plant: equipment, instrumentation, piping and plumbing, electrical,
heating, ventilating, air conditioning, structures, buildings, outside storage and
conveyance, interconnecting process lines, tankage
2. Ancillary utilities and service: electrical, steam, fuel
3. Site improvements: ancillary structures and buildings, engineering, owner admin-
istration, startup and operator training, land, contingencies.
Estimates of annual costs should include:
1. Annual capital charges
2. Interest of nondepreciable capital investment
3. Amortization of royalties, licenses, and fees
4. Direct operating costs
5. Operating and maintenance labor
6. Fuel, power, steam, utility water
7. Supplies and maintenance materials
8. Contract services
9. Raw materials
10. Chemicals
17-5
-------
11. Transportation
12. Residual waste disposal
13. Indirect operating costs
14. Administration and staff
15. Taxes
16. Insurance
Each of the above capital and annual cost estimating items is explained in detail in reference
(4).
17.5 Cost Effectiveness of Infiltration/Inflow Reduction
The Federal Water Pollution Control Act Amendments of 1972 require all applicants for a
treatment works grant to demonstrate that each sewer system discharging into such treat-
ment works is not subject to excessive infiltration/inflow (I/I). The objectives are to:
1. Eliminate untreated wastewater bypasses and overflows
2. Lower total costs of treatment works
3. Avoid construction of unnecessary treatment works capacity
4. Reduce total wastewater volume
The last three objectives are based on cost effectiveness. Such a study should include 1) an
in-depth, rigorous, economic analysis, based on detailed records of all costs plus the docu-
mentation of corrections made, the reduction in I/I resulting from the corrections, the
durability of repairs made by sealing, and overall performance of all correction methods
used; and 2) an evaluation of the costs of all alternatives with and without correction.
I/I Sufficient detailed cost information relating to specific quantities of I/I treated and
quantitative reductions in I/I achieved must be available before this evaluation can be made.
It may be desirable to make periodically (at 5-year intervals), for example, such an evalua-
tion to check progress, upgrade program, and verify economics. This study should be
required for all applicants prior to stage 2 work, even if the preliminary analysis did not
demonstrate the need for a preliminary evaluation survey and I/I correction program prior
to stage 1 work. The economic evaluation study allows confirmation of correction of all
decisions made previously.
17.6 Costs
Average initial and annual costs for small wastewater treatment facilities are shown on
Figures 17-1 and 17-2. These costs are based on generalized 1973 values. Technological
advancements, more reliability, and changes in regulation make invalid the use of these
curves for other than broad conclusions. Cost indices can be used to convert cost data based
on past conditions to present conditions. Because they cannot take into account all techno-
logical and economic changes, these indices must be considered general. Cost indices should
be used only to update costs no more than 10 years old. Indices commonly used in the
United States for wastewater treatment facilities are presented in Table 17-2. For costs of
wastewater treatment processes, the EPA cost index is usually most suitable.
17-6
-------
320
300
280
V)
CC
o
o
to
c-
to
o
o
a.
<
o
-------
V)
tr.
o
Q
ro
s-
o>
I-
V)
o
o
I-
CL
O
IT
UJ
Q.
32
30
28
26
24
22
18
16
14
12
10
\
A STABILIZATION PONDS, OftM
B PRIMARY PLANTS, 08M
C PRIMARY PLANTS, 08M
D ACTIVATED SLUDGE PLANT, 08M
E ACTIVATED SLUDGE PLANT, 08M
F TRICKLING FILTER PLANT, 08M
G TRICKLING FILTER PLANT, 08 M
H CUSTOMER SERVICE 8 ACCOUNTING
I GENERAL AND ADMINISTRATIVE
100
200
500 1000 2000
COMMUNITY SIZE (PERSONS)
5000
10000
FIGURE 17-2
ANNUAL COSTS OF WASTEWATER SYSTEM COMPONENTS (4) (5)
17-8
-------
TABLE 17-2
COST INDICES (Average Per Year)
Engineering
Marshall & Stevens News-Record Handy-Whitman
Installed Equipment Construction Index for Water
Indices Index Treatment Plants^
Year
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1926-100
All
Industry
168
180
181
183
185
191
209
225
229
235
238
237
239
239
242
245
252
263
273
285
303
321
332
344
398
444
472
4914
1913=100
510
543
569
600
628
660
690
724
759
797
824
847
872
901
936
971
1021
10433
llll3
12063
13113
14653
16663
18123
19353
21253
23003
25405
1936=100
Large
Plant
210
225
235
246
251
258
275
288
296
311
317
315
324
330
340
350
368
3803
398
441
480
Small
Plant
213
229
235
246
251
257
276
289
296
309
317
315
322
327
336
346
362
3743
389
424
462
Engineering
News-Record
Building Cost
1 Index
1913=100
375
400
416
431
446
465
491
509
525
548
559
568
580
594
612
627
652
660
695
760
801
877
1013
1113
11503
12603
13673
15205
Chemical
Engineering
Plant
Construction
Index
1957-1959=100
74
80
81
85
86
88
94
99
100
102
102
101
102
102
103
104
107
110
114
119
126
132
137
144
165
182
192
1992
EPA
Sewage
Treatment
Plant
Construction
Index
1957-1959=
100
102
104
105
106
107
109
110
112
116
119
123
132
143
160
172
182
217
250
262
2712
1
Based on July of year.
