United States
Environmental Protection
Agency
Center for Environmental
Research Information
Cincinnati, OH 45268
EPA/625/6-89/020
July 1989
Technology Transfer
&EPA
Handbook
Retrofitting POTWs
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EPA/625/6-89/020
July 1989
Handbook
Retrofitting POTWs
U.S. Environmental Protection Agency
Office of Research and Development
Center for Environmental Research Information
Cincinnati, OH 45268
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Notice
This document has been reviewed in accordance with the U.S. Environmental Protection
Agency's peer and administrative review policies and approved for publication. Mention of trade
names or commercial products does not constitute endorsement or recommendation for use.
This document is not intended to be a guidance or support document for a specific regulatory
program. Guidance documents are available from EPA and must be consulted to address
specific regulatory issues.
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Contents
Chapter
Page
Figures ix
Tables . . . xi
Acknowledgments xiii
1 Introduction 1
1.1 Purpose 1
1.2 Background 1
1.3 Scope 1
1.4 Using the Manual , 3
1.5 References 3
2 Comprehensive Performance Evaluations 5
2.1 Introduction 5
2.2 Approach to Conducting CPEs . 5
2.2.1 Methodology
2.2.1.1 Evaluation of Major Unit Processes 5
2.2.1.2 Identification of Performance-Limiting Factors 6
2.2.1.3 Prioritization of Performance-Limiting Factors 7
2.2.1.4 Assessing Approach to Improve Performance 8
2.2.1.5 CPE Report 9
2.3 How to Conduct a CPE 9
2.3.1 Initial Activities 9
2.3.1.1 Personnel 9
2.3.1.2 Wastewater Treatment Plant 9
2.3.1.3 Scheduling 10
2.3.2 Data Collection 10
2.3.2.1 Kickoff Meeting 10
2.3.2.2 Plant Tour . . . 10
2.3.2.3 Detailed Data Gathering 12
2.3.3 Evaluation of Major Unit Processes . 13
2.3.3.1 Suspended Growth Major Unit Processes 13
2.3.3.2 Fixed Film Major Unit Processes 19
2.3.3.3 Stabilization Pond Processes 23
2.3.4 Evaluation of Performance-Limiting Factors 24
2.3.4.1 Administration Factors 25
2.3.4.2 Design Factors 26
2.3.4.3 Operational Factors 27
2.3.4.4 Maintenance Factors 29
2.3.5 Performance Evaluation 29
2.3.5.1 Magnitude of the Performance Problem 29
2.3.5.2 Projected Improved Performance . 32
2.3.6 Presentation to POTW Administrators and Staff 32
2.3.7 CPE Report 32
2.3.7.1 Introduction 33
in
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Contents (continued)
Chapter
Page
2.3.7.2 Facility Background 33
2.3.7.3 Major Unit Process Evaluation . 33
2.3.7.4 Performance-Limiting Factors . '. . . . 34
2.3.7.5 Performance Improvement Activities 34
2.3.7.6 Costs 34
2.3.8 Example CPE . . 34
2.3.8.1 Plant Data 34
2.3.8.2 Major Unit Process Evaluation 37
2.3.8.3 Performance-Limiting Factors 38
2.3.8.4 Performance Improvement Activities 39
2.3.9 CPE Worksheets 39
2.3.10 CPE Results : 39
2.4 Personnel Capabilities for Conducting CPEs 39
2.5 Estimating CPE Costs 39
2.6 References 40
3 Conducting Composite Correction Programs 43
3.1 Objective 43
3.2 CCP Methodology . 43
3.2.1 CPE Results 43
3.2.2 Process Control Basis 43
3.2.3 Long-Term Involvement 44
3.3 CCP Activities 44
3.3.1 General 44
3.3.2 Initial Site Visit 45
3.3.2.1 CPE Results 45
3.3.2.2 Monitoring Equipment 45
3.3.2.3 Process Control Summaries 45
3.3.2.4 Process Control Adjustments ; 45
3.3.2.5 Minor Design Changes 45
3.3.2.6 Action Lists 45
3.3.3 Improving Design Performance-Limiting Factors . . . 46
3.3.3.1 Identification and Justification 46
3.3.3.2 Implementation . 46
3.3.3.3 Assessment 46
3.3.4 Improving Maintenance Performance-Limiting Factors 48
3.3.5 Improving Administrative Performance-Limiting Factors . 50
3.3.6 Improving Operational Performance-Limiting Factors 50
3.3.6.1 Suspended Growth Process Control . ; 50
3.3.6.2 Fixed Film Process Control 56
3.3.6.3 ABF Process Control 57
3.4 Example CCP 57
3.4.1 Addressing Performance-Limiting Factors 57
3.4.2 Plant Performance 58
3.4.3 CCP Costs 58
3.4.4 Summary 58
3.5 CCP Results 58
3.6 Personnel Capabilities for Conducting CCPs 59
3.7 Estimating CCP Costs ,. 59
3.8 References 60
IV
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Contents (continued)
Chapter
Page
4 Facility Modifications • 63
4.1 Introduction . • • 63
4.1.1 Identifying Alternatives 63
4.1.2 Selecting A Modification 64
4.2 Preliminary/Primary Treatment Processes ;. 67
4.2.1 Excessive Flow Variation 67
4.2.1.1 Pump Station Modification 68
4.2.1.2 Flow Equalization Basin . 68
4.2.2 Inadequate Flow Splitting and Control 70
4.2.3 Inadequate Screening Equipment . 70
4.2.3.1 Coarse Screens 70
4.2.3.2 Fine Screens - 72
4.2.3.3 Comminutors 72
4.2.4 Inadequate Grit Removal Equipment 72
4.2.4.1 Horizontal-Flow Grit Chambers 72
4.2.4.2 Aerated Grit Chamber 72
4.2.4.3 Vortex Type Grit Chamber 72
4.2.5 Undersized Primary Clarifier 72
4.2.5.1 Relocation of Recirculation or Secondary Sludge Flows 73
4.2.5.2 Flow Equalization 73
4.2.5.3 Flow Splitting and Control 73
4.2.5.4 Additional Clarifier . 73
4.2.6 Inadequate Scum and Sludge Removal 73
4.2.7 Inadequate Septage Handling Facilities 73
4.2.7.1 Separate Treatment 73
4.2.7.2 Septage Receiving Station 74
4.3 Fixed-Film Treatment Processes 74
4.3.1 Undersized Trickling Filter . . . 74
4.3.1.1 Replacement of Rock Media With Plastic Media 75
4.3.1.2 Increase Height of Existing Trickling Filter 76
4.3.1.3 Additional Fixed-Film Facilities .....' 76
4.3.1.4 Trickling Filter/Suspended Growth Systems . . .'. 76
4.3.1.5 Additional Recirculation Capacity 79
4.3.2 Inadequate Oxygen Transfer 79
4.3.3 Inadequate Hydraulic Wetting for TF 81
4.3.4 Lack of Cold Weather Protection for Trickling Filter 81
4.3.5 Undersized RBC System 82
4.3.5.1 Additional Media Surface Area 82
4.3.5.2 Preaeration 82
4.3.5.3 Additional Biological Process 82
4.3.6 Undersized First Stage RBCs 82
4.3.6.1 Organic Load Redistribution . 83
4.3.6.2 Positive Control of Flow Distribution . . . 83
4.3.7 Inadequate Oxygen Transfer for RBCs 83
4.3.8 Inadequate Control With RBC Air Drive Units . . 84
4.3.9 Undersized ABF 84
4.3.10 Inadequate Oxygen Transfer for an ABF System . . 84
4.3.11 Inadequate Return/Waste Sludge Capability for ABF System 86
4.3.12 Inadequate Hydraulic Wetting for ABF 86
4.3.13 Undersized Clarifier 86
4.3.13.1 Relocation of Recirculation Flows 86
4.3.13.2 Flow Equalization 86
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Contents (continued)
Chapter
Page
4.3.13.3 Additional Clarifier 86
4.3.14 Excessive Clarifier Hydraulic Currents 86
4.4 Suspended Growth Treatment Processes 87
4.4.1 Inadequate Waste Sludge Flexibility/Capacity 87
4.4.1.1 Sludge Disposal/Utilization Management .'. . : 88
4.4.1.2 Sludge Treatment Facilities 88
4.4.1.3 Separate Waste Sludge Pumps 88
4.4.1.4 Sludge Wasting to Thickener 88
4.4.1.5 Separate Waste Sludge Hopper in Clarifier 88
4.4.1.6 Flow Measurement 88
4.4.2 Inadequate Process Flexibility 89
4.4.2.1 Step Feed 89
4.4.2.2 Contact Stabilization 91
4.4.2.3 Plug Flow 93
4.4.2.4 Chemical Addition 93
4.4.2.5 Selector Basin 94
4.4.3 Undersized Clarifier 94
4.4.3.1 Flow Equalization 94
4.4.3.2 Additional Clarifiers 95
4.4.3.3 Intrachannel Clarifiers 95
4.4.4 Undersized Aeration Basin 95
4.4.4.1 Flow Equalization 96
4.4.4.2 Fixed Film/Suspended Growth Systems 96
4.4.4.3 Additional Aeration Volume 97
4.4.4.4 Biological Aerated Filter (BAF) 97
4.4.4.5 Two-Zone Process 98
4.4.4.6 Powdered Activated Carbon Treatment (PACT) 99
4.4.4.7 Porous Biomass Support Systems 99
4.4.5 Inadequate Oxygen Transfer Equipment 100
4.4.5.1 Additional Blowers 100
4.4.5.2 Diffused Air System Upgrade 100
4.4.5.3 Mechanical Aerator Upgrade 101
4.4.5.4 Roughing Filters 101
4.4.6 Inadequate Return Sludge Flexibility 101
4.4.6.1 Flow Recycle Around Pump 101
4.4.6.2 Adjustable Speed Drives 101
4.4.6.3 Time Clocks 102
4.4.6.4 Multiple Pumps for Return Sludge Pumping 102
4.4.6.5 Flow Measurement 102
4.4.7 Excessive Clarifier Hydraulic Currents . 102
4.4.7.1 Inlet Modifications 102
4.4.7.2 Baffle Addition 103
4.4.7.3 Weir Relocation/Addition 104
4.4.8 Basin Mixing 105
4.4.9 Inadequate Scum Removal 105
4.5 Wastewater Stabilization Ponds . 106
4.5.1 Undersized Ponds 106
4.5.1.1 Improved Utilization of Existing Volume 108
4.5.1.2' Additional Treatment Cell Volume 108
4.5.1.3 Hydrograph Controlled Release 109
4.5.1.4 Aquaculture 109
4.5.2 Inadequate Oxygen Transfer Capability 110
4.5.2.1 Mechanical Mixers 110
4.5.2.2 Additional Aeration Equipment 110
VI
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Contents (continued)
Chapter
Page
4.5.3 Short-Circuiting 110
4.5.3.1 Baffles 110
4.5.3.2 Mixing Equipment 111
4.5.3.3 Multiple Inlets and Outlets . . . . . 111
4.5.4 Inadequate Suspended Solids Control 111
4.5.4.1 Controlled Discharge 112
4.5.4.2 Variable Level Draw-Off 112
4.5.4.3 Aquaculture 112
4.5.5 Inflexible Mode of Operation 113
4.5.5.1 Series/Parallel Operation 113
4.5.5.2 Recycle Capability 113
4.5.6 Low Pond Temperature 114
4.5.6.1 Short-Term Diffused Aeration Cells 114
4.6 Sludge Treatment and Disposal 115
4.6.1 Background 115
4.6.1.1 Sludge Production . . 115
4.6.1.2 Flexibility ....'.' 119
4.6.2 Selecting Modification Approaches 120
4.6.3 Inadequate Sludge Thickening Capability 120
4.6.3.1 Chemical Addition to Existing Thickener 121
4.6.3.2 Additional or Alternate Thickening Process 121
4.6.4 Inadequate Sludge Stabilization Capability 121
4.6.4.1 Improved Thickening/Dewatering 122
4.6.4.2 Additional Blowers or Diffuser Replacement (Aerobic Digestion) 122
4.6.4.3 Lime Stabilization . . 122
4.6.4.4 Additional Heat Exchanger Capacity For Anaerobic Digesters .. 122
4.6.4.5 Improved Mixing Capability . . . 122
4.6.4.6 Additional or Alternate Stabilization Process . . . . , 123
4.6.5 Inadequate Sludge Dewatering Capability 123
4.6.5.1 Additional or Alternate Dewatering Process 123
4.6.6 Sidestream Return to Wastewater 123
4.6.6.1 Pretreatment 123
4.6.7 Inadequate Sludge Storage Capability 123
4.6.7.1 Mechanical Thickening 124
4.6.7.2 Additional or Alternate Storage Facilities 124
4.6.8 Inadequate Sludge Transportation Capability 124
4.6.9 Inadequate Sludge Utilization/Disposal Capability 124
4.6.9.1 Additional or Alternate Sludge Utilization/Disposal Facilities .... 124
4.7 Additional Facility Modifications 124
4.7.1 Disinfection System Modifications • • • •, • • • 124
4.7.1.1 Chlorine Disinfection Systems . 125
4.7.1.2 Ozone Disinfection Systems . . . 127
4.7.1.3 Ultraviolet (UV) Radiation Disinfection Systems 128
4.7.2 Modifications Directed Toward the Wastewater Stream 129
4.7.2.1 Modifications to Wastewater Characteristics . 129
4.7.2.2 Modifications to Provide Instrumentation 129
4.7.3 Modifications Directed Toward Non-Routine or Emergency Operation . . . 130
4.7.3.1 Modifications to Respond to Non-Routine Operation 130
4.7.3.2 Modifications to Respond to Emergency Conditions 130
4.8 References 130
VII
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Contents (continued)
Chapter
Appendices
A
B
C ...
D
E
F
G ...
H
I
J
K
L
M . . .
N
0
P
Page
135
147
151
155
217
221
223
229
235
239
243
247
257
265
273
281
VIII
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Figures
Number
Page
1-1 Methodology for achieving POTW compliance 2
2-1 CPE/CCP schematic of activities 6
2-2. Effect of aeration basin DO concentrations on sludge settling characteristics 26
2-3. Typical return sludge flow rates with various clarifier surface overflow rates. ....... 28
2-4. Example performance potential graph .- 35
2-5. Flow diagram of POTW in example CPE. 36
3-1. Relationship of performance-limiting factors to achieving a performance goal 43
3-2. Typical scheduling of CCP activities 46
3-3 Sample process control and performance monitoring program form
for a small plant • 47
3-4. Example format for summarizing CCP action items 49
3-5. Relationship between suspended growth process control parameters
and effluent quality 51
3-6. Representations of activated sludge floe 51
3-7. Activated sludge mass control using MCRT. 53
3-8. Activated sludge mass control using total sludge mass 53
3-9. Simplified activated sludge process diagram. 54
3-10. Graphical presentation of improved performance from a successful CCP 60
4-1. Methodology for achieving POTW effluent compliance 64
4-2. Decision-making approach for selecting a modification
for an existing facility 67
4-3. Recycle system for variable flow control 69
4-4. Flow equalization system for Frisco, CO WWTP 71
4-5. Flow splitting from a Parshall flume 71
4-6. Flow schematics of two-stage fixed-film processes ' 77
4-7. Three modes of operation for the trickling filter/solids contact (TF/SC) Process 78
4-8. Flow schematics for the roughing filter/activated sludge
and biofilter/activated sludge processes. . 79
4-9. Fort Collins, Colorado RF/AS Wastewater Treatment Plant 80
4-10. Flow schematics for RBC modification options that provide
for load redistribution 83
4-11. Activated Biofilter (ABF) System 85
4-12. Relationship of variables for optimizing suspended
growth facility performance. . . 87
4-13. Flow schematics of conventional and step feed activated sludge processes. 89
4-14. Advantages of the step feed process in treating organic shock loads 90
4-15. Managing hydraulic shock loads with the step feed process 90
4-16. Use of step feed to reduce solids loading to the secondary clarifier 91
4-17. Organic loading configuration for Keokuk, Iowa
Wastewater Treatment Plant 91
4-18. Modified loading configuration for Keokuk, Iowa
Wastewater Treatment Plant 92
4-19. Contact stabilization mode of the activated sludge process 92
4-20. Conversion of complete mix to plug flow 93
4-21. Selector basin modification 95
IX
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Figures (continued)
Number Page
4-22. Conventional vs. intrachannel oxidation ditch systems 95
4-23. Roughing filter/activated sludge 97
4-24. BF/AS system at Casper, Wyoming 97
4-25. Biological Aerated Filter (BAF) 98
4-26. Flow diagram of the Two-Zone System 99
4-27. Typical flow pattern for center-feed, peripheral overflow clarifier 102
4-28. Typical flow pattern in rectangular clarifier 102
4-29. Secondary clarifier with flocculating center well 103
4-30. Mid-radius baffle for circular clarifier . 103
4-31. Mid-tank baffle for rectangular clarifier. 104
4-32. Baffled weir configurations . . 104
4-33. Typical rectangular clarifier. . . 105
4-34. Center-feed clarifier with cantilevered weir. • '. 105
4-35. Schematic of Sterling, Colorado wastewater stabilization pond system .. 107
4-36. Effect of short-circuiting on organic loading 108
4-37. Wastewater stabilization pond system with HCR flexibility. 109
4-38. Modification of ponds with baffle walls 111
4-39. Inlet/outlet configurations . 111
4-40. Variable level draw-off structure 112
4-41. Stabilization pond system utilizing duckweed cover. 113
4-42. Series and parallel operational flexibility. 113
4-43. Stabilization pond system with recirculation flexibility 114
4-44. Modifications to existing two-cell pond. 114
4-45. Schematic of Stevensville, Montana WWTP , • • • 115
4-46. Stevensville, Montana performance potential graph 117
4-47. Tuscaloosa, Alabama 1986 sludge production . 117
4-48. Typical sludge handling, treatment, and disposal processes 120
4-49. Addition of baffles to existing circular contact basin 125
4-50. Modifications for cross-baffled contact basins 126
4-51. Plan view of secondary clarifier with chlorine solution diffuser. 126
4-52. Schematic of a 3-stage ozone contact basin 127
4-53. Inlet and outlet considerations for quartz UV systems 129
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Tables
Number
Page
2-1. Classification System for Prioritizing Performance-Limiting Factors 7
2-2. Parameters for Scoring Capability of Aeration Basins
in Suspended Growth POTWs 13
2-3. Typical Clean Water Standard Oxygen Transfer Values . . 14
2-4. Parameters for Scoring Capability of Clarifiers in Suspended Growth POTWs 16
2-5. Typical Ranges for Return Activated Sludge Pumping Capacities 17
2-6. Criteria for Scoring Sludge Handling Capability for Suspended Growth POTWs ..... 17
2-7. Unit Sludge Production Values for Projecting Sludge Production
From Suspended Growth POTWs 18
2-8. Sludge Concentrations for Projecting Sludge Production
From Suspended Growth POTWs 18
2-9. Guidelines for Evaluating Capability of Existing Sludge Handling Processes 19
2-10. Miscellaneous Unit Values Used in Evaluating Sludge Handling Capability 19
2-11. Suspended Growth Major Unit Process Capability Evaluation 20
2-12. Parameters for Scoring Aerator Capability for Trickling Filter POTWs 20
2-13. Parameters for Scoring Aerator Capability for RBC POTWs 21
2-14. Parameters for Scoring Aerator Capability for ABF POTWs 21
2-15. Parameters for Scoring Capability of Clarifiers in Trickling Filters and RBCs 21
2-16. Criteria for Scoring Sludge Handling Capability for Fixed Film POTWs 22
2-17. Unit Sludge Production and Sludge Concentration Values
for Projecting Sludge Production From Fixed Film POTWs 22
2-18. Trickling Filter Major Unit Process Capability Evaluation 22
2-19. RBC Major Unit Process Capability Evaluation 22
2-20. ABF Major Unit Process Capability Evaluation 23
2-21. Parameters for Scoring Capability of Facultative Stabilization
Pond Systems 23
2-22. Parameters for Scoring Capability of Aerated Stabilization
Pond Systems 24
2-23. Stabilization Pond Process Capacity Evaluation 24
2-24. Typical Mean Cell Residence Times for Suspended Growth POTWs 28
2-25. Suspended Growth Major Unit Process Capacity Evaluation for Example CPE 39
2-26. Typical Cost for Conducting CPEs 40
3-1. Typical CCP Facilitator Involvement 46
3-3. Acronyms Used in Figure 3-3 48
3-3. Process Control Monitoring at a Small Activated Sludge Plant 56
3-4. Typical Cost for Conducting a CCP 60
4-1. Summary of Design Limitations 65
4-2. Preliminary Treatment/Primary Treatment Design Limitations
and Potential Modifications 67
4-3. Design Criteria for Frisco, CO WWTP Flow Equalization System 71
4-4. Fixed-Film Design Limitations and Potential Modifications 74
4-5. Design Criteria for Fort Collins, Colorado TF/AS Wastewater Treatment Plant 81
4-6. Suspended Growth Design Limitations and Potential Modifications 87
4-7. Design Criteria for Casper, WY BF/AS System 97
4-8. Aeration Basin Mixing Requirements 105
XI
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Tables (continued)
Number Page
4-9. Stabilization Pond Design Limitations and Potential Modifications 106
4-10. Design Organic Loading Rates for Wastewater Stabilization Ponds 106
4-11. Loading to the Sterling, Colorado Wastewater Stabilization
Treatment Facility 107
4-12. Sludge Production Data for Tuscaloosa, Alabama WWTP 118
4-13. Summary of Reported Minimum and Maximum Variations
with a Sustained Mass Loading for Selected Periods 118
4-14. Guidelines for Modifying Existing Sludge Handling Facilities 118
4-15. Sludge Handling Design Limitations and Potential Modifications 120
4-16. Additional Design Limitations 124
4-17. Effect of Short-Circuiting on Disinfection Performance 125
4-18. Fecal Coliform Counts Before and After Installation of Diffuser 127
XII
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Acknowledgments
Many individuals contributed to the preparation and review of this handbook. Contract
administration was provided by the Center for Environmental Research Information, Cincinnati,
Ohio.
Major Authors:
Bob A. Hegg, Larry DeMers, John Barber - Process Applications Inc., Ft. Collins, Colorado
Contributing Authors:
Edwin F. Barth, Jr. - Barth Tec, Inc., Cincinnati, Ohio
David T. Chapman - Canada Centre for Inland Waters, Burlington, Ontario
Dais Chaudhary - Hazen and Sawyer, New York, New York
Alan Cooper - Black and Veatch, Inc., Rockville, MD
Glen Daigger - CH2M Hill, Denver, Colorado
E. J. Middlebrooks - University of Tulsa, Tulsa, Oklahoma
Ms. Tyler Richards - Pollution Control Bureau, Atlanta, Georgia
Rudy J. Tekippe - James M Montgomery Engineers, Pasadena, California
Ronald F. Wukasch - Purdue University, West Lafayette, Indiana
James Young - University of Arkansas, Fayetteville, Arkansas
Sherwood Reed - U.S. Army Corps of Engineers, Hanover, New Hampshire
Reviewers:
Richard C. Brenner - U.S. EPA-RREL, Cincinnati, Ohio
Arthur J. Condren - James M. Montgomery Engineers, Pasadena, California
James A.Heidman - U.S. EPA-RREL, Cincinnati, Ohio
Peer Reviewers:
Eric Cohen - U.S. EPA, Office of Water, Washington, DC
Charles Conway - U.S. EPA-Region 1, Boston, Massachusetts
John Esler - NYS Dept. of Environmental Conservation, Albany, New York
Technical Direction/Coordination:
James F. Kreissl - U.S. EPA-RREL, Cincinnati, Ohio
Denis J. Lussier - U.S. EPA-CERI, Cincinnati, Ohio
xiii
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Chapter 1
Introduction
1.1 Purpose
This manual is intended as a source document for
individuals responsible for improving the performance
of an existing, non-complying wastewater treatment
facility. Described are: 1) methods to evaluate an
existing facility's capability to achieve improved
performance, 2) a process for systematically
improving its performance, and 3) details on how to
modify the facility to achieve the required levels of
performance. The manual emphasizes meeting
National Pollutant Discharge Elimination System
(NPDES) permit requirements for secondary treatment
facilities (30 mg/L BOD5 and TSS). Though the
manual is not intended to describe cost saving options
or to present alternatives for designing new facilities
for expansion purposes (i.e., to provide increased
hydraulic and/or BOD loading capacity), in some
cases the approach and modifications described may
result in cost savings and/or increased capacity.
1.2 Background
The need for better treatment performance from
existing facilities is widespread. Usually the most cost
effective approach for owners to achieve compliance
is to optimize existing facilities either in terms of
capital or operational improvements. Recent events
have provided impetus to this approach by increasing
incentives for improving existing plant performance.
The 1986 Needs Survey (1), conducted by the U.S.
Environmental Protection Agency (EPA), identified
10,131 of 15,438 operating treatment plants as having
documented water quality or public health problems.
The study determined that $36.2 billion is needed to
meet the needs of secondary treatment, advanced
treatment, infiltration/inflow, and new interceptor
sewers. However, the funds available to support these
projects are decreasing. Directions provided by the
Clean Water Act (CWA) (2), which became law in
February 1987, increased local responsibility for
financing wastewater treatment projects by decreasing
grant funding. The CWA provided $9.6 billion for
construction grants, which falls short of the projected
needs. A major aspect of the CWA was that these
grants would end after 1990.
An alternative source of funding assistance was
addressed by the CWA and involved providing seed
money for state implemented, revolving loan
programs. Funds allocated for this program are as
follows: $1.2 billion annually for FY 89 and FY 90;
$2.4 billion for FY 91; $1.8 billion for FY 92; $1.2
billion for FY 93; and $0.6 billion for FY 94 (a total of
$7.2 billion). Even with these funds, the requirements
identified in the Needs Survey will not be met.
The combined effects of a significant need for capital
improvements at wastewater treatment projects,
decreased grant funding, and more local responsibility
for financing encourages optimization of existing
facilities.
Large numbers of wastewater treatment plants are still
in serious violation of final effluent permit
requirements. A Management Advisory Group,
consisting of an independent group of wastewater
professionals, examined the municipal compliance
picture for the U.S. EPA. Their 1987 report (3)
indicated that the percentage of major dischargers still
not in compliance could be as high as 37 percent.
U.S. EPA's National Municipal Policy, issued in early
1984, called for every wastewater treatment system to
be in compliance with permit requirements by July 1,
1988 (4). A major provision of the Policy was the
requirement of a non-complying facility to develop a
Composite Correction Plan. The purpose of this plan
was to identify the causes of non-compliance and to
outline corrective actions and a schedule for
completing the corrective actions in order to achieve
compliance. Provisions were also required in the plan
for interim steps to optimize existing facilities. As
such, the Policy focuses heavily on optimizing existing
facilities.
1.3 Scope
Figure 1-1 depicts the methodology for achieving
POTW compliance used in this manual. As shown, an
evaluation procedure must be implemented to
establish existing facility capability. An evaluation
approach called a Comprehensive Performance
Evaluation (CPE) is described in Chapter 2. If
compliance appears possible through optimization of
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Figure 1-1. Methodology for achieving POTW compliance.
POTW Out Of
Compliance
Wilh Permit
Requiremenls
^
w
Conduct Plant
Evaluation To
Identify
Reasons for
Non-
Compliance
Compliance Possible
Through
Optimization of
Existing Facilities
Without Major Capital
Expenditures
Existing
Facilities
Inadequate -
Major Capital
Expenditures
Required
Abandon Existing
Facilities And
Construct New
Facilities
Yes
existing facilities, then a systematic approach to
address identified deficiencies should be implemented
to achieve compliance (i.e., Chapter 3 - Composite
Correction Program [CCP]). If compliance has not
been achieved after completing the evaluation step
and implementing corrective actions to optimize
performance of the existing plant, facility modifications
are indicated. Either: 1) totally new facilities must be
designed and constructed or 2) capital expenditures
must be made at the existing facility. A fundamental
difference between these options is that the first
approach includes activities that assess the need for
increased capacity as well as improved performance,
and includes an alternative analysis that may lead to
abandoning present facilities. The second approach, a
facility modifications or retrofit approach, emphasizes
existing facility changes to meet desired performance
with current loads. Chapter 4 presents facility
modifications that may be suitable for implementation
to assist in plant optimization efforts.
The intended users of this manual are owners,
designers, and others associated with the respon-
sibility of achieving compliance or more reliable
performance from their existing facilities. The manual
provides procedures for: 1) conducting performance
evaluations, 2) implementing performance improve-
ment activities, and 3) selecting facility modifications
for secondary treatment plants that have identified
design limitations. Relative to implementing facility
modifications (i.e., Chapter 4), only general design
criteria and guidelines are discussed. The manual
user will need to refer to the specific references for
the more detailed design information.
The manual focuses on conventional secondary
treatment facilities which are broadly categorized as
suspended growth facilities (e.g., activated sludge),
fixed film facilities (e.g., trickling filter), and
stabilization ponds.
As defined by the EPA, a secondary treatment plant is
a facility designed to provide an effluent with monthly
average BOD5 and TSS concentrations of 30 mg/L.
Stabilization pond modifications described in the
manual are those that allow ponds to meet the relaxed
standards for facilities less than 88 L/s (2 mgd)
capacity (i.e., typically 30 mg/L BOD5 and 75 mg/L
TSS).
Preliminary and primary treatment and sludge
treatment processes are presented, but only from the
perspective of their impact on achieving desired
secondary effluent quality. Nutrient removal facilities
are specifically excluded from this manual except
where aspects of these processes (i.e., oxygen
requirements for nitrification) directly impact
secondary treatment processes. Innovative/alternative
(I/A) technologies are presented, where applicable,
with respect to modifying the capabilities of existing
conventional facilities. However, facility modifications
directed toward improving performance of
innovative/alternative technologies are beyond the
scope of the manual. Also, land treatment
technologies are not addressed in this manual.
Readers interested in this technology are directed to
other sources (5,6).
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1.4 Using the Manual
This manual represents an expanded update of the
EPA Handbook Improving POTW Performance Using
the Composite Correction Program Approach (7)
coupled with additional materials which address
methods of selecting and implementing facility
modifications. The manual is intended to be used in
whole or in part to pursue improved performance with
existing facilities.
Text of the manual. closely parallels the major steps
depicted in Figure 1-1. Chapter 2 discusses an
evaluation approach to identify reasons for non-
compliance and to assess the suitability of existing
facilities for improved performance. The procedure
described is called a CPE. At non-complying facilities,
this (or similar) evaluation procedure should be
implemented before a decision is made to pursue the
next phase of performance improvement.
Chapter 3 discusses the CCP approach which details
methods of optimizing existing facilities without major
capital expenditures. Procedures to address design,
operation, maintenance, and administrative factors
limiting performance are outlined. Implementation of a
CCP ensures optimization of existing facilities, and, if
compliance is not achieved, the design factors limiting
performance are identified.
Chapter 4 has been added to the manual to provide
the reader with a more detailed description of facility
modifications that can result in improved perfor-
mance. This portion of the manual can be used in
conjunction with the CPE/CCP steps outlined in
Chapters 2 and 3 or it can be used alone if the reader
has already identified that a facility modification is
necessary to achieve desired effluent quality. In both
cases the user of this portion of the manual must
have already established the design factors or
deficiencies limiting performance of the treatment
facility. Chapter 4 utilizes a tabular format to list
design factors that can potentially limit compliance of
major unit processes, describes briefly the manner in
which compliance is affected, and directs the reader
to those sections of the chapter where specific
information regarding suggested facility modifications
are discussed.
1.5 References
1. 1986 Needs Survey.
2. Water Quality Act of 1987, P.L. 100-4.
3. 1987 Management Advisory Group Report.
Municipal Construction Grants Division, Office of
Municipal Pollution Control, U.S. Environmental
Protection Agency, Washington, DC, October,
1987.
4. 1984 National Municipal Policy. Federal Register,
January 30, 1984.
5. Process Design Manual: Land Treatment of
Municipal Wastewater. EPA-625/1-81-013, U.S.
Environmental Protection Agency, Cincinnati, OH,
1981.
6. Process Design Manual: Land Treatment of
Municipal Wastewater - Supplement on Rapid
Infiltration and Overland Flow. EPA-625/1-81-013a,
U.S. Environmental Protection Agency, Cincinnati,
OH, 1984.
7. Handbook: Improving POTW Performance Using
the Composite Correction Program Approach. EPA-
625/6-84-008, U.S. Environmental Protection
Agency, Cincinnati, OH, 1984.
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Chapter 2
Comprehensive Performance Evaluations
2.1 Introduction
This chapter provides information on the evaluation
phase of a two-step process to economically improve
the performance of existing publicly owned treatment
works (POTWs). The evaluation phase, called a
Comprehensive Performance Evaluation (CPE), is a
thorough review and analysis of a POTW's design
capabilities and associated administrative, operational,
and maintenance practices. It is conducted to provide
information for POTW administrators to make
decisions regarding efforts necessary to improve
performance. The primary objective is to determine if
significant improvements in treatment can be achieved
without major capital expenditures. This objective is
accomplished by assessing the capability of major unit
processes and by identifying and prioritizing those
factors that limit performance and which can be
corrected to improve performance.
The second step of the process is called a Composite
Correction Program (CCP) and represents the
performance improvement phase. It is a systematic
approach to eliminating those factors that inhibit
performance in existing POTWs. A CCP focuses on
optimizing the capability of existing facilities to perform
better. This phase is described in Chapter 3.
It is assumed that POTW owners and administrators
have already recognized a need to improve the
performance of their wastewater treatment facilities
and will use this manual to economically accomplish
the required wastewater effluent quality.
2.2 Approach to Conducting CPEs
2.2.1 Methodology
A CPE involves several activities: evaluation of the
major unit processes; identification of performance-
limiting factors; prioritization of performance-limiting
factors; assessment of approach to improve
performance; and reporting results of the evaluation.
Although these are distinct activities, some are
conducted concurrently with others. For example,
evaluation of the major unit processes and
identification of performance-limiting factors are
generally undertaken at the same time.
Although this chapter presents all the information
required to conduct a CPE, many references are
available on techniques for evaluation of treatment
plant performance, reliability, etc. (1-14). It is
recommended that these references be consulted for
further specifics on the subject.
2.2.1.1 Evaluation of Major Unit Processes
Major unit processes are evaluated to assess their
potential to achieve desired performance levels. If the
CPE indicates that the major unit processes are
adequate or potentially adequate, a major plant
expansion or upgrade may not be necessary and a
properly conducted CCP should be implemented to
achieve optimum performance. If, on the other hand,
the CPE shows that major unit processes are
inadequate, owners should consider modification of
these processes as the focus for achieving desired
performance.
Results of evaluation of major unit processes can be
summarized by categorization of plant type, as
illustrated in Figure 2-1.
