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

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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

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      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

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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

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      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

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   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

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 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

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                                   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

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                             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

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                                                   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

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                                                   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

-------
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

-------
                                              HOUSEHOLD  WATER  USE  G.P.H./HOME
K)
       x
       O
       c
       V)
       m
       X
       O
       m
       33

       Crt
       m
       CJ1
C
:o
m
ro
                         ro
                       >
                       2
m
o
                  o
                  >
                         ro
                       "O

-------
       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

-------
     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

-------
            IS   .2   25  .3   .4   .5  .6 .7 .8 .9 LO     1.5   2
o
E
o
o

0



5


o

Ul
o

tr.
UJ
o

H
<
                  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

-------
              450
             400
to
to
          Q.

          3


          o
Ul
QL

LU
_l
CO
<
CD
O
oc
a.
              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

-------
                                     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

-------
N)
OO

GENERAL ANALYSIS
Color
Turbidity
Temperature
Toxicity
Odor



1                                                                               Organic
                                                                               Anoly»i«
1
£2,- ESS
Dwond Concwitration
I
Calculattd ......
ThOD TOC


]
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
I]
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«:


















                                                                                                     o»r>quir«d|
                                                        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

-------
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

-------
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

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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-

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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.

                                        2-36

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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.
                                      2-37

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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
                                       2-39

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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
                                         2-42

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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).

                                        2-54

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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.

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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.

                                         2-56

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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.
                                        2-57

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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.
                                          2-58

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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

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                                                     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

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                                                              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

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                                                        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

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      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

-------
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

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                                    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

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                                                                  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

-------
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

-------
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

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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

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 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

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     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

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                                                      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

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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

-------
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

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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

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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

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                                     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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-------
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

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     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

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    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

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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

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 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

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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

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                        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

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     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

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     ^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

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     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

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 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

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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

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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

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                                                           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

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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.

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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

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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

-------
    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

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                             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

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     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

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                                       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

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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

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     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

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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

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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

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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

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                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

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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

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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

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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

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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

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                                            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

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                                               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

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                                        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

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      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

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     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

-------
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

-------

^.

-------
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

-------
 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

-------
      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

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                                  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

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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

-------
       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

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                                                       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.

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       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

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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

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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
                                        9-36

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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

                                        9-37

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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

-------
                                      •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

                                        9-40

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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

                                        9-41

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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

                                        9-42

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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

                                        9-43

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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.

                                        9-44

-------
           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

                                        9-45

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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).

                                        9-46

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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).

                                        9-47

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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).
                                       9-48

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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).
                                       9-49

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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

-------
 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

-------
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

-------
                 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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                     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

-------
                                   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

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       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

-------
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                                                      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

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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

-------
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      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















































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S
S
























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J A S 0 N D
1969





./
/
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INFLUENT
^>

















^^~
	 L
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1













































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EFFLUENT
--J
^»—
^ —



JFMAMJJ ASOND
1970















_ — •














































^— •























— — .























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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

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    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

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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

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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

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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

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     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

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     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

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                                                                 INFLUENT
EFFLUENT
TRANSFER
PIPE
                             WASHWATER
                              STORAGE
                            COMPARTMENT
               FILTER   \\  FILTER MEDIA
             COMPARTMENT
      AIR FOR WASH
                SHl
                            FIGURE 11-6
               AUTOMATIC GRANULAR MEDIA FILTER
                               11-13

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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

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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.
                                        11-18

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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:
                                       12-1

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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.
                                         12-2

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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.
                                        12-3

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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.
                                        12-4

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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

                                        12-5

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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

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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).

                                       13-5

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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

                                         14-2

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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.

                                         14-3

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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.
                                         14-4

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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
                                     14-5

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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

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                                                   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.

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                                                                    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.

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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

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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

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                                                                           EFFLUENT WEIR
                                                                               EFFLUENT
RAISED POSITJON
 OF TRUSS ARM
                                                           SCRAPER BLADES


                                                      UNDERFLOW
                                      ELEVATION
                                      FIGURE 14-1
                                 GRAVITY THICKENER (8)

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                                           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

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 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

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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)

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                                           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

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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

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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

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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

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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

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                                                    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

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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

-------
 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

-------
 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
 cc.
 o
  : 005
 LL)
 CC
   002
   OOI
























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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

-------
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

-------
                                    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

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                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

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-------
     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

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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

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                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

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                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

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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

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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

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                              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

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                    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

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                         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

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                                       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

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 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

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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

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                                    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

-------
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.
                                     17-12

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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
                                     17-13

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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.
                                       G-6

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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.
                                       G-7

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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.

                                          Go
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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.

                                       G-9

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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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