z Based on March of year.
Based on January of year.
Based on first quarter of year.
Based on June of year.
17-9
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Tchobanoglous, converting costs to July 1973 values, prepared comparative costs for the
most commonly used wastewater treatment processes at a design flow of 1 mgd (5).
Estimated initial capital and total annual costs, adjusted to an EPA STP cost index, are
presented in Tables 17-3 and 17-4. The ratio of costs for the population of a specific
community to costs for a population of 10,000 may be determined from Table 17-4 and
Figure 17-1.
A survey was conducted (6) of unit costs of the unit processes at 40 wastewater treatment
plants. The results, adjusted to an EPA STP cost index of 225, are presented in Table 17-5.
In 1970 Michel (7) statistically analyzed data on municipal waste treatment and developed
cost equations, based on design population for different types of treatment, as follows:
Cc = a?*1
C0 = dF
where
Cc = # capital costs
C0 = # annual costs
P = design population
a, b, d, and e = Constants (see Table 17-6)
In 1974, Tihansky (8) summarized previous cost information on wastewater treatment
costs and described the state-of-the-art on cost formulations from a historical perspective.
Reference (8) contains a comprehensive list of references on municipal wastewater treatment
costs.
Such factors as ground-water or poor soil conditions could necessitate more costly
structures, if site conditions are not fully investigated. For example, uplift of structures,
because of rising groundwater, foundation requiring piling, tight sheeting for excavation and
dewatering, pumping to keep groundwater levels low, and many other conditions must be
included in estimates for the structures required for a specific location.
17-10
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TABLE 17-3
ESTIMATED CAPITAL COSTS FOR ALTERNATIVE TREATMENT PROCESSES
WITH A DESIGN FLOW OF 1 mgd (6)
Process Cost Range
$x 103
Stabilization pond 200 - 300
Extended aeration, aerated pond, or oxidation ditch1' 400 - 600
Rotating Biological Contactor (RBC)l • 2 > 3 700-1,000
Trickling Filter1' 2>3 200 - 1,000
Complete Mix or Contact Stabilizationl >2' 3 900-1,000
Land Disposal (Infiltration/Percolation)
Including Primary Treatment 760 - 900
Including Secondary Treatment 580 - 1,320
Land Disposal (Irrigation or Overland Flow)
Including Primary Treatment 880 - 1,040
Including Secondary Treatment 700 - 1,460
Note: Based on a July 1973 EPA STP Cost Index of 183. Excludes disinfection, land, and
wastewater collection and transmission costs. Includes contractor's profit and allow-
ance for contingencies and engineering.
1 Includes Screening.
Includes secondary sedimentation and sludge drying beds.
Includes primary sedimentation and digestion.
17-11
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TABLE 17-4
ESTIMATED TOTAL ANNUAL COSTS FOR ALTERNATIVE
TREATMENT PROCESSES WITH A DESIGN FLOW OF 1 mgd (5)
Process
Initial Capital
Cost
Annual Cost1
Capital2 O&M3
Total
$x 103
$x 103
Stabilization Pond
250
27.45
23.68 51.13
Extended Aeration, Aerated Pond,
or Oxidation Ditch 500
Rotating Biological Contactor 800
Trickling Filter 800
Complete Mix or Contact
Stabilization 1,000
Land Disposal (Infiltration/Percolation)
Including Primary Treatment 800
Including Secondary Treatment 1,000
Land Disposal (Irrigation or
Overland Flow)
Including Primary Treatment 940
Including Secondary Treatment 1,240
54.90
87.83
109.79
103.30
136.14
48.80 103.70
87.83 57.68 145.51
87.83 58.48 146.31
109.79 74.41 184.20
65.10 152.93
99.51 209.30
81.54 184.84
115.95 252.09
1 Based on Engineering News-Record Construction Cost Index of 1900
(EPA STP cost index = 183).