Type 1 plants are those POTWs where a CPE shows
that current performance difficulties are not caused by
limitations in the size "or capabilities of the existing
major unit processes. In these cases, the major
problems are related to plant operation, maintenance,
or administration, or to problems that can be corrected
with only minor facility modifications. POTWs that fall
into this category are most likely to achieve desired
performance through the implementation of a
nonconstruction-oriented CCP.
Identification of a POTW as Type 2 represents a
situation where the marginal capacity of major unit
processes will potentially prohibit the ability to achieve
the desired performance level. For Type 2 facilities,
implementation of a CCP will lead to improved
performance but may not achieve required
performance levels without facility modifications to the
major treatment units.
A Type 3 plant is one in which the existing major unit
processes are inadequate. Although other limiting
factors may exist, such as the operators' process
control capability or the administration's unfamiliarity
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Figure 2-1 CPE/CCP schematic of activities.
POTW Administrators
Recognize Need To
Improve Plant Performance
CPE Evaluation
of
Major Unit Processes
Type 1
Major Unit Processes
Are Adequate
Type 2
Major Unit Processes
Are Marginal
Type 3
Major Unit Processes
Are Inadequate
Implement CCP to
Achieve Desired
Performance
From Existing Facilities
Implement CCP to
Optimize Existing Facilities
Before Initiating
Facility Modifications
Do Not Implement CCP
Evaluate Options For
Facility Modifications
Desired Performance Achieved
Abandon Existing
Facilities arid
Design New
Ones
with plant needs, performance cannot be expected to
improve significantly until physical limitations of major
unit processes are eliminated. In this case,
implementation of a nonconstruction-oriented CCP
may only be of limited value and is not recommended.
Owners with a Type 3 plant could meet their
performance requirements by pursuing modifications
of existing wastewater treatment facilities. However,
depending on future waste loads, more detailed study
of treatment alternatives and financing mechanisms
may be warranted. CPEs that identify Type 3 facilities
are still of benefit to POTW administrators in that the
need for construction is clearly defined for facility
owners. Additionally, the CPE provides an under-
standing of the capabilities and weaknesses of
existing operation and maintenance practices and
administrative policies. POTW owners can use this
information to evaluate use of existing facilities as part
of any plant modification and as a guideline for
optimizing operational, maintenance, and admini-
strative practices.
2.2.1.2 Identification of Performance-Limiting
Factors
Whereas the evaluation of major unit processes in a
plant is used to broadly categorize performance
potential by assessing only physical facilities, the
identification of performance-limiting factors focuses
on one facility and the factors unique to that facility.
To assist in this identification, a list of 70 different
factors that could potentially limit a POTW's
performance is provided in Appendix A (1). These
factors are divided into the categories of
administration, maintenance, design, and operation.
Suggested definitions of each factor are also provided.
This list was developed as a result of many plant
studies and is provided for convenience and
reference. If alternate names or definitions provide a
clearer understanding to those involved in conducting
a CPE, they should be used. If different terms are
used, each factor should be defined and these
definitions should be readily available to those
conducting the CPE and those interpreting the results.
Note that the list includes factors on capacity of major
unit processes. If the evaluation of major unit
processes results in a Type 2 or 3 classification,
these same limitations should be documented in the
list of factors limiting the POTW's performance.
Completing the identification of factors is difficult in
that true problems in a POTW are often masked. This
concept is illustrated in the following discussion.
A contact stabilization plant was routinely losing
sludge solids over the final clarifier weirs, through
the chlorine contact tank, and to the receiving
stream, resulting in noncompliance with the plant's
permit. Initial observations could lead to the
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conclusion that the plant had an inadequately sized
final clarifier. However, further investigation
indicated that the solids loss was a result of the
operator's practice of routinely wasting less sludge
than was produced. It was determined, that
increased operator time and additional monitoring
equipment would be required to properly control the
sludge mass. It was further determined that the
digester was undersized and would not provide
adequate residence time for complete digestion of
the waste activated sludge.
The most obvious problem is the operator's lack of
knowledge of how to apply the concept of sludge
mass control. The needed laboratory equipment
was within the approved budget for the facility and
therefore was not assessed as a major problem.
Plant administrators indicated that they could not
afford additional operator time. This administrative
policy was a significant factor limiting performance.
The undersized digester was a less significant
problem in this case because unlimited cropland for
disposal of partially digested sludge was available.
(Note: Disposal of partially digested sludge on
cropland can no longer be considered a permanent
solution since promulgation of Federal regulations
for land disposal of POTW sludges). It was
concluded that four factors contributed to the solids
loss that caused poor plant effluent quality:
1. Inadequate operator knowledge to apply the
concept of sludge mass control.
2. Restrictive administrative policy that prohibited
needed operator time.
3. Inadequate test equipment.
4. Inadequate digester capacity.
The above discussion illustrates that a comprehensive
analysis of a performance problem is essential to
identify the true performance-limiting factors. If the
initial obvious problem of lack of clarifier capacity had
been identified, improper corrective actions and
unnecessary expenditures of funds would likely have
occurred.
It is emphasized that the purpose of identifying
performance-limiting factors is to identify, as
accurately as possible, causes of poor performance
unique to a particular plant. Observation that a factor
does not meet the "industry standard" does not
necessarily constitute cause for identifying that factor
as limiting the POTW's performance. An actual link
between poor plant performance and an identified
factor must exist.
2.2.1.3 Prioritization of Performance-Limiting
Factors
In almost all CPEs, several factors are identified as
limiting performance. After these factors have been
identified, they are prioritized as to their adverse effect
on achieving desired plant performance. The purposes
of this prioritization are to establish the sequence
and/or emphasis of follow-up activities necessary to
achieve compliance. If the highest ranking factors
(i.e., those having the most negative impact on
performance) are related to physical limitations in unit
process capacity, initial corrective actions are directed
toward defining plant modifications and obtaining
administrative funding for their implementation. If the
highest ranking factors are process control oriented,
the initial emphasis of follow-up activities would be
directed toward plant-specific operator training.
The prioritization of factors is accomplished by a two-
step process. First, all factors that have been
identified are individually assessed with regard to
adverse impact on plant performance and assigned an
"A," "B," or "C" rating (Table 2-1). The checklist of
factors in Appendix A includes a column to enter this
rating. Second, those factors receiving "A" or "B"
ratings are listed in order of priority, since typically all
"A" and "B" factors must be eliminated before a plant
will achieve consistent desired performance.
Table 2-1. Classification System for Prioritizing
Performance-Limiting Factors
Rating
Adverse Effect of Factor on Plant Performance
A Major effect on long-term repetitive basis
B Minimum effect on routine basis or major effect on a
periodic basis
C Minor effect
Factors that are assigned an "A" are the major
problems that cause a performance deficiency. They
should be the central focus of any subsequent
program to improve plant performance. An example of
an "A" factor would be "ultimate sludge disposal"
facilities (e.g., drying beds) that are too small to allow
routine wasting of sludge from an activated sludge
POTW.
Factors are assigned a "B" if they fall in one of two
categories:
1. Those that routinely contribute to poor plant
performance but are not the major problems. An
example would be a shortage of staff time to
complete required process control testing in a small
activated sludge plant where the underlying
problem is that the operator does not understand
how to run or interpret the tests or understand the
need for a better testing program.
2. Those that cause a major degradation of plant
performance, but only on a periodic basis. Typical
examples are infiltration or inflow that cause
periodic solids loss from final clarifiers, or marginal
oxygen transfer capacity that causes an oxygen
shortage only during the hottest month of the year.
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Factors that receive a "C" rating can be shown to
contribute to a performance problem, but their effect
is minor. For example, if a critical process stream
were accessible, but difficult to sample, it could
indirectly contribute to poor performance by making
process control testing less convenient and more time
consuming. The problem would not be a major focus
of a subsequent corrective program. As a comparison
of the different ratings, the example "A" factor above
("ultimate sludge disposal") would receive a "B"
rating if adequate drying bed capacity were available,
in the summer but winter weather inhibited drying bed
use. The factor would receive a "C" rating if adequate
drying bed capacity were available but cleaning the
beds with a front loader has crushed several
underdrain tiles.
In the illustration presented in Section 2.2.1.2,
"inadequate operator knowledge to apply the concept
of sludge mass control" is assigned an "A" because
of its continuous detrimental effect on plant
performance; "administrative policy" a "B" because of
its routine effect; and "testing equipment" a "C"
because its effect is only a minor contributing factor.
"Inadequate digester size" is given a "B" because it
made proper sludge mass control more difficult and
labor intensive. It is not given an "A" because it did
not limit performance in a major way since adequate
sludge disposal capacity is available by utilizing
nearby cropland.
During the conduct of a CPE, the factors that are not
identified as performance limiting also provide very
useful information for POTW owners. For example, in
the illustration presented in Section 2.2.1.2, the
clarifier was not identified as a performance-limiting
factor. Since it was not identified, plant personnel do
not need to focus on the clarifier as a problem.
Typically 5 to 15 factors are identified during a CPE.
The remaining 55 to 65 factors outlined in Appendix A
that are not identified represent a significant finding
and also a source of providing recognition to plant
personnel for adequately addressing these sources of
problems.
Once each identified factor is assessed individually
and assigned an "A," "B," or "C" classification, those
receiving "A" or "B" ratings are listed on a one-page
summary sheet in order of priority. This requires that
the evaluator assess all the "A" and "B" factors to
determine the most serious cause of poor
performance, second most serious, etc. A summary
sheet for ranking the prioritized factors limiting plant
performance in order of severity is presented in
Appendix B. This process is effective in reducing the
identified factors to a one-page summary and serves
as a valuable reference for the next step of the CPE:
assessing ability to improve plant performance.
All factors limiting facility performance typically cannot
be, nor are they intended to be, identified during the
CPE phase. It is often necessary to later modify the
original corrective steps and requirements as new or
additional information becomes available during the
conduct of a performance improvement (CCP) phase.
This concept is illustrated by the following:
A CPE conducted at an activated sludge plant
identified the major performance-limiting factors as;
1. Inadequate operator understanding to make
process adjustments to control sludge settling
characteristics ("A").
2. Inadequate staffing to make operational
adjustments ("B").
3. Inadequate maintenance program to keep
equipment functioning continuously ("C").
Based on these factors, a CCP was implemented
to improve performance of the existing facilities. It
was decided that this plant could perform best
when the activated sludge settling rate was
relatively 'slow. The plant operator's understanding
was improved through training, and he became
capable of making process control adjustments to
achieve the desired slower sludge settling rate.
Once the desired slower sludge settling rate was
achieved, poor clarifier performance was observed
and effluent quality deteriorated. Further
investigation indicated that modifications made a
year earlier to the clarifier inlet baffles were allowing
short-circuiting to occur. This short-circuiting only
became apparent after the slower settling sludge
solids predominated in the system. These baffle
modifications were reassessed and changed to
reduce short-circuiting, and effluent quality
improved dramatically.
In this illustration, a minor design modification was
determined to be a performance-limiting factor. This
factor was not identified in the original CPE. An
awareness that it may not be possible to identify all
performance-limiting factors in the CPE, as well as an
awareness that the performance improvement phase
allows further definition and identification of factors
during its implementation, is an important aspect of
understanding the approach to conducting a CPE.
2.2.1.4 Assessing Approach to Improve
Performance
All performance-limiting factors can be eliminated.
Therefore, it is necessary to specifically assess the
ability to achieve improved performance in each
POTW. If performance improvement efforts are
restricted to existing facilities, then a CCP with facility
modifications or facility modifications can be employed
to eliminate identified performance-limiting factors
(Figure 2-1). Another option is to abandon the exi
sting plant and design new facilities.
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An assessment of the list of prioritized factors helps
assure that all factors can realistically be addressed
given the unique set of factors noted at the facility
being evaluated. Often there are practical reasons
why a factor cannot be addressed in a straightforward
manner. Examples of factors that may not be feasible
to address directly are replacement of key personnel,
required increases in funding, or training of
uncooperative owners or administrators to support
POTW needs. As such, alternative solutions or
combinations of corrective actions that can effectively
address the problem may have to be developed. For
example, an activated sludge secondary clarifier may
be improved by installing baffles to decrease short-
circuiting, by utilizing partial flow equalization to
reduce hydraulic peaks, or by switching to other
activated sludge modes to modify sludge settling
characteristics. Often a combination of these
corrective actions would be appropriate.
Proper interpretation of the CPE findings is necessary
to provide the basis for recommendations of which
alternatives to pursue for the performance
improvement phase. It is at this assessment phase
that the maximum application of the evaluator's
judgment and experience is required.
2.2.1.5 CPE Report
The results of a CPE should be summarized in a brief,
written report to provide guidance for facility owners
and administrators. An example is included in
Appendix C. A typical CPE report is 8-12 pages in
length and includes the following topics:
• Facility background
• Major process evaluation
• Performance-limiting factors
• Performance improvement activities
• Costs
A CPE report should not provide a list of specific
recommendations for correcting individual
performance-limiting factors. This often leads to a
piecemeal approach to corrective actions where the
goal of improved performance is not met. For Type 1
and 2 plants, the necessity of comprehensively
addressing the combination of factors identified by the
CPE through the implementation of a CCP should be
stressed. For Type 3 plants, a recommendation for a
facility modification or a more detailed study to
support the anticipated upgrade may be warranted.
2.3 How to Conduct a CPE
2.3.1 Initial Activities
To determine the magnitude of the fieldwork required,
and to make the on-site activities most productive,
specific initial information should be gathered. This
information includes basic data on the POTW and
sources for any needed additional information. If a
person associated directly with the POTW is the
evaluator conducting the CPE, some of the steps may
not be necessary.
2.3.1.1 Personnel
The evaluator should obtain the names of those
persons associated with the POTW who will be the
primary sources of information for the CPE. The
POTW superintendent, manager, or other person in
charge of the wastewater treatment facility should be
identified. If different persons are responsible for plant
maintenance and process control, they should also be
identified.
The person most knowledgeable about the details of
the POTW budget should be identified by name,
position, and physical location. A one- to two-hour
meeting with this person during the fieldwork will have
to be scheduled to obtain a copy of the budget and
discuss it. In many small communities, this person is
most often the city clerk; in larger communities, the
utilities director or wastewater superintendent can
usually provide the best information on the budget.
Key administrative personnel should also be identified.
In many small communities or sanitation districts, an
operator or plant superintendent may report directly to
the elected governing administrative body, usually the
city council or district board. In larger communities,
the key administrative person is often the director of
public works, city manager, or other non-elected
administrator. In all cases, the admiriistrator(s) who
has the authority to effect a change in policy or
budget for the POTW should be identified.
If a consulting engineer is currently involved with the
POTW, that individual should be informed of the CPE
and be provided a copy of the final report for
comment. Normally, the consulting engineer will not
be directly involved in conduct of the CPE. An
exception may occur if there is an area of the
evaluation that could be supplemented by the
expertise available through the consultant.
2.3.1.2 Wastewater Treatment Plant
The initial information outlined in Appendix D (Form D-
1) can be used to estimate field time required. The
plant superintendent and/or chief operator typically
would be the contact for this information. This
information should be collected bearing in mind that
some of the data may later be found to be inaccurate.
As such, the data that a chief operator can provide
from memory or from a readily available reference are
sufficient at this time.
Irregularities that may warrant special consideration
when planning or conducting the fieldwork should be
identified, and more specific questions should be
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asked to define the potential effect on the evaluation.
Frequently occurring irregularities include: major
process or pieces of equipment out of service; key
persons on vacation or scheduled for other priority
work; and new or uncommon treatment processes.
An out-of-service single trickling filter, aeration basin,
or final clarifier will probably necessitate postponing
fieldwork in small plants. In plants with duplicate unit
processes, a CPE can be conducted with one unit out
of service if the results of the evaluation are needed
before normal operation can be resumed.
2.3.1.3 Scheduling
Interviews of personnel associated with the
wastewater treatment facilities are a key component of
conducting a CPE. As such, the major criterion for
scheduling the time for a CPE should be local
personnel availability. Usually, one-half to two-thirds of
the time scheduled for fieldwork will require the
availability and help of these persons.
Scheduling should be coordinated with the availability
of at least the major process control decision-maker,
the major administrative decision-maker, and the
person most knowledgeable of the plant budget. A
commitment of time from these key persons is
essential to the successful conduct of a CPE. It may
also be beneficial to inform State and Federal
regulatory personnel of the CPE schedule.
Responsibility for this task should be clearly identified
between the evaluator and local personnel during the
scheduling of activities.
During the fieldwork, the process control decision-
maker should be prepared to devote at least half of
his/her time to the evaluation. The administrative
decision-maker should be available for one hour for a
kickoff meeting, several hours for reviewing the
budget, another several hours for an interview
concerning plant administration issues, and one to two
hours for a summary meeting.
2.3.2 Data Collection
Onsite CPE activities are largely devoted to collection
and evaluation of data. As a courtesy to the facility
owner, and to promote efficient data collection, the
fieldwork is initiated with a kickoff meeting and a plant
tour. These activities are followed by a period of time
where a large amount of detailed data on the POTW
are gathered and analyzed.
2.3.2.1 Kickoff Meeting
A short meeting between key POTW personnel
(including key administrators) and the evaluator should
be held to initiate the fieldwork. The major purposes of
this meeting are to explain and gain support for the
CPE effort, to coordinate and establish the schedule,
and to initiate the administrative evaluation activities.
The objectives of the CPE should be presented along
with the proposed activities. Specific meeting times
for interviews with non-plant and plant personnel
should be scheduled, information and resource
requirements should be spelled out. Specific items
that are required and may not be readily available are:
budget information to provide a complete overview of
costs associated with wastewater treatment; schedule
of sewer use and tap charges; discharge permit
(NPDES) for the POTW; historical monitoring data (1
year); utility bills (1 year); sewer use ordinance (if
applicable); and any facility plans or other engineering
studies completed on the existing facility.
Administrative factors that may affect plant
performance should be noted during this meeting,
such as the priority put on permit compliance,
familiarity with plant needs, communication between
administration and plant staff, and policies on plant
funding. These initial perceptions often prove valuable
when formally evaluating administrative factors later in
the CPE effort.
2.3.2.2 Plant Tour
A plant tour should follow the kickoff meeting. The
objectives of the tour are to familiarize the evaluator
with the physical plant, make a preliminary
assessment of design operational flexibility of the
existing unit processes, and provide an initial basis for
discussions on performance, process control, and
maintenance. A walk-through tour following the flow of
wastewater is suggested. It is then appropriate to tour
the sludge treatment and disposal facilities, followed
by the support facilities such as maintenance areas
and laboratories. The evaluator should note the
sampling points established throughout the plant for
both process control and compliance monitoring.
Suggestions to help the evaluator meet the objectives
of the plant tour are provided in the following sections.
a. Preliminary Treatment
Major components of preliminary treatment typically
include coarse screening or comminution, grit
removal, and flow measurement.
Although inadequate screening rarely has a direct
effect on plant performance, it can become a
significant factor. For example, if surface mechanical
aerators must be shut down twice a day to remove
rags in an activated sludge plant with marginal oxygen
transfer capacity, screening could be a major
limitation. Indications of screening problems are:
• Plugging (with rags) of raw sewage or primary
sludge pumps
• Plugging of trickling filter distributors
• Rag buildup on surface mechanical aerators or
submerged diffusers
• Plugging of activated sludge return pumps where
primary clarifiers are not used
10
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Grit removal generally only has an indirect effect on
plant performance. For example, inadequate grit
removal can cause excessive wear on pumps or other
downstream and sludge processing equipment,
resulting in excessive downtime which could impact
plant performance and reliability.
Wastewater flow measurement facilities are important
to accurately establish plant loadings. The plant tour
should be used to observe the primary measuring
devices and to ask several questions regarding plant
flows. If flow is turbulent or non-symmetrical through
flumes and over flow measurement weirs, the flow
records are immediately questionable. If flow is non-
turbulent and symmetrical, there is a good chance the
flow measurement device is sufficiently accurate,
provided the flow recorder and totalizer prove to be
properly calibrated. The evaluator should always plan
to verify the accuracy of flow measurement during the
fieldwork.
Sources of wastewater and the nature of the waste
contributions should initially be discussed when
observing preliminary treatment facilities. Impacts of
infiltration and inflow on plant flows should also be
discussed.
b. Primary Clarification
The value of primary clarification in relation to overall
plant performance is in decreasing the load on
subsequent secondary treatment processes. As such,
the evaluator should determine what performance
monitoring of the primary processes is conducted. At
a minimum, sufficient data to calculate average BOD5
loadings on the secondary portion of the plant should
be available. The areas of major concern that should
be discussed during the tour are flexibility available for
changing operational functions and clarifier
performance.
The major operational variable that affects primary
clarifier performance is sludge removal. The evaluator
should discuss the process control method used to
adjust sludge withdrawal. In general, primary clarifiers
work best with a minimum of sludge in ,'the clarifier
(low sludge detention times arid low blanket level).
The practical limit for minimizing the sludge in the
clarifier is when the sludge becomes too thin (i.e., too
much water) such that it adversely affects the
capacity and/or performance of the sludge handling
facilities. A primary sludge concentration of less than
3 percent total solids often indicates there, is
opportunity for improved sludge handling facilities
performance with decreased sludge pumping. On the
other hand, a primary sludge concentration of greater
than 6 percent total solids can be an indication that
primary clarifier performance may be improved by
increased sludge pumping. The operational approach
used to improve primary clarifier performance must be
balanced with the capacity and performance
requirements of the sludge handling processes.
The surface overflow rate (SOR), which is the daily
average flow divided by clarifier surface area (CSA),
can be used as an indicator to estimate the
performance that can be expected from a primary
clarifier handling typical domestic wastewater. A
clarifier operating at an SOR of less than 24 m3/m2/d
(600 gpd/sq ft) will typically remove 35-45 percent of
the BOD5 in domestic wastewater. A clarifier operating
at an SOR of 24-40 m3/m2/d (600-1,000 gpd/sq ft) will
typically remove 25-35 percent of the BOD5.
c. Aerator
The term "aerator" is used in this manual to describe
the unit process that provides the conversion of
dissolved and suspended organic matter to settleable
microorganisms. Examples of an aerator are: aeration
basin, trickling filter, and rotating biological contactor
(RBC). The aerator represents a critical process in the
wastewater flow stream in determining overall plant
performance capability. During the plant tour, the
evaluator should determine if current operating
conditions represent normal conditions and inquire
about what operational flexibility is available. For
example: Can trickling filters be run in parallel as well
as series? Can recirculation be provided around the
filter only? Can aeration basins be operated in a step
loading mode as well as a plug flow mode?
d. Secondary Clarification
In all biological wastewater treatment plants, the main
function of secondary clarification is to separate the
sludge solids from the treated wastewater. Another
purpose is to thicken the sludge before removal from
the clarifier. Characteristics that should be noted on
the plant tour are configuration, depth, and operational
flexibility.
The evaluator should note the general configuration of
the clarifier, including shape, sludge removal
mechanism, and weir and launder arrangement. A
circular clarifier with a "donut" launder located several
feet from the clarifier wall and a siphon-type,, rapid
withdrawal sludge collector often provides satisfactory
performance. A long, narrow, shallow rectangular
clarifier with effluent weirs only at the end often
provides marginal solids separation and thickening
capability. Clarifiers with a depth of less than 3 m (10
ft) provide limited sludge storage and thickening
capability and create concerns about capacity,
especially in activated sludge plants.
The SOR can be used to roughly estimate final
clarifier performance capability. A SOR, based on
average daily flow less than 24 m3/m2/d (600 gpd/sq
ft), typically can be operated to achieve desired
performance. A significantly higher SOR would mean
11
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that other processes would have to be fairly
conservative to make the system perform adequately.
When touring activated sludge facilities, the evaluator
should become familiar with operation and flexibility of
the return sludge scheme: how sludge is withdrawn
from the clarifier; ability to operate at higher or lower
recycle rates; availability of return sludge flow
measurement; and flexibility to direct return sludge to
different aeration basins or points in the basins.
e. Disinfection
The evaluator should tour disinfection facilities to
become familiar with the process and equipment
available and because inspection of disinfection
facilities often provides insight into performance of the
secondary treatment process. Where disinfection is
required, nearly all POTWs use chlorine as the
disinfectant and incorporate a chlorine contact basin
of sufficient size to provide 10 minutes to 2 hours of
contact time.
Poorly performing biological wastewater treatment
facilities periodically lose sludge solids over the final
clarifier weirs. Chlorine contact basins generally will
capture a portion of these solids. If more than 5-10
cm (2-4 in) of sludge has built up on the bottom of the
basin, there is a good chance that significant solids
loss is occurring from the secondary clarifier.
f. Sludge Handling Capability
During the tour of sludge handling facilities, the
evaluator should become familiar with primary and
secondary sludge management practices, including: 1)
methods used to determine waste sludge quantities;
2) equipment used to thicken, stabilize, and dewater
sludge; and 3) available options for final disposal and
reuse. The evaluator's major concern with sludge
handling facilities is identifying any potential
"bottlenecks" and possible alternatives if problems
that may limit performance are indicated.
All recycle streams should be identified during the tour
and the plant personnel should be questioned
regarding the availability of data concerning each
stream's volume and strength. Return supernatant
streams from anaerobic digesters and heat treatment
conditioning processes are the most common return
streams that cause performance problems. Super-
natant from aerobic digesters and filtrate from
dewatering operations typically have a lesser impact
on plant performance.
g. Stabilization ponds
Municipal wastewater stabilization ponds are typically
classified as facultative or aerated, depending on the
method used to provide oxygen transfer in the facility.
Characteristics that should be noted include the
number of ponds, type of aeration and mixing, and
flexibility in flow patterns and discharge structures.
System flexibility should investigated (i.e., Can the
ponds be operated in both series and parallel? Is
recirculation flexibility provided between ponds? Does
the discharge structure for a pond allow draw-off at
various levels?).
h. Laboratory
The laboratory facilities should be included as part of
the plant tour. Performance monitoring, process
control testing, and quality control procedures should
be discussed with laboratory personnel. Available
analytical capability should also be noted. Sampling
and analytical support are often essential parts of the
evaluation effort and the evaluator should determine
what level of support is available from the laboratory
during the CPE.
/. Maintenance Facilities
Maintenance facilities should be included as part of
the plant tour. Tools, spare parts availability and
storage, filing systems for equipment catalogs, general
plant appearance, and condition of equipment should
be observed during the tour. Questions on the
preventive maintenance program, including methods
of initiating work (e.g., work orders), are appropriate.
2.3.2.3 Detailed Data Gathering
Following the plant tour, a major effort is initiated to
collect all data necessary to assess the performance
potential of the existing facilities. This data collection
effort may require two or three persons for 3-7 days in
a larger plant, and one or two persons for 1 -2 days in
a smaller plant.
Information, is collected to document past perfor-
mance, process design, maintenance, management,
budget, process control, and administrative policies.
Collecting information for many of these items
requires the assistance of POTW and other per-
sonnel. As such, the data gathering should be
scheduled around their availability. The time when key
personnel are not available should be used to initially
review documents such as O&M manuals and
construction plans, to summarize notes and questions
for POTW personnel, and to check completeness of
data collection. ,
The forms in Appendix D have proven to be valuable
working guidelines for the data collection effort (1).
Items covered by these forms are listed below:
Preliminary Plant Information, Form D-1
Administration Data, Form D-2
Design Data, Form D-3
Operations Data, Form D-4
Maintenance Data, Form D-5
Performance Data, Form D-6
Interview Data, Form D-7
When collecting information using these forms, the
evaluator should be aware that the data are to be
used to evaluate the performance capability of the
12
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existing POTW. The evaluator should continuously be
asking "How does this affect plant performance?" If
the area of inquiry is directly related to plant
performance, such as a clarifier design or an
administrative policy to cut electrical costs to an
unreasonable level, the evaluator should spend
sufficient time and effort to fully understand and define
the effect on plant performance. If the area of inquiry
is not directly related to plant performance, such as
the appearance of the grounds, the condition should
be noted and efforts directed toward areas that
specifically impact performance.
Completion of Form D-3 requires that values be
selected to represent current plant hydraulic and
BOD5 loadings. Typically, data for the most recent 12
months are used.
2.3.3 Evaluation of Major Unit Processes
Early in the on-site activities, an evaluation of the
POTW's major unit processes is conducted to
determine the performance potential of existing
facilities at current loadings (i.e., define the facility as
Type 1, 2, or 3 as described in Section 2.2). The
three unit processes whose capabilities most
frequently affect biological wastewater treatment plant
performance are: the aerator, the secondary clarifier,
and the sludge handling system (1,15,16).
A point system is used to quantify the evaluation of
these three basic unit processes. Key loading and
process parameters are calculated and results for
each parameter assigned points by comparison with
standard tables. Subsequently, each of the three
major unit processes receives a total score by adding
together the value of the points assigned the loading
and process parameters. The totals are then
compared with standards to assess whether a Type 1,
2, or 3 capability is indicated for that unit process (see
Figure 2-1). The overall plant type is determined by
the "weakest link" among the thtee major process
areas. It must be remembered in using this point
system that this simplification can provide valuable
assistance but cannot replace the overall judgment
and experience of the evaluator.
2.3.3.1 Suspended Growth Major Unit Processes
Suspended growth facilities include those plants using
variations of the activated sludge process. The three
significant unit processes within these types of
facilities that determine capacity and performance are
the aeration basin, secondary clarifier, and sludge
handling system.
a. Aeration Basin
Parameters that are used for scoring the capability of
an aeration basin are: hydraulic detention time, BOD5
loading, and oxygen availability. The point system for
scoring these parameters is presented in Table 2-2.
To obtain the necessary parameters, information is
required on wastewater flow to the aeration basin,
aeration basin BOD5 loading, aeration basin liquid
volume, and oxygen transfer capacity.
Table 2-2. Parameters for Scoring Capability of Aeration
Basins in Suspended Growth POTWs*
Current Operating Condition Points Points
Hydraulic Detention Time, hr:
24
10
5
3
BOD5 Loading, kg/m3/d
(lb/d/1,000 cuft):
0.24(15)
0.40 (25)
0.80 (50)
1.28 (80)
Oxygen Availability, kg O2/kg
BODS applied:
2.0
1.5
1.2
1.0
0.8
0.6
10 (max,)
6
0
-6
10 (max.)
6
0
-6
w/ nitrification
10 (max.) '
5
0
-5
. -10
-10
w/o nitrification
10 (max.)
1 0 (max)
5
0
-5
-10
* Interpolate to nearest whole number between loadings listed.
Oxygen transfer capacity is usually the most difficult
information to obtain if the original engineering data
are not available or if there is some reason to
question the original design data based on current
conditions. Generally, the evaluation proceeds by
using available data on oxygen transfer capacity and
assuming it is correct unless the transfer capacity
appears to be marginal. If oxygen transfer capacity
appears marginal, further investigation is warranted.
Any of the following conditions would lead an
evaluator to suspect marginal oxygen transfer:
• Difficulty in maintaining minimum desired dissolved
oxygen concentrations in the aeration basin
• Continuous operation of all blowers or all aerators
set at high speed
• Design data showing less than 1.2 kg oxygen
transfer capacity per kg actual BOD5 load
If design oxygen transfer numbers are unavailable or
are believed suspect, oxygen transfer rates presented
in Table 2-3 can be used to estimate oxygen transfer
capacities.
Typically, the oxygen transfer efficiency (percent) is
used when evaluating different diffused air systems,
and oxygen transfer rate (Ib CVhp-hr) is used when
evaluating surface mechanical aerators. The
evaluation of both diffused air and surface mechanical
aerators is described in more detail below.
13
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Table 2-3. Typical Clean Water Standard Oxygen Transfer
Values
System
Oxygen Oxygen
Transfer Transfer
Efficiency^ Rateb
Fine bubble diffusers, total floor
coverage
Fine bubble diffusers, side wall
installation
Jet aerators (fine bubble)
Static aerators (medium-size
bubble)
Mechanical surface aerators
Coarse bubble diffusers wide band
pattern
Coarse bubble diffusers, narrow
band pattern
percent
28-32
18-20
18-25
10-12
8-12
6-8
Ib/wire hp-hr
6.0-6.5
3.5-4.5
3.0-3.5
2.3-2.8
2.5-3.5
2.0-3.0
1.5-2.0
* at 15 feel submergence.
l> 1 Ib/hp-hr - 0.61 kg/kW-hr.
When evaluating oxygen transfer capability of diffused
aeration systems it is necessary to assess the
capacity of the aeration blowers and the standard
transfer efficiency of the diffusers. This information is
often available from O&M manuals, specifications, and
manufacturers literature. If questionable information is
available, typical values for various systems are
shown in Table 2-3. The blower capacity and diffuser
transfer efficiency can then be utilized to determine
the amount of oxygen (Ib/d) that can be transferred
into the wastewater by the existing aeration system.
To determine the aeration system oxygen transfer in
Ib/day, the diffuser standard transfer efficiency or
standard oxygen transfer rate at standard conditions
(14.7 psia, 20°C, and clean water) must be converted
to transfer efficiency at actual site conditions including
adjustments for site elevation, wastewater tempera-
ture, and wastewater characteristics. The procedure
to convert standard oxygen transfer rates to actual
oxygen transfer rates is presented in Appendix E.
In addition, blower capacity must be determined in
standard cubic feet per minute (SCFM) to determine
the mass of air/oxygen that the blowers are capable of
discharging. There is not a standard method of
presenting blower output. Some manufacturers
provide the blower rating in standard cubic feet per
minute (SCFM), which is a term that describes airflow
at standard conditions of 14.7 psia and 20 °C. It is
noted that different air temperatures, such as 70°F,
are used by other manufacturers to describe standard
conditions. Also, the blower output rating is often
presented in terms of ICFM (inlet cfm) or ACFM
(actual cfm), which is CFM at site conditions. ICFM,
ACFM, or SCFM at standard conditions other than the
conditions chosen for the evaluation must be
converted to SCFM. The procedure to convert ICFM
or ACFM to SCFM for the standard condition of 14.7
psia, 20 °C and clean water, is presented in Appendix
E.
Utilization of the procedures presented in Appendix E
to calculate the oxygen transfer capability of a diffused
air system is shown in the following example.
In Plant A there are four centrifugal blowers, each with
a capacity of 1,550 acfm. Three are utilized, with one
as standby. The standard oxygen transfer efficiency
(SOTE) or efficiency of the coarse bubble diffusers is
12 percent at 15-ft water depth based on
manufacturer's data. Plant A is located at 2,750 feet
above sea level.
1. Convert SQTE = 12 percent to AOTE using:
AOTE = (SOTE)a
PC -C.
^ sw I
9
T-20
Where,
site
AOTE = actual oxygen transfer efficiency at
conditions, percent.
SOTE = standard oxygen transfer efficiency at
standard conditions in clean water, percent.
a = 0.85 for coarse bubble diffuser, from Table
E-1.
0 = 0.95 for domestic wastewater.
9 = 1.024
Cs =9.17 mg/L oxygen saturation at standard
temperature and pressure.
= C14.7 (P/14.7), mg/L
-"SW
CL =
Assume maximum summer wastewater
temperature = 25 °C at Plant A. From Table
E-2, C14.7 = 8.38 mg/L @ 25°C. From Figure
E-1, P = 13.25 psia @ 2,750 ft above mean
sea level. A potential depth correction can be
applied to this term, as noted in Appendix E.
However, to be more conservative in the
evaluation, utilize atmospheric pressure:
Csw = 8.38 [13.25/14.7) = 7.55 mg/L
2.0 mg/L (mixed liquor DO concentration)
AOTE = {(0.12)(0.85) [(0.95)(7.55)-2] 1.02425-20}-=. 9.17
AOTE = 6.5 percent
2. Convert blower output of 1,550 acfm to scfm:
acfm = scfm (Ta/Ts)(Ps/Pa)
Where, ''..
acfm = 1,550 cfm.
14
-------
Ta = 100°F + 460°F = 560°R (temperature at
which manufacturer rated blowers).
Ts = 68°F + 460°F = 528°R (standard
temperature).
Ps = 14.7 psia (standard pressure).
Pa = 13.25 psia (pressure @ 2,750 ft above mean
sea level).
scfm = (1,550) (528/560) (13.25/14.7) = 1,317 cfm
3. Calculate Ib Op/d from 3 blowers using diffuser
actual oxygen transfer efficiency of 6.5 percent and
blower capacity of 1.317 cfm:
Peak air flow = 3 x 1,317 = 3,951 scfm
Ib 02/d = (scfm)(1,440min/d)(23.2 Ib O2/100lbair)
x (0.075 Ib air/cu ft air)(AOTE)
= (3,951 )(1,440)(23.2/100)(0.075)(6.5)
= 6,435 Ib/d
4. Therefore. 3 blowers (5) 1,317 cfm each will
transfer 6.435 Ib Op/d. Compare the oxygen transfer
capability with the BOD?; loading applied to determine
the Ib Op/lb BODs that the diffused air system can
provide.
When evaluating the oxygen transfer capability of a
surface mechanical aeration system, the power usage
of the motor (whp) and the oxygen transfer rate of the
aerator (Ib CVwhp-hr) must be determined. Various
techniques for estimating motor power usage based
on actual power measurements are presented in
Appendix F. If power measuring equipment is not
available, wire horsepower may be estimated by
assuming the motor is 90 percent efficient and the
surface mechanical aerator gear box is 85 percent
efficient. Using these estimates, the evaluator may
assume that 75 (appropriately 0.9 x 0.85) percent of
the motor horsepower (mhp) is being converted to
oxygen transfer energy, or wire horsepower. For
example, if a surface mechanical aerator motor is
rated at 50 mhp, the wire horsepower could be
estimated to be 0.75 x 50 = 37.5 whp. Actual power
measurements should be taken if the oxygen transfer
capability of the system determined by estimating wire
horsepower appears inadequate.
The aerator oxygen transfer rate may be determined
from the O&M manual, specifications, and equipment
manufacturer's literature. If questionable information is
available, a typical value for surface mechanical
aeration systems can be found in Table 2-3. The
standard oxygen transfer rate (SOTR) is typically
provided and this must be converted to the actual
oxygen transfer rate (AOTR) as shown in Appendix E.
Utilization of the procedures presented in Appendices
E and F to determine actual oxygen transfer rate and
motor wire horsepower are presented in the following
example.
In Plant B there are two 50-hp surface mechanical
aerators. Both units are utilized. The SOTR is 3 Ib
Oa/whp-hr based on manufacturer's data. Plant B is
located at 2,750 ft above sea level.
1. Convert SOTR = 3 Ib Op/whp-hr to AOTR using
AOTR = (SOTR) a
J3C -Cr
^ sw L
Where,
AOTR = actual oxygen transfer rate at site
conditions, percent.
SOTR = standard oxygen transfer rate at standard
conditions in clean water, percent.
a =0.90 for surface mechanical aerator, from
Table E-1.
P = 0.95 for domestic wastewater.
9 = 1.024
Cs = 9.17 mg/L oxygen saturation at standard
temperature and pressure.
Csw = C14.7 (P/14.7), mg/L
Assume maximum summer wastewater
temperature = 25°C at Plant B. From Table
E-2, C14.7 = 8.38 mg/L @ 25°C. From Figure
E-1, P = 13.25 psia @ 2,750 feet above mean
sea level.
Csw = 8.38 [13.25/14.7) = 7.55 mg/L
CL = 2.0 mg/L (mixed liquor DO concentration)
AOTR = {(3)(0.9) [(0.95)(7.55)-2] 1.02425-20} -=-9.17
AOTR = 1.7 Ib O2/whp-hr
2. Determine surface mechanical motor power usage:
a. Determine whp of motor based on the assumption
that whp is 75 percent of mhp.
whp = 0.75 (50 mhp) = 37.5 whp
b. Determine whp based on actual power
measurements and assumption that power factor is
0.90, as shown in Appendix F.
Voltage measurement = 480 volts
Amperage measurement = 37.4 amps
kVA = (V) (A) (3)1/24-1,000 (3-phase power)
= (480) (37.4) (3)1/2 -M,000 = 31.1kW
15
-------
kW = kVA x PF = 31.1 (0.9) = 28 kW
whp = kW * 0.746 = 28 * 0.746 = 37.5 hp
Total whp = 2 motors x 37.5 whp = 75 whp
3. Determine oxygen transferred based on AQTR and
whp:
O2 transfer = (1.7 Ib O2/whp-hr) (75 whp) (24 hr/d)
= 3,060 Ib O2/d
4. Compare the oxygen transfer rate with the plant
BQD.g applied loading to determine the Ib Og/lb BOD^
that the surface mechanical aeration system can
provide.
Once data are available on wastewater flows, BOD5 of
influent to the aeration basin, aeration basin volume,
and oxygen transfer capacity, the following
calculations should be completed by the evaluator:
Aerator Hydraulic Detention Time =
Aeration Basin Vol. T Peak Month ADWF
Peak month average daily wastewater flow is
typically for the most recent 12 months. Peak
month flow is used to ensure that the aerator is
capable of meeting monthly average effluent
requirements.
BOD5 Loading =
BODg Applied Loading •=• Aeration Basin Volume
BODs loading is typically the daily average value for
the most recent 12 months. Peak value is not used
since aerator capability is not as sensitive to
"normal" BODs loading variations.
Table 2-4.
Parameters for Scoring Capability of Clarifiers
in Suspended Growth POTWs
Current Operating Condition
Points
Oxygen Availability =
Oxygen Transfer Capacity
Loading
BOD5 Applied
When the above calculations have been completed for
the subject POTW, the results are compared to the
values given in Table 2-2 and appropriate points are
assigned each parameter. If the parameters for the
subject POTW fall between the values listed,
interpolation is used to assign appropriate points.
b. Secondary Clarifiers
Parameters that are used for scoring the capability of
suspended growth secondary Clarifiers are:
configuration, surface overflow rate (SOR), depth,
return sludge removal mechanism, and return sludge
control. The scoring system for these parameters is
presented in Table 2-4.
The configuration score is applied to assist in
assessing those Clarifiers that have diminished
Configuration
Circular with "donut" or interior launders
Circular with weirs on walls
Rectangular with 33% covered with launders
Rectangular with 20% covered with launders
Rectangular with launder at or near end
Surface Overflow Rate, m3/m2/d' (gpd/sq ft)*1):
12 (300)
20(550)49(1,200)
27 (650)
33 (800)
41 (1,000)
49(1,200)
Depth at Weirs, m (ft):
4.6(15)
3.7 (12)
3.0 (10)
2.4 (8)
2.1 (7)
Return Sludge Removal:
Circular, rapid withdrawal
Circular, scraper to hopper
Rectangular, cocurrent scraper
Rectangular, countercurrent scraper
No mechanical removal
Return Activated Sludge Control:
Actual RAS flow range completely within
typical RAS flow range; capability to measure
RAS flow
Actual RAS flow range completely within
typical RAS flow range; no capability to
measure RAS flow
50% of typical RAS flow range covered
by actual RAS flow range; capability to measure
RAS flow
50% of typical RAS flow range covered
by actual RAS flow range; no capability to
measure RAS flow
Actual RAS flow range completely outside
typical RAS flow range
capabilities due to poor weir locations or poor surface
development with weirs. For example, a clarifies 15-m
long and 3-m wide (total surface area of 45 m2)iwith a
two-sided 1-m wide weir located 1 m from the end is
judged to have 9 m2 of launder coverage [(3 m wide)
x(1 m + 1 m + 1 m)], or only 20 percent of the surface
area developed. This clarifier's score would be :;low
because of the configuration. •
Surface overflow rate is calculated independently of
the configuration evaluation and is based on the total
clarifier surface area, as follows:
SOR = Wastewater Flow from the Clarifier .
-r Clarifier Surface Area
Typically, the peak month daily average wastewater
flow for the most recent 12 months is used to
calculate SOR. Peak flow is used to ensure that
10
5
5
0
-10
15
10
5
0
-10
-15
10
4
0
-5
-10
10
8
2
0
-5
10
7
5
0
16
-------
clarifier is capable of meeting monthly average
effluent requirements.
Note: If diurnal flow variations are greater than 2:1
(peak daily flow:daily average flow) the points for SOR
must be adjusted to lower values. Conversely, if a
clarifier is loaded at a relatively constant rate due to
the availability of flow equalization, the points for SOR
can be adjusted upward. It must be remembered that
the assessment is not a design evaluation but an
assessment of whether the clarifier can be made to
perform under the desired conditions. In POTWs
where special allowance has been made for high
infiltration/inflow, such as permitted bypassing above a
certain flow, the flow at which secondary treatment is
required should be used.
Depth and return sludge removal scores are derived
by comparing existing facilities with the conditions
shown in Table 2-4. Evaluation of return activated
sludge control is based on the ability to control the
return flow rate within the typical range for the
particular type of activated sludge plant. Typical
ranges for return activated sludge pumping rates are
presented in Table 2-5.
Table 2-5. Typical Ranges for Return Activated Sludge
Pumping Capacities
Process Type Return Activated Sludge
Table 2-6. Criteria for Scoring Sludge Handling Capability
for Suspended Growth POTWs
Conventional A. S. and Activated Bio-
filters (plug flow or complete mix)
Extended Aeration
(including oxidation ditches)
Contact Stabilization
% of average daily
wastewater flow
25-100
50-100
50-125
c. Sludge Handling Capability
The capability of sludge handling facilities associated
with an activated sludge plant is scored by the
controllability of the wasting process and the capability
of the available sludge treatment and ultimate disposal
facilities. Scoring for sludge handling capability is not
straightforward because existing facilities cannot be
easily assessed due to the variability that exists in
design and operational "standards" for unit process
capability. To evaluate the sludge handling capability,
the evaluator must first calculate expected sludge
production based on current loadings to the
wastewater treatment processes. The evaluator then
assesses the capability of the existing sludge facilities
to handle the expected sludge production.
The criteria and point system for evaluating sludge
handling capability are presented in Table 2-6. As
indicated by the lower points allocated, controllability
is much less important than capacity.
Current Operating Condition
Points
Controllability:
Automated sampling and volume control
Metered volume and hand sampling
Hand measured volume and hand sampling
Sampling or volume measurement by hand not
practical
Capability:
150% of calculated sludge production
125% of calculated sludge production
100% of calculated sludge production
75% of calculated sludge production
50% of calculated sludge production
25
20
15
0
-10
Controllability of the wasting process is indicated by
the type of waste sludge volume measurement and
the type of waste sludge sampling available. The
optimum control for an activated sludge wasting
system includes automatic volume control and
automatic sampling. A positive displacement pump
and automatic sampler, both controlled by an accurate
and precise clock, is an example of this type of
control.
Most small activated sludge plants can manually
measure a wasted volume (depth increase in holding
tank or digester, or the number of tank trucks filled)
and manually sample (from a tap or the open end of
the waste sludge line). Most larger plants have flow
measuring and totaling devices on waste sludge lines.
Capability of existing sludge handling facilities is
evaluated using the following procedures:
• Determine current plant loadings and calculate
expected sludge production.
• Establish capability of existing sludge handling unit
processes.
• Determine percentage of the expected sludge
production each unit process can handle.
« Identify the "weakest link" process as the overall
capability of the existing sludge handling facilities
and compare to scoring values in Table 2-6.
Expected sludge production is calculated using
current BOD5 loadings (unless believed inaccurate)
and typical unit sludge production values for the
existing wastewater treatment processes (17). Typical
unit sludge production values for various processes
are shown in Table 2-7. For example, an oxidation
ditch removing about 1,000 kg BOD5/d would be
expected to have an average sludge production of
about 650 kg TSS/d (1,000 kg BOD5/d x 0.65 kg
TSS/kg BOD5 removed).
When plant records include sludge production data,
the actual unit sludge production value should be
compared to the typical value. If a discrepancy greater
than 15 percent exists between these values, further
17
-------
Table 2-7. Unit Sludge Production Values for Projecting
Sludge Production From Suspended Growth
POTWs
kg TSS (sludge)/
Process Type kg BOD5 removed
Table 2-8. Sludge Concentrations for Projecting Sludge
Production From Suspended Growth POTWs
Waste
Sludge Type Concentration
Activated Sludge w/Primary Clarification
Activated Sludge w/o Primary Clarification
Conventional"
Extended Aeration13
Contact Stabilization
0.7
0.85
0.65
1.0
8 Includes tapered aeration, step feed, plug flow, and complete mix
with wastewater detention times < 10 hours.
b Includes oxidation ditch.
evaluation is warranted. If actual plant data fall within
the 15 percent range, these data can be used for the
evaluation of sludge handling capability. A detailed
example of calculating expected sludge production
and comparing it with plant data is included in Section
2.3.5.
Often plant sludge production data is not reliable and
cannot be used to accurately assess sludge handling
capability. The most common causes of inaccurate
recorded sludge production are:
• Excessive solids loss over the final clarifier weirs
• Inaccurate waste volume measurement
• Insufficient waste sampling and concentration
analyses
• Inaccurate determination of BOD5 removed
Using the unit sludge production values and projected
desired BODg removals for the subject plant (desired
effluent BODs should meet effluent requirements), the
expected mass of sludge produced per day can be
calculated. To complete the scoring of sludge
handling capability, the expected volume of sludge
produced per day must also be calculated. Typical
waste sludge concentrations for activated sludge
plants are presented in Table 2-8 and can be used to
convert the expected mass of sludge produced per
day to the expected volume of sludge produced per
day.
Variations in sludge production values have been
observed throughout the year. Additionally, operation
decisions to lower sludge inventories in the plant can
place increased requirements on the sludge handling
facilities. It is not uncommon for these variations to
require 125-150 percent of the long-term average
sludge production value (17). For this reason, a factor
of 1.25 is applied to the calculated sludge mass and
volume values to ensure reliable capability under most
operational situations throughout the year.
The capability of each of the components of the
sludge handling process are evaluated with respect to
its ability to handle the calculated sludge production
Primary
Activated
Return Sludge/Conventional
Return Sludge/Extended Aeration
Return Sludge/Contact Stabilization
Return Sludge/small plant with low SOR*
Separate waste hopper in sec. clarifier
mg/l
50,000
6,000
7,500
8,000
10,000
12,000
a Returns can often be shut off for short periods to thicken waste
sludge in clarifiers with SORs less than 20 m3/m2/d (500 gpd/sq
ft).
based on current loadings (the mass and volume
values adjusted by the 1.25 factor are used in this
evaluation). Using this evaluation approach, sludge
handling "bottlenecks" can be identified.
Typical components found in activated sludge facilities
are: thickening, digestion, dewatering, hauling, and
disposal. Guidelines for the capability evaluation of the
components of the existing sludge handling processes
are provided in Tables 2-9 and 2-10. The guidelines
provided in Table 2-9 are used to compare existing
facility capability to calculated sludge production. For
example, an exist-ing aerobic digester with a volume
of 380 m.3 (100,000 gal) in a plant with a calculated
waste sludge volume of 19 m3/d (5,000 gpd) would
have a hydraulic detention time of 20 days. This is
133 percent of the guidelines (15 days) provided for
aerobic digesters in Table 2-9. Thus, this compo-nent
of the sludge handling process in this particular
POTW would have capability for 133 percent of the
calculated sludge production. If the aerobic digester
proved to have the lowest capability to handle the
calculated sludge production of all the components of
the sludge handling processes in this POTW, sludge
handling capability would score 22 points (interpolated
from Table 2-6). The sludge handling capability
evaluation is illustrated as part of the CPE example
presented in Section 2.3.8.
d. Suspended Growth Major Unit Process Analysis
A worksheet is presented in Appendix L to facilitate
the suspended growth major unit process evaluation.
Once individual major unit processes are evaluated
and given a score, these results should be recorded
on a summary sheet, as shown in Table 2-11, and
compared with standards for each major unit process
and the total plant. This analysis results in the subject
POTW being rated a Type 1, 2, or 3 facility, as
described in Section 2.2.1.1. The sum of the points
scored for aeration basin, secondary clarifier, and
sludge handling capability must be 60 or above for the
subject POTW to be designated a Type 1 facility.
18
-------
Furthermore, regardless of total points, the aerator
must score at least 13 points, the secondary clarifier
at least 25 points, and sludge handling capability at
least 10 points for the plant to be considered Type 1.
If the subject POTW meets the criteria for a Type 1
plant, the evaluation has indicated that all major
processes have adequate capability for the plant to
provide desired performance. If the total is less than
60 points, or if any one major unit process scores less
than its minimum, the facilities must be designated as
Type 2 or 3.
The minimum criteria for a Type 2 plant are 20 total
points and zero for each individual process. If the total
is less than 20, or if any major process scores a
negative value, the POTW must be considered
inadequate and the plant designated as Type 3. Type
3 plants generally require major modifications before
they can be expected to meet secondary treatment
effluent limits.
Table 2-10. Miscellaneous Unit Values Used in Evaluating
Sludge Handling Capability3
Table 2-9.
Process
Guidelines for Evaluating Capability of Existing
Sludge Handling Processes3
Parameters That Can Be Used
to Represent 100% of Required
Sludge Handling Capability^
Gravity Thickeners
Primary Sludge
Activated Sludge
Primary + Activated
Fixed Film
Primary + Fixed Film
Dissolved Air Flotation
Activated Sludge
Primary + Activated
Fixed Film
Primary + Fixed Film
Digesters
Aerobic
Anaerobic
Single Stage
Two Stage
Drying Beds
Mechanical Dewatering
Single Unit
Multiple Units
Liquid Sludge Haul
Short Haul (<3 km)
Long Haul (>20km)
125 kg/nvVd (25 Ib/d/sq ft)
20 kg/m2/d (4 Ib/d/sq ft)
50 kg/m2/d (10 Ib/d/sq ft)
40 kg/m2/d (8 Ib/d/sq ft)
75 kg/m2/d (15 Ib/d/sq ft)
50 kg/m2/d (10 Ib/d/sq ft)
100 kg/m2/d (20 Ib/d/sq ft)
75kg/m2/d (15 Ib/d/sq ft)
125 kg/m2/d (25 Ib/d/sq ft)
15 days' HDTc
40 days' HOT
30 days' combined HOT
Worst season turnover time
30 hours of operation/week
60 hours of operation/week
(with one unit out of service)
6 trips/day maximum
4 trips/day maximum
a These guidelines are not developed to meet the proposed new
Federal sludge regulations.
b Capability of existing unit processes should not be downgraded to
these values if good operation and process performance are
documented at higher loadings. For example, if records appear
accurate and show that all sludge production has been
successfully thickened in a gravity activated sludge thickener for
the past year at an average loading of 25 kg/m3/d (5 Ib/d/sq ft),
the existing thickener should be considered to have 100% of
required capability.
c HOT = Hydraulic detention time = Volume of digester •=- Volume
of waste sludge calculated to be produced.
Aerobic Digesters Following
Extended Aeration
(MCRT >20 d)
Aerobic Digesters Following
Conventional A. S.
(MCRT <12 d)
Anaerobic Digesters for
Activated + Primary, and
Fixed Film + Primary
(Supernating Capability
Usable)
WAS Volatile Solids Content
Conv. (MCRT <12d)
Ext. Aer. (MCRT >20 d)
Digester
HDT&
days
10
15
20
>30
10
15
>20
20
30
40
Total
Solids
Reduction
%
10
20
30
35
20
35
40
25
35
45
80%
70%
Output
Solids
Cone.
mg/l
12,000
15,000
17,000
20,000
12,000
15,000
17,000
= input
= input
= input
a Values in table are intended for use in allowing an evaluation of
sludge handling capability to proceed in the absence of available
plant data. Many other variables can affect the values of the
parameters shown.
b HOT = Hydraulic detention time = Volume of digester * Volume
of waste sludge expected to be produced.
A suspended growth POTW that scored the following
during the evaluation of major unit processes would
meet the criteria for a Type 3 plant:
Aeration Basin
Secondary Clarifier
Sludge Handling
Total
Points
Scored
14
-8
10
16
Points Required
Type 1
13-30
25-55
10-30
60-115
Type 2
0-12
0-24
0-9
20-59
Type 3
<0
<0
<0
<20
The point system in Table 2-11 has been developed
to aid in assessing the capability of a POTW's major
physical facilities. It cannot replace the overall
judgment and experience of the evaluator. which
should be the deciding factor in determining the
capability of facilities to provide improved
performance.
2.3.3.2 Fixed Film Major Unit Processes
Fixed film facilities covered in this manual include
those trickling filter plants using rock or plastic media
plus those using the RBC or activated bio-filter (ABF)
variations of the basic process. The unit process in
fixed film wastewater treatment plants that most
significantly affects capacity and performance is the
"aerator" portion of the plant (i.e., the amount and
type of trickling filter media, RBC media, etc.) Other
significant unit processes are the secondary clarifier
and sludge handling capability.
19
-------
Table2-11. Suspended Growth Major Unit Process
Capability Evaluation
Dn!nt« Points Required-
Aeration Basin
Secondary Clarifier
Sludge Handling
Total
Scored Type 1
13-30
25-55
10-30
60-115
Type 2 Type 3
0-12
0-24
0-9
20-59
<0
<0
<0
<20
Table 2-12. Parameters for Scoring Aerator Capability for
Trickling Filter POTWs
Points
* Each unit process as well as the overall points must fall in the
designated range for the plant to achieve the Type 1 or 2 rating.
a. Aerator
Trickling Filters
An approach to develop "equivalency" is used to
allow a comparable evaluation of the potential
performance capability of trickling filters of varying
media types. It is not intented that this equivalency
approach be used as a basis of design. The unit
surface area for common rock media is typically 43
m2/m3 (13 sq ft/cu ft) (3). This information can be
used to convert data from trickling filters with artificial
media to roughly equivalent volumes of common rock
media. For example, 1,000 ms (3,500 cu ft) of a
plastic media with a specific surface area of 89 m2/m3
(27 sq ft/cu ft ) is roughly equivalent to [(89/43) x
(1,000 rr»3)] or 2,070 m3 (7,300 cu ft) of common rock
media. Unit surface area information for various media
types is generally available in manufacturers'
literature.
Using the equivalency calculation, BOD5 loadings can
be calculated for all types of media. Loadings for
trickling filters are typically expressed as mass of
BODs per volume of media. The volumetric loading
can be calculated using the equivalency calculation
presented above. Results can be compared with
criteria in Table 2-12 to compute a "score" for the
trickling filter.
The capability of a trickling filter can be significantly
decreased if plugging occurs. Ponding on the filter is
a common indicator of plugging and can be due to
overgrowth of microorganism mass, disintegration of
the media, or underdrain blockage or damage. The
evaluator should inspect the filter in several places
(removing media where possible) to ensure that
ponding underneath the upper layer of rocks is not
occurring.
RBCs
Parameters for scoring RBCs are presented in Table
2-13 (18). The key parameters to be evaluated are:
BODs loading on the first stage and on the entire
system; number of stages provided; and whether or
not sidestreams from anaerobic sludge treatment are
received. BOD5 loading used for evaluating RBCs is
Current Operating Condition
BOD5 Loading,
kg BOD5/m3/d (lb/d/1 ,000 cu ft):a
0.16 (10)
0.32 (20)
0.48 (30)
0.80 (50)
1.12 (70)
Recirculation, ratio to raw flow:
2:1
1:1
None
Freezing
Temp.b
20
15
0
-10
-20
Covered
Filter or
Nonfreezing
Temp.
20
20
10
-5
-10
3
2
0
Anaerobic Sidestreams:0
Not returned to plant 0
Returned to wastewater stream
ahead of the TF:
Returned to flow equalization
tank or prior to primary -6
clarifier -10
Returned directly ahead of TF
a Based on primary effluent and common rock media having a
specific surface area of about 43 m2/m3 (13 sq ft/cu ft).
b Temperatures below freezing for more than one month.
c Supernatant from anaerobic digesters or filtrate/concentrate from
the dewatering processes following anaerobic digesters.
soluble BOD5 (SBOD5) per unit of media. If data are
not available, SBOD5 is estimated for typical domestic
wastewater as 0.4 of the primary effluent total BODg
(TBOD5). If significant industrial contributions are
present in the system, SBOD5 should be determined
by testing.
Surface area data for RBCs are generally available in
manufacturers' literature or in plant O&M manuals. If
these sources are unavailable or do not contain the
needed information, the manufacturer's representative
or the manufacturer should be contacted to obtain the
data.
First-stage media loading is calculated by dividing the
mass of SBOD5 going to the first stage by the total
surface area of only the first-stage media. System
media loading is calculated by dividing the total
SBOD5 load to the RBCs by the total surface area of
all RBC media. In most cases, the mass of SBOD5
will be the same for these calculations. They should
only be different in plants where some of the SBOD5
load is bypassed around the first stage.
ABFs
Parameters for evaluating ABF aerators are presented
in Table 2-14 (19). The key parameters are: biocell
BOD5 loading and aeration basin detention time. A
criterion of lesser importance is recirculation directly
around the biocell.
20
-------
Table 2-13. Parameters for Scoring Aerator Capability for
RBC POTWsa (7)
Current Operating Condition
Points
First-Stage Loading, g SBOD5/m2/d (lb/d/1,000 sq
ft):
12(2.5) 10
20 (4.0) 0
29 (6.0) -6
System Loading g SBOD5/m2/d (lb/d/1,000 sq ft):
2.9 (0.6) 10
4.9(1.0) 0
7.3 (1.5) -6
Number of Stages:
4
3
2
Anaerobic Sidestreams:
Not returned to plant
Returned to wastewater stream ahead of RBC
Returned to flow equalization
tank or prior to primary clarifier
Returned directly ahead of RBC
10
7
4
0
-6
-10
a Includes mechanical and air drive RBCs.
b Supernatant from anaerobic digesters or filtrate/concentrate from
the dewatering processes following anaerobic digesters.
Table 2-14. Parameters for Scoring Aerator Capability for
ABF POTWs (19)
Current Operating Condition
Biocell BOD5 Loading, kg BOD5/m3/d (lb/d/1,000
cuft):
1.6(100)
2.4(150)
2.8 (1 75)
3.2 (200)
4.0 (250)
4.8 (300)
Aeration Basin Detention Time, hours:
4
3
2
1
0.75
0.5
Points
15
10
5
0
-5
-10
20
15
12
5
0
-10
Oxygen Availability in Aeration Basin, kg O2/kg
BOD5 to Biocell:
1.0 10
0.75 7
0.5 3
0.4 0
0.3 -15
Recirculation - Directly Around Biocell, ratio to raw
flow:
1:1 3
None 0
loadings on the biocell are calculated in a
manner similar to trickling filter loadings: primary
effluent BOD5 mass is divided by the volume of the
biocell media. Aeration basin detention time is
calculated in a manner similar to activated sludge
aeration basin hydraulic detention time: the aeration
basin liquid volume is divided by the average daily
wastewater flow. Sludge recirculation is not included
in this calculation. Typically, peak month daily average
flow for the most recent 12 months is used to ensure
that adequate detention time exists to meet the
monthly average effluent requirements.
Oxygen availability in the aeration basin of an ABF
plant is calculated by dividing mass of oxygen transfer
capacity by the total mass of BOD5 applied to the
biocell. This is done because the removal attributed to
the biocell versus that occurring in the aeration basin
is not easily distinguished. Most ABF plants provide
for recirculation directly around the biocell.
Recirculation is calculated as a ratio to raw flow
(typically, the peak month daily average flow).
b. Secondary Clarifier
Criteria for scoring the capability of secondary
clarifiers in trickling filter and RBC plants are
presented in Table 2-15. The calculations require that
wastewater flow rate and the clarifier configuration,
surface area, and depth be known (see Section
2.3.3.1). For ABF plants, the criteria for suspended
growth secondary clarifiers presented in Tables 2-4
and 2-5 are more appropriate and should be used.
Table 2-15. Parameters for Scoring Capability of Clarifiers
in Trickling Filters and RBCs*
Current Operating Condition
Points
Configuration:
Circular with "donut" or interior launders
Circular with weirs on walls
Rectangular with 33% covered with launders
Rectangular with 20% covered with launders
Rectangular with launder at or near end
Surface Overflow Rate, m3/m2/d (gpd/sq ft):
12(300)
20 (500)
27 (650)
33 (800)
41 (1,000)
49 (1,200)
Depth at Weirs:
3.7 (12)
3.0 (10)
2.1 (7)
10
5
5
0
-10
15
10
5
0
-10
-15
* For ABF plants, criteria for suspended growth clarifiers (Table 2-
4) should be used.
c. Sludge Handling Capability
Criteria for scoring sludge handling capability
associated with fixed film plants are presented in
Table 2-16. The criteria for controllability in Table 2-16
are self-explanatory. The capability of sludge handling
associated with fixed film facilities is evaluated using
the same approach presented in Section 2.3.3.1 for
suspended growth POTWs.
Different unit sludge production values are used in
calculating expected sludge production from fixed film
facilities. Typical unit sludge production values for the
various types of fixed film plants are summarized in
21
-------
Table 2-16.
Criteria for Scoring Sludge Handling Capability
for Fixed Film POTWs
Table 2-18. Trickling Filter Major Unit Process Capability
Evaluation
Current Operating Condition
Points
Controllability:
Automated sampling and volume control
Motored volume and hand sampling
Hand measured volume and hand sampling
Sampling or volume measurement by hand not
practical
Capability:
125% of calculated sludge production
100% of calculated sludge production
75% of calculated sludge production
50% of calculated sludge production
25
15
5
-10
D/Jnte Points Required"
"Aerator"
Secondary Clarifier
Sludge Handling
Total
Scored Type 1
17-23
17-30
10-30
45-83
Type 2 Type 3
0-11 <0
0-16 <0
0-9 <0
15-44 <15
' Each unit process as well as the overall points must fall in the
designated range for the plant to achieve the Type 1 or 2 rating.
Table2-17. Unit Sludge Production and Sludge
Concentration Values for Projecting Sludge
Production From Fixed Film POTWs (1,21,26)
Process Type
Trickling Filler
RBC
ABF
Sludge Type;
Primary
Primary + Trickling Filter
Pnmary + RBC
Primary + ABF
Trickling Filter
RBC
ABF
kg TSS (sludge)/
kg BOD5 removed
0.9
1.0
1.0
Waste Cone., mq/l
50,000
35,000
35,000
30,000
20,000
20,000
10,000
Table 2-17. A detailed example of calculating
expected sludge production and comparing it with
data is included in Section 2.3.5.
Frequently, secondary sludge from fixed film facilities
is returned to the primary clarifiers. Typical underflow
concentrations of the combined sludge from the
primary Clarifier are shown in Table 2-17 as well as
sludge concentrations from the individual fixed film
processes.
The guidelines presented in Tables 2-9 and 2-10 can
be used to help an evaluator assess the performance
potential of existing sludge treatment and disposal
facilities.
d. Fixed Film Major Unit Process Analysis
Worksheets are presented in Appendices M, N, and O
to facilitate the fixed film major unit process
evaluation. Once major fixed film unit processes are
evaluated, they should be summarized and compared
to standards for each type of fixed film facility. Tables
2-18, 2-19 and 2-20 can be used for this purpose.
This analysis results in the subject POTW being rated
Type 1, 2, or 3, as described in Section 2.3.3.1. Using
these tables, the subject plant must score the
minimum number of points listed for each individual
process and the minimum number total points for all
processes for the plant to qualify for a specific plant
type. For example, a trickling filter plant scoring the
following would meet the criteria for a Type 1 facility
for overall points, aerator, and secondary clarifier, but
would be classified Type 2 because of its score for
sludge handling capability:
Points Required
"Aerator"
Secondary Clarifier
Sludge Handling
Total
Scored
21
27
6
54
Type 1
17-23
17-30
10-30
45-83
Type 2
0-11
0-16
0-9
15-44
Type3
<0
<0
<0
<15
Table 2-19. RBC Major Unit Process Capability Evaluation
Points Required''
"Aerator"
Secondary Clarifier
Sludge Handling
Total
Scored Type 1
14-30
17-30
10-30
48-90
Type 2
0-13
0-16
0-9
15-47
TypeS
<0
< 0
<0
<15
Each unit process as well as the overall points must fall in the
designated range for the plant to achieve the Type 1 or 2 rating.
Table 2-20. ABF Major Unit Process Capability Evaluation
Points Required"
"Aerator"
Secondary Clarifier
Sludge Handling
Total
Scored Type 1
15-48
20-55
10-30
50-133
Type 2
0-14
0-19
0-9
15-49
Types
<0
<0
<0
<15
* Each unit process as well as the overall points must fall in the
designated range for the plant to achieve the Type 1 or 2 rating.
22
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2.3.3.3 Stabilization Pond Processes
Stabilization pond treatment systems combine the
"aerator" function, secondary clarification, and sludge
handling into one unit process. Typical pond facilities
include facultative and aerobic systems. Process
evaluation is based on the relaxed effluent limitation
for total suspended solids available for pond systems
(e.g., 75 mg/L effluent TSS for facilities less than 2
mgd capacity).
a. Facultative Pond Facilities
The parameters that are used for scoring the
capability of facultative stabilization pond facilities and
the point system for scoring these parameters is
presented in Table 2-21. To obtain the necessary
parameters, the evaluator must collect information on
wastewater flow, BOD5 strength, the minimum
average winter air temperature, pond dimensions, and
the type of flexibility provided with the facility.