^Capital Recorvery Factor = 0.1098 (15 years at 7 percent interest).
3Based on values from Tables 17-5 and 17-6.
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TABLE 17-5
CONSTRUCTION COSTS FOR UNIT PROCESSES
FOR WASTEWATER TREATMENT PLANTS
Process Cost
$ x 103
Raw Wastewater Pumping (1 mgd) 82
Degritting and Flow Gaging (1 mgd) 19
Screening, Degritting, and Flow Gaging (mgd) 38
Sedimentation (1,000 ft2 surface area) 61
Trickling Filter (5,000 ft3 media) 38
Aeration Structures (3,000 ft3 ) 19
Diffused Aeration (100-cfm blower) 27
Mechanical Aerators (20-hp installed capacity) 39
Recirculation Pumping (0.5 mgd) 37
Chlorination Feed System (10 Ib/day) 16
Chlorination Contact Basin (2,000 ft3) 15
Primary Sludge Pumping (40 gpm) 45
Sludge Digestion (2,000 ft3) 194
Sludge Drying Beds (7,000 ft2) 19
Administration and Laboratory Buildings (1 mgd) 50
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TABLE 17-6
COST FUNCTIONS OF MUNICIPAL WASTE TREATMENT (7) (8)
Regression Coefficient
Capital Costs O & M Costs
Technology a b d
Ordinary Treatment
Primary Sedimentation 675.7, -0.33 25.0 -0.26
Activated Sludge 912.7 -0.31 30.1 -0.25
Trickling Filter 942.0 -0.31 55.0 -0.36
Waste Stabilization Ponds 2,863.1 -0.61 17.4 -0.42
Upgrading Primary to
Activated Sludge 1,484.0 -0.41
Ancillary Works1 86.3 -0.09
Tertiary Treatment
Microscreening 9.4 -0.12 0.3 -0.04
Filtration 207.1 -0.34 51.3 -0.38
Two-stage Lime Clarification
<10mgd 140.9 -0.26 148.6 -0.44
> lOmgd 50.1 -0.18 11.0 -0.23
Lime Precalcination
<10mgd 1,903.2 -0.50 30.0 -0.30
> lOmgd - - 9.4 -0.21
Ammonia Stripping
< 10 mgd - - 35.5 -0.33
> 10 mgd 22.7 -0.10 3.5 -0.13
Carbon Adsorption
< 10 mgd 1,439.6 -0.40 1,418.9 -0.55
> 10 mgd 79.0 -0.14 23.9 0.20
1 Includes interceptors, outfalls, and pumping stations.
NOTE: m3/day = mgd x 3,785.
17-14
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17.7 References
1. Cost Effectiveness in Water Quality Programs. U.S. EPA, Office of Air and Water
Programs (October 1972).
2. "Cost Effectiveness Analysis." Federal Register, U.S. EPA, vol. 38, No. 174, title 40,
part 35, appendix A, p. 24639. (10 September 1973).
3. Electrical Power Consumption for Municipal Wastewater Treatment. U.S. EPA, Office
of Research and Development, EPA-R2-73-281 (July 1973).
4. Cost Estimating Guidelines for Wastewater Treatment Systems. Bechtel Corporation
for FWQA, WPCF Series ORD 17090DRU07 (1970).
5. Tchobanoglous, G., Wastewater Treatment for Small Communities. Conference on
Rural Environmental Engineering, Warren, Vermont (September 1973).
6. Patterson, W. L., and Banker, R. F., Estimating Costs and Manpower Requirements for
Conventional Wastewater Treatment Facilities. U.S. EPA, Office of Research and
Monitoring (October 1971).
7. Michel, R.L., et al., "Operation and Maintenance of Plants: Municipal Waste Treatment
Plants." Journal Water Pollution Control Federation, vol. 41, p. 335 (1969).
8. Tihansky, D.P., "Historical Development of Water Pollution Control Cost Functions,"
Journal Water Pollution Control Federation, vol. 46, p. 813 (1974).
17-15
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GLOSSARY
ACIDITY — Quantitative capacity of aqueous solutions to react with hydroxylions. Measur-
ed by titration, with a standard solution of a base to a specified end point. Usually
expressed as milligrams per liter of calcium carbonate.
ACTIVATED CARBON - Carbon "activated" by high-temperature heating with steam or
carbon dioxide, producing an internal porous particle structure. Total surface area of
granular activated carbon is estimated to be 1,000 m^/gm.