Once data are available on these parameters, the
following calculations should be completed by the
evaluator:
BOD5 Loading = Influent BOD5
-r Surface Area of All Ponds
(Influent BOD5 mass is typically the daily average
value for the most recent 12 months)
Detention Time = Total Pond Volume
* Average Daily Flow
(Flow is typically the daily average value for the
most recent 12 months)
To evaluate the possibility of short-circuiting, the
following ratios should be calculated:
Short-Circuiting Ratio =
Distance From Inlet To Outlet
-5- Maximum Pond Dimension
Length-to-width Ratio =
Maximum Pond Dimension
•f Minimum Pond Dimension
(The ratios for each pond should be calculated,
then these values should be averaged)
When the above calculations have been completed for
the subject POTW, the results are compared to the
values given in Table 2-21 and appropriate points are
assigned each parameter. If the parameters for the
subject POTW fall between the values listed,
interpolation is used to assign appropriate points.
b. Aerated Pond Facilities
The parameters used for scoring the capability of
aerobic stabilization pond facilities and the point
system for scoring these parameters are presented in
Table 2-21. Parameters for Scoring Capability of Facultative
Stabilization Pond Systems* (24,25)
Current Operating Condition Points
Average Winter Air Temperature > 15°C
BOD5 Loading, kg BOD5/ha/d (Ib/ac/d):
45 (40)
67 (60)
90 (80)
112(100)
134 (120)
1st Pond Loading >101 kg BOD5/ha/d (>90 Ib/ac/d)
Detention Time, days
30
20
15
Average Winter Air Temperature 0-15°C
BODS Loading, kg BOD5/ha/d (Ib/ac/d):
11 (10)
22 (20)
45 (40)
67 (60)
90 (80)
1st Pond Loading >73 kg BOD5/ha/d (>65 Ib/ac/d)
Detention Time, days
80
30
20
Average Winter Air Temperature < 0°C
BOD5 Loading, kg BOD5Aia/d (Ib/ac/d):
11(10)
17(15)
22 (20)
34 (30)
45 (40)
1st Pond Loading >39 kg BOD5/ha/d (>35 Ib/ac/d)
Detention Time, days
100
50
40
Number of Ponds in Series:
3
2 Applies to # ponds in series
1
Length to Width Ratio:
>2
1.5-2 Applies to length-to-width
<1.5
Ratio of Inlet-Outlet Distance to Max. Pond Dimension
(Short Circuiting Ratio):
>0.75
0.5-0.75
<0.5
Flexibility to Operate in Series and Parallel:
Available
Not Available
Variable Level Draw-Off:
Available
Not Available
10
5
0
-5
-10
-3
5
0
-5
10
5
0
-5
-10
-3
5
0
-5
10
5
0
-5
-5
-3
5
0
-5
5
0
-5
2
0
-2
2
0
-2
3
0
3
-3
Interpolate to nearest whole number between loadings listed..
Table 2-22. They are similar to parameters used to
evaluate facultative facilities with the exception of the
23
-------
oxygen availability and mixing parameters. To obtain
the necessary parameters, the evaluator must collect
information on wastewater flow, BOD5 strength,
oxygen transfer capacity, mixing energy, pond
dimensions, and the type of flexibility provided with
the facility. With the exception of oxygen availability
and mixing energy, the calculations for these
parameters are discussed in Section 2.3.3.3a. For
information on evaluating oxygen availability, refer to
Section 2.3.3.1.
Table 2-22. Parameters for Scoring Capability of Aerated
Stabilization Pond Systems* (24, 25)
Current Operating Condition Points
BODS Loading, kg BOD5/ha/d (Ib/ac/d):
[based on aerated ponds only]
56 (50) 16
112(100) 8
168 (150) 0
224 (200) -5
280 (250) -10
Detention Time, days
40 5
15 0
10 -5
Number of Ponds in Series:
3 10
2 0
1 -10
Length to Width Ratio:
>2 2
1.5-2 0
<1.5 -2
Ratio of Inlet-Outlet Distance to Max. Pond Dimension
(Short Circuiting Ratio):
>0.75 2
0.5-0.75 0
<0.5 -2
Oxygen Availability, kg O2/kg BODS load:
2.0 10
1.5 5
1.2 0
1.0 -5
0,8 -10
Mixing Energy (aerated ponds only), kW/1,000 m3
(hp/lQS gal):
3 (15) 5
2 (10) 3
1 (5) 0
Flexibility to Operate in Series and Parallel:
Available 3
Not Available 0
Variable Level Draw-Off Discharge:
Available 3
Not Available -3
* Interpolate to nearest whole number between loadings listed..
The mixing energy parameter is calculated as follows:
Mixing Energy =
Total Energy in Primary Pond
* Primary Pond Volume
(Total energy includes energy used for aeration and
mixing)
When the above calculations have been completed for
the subject POTW, the results are compared to the
values given in Table 2-22 and appropriate points are
assigned each parameter. If the parameters for the
subject POTW fall between the values listed,
interpolation is used to assign appropriate points.
c. Stabilization Pond Process Analysis
A worksheet is presented in Appendix P to facilitate
the stabilization pond process evaluation. Once the
stabilization pond process has been evaluated and
given a score, results should be compared with the
categories shown in Table 2-23. This analysis results
in the subject POTW being rated a Type 1, 2, or 3
facility, as described in Section 2.2.1.1. The sum of
the points scored for a facultative pond system must
be 16 or above for the subject POTW to be
designated a Type 1 facility. For an aerobic pond
system the total points scored must be 21 or above
for this same ranking. If the subject POTW meets the
criteria for a Type 1 plant, the evaluation has
projected that the facility has adequate capability
within the physical facilities to achieve compliance
with the related definition of secondary treatment for
pond systems. If the total points for the facility are
less than 16 for a facultative pond system or 21 for an
aerobic pond system, the facility must be designated
as Type 2 or 3.
Table 2-23. Stabilization Pond Process Capacity Evaluation
Points Points Required
Scored Type 1 Type 2 Type 3
Facultative Facilities
Aerobic Facilities
>20
0-15
5-20
<0
<5
2.3.4 Evaluation of Performance-Limiting Factors
Identification of performance-limiting factors should be
completed at a location that allows all potential factors
to be discussed openly and objectively (e.g., away
from the plant staff). The checklist of performance-
limiting factors presented in Appendix A, as well as
the guidelines for interpreting these factors, provide
the structure for an organized review of problems in
the subject POTW. The intent is to identify as clearly.
as possible the factors that most accurately describe
the causes of limited performance. For example, poor
activated sludge operation may be causing poor plant
performance because the operator is improperly
applying activated sludge concepts. If the operator is
solely responsible for process control decisions as
well as for testing for these decisions, the factor of
improper application of concepts should be identified.
Often, operator inability can be traced to another
source, such as an O&M manual containing
inaccurate information or a technical consultant who
provides routine assistance to the operator. In this
case, improper application of concepts plus the
source of the problem (O&M manual or inappropriate
technical guidance) should be identified as
24
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performance-limiting factors, since both must be
corrected to achieve desired plant performance.
Whereas the checklist and guidelines in Appendix A
provide the structure for the identification of
performance-limiting factors, notes taken during the
plant tour and detailed data-gathering activities
(including the completed forms from Appendix D)
provide the resources for identifying these factors.
Each factor identified as limiting performance should
be assigned an "A," "B," or "C" rating as discussed
in Section 2.2.1.3. Further prioritization is
accomplished by completing the summary sheet
presented in Appendix B. Only those factors receiving
either an "A" or "B" rating are prioritized on this
sheet. Additional guidance for identifying and
prioritizing performance-limiting factors is provided in
the following sections for the general areas of
administration, design, operation, and maintenance.
2.3.4.1 Administration Factors
Budgeting and financial planning are the mechanisms
whereby POTW owners/administrators generally
implement their objectives. Therefore, evaluation and
discussion of these aspects is an integral part of
efforts to identify the presence of administrative
performance-limiting factors. For this reason, early
during the on-site fieldwork, the evaluator should
schedule a meeting with the key POTW decision-
maker and the "budget person." This meeting should
be scheduled after the evaluator is familiar with the
plant.
Nearly every POTW's financial information is set up
differently so it helps to review the information with
the assistance of plant personnel to realistically
rearrange the line items into categories understood by
the evaluator. Forms for collecting financial data are
presented in Appendix D. Analysis of these data can
be supported by comparison with typical values for
wastewater treatment plants (16,20,21). POTWs with
flows greater than 88 L/s (2 mgd) usually have
separate financial information for the wastewater
treatment facilities. Smaller POTWs often have
financial information combined with other utilities, such
as wastewater collection, water treatment and
distribution, or even street repairs and maintenance.
For this reason, it is often more difficult and time
consuming to assess, the financial status for small
POTWs.
Key POTW administrators should be identified and
individual interviews scheduled with them as
described in Section 2.3.3.1. As a general rule, the
individual interviews should be held in an environment
that allows for open discussion.
The evaluation of administrative performance-limiting
factors is by nature subjective. Typically, all
administrators verbally support goals of low costs,
safe working conditions, good treatment performance,
high employee morale, etc. An important question that
the evaluator must ask is, "Where does good
treatment fit in?" Often this question can be answered
by observing the priority of items implemented or
supported by administrators. The ideal situation is one
in which the administrators function with fu|l
awareness that they want to achieve desired
performance as an end product of their wastewater
treatment efforts. Improving working conditions,
lowering costs, and other similar goals would be
pursued within the realm of first achieving adequate
performance.
At the other end of the spectrum is an administrative
attitude that "we just raised the monthly rates 100
percent last year; we can't afford to spend another
dime on that plant." POTW administration can be
judged by the following criteria:
Excellent: Reliably provides adequate wastewater
treatment at lowest reasonable cost.
Normal: Provides best possible treatment with the
money available.
Poor: Spends as little as possible with no
correlation made to achieving adequate
plant performance.
Administrators who fall into the "poor" category
typically are identified as contributing to inadequate
performance during the factor identification activities.
Technical problems identified by the plant staff or the
CPE evaluator, and the potential costs associated with
correcting these problems, often serve as the basis
for assessing administrative factors limiting plant
performance. For example, the plant staff may have
correctly identified needed minor modifications for the
facility and presented those needs to the POTW
administrators, but had their request turned down. The
evaluator should solicit the other side of the story from
the administrators to see if the administrative policy is
indeed non-supportive in correcting the problem.
There have been many instances in which operators
or plant superintendents have convinced
administrators to spend money to "correct" problems
that resulted in no improvement in plant performance.
Another area in which administrators can significantly,
though indirectly, affect plant performance is through
personnel motivation. A positive influence exists if
administrators encourage professional growth through
support of training, tangible awards for initial or
upgrading certification, etc. If, however, administrators
eliminate or skimp on essential operator training,
downgrade operator positions through substandard
salaries, or otherwise provide a negative influence on
operator morale, administrators can have a significant
detrimental effect on plant performance.
25
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2.3.4.2 Design Factors
Data gathered during the plant tour, completion of
forms in Appendix D, and the completed evaluation of
major unit process capabilities provide the basic
information needed to complete the identification and
prioritization of design-related performance-limiting
factors. Often, to complete the evaluation of design
factors, the evaluator must make field investigation of
the operational flexibility of the various unit processes.
Field investigations should be completed in
cooperation with the POTW operator. The evaluator
must not make any changes unilaterally. Any field
testing desired should be discussed with the operator,
whose cooperation should be obtained in making any
needed changes. This approach is essential since the
evaluator may wish to implement changes that, while
improving plant performance, could be detrimental to
specific equipment at the plant. The operator has
worked with the equipment, repaired past failures, and
read the manufacturers' literature, and is in the best
position to ascertain any adverse impact of proposed
changes.
Field investigation of process flexibility defines the
limitations of the equipment and processes and also
promotes a better understanding of the time and
difficulty required to implement better process control.
This is illustrated by the following discussion:
A 4.4-L/s (0.1-mgd) extended aeration facility has
airlift sludge return pumps that have been operated
to provide return rates of 200-300 percent of
influent flow rates. The evaluator desired to know if
returns could be held under 100 percent since this
would substantially reduce solids loading on the
final clarifier and potentially improve clarifier
performance.
Discussions with the plant operator revealed that he
had previously tried to reduce the return rate by
reducing the air to the airlift return pumps. The
operator abandoned the idea because the airlifts
repeatedly plugged overnight when left at the lower
rates. The evaluator convinced the operator to
again try reducing the return rate so that the limits
of return sludge flow control available could be
defined.
Air flow rate was initially reduced to produce a
return flow rate of 100 percent of incoming
wastewater flow as measured by a bucket and
stopwatch. The airlift return pumps plugged
completely in less than 2 hours. The return flow
rate was reset to 100 percent by increasing the
airflow substantially above the previous setting. An
hour later the return flow rate was measured as
220 percent. These results supported the
operator's contention that return flow rates could
not be controlled at reasonable levels.
The air supply was again adjusted to provide a flow
rate halfway between the current and the desired
rate. This setting allowed better control to be
exercised, but plugging still occurred with existing
sludge characteristics at return sludge flows of less
than about 125 percent. It was concluded that this
was the practical lower limit for return sludge flow
rate control with the existing facilities and sludge
character. To maintain a return sludge in the range
of 125-150 percent required frequent checking,
including an evening check not before asked of the
operator. In this manner, part - but not all - of the
design limitation could be overcome with increased
operator attention.
The areas in a POTW that frequently require field
investigations to determine process flexibility are:
1. Suspended Growth Systems
• Control of return sludge flow rate within the
ranges presented in Table 2-5
• Control of aeration basin DO within the ranges
presented in Figure 2-2
• Sludge mass control by wasting expected sludge
production (mass and volume) presented in
Tables 2-7 and 2-8 f
• Flow splitting to prevent unnecessary
overloading of individual process units
Figure 2-2.
Bulk DO, mg/l
4 r-
Effect of aeration basin DO concentrations on
sludge settling characteristics.
Tendency to Increase
Sludge Settling Rate
Tendency to Decrease
Sludge Settling Rate
I
I
I
10 20 30 40 50 60
Oxygen Uptake Rate, mg/L/hr
70
26
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• Available mode changes to provide maximum
use of existing facilities:
- Step feed or contact stabilization when the
final clarifier appears to be a limiting unit
process
- Step feed when oxygen transfer is marginal
2. Trickling Filters
• Alternate disposal methods for anaerobic
digester supernatant
• Ability to control sludge levels in ciarifiers without
adversely impacting sludge handling facilities
• Recirculation to the filter without excess
hydraulic loads on the primary or secondary
clarifiers
3. RBCs
• Alternate disposal methods for anaerobic
digester supernatant
• Ability to control sludge levels in clarifiers without
adversely impacting sludge handling facilities
• Ability to redistribute individual stage loadings to
provide unit loadings within the ranges shown in
Table 2-13
4. ABFs
• Control of return sludge flow rates within 50-100
percent of influent flow
• Ability to waste a desired mass of sludge on a
daily basis
• Ability to spread a day's sludge wasting over a
24-hour period, or at least an extended period of
time
• Ability to provide recirculation directly around the
biocell
• Ability to maintain aeration basin DO at 2-3 mg/L
2.3.4.3 Operational Factors
Operational factors are those factors that relate to the
unit process control functions implemented at a
POTW. Significant performance-limiting factors often
exist in these areas (16). The approach and methods
used in maintaining process control can significantly
affect the performance of plants that have adequate
physical facilities. This section provides guidance to
evaluators for identification and prioritization of
operational factors that limit plant performance.
The evaluator starts collecting data for the process
control evaluation by identifying the key POTW person
for process control strategies implemented at the
plant. The plant tour and data-gathering phases also
provide opportunity to assess the process control
applied. In addition, the process control capability of
an operator can be subjectively assessed during the
major unit process evaluation. If an operator
recognizes the unit process functions and their
relative influences on plant performance, a good grasp
of process control is indicated. An approach to
evaluating process control is discussed in the
following sections.
a. Suspended Growth Facility Process Control
The process controls that should be available to an
operator of an activated sludge facility are control of:
sludge mass, aeration basin DO, and return sludge
rate. Techniques and approaches to improving these
controls are presented in Chapter 3.
Sludge Mass Control
The activated sludge process removes colloidal and
dissolved organic matter from wastewater resulting in
a net increase in the sludge solids in the system.
Control of the amount of sludge maintained in the
system by wasting (removing) excess sludge is a key
element in controlling plant performance. All variations
of the activated sludge process require sludge mass
control and periodic wasting. In line with this
requirement, an operator who properly understands
activated sludge mass control should be able to show
the evaluator a recorded history of a controlled sludge
mass (e.g., records of mean cell residence time
[MCRT], mixed liquor volatile suspended solids
[MLVSS], plots of MLSS/MLVSS concentrations in the
aeration basin, total mass of sludge in the plant, etc.).
The following are common indicators that sludge mass
control is not adequately practiced at an activated
sludge plant:
• A sludge mass indicator parameter or calculation
(MLVSS, MCRT, total sludge units) is not obtained
on a routine basis (22). "Routine" would be at least
daily for an 88-L/s (2-mgd) or larger plant and 2-3
times a week for a 4.4-L/s (0.1-mgd) plant.
• Only a settled sludge test is used to determine
wasting requirements (e.g., waste if the 30-minute
settled sludge volume in a graduated cylinder is
greater than 600 ml/L).
• The operator does not relate mass control to
control of sludge settling characteristics and sludge
removal performance (i.e., sludge character).
• Significantly less mass is wasted than calculations
indicate should be produced (i.e., the clarifiers lose
solids over the weirs routinely).
• Poor performance persists and the mass of sludge
maintained provides an MCRT significantly out of
the ranges in Table 2-24.
27
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Tabla2-24. Typical Mean Cell Residence Times for
Suspended Growth POTWs
Process Type Typical MCRT
Conventional Aeration
Extended Aeration
Contact Stabilization
days
4-12
20-40
10-30
Aeration Basin DO Control
The aeration basin DO level is a significant factor in
promoting the growth of either filamentous or
zoogleal-type sludge organisms (23). Higher DO tends
to speed up or slow down the relative populations of
these major organism types toward primarily zoogleal.
Conversely, lower DO encourages the growth of
filamentous organisms and a bulky, slow settling
sludge. A general guideline for relating sludge
characteristics to DO concentration in an aeration
basin is presented in Figure 2-2. This information can
be used to evaluate the DO control approach at the
POTW under study.
The following are common indicators that aeration
basin DO control is not properly practiced at an
activated sludge plant:
• DO testing is not run routinely on the aeration
basin. "Routine" ranges from daily for an 88-L/s (2-
mgd) or large plant to weekly for a 4.4-L/s (0.1-
mgd) plant.
• The operator does not understand or use the
relationship between DO and sludge character
(e.g., sludge settling is very slow and DO is very
low, or sludge settling is very fast, effluent is turbid,
and DO is very high).
Return Sludge Control
The objective of return sludge flow control is to
optimize sludge distribution between the aerator and
secondary clarifier to achieve and maintain good
sludge characteristics. Thus, return sludge flow rate
control should be used to maximize the sludge mass
and sludge detention time in the aeration basins and
minimize the sludge mass and sludge detention time
in the clarifiers.
The following are common indicators that return
sludge flow rate control is not properly practiced at an
activated sludge plant:
• Returns are operated outside the ranges (especially
higher) indicated in Table 2-5 and Figure 2-3.
• The operator believes that a high sludge blanket
condition in a final clarifier can be categorically
lowered by increasing the sludge return rate. (E.g.,
the operator does not realize that increasing the
return sludge flow rate increases the solids loading
Figure 2-3. Typical return sludge flow rates with various
clarifier surface overflow rates.
RAS Flow, % of
wastewater flow
180 r-
160
140
120
100
80
60
40 -
20 -
High Return Range
(Poor Process Control)
Normal Operating Range
8
(196)
16
(393)
24
(589)
32
(785)
40
(982)
Sec. Clarifier SOR, m3/m2/d (gpd/sq ft)
to the final clarifier and decreases the settling time
in the final clarifier.)
• MLSS concentrations fluctuate widely on a diurnal
basis, but return rates are not adjusted throughout
the day to account for diurnal flow variations.
« The operator has not devised a method to estimate
or measure the return sludge flow rate if
measurement was not provided for in the original
design.
b. Fixed Film Facility Process Control
There is a lesser amount of process control that can
be applied to fixed film facilities than to suspended
growth facilities. However, because fixed film facility
performance is so dependent on media loading,
process control, which may at first seem unimportant,
can make a significant difference in plant
performance. The following are common indicators
that process control at a fixed film facility is not
optimum (1): .
« Sludge blankets in either the primary or secondary
clarifiers are maintained at a high level [i.e., >0.3
m (1 ft)].
« Organic loads from return process streams are not
minimized.
28
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• Lack of good maintenance, indicated by:
- Distributors on trickling filters are plugged, or
leaky distributor seals are not fixed.
- Filter media is partially plugged and measures
such as chlorination, flooding, and recirculation
are not used to address the problem.
- Trickling filter underdrain collector outlets are
submerged or air vents are plugged.
• High recirculation, which increases primary or
secondary clarifier overflow rates, is provided
without regard to clarifier overloading. Some
trickling filter plants provide recirculation that is
directed to the influent wastewater wet-well and
must pass through the primary clarifier a second
time. Likewise, some trickling filters provide
recirculation through the secondary clarifier sludge
return to the head of the plant. Recirculation
provided by these methods should not be
practiced.
2.3.4.4 Maintenance Factors
General information on POTW maintenance is
gathered during the detailed data collection phase and
is recorded on Form D-5. However, the evaluation of
maintenance performance-limiting factors is done
throughout the CPE by observation and questions
concerning the reliability and service requirements of
pieces of equipment critical to process control and'
thus performance. If units are out of service routinely
or for extended periods of time, maintenance
practices may be a significant contributing cause to a
performance problem. An adequate spare parts
inventory is essential to a good maintenance program.
Equipment breakdowns are often used as excuses for
process control problems. For example, one operator
of an activated sludge plant blamed the repeated loss
of sludge over the final clarifier weirs on the periodic
breakdown of one sludge return pump. Even with one
pump out of service, the return sludge capacity was
over 200 percent of influent flow. The real cause of
the sludge loss was improper process control,
including inadequate sludge mass control and
excessively high return sludge flow rates.
Observation and documentation are necessary
portions of the approach utilized to evaluate
emergency and preventive maintenance practices.
Important aspects are examination and verification of
spare parts inventories and recordkeeping systems.
An example approach to a preventive maintenance
scheduling system that has been applied successfully
at several plants is presented in Appendix G. A good
preventive maintenance program includes a schedule
to distribute the workload evenly. Evaluation of these
items provides a basis from which the specific results
of -maintenance, or lack thereof, can be assessed.
This approach is illustrated by the following:
A poorly performing trickling filter plant was
assessed to have acceptable BOD5 loadings to the
filter, capable secondary clarifiers, and adequate
sludge handling facilities. However, a large buildup
of sludge was maintained in both the primary and
secondary clarifiers. Questioning of the operator
revealed that sludge was not removed adequately
because the heated anaerobic digesters were upset
if too much sludge is added. Further investigation
indicated that adequate temperature control of the
digester contents was not being achieved. The
operator pointed out that the boiler for the heat
exchanger was operated manually and just during
the day because he had tried unsuccessfully to fix
the automatic controls. Ultimately, inadequate
maintenance was identified as a cause of poor
plant performance.
The above discussion illustrates how a detailed
evaluation of process control activities was necessary
to properly identify a maintenance-related factor as a
cause of poor plant performance. The evaluator must
evaluate maintenance during all phases of the CPE
and should not expect to identify these factors solely
in a formal evaluation of maintenance procedures.
2.3.5 Performance Evaluation
The plant performance evaluation is directed toward
two goals: 1) establishing, or verifying, the magnitude
of a POTW's performance problem; and 2) projecting
the level of improved treatment that can be expected.
2.3.5.1 Magnitude of the Performance Problem
During the CPE, the evaluator should develop a clear
understanding of the performance problem associated
with the subject POTW. As a first step of this
assessment, recorded historical performance data can
be used. These data are available from copies of
NPDES permit reports in small POTWs and from
monthly monitoring summary sheets in larger POTWs.
Once historical data are reviewed, the evaluator
should attempt to verify the accuracy of the reported
plant performance. It should be stressed that the
purpose is not to blame the plant staff, but rather to
assist in identifying and substantiating the true
cause(s) of poor plant performance.
The evaluator can indirectly collect data to establish
authenticity of the monitoring results throughout the
CPE. For example, major unit processes are assessed
for their capability to achieve desired performance. If a
POTW is rated a Type 3 plant (inadequate major
process capability), reported excellent effluent quality
should be suspect. If reported performance is
consistent with the results of the overall evaluation,
the validity and accuracy of the data are reinforced.
29
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Limitations of these comparisons are their subjective
nature.
Major test parameters critical for completion of the
CPE are influent BOD5 and flow. The evaluator can
roughly check both BODs and flow data by calculating
a per capita BOD5 contribution. Per capita BOD5
contributions are usually 0.07-0.09 kg (0.15-0.20 lb)/d
for typical domestic wastewater. When estimating
BODs loads to a plant without actual data, or checking
reasonableness of existing plant data, loads from
significant industrial contributors must be added to the
calculated per capita loads.
Small activated sludge plants have been shown to
have the most variance between historical records
and actual performance. In small activated sludge
plants - such as package extended aeration plants,
contact stabilization plants, and oxidation ditches -
several days' or even an entire week's sludge
production can be lost as the result of sludge bulking
in several hours. Effluent TSS may be less than 10
mg/L before and after bulking occurs, but may reach
1,000-2,000 mg/L while bulking. The operator has
ample opportunity between bulking periods to collect
more than enough samples to meet permit monitoring
requirements and indicate a good effluent quality.
Another sampling procedure that can result in
nonrepresentative monitoring is sometimes seen in
fixed film facilities where performance degrades
significantly during peak daytime loads. Samples
collected from 6 a.m. to 10 a.m. may meet the
required compositing criteria (e.g., three samples at 2-
hour increments), but would probably indicate better
than overall average effluent quality. Likewise,
samples collected from noon to 4 p.m. may indicate
worse than actual average effluent quality.
To verify good data or determine the magnitude of a
performance problem, a comparison of expected vs.
actual sludge production should be made during the
CPE. This comparison has proven invaluable in
conducting a CPE and is termed a Sludge
Accountability Evaluation. A detailed example of this
evaluation is presented.
a. Example Sludge Accountability Evaluation
A 44-Us (1-mgd) oxidation ditch activated sludge plant
is bring evaluated. The plant's NPDES discharge
monitoring reports indicate that the plant is in
compliance. Information collected during the CPE is
as follows:
• The plant operator felt the final clarifier was not
large enough to capture all the solids during high
flows - he suggested that a flow equalization basin
be constructed.
• The plant has 1 oxidation ditch, 2 final clarifiers, 1
chlorine contact tank, and 6 sludge drying beds.
• The plant has limited infiltration/inflow (I/I) and no
significant industrial waste contribution. I/I occurs in
the early spring, causing the highest average
monthly flow to be about 20 percent greater than
the annual average flow.
• The City Clerk reported that there were 3,520 taps
that were billed quarterly.
• The plant does not have return sludge flow
measurement. Two constant-speed return sludge
pumps are available to return sludge to the
oxidation ditch. Both pumps are being operated at
full speed to "get the solids out of the clarifier and
back to the oxidation ditch." The R/Q ratio was
estimated to be greater than 500 percent by
measuring the return sludge flow rate utilizing draw-
down of the final clarifier. Return sludge flow can
be reduced by adjusting a plug valve on the
discharge .side of the pump, and by operating 1 of
the 2 pumps. It is difficult to operate the pump at
lower flow rates because the valve plugs with rags.
• Excellent laboratory facilities were available and the
operators run all tests required by the NPDES
permit. These include tests for: plant influent and
effluent BOD5 and TSS; plant effluent residual
chlorine; and fecal coliform. Oxidation ditch MLSS
tests are run once per week along with a settling
test in a graduated cylinder to determine SVI.
• The plant superintendent reported that he wasted
sludge to his drying beds when he had trouble
keeping the sludge in the final clarifiers.
• The population of the community was estimated by
the Mayor to be about 8,500.
• Influent plant flow was checked by measuring the
depth of flow in the Parshall flume and comparing
the calculated flow to the flow indicator. The
reading was within 10 percent of the measured
flow.
• The plant superintendent said that he "filled up" all
of his drying beds "about 10 times" last year.
There are 6 beds, each measuring 100' x 50' x 18"
deep.
• NPDES permit monitoring requirements and plant
effluent limits are as follows:
• Plant performance data are shown in the table
below:
Plant Loading Evaluation
Plant loadings should be verified by comparison to
typical per capita contributions for domestic
wastewater. Since industrial loadings can significantly
effect this evaluation, plants where significant
30
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Permit Limits
Monitoring Requirements
Parameter
Influent BOD5
Effluent BOD5
Influent TSS
Effluent TSS
Effluent pH
Fecal Coliform
30-day
avg.
30 mg/L
30 mg/L
6.5-9
200 per
100 mL
7-day
avg.
45 mg/L
45 mg/L
6.5-9
400 per
100 mL
Frequency
Monthly
Monthly
Monthly
Monthly
Weekly
Monthly
Sample
Type*
Composite
Composite
Composite
Composite
Grab
Grab
A composite sample is defined as being comprised of a minimum
of 4 samples collected 2 hours apart with the individual sample
volume being proportioned to plant flow.
Performance Data for Example
Sludge Accountability Evaluation
Raw Final Effluent3
Conclusion: measured plant flow appears to be within
expected range - therefore use actual plant flow in
evaluation.
3. Organic Loading Evaluation (assume 0.15-0.20 Ib
BOD5/capita/d for typical domestic wastewater - use
0.17).
Projected plant organic load = 8,650x0.17
= 1,471 Ib BODg/d
Plant organic load from plant data = 1,534 Ib BOD5/d
Conclusion: plant BOD5 data is higher than projected
BOD but sampling is during peak load portions of the
day - use projected organic load.
Calculated influent BOD5:
Date
6/84
7/84
8/84
9/84
10/84
11/84
12/84
1/85
2/85
3/85
4/85
5/85
Avg.
1 ^i
Flow,
mgd1
0.791
0.762
0.781
0.720
0.759
0.747
0.715
0.729
0.761
0.813
0.938
0.880
0.783
BOD5,
mg/L2
276
281
270
274
251
225
231
197
263
201
173
197
237
TSS,
mg/L2
290
205
232
190
237
241
187
215
217
245
198
220
223
BOD5
Load, Ib
1,820
1,785
1,758
1,645
1,589
1,402
1,377
1,198
1,669
1,363
1,353
1,446
1,534
BOD5,
mg/L2
8
7
5
7
4
9
7
6
4
7
11
9
7
TSS,
mg/L2
10
12
11
12
9
10
9
9
10
14
18
16
12
(1,471) * (8.34 x 0.783) = 225 mg/L
Sludge Accountability Evaluation
1. Determine anticipated sludge production.
BOD5 cone, removed = 225 - 7 = 218 mg/L
BOD5 mass removed =218 (0.783) (8.34)
=• 1 ,424 Ib BOD5/d
From Table 2-7, expected Ib TSS (sludge)/lb
BOD5
removed = 0.65 for an oxidation ditch. Therefore:
Expected sludge = 0.65 (1,424) (365)
= 337,990 Ib/yr
2 Based on one composite sample/month. Composite sampling
usually done on Thursday. Samples collected at 8:00 am, 9:00
am, 11:00 am, and 1:00 pm. Influent BOD5 and TSS values may
be high since this is the peak load period of the day.
3 The operator reported the need for flow equalization since, during
high flows, some solids were lost. No indication of solids loss is
apparent in plant effluent data.
Source of Data: Plant NPDES Monitoring Records
industrial contributions are known to be present
cannot be evaluated in this manner.
1. Population Served (assume 2.5 persons/tap).
(2.5 persons/tap) x (3,520 taps) = 8,800 people
Use average of reported population and estimated
population:
(8,500 + 8,800) T 2 = 8,650 people
2. Plant Flow Evaluation (assume 100 gal/capita/d for
typical domestic wastewater).
Projected plant flow = 8,650 x 100 = 865,000 gpd
Measured plant flow = 783,000 gpd
2. Estimate Accounted-For Sludge Wasted From
Plant.
Effluent "Waste Sludge" = (0.783)(12)(8.34)(365)
= 28,602 Ib/yr
Intentionally Wasted Sludge (operator said he filled
sludge beds "about ten times" last year):
Sludge bed volume = (6 beds) (100') (50') (18"/12)
= 45,000 cu ft
Waste sludge volume = (10 times) (45,000) (7.48)
= 3,366,000 gal/yr
From Table 2-8, expected waste sludge concentration
is 7,500 mg/L. (Assume that return sludge flow can be
controlled within a reasonable range.) Therefore:
Wasted Sludge = 3.366 (7,500) (8.34)
= 210,543 Ib/yr
Total Accounted-For Sludge = 28,602 + 210,543
= 239,145 Ib/yr
31
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Complete Sludge Accountability Summary Sheet
From Appendix D as Shown on Following Page
As shown in this example, the sludge accountability
analysis indicates that the plant effluent data are
probably not a reliable indicator of true plant
performance. Effluent quality could have been quite
good when samples were collected, yet periodic solids
loss could have been occurring during the relatively
infrequent effluent monitoring events. Use of the
sludge accountability evaluation often provides
significant insight to the CPE effort.
2.3.5.2 Projected Improved Performance
The plant performance that is achievable is initially
estimated by evaluating the capability of major unit
processes. This concept is schematically shown in
Figure 2-1. If major unit processes are deficient in
capacity, secondary treatment may not be achievable
with the existing POTW (i.e., it is a Type 3 plant).
If the evaluation of major unit processes shows that
the major facilities have adequate capacity, then an
approach, like the CCP approach presented in
Chapter 3 likely can be used to achieve improved
POTW performance (i.e., it is a Type 1 or 2 plant). For
plants of these types, all performance-limiting factors
are considered as possible to correct with adequate
training of the appropriate POTW personnel. The
training is addressed toward the operational staff for
improvements in plant process control and
maintenance; toward the POTW administration for
improvements in administrative policies and budget
limitations; and toward both operators and
administrators to achieve minor facility modifications.
"Training" as used in this context describes activities
whereby information is provided to facilitate
understanding and facilitate implementation of
corrective actions.
Once the plant's major unit process capability has
been established and the performance-limiting factors
have been identified and prioritized, the evaluator is in
a position to assess the potential for improved
performance. During this effort, the evaluator must
assess the practicability and potential time frame
necessary to address each identified factor.
Additionally, it is necessary to project levels of effort,
activities, time frame, and costs associated with
implementation of activities to optimize existing
facilities (e.g., by implementing a CPE). Projecting
costs for modifying a Type 3 plant are beyond the
scope of a CPE effort.