ADSORPTION — Adhesion of an extremely thin layer of molecules (gas or liquid) to the
surfaces of solids (e.g., granular activated carbons) or liquids with which they are in contact.
AERATE — To permeate or saturate a liquid with air.
ALKALINITY — Capacity of water to neutralize acids, imparted by the water's content of
carbonates, bicarbonates, hydroxides, and occasionally borates, silicates, and phosphates.
Expressed in milligrams per liter of equivalent calcium carbonate.
ANAEROBIC WASTE TREATMENT - Waste stabilization brought about by the action of
microorganisms in the absence of air or elemental oxygen. Usually refers to waste treatment
by methane fermentation.
ASSIMILATIVE CAPACITY - Capacity of a natural body of water to receive 1) waste-
waters, without deleterious effects; 2) toxic materials, without damage to aquatic life or
humans consuming the water; and 3) BOD, within prescribed dissolved oxygen limits.
BACKWASH — Process by which water is forced through a filtration bed in the direction
opposite to the normal flow (usually upward). During backwashing, the granular bed
expands, allowing material previously filtered out to be washed away.
BIO ASSAY - Assay method using a change in biological activity as a qualitative or quantita-
tive means of analyzing the response of biota to industrial wastes and other wastewaters.
Viable organisms, such as live fish or daphnia, are used as test organisms.
BIOCHEMICAL OXYGEN DEMAND (BOD) - Measure of the concentration of organic
impurities in wastewater. The amount of oxygen required by bacteria while stabilizing
organic matter under aerobic conditions, expressed in milligrams per liter, is determined
entirely by the availability of material in the wastewaters to be used as biological food and
by the amount of oxygen utilized by the microorganisms during oxidation.
BIOLOGICAL OXIDATION - Process in which living organisms in the presence of oxygen
convert the organic matter contained in wastewater into a more stable or mineral form.
BUFFER - Any combination of chemicals used to stabilize the pH or alkalinities of
solutions.
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BYPASS — Pipe or channel that permits wastewater to be transported around a wastewater
facility or any unit of the facility. Usually found in facilities receiving combined flow or
high infiltration rates, it is utilized to prevent flooding of units; or, in case of shutdown for
repair work, allows the flow to be moved to parallel units.
CALIBRATION — Determination, checking, or rectifying of the graduation of any
instrument giving quantitative measurements.
CHEMICAL COAGULANT - Destabilization and initial aggregation of colloidal and finely
divided suspended matter by the addition of floe-forming chemical.
CHEMICAL OXYGEN DEMAND (COD) - Measure of the oxygen-consuming capacity of
inorganic and organic matter present in water or wastewater, expressed as the amount of
oxygen consumed from a chemical oxidant in a specific test. It does not differentiate
between stable and unstable organic matter and thus, does not necessarily correlate with
biochemical oxygen demand.
CHEMICAL PRECIPITATION - Separating a substance from a solution, resulting in the
formation of relatively insoluble matter.
CHLORINATION — Application of chlorine to water or wastewater, generally for the
purpose of disinfection, but frequently for accomplishing other biological or chemical
results.
CHLORINE CONTACT CHAMBER - Detention basin in which a liquid containing diffused
chlorine is held for a sufficient time to achieve a desired degree of disinfection.
CHLORINE DEMAND — Difference between the amount of chlorine added to the waste-
water and the amount of residual chlorine remaining at the end of a specific contact time.
The chlorine demand for given water varies with the amount of chlorine applied, time of
contact, temperature, pH, and nature and amount of impurities in the water.
CLARIFICATION — Any process or combination of processes to reduce the concentration
of suspended matter in a liquid.
COAGULATION — Process by which chemicals (coagulants) are added to an aqueous
system, to render finely divided, dispersed matter with slow or negligible settling velocities
into more rapidly settling aggregates. Forces that cause dispersed particles to repel each
other are neutralized by the coagulants.
COLLOIDAL MATTER - Dispersion of very small (1 m/n to 0.5 n) particles that will not
settle but may be removed by coagulation or biochemical action or membrane filtration.
COMMINUTION - Process of cutting and screening solids contained in wastewater flow
before it enters the pumps or other units in the treatment plant.
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COMPOSITE WASTEWATER SAMPLE - Combination of individual samples of water or
wastewater taken at selected intervals (generally hourly or some similar specified period),
to minimize the effect of the variability of the individual sample. Individual samples may
have equal volume or may be roughly proportional to the flow at time of sampling.