2.3.6 Presentation to POTW Administrators and
Staff
Once the evaluator has completed the fieldwork for
the CPE, an exit meeting should be held with the
POTW administrators and staff. A presentation of
preliminary CPE results should include brief
descriptions of the following:
• Sludge Accountability Analysis
• Evaluation of major unit processes (Type 1, 2, or 3)
• Prioritized performance-limiting factors
• Plant performance potential
In general, it is desirable to present a[l findings at the
exit meeting with local officials. This approach
eliminates surprises when the CPE report is received
and begins the cooperative approach necessary for
any follow-up activities. In situations where
administrative or staff factors are difficult to present,
the evaluator must be sensitive and use
communication skills to successfully present the
results. Throughout the discussions, the evaluator
must remember that the purpose of the CPE is to
identify and describe facts to be used to improve the
current situation, not to place blame for any past or
current problems.
It should be made clear at the exit meeting that other
factors likely will be identified during the conduct of
any follow-up activities. These factors will also have to
be addressed to achieve the desired performance.
This understanding of the short-term CPE evaluation
capabilities is often missed by local and regulatory
officials, and efforts may be developed to address
only the items prioritized during the CPE. The
evaluator should stress that a commitment must be
made to achieve the desired improved performance,
not to addressing a "laundry list" of currently identified
problems.
2.3.7 CPE Report
The objective of a CPE report is to summarize the
CPE findings and conclusions. It is particularly
important that the report be kept brief so that the
maximum amount of resources are used for the
evaluation rather than for preparing an all-inclusive
report. The report should present the important CPE
conclusions necessary to allow the decision-making
officials to progress toward achieving desired
performance from their facility. Eight to twelve typed
pages are generally sufficient for the text of a CPE
report. A brief example CPE report is presented in
Appendix C. Typical contents are:
Introduction
Facility background
Major unit process evaluation
Performance-limiting factors
Performance improvement activities
Costs
As a minimum, the CPE report should be distributed
to POTW administrators and key plant personnel.
Further distribution of the report (e.g., to the design
engineer or regulatory agencies) depends on the
circumstances of the CPE, but should be done at the
direction, or with the awareness, of local
administrators. '
32
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Performance Monitoring/Sludge Accountability Summary Sheet (Example)
1. Sludge Accountability
• Anticipated Sludge Production (see Table 2-7, note: unit production
values include solids lost in plant effluent)
• Accounted-for Sludge
- wasted intentionally
- effluent sludge
Total: 2 + 3
• Unaccounted-for Sludge: 1-4
5 * 365
• Unaccounted-for Sludge Percentage: 100 x 5 -r 1
if -15<7<15then not possible to conclude that a problem with
sludge wasting exists.
if 7 > 15 then problem with effluent monitoring indicated.
if 7 < -15 then may indicate organic loading greater than
typical domestic (i.e., industrial loading).
2. Performance Monitoring Assessment
• Projected Actual Effluent TSS
- recorded effluent TSS
- projected increase in effluent TSS: 6 -=• (8.34 x flow in mgd)
- estimated actual increase in effluent TSS: 8 + 9
• Projected Actual Effluent BOD5
- recorded effluent BOD5
- projected increase in effluent BOD5: 0.5 x 9
- estimated actual increase in effluent BOD5: 10+ 11
337,990 Ib/yr
210,543 Ib/yr
28,602 Ib/yr
239,145 Ib/yr
98,845 Ib/yr
271 Ib/d
29.2 %
CONDITION
FOUND
Item
1
2
3
4
5
6
7
12mg/L
41 mg/L
53 mg/L
7 mg/L
21 mg/L
28 mg/L
8
9
10
11
2.3.7.1 Introduction
The introduction of the CPE report should cover the
following topics:
• Reason(s) for the CPE
• Objectives of the CPE
• Plant effluent performance requirements
2.3.7.2 Facility Background
This section should include general information about
the POTW that will serve as the reference basis for
the remainder of the report. The following information
should be included:
POTW description (oxidation ditch, RBC, etc.)
Design and current flows
Age of plant and dates of upgrades
Service population
Significant industrial wastes
Significant infiltration/inflow
Unit processes diagram
2.3.7.3 Major Unit Process Evaluation
This section should include a description of the
assessed plant type (Type 1, 2, or 3) and a summary
of data sources for calculating current loading; for
example, "current loadings were calculated using
plant laboratory results for concentrations and plant
flow records lowered by 10 percent to adjust for high
calibration of flow recording equipment." Other
significant evaluations should be included in this
section, such as calibration of flow measuring devices,
sludge accountability analysis, evaluation of loads
from industrial contributors, etc.
Results should be presented for each major unit
process (aerator, secondary clarifier, sludge handling
processes). The evaluator may choose to present
capabilities of other unit processes if these data are
pertinent to assessing the POTW's treatment
capability.
An effective method of presenting results of a plant's
unit process evaluation relative to current and design
loadings is the use of a Performance Potential Graph.
33
-------
An example of this type of graph is shown in Figure 2-
4. The plant's unit processes are listed in the left-
hand column. Plant loadings, typically flow or BOD or
both, are shown horizontally across the top of the
graph. Current and design loadings are depicted by
vertical dashed lines. Rated capabilities of the various
unit processes are projected using horizontal bars.
Specific evaluation criteria are listed under each unit
process. Assumptions for each set of calculations
used to determine the values of the selected
evaluation criteria are noted on the bottom of the
graph. In addition, the value of the parameter used to
project the unit's capability is shown.
The graph is time consuming to develop, but a great
deal of clarity is provided to facility administrators and
personnel when the plant's capabilities are thus
depicted. It is important to note that the capabilities
are not established using "typical" design values for
unit process capacity. Judgment of the evaluator,
each plant's unique circumstances, and experience
with other similar facilities are factors that affect
projections of the unit process capability to meet the
plant's performance requirements.
As might be expected, rating a unit process'
capabilities less than at its "design capacity" is
something that requires full awareness of the possible
ramifications. A first question of some administrators
is "Why didn't I get what I paid for?" Many reasons
may exist, such as industrial loadings not anticipated
in design, inaccurate loading assumptions, changed
criteria for capabilities of unit processes based on
experience, etc. As such, the evaluator who chooses
to use the Performance Potential Graph must be
prepared to support the projections. Despite the
potential ramifications, the effectiveness of the graph
in presenting CPE results makes attempts to present
information in this manner worthwhile.
2.3.7.4 Performance-Limiting Factors
Factors limiting performance that were identified
during the CPE should be listed. The more serious
factors (those receiving "A" or "B" ratings) are listed
in order of priority and short, one- or two-paragraph
explanations of each factor are included. Factors
receiving a "C" rating are normally also listed. Often it
is appropriate to summarize factors not identified as
performance-limiting (i.e., areas where the POTW was
meeting or exceeding expectations) in this section of
the report.
2.3.7.5 Performance Improvement Activities
Activities required to achieve required plant
performance are briefly discussed. If a CCP activity
could improve performance, this should be stated. If
facility modifications are indicated (i.e., Type 3 plant),
then a recommendation for a more detailed evaluation
of treatment alternatives should be recommended.
Any additional benefits, such as reduction in energy
consumption, improved safety, etc., should also be
indicated.
2.3.7.6 Costs
Costs associated with follow-up activities should be
projected as accurately as possible. Ranges of costs
can be used if an evaluator does not feel comfortable
projecting specific dollar amounts. Each cost
projected should be indicated as a "one-time" or
"annual" cost. Costs for a CCP facilitator (consultant)
or for a piping modification are examples of "one-
time" costs. Increased sludge handling and electrical
or chemical costs are examples of "annual" costs.
2.3.8 Example CPE
A 52.6-L/s (1.2-mgd) oxidation ditch serves a primarily
residential community with a population of 8,500. The
City Council was notified by the State Health
Department that the City's self-monitoring reports
indicated the City is not meeting its NPDES permit
requirements of 30 mg/L (30-day average) for BOD5
and TSS.
After researching several alternatives, the Public
Works Director recommended to the City Council that
a CPE be conducted to determine the causes of their
performance problem and provide direction in
selecting corrective actions. A consultant who
specializes in conducting CPEs was subsequently
hired.
2.3.8.1 Plant Data
A flow diagram is presented in Figure 2-5. The
following data were extracted from the completed data
collection forms as presented in Appendix D.
DESIGN DATA
Design Flow: 52.6 L/s (1.2 mgd)
Hydraulic Capacity: 131.5 L/s (3.0 mgd)
Organic Loading: 900 kg (2,000 Ib) BOD5/d
900 kg (2,000 Ib) TSS/d
Preliminary
Treatment:
Mechanical Bar Screen,
Aerated Grit Chamber
Flow Measurement: Parshall Flume, Sonic Level
Sensor, Strip Chart Recorder
Oxidation Ditch: Volume - 4,500 m3 (160,000
cuft)
02 Transfer - Brush Rotors
rated at 1,800 kg (4,000
lb)/d @ 38°C (100°F) with 2.0
mg/L residual DO
Final Clarifiers:
Number - 2 with Center Feed
and Peripheral Weirs
34
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Figure 2-4. Example performance potential graph.
Wastewater Flow1 ,
Unit Process 02 Q4 Q6
Aeration Basin
Detention Time, hr
BOD5 Load, lb/d/1 ,000 cu ft
Oxygen Supply2, Ib O2/lb BOD5
Clarifier
Surface Overflow Rate3,
gpd/sq ft
Solids Loading Rate4, Ib/d/sq ft
Sludge Handling and Disposal
Holding Tank5, cycles/day
Sludge Drying Beds6, days
1 Assume average influent BOD cone
2 Assume 83 Ib/hr oxygen transfer ra
3 Use maximum overflow rate of 600
4 Assume 2,500 mg/L MLSS; 50% F
5 Waste MLSS from aeration basin a
Influent BOD5 = 254 mg/L; Effluen
6 Waste from holding tank at 7,500 rr
mgd
0.8 1.0 1.2
i
120 60 40
30 24 20
i
3.2 6.3 9.5
i
4.89 2.45 1.63 j
i
i
i
75 149 224
12.7 19.8 19.0
1.22 O.S8 0.82
2 39 323 448
i
2.3 4.7 7.0
i
i
I
0.9 1.8 2.7
i
20.0 10.0 6.7
9.4 11.7 14.0
3.6 4 5 5.4
5.0 4 0 3.3
i
i
Current Flow Design Flow
•entration of 254 mg/L based on current plant records.
e per manufacturer's literature and 1 .5 Ib O2 required/lb BOD applied.
gpd/sq ft at 2:1 diurnal peaking.
!/Q; consider 2:1 peaking; and maximum solids loading of 20 Ib/d/sq ft; includes R + Q.
1 2,500 mg/L; assume sludge production ratio = 0.65 Ib TSS/lb BODR;
t BODS = 10 mg/L; 14,000-gal holding tank; 2 cycles per day maximum with existing staff.
ig/L; 9-inch pour; 21 -day turnover time required.
Disinfection:
Sludge Return:
Aerobic Digester:
Diameter - 10.7 m (35.0 ft)
Area - 90 m2 (962 sq ft) each
Sidewater Depth - 2.7 m (9.0
ft)
Center Depth - 3.1 m (10.5 ft)
Clarifier Scraper to Center
Hopper
Number of Chlorinators - 2
Capacity - 113 kg (250 lb)/d
each)
Contact Basin - 142 m3
(37,500 gal)
Number of Vortex Pumps - 2
Flow Control - 1.9-5.7 m3
(500-1,500 gal)/min
Measurement - 90° V-notch
Weir w/o Recorder
Sampling - Manual @ Weir
Volume - 680 m.3 (220,000 gal)
Sludge Removal - Bottom Pipe
to Drying Beds
Supernatant Removal -
Muliple-Port Drawoff to
Oxidation Ditch
Sludge Drying Beds: Number of Beds - 8
Size - 18.2 m x 45.0 m (60 ft x
150ft)
Summer Drying Time - 3
weeks
Winter Drying Eliminated -
Storage Required for
December-March
Subnatant Returned to Head of
Plant
CURRENT LOADING
Flow:
Annual Average
Minimum Month
Peak Month
Influent BOD5:
41.6 L/s (0.95 mgd)
37.5 Us (0.855 mgd)
50.0 L/s (1.14 mgd)
190 mg/L or 680 kg (1,500 lb)/d
35
-------
Figure 2-5. Flow diagram of POTW in example CPE.
Raw Wastewater
Mechanical
Bar Screen
Aerobic
Digester
Waste Sludge
Sludge Drying Beds
Dewatered Sludge
Land Spreading on Public Property
36
-------
Influent TSS:
205 mg/L or 740 kg (1,600 Ib)/d
From Table 2-4, Score = 5 points
2.3.8.2 Major Unit Process Evaluation
Aerator
Hydraulic Detention Time: [(160,000 cu ft x
7.48)^(950,000 gpd)] x
24 = 30 hr
From Table 2-2. Score = 10 points
BOD5 Loading: [(1,500 Ib/d)-M160,000 "cu ft)] x
1,000 = 9.4 lb/d/1,000 cu ft
From Table 2-2. Score = 10 points
Oxygen Availability: (4,000 Ib O2/d) * (1,500 Ib
BOD5/d) = 2.6 Ib O2/lb BOD5
From Table 2-2. Score = 10 points
Aerator Subtotal =. 10 + 10 + 10 = 30 points
Secondary Clarifier
Configuration: Circular With Weirs on Wall
From Table 2-4. Score = 5 points
Surface Overflow Rate: (1,140,000 gpd) * (1,924 sq
ft) = 592 gpd/sq ft
From Table 2-4, Score = 7 points
Depth at Weirs: 9.0.ft
From Table 2-4. Score = -3 points
Return Sludge Removal: Scraped to Center Hopper
From Table 2-4. Score = 8 points
Return Sludge Control:
From Table 2-5, Typical Range is 50-100% of Influent
Wastewater Flow.
Desired Range = (50% x 0.855 mgd) to (100% x
1.14mgd) = 0.43-1.14 mgd
Actual Range = (500 gpm x 1,440 x 10-6) to
(1,500 gpm x 1, 440 x 10-6) =
0.72-2.16 mgd
Actual Return Sludge Control is 59% (0.72 to 1.14
mgd) of Desired Range.
Return Sludge Measurement Provided.
Sec. Clarifier Subtotal = 5 + 7-3 + 8 + 5
= 22 points
Sludge Handling Capability
Controllability: Waste Volume Manually Calculated
Waste Stream Manually Sampled
From Table 2-6. Score = 2 points
Capacity:
a. Expected Sludge Production
Effluent BOD5 achievable is assumed value for a well
operated oxidation ditch facility. Unit sludge
production, from Table 2-7 is 0.65 Ib TSS/lb BOD5
removed.
BOD5 Removed = (Influent BOD5 - Effluent BOD5
Achievable) x Flow
= (190 mg/L - 15 mg/L) x (0.95 mgd) x (8.34)
= 1,385 Ib/d
Expected Sludge Mass = , (0.65 Ib TSS/lb BOD5) x
(1,385lb BOD5/d)
= 900lbTSS/d
Expected Sludge Concentration, From Table 2-8:
7,500 mg/L.
Expected Sludge Volume = (900 Ib/d) * (7,500 mg/L
x 8.34 x 10-6)
= 14,400 gpd
Increase by 25 percent to allow operation flexibility:
Expected Sludge Volume = 1.25x14,400
= 18,000 gpd
b. Percentage of Expected Sludge .Production Each
Process Can Handle
1. Aerobic Digester
From Table 2-9, standard for evaluating aerobic
digesters is a hydraulic detention time of 15 days.
Sludge Volume Existing Digester Can Handle
= (220,000 gal) 4 (15 days) = 14,670 gpd
Percentage of Expected Sludge Production
= (14,670 gpd)-^(18,000 gpd)
= 81 percent
37
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2. Drying Beds
4. Land Application
From Table 2-9, the standard for evaluating drying
beds is the worst season turnover time as
demonstrated by past experience. Essentially, no
drying is experienced from December through March
so that beds operate only as storage during that
period. Storage volume required must first be
calculated.
Digester Hydraulic Detention Time (HOT)
= Digester Volume/Sludge Volume
HOT = (220,000 gal) *(18,000 gpd) = 12.2 days
From Table 2-10, for HOT = 12 days, total solids
reduction of 14% and output solids concentration of
about 13,000 mg/l is expected.
Sludge to Drying Beds =(900 Ib TSS/d) x (1.00 -
0.14) x (1.25 flex, factor)
= 967 Ib/d
Sludge Volume = (967 Ib/d) 4- (13,000 mg/L x 8.34 x
10'6)
= 8,919 gpd
Storage Capacity of Existing Beds = (8) x (60 ft x
150 ft x 1.5ft)
= 108,000 cu ft
Storage Capacity Available =(108,000 cu ft x
7.48)* (8,919 gpd)
=91 days
Storage Capacity Required = 31 (December)
31 (January)
28 (February)
30 (March)
121 days
Drying bed capacity is available for 8 months of the
year, but only 75% (91-f 121) of required storage
capacity is available during the winter 4 months.
Existing Drying Bed Adequacy
= Worst Case - Use
Winter Capacity of
75 percent
3. Hauling
From discussions with the POTW staff and
administrators, "Hauling dried sludge is not a problem.
If we have to, we can get the street crew down to the
plant to help out."
Hauling Adequacy = 100 percent
From discussions with the POTW staff and
administrators, "If we can get it through the beds, we
can get rid of it. We can go to the landfill if we have
to."
Land Application Adequacy = 100 percent
From the capacity evaluation, the drying beds are the
"weakest link" at 75 percent capacity.
From Table 2-6, Score = 0 points
Sludge Handling Capability Subtotal = 2 + 0
= 2 points
Scores for each major unit process are presented in
Table 2-11.
The data in Table 2-11 indicate that the aerator points
are sufficient to receive a Type 1 rating. However, the
points scored for the secondary clarifier, sludge
handling capability, and the total plant are only
sufficient for a Type 2 rating. Therefore, the overall
plant rating is Type 2. This rating indicates that
improvement in plant performance without any
upgrade of major processes is likely.
2.3.8.3 Performance-Limiting Factors
The following performance-limiting factors were
identified during the CPE and given rankings of "A" or
"B." Further prioritization of these identified factors
was also completed, as indicated by the number
assigned to each factor.
1. Operator Application of Concepts and Testing to
Process Control ("A")
Less sludge was wasted than was produced on a
routine basis. Excess sludge periodically bulked from
the final clarifiers. Mixed liquor concentrations were
monitored routinely, but the concept of controlling total
sludge mass at a desired level was not implemented.
Operation of return sludge flow at excessively high
rates, typically 150-200 percent of wastewater flow,
contributed to solids loss.
2. Sludge Wasting Capability ("B")
An undersized digester and drying beds that do not
provide adequate sludge disposal capability during
winter months result in inadequate sludge wasting
capacity.
3. Improper Technical Guidance ("B")
The above process control and inadequate sludge
disposal situation continued despite annual plant
inspections by the State district engineer. "Periodic
solids loss" was given the same emphasis in the
annual inspection reports as plant housekeeping,
timely submittal of monitoring reports, leveling and
38
-------
Table 2-25. Suspended Growth Major Unit Process Capacity
Evaluation for Example CPE
Aeration Basin
Secondary Clarifier
Sludge Handling
Total
Points .
Scored
30
24
2
56
Points Required*
Typel
13-30
25-55
10-30
60-115
Type 2
0-12
0-24
0-9
20-59
Type 3
<0
<0
<0
<20
* Each unit process as well as the overall points must fall in the
designated range for the plant to achieve the Type 1 or 2 rating..
cleaning of clarifier weirs, and other items far removed
from the true performance-limiting problems and
potential solutions.
4. Performance Monitoring ("B")
Performance monitoring samples were collected
routinely in the morning hours. Periods when solids
loss occurred from the clarifiers were generally in the
afternoon.
5. Familiarity With Plant Needs ("B")
Administrators were not familiar enough with the plant
requirements for performance and operations to
adequately make decisions on the plant budget yet
they did not allow the plant superintendent to have
input to the budgeting process.
6. Process Controllability ("B")
Oversized return activated sludge pumps were
provided in the plant design. This promoted poor
operation with excessively high return flows and would
require a modification to improve process control.
2.3.8.4 Performance Improvement Activities
The most serious of the performance-limiting factors
identified were process control oriented. The
evaluation of major unit processes resulted in a Type
2 rating because of marginal, but not drastically
deficient, sludge handling capability. The POTW
appears to be a good candidate for improved
performance through implementation of a CCP. This
recommendation should be presented to the City
Council. Continual compliance will depend on the
ability to dispose of adequate quantities of waste
sludge. Documentation of improved performance may
be difficult because existing monitoring data do not
reflect true past effluent quality.
2.3.9 CPE Worksheets
Worksheets for evaluating POTW capability are
presented in Appendices L through P. These
worksheets can be used to assess the capability of
existing major unit processes (i.e., aerator, to perform
secondary clarifier, and sludge handling system) and
to determine whether the POTW is a Type 1, 2, or 3
plant. It should be noted that these worksheets
simplify the steps presented in this chapter. However,
this simplification cannot be substituted for the
judgment of the CPE evaluator in assessing a
particular facility.
2.3.10 CPE Results
The success of conducting CPE activities can be
measured by POTW administrators selecting an
approach and implementing activities to achieve the
required performance from their wastewater treatment
facility. If definite followup activities are not initiated
within a reasonable time frame, the objectives of
conducting a CPE have not been achieved.
2.4 Personnel Capabilities for
Conducting CPEs
Persons responsible for conducting CPEs should have
a knowledge of wastewater treatment, including the
following areas: '
• Regulatory requirements
« Process control
• Process design
• Sampling
• Laboratory testing
• Microbiology
• Hydraulic principles
• Operator training
« Wastewater facility budgeting
• Safety
• Maintenance
• Management
Consulting engineers who routinely work with POTW
design and start-up, and regulatory agency personnel
with experience in evaluating wastewater treatment
facilities, represent the types of personnel with
adequate backgrounds to conduct CPEs.
2.5 Estimating CPE Costs
The cost of conducting a CPE depends on the size
and type of facility. Activated sludge plants tend to be
more complex than trickling filter plants or other fixed
film facilities. Guidelines for estimating CPE costs and
person-days are presented in Table 2-26. These
estimates are for contracting with a consultant who
normally performs this type of service. The cost to a
community for conducting a CPE with municipal
employees would probably be less than the amounts
shown in Table 2-26. However, municipal employees
may not have the necessary qualifications or may be
too close to the existing operation to be able to
perform a truly objective evaluation.
39
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Tablo 2-26. Typical Costs for Conducting CPEsa
Typo and Size of Facility
Suspended Growlh".c
<9 Us (0.2 mgd)
9-88 Us (0.2-2 mgd)
88-440 Us (2-10 mgd)
Fixed Fi!m<*
<22 Us (0.5 mad)
22-440 Us (0.5-10 mgd)
Stabilization Ponds
Person-
Days
Onsite
2
5
7
2
, 5
2
Typical Cost
(1988$)
2,000-5,000
3,000-12,000
4,000-18,000
2,000-5,000
3,000-12,000
2,000-5,000
• For contract consultant.
b Includes all variations of activated sludge.
c ABF systems, which combine suspended and fixed growth,
require an effort similar to activated sludge.
d Includes trickling filters with both plastic and rock media as well as
RBCs.
Agency, Municipal Environmental Research
Laboratory, Cincinnati, OH, 1982.
6. Niku, S., E.D. Schroeder, G. Tchobanoglous, and
F.J. Samanieqo. Performance of Activated Sludge
Processes: Reliability, Stability and Variability.
EPA-600/2-81-227, NTIS No. PB-82-108143, U.S.
Environmental Protection Agency, Municipal
Environmental Research Laboratory, Cincinnati,
OH, 1981.
7. Haugh, R., S. Niku, E.D. Schroder, and G.
Tchobanoglous. Performance of Trickling Filter
Plants: Reliability, Stability and Variability. EPA-
600/2-81-228, NTIS No. PB 82-109174, U.S.
Environmental Protection Agency, Municipal
Environmental Research Laboratory, Cincinnati,
OH, 1981.
2.6 References
When an NTIS number is cited in a reference, that
reference is available from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
(703)487-4650
1. Handbook: Improving POTW Performance Using
the Composite Correction Program Approach.
EPA-625/6-84-008, U.S. Environmental Protection
Agency, Center for Environmental Research
Information, Cincinnati, OH, 1984.
2. Gulp, G.L. and N.F. Helm. Field Manual for
Performance Evaluation and Troubleshooting at
Municipal Wastewater Treatment Facilities: EPA-
430/9-78-001, NTIS No. PB-279448, U.S.
Envirommental Protection Agency, Office of Water
Program Operations, Washington, DC, 1978.
3. Process Design Manual: Wastewater Treatment
Facilities for Sewered Small Communities. EPA-
625/1-77-009, NTIS No. PB-299711. U.S.
Environmental Protection Agency, Center for
Environmental Research Information, Cincinnati,
OH, 1977.
4. Hinrichs, D.J. Inspectors' Guide for Evaluation of
Municipal Wastewater Treabment Plants. EPA-
430/9-79-010, NTIS No. PB-80-138605, U.S.
Environmental Protection Agency, Office of Water
Program Operations, Washington, DC, 1979.
5. Schultz, D.W. and V.B. Parr. Evaluation and
Documentation of Mechanical Reliability of
Conventional Wastewater Treatment Plant
Components. EPA-600/2-82-044, NTIS No. PB-
82-227539, U.S. Envirommental Protection
8. Handbook for Identification and Correction of
Typical Design Deficiencies at Municipal
Wastewater Treatment Facilities. EPA-625/6-82-
007. U.S. Environmental Protection Agency,
Center for Environmental Research Information,
Cincinnati, OH, 1982.
9. Ball, R.O., M. Harris,. and K. Deeny. Evaluation
and Control of Sidestreams Generated in Publicly
Owned Treatment Works. EPA-600/2-82-016,
NTIS No. PB-82-195272, U.S. Environmental
Protection Agency, Municipal Environmental
Research Laboratory, Cincinnati, OH, 1982.
10. Energy Management Diagnostics. EPAi430/9-82-
002, NTIS No. PB-82-198219, U.S. Environmental
Protection Agency, Office of Water Program
Operations, Washington, DC, 1982.
11. Comprehensive Diagnostic Evaluation and
Selected Management Issues. EPA-430/9-82-003,
NTIS No. PB-82-212770, U.S. Environmental
Protection Agency, Office of Water Program
Operations, Washington, DC, 1982.
12. Contract Operations. EPA-430/9-82-004, NTIS No.
PB-82-197161, U.S. Environmental Protection
Agency, Office of Water Program Operations,
Washington, DC, 1982.
13. Wastewater Utility Recordkeeping, Reporting, and
Management Information Systems. EPA-430/9-82-
006, NTIS No. PB-83-109348, U.S. Environmental
Protection Agency, Office of Water Program
Operations, Washington, DC, 1983.
14. New England Wastewater Management Guide -
Utility Management System. Available from the
New England Water Pollution Control
Commission.
40
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15. Hegg, B.A., K.L. Rakness, and J.R. Schultz. A
Demonstrated Approach for Improving
Performance and Reliability of Biological
Wastewater Treatment Plants. EPA-600/2-79-035,
NTIS No. PB-300476, U.S. Environmental
Protection Agency, Municipal Environmental
Research Laboratory, Cincinnati, OH, 1979.
16. Hegg, B.A., K.L. Rakness, J.R. Schultz, and LD.
DeMers. Evaluation of Operation and Maintenance
Factors Limiting Municipal Wastewater Treatment
Plant Performance - Phase II. EPA-600/2-80-129
NTIS No. PB-81-112864, U.S. Environmental
Protection Agency, Municipal Environmental
Research Laboratory, Cincinnati, OH, 1980,
17. Schultz, J.R., B.A. Hegg, and K.L. Rakness.
Realistic Sludge Production for Activated Sludge
Plants Without Primary Clarifiers. JWPCF
54(10): 1355-1360, 1982.
18. Brenner, R.C., J.A. Heidman, E.J. Opatken, and
A.C. Petrasek, Jr. Design Information on Rotating
Biological Contactors. EPA-6QO/2-84-106, NTIS
No. PB-84-199561, U.S. Environmental Protection
Agency, Municipal Environmental Research
Laboratory, Cincinnati, OH, 1984.
19. Rakness, K.L., J.R. Schultz, B.A. Hegg, J.C.
Cranor, and R.A. Nisbet. Full Scale Evaluation of
Activated Bio-Filter Wastewater Treatment
Process, EPA 600/2-82-057, NTIS No. PB-82-
227505, U.S. Environmental Protection Agency,
Municipal Environmental Research Laboratory,
Cincinnati, OH, 1982.
20.- A Guide to the Selection of Cost-Effective
Wastewater Treatment Systems. EPA-430/9-75-
002, NTIS Report No. PB-244417, U.S.
Environmental Protection Agency, Office of Water
Program Operations, Washington, DC, 1975.
21. Process Design Manual: Sludge Treatment and
Disposal. EPA-625/1-79-011, NTIS No. PB80-
200546. U.S. Environmental Protection Agency,
Center for Environmental Research Information,
Cincinnati, OH, 1979.
22. West, A.W. Operational Control Procedures for
the Activated Sludge Process - Part III A,
Calculation Procedures. EPA-330/9-74-001C,
NTIS No. PB:231598, U.S. Environmental
Protection Agency, Cincinnati, OH, 1973.
23. Palm, J.C., D. Jenkins, and D.S. Parker. The
Relationship Between Organic Loading, Dissolved
Oxygen Concentration, and Sludge Settleability in
the Completely-Mixed Activated Sludge Process.
Presented at the 51st Annual Conference, Water
Pollution Control Federation, Anaheim, CA, 1978.
24. Personal Communication with Mr. Richard
Pedersen, Environmental Specialist, Montana
Department of Health and Environmental
Sciences, Helena, Montana, May, 1988.
25. Design Manual: Municipal Wastewater
Stabilization Ponds. EPA-625/1-83-015, U.S.
Environmental Protection Agency, Cincinnati,
Ohio, 1983.
26. Design Manual: Dewatering Municipal
Wastewater Sludges. EPA-625/1-87-014, U.S.
Environmental Protection Agency, Cincinnati,
Ohio, 1987.
41
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Chapters
Conducting Composite Correction Programs
3.1 Objective
The objective of a Composite Correction Program
(CCP) is to improve the performance of an existing
POTW (1). If the results of a Comprehensive
Performance Evaluation (CPE) indicate a POTW is a
Type 1 plant (see Figure 2-1), then the existing major
unit processes have been determined to be adequate
to meet current treatment requirements. For Type 1
facilities, the CCP focuses on systematically
addressing performance-limiting factors to achieve the
desired effluent quality. This can be done without
major plant modifications (2).
For Type 2 plants, the existing major unit processes
have been determined to be marginal but improved
performance is likely through the use of a CCP, and
the POTW may or may not meet performance
objectives without major facility modifications. For
these plants, the CCP focuses on clearly defining the
optimum capability of existing facilities. Even if the
CCP does not achieve the desired effluent quality, unit
process deficiencies will be identified and plant
administrators can be confident in pursuing the facility
modifications indicated.
For Type 3 plants, major construction is often
indicated and a more comprehensive study is
warranted. A study of this type could look at long-term
needs, treatment alternatives, potential location
changes, and financing mechanisms. Chapter 4
provides alternatives for making facility modifications
at existing facilities where specific design deficiencies
have been identified. Typically, a CCP would not be
implemented at a Type 3 facility until adequate
modifications have been completed.
3.2 CCP Methodology
The methodology for conducting a CCP is a
combination of: 1) implementing activities that support
process requirements; and 2) systematically training
the staff and administrators responsible for wastewater
treatment (2-4).
3.2.1 CPE Results
The basis for implementation and training efforts is the
prioritized list of performance-limiting factors that was
Figure 3-1. Relationship of performance-limiting factors to
achieving a performance goal.
Effluent in Compliance
Operation (Process Control)
Capable Plant
Administration
Design
Maintenance
developed during the CPE (see Section 2.2.1.3). The
list provides a plant-specific outline of activities that
must be addressed during the CCP. It is important to
note, however, that performance-limiting factors not
identified in a CPE often become apparent during
conduct of the CCP and must be addressed to
achieve the desired level of performance (3).
3.2.2 Process Control Basis
The areas in which performance-limiting factors have
been broadly grouped (administration, design,
operation, and maintenance) are all important in that a
factor in any one of these areas can individually cause
poor performance. However, when implementing a
CCP, the relationship of these areas to achieving the
goal of an effluent in compliance must be understood.
Administration, design, and maintenance activities all
lead to a plant physically capable of achieving desired
performance.. It is the operation, or more specifically
the process control activities, that take a physically
capable plant and produce adequately treated
wastewater, as indicated in Figure 3-1. By focusing on
the needs of the biological treatment process, as
established through process control efforts, priorities
for changes to achieve improved performance are
thus developed.
For example, if good performance in an extended
aeration activated sludge plant cannot be maintained
because bulky sludge has developed as a result of
43
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inadequate oxygen transfer capability, better
performance requires meeting the oxygen deficiency
(5). In this case, limitations in meeting process needs
(inadequate DO) establish a high priority for design
changes (i.e., oxygen transfer equipment). This
example illustrates how the process control basis can
be used to prioritize improvements in physical
facilities. Proposed improvements must alleviate a
deficiency in the existing "incapable plant," as
identified by process requirements, so that progress
toward the performance goal can be pursued. In this
way the most direct approach to improve performance
is implemented. Nonperformance-related improve-
ments can be delayed properly until the plant has
achieved the treatment objective for which it is
intended.
3.2.3 Long-Term Involvement
Implementation of a CCP is a long-term effort,
typically involving one year, for several reasons:
• Greater effectiveness of repetitive training
techniques. Operator and administrator training can
be conducted under a variety of actual operating
(e.g., sasonal) and administrative experiences.
Time is also necessary for the staff to develop
confidence in new techniques.
• Inherently long response times associated with
making changes and achieving stability in biological
systems. Biological systems typically respond
slowly to process control adjustments that affect
the environment in which the microorganism
population lives. New environmental conditions
eventually result in changes in the relative numbers
of different microorganisms. For example, for
activated sludge systems, some changes can be
accomplished in the period of three to five MCRTs,
but it is not uncommon for some changes to take
weeks and even several months before desired
shifts in microorganism populations are
accomplished (6).
• Time required to make physical and procedural
changes. This is especially true for those changes
requiring financial expenditures where
administrative (e.g., city council) approval is
necessary.
• Attitude of staff. If the staff is not supportive of the
CCP approach, the CCP will require additional
effort and may have to include some personnel
changes to be successful.
• Time required for identification and elimination of
any additional performance-limiting factors that may
be found during the CCP.
3.3 CCP Activities
3,3.1 General
This section presents techniques that have been
successfully used in implementing CCPs. The
methods presented should not be considered as the
only workable methods, since experience has shown
that no single approach will work at every POTW (7).
The concept of correcting performance-limiting
factors, until the desired POTW performance is
achieved, must remain the controlling guidance when
implementing a CCP. Details of implementation are, of
necessity, site-specific and should be left to the
individual implementing the CCP.