DENITRIFICATION — Chemically-bound oxygen in nitrate or nitrate ions stripped away
by microorganisms, producing nitrogen gas, which can cause floe to rise in the final
sedimentation process. An effective method of removing nitrogen from wastewater.
DETENTION TIME — Average period of time a fluid element is retained in a basin or tank
before discharge.
DIALYSIS — Separation of a colloid from a substance in true solution, by allowing the
solution to diffuse through a semi-permeable membrane.
DUAL MEDIA FILTRATION — Filtration process that uses a bed composed of two dis-
tinctly different granular substances (such as anthracite coal and sand), as opposed to
conventional filtration through sand only.
EFFECTIVE SIZE — Size of the particle that is coarser than 10 percent, by weight, of the
material; i.e., the size sieve that will permit 10 percent of the granular sample to pass while
retaining the remaining 90 percent. Usually determined by the interpolation of a
cumulative particle size distribution.
ELECTRICAL CONDUCTIVITY — Reciprocal of the resistance in ohms measured between
opposite faces of a centimeter cube of an aqueous solution at a specified temperature.
Expressed as microhms per centimeter in degrees Celsius.
ENDOGENOUS RESPIRATION - Auto-oxidation of cellular material, which takes place
inside a cell in the absence of assimilable external organic material, to furnish the energy
required for the replacement of exhausted components of protoplasm.
ENERGY HEAD - Height of the hydraulic grade line above the center line of a conduit
plus the velocity head resulting from the mean velocity of the water in that section.
FATS — Triglyceride esters of fatty acids. Erroneously used as synonym for grease.
FLOC - Agglomeration of finely divided or colloidal particles resulting from certain
chemical-physical or biological operations.
FOOD TO MICRO-ORGANISM RATIO (F/M) - Aeration tank loading parameter. Food
may be expressed in pounds BOD added per day to the aeration tank; micro-organisms may
be expressed as mixed liquor volatile suspended solids (MLVSS) in the aeration tank.
FREEBOARD - Vertical distance from the top of a tank, basin, column, or wash trough
(in the case of sand filters) to the surface of its contents.
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GAGING STATION — Point on a stream or conduit at which measurements of flow are
customerily made and that includes a stretch of channel through which the flow is uniform
and a control downstream from this stretch. The station usually has a recorder or gage for
measuring the elevation of the water surface in the channel or conduit.
GRAB SAMPLE — Single sample of wastewater taken at neither set time nor flow.
GREASE — In wastewater, a group of substances, including fats, waxes, free fatty acids,
calcium and magnesium soaps, mineral oils, and certain other nonfatty materials. The type
of solvent and method used for extraction should be stated for quantification.
GREASE SKIMMER — Device for removing floating grease or scum from the surface of
wastewater in a tank.
GRIT CHAMBER - Detention chamber or an enlargement of a sewer, designed to reduce
the velocity of flow of the liquid, to permit the separation of mineral from organic solids
by differential sedimentation.
HARDNESS — Characteristic of water imparted by salts of calcium, magnesium, and iron
(such as bicarbonates, carbonates, sulfates, chlorides, and nitrates), which causes curdling of
soap, deposition of scale in boilers, damage in some industrial processes, and sometimes
objectionable taste. It may be determined by a standard laboratory procedure or computed
from the amounts of calcium, magnesium, iron, aluminum, manganese, barium, strontium,
and zinc, and is expressed as equivalent calcium carbonate.
HYDRAULIC LOADING — Quantity of flow passing through a column or packed bed,
expressed in the units of volume per unit time per unit area; e.g., gal/min/ft^ (m3/m2 -s).
HYDROPHILIC - Having a high affinity for water.
HYDROPHOBIC - Having a low affinity for water.
HYGROSCOPIC — Solid capable of absorbing moisture from the air without eventual
dissolution in that moisture.
HYPERFILTRATION — High pressure reverse osmosis process using a membrane that will
remove dissolved salts as well as suspended solids.
INDUSTRIAL WASTES — Liquid wastes from industrial processes, as distinct from
domestic or sanitary wastes.
INFILTRATION — Ground water that seeps into pipes, channels, or chambers through
cracks, joints, or breaks.
INFLUENT — Wastewater or other liquid (raw or partially treated) flowing into a reservoir,
basin, treatment process, or treatment plant.
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INORGANIC MATTER — Chemical substances of mineral origin; not of basically carbon
structure, with animal or vegetable origin.
IONS — Atom or group of atoms with an unbalanced electrostatic charge.
POND — 1) Shallow body of water, i.e., lagoon or lake; or 2) pond containing raw or
partially treated wastewater in which aerobic or anaerobic stabilization occurs.