The individual that implements a CCP is called a
facilitator. This individual is typically an "outsider" and
accomplishes the CCP objectives utilizing periods of
on-site involvement interspersed with off-site limited
involvement. The facilitator assumes a leadership role
in making process control decisions, assigning
responsibilities, training POTW staff, and checking
progress. When not onsite, POTW personnel are
responsible for this leadership and the CCP facilitator
monitors their progress as well as the process control
and performance of the plant.
The following tools have been successfully used in
implementing a CCP:
• Telephone calls to routinely monitor CCP progress.
Routine telephone contact can be used to train and
encourage POTW personnel concerning their
responsibility for making critical plant observations,
interpreting data, and summarizing important
indicators and conclusions. The effectiveness of
telephone calls is limited in that the CCP facilitator
must rely on observations of the POTW personnel
rather than his/her own. To ensure common
understanding of the telephone conversations, the
CCP facilitator should always summarize important
points, decisions that have been reached, and
actions to be taken subsequent to the call. Both the
CCP facilitator and POTW personnel should keep
written phone logs.
• Site visits to verify or clarify plant status, initiate
major process control changes, test completed
facility modifications, provide onsite operator
training, and report progress to POTW
administrators. Specific dates for site visits should
be scheduled as indicated by the plant status and
training requirements.
• Written reports to promote clarity and continuity.
Development of' written reports depletes funds
available for action-oriented work by both the
POTW staff and the CCP facilitator; therefore, only
concise status reports are recommended. Short (1-
page) written summaries should also be prepared
for each facility modification. Initially, these may be
prepared by the CCP facilitator, but this
responsibility should ultimately be transferred to the
POTW staff.
44
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• Final CCP report to summarize activities. Since all
major recommendations should have been
implemented during the CCP, only current status of
the POTW performance should be presented in this
report.
The approach of interspersing on-site with off-site
involvement is illustrated in Figure 3-2. As the CCP
progresses, fewer site visits and telephone calls will
typically be necessary. This is in line with the transfer
of responsibility to the permanent POTW staff. Typical
levels of effort required by CCP facilitators are
presented in Table 3-1. For any POTW, the level of
effort is dependent on the specific performance-
limiting factors.
3.3.2 Initial Site Visit
The working relationship between the CCP facilitator
and the POTW staff and administration is established
during the first site visit. A good working relationship -
based on mutual respect, communication, and
understanding of the CCP - greatly enhances the
potential for success.
3.3.2.1 CPE Results
A CCP is often implemented by individuals more
experienced in identifying and correcting factors
limiting POTW performance than those who
conducted the CPE. During the initial site visit, the
CCP facilitator should allow time to confirm and/or
modify the original performance-limiting factors
identified in the CPE. Time to assess the Type 1 or 2
"rating" of the POTW should also be allowed.
The initial site visit is used to begin activities for
addressing all major performance-limiting factors
(rated "A" or "B" in the CPE). The process control
focus of the CCP activities should be made apparent
during this visit. Existing process control testing
should be reviewed and modified so that all necessary
process control elements are adequately monitored.
Sampling frequency and location, collection
procedures, and laboratory analyses should be
reviewed and, if necessary, standardized so that data
collected can be used for evaluating progress. New or
modified sampling or analyses procedures should be
demonstrated by the CCP facilitator and then
performed by plant personnel under the supervision of
the CCP facilitator.
3.3.2.2 Monitoring Equipment
Any needed sampling or testing equipment should be
obtained as quickly as possible. Rental or "loaner"
equiupment should be made available immediately.
The CCP facilitator should assist the POTW personnel
in obtaining administrative approvals.
3.3.2.3 Process Control Summaries
The CCP facilitator should, with the help of plant
personnel, draft a precise summary form for process
control parameters and performance monitoring
results. Monthly records are often available, but
monthly data are too infrequent to allow timely
process control adjustments. POTW personnel should
provide data from the summaries to the CCP facilitator
throughout the CCP. Often, weekly summaries of data
are used. However, if computer capability is available,
electronic transfer of data can be used to allow daily
data exchange.
In small plants, where process control and monitoring
activities are not conducted on a daily basis, a single
page can often be used to record results. A sample
process control form for a small plant used both for in-
plant records and as a summary sent to the CCP
facilitator is shown in Figure 3-3. Terms used in this
figure are defined in Table 3-2.
3.3.2.4 Process Control Adjustments
The CCP facilitator should begin directing process
control adjustments during the site visit. Where
process control adjustments are grossly out of line
(e.g., 300 percent estimated return sludge flows), the
CCP facilitator should direct changes toward more
reasonable values. Fine tuning of process control
procedures and training of the POTW staff cannot
legitimately progress until this first level of effort is
initiated.
When implementing major changes in process control
adjustments, the facilitator must be very aware of the
potential adverse impact on the POTW operators'
morale. All recommendations for process control
changes should be thoroughly explained prior to
implementation. Even with this approach, a CCP
facilitator should not expect to obtain immediate
support from POTW personnel. A response such as
"well; let's try it then and see" is often the best that
can be expected. Some changes may have to be
made with only the degree of consensus expressed
by the statement: "I don't think it'll work, but we can
try it."
3.3.2.5 Minor Design Changes
Any minor design changes identified as necessary by
the CPE and confirmed by the CCP facilitator should
be initiated during the site visit. Some design changes
often require significant amounts of time for approvals,
delivery of parts or equipment, or construction. It is
necessary, therefore, to initiate changes as soon as
needs are identified so that their effect can be
evaluated during the majority of the CCP.
3.3.2.6 Action Lists
An important aspect of a CCP is implementation of
activities to improve plant performance. As such, it is
helpful to "inventory" the action items to be
accomplished. A summary of this inventory is
developed and updated throughout the CCP by the
CCP facilitator. The summary is distributed to the
plant staff and administration. An example format for
an action list is shown in Figure 3-4.
45
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Figure 3-2. Typical scheduling of CCP activities.
1234
10
11
12
Telephone
Consultation
On-site
Consultation
Months of Involvement
Table 3-1. Typical CCP Facilitator Involvement
Initial
Facility Size and Type' Visit
Suspended Growth:
44 Us (1 mgd)
440 Us (10 mgd)
Fixed Film:
44 Us (1 mgd)
440 Us (10 mgd)
days
3-5
4-10
2-5
4-10
Telephone
Consultation
number/week
initial
2-6
3-8
1-3
2-3
end
2-4
2-4
1-4
1-4
Additional
Site
Visits
days
4-12
6-20
3-8
5-12
* Suspended growth facilities have greater process control flexibility
and typically require a greater level of effort by the CCP
facilitator.
3.3.3 Improving Design Performance-Limiting
Factors
The performance of Type 1 and 2 POTWs can often
be improved by making modifications or additions to
the original design. A detailed discussion of facility
modifications that can be used to improve plant
performance is presented in Chapter 4. Only the
conceptual approach to improving design perfor-
mance-limiting factors will be presented here.
3.3.3.1 Identification and Justification
Initially, proper identification of a design perfor-mance-
limiting factor is required. CPE results or findings
during the conduct of a CCP are excellent methods to
identify design limitations. Once design factors have
been identified, the process of selecting facility
modification alternatives for implementation can begin.
An indexed guide is presented in Chapter 4 (Table 4-
1) to assist in evaluating alternatives.
The CCP facilitator and POTW personnel must be
able to justify each proposed modification based on
the resulting increased performance capability that the
modification will provide. A sound basis is to relate
design modifications to the items needed to provide a
capable plant such that process control objectives can
be met (see Figure 3-1). The degree of justification
required usually varies with the associated costs and
specific plant circumstances. For example, little
justification may be required to add a sampling tap in
a sludge line. Whereas justification for modifications to
the aeration basin to allow use of several modes of
the activated sludge process would require much
more emphasis. Additionally, extensive justification
may be required for a facility where sewer rates are
high and have recently been raised, yet there is no
money available for an identified modification.
3.3.3.2 Implementation
The CCP facilitator should ensure that each
modification is formally documented in writing. This
documentation is more valuable in terms of training
and commitment if it is completed by POTW
personnel. It should include:
• Purpose of the proposed change
(Identification/Justification)
Detailed description of the change
Quantitative criteria for evaluating success or failure
of the change
Individual(s) responsible for completing the change
Cost estimate
Anticipated improvement in plant performance
Schedule
Another role of the CCP facilitator is to assist POTW
personnel in understanding and implementing their
responsibility in regard to the modification. Ideally, the
CCP facilitator should be a technical and managerial
reference throughout the implementation of the
modification, and the POTW staff should have, or
develop, the technical expertise, available time, and
motivation to complete the modification. If there is a
breakdown in completing assigned responsibilities, the
CCP facilitator must become more aggressive in
assuring completion of the modification.
3.3.3.3 Assessment
Following completion of a facility modification, the
CCP facilitator should ensure that an evaluation of the
46
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Figure 3-3. Sample process control and performance monitoring program form for a small plant.
PROCESS CONTROL SUMMARY
OPERATIONAL OBJECTIVES:
TSU
RSP
RSP
DO
DCmjn
- lowest possible setting
SSC6o(min)
DCmax
without plugging
DATE__
TIME
INITIALS
PROCESS CONTROL TEST INFORMATION:
TIME
(min.)
0
5
30
60
SSV
(cc/L)
1,000
SSC
(%)
ATC
DC
DO
RSC
DOB
FLOW
CSC
TEMP
O
WASTING INFORMATION:
ASU = ATC x 264,000 = ASU
SDR = ASU* CSU =
CSU = CSC x 37,700 =
CSU
RSP = 100 x ATC -5- (RSC - ATC) =
Current TSU = ASU + CSU =
TSU
Objective WSU = Current ASU - Objective TSU =
WSU-5-WSCE = -5- =
WSU
Gallons * Gallons per Inch =
Gallons Needed to Waste
Inches Estimated to Waste
Gallons per Inch
306
306
306
Actual Inches
Gallons
Actual WSC, %
Total Actual WSU
WSU, Gal x %
ULTIMATE DISPOSAL INFORMATION:
DATE
PLANT MONITORING INFORMATION:
DATE
Gal/Load
Total Gals •
Avg. UDC
App. Rate (Gal/Acre)
NOTES:
LOADS
Inf. BOD5
Inf. TSS _
MLSS
Eff. FC
Eff. BOD5
Eff. TSS
RATIO
47
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Tablo 3-2.
Term
Acronyms Used in Figure 3-3
Definition
Calculation
Operational Objectives
TSU
DO
SSC60
RSP
DC
Process Control Test Information
SSV
ATC
RSC
CSC
DOB
Wasting Information
ASU
CSU
SDR
WSU
WSCE
Gallons per Inch
Ultimate Disposal Information
UDC
Plant Monitoring Information
MLSS
Ratio
FC
Total Sludge Units
Dissolved Oxygen
Settled Sludge Concentration in 60 Minutes
Return Sludge Percentage
Digester Concentration
Settled Sludge Volume
Aeration Tank Concentration
Return Sludge Concentration
Clarifier Sludge Concentration
Depth of Sludge Blanket in Clarifier
Aerator Sludge Units
Clarifier Sludge Units
Sludge Distribution Ratio
Waste Sludge Units
Estimated Waste Sludge Concentration
Incremental Volume of Waste Receiving Tank
Ultimate Disposal Concentration
Mixed Liquor Suspended Solids Concentration
Fecal Coliform
ASU + CSU
1,OOOxATC*SSV60
100 x ATC * (RSC - ATC)
Centrifuge Spin
cc/L from Mallory Settleometer
Centrifuge Spin
Centrifuge Spin
Centrifuge Spin of Clarifier Core Sample
Total Aeration Tank Volume x ATC
Total Clarifier Tank Volume x CSC
ASU -s- CSU
WSC x Waste Flow
Centrifuge Spin
MLSS * % Centrifuge Spin (mg/L per %)
# per 100 mL
improved POTW capability is completed and doc-
umented. This assessment should compare the
quantitative criteria established for the project with the
capability of the actual modification. A short summary
(1-2 pages) is helpful in informing and maintaining
support from POTW personnel and administrators.
3.3.4 Improving Maintenance Performance-
Limiting Factors
Plant maintenance can generally be improved in
nearly all POTWs, but it is a significant performance-
limiting factor in only a small percentage of plants
(1,4,8). Nevertheless, adequate maintenance is
essential to achieve consistent effluent quality. As
such, a CCP facilitator may end up improving the
maintenance program to ensure that improved
performance achieved during a CCP is maintained.
The first step in addressing maintenance factors is to
document any undesirable results of the current
maintenance effort. If plant performance is degraded
as a result of maintenance-related equipment
breakdowns, the problem is easily documented.
Likewise, if extensive emergency maintenance events
are experienced, a need for improved preventive
maintenance is easily recognized. Ideally, these
factors should have been previously identified and
prioritized during a CPE. However, most POTWs do
not have such obvious evidence directly correlating
poor maintenance practices with poor performance.
For these POTWs, maintenance factors would not
have been identified as limiting performance.
Simply formalizing recordkeeping will generally
improve maintenance practices to an acceptable level
in many POTWs, particularly smaller ones. A
suggested four-step procedure for developing a
maintenance recordkeeping system is to: 1) list all
equipment; 2) gather manufacturers' literature on all
equipment; 3) complete equipment information
summary sheets for ail equipment; and 4) develop
time-based preventive maintenance scheduler.
Equipment lists can be developed by touring the
POTW. As new equipment is purchased it can be
added to the list. Existing manufacturers' literature
should be inventoried to identify missing but needed
materials. Maintenance literature can be obtained from
the factory (usually a source is identified on the
equipment name plate) or from local equipment
representatives. An equipment information sheet is
presented in Appendix G. Once sheets are completed
for each piece of equipment, a time-based schedule
can be developed. This schedule typically includes
daily, weekly, monthly, quarterly, semiannual, and
annual checkoff lists of required maintenance tasks.
An example of this scheduling system is also
presented in Appendix G.
The above system for developing a maintenance
recordkeeping system has worked successfully at
several POTWs. However, there are many other good
maintenance references available for use by CCP
facilitators and POTW staffs (9-11).
48
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Figure 3-4. Example format for summarizing CCP action items.
DATE: CONCERNS/ACTIONS SUMMARY
PAGE NO. 1
No.
Concern
Action Implementation Plan
Assigned To
Date Assigned
Date Due
rocess Control Spreadsheet
lump pump for RAS vault
Update daily data on a weekly basis
urchase and install,
Coordinate electrical requirements
Larry
Bob
3/15
2/26
3/22
4/2
49
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3.3.5 Improving Administrative Performance-
Limiting Factors
Administrators who are unfamiliar with plant needs
and thus implement policies that conflict with plant
performance are a commonly identified factor. For
example, such items as implementing minor
modifications, purchasing testing equipment, or
expanding operator coverage may be recognized by
plant operating personnel as needed performance
improvement steps but changes cannot be pursued
due to lack of support by non-technical administrators.
Administrative support and understanding is essential
to the successful implementation of a CCP. The
following techniques have proven useful in
overcoming administrative limitations:
• Involve plant administrators from the start of the
CCP. The initial site visit should include time with
key administrators to explain the CCP process and
possibly include a joint plant tour to increase their
understanding of plant processes and problems.
• Focus administrators on their responsibility to
produce a "product" that meets regulatory
requirements. Administrators may be reluctant to
pursue corrective actions because of lack of
understanding of their responsibilities in producing
clean water from the plant's treatment processes.
• Listen carefully to the concerns of administrators so
that they can be addressed during the CCP. Some
of their concerns or ideas may be technically
unimportant, but are very important "politically."
Political influence as well as technical limitations
must be addressed and are considered to be an
integral part of the activities of a CCP facilitator.
• Use technical data based on process needs to
convince administrators to take appropriate actions;
do not rely on "authority." Alternatives should be
presented, when possible, and the administrators
left with the decision.
* Initiate a process control coordination committee. In
larger plants it is often advantageous to establish a
process control coordination committee. The
purpose of this committee is to meet routinely
(weekly) to discuss process control decisions and
direction. It should include, as a minimum, one
representative each from operations, maintenance,
laboratory, and administration. These meetings
encourage communication and understanding since
each party has a different perspective yet is
focused on the common objective of effluent
quality.
• Encourage intimate involvement of plant staff in the
budgeting process. Budget involvement has been
effective in motivating plant staff as well as
encouraging more effective communication.
• • Include a plant-specific "management audit" as a
portion of the CCP. An example audit that has been
used to more effectively describe administrative
factors is shown in Appendix H. Results from an
audit of this type have been used to effectively
encourage improvement of administrative factors
that are limiting performance, since the plant staff
has an opportunity to provide constructive,
confidential feedback on many topics that are often
sensitive.
« Encourage financial planning for modification and
replacement of POTW equipment structures. This
type of planning encourages communication
between administrators and plant staff through the
need to accomplish both short- and long-term
planning. Many reference materials are available to
assist the CCP facilitator in guiding activities in this
area (12,13).
3.3.6 Improving Operational Performance-
Limiting Factors
Improvement of POTW operations during a CCP is
achieved by providing training while improved process
control procedures, tailored for the particular plant, are
developed and implemented. The initial training efforts
should be directed at the key process control
decision-makers. In most plants with flows less than
22 Us (0.5 mgd), one person typically makes and
implements all major process control decisions. In
these cases, on-the-job training is usually more
effective than classroom training and is recom-
mended. If possible in plants of this size, a "back-up"
person should also be trained. As the number of
operators to be trained increases with plant size, the
need for and effectiveness of combining classroom
training with on-the-job training also increases.
As discussed in Section 3.2, process control is a key
aspect of implementing a CCP because it represents
the essential step that enables a capable plant to
achieve the ultimate goal of producing a plant effluent
in compliance with regulatory requirements. A detailed
discussion of process control for suspended growth
and fixed film facilities is presented.
3.3.6.1 Suspended Growth Process Control
Process control of suspended growth facilities can be
achieved through control of the following important
parameters associated with the process:
• Activated sludge mass
• Return sludge flow
• Aeration basin DO
• Aeration basin configuration
These items can be utilized to apply "pressure" to the
biological environment. If a particular pressure is held
for an adequate length of time to get biological system
response, a desired change in activated sludge
characteristics - such as settling velocity - will result.
50
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The relationships between sludge characteristics,
pressure, and time for biological system response
relative to process control parameters are graphically
depicted in Figure 3-5.
Figure 3-5. Relationship between suspended growth
process control parameters and effluent quality.
Good Sludge Characteristics
a. Suspended Growth Characteristics
The primary objective of suspended growth process
control is achieving good performance by maintaining
proper sludge characteristics (e.g., those physical and
biological characteristics of a sludge that determine its
ability to remove organic material from wastewater).
Obtaining good sludge characteristics requires that
filamentous and zoogleal bacteria be in proper
balance. Enough filaments should be present to form
a skeleton for the floe particles, but the filaments
should not extend significantly beyond the floe.
More filaments tend to produce a slower settling,
larger sludge floe that produces a clearer supernatant.
Too many filaments, however, produce a sludge that
will not adequately settle and thicken in the final
clarifier, often causing sludge to be carried over the
clarifier weirs. Having fewer filaments produces a
more rapid settling sludge but also leaves more
turbidity. The faster settling, small sludge floe exhibits
discrete settling and produces "pin floe" or "straggler
floe" as well as higher turbidities. A representation of
a microscopic view of this desirable type of sludge is
shown in Figure 3-6 (14).
It is desirable to obtain good solids/liquid separation
and the good sludge thickening characteristics of a
faster settling sludge along with the high quality
effluent produced by a slower settling sludge. This is
achieved by process control to obtain the best
balance of fast- and slow-settling characteristics.
Settling tests and microscopic examinations can be
used to monitor the sludge conditions shown in Figure
3-6.
Figure 3-6. Representations of activated sludge floe.
Desirable Activated Sludge Floe
Filament
Backbone
Undesirable Slow-Settling Activated Sludge Floe
Extended
Filament
Filament
Backbone
Undesirable Fast-Settling Activated Sludge Floe
Dispersed
Particle
b. Suspended Growth Mass Control
Suspended growth mass is controlled to achieve and
maintain desired sludge characteristics and, as such,
represents a critical aspect of good process control.
There are several ways to control sludge mass in a
POTW. These variations put emphasis on different
calculations or different control parameters, but the
basic objective of each is to obtain the desired mass
of desirable microorganisms in the system.
A common mass control technique is based on
maintaining a relatively constant MLSS concentration.
Another technique attempts to adjust sludge mass to
produce a desired food to microorganism ratio (F/M).
Yet another attempts to maintain a consistent average
age of the activated sludge in the system, (e.g., mean
cell residence time [MCRT]).
Mass control schemes based on the mass of sludge
in the aeration basin (e.g., MLSS control) assume that
variations in the amount of sludge in the secondary
clarifiers is insignificant. A preferred approach includes
secondary clarifier sludge in the mass control
monitoring program.
51
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The F/M method of sludge mass control is difficult to
implement because a method to quickly and
accurately monitor the food portion of this parameter
is not commonly available. Typically, BOD5 or
chemical oxygen demand (COD) are used to indicate
the amount of food available. The BOD5 test requires
five days to complete and is therefore unsatisfactory
for process control purposes. Although the COD test
can be completed in only several hours, it requires
equipment and laboratory capabilities that are not
usually available in smaller plants.
Mass control using the MCRT approach can be set up
to include the mass of sludge in the aeration basin
and the secondary clarifier (6). A variation of this
technique is to select a desired level of total mass for
the system (e.g., both the aeration basin and
secondary clarifier) and adjust the amount of sludge
wasted to approach the selected total mass. It is
recommended that one of these two strategies be
selected for controlling sludge mass. The following
discussion identifies the differences between the two
strategies.
Suspended growth mass control using the MCRT
approach requires that the total sludge mass be
measured each day and that total be divided by the
target MCRT. This calculated mass is then attempted
to be wasted. Actual MCRTs are calculated by
dividing the total sludge mass in the system by the
actual sludge mass wasted. Actual data for a 3-week
period of sludge mass control using the MCRT
approach are shown in Figure 3-7. During this period
the target MCRT was kept constant at 10 days. The
data in Figure 3-7 show that fairly constant MCRT can
be maintained. From a process control viewpoint, an
advantage of mass control by this method is that it
requirns daily wasting.
Sludge mass control using the total mass in the
system approach requires that wasting be varied
depending on increases or decreases in the total
sludge mass. For example, if the total sludge mass
was increasing above the selected target level,
wasting would be increased until the desired sludge
mass was again achieved. Actual data for a 3-week
period of sludge mass control using the target total
mass approach are shown in Figure 3-8. An important
observation from Figure 3-8 is that total mass was
held relatively constant despite individual MCRTs
ranging from 10 to infinity (no wasting that day).
Control of total sludge mass can be a useful process
control parameter, especially in activated sludge
plants where wasting cannot be completed every day.
c. Return Sludge Flow Rate Control
The distribution of sludge between the aeration basin
and secondary clarifier can be controlled by adjusting
the return sludge flow rate. In general, return sludge
flow rate control should be used to maximize the
sludge mass and sludge detention time in the aeration
basin and minimize the sludge mass and sludge
detention time in the final clarifier. This represents the
optimum condition for an aerobic biological treatment
system and can be summarized as maximizing the
sludge distribution ratio (aerator sludge mass divided
by clarifier sludge mass).
A general misconception concerning the use of return
sludge flow rates for process control is that increasing
the flow of return sludge decreases the sludge blanket
level in the secondary clarifier. This is not as
straightforward as it first appears since the return
sludge ultimately contributes to the total hydraulic and
total solids load to the clarifier (see Figure 3-9).
Depending on the sludge settling characteristics.
increased solids loading on the clarifier may or may
not increase the solids mass in the clarifier in
conjunction with the faster solids removal rate. If
sludge settling characteristics are such that the mass
of sludge in the clarifier is increased as a result of
increasing the return sludge flowrate, the objective of
maximizing the sludge distribution ratio is not
achieved.
Two levels of improved sludge return control are
typically encountered when implementing a CCP:
gross adjustments to achieve normal operating ranges
followed by fine tuning to further optimize the sludge
distribution ratio. Thus, a grossly out-of-line return rate
should first be adjusted to at least fall within the
appropriate range presented in Table 2-5. Most
suspended growth plants with design flows of less
than 88 L/s (2 mgd) are designed conservatively
enough (e.g., Type 1 plants) that gross adjustments to
bring the return sludge flow rate within normal ranges
often provide sufficiently improved control. The
applicability and results of gross sludge return
adjustments are illustrated by the following example:
An activated sludge plant was experiencing almost
continuous problems with solids loss in the final
clarifier effluent. This continued despite repeated
efforts by the plant superintendent to control the
filamentous nature of the sludge. The
superintendent had tried chlorinating the aeration
basin contents and had even dumped the entire
contents of the aeration basin to polishing ponds.
Review of plant operation records indicated that the
return sludge flow rates were about 150 percent of
the raw wastewater flow rate. After a discussion of
the advantages of a lower return rate, the
superintendent reduced the return sludge rate to
about 50 percent of the raw wastewater flow. Solids
loss from the clarifiers stopped in about 3 hours.
This gross return rate adjustment did not solve the
filamentous sludge problem, but it did significantly
improve plant performance. At the higher return
rate, hydraulic loading to the final clarifier had been
2.5 times the raw wastewater flow. After the
adjustment, the hydraulic loading was reduced to
52
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Figure 3-7. Activated sludge mass control using MCRT.
Total Activated
Sludge Mass, kg
10,000 r-
8,000
6,000
4,000
2,000
_ o o o
MCRT, days
Total Activated
Sludge Mass
oo°°oooooo
MCRT
o o o —
_L
I I I
10
8
6
10 12
Days
14
16 18 20
Figure 3-8. Activated sludge mass control using total sludge mass.
Total Activs
Sludge Mas
20,000
16,000
12,000
tpri
s,kg MCRT MCR
- O O O O OO O OO
0 -
- o
_ Total Activated O -
_ Sludge Mass -
- O ~
o -
o o o
- o o o
I I I 1 I I I I ! I 1 1 1 1 1 1 1 1 1 1 1
2 4 6 8 10 12 14 16 18 20
Days
T, d
00
30
26
22
18
14
10
53
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Figure 3-9. Simplified activated sludge process diagram.
Q = Wastewater Flow
R » Return Sludge Flow
1.5 times the raw wastewater flow. Although
clarifier surface overflow rates were not affected,
detention time in the clarifier for settling was
increased by 67 percent and solids loading to the
clarifier was reduced by 40 percent. These
changes greatly enhanced the solids/liquid
separation capabilities of the clarifier to be more
compatible with the existing sludge settling
characteristics.
Plants where gross return sludge flow adjustments do
not produce the desired results require a higher level
of return sludge flow control. Return sludge flow
adjustments need to be compatible with changes such
as diurnal variations in wastewater flow and/or
variations in sludge settling characteristics that occur
due to diurnal variations in POTW loadings. The
selection of a specific fine-tuning technique, and
evaluation of the results, is best left to the skill and
judgment of an experienced CCP facilitator and is not
discussed in this document.
d. Aeration Basin DO Control
Oxygen levels in an aeration basin can be used to
promote or hinder the growth rates of filamentous
organisms in suspended growth processes
(5,6,14,15). DO control can therefore be used to
promote the desired balance between filamentous and
zoogleal microorganisms, which controls sludge
settling characteristics and ultimately leads to
improved plant performance.
In activated sludge plants, the greatest single use of
energy is for aeration and mixing in the aeration basin.
The desire to cut energy and associated costs while
maintaining good performance makes the decision as
to how much oxygen to use a critical one. Some
guidelines and tests that have been used to aid in
making this decision in other plants are discussed
below.
Oxygen supply in an aeration basin can be thought of
as satisfying two needs: oxygen demand and residual
DO. Typically, these are satisfied without
differentiation, but an understanding of both may be
helpful when developing a DO process control
approach. Oxygen demand is the mass of oxygen
required to meet BOD5 and nitrification demands and
to maintain a viable microorganism population. The
required residual DO is that mass of oxygen needed
to provide the environment that produces desired
sludge settling characteristics. The residual DO, which
exists in an aeration basin when the oxygen demand
is satisfied, varies with the type of process. Generally,
the higher the BOD5 loading rate on the activated
sludge system, the higher the aeration basin oxygen
uptake rates and the higher the residual DO. A
general guideline for residual bulk DO is shown in
Figure 2-2.
Operating experience has shown that DO becomes a
growth-limiting factor for zoogleal-type micro-
organisms before becoming a limiting factor for
filamentous microorganisms. DO control at low levels
in an aeration basin can therefore be used to apply
"pressure" to shift sludge characteristics toward
slower settling. Conversely, higher DO levels can be
used to apply "pressure" for faster settling.
If a decision is made to lower DO, proper testing is
essential to ensure that adequate oxygen is being
transferred. Tests that will be most beneficial are
residual DO measurements and oxygen uptake rate
tests (16). Residual DO measurements should be
taken initially at several locations throughout the
aeration basin and verified periodically to determine a
sample point that can be considered "average." When
determining residual DO, it is important to take
measurements several times during the day to be
coincident with diurnal BOD5 loading variations. In
general, plants operating at low DO levels during peak
loading may still provide good treatment if
considerably higher DO residuals exist before the
day's peak loading is received. For example, a plant
may operate very successfully with a DO of 0.4-0.6
mg/L during the day if the morning DO is 1.0-1.5
mg/L. This daily fluctuation in DO levels can produce
the desired mix of zoogleal and filamentous
organisms.
The oxygen uptake test can also be used as a
measure of adequacy of oxygen transfer (17). If the
oxygen uptake test indicates an oxygen demand
significantly greater than 0.65 kg 02/kg BOD5
removed plus 0.1 kg 02/kg total sludge in an activated
sludge system, the test may be indicative of an
inadequate oxygen supply. For example, an activated
sludge facility was removing approximately 240 kg
(530 Ib) BOD5/d with a total sludge mass in the
aeration basin and secondary clarifier of about 2,000
kg (4,500 Ib). The calculated oxygen demand is [(240
kg BOD5/d) x (0.65 kg O2/kg BOD5)] + [(2,000 kg
sludge) x (0.1 kg 02/kg sludge/d)], or 356 kg (783 Ib)
O2/d. However, the measured oxygen uptake in the
760-m3 (200,000-gal] aeration basin was 30 mg/L/hr,
or 550 kg (1,200 Ib) O2/d (150 percent of the
calculated oxygen demand). These results indicated
54
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that the realistic oxygen requirements are not being
met with the current residual DO of 0.5-0.8 mg/L.
Oxygen supply was increased, turbidity of the effluent
dropped, and the oxygen demand measured by the
oxygen uptake rate decreased to 110 percent of the
calculated demand.
The above illustrates the use of a successful
troubleshooting technique for identifying and
correcting a DO deficiency. Like return sludge control,
the capability to use DO control to fine tune activated
sludge processes is a function of the experience and
technical judgment of the CCP facilitator.
e. Aeration Basin Configuration
Sludge characteristics and thus plant performance can
often be improved by utilizing different aeration basin
loading configurations. For example, the adverse
impacts of extremes in flow and BOD5 loading
variations can be minimized by operating in the step
feed or contact stabilization mode as opposed to plug
flow. For a more detailed discussion of utilizing
aeration basin configurations to improve process
performance, see Section 4.4.2.
f. Process Control Pressure
As discussed in Section 3.3.6.1 a, overall suspended
growth process performance is primarily a function of
the sludge characteristics. Process control tests and
adjustments should be made with the purpose of
achieving desired sludge characteristics. The specific
process controls discussed earlier (sludge mass,
sludge returns, aeration basin loading configuration,
and aeration basin DO) are used to apply "pressure"
to develop desired sludge characteristics by changing
the environment for the sludge mass. A combination
of operational adjustments may be necessary to
provide enough pressure to achieve the desired
changes. For example, if sludge settling had slowed to
an undesirable level and a wet weather season (which
will cause higher average and peak clarifier hydraulic
loadings) was approaching, it would be advantageous
to expedite efforts to increase the settling rate.
Simultaneous adjustments of several process control
parameters could be used to provide more pressure in
the desired direction. In general, a raise in aeration
basin DO, more frequent return rate adjustments to
minimize sludge mass and sludge detention time in
the clarifier, and converting to a step loading mode
would all be appropriate to achieve faster settling.
g. Time for Biological System Response
When making process control adjustments at
suspended growth facilities, it is important to realize
that, although some changes take place quite rapidly,
some changes in sludge characteristics develop
slowly and adequate time must be allowed for the
biological system to respond to the pressures applied.
Adjustments change the environment of the activated
sludge very quickly, but a considerably longer period
of time may be required before sludge characteristics
change to reflect the new environment. For example,
if low DO in a diffused air aeration basin is believed to
be a cause of slow-settling sludge, it would be
appropriate to increase the oxygen transfer by
increasing blower output. Two changes should be
monitored, one immediate and one long-term. Mixed
liquor DO measurements a few hours after the change
as well as the next day should indicate whether the
increased blower output selected was sufficient to
change the environment (DO level) in the aeration
basin, but it may take several weeks of sludge settling
tests to determine if that new environment applied
enough pressure to cause the sludge to settle more
rapidly.
A tendency to return to status quo if a desired result
is not achieved quickly has been observed at many
plants. In the above example, a person using a trial
and error approach may decide after 3 days of higher
DO concentration in the aeration basin that additional
aeration was the wrong adjustment and a waste of
energy. However, a person directing a CCP must
have enough experience and confidence to hold the
changed environmental conditions long enough to
produce the desired result. If the desired change in
sludge characteristics has not started to take place in
a length of time equal to two or three MCRTs,
additional pressure should be applied. As a general
reference, a time equal to three to five MCRTs is
necessary to establish changes in sludge
characteristics.
An acceptance of the time required for a biological
system to respond to new process control
adjustments should be a major training objective of
the CCP effort. Graphing process monitoring results
to produce trend charts can enhance this acceptance.
h. Suspended Growth Testing
Several references are available for selecting tests
and their frequency for suspended growth plants
(15,18,19). Appendix I contains an example process
control daily data sheet that has proven useful in
monitoring activated sludge POTWs. To achieve
adequate process control, aH activated sludge plants
should include monitoring for at least the following:
Sludge settling
Total sludge mass control
Sludge wasting .
Return sludge concentration and flow control
Aeration basin DO control
Figure 3-3 illustrates a process control test recording
sheet for an 11-L/s (0.25-mgd) extended aeration
package plant. Process control tests for this plant
were conducted once per day, 2 days per week.
Larger activated sludge POTWs require that similar
parameters be monitored. However, since larger
plants are often designed less conservatively, they
require more frequent monitoring and more frequent
55
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process adjustments. For example, at a 241 -L/s (5.5 -
mgd) activated sludge plant, settling and mass control
tests were conducted once per 8-hour shift, 7 days
per week.