METHYL-ORANGE ALKALINITY - Measure of the total alkalinity of an aqueous
suspension or solution, determined by the quantity of sulfuric acid required to bring the
water pH to a value of 4.3, as indicated by the change in color of methyl orange. Expressed
in milligrams CaCO3 per liter.
MICROSCREENING — Form of surface filtration using specially woven wire fabrics
mounted on the periphery of a revolving drum.
MIXED LIQUOR — Mixture of activated sludge and wastewater undergoing activated sludge
treatment in the aeration tank.
MIXED LIQUOR SUSPENDED SOLIDS (MLSS) - Concentration of suspended solids
carried in the aeration basin of an activated sludge process.
MONITORING — 1) measurement, sometimes continuous, of water or wastewater quality;
or 2) procedure or operation of locating and measuring radioactive contamination, by means
of survey instruments that can detect and measure, as dose rate, ionizing radiations.
MOST PROBABLE NUMBER (MPN) - Number of organisms per unit volume that, in
accordance with statistical theory, would be more likely than any other number to yield the
observed test result with the greatest frequency. Expressed as density of organisms per 100
ml. Results are computed from the number of positive findings of coliform organisms
resulting from multiple-portion decimal-dilution plantings.
NEUTRALIZATION — Reaction of acid or alkali with the opposite reagent until the con-
centrations of hydrogen and hydroxyl ions in the solution are approximately equal.
NITRIFICATION — Conversion of nitrogenous matter to nitrates.
NONSETTLEABLE SOLIDS - Suspended matter that does not settle or float to the surface
of water in a period of 1 hour.
ORGANIC MATTER — Chemical substances of animal or vegetable origin of basically
carbon structure, comprising compounds consisting of hydrocarbons and their derivatives.
ORGANIC NITROGEN — Nitrogen combined in organic molecules, such as protein, amines,
and amino acids.
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OVERFLOW RATE — One of the criteria for the design of settling tanks in treatment
plants, expressed in gallons per day per square foot (m3 /m2 -d) of surface area in the settling
tank.
OXIDATION — Addition of oxygen to a compound. More generally, any reaction involving
the loss of electrons from an atom.
OXIDATION POND OR LAGOON - Basin used for retention of wastewater before final
disposal, in which biological oxidation of organic material is effected by natural or
artificially accelerated transfer of oxygen to the water from air.
OXIDATION-REDUCTION POTENTIAL (ORP) - Potential requked to transfer electrons
from the oxidant to the reductant; used as qualitative measure of the state of oxidation in
wastewater treatment systems.
OXYGEN UPTAKE RATE — Amount of oxygen utilized by an activated sludge system
during a specific time period.
PARSHALL FLUME — Calibrated device developed by Ralph Parshall for measuring the
flow of liquid in an open conduit, which consists essentially of a contracting length, a
throat, and an expanding length. A sill, over which the flow passes at critical depth, is
located at the throat. The upper and lower heads are individually measured at a definite
distance from the sill. The lower head need not be measured unless the sill is submerged
more than about 67 percent.
PATHOGENIC ORGANISMS — Organisms, usually microscopic in size (e.g., bacteria and
viruses), that may cause disease in the host organisms by their parasitic growth.
pH - Reciprocal of the logarithm of the hydrogen ion concentration. The concentration is
the weight for hydrogen ions, in grams per liter of solution. Neutral water, for example, has
a pH value of 7 and hydrogen ion concentration of 1CT7 .
PHENOLPHTHALEIN ALKALINITY - Measure of the hydroxides plus one-half the
normal carbonates in aqueous suspension. Measured by the amount of sulfuric acid required
to bring the water to a pH of 8.3, as indicated by a change in color of phenolphthalein.
Expressed in parts per million of calcium carbonate.
PHYSICAL-CHEMICAL TREATMENT (PCT) PLANT - Treatment sequence in which
physical and chemical processes are used to the exclusion of explicitly biological process
(including incidential biological treatment obtained on filter media or absorptive surfaces).
In this sense, a PCT scheme is a substitute for conventional biological treatment. A PCT
scheme following an existing biological plant may, by contrast, be termed simply a tertiary
plant, although it is also a PCT in a general sense.
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POLYELECTROLYTES — Chemicals consisting of high molecular weight molecules with
many reactive groups situated along the length of the chain. Polyelectrolytes react with the
fine particles in the waste and assist in bringing them together into larger and heavier masses
for settling.
POSTCHLORINATION - Application of chlorine to the final treated wastewater or
effluent following plant treatment.