As a further example, Table 3-3 illustrates a testing
schedule for a 2-L/s (50,000-gpd) plant that is
subjected to highly variable conditions due to drastic
climate changes and wide seasonal population
fluctuations. The concept of providing two different
frequency schedules is a compromise between the
desirable higher frequencies and the minimum
operator time typically allocated to this function in
small facilities. Under normal operating conditions with
little stress on the processes, the "routine" frequency
is adequate. When the system is under stress (e.g.,
peak seasonal populations), the "critical" frequency is
appropriate.
Tabla 3-3. Process Control Monitoring at a Small Activated
Sludge Plant
Frequency
Test, Parameter, or Evaluation
Routine Critical*
Activated Sludge*.
Control Tests
Centrifuge Spins (Aeration Tank
ConcJ Return Sludge Conc./Clarifier
Core Sample Cone.), Settleometer
Test, Depth to Blanket, Aeration Basin
DO
Control Calculations
Total Sludge Mass, Aerator Sludge
Mass, Clarifier Sludge Mass, Return
Sludge Percentage, Sludge Distribution
Ratio, Clarifier Solids Loading, MCRT
Control Plots
Graph 1: Settling Results, Return
Sludge Cone., Aerator Cone.
Graph 2: Total Sludge Mass (Aerator
and Clarifier}, Wasted Sludge Mass
Wasting
Volume, Concentration, Mass
Digester:
DO, Concentration, Temperature, pH
Waste Activated Sludge, Digested
Sludge,Volatile Solids Percentage,
Volatile Solids Reduction
Chlorine Residual:
3/week
Daily
3/week Daily
3/week Daily
3/week Daily
Weekly 2/week
Monthly 2/month
5/week Daily
* "Critical" refers to periods of transition to higher loadings and
during oeak loadings and periods of stressed operation, i.e., bulky
sludge, process out of service, or major change in process control.
3.3.6.2 Fixed Film Process Control
Fixed film (trickling filter and RBC) POTWs are not
impacted to the same degree by process adjustments
as are suspended growth facilities (7,8) since there
are only a limited number of process adjustments that
can be optimized. The potential improvement in
effluent quality due to improved process control is
accordingly less. Areas of process control that can be
optimized to improve fixed film facility performance
are discussed below.
a. Reducing Return Process Stream Loadings
The CCP facilitator should strive to reduce the BODg
loading returned through the plant from anaerobic
digestion and from sludge dewatering operations.
Disposal of all digester supernatant with the digested
sludge can significantly reduce plant BOD5 loadings.
This practice has been implemented most frequently
in smaller POTWs where sludge disposal is by liquid
haul to nearby farmland. Another way to achieve
partial BOD5 load reduction is by "filtering" the
digester supernatant through a drying bed.
When dewatering digester sludge with a belt press,
vacuum filter, or centrifuge, chemical dosages are
often minimized to lower costs. If a low solids capture
is being accomplished, increased chemical usage to
increase capture and reduce the impact of return flow
on the plant should be considered. A more detailed
description of alternatives for reducing sidestream
loading is included in Chapter 4.
b. Optimizing Clarifier Operation
Process control adjustments can be used to optimize
primary clarifier performance (e.g., decrease BOD5
loading on subsequent fixed film processes). Similarly,
process control adjustments can be used to optimize
secondary clarifier performance. BOD5 and TSS
removals in both primary and secondary clarifiers can
be typically improved by minimizing surface overflow
rates and controlling sludge quantities in the clarifiers.
Surface overflow rates can be minimized by
eliminating any unnecessary flow through the
clarifiers. A common situation occurs when trickling
filter recycle is directed through either the primary or
secondary clarifier. In this case, a facility modification
to provide recirculation only through the fixed film
process is typically justified.
Keeping sludge blanket levels and sludge detention
times low in both primary and secondary clarifiers
typically enhances BOD5 and TSS removals, these
operational objectives can often be accomplished by
increasing sludge pumping. Care must be exercised to
ensure that removed sludge is not so thin that it
adversely affects sludge treatment processes.
Experience and judgment of the CCP facilitator must
be used to achieve the best compromise.
c. Media Cleaning/Flushing
Solids accumulation in fixed film facilities can
decrease BOD5 removal efficiency and result in
uncontrolled sloughing events that can overload
secondary clarifiers. Various process control pro-
cedures can be used to regulate these occurrences.
Varying the recycle rate, and thus the hydraulic
application rate, can promote sloughing. Additionally,
56
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increased recycle can help distribute the BODs load
throughout the filter depth. At plants having multiple
filters, all of the wastewater flow can be directed to
one filter on a periodic basis (e.g., weekly). Another
practice, called "walking the filter," hydraulically
overloads a section of the filter by physically
restraining the rotational speed of the distributor arm.
One method of accomplishing this procedure involves
the plant operator tying a rope or cable to the arm and
slowly "walking" the arm around the filter on a
periodic basis.
In addition to increasing the hydraulic loading,
"flooding" of fixed film filters has been used to aid in
controlling solids accumulation. Also, if the media is in
good condition, it can be removed and cleaned and
then placed back in service (21).
d. Fixed Film Testing
Process control monitoring for fixed film facilities is
generally less extensive than for suspended growth
systems. The performance of the primary ciarifier,
fixed film reactor, and final ciarifier should be
monitored on a routine basis. Fixed film reactor
performance can best be monitored by measuring
soluble BOD5 removals since this test directly
measures the unit's capability to convert dissolved
and colloidal organics to microbial solids. An example
process control summary sheet developed for a 88-
L/s (2-mgd) RBC POTW is presented in Appendix J.
3.3.6.3 ABF Process Control
The ABF process contains elements of both
suspended growth and fixed film facilities. Sludge
characteristics (settling rates, compaction capability,
appearance, etc.) are more like that of fixed film
sludge in that wide fluctuations are not common.
Consequently, overall sludge return rates and diurnal
adjustments are not as critical for the ABF process as
they are for activated sludge processes. The same is
true for aeration basin DO. DO must be provided to
meet the demand in an ABF system without special
consideration for the residual DO and its effect on
sludge settling characteristics (20). A DO residual of
2-3 mg/L is usually sufficient.
The process control parameter in an ABF system that
is similar to an activated sludge system and slightly
more susceptible to problems is suspended growth
sludge mass control. Mass control is critical in an ABF
system because a large fraction of the mass, usually
one-quarter to one-half, is wasted daily. Process
control parameters monitored in two ABF POTWs are
presented in Appendix K.
3.4 Example CCP
An example CCP is difficult to present because many
of the performance-limiting factors are addressed
through training, interpersonnel relationships, weekly
data review, phone consultations, and other activities
conducted over a long period of time. These activities
do not lend themselves readily to an abbreviated
discussion (3). Despite these limitations, an overview
of a CCP is presented based on the example CPE
presented in Section 2.3.8.
3.4.7 Addressing Performance-Limiting Factors
The most serious performance-limiting factors
identified in the CPE were process control oriented.
The major emphasis, therefore, of the initial portion of
the CCP was directed at improving plant operations
(process control).
1. Operation (Process Control)
• A process control testing schedule to monitor
sludge settling, sludge mass, sludge wasting,
sludge return concentration and flow, and
aeration basin DO was .established using the
guidelines in Table 3-3. On-the-job training was
provided in the areas of specific process control
sampling and testing requirements (see Section
3.3.6).
• A process control summary sheet was
developed and process control calculations were
implemented as shown in Appendix I.
« Trend graphs were initiated to monitor activated
sludge mass inventory and wasting, and
activated sludge settling characteristics and
return sludge concentrations.
• Sludge wasting requirements were documented
to provide justification for adequate sludge
disposal capability.
Results of the improved process control activities
led to the following sequence of events:
« Operational tests showed that actual sludge
production averaged 0.81 kg TSS produced/kg
BOD5 removed. This actual value was higher
than the projected sludge production of 0.65 kg
TSS/kg BOD5 removed used for evaluation in
the CPE, further aggravating the capacity
limitation of the anaerobic digester.
• POTW administrators were presented with the
sludge production results by using the following
explanation: "Your POTW treats about 350
million gallons of wastewater a year which
results in about 5.5 million gallons of sludge.
This sludge must be disposed of properly. The
existing aerobic digester is too small to handle
the total sludge produced. This one deficiency
negates a significant portion of the pollution
control already accomplished in the rest of the
plant. If you want to bring your plant into
compliance and obtain full benefit from the rest
of the plant, additional acceptable sludge
handling capacity will have to be provided."
57
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• After considering various options, including
construction, it was decided to utilize a contract
hauler to dispose of liquid sludge in a nearby
large POTW at a charge of $0.22/L ($0.06/gal).
* The first month of contract hauling resulted in a
supplemental sludge disposal cost of $4,500 and
all involved believed a significant effort to reduce
this cost was justified. An effort was made to
increase the concentration of the sludge fed to
the digester by thickening the sludge in an old
clarifier available on the plant site. Polymer was
used to aid in the sludge thickening. After
several trial tests, a polymer was found that
significantly improved waste sludge
concentrations from the "thickening tank." The
concentration fed to the digester was increased
about 250 percent by adding 20-25 Ib
polymer/ton sludge solids. The net effect was to
decrease supplemental sludge disposal cost by
56 percent from the $4,500/month initially
incurred.
2. Design
• Minor piping changes and a polymer feed
system had to be provided to use the available
tankage at the plant as a "thickener." Major
facility modifications, such as enlarging the
aerobic digester, were avoided.
3. Maintenance
• Suggested preventive maintenance forms
(similar to those in Appendix G) were provided
the plant superintendent. However, lack of a
documented preventive maintenance program
had not been a significant performance-limiting
problem and consequently, no additional
emphasis was placed on plant maintenance.
4. Administration
• Administrators' familiarity with plant needs and
their ability to make appropriate decisions
regarding the plant was increased by describing
process requirements, providing oral status
reports, and involving them in correction of the
sludge capacity deficiency.
3.4.2 Plant Performance
Plant performance was improved dramatically by
implementation of the CCP. The results are
summarized below:
The reported values prior to the CCP were collected
only during periods when the clarifiers were not losing
solids. The estimated actual effluent quality was
projected by comparing sludge wasted prior to trie
CCP with sludge wasted after the CCP Was initiated.
The difference in these values was projected to have
been consistently lost in the plant effluent. Actual
results are based on proper testing and represent a
true picture of plant performance.
3.4.3 CCP Costs
The costs for the example CPE and CCP described in
Sections 2.3.8 and 3.4 are summarized below:
Item
CPE Consultant
CCP Consultant
Test Equipment
Polymer Addition Equipment
Sludge Disposal
Polymer
Total
3,500 (one-time)
12,000 (one-time)
700 (one-time)
550 (one-time)
26,500 (annual)
2,500 (annual)
45,750 (first year)
29,000 (ongoing annual costs)
3.4.4 Summary
This example illustrates several important points of the
CCP approach and includes several problems and
associated solutions that occur frequently during CCP
implementation. These are:
• The primary objective of a CCP is attaining
adequate performance. A secondary objective can
be minimizing costs within the framework of
adequate treatment.
• Some potential performance-limiting factors
identified during a CPE are later found to be
incorrect or less significant when actually
eliminating problems" with a CCP. This was true of
the digester design limitations in this plant.
• The degree of administrative support is sometimes
difficult to assess during a CPE but often becomes
a major concern during a CCP. This was true when
the administrators were faced with supporting
dramatically increased sludge handling costs in the
example CCP.
• A Type 2 POTW was brought into compliance
without major plant modifications.
Effluent Effluent TSS,
BOD5, mg/L mg/L
Before CCP
Reported
Estimated Actual
After CCP
Actual
14
44
14
15
75
17
3.5 CCP Results
The success of conducting CCP activities can be
measured by a variety of parameters, such as
improved operator capability, cost savings, improved
maintenance, etc. However, the true success of a
CCP should be documented improved performance to
the degree that the plant has achieved compliance.
Given this measure, the results of a successful CGP
58
-------
effort can be easily depicted in graphical form. Results
from an actual CCP are presented in Figure 3-10. It is
desirable to present CCP results in this format.
3.6 Personnel Capabilities for
Conducting CCPs
Persons responsible for conducting a CCP must have
a comprehensive understanding of wastewater
treatment (see Section 2.4), extensive hands-on
experience in biological wastewater treatment
operations, and strong capabilities in personnel
motivation. Comprehensive understanding of, and
experience in, biological wastewater processes are
necessary because the current state-of-the-art in
biological treatment leaves room for, individual
judgment in both design and process control. For
example, references can be found to support, the use
of a variety of activated sludge process control
strategies. Those responsible for implementing a CCP
must have sufficient process experience to determine
appropriate application of a strategy to the personnel
capabilities of the POTW in question. Leadership and
motivational skills are required to fill the multi-faceted
"facilitator" role required of individuals responsible for
implementing a CCP.
Individuals who routinely work in the area of improving
wastewater treatment plant performance likely will be
best qualified to be CCP facilitators. These persons
are, typically, engineers or operators who have
focused their careers on wastewater treatment plant
troubleshooting and have gained experience in
correcting deficiencies at several plants of various
types. It is important that CCP facilitators have
experience in a variety of plants because the ability to
recognize true causes of limited performance is a skill
developed only through experience. Similarly, the
successful implemen-tation of a cost-effective CCP is
greatly enhanced by experience.
By the very nature of the CCP approach, the CCP
facilitator must often address improved operation,
maintenance, and minor design modifications with
personnel already responsible for these wastewater
treatment functions. A "worst case situation" is one in
which the POTW staff is trying to prove that "the
facilitator can't make it work either." The CCP
facilitator must be able to deal with this personal issue
in such a manner that allows all parties involved to
focus on the common goal of achieving desired plant
performance.
A CCP facilitator must be able to conduct training in
both formal classroom and on-the-job situations.
Training capabilities must also be broadly based (i.e.,
effective with both the operating as well as the
administrative personnel). When addressing process
control limitations, training must be geared to the
specific process control decision-makers. Some may
be inexperienced and uncertified; others may have
considerable experience and credentials.
Administrative "training" is often a matter of clearly
providing information to justify or support CCP
activities. Although many administrators are
competent, successful, and experienced, some may
not know what their facilities require in terms of
manpower, minor modifications, or specific funding
needs.
CCP facilitators can be either consultants, including
state and federal personnel, or utility employees.
When local administrators decide to use a consultant
to implement the CCP, they should conduct interviews
and check references thoroughly. A substantial
construction cost could be incurred if an
inexperienced facilitator is not capable of bringing a
capable POTW to the desired level of treatment.
Another important attribute of a consultant providing
CCP services is the ability to explain problems and
potential solutions clearly in a non-threatening
manner.
When local administrators decide to conduct a CCP
without the services of outside personnel, they should
recognize that some inherent problems may exist. The
individuals implementing the CCP, for example, often
find it difficult to provide an unbiased assessment of
the area in which they normally work: operating
personnel tend to look at design and administration as
problem areas; administrators typically feel the
operating personnel should be able to do better with
what they have; the engineer who designed a facility
is often reluctant to admit design limitations, etc.
These biases should be recognized and discussed
before personnel closely associated with the POTW
initiate a CCP.
3.7 Estimating CCP Costs
CCP costs vary widely depending on: 1) the size and
complexity of the facility; 2) who implements the CCP;
3) the number and nature of performance-limiting
factors; and 4) the capability and cooperation available
from the POTW technical and administrative staff. The
cost of a CCP falls into two main areas: 1) cost of a
consultant to implement the CCP; and 2) cost of
implementing activities to support the CCP effort,
such as minor plant modifications, additional staffing,
more testing equipment, and certain process changes.
Estimated costs for using a CCP consultant are
presented in Table 3-4.
Wide ranges are presented in Table 3-4 because the
performance-limiting factors generally vary widely from
plant to plant and require different types and amounts
of training before they can be eliminated.
The costs of implementing activities to support the
CCP effort and for operating the POTW at a higher
level of performance are difficult to generalize. They
must be developed on an individual POTW basis
59
-------
Figure 3-10. Graphical presentation of improved performance from a successful CCP.
100-
80-
1 1
!i
Jl A
ifA
/ r
\
v*-\
\ TSS
\
Hi
Table 3-4. Typical Costs for Conducting a CCPa
Type and Size of Facility
CCP Consultant Cost
(1988$)
Suspended Growth^
<9L/s(0,2mgd)
9-88 Us (0.2-2 mgd)
88-440 Us (2-10 mgd)
Fixed Film:6
<22 Us (0.5 mgd)
22-440 Us (0.5-10 mgd)
, 3,000-20,000
5,000-50,000
15,000-100,000
3,000-25,000
5,000-80,000
a For contract consultant
b Includes all variations of activated sludge, and ABF systems.
c Includes trickling filters with both plastic and rock media and
RBCs.
since they are more dependent on the particular
performance-limiting factors than the size or type of
facility. In most CCPs these costs equal or exceed the
typical costs of a CCP consultant, as presented in
Table 3-4.
3.8 References
When an NTIS number is cited in a reference, that
reference is available from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
(703) 487-4650
1. Hegg, B.A., K.L. Rakness, and J.R. Schultz.
Evaluation of Operation and Maintenance Factors
Limiting Municipal Wastewater Treatment Plant
Performance. EPA-600/2-79-034, NTIS No. PB-
300331, U.S. Environmental Protection Agency,
Municipal Environmental Research Laboratory,
Cincinnati, OH, 1979.
2. Hegg, B.A., K.L. Rakness, and J.R. Schultz. The
CCP Way to Better Effluents. Water Engineering
and Management, 129(10):40-43, 1982.
3. Hegg, B.A., K.L. Rakness, and J.R. Schultz. A
Demonstrated Approach for Improving
Performance and Reliability of Biological
Wastewater Treatment Plants. EPA-600/2-79-035,
NTIS No. PB-300476, U.S. Environmental
Protection Agency, Municipal Environmental
Research Laboratory, Cincinnati, OH, 1979.
4. Gray, A.C., Jr., P.E. Paul, and H.D. Roberts.
Evaluation of Operation and Maintenance Factors
Limiting Biological Wastewater Treatment Plant
Performance. EPA-600/2-79-087, NTIS No. PB-
297491, U.S. Environmental Protection Agency,
Municipal Environmental Research Laboratory,
Cincinnati, OH, 1979,
5. Palm, J.C., D. Jenkins, and D.S. Parker. The
Relationship Between Organic Loading, Dissolved
Oxygen Concentration, and Sludge Settleability in
the Completely-Mixed Activated Sludge Process.
Presented at the 51st Annual Conference, Water
Pollution Control Federation, Anaheim, CA, 1978.
6. Jenkins, D. and W.E. Garrison. Control of
Activated Sludge by Mean Cell Residence Time.
JWPCF 40(11):1905-1919, 1968.
60
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7. Schultz, J.R., B.A. Hegg, and C.S. Zickefoose.
Colorado CCP Demonstration and Development
of Areawide Compliance Strategy. Draft Report,
U.S. Environmental Protection Agency, Municipal
Environmental Research Laboratory, Cincinnati,
OH, 1984.
8. Hegg, B.A, K.L. Rakness, J. R. Schultz, and L.D.
DeMers. Evaluation of Operation and Maintenance
Factors Limiting Municipal Wastewater Treatment
Plant Performance - Phase II. EPA-600/2-80-129,
NTIS No. PB-81-112B64, U.S. Environmental
Protection Agency, Municipal Environmental
Research Laboratory, Cincinnati, OH, 1980.
9. Maintenance Management Systems for. Municipal
Wastewater Facilities. EPA-430/9-74-004, NTIS
No. PB-256611, U.S. Environmental Protection
Agency, Office of Water Program Operations,
Washington, DC, 1973.
10. Plant Maintenance Program. Manual of Practice
OM-3, Water Pollution Control Federation,
Washington, DC, 1982.
11. Roberts, H.D., A.C. Gray, Jr., and P.E. Paul.
Model Protocol for the Comprehensive Evaluation
of Publicly Owned Treatment Works Performance
and Operation. EPA-600/2-82-015, NTIS No. PB-
82-180480, U.S. Environmental Protection
Agency, Municipal Environmental Research
Laboratory, Cincinnati, OH, 1981.
12. EPA Financial Capability Handbook. Office of
Water Program Operations, U.S. Environmental
Protection Agency, Washington, DC.
13. Financing and Changes for Wastewater Systems.
American Public Works Association, American
Society of Civil Engineers, and Water Pollution
Control Federation, 1984.
14. Jenkins, D., M. Sezgin, and D.S. Parker. A Unified
Theory of Filamentous Activated Sludge Bulking.
Presented at the 49th Annual Conference, Water
Pollution Control Federation, Minneapolis, MN,
1976.
15. Updated Summary of the Operational Control
Procedures for the Activated Sludge Process.
EW006973, U.S. Environmental Protection
Agency, Instructional Resources Center,
Columbus, OH.
16. Wooley, J.F. Oxygen Uptake: Operational Control
Tests for Wastewater Treatment Facilities.
EW008446, U.S. Environmental Protection
Agency, Instructional Resources Center,
Columbus, OH, 1981.
17. McKinney, R.E. Testing Aeration Equipment in
Conventional Activated Sludge Plants. JWPCF
53(1):48-58, 1981.
18. Process Control Manual for Aerobic Biological
Wastewater Treatment Facilities. EPA-430/9-77-
006, NTIS No. PB-279474, U.S. Environmental
Protection Agency, Office of Water Program
Operations, Washington, DC, 1977.
19. Gulp, G.L. and N.F. Helm, Field Manual for
Performance Evaluation and Troubleshooting at
Municipal Wastewater Treatment Facilities. EPA-
430/9-78-001, NTIS No. PB-279448, U.S.
Environmental Protection Agency, Office of Water
Progra'm Operations, Washington, DC, 1978.
20. Rakness, K.L, J.R. Schultz, B.A. Hegg, J.C.
Cranor, and R.A. Nisbet. Full Scale Evaluation of
Activated Bio-Filter Wastewater Treatment
Process. EPA 600/2-82-057, NTIS No. PB-82-
227505, U.S. Environmental Protection Agency,
Municipal Environmental Research Laboratory,
Cincinnati, OH, 1982.
21. Dow Chemical Company. A Literature Search and
Critical Analysis of Biological Trickling Filter
Studies. U.S. Environmental Protection Agency,
Office of Research and Monitoring. Project'No.
17050 DDY, 1971.
61
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Chapter 4
Facility Modifications
4.1 Introduction
If a POTW is having difficulty in complying with its
permit requirements, a formalized problem
identification and correction procedure will ensure a
cost effective approach for achieving facility
compliance. Figure 4-1 presents such an approach.
The first step taken should be a thorough evaluation
of all factors that may be limiting performance at the
POTW. Such an evaluation, called a Comprehensive
Performance Evaluation (CPE), is described in
Chapter 2. A portion of the CPE includes an
evaluation of major unit processes that results in the
plant being categorized as Type 1, 2, or 3. If it is
determined that performance cannot be expected to
improve significantly until facility modifications are
constructed, the plant is categorized as Type 3.
Compliance is possible with Type 1 and 2 plants
through optimization of existing facilities. A facility
optimization approach, called a Composite Correction
Program (CCP), is described in Chapter 3. Often
during the conduct of the CCP, design limitations are
identified. Facility modifications are also required to
achieve compliance at these plants.
This chapter describes various modifications that can
be utilized at an existing POTW to eliminate identified
design limitations. This manual is not intended to
replace a detailed engineering study that may be
necessary to evaluate factors beyond the scope of
this manual such as site constraints, pressure for
growth, and more stringent effluent requirements.
These conditions may warrant the replacement of
existing facilities with new facilities.
4.1.1 Identifying Alternatives
Facility modifications that result in improved
compliance with effluent BOD5, TSS, and coliform
permit requirements (i.e., secondary treatment
standards) are the emphasis of this chapter.
Compliance-limiting unit processes are most often the
secondary treatment units (i.e., the "aerator" and the
secondary clarifier) and, therefore, modifications to
these unit processes are presented in detail. In some
instances, the poor performance of a secondary unit
process may be due to inadequacies in other unit
processes. An example would be an undersized
sludge treatment process that restricts sludge wasting
such that effluent TSS violations occur. For cases
where preliminary, primary, or sludge treatment unit
processes contribute to non-compliance, facility
modification alternatives are presented.
Once design limitations have been identified, it is
possible to begin the process of selecting facility
modification alternatives for implementation. The
indexed guide presented in Table 4-1 was developed
to assist the reader in this selection. Column 1 lists
various treatment components grouped by unit
processes to assist the reader in identifying those
components applicable to their treatment facility.
Design limitations that have the potential of limiting
overall facility compliance are shown in column 2.
Column 3 briefly describes how compliance is typically
affected by the design limitation.
After locating the identified design limitations in Table
4-1, the reader is directed by page number to
information on specific modifications. The modification
alternatives presented in Sections 4.2 through 4.7 are
categorized as follows:
Section 4.2 - Preliminary/Primary Treatment
Processes - includes modifications to flow
equalization, primary clarification, screening, grit
removal, and septage handling.
Section 4.3 - Fixed Growth Treatment Processes -
includes modifications to trickling filters, rotating
biological contactors (RBC), and activated biofilters
(ABF).
Section 4.4 - Suspended Growth Treatment
Processes - includes modifications to activated
sludge systems.,
Section 4.5 - Stabilization Ponds - includes
modifications to aerobic and facultative stabilization
ponds and aerated lagoons.
Section 4.6 - Sludge Treatment and Disposal -
includes modifications to sludge thickening,
stabilization, dewatering, and disposal processes.
63
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Figure 4-1. Methodology for achieving POTW effluent compliance.
Type 1
and
Type 2
POTW Out Of
Compliance
With Permit
Requirements
w
w
Evaluation To
Identify
Reasons for
Non-
Compliance
Types
Compliance Possible
Through
Optimization of
Existing Facilities
Without Major Capital
Expenditures
Type 2
Existing
Facilities
Inadequate -
Major Capital
Expenditures
Required
Abandon Existing
Facilities;
Construct New
Facilities
Section 4.7 - Additional Facility Modifications -
includes modifications to disinfection systems,
analytical and process instrumentation, and
emergency systems.
Each section contains a process description,
important process control parameters, and a table that
summarizes design limitations and associated
modifications. The order in which design limitations
are presented in the table is based on their potential
for improving performance at the POTW. Where
applicable, potential operation- and maintenance-
related solutions to the identified design limitations are
presented.
Each construction-related modification presented
includes general design criteria plus advantages,
disadvantages, and a physical description of the
modification. Case studies and operational guidelines
are included when available. References are included
for detailed information concerning the specific
modification. Specific product and manufacturer
references are not provided but can be obtained
through the use of the following as well as similar
publications:
• JWPCF Buyers Guide (part 2 of the March issue)
• AWWA Buyers Guide (part 2 of the November
issue)
• Pollution Engineering (October issue)
• Pollution Equipment News Buyers Guide
(November issue)
• Municipal Index Purchasing Guide (published by
American City and County Magazine)
4.7.2 Selecting A Modification
Modifications presented in this manual generally fall
into one of the following categories:
• addition of unit processes in series or parallel
• "in-tank" modifications (e.g., baffles, aeration
modifications)
• modification of wastewater characteristics (e.g.,
chemical addition, upgrading pretreatment
requirements)
• modification of auxiliary processes (e.g., wasting
and recycle pumps, piping and valves)
The final selection of a modification is determined by
evaluating several conditions. Figure 4-2 presents a
decision-making approach for selecting a modification
for existing facilities. The decisions are divided into
Level I and Level II categories. Level I decisions are
more straightforward and relate to site requirements
and cost effectiveness. Level II decisions require
increased judgment since they involve consideration
for such things as reliability and flexibility. Assistance
in making Level II decisions is provided in the
following sections.
64
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Table 4-1. Summary of Design Limitations
Treatment Component Page No.
Design Limitation
Associated Performance Problems
4.2 Preliminary/Primary Processes
Flow Control
Flow Splitting
Screening
Grit Removal
Primary Clarification
Septage Handling Facilities
4.3 Fixed Film Processes
Trickling Filter
RBC
ABF
Clarifier
67
82
84
86
4.4 Suspended Growth Processes 87
Excessive flows/load variations
70 Inadequate flow splitting and control
70 Inadequate screening efficiency
72 Undersized grit removal process
inadequate velocity control
72 Undersized basin
Inadequate scum and sludge removal
73 Undersized or lack of septage handling
facilities
74 Undersized unit process
Inadequate oxygen transfer capacity
Inadequate hydraulic wetting
Lack of cold weather protection
Undersized unit process
Undersized first stage
Inadequate oxygen transfer capacity
Inadequate control of air drive units .
Undersized unit process
Inadequate oxygen transfer
Inadequate return/waste sludge capability
Undersized clarifier
Excessive clarifier hydraulic currents
Inadequate waste sludge flexibility/capacity
Inadequate process flexibility
Undersized clarifier
Undersized aeration basin
Solids washout from secondary clarifier
during peak flow events; hydraulic/organic
shocks to biological processes; poor solids
removal by primary clarifier
Inconsistent treatment efficiency due to
excessive flow variations
Damage or plugging of critical equipment
Fouling of critical equipment; decrease in
basin volume due to grit deposition
Fouling of critical equipment; decrease in
basin volume due to grit deposition
Decreased solids removal efficiency
causing increased loading to secondary
Decreased solids removal efficiency
causing increased loading to secondary
Organic toxic shocks to biological
processes without primary clarifiers
Organic overload; inadequate SBOD
removal, potential odors
Inadequate removal of SBOD, odors
. Media plugging and inconsistent sloughing
Inadequate remoyel of SBOD
Inadequate removal of SBOD, potential •
mechanical failure
Organic overload; inadequate removal of
SBOD; mechanical problems
Organic overload; inadequate SBOD
removal
Inadequate removal of SBOD
Uneven growth; rotational problems;
potential mechanical failure
Organic overload; inadequate SBOD
removal
Inadequate removal of SBOD
Inability to control solids distribution
between aerator and clarifier.
High effluent suspended solids
Solids washout; periodic high effluent SS
Inability to control sludge mass in system;
periodic high effluent SS and SBOD
Inability to recover from process upsets;
solids washout; inability to control sludge
characteristics (e.g., bulking); periodic high
effluent TSS and BOD
High effluent SS
Organic or hydraulic overload; inadequate
removal of SBOD; inability to control
sludge characteristics
65
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Table 4-1. Summary of Design Limitations (continued)
Treatment Component Page No. Design Limitation
Associated Performance Problems
4.4 Suspended Growth Processes
(continued)
4.5 Stabilization Ponds
4.6 Sludge Treatment and Disposal
Thickening
Stabilization
Dewatering
Sidestream Handling
Sludge Storage
Sludge Transportation
Ultimate Disposal
4.7 Miscellaneous
Disinfection System
Miscellaneous Systems
Emergency Systems
87 Inadequate oxygen transfer equipment
Inadequate return sludge flexibility
Excessive clarifier hydraulic currents
Inadequate basin mixing
Inadequate scum removal
106 Undersized ponds
Inadequate oxygen transfer capability
Short-circuiting
Inadequate suspended solids control
Inflexible mode of operation
Low pond temperature
115
88 Inadequate capacity and/or flexibility
91
122
123
123
124
124
124 Short-circuiting
Inadequate clarifier
Inadequate feed capacity
Undersized basin
129 Inadequate instrumentation
Inadequate flow measurement
Inadequate flow splitting
Wastewater characteristics limit treatability
130 Lack of alarm system
Lack of unit bypass
Lack of standby units for key equipment
Inability to control sludge characteristics;
inadequate removal of SBOD
Inability to control solids distribution
between aeration basin and clarifier
Solids washout; periodic high effluent SS
Short-circuiting; decrease in basin volume
due to sludge deposition
High effluent SS
Organic overload; inadequate removal of
SBOD
Inadequate removal of SBOD
Inadequate removal of SBOD
Periodic high effluent SS
Inadequate removal of SBOD; periodic
high effluent SS
Inadequate removal of SBOD
Periodic inadequate effluent TSS and BOD
removal; process upset due to retention of
high solids inventory in secondary units
High effluent fecal conforms
High effluent BOD, TSS, and fecal
coliforms
Periodic high effluent fecal coliforms
Sustained periods with high effluent fecal
coliforms
Unreliable effluent SS and/or SBOD due to
unavailable or incorrect data
Unreliable effluent SS and/or SBOD due to
unavailable or incorrect data
Unreliable effluent SS and/or SBOD due to
uneven flow distribution to process
Inadequate removal of SBOD or TSS
Process upset due to equipment failure
Process upset due to equipment failure
Process upset due to equipment failure
66
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Figure 4-2.
Decision-making approach for selecting
modification for an existing facility.
Design Factors Limiting
POTW Performance Identified
Potential Facility Modifications
• Modification 1
• Modification 2
• Modification 3
Eliminate Modifications
Limited By Existing
Site Conditions
Eliminate Modifications
Incompatible With Existing
Process or Facilities
Eliminate' Modifications
That Are Not
Cost Effective
Preference for
Modifications That
Provide Flexibility
In Operations
Increasing
Judgment
\
Preference for
Modifications
That Benefit
Other Processes
Preference for
Modifications
With
Proven
Reliability
Level II
Decisions
Select
Facility Modification
Implement
Facility Modification
Optimize
Modified Facility
4.2 Preliminary/Primary Treatment
Processes
Improperly operated or poorly designed preliminary
and primary unit processes often contribute to
problems encountered in the secondary treatment unit
processes that result in plant non-compliance. The
processes that can impact overall plant performance
in this fashion include flow equalization, screening, grit
removal, and primary clarification. In addition, POTWs
that receive septage can experience problems if the
plant was not designed to adequately handle this type
of waste. Table 4-2 summarizes design limitations/
corrective modifications that are prioritized based on
their potential for achieving improved performance.
Modifications with proven reliability and those that are
less construction intensive appear at the beginning of
the listing.
Table 4-2.
Preliminary Treatment/Primary Treatment Design
Limitations and Potential Modifications
Design Limitation/Potential Modification
Page. No.
Flow Control
4.2.1 Excessive Flow/Load Variations ^67
4.2.1.1 Pump Station Modification 68
4.2.1.2 Flow Equalization Basin 68
4.2.2 Inadequate Flow Splitting and Control 70
Screening
4.2.3 Inadequate Screening Equipment 70
4.2.3.1 Coarse Screens 70
4.2.3.2 Fine Screens 72
4.2.3.3 Comminutors 72
Grit Removal
4.2.4 Inadequate Grit Removal Equipment 72
4.2.4.1 Horizontal-Flow Grit Chamber 72
4.2.4.2 Aerated Grit Chamber 72
4.2.4.3 Vortex Type Grit Chamber 72
Primary Clarification
4.2.5 Undersized Basin 72
4.2.5.1 Relocation of Recirculation or Secondary 73
Sludge Flows
4.2.5.2 Flow Equalization ' 73
4.2.5.3 Flow Splitting and Control 73
4.2.5.4 Additional Clarifier 73
4.2.6 Inadequate Sludge and Scum Removal 73
Septage Handling
4.2.7 Inadequate Septage Handling Facilities 73
4.2.7.1 Separate Treatment 73
4.2.7.2 Septage Receiving Station 74
4.2.1 Excessive Flow Variation
Hydraulic surges, such as those resulting from
infiltration or inflow, can adversely affect the
performance of practically every unit process,
sometimes to the point of non-compliance. These
variations can be aggravated by operational factors
such as the on-off operation of constant speed pumps
or the recirculation of flows within the plant.