PRECHLORINATION - Chlorination at the headworks of the plant; influent chlorination
prior to plant treatment.
PRIMARY SETTLING TANK - First settling tank for the removal of settleable solids
through which wastewater is passed in a treatment works.
PRIMARY TREATMENT - 1) First (sometimes only) major treatment in a wastewater treat-
ment works, usually sedimentation; or 2) removal of a substantial amount of suspended
matter, but little or no colloidal and dissolved matter.
RAW SLUDGE — Settled sludge promptly removed from sedimentation tanks before
decomposition has much advanced. Frequently referred to as undigested sludge.
RECALCINATION — Process for recovering lime for reuse by heating spent lime to high
temperatures, thereby driving off water of hydration and carbon dioxide.
RECARBONATION — Addition of carbon dioxide to lime-treated water, to reduce the pH
of the waste for further calcium removal and/or stabilization of the water.
RECIRCULATION RATE — Rate of return of part of the effluent from a treatment process
to the incoming flow.
RESIDUAL CHLORINE — Chlorine remaining in water or wastewater at the end of a
specified contact period as combined or free chlorine.
SALINITY — 1) Relative concentration of salts, such as sodium chloride, in a given water,
usually expressed in terms of the number of parts per million of chloride (Cl); or 2)
measure of the concentration of dissolved mineral substances in water.
SAMPLER - Device used with or without flow measurement, to obtain an adequate portion
of water or waste for analytical purposes. May be designed for taking a single sample (grab),
composite sample, continuous sample, or periodic sample.
SANITARY SEWER - Sewer that carries liquid and water-carried human wastes from
residences, commercial buildings, industrial plants, and institutions, together with minor
quantities of storm, surface, and groundwater(s) that are not admitted intentionally.
Significant quantities of industrial wastewater are not carried in sanitary sewers.
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SCREEN — Device with openings, generally of uniform size, used to retain or remove
suspended or floating solids in flowing water or wastewater and to prevent them from
entering an intake or passing a given point in a conduit. The screening element may consist
of parallel bars, rods, wires, grating, wire mesh, or perforated plate; the openings may be of
any shape, although they are usually circular or rectangular. Also a device used to segregate
granular material, such as sand, crushed rock, and soil, into various sizes.
SECONDARY SETTLING TANK - Tank through which effluent from some prior
treatment process flows for the purpose of removing settleable solids.
SECONDARY WASTEWATER TREATMENT - Treatment of wastewater by biological
methods after primary treatment by sedimentation.
SECOND-STAGE BIOLOGICAL OXYGEN DEMAND - Part of the oxygen demand
associated with the biochemical oxidation of nitrogenous material. As the term implies, the
oxidation of the nitrogenous materials usually does not start until a portion of the
carbonaceous material has been oxidized during the first stage.
SEDIMENTATION — Process of subsidence and deposition of suspended matter carried by
water, wastewater, or other liquids, by gravity. Usually accomplished by reducing the
velocity of the liquid to below the point at which it can transport the suspended material.
Also called settling.
SELF-PURIFICATION — Natural processes occurring in a stream or other body of water
resulting in the reduction of bacteria, satisfaction of the BOD, stabilization of organic
constituents, replacement of depleted dissolved oxygen, and the return of the stream biota
to normal. Also called natural purification.
SEMIPERMEABLE MEMBRANE - Barrier, usually thin, that permits passage of particles
up to a certain size or of special nature. Often used to separate colloids from their
suspending liquid, as in dialysis.
SETTLEABLE SOLIDS — 1) Matter in wastewater that will not stay in suspension during a
preselected settling period (such as 1 hour) but settles to the bottom or floats to the top; 2)
in the Imhoff cone test, the volume of matter that settles to the bottom of the cone in 1
hour.
SKIMMING TANK — Tank so designed that floating matter will rise and remain on the
surface of the wastewater until removed, while the liquid discharges continuously under
certain walls or scum baffles.
SLOUGHINGS - Trickling filter slimes that have been washed off filter media. They are
generally quite high in BOD and will degrade effluent quality unless removed.
SLUDGE AGE - In the activated sludge process, a measure of the length of time (expressed
in days) a particle of suspended solids has been undergoing aeration. Usually computed by
dividing the weight of the suspended solids in the aeration tank by the daily addition of new
suspended solids having their origin in the raw waste.
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SLUDGE VOLUME INDEX (SVI) - Numerical expression of the settling characteristics of
activated sludge. The ratio of the volume in milliliters of sludge settled from a 1,000-ml
sample in 30 minutes to the concentration of mixed liquor in milligrams per liter multiplied
by 1,000.
SODA ASH - Sodium Carbonate (Na2CO3)
STABILIZATION POND - Type of oxidation pond in which biological oxidation of organic
matter is effected by natural or artificially accelerated transfer of oxygen to the water from
air.
STAFF GAGE — Graduated scale, vertical unless otherwise specified, on a plank, metal
plate, pier, wall, etc., used to indicate the height of a fluid surface above a specified point or
datum plane.
STAGE-DISCHARGE RELATION - Relation between the water height, as indicated on the
staff gage, and the discharge of a stream or conduit at a gaging station. This relation is
shown by the rating curve or rating table for such stations.
STATIC HEAD — Total head, without reduction for velocity head or losses; for example,
the difference in the elevation of headwater and tailwater of a power plant. Also the vertical
distance between the free level of the source of supply and the point of free discharge of the
level of the free surface.
STILLING WELL — Pipe, chamber, or compartment with comparatively small inlets
communicating with a main body of water. Used to dampen waves or surges while permit-
ting the water level within the well to rise and fall with the major fluctuations of the main
body of water. Used with water-measuring devices, to improve accuracy of measurement.
SUBMERGED WEIR —Weir that, when in use, results in the water level on the downstream
side rising to an elevation equal to, or higher than, the weir crest. The rate of discharge is
affected by the tailwater. Also called drowned weir.
SURFACE AREA — Amount of surface area per unit weight of carbon, usually expressed in
square meters per gram of carbon. The surface area of activated carbon is usually
determined from the nitrogen adsorption isotherm by the Brunauer, Emmett, and Teller
method (BET method).
SUSPENDED SOLIDS - Solids that float on the surface of, or are in suspension in, water,
wastewater, or other liquids, and that ar6 largely removable by laboratory filtering. Also the
quantity of material removed from wastewater in a laboratory test, as prescribed in
Standard Methods for the Examination of Water and Wastewater and referred to as non-
filterable residue.
THRESHOLD ODOR NUMBER - Test is based on comparison with an odor-free water,
obtained by passing tap water through a column of activated carbon. The water being tested
is diluted with odor-free water until the odor is no longer detectable. The last dilution at
which an odor is observed is the threshold odor number.
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TOTAL ORGANIC CARBON (TOC) - Measure of the amount of organic material in a
water sample, expressed in milligrams per liter of carbon. Measured by Beckman
carbonaceous analyzer or other instrument in which the organic compounds are catalytically
oxidized to CO2 and measured by an infrared detector. Frequently applied to wastewaters.
TITRATION — Determination of a constituent in a known volume of solution, by the
measured addition of a solution of known strength to completion of the reaction, as
signaled by observation of an end point.
TOTAL SOLIDS — Sum of dissolved and undissolved constituents in water or wastewater,
usually expressed in milligrams per liter.
TRACER — Foreign substance mixed with, or attached to, a given substance for the
determination of the location or distribution of the substance. Also an element or
compound that has been made radioactive so it can be easily followed (traced) in biological
and industrial processes. Radiation emitted by the radioisotope pinpoints its location.
TURBIDIMETER — Instrument for measurement of turbidity, in which a standard
suspension is generally used for reference.
TURBIDITY — Condition in water or wastewater caused by the presence of suspended
matter, resulting in the scattering and absorption of light rays. Measure of fine suspended
matter in liquids. Analytical quantity, usually expressed in Jackson turbidity units (Jtu),
determined by measurements of light diffraction.
TURBULENT FLOW — Flow of a liquid past an object so that the velocity at any fixed
point in the fluid varies irregularly. Type of fluid flow in which there is an unsteady motion
of the particles and the motion at a fixed point varies in no definite manner. Sometimes
called eddy or sinuous flow.
ULTIMATE BIOCHEMICAL OXYGEN DEMAND (UBOD) - Quantity of oxygen required
to satisfy completely both first-stage and second-stage biochemical oxygen demands.
UNIFORMITY COEFFICIENT - Obtained by dividing the sieve opening in millimeters
that will pass 60 percent of a sample by the sieve opening in millimeters that will pass 10
percent of the sample. These values are usually obtained by interpolation of a cumulative
particle size distribution.
VOLATILE SOLIDS — Quantity of solids in water, wastewater, or other liquids lost on
ignition of the dry solids at 600° C.
WET WELL — Compartment in which a liquid is collected and held for flow equalization
and then pumped (by system pumps) for transmission through the plant.
-fj U S. GOVERNMENT PRINTING OFFICE- 1977-758-959
G-10
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