Operational changes can frequently decrease these
impacts. The following examples illustrate this point:
67
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* Constant speed pumps in an influent pump station
can be throttled to operate at a lower flow rate over
a longer period of time. This practice will increase
the water level in the wet well and sewer;
consequently, potential adverse effects should be
evaluated before implementing this procedure.
• The step feed and contact stabilization modes can
be used to alleviate the impact of excessive I/I on
suspended growth treatment plants (see Section
4.4).
* Anaerobic digester supernatant can be returned to
the plant flow stream during low flow periods (e.g.,
during the night).
If operational changes do not provide the degree of
flow and load stability required, modifications will be
necessary. The following discussion includes potential
modifications for equalizing flow and loading to a
treatment plant.
4.2.1.1 Pump Station Modification
Diurnal variations exhibited by domestic wastewater
flows are often compounded by lift stations equipped
with constant speed pumps. Modifications to reduce
the hydraulic surges caused by the on-off operation of
these pumps include:
1. using a recycle system for variable flow control
2. replacing the constant speed pumps with adjustable
speed pumps
3. replacing the constant speed pumps with screw
pumps
4. using multiple constant speed pumps
A recycle system with constant speed pumps can
achieve variable flow control for some applications. A
flow diagram of this concept is shown in Figure 4-3.
When the constant speed pump turns on, water is
pumped from the wet well to a flow control structure.
Excess flow is recycled back to the wet well by using
an adjustable weir. The weir can be adjusted manually
or, if more precise control is desired, automatically
using a control system incorporating a flow set point
and the wet well liquid level. A pump recycle system
could also be installed utilizing control valves instead
of a flow control structure.
Replacing constant speed pumps with adjustable
speed pumps or screw pumps can also be used to
dampen hydraulic surges. Additionally, installation of
multiple constant speed pumps of a lower capacity
can be used to minimize the magnitude of hydraulic
surges. Implementation of any of these pumping
modifications will reduce the impact of on-off
operation but the diurnal variations will continue.
Equipment commonly used to achieve adjustable
speed control include: 1) mechanical speed drives, 2)
adjustable frequency drives, 3) adjustable voltage
drives, 4) wound-rotor induction motor with liquid
rheostat, 5) eddy current coupling, and 6) hydraulic
coupling. The selection of the appropriate type of
, speed control is dependent on desired speed range,
cost, equipment mounting, compatibility with existing
motor, and efficiency. The main advantage of an
adjustable speed pump is the ability to match pumping
rate with the influent flow rate. Space requirements
are typically less than that for either screw pumps or
multiple constant speed pumps. Disadvantages
include capital cost and specialized maintenance
requirements. Additional information can be found in
other sources (1-3).
Replacing constant speed pumps with screw pumps
requires major structural changes. These pumps are
advantageous in that they do not require a
conventional wet well, and they automatically lift
variable wastewater flow up to the design capacity of
the pump. Screw pumps require more space and are
generally limited to applications where the pumping
head does not exceed 30 feet. Additional information
can be found in other sources (1,3).
Installation of additional smaller constant speed pumps
will require structural changes if space has not been
provided. Multiple constant speed pumps have simple
controls and are less expensive than other pump
options. The major disadvantage is the limited flow
matching capability of this option relative to those
mentioned above.
4.2.1.2 Flow Equalization Basin
An equalization basin temporarily stores flow in
excess of a pre-set value and returns the stored
wastewater to the plant flow stream during low flow
periods. Additionally, equalization of organic and toxic
compounds is accomplished, thereby dampening the
impact of these loads on downstream processes.
Since major construction is typically involved, this
option should be evaluated relative to increasing the
size of other unit processes.
Installation of an equalization basin is dictated by the
unique characteristics of each plant. Plants with
adequately sized primary treatment facilities should
consider locating the unit between the primary and
secondary treatment processes. This will result in
fewer problems with solids deposition and scum
accumulation. If primary clarifier performance is a
consideration, the unit should be installed following
the plant headworks. At this location, consideration
must be given to providing aeration and mixing
equipment.
If the major cause of flow variability is an industrial
contributor, the preferred location for an equalization
basin is at the point of discharge. This allows for a
68
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Figure 4-3. Recycle system for variable flow control.
Control Structure with
Adjustable Weir
Influent
Flow w
Recycle
^ Flow f
^
O *
i^_
N
n n
•i
r
Flow
Measurement
tol 1
H I
^
w
Discharge to Process
Wet Well
smaller basin since only the point source flow would
need to be considered when sizing the basin.
Equalization basin sizing is dependent upon the
degree of equalization desired. The basin can be
sized to equalize dry weather diurnal flows or wet
weather flows (e.g., infiltration/inflow). In addition, the
basin can be sized to receive the total flow stream (in-
line basins) or only flow that is above the daily
average plant flow (side-line basins). Since side-line
basins only handle those flows in excess of the daily
average, the required tank size and pumping
requirements are less than those of in-line units. In-
line equalization basins dampen the variability in
constituent concentration to a greater degree than
side-line units.
The methods for calculating the size of an
equalization basin are described elsewhere (4,77).
Typically, a volume equivalent to 10-20 percent of the
average daily dry weather flow is required for total
flow equalization. The design volume must be
increased to accommodate any recycle streams that
are intended to pass through the equalization basin. In
addition to size, design considerations include basin
construction, air and mixing requirements, pumping
and control systems, solids removal, and cleaning.
Flow equalization can be achieved by utilizing existing
tankage or by constructing new metal or concrete
tanks or earthen basins. Existing tankage may not be
adequate for equalizing flows totally but often can
improve the situation such that compliance is
achieved. If a new flow equalization basin is proposed,
an analysis of concrete, metal, or earthen basin
construction should include space availability, ground
water level, ground-water protection, basin enclosure,
capital cost, and O&M requirements.
Flow equalization basins prior to primary clarification
are provided with aeration and mixing equipment to
prevent septic conditions and deposition of solids. For
normal domestic wastewater, aerobic conditions can
be maintained by supplying 0.16-0.25 L/m3/s (1.25-2.0
cfm/1,000 gal) of wastewater. For example, to satisfy
an oxygen uptake rate of 8 mg/L/hr would require
about 0.23 L/m3/s (1.8 cfm/1,000 gal), given a 10
percent standard oxygen transfer rate. If return
process streams are to pass through the equalization
basin, the aeration requirements must be modified to
account for this additional oxygen demand. Mixing
requirements for domestic wastewater are 0.004-
0.008 W/L (20-40 hp/mil gal) storage.
Normally, because of head requirements, pumping will
be required preceding, following, or in some cases,
both preceding and following the equalization basin.
Flow measurement devices, control valves, and
instrumentation are required to divert and release
wastewater to and from the equalization basin.
Flow equalization facilities provide significant benefits
relative to certain unit process operation and
performance. Peak overflow rates at downstream
clarifiers are equalized, thereby improving solids
removal capabilities. Return activated sludge pumping
rates can be held at a more constant level. Where
chemical addition is used, equalized flow allows for
greater control of dosages and lower chemical costs.
Disadvantages are capital cost and additional
operational and maintenance requirements. In some
plants, it is more advantageous to use an existing
aeration basin as an equalization basin even though it
reduces aeration time, especially with extended
aeration plants with dual aeration.
The following case history illustrates the application of
the flow equalization process to an existing
wastewater treatment plant.
Case History - Frisco, CO Sanitation District
Wastewater Treatment Plant
The Frisco wastewater treatment plant serves a
summer and winter recreational area located in
Summit County, Colorado. Peak hourly flow to the
plant frequently exceeded twice the average daily flow
during the winter ski season. In addition, constant
speed pumps were used to transfer incoming
wastewater to the 32.9-L/s (0.75-mgd) activated
sludge plant. The expansion of the plant to 52.6 L/s
(1.2 mgd) included the addition of an aerated flow
69
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equalization basin. The basin was designed to
equalize 100 percent of the daily average flow, and
additional volume was included for storage of sludge
thickening filtrate water and backwash water from
tertiary treatment.
A flow diagram for the modified Frisco plant is shown
in Figure 4-4, and design criteria for the flow
equalization system are shown in Table 4-3. Following
the headworks, the wastewater flows by gravity to a
wet well. At this location, peak flows can be diverted
to a flow equalization basin. Two constant speed
pumps transfer the wastewater from the wet well to
the aeration basin. Aeration basin effluent flows by
gravity to a secondary clarifier. Secondary clarifier
effluent then flows to a tertiary treatment facility for
phosphorus removal.
A flow control valve/flow meter arrangement following
the raw wastewater wet well is used to control the
flow rate to the aeration basin. These devices are
used to limit the pump output to not exceed the
selected set-point. Excess wastewater in the wet well
flows through the overflow pipe to an equalization
basin wet well. Two pumps transport this excess flow
to the equalization basin. If the water level in the raw
wastewater wet well decreases below a set-point, a
second control valve on the equalization basin outlet
pipe opens to allow stored wastewater to flow back to
the raw wastewater wet well.
Expansion of the aeration basin and clarifier was
avoided by the addition of the equalization basin.
Capability was included to operate the activated
sludge system in the plug flow, contact stabilization,
and step feed operational modes.
4.2.2 Inadequate Flow Splitting and Control
Flow splitting is often required in treatment plants,
especially at larger facilities that utilize multiple basins.
A symmetrical design is often utilized to evenly
distribute the flow to multiple units and typically no
means are provided to control or measure each flow
stream. If flow splitting between units is not even, the
hydraulic capacity of one of the treatment units and
thus the performance can be affected (e.g., solids
loss from a secondary clarifier).
Ideal flow splitting flexibility requires: 1) a flow control
structure, and 2) flow measurement. Flow control can
be incorporated into an existing facility through the
addition of adjustable control weirs or appropriate
valves (e.g., plug) to each flow stream. If a control
weir structure can be utilized, flow measurement can
be estimated by measuring the height of water over
the weir. In situations where control valves are
utilized, some type of meter (e.g., magnetic) or a
downstream control structure (e.g., effluent weir from
each aeration basin) is necessary.
At plants that include a Parshall flume immediately
prior to flow splitting to two treatment trains, the
insertion of a thin splitter wall into the throat of the
flume can provide reasonably accurate flow
distribution. This type of arrangement, shown in Figure
4-5, requires construction modification to the existing
exit channel and the addition and support of the
splitter wall in the flume. A potential problem with this
arrangement is the accumulation of rags on the
splitter wall.
4.2.3 Inadequate Screening Equipment
The plugging of sludge pipelines, valves, and pumps
are typical problems at treatment plants that have
inadequate screening equipment. These problems can
ultimately limit overall plant performance in addition to
greatly increasing O&M requirements.
Screens utilized for preliminary treatment are
generally classified as either coarse or fine, depending
on size of the screen opening. Manually-cleaned
coarse screens typically have openings of 25-50 mm
(1-2 in) and are inclined 30-45 degrees from the
vertical. Mechanically cleaned coarse screens have
openings of 13-38 mm (0.5-1.5 in) and are inclined 0-
30 degrees from the vertical. Approach velocities of
0.6-0.9 m/s (2-3 fps) are recommended, however, to
minimize penetration of debris at peak flows, velocity
between bars should not exceed 0.9 m/s (3 fps) (5). If
fine screens are used for preliminary treatment, they
are typically located after a coarse screen. Openings
for fine screens range from less than 0.25 mm (0.01
in) to about 6.4 mm (0.25 in), and they can be fixed or
utilize a rotating drum. Once screenings have been
removed, reintroducinq them into the wastewater is
not recommended.
Comminutors retain solids on a screen and shred
them until they become small enough to pass through
the screen openings [typically 6.4-9.6 mm (1/4 to 3/8
in)]. For maintenance purposes, comminutors are
usually located in parallel with a manually cleaned
screen. Comminutors contribute unnecessarily to
downstream loading and maintenance problems when
operational and are relatively expensive to repair.
4.2.3.1 Coarse Screens
Practically all wastewater treatment plants are
equipped with some type of coarse screening device
such as trash racks, manually cleaned screens, and
mechanically cleaned screens. If an existing plant
headworks utilizes only a trash rack or a coarse
screen with large openings [e.g., 50 mm (2 in)], the
headworks could be modified by the addition of a
manually or mechanically cleaned coarse screen with
smaller openings. The new screen could either
repjace or follow the existing screen. Manually
cleaned screens, because of their operational
requirement, are typically only used in treatment
plants with capacities less than 44 L/s (1 mgd).
Mechanically cleaned screens are extensively used for
70
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Figure 4-4. Flow equalization system for Frisco, CO WWTP.
Influent
Mechanical
Bar Screen
Raw Wastewater
Wet Well
Aerated Grit
Chamber
Parshall
Flume
Chlorine
Contact
Chamber
i.
Effluent
Waste Sludge
to Thickener
Table 4-3. Design Criteria for Frisco, CO WWTP Flow
Equalization System
Parameter
Design Capacity, m3/d
Equalization Basin Volume, m3
Influent EQ
Spent Backwash EQ
Sidestream EQ
Total EQ
Equalization Pumps
Control Valves
Flow Meter Type
Aeration Type
Design Criteria
4,540
545
628
151
1,324
2 @ 8,720 m3/d each
Motorized Plug Valves
Magmeter
Flexible Tube Diffusers
Figure 4-5. Flow splitting from a Parshall flume.
Exit Channels
Approach Channel Parshall Flume
71
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preliminary treatment in all sizes of plants. Several
different types of mechanically cleaned screens are
available including: chain and rake, reciprocating rake,
and cable driven. An extensive review by Melbinger
discusses the design characteristics and operating
experiences of the various types of mechanically-
cleaned screens (78).
4.2.3.2 Fine Screens
Rag and debris removal can be increased through the
addition of a fine screen following a coarse screen. A
fine screen with openings of 6.4 mm (0.25 in) will tend
to remove increased amounts of debris as well as
organic material; consequently, the plant should be
prepared to handle and dispose of this material. This
type of fine screen must be mechanically cleaned.
Extremely fine screens [e.g., openings of 0.25-1.52
mm (0.01-0.06 in)] provide a level of treatment
approaching primary. Static and rotating wedgewire
screens are included in this category. Because of the
small size of screen opening, these screens typically
require some type of high pressure water (and often
steam) for periodic cleaning. This type of screening
can be considered as an alternative to providing
primary treatment at an existing plant, even though
BOD and SS removals are generally less than in
primaries. Additional information can be found in other
sources (5).
4.2.3.3 Comminutors
Comminutors can eliminate screenings handling;
however, problems can occur with downstream
equipment such as pumps and mechanical -aerators
due to the "recombining" of shredded material.
Additionally, Comminutors frequently break down
because of their working environment and function,
thus requiring bypass of the unit. The use of
Comminutors at treatment plant headworks is not
recommended.
4.2.4 Inadequate Grit Removal Equipment
Typically, the presence of grit in a treatment plant will
increase operation and maintenance requirements due
to the need for equipment replacement and/or for grit
removal from basins. However, in some situations, grit
accumulation can directly affect plant performance.
For example, grit can accumulate in aeration basins
and digesters and subsequently reduce the available
treatment volume. Individual types of grit chambers
are discussed in the following sections.
4.2.4.1 Horizontal-Flow Grit Chambers
In the horizontal-flow chamber, flow velocities of 0.15-
0.3 m/s (0.5-1.0 fps) are maintained in a long channel
or a square, shallow sedimentation (detritus) tank.
This velocity allows the grit to settle out while the
lighter organics remain in suspension. The units are
designed to provide a detention time of approximately
one minute (5). Minimal control and high O&M
requirements are associated with these units;
consequently, they are usually not considered as an
alternative when modifying a treatment plant. If an
existing horizontal-flow chamber is ineffective, aerated
or vortex type grit chambers can be considered as
replacements.
4.2.4.2 Aerated Grit Chamber
Aerated grit chambers create a spiral rolling motion
with the addition of air through diffusers located on
one side of the chamber. The diffused air can be used
as a method of controlling the velocity within the
chamber and thereby allows control of the size of
particle that will be removed. Because of the spiral
flow pattern, grit particles make several passes across
the tank bottom, thus improving the chances for
particle removal. Hydraulic detention times in aerated
grit chambers range from 2 to 5 minutes at the
maximum flow rate (5); however, recent research
suggests that detention times of 10 minutes or greater
should be considered during peak wet weather flow
conditions (76). Because of their low head
requirements, aerated grit chambers can be a viable
alternative for providing grit removal in an existing
treatment plant.
If an existing aerated grit chamber is experiencing
problems, the installation of longitudinal or transverse
baffles or modifying air flow can sometimes improve
performance. Specific baffle locations include: 1)
adjacent to the grit hopper, 2) near the chamber
outlet, and 3) at the inlet to control direction of flow.
Since air flow is one of the main parameters affecting
the performance of aerated grit chambers, air flow
control is essential. Aerated grit chambers should be
equipped with a means of controlling the air rates
within a range of 1.6-2.6 L/s/m (1-8 cfm/ft) of channel
length (2). Air control valves should be provided if
none exist. For chambers with several air headers,
individual control valves for each air header should be
provided.
If grit problems are occuring in a plant and the grit
chamber appears to be adequately designed, the
problem may be with the grit removal system.
Components such as pumps, chain and flight
conveyors, screw conveyors, or bucket elevators may
be inadequately designed, installed, or maintained.
4.2.4.3 Vortex Type Grit Chamber
Vortex type grit chambers have been introduced within
the last ten years. The two principal vortex devices
are the PISTA and the Teacup, both proprietary. In
both units, wastewater tangentially enters the grit
chamber, creating a vortex flow pattern. Grit settles by
gravity to the bottom of the unit while the lighter
organics remain in suspension.
4.2.5 Undersized Primary Clarifier
Primary clarifiers can contribute to plant performance
problems if poor removal efficiencies and resulting
increased secondary loadings negatively affect total
72
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plant performance.. Typically, primary clarifiers with
surface overflow rates in excess of 32-48 m3/m2/d
(800-1,200 gpd/sq ft), based on daily average flow,
will have diminished removal efficiencies. Primary
clarifiers loaded within the specified range can be
expected to achieve 50-70 percent SS removal (2,4).
In some cases, the addition of coagulants to an
undersized primary clarifier can improve process
efficiency. However, the surface overflow rate of .the
clarifier should be below about 48 m3/m2/d (1,200
gpd/sq ft) to avoid carryover of chemical floe. The
coagulants most commonly used in wastewater
treatment include lime, alum, ferric chloride, ferrous
sulfate, ferric sulfate, and sodium aluminate (2). Other
plant modifications to correct an undersized primary
clarifier are presented in the following sections.
4.2.5.1 Relocation of Recirculation or Secondary
Sludge Flows
Fixed film treatment facilities that require filter
recycled flows to pass through a primary clarifier can
exert a significant hydraulic load on the clarifier. A
modification to move the recirculation flow to a
location after the primary clarifier is often warranted.
With configurations that utilize common piping for
recirculation flows and returning secondary sludge
flows to the primary clarifier, consideration should be
given to separating these functions. The potential
improved performance from eliminating recirculation
through a primary clarifier can be evaluated by
stopping recirculation flow for a sufficient period, and
monitoring SS removals.
If secondary sludge is directed back to an undersized
primary clarifier for co-settling with the primary sludge,
excessive scouring of the secondary sludge may
occur. This situation can result in the accumulation of
solids in the secondary treatment process and,
eventually, degradation of plant performance. Other
methods of secondary sludge thickening should be
investigated. Section 4.6.2 provides information on
sludge thickening alternatives. •
4.2.5.2 Flow Equalization
The addition of flow equalization facilities can improve
performance of a primary clarifier by providing a
consistent hydraulic loading rate. The addition of flow
equalization facilities to a treatment facility will also
benefit other treatment process. Information on flow
equalization can be found in Section 4.2.1.2.
4.2.5.3 Flow Splitting and Control
In situations where multiple primary clarifiers are
utilized at a plant, inadequate flow splitting could
overload some of the units. Refer to Section 4.2.2 for
information on flow splitting and control.
4.2.5.4 Additional Clarifier
In treatment plants where the existing primary clarifier
is experiencing high surface overflow rates and/or
poor removal efficiencies, the construction of an
additional clarifier should be evaluated. Design
information can be found in other sources (2,4).
4.2.6 Inadequate Scum and Sludge Removal
Inadequate removal of scum from primary clarifiers
can impact downstream processes through additional
organic loading and plugging of media surfaces (e.g.,
rotating biological contactor media). Often large
amounts of grease and oils in the treatment plant can
be attributed to commercial and industrial users.
Pretreatment at the source is usually more effective
than plant modifications to reduce the amount of
scum. When sludge treatment and disposal capacity is
limited in a treatment plant, sludge can frequently
accumulate in the primary clarifier resulting in
abnormally high sludge blanket levels and reduced
clarifier performance. In these situations it becomes
necessary to address the sludge treatment and
disposal limitations. This topic is discussed in Section
4.6.
If the above factors do not significantly affect sludge
or scum removal, modification or replacement of the
respective removal equipment may be necessary.
Steam removal systems should be capable of operat-
ing under extreme (winter) conditions and designed to
quickly concentrate the scum for separate removal
and disposal. Information on equipment modifications
can be obtained from the manufacturers.
Scum removal systems should be capable of
operating under extreme (winter) conditions and
designed to quickly concentrate the scum for separate
removal and disposal.
4.2.7 Inadequate Septage Handling Facilities
Some treatment plants receive waste from septic
tanks (septage), vaults, chemical toilets, and sand and
grease traps. If the treatment plant is not designed to
handle these types of wastes, significant operation,
maintenance and performance problems can occur
(e.g., increased organic loading). Ideally, only septage
from septic tanks treating domestic wastewater should
be allowed in most POTWs. Other wastes, such as
those from sand and grease traps and chemical
toilets, can have a detrimental effect on biological
treatment systems and should be handled by separate
treatment facilities.
4.2.7.1 Separate Treatment
If organic loading due to septage is negatively
impacting plant performance, consideration should be
given to separate treatment. Processes utilized for
separate treatment include: 1) lagoons, 2)
composting, 3) biological secondary treatment, 4)
aerobic digestion, 5) anaerobic digestion, 6) lime
stabilization, and 7) chlorine oxidation. The Handbook:
Septage Treatment and Disposal provides information
on the design of these various systems (6).
73
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4.2.7.2 Septage Receiving Station
The impact of septage on a treatment plant can be
minimized by utilizing a properly designed receiving
station. Receiving stations can be located at the
treatment plant site or remotely where they discharge
into an interceptor. Fundamental components of most
receiving stations include: 1) dumping station, 2) flow
measurement, 3) screening, 4) grit removal, 5)
storage, and 6) odor control. Design considerations
for septage receiving stations are outlined in the EPA
Handbook: Septage Treatment and Disposal (6).
4.3 Fixed-Film Treatment Processes
Fixed-film processes have been installed at many
wastewater treatment plants because of their
predictable and reliable performance and their
Characteristic ease of operation. A disadvantage of
fixed-film processes is their limited operational
flexibility. Because of this characteristic, facility
modifications are often necessary before compliance
can be reached. This section outlines modifications
for trickling filter (TF) and rotating biological contactor
(RBC) fixed-film systems, as well as for activated
biofilters (ABFs).
Table 4-4 summarizes design limitations/corrective
modifications that are prioritized based on their
potential for achieving improved performance.
Modifications with proven reliability and those that are
less construction intensive appear at the beginning of
the listing.
4.3.1 Undersized Trickling Filter
Trickling filter plant non-compliance is often related to
insufficient media surface area to adequately treat
organic loads. There are essentially two approaches
that can be taken to address this limitation: 1) reduce
the organic load, or 2) increase the size of the filter.
Reduction of organic load can be accomplished
through operational changes or administrative
direction (e.g., pretreatment, sidestream control, and
chemical addition to a primary clarifier). These
approaches should be investigated before
construction-intensive alternatives are evaluated.
Reduction of organic loads from industrial sources can
be particularly effective for extending the capacity of
an existing facility. Pretreatment processes that
remove soluble BOD from the waste stream are the
most effective. Pretreatment options are particularly
appealing where industrial contributions make up a
significant portion of the POTW's organic load.
Locating pretreatment processes at the source allows
a reduced organic load to be treated. Typically, capital
costs are lower with this approach than if capacity has
to be provided at the POTW to treat the combined
waste stream.
Table 4-4.
Fixed-Film Design Limitations and Potential
Modifications
Design Limitation/Potential Modification
Trickling Filters
4.3.1 Undersized Trickling Filter
4.3.1 .1 Replacement of Rock Media with Plastic
4.3.1.2 Increase Height of Existing TF
4.3.1.3 Additional Fixed-Film Facilities
4.3.1 .4 Trickling Filter/Suspended Growth System
4.3.1.5 Additional Recirculation Capacity
4.3.2 Inadequate Oxygen Transfer for TF
4.3.3 Inadequate Hydraulic Wetting for TF
4.3.4 Lack of Cold Weather Protection for TF
Rotatina Bioloaical Contactors
4.3.5 Undersized RBC System
4.3.5.1 Additional Media Surface Area
4.3.5.2 Preaeration
4.3.5.3 Additional Biological Process
4.3.6 Undersized First Stage RBCs •
4.3.6.1 Organic Load Redistribution
4.3.6.2 Positive Control of Flow Distribution
4.3.7 Inadequate Oxygen Transfer for RBCs
4.3.8 Inadequate Control with RBC Air Drive Units
Activated Bio-filters
4.3.9 Undersized ABF
4.3.10 Inadequate Oxygen Transfer for ABF
4.3.1 1 1nadequate Return/Waste Sludge Capability for
ABF
4.3.12 Inadequate Hydraulic Wetting for ABF
Fixed-Film Clarifiers
4.3.13 Undersized Clarifier
4.3.13.1 Relocation of Recirculation Flows
4.3.13.2 Flow Equalization
4.3.13.3 Additional Clarifier(s)
4.3.14 Excessive Clarifier Hydraulic Currents
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The reduction or elimination of certain sidestreams is
also effective in reducing organic load. Sidestreams
that are often returned to the plant headworks include
those from aerobic and anaerobic digestion, decant
from thermal sludge conditioning, and sludge
dewatering operations. Typically, aerobic digester
supernatant is of good quality and can be returned
ahead of a trickling filter process without significant
increase in organic loading to the process. Super-
natant from an anaerobic digester typically contains
high concentrations of BOD5, SS and ammonia
nitrogen, and often this sidestream has a detrimental
effect on trickling filter performance. Recycle BOD5
load from thermal conditioning can range from 20 to
40 percent of the plant influent load. If the efficiency
of solids removal during dewatering operations is
poor, sidestreams from these processes will contain
solids that can accumulate in the plant and have a
negative effect on plant performance. In some cases,
cleaning of the filter media may be necessary
following elimination of the sidestream to accurately
assess the impact of the sidestream load reduction.
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Operational changes to reduce the impact of
sidestreams include: 1) optimizing sidestream quality,
2) reducing variability of sidestream return, and 3)
land application of the sidestream. With single- and
two-stage digesters treating biological sludge, it is
generally difficult to obtain a good quality supernatant.
In a two-stage digester system, supernatant quality
can be optimized by ensuring sufficient settling time
for liquid/solids separation. The supernatant drawoff
point should be located as far from the sludge feed
point as possible to minimize short-circuiting, and not
in the scum or sludge. The addition of a telescoping
supernatant pipe or multiple drawoff ports will help the
operator to accurately locate the best drawoff point;
however, even with this flexibility, the quality of
supernatant recycle is not easily controlled. This
approach should not be expected to significantly
improve the performance capability of a fixed-film
facility.
The impact of sidestream shock loads can be reduced
operationally by allowing the return of the sidestream
on a fairly continuous basis or during the night when
the incoming BOD5 loading is low. If the plant has an
existing flow equalization basin, or unused tankage
capable of being converted for use in flow
equalization, these facilities can be used to dampen
sidestream shock loads. The better solution for
negating the adverse impact of sidestreams on facility
performance may be to eliminate their return
altogether. In some situations, anaerobic digester
supernatant can be handled through co-disposal with
the digested sludge (e.g., land application).
Before major modifications are planned, it may be
advantageous to add coagulants to a primary clarifier
to reduce the organic loading to the fixed-film process.
The coagulants most commonly used in wastewater
treatment include: lime, alum, ferric chloride, ferrous
sulfate, ferric sulfate, and sodium aluminate (2). A
special short-term (i.e., two or three months) study on
chemical addition to a primary clarifier is
recommended to determine the impact on
performance and to evaluate the effect of increased
sludge production.
If operational and administrative approaches have not
produced desired performance, modifications to the
fixed-film processes will be necessary. The following
sections describe modifications for trickling filters in
the order of their ease of implementation and potential
for improved performance.
4.3.1.1 Replacement of Rock Media With Plastic
Media
The organic loading capacity of a trickling filter can be
increased by replacing rock media with plastic media.
Common rock media has a unit surface area of about
43 m3/m2 (13 sq ft/cu ft), whereas plastic media
designed for BOD5 removal has a unit surface area of
approximately 98 m3/m2 (30 sq ft/cu ft). Owners of
rock trickling filters can effectively cut their current
organic loading rate in half by replacing the rock with
plastic media. Some plastic media with a specific
surface area greater than 131 m3/m2 (40 sq ft/cu ft)
have demonstrated tendencies for media plugging and
limited oxygen transfer (10).
Plastic trickling filter media comes in four basic forms:
cross-flow, vertical-flow, tubular, and random dump.
Cross-flow media consist of ridged sheets that are
joined to form an X pattern. Vertical-flow media
contain vertical sheets with both wavy and straight
patterns. Tubular media consist of only straight
vertical tubes. These three plastic media types are
constructed of modules of varying volume. Random
dump media comes in a variety of sizes and shapes,
and the largest dimension of each module is generally
less than 20 cm (8 in). Comparative testing has been
completed on these various plastic media types
(8,28,90). These test results provide information on
oxygen transfer, plugging, wetting characteristics, and
loading rates.
Two approaches have been generally used to design
trickling filters. The first relies on the performance of
similar systems (i.e., the NRC equation). The second
approach is based on kinetic equations (i.e., the
modified Velz equation). This approach was used to
characterize the relative performance of six different
media types under a variety of loading conditions (8).
A recently developed model predicts soluble BOD5
removal in a trickling filter for several cross-flow and
vertical-flow media types under many loading and
design conditions (28,29). This model suggests that
soluble BOD5 removal is largely governed by the
mass transfer of organic compounds to the biofilm
surface and not by substrate kinetics. Model
predictions compared favorably with various pilot- and
full-scale trickling filter studies.
Total media replacement enables the owner to
evaluate the need for renovation of the ventilation and
underdrain systems. The clay block underdrain
system typically used in rock filters may not be
capable of providing the greater ventilation
requirements of plastic media filters. When modifying
through total media replacement, the clay block
underdrain system should be removed and a new
support structure installed. For plastic block module
use, a simple underdrain and support structure can
consist of 15-cm (6-in) wide concrete beams spaced
at 60 cm (2 ft) on center, with the beams themselves
supported by short concrete walls or piers (10).
Random media is generally supported by metal grates
(aluminum or stainless steel) placed on top of short
concrete walls. The air inlet ports at the base of the
filter should be sized to provide adequate natural
ventilation throughout the media. As a rule-of-thumb,
1 sq ft of ventilating area should be provided for every
10 to 15 ft of filler periphery, with a minimum of 1 ft
provided for a plenum at the filter bottom (2).
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Depending on the size of the trickling filter, the rock
media can be removed either by hand, by clamshell
bucket and crane, or by conveyer. If a clamshell
bucket is being used, the bottom layers of rock and
debris should be removed by hand to avoid damage
to the underdrain blocks. Any remaining material (e.g.,
rock fragments and biological solids) can be flushed
out with a high pressure hose and water. A relatively
new technique utilizing a concrete-type pump has
been used to remove rock media from trickling filters.
This method is relatively fast and avoids any potential
damage to the underdrain system that could be
caused by a clamshell.
Corrugated sheet media is installed by transferring the
block modules by conveyor into the basin and then
stacking them by hand in a herringbone pattern.
Blocks that go along the perimeter of the basin must
be cut to conform to the basin's shape. Random
media is typically "dumped" into the basin using a
bucket and crane. The media must be gently dumped
since it has a tendency to break apart if handled
roughly. Additional random media will be necessary to
"top off" the trickling filter after settling occurs.
4.3.1.2 Increase Height of Existing Trickling Filter
Increasing the height of an existing trickling filter could
provide additional capacity. Considerations for this
modification include: 1) additional pumping
requirements, 2) structural integrity of the existing
media and underdrain systems, 3) available ventilation
capacity, and 4) structural integrity of the existing
foundation. For rock filters, the addition of rock media
above the existing media surface may be limited
because of support limitations of the underdrain
system. Additional height can readily be achieved by
utilizing plastic media; however, the ventilation
requirements for the plastic media should be
compared with the available ventilating area.
4.3.1.3 Additional Fixed-Film Facilities
Additional fixed-film facilities, either in parallel or
series, can be added to an organically overloaded
trickling filter facility. Adding a trickling filter in parallel
has the advantage of not significantly changing the
hydraulic grade line. However, a flow splitting
structure would be necessary to distribute the loading
to each filter.
If a two-stage (i.e., series) fixed-film process is added,
the option may exist to include an intermediate
clarifier, as shown in Figure 4-6. New construction of
two-stage, fixed-film systems often includes the
installation of intermediate clarifiers to remove SS
before applying the effluent to the second-stage unit.
The benefits are not definite, however, since many
two-stage plants function well without an intermediate
clarifier. A prudent approach would be to design the
modification so that it can be constructed with or
without the intermediate clarifier. During initial
construction, only the additional fixed-film unit would
be installed. If the desired performance is achieved,
construction of the intermediate clarifier is avoided. If
the desired performance is not attained, the additional
clarifier can be added.
RBCs represent an option for adding additional fixed-
film capability. Because of the difference in head
requirements, RBCs would typically be more
appropriate in a series configuration with a trickling
filter than in a parallel one. As a general guideline,
RBCs shouldn't be used as upstream roughing filters
since the first RBC stage must not become
organically overloaded, causing treatment and
equipment structural problems (16). ,lf the two-stage,
fixed-film process is to include RBCs, the design
should provide for soluble BOD loads to the RBCs
first stage below 12.2 kg/1,000 m2/d (2.5 lb/d/1,000 sq
ft) of media surface area (7). RBCs are often cited as
requiring less area than conventional trickling filters,
however, plastic media block modules have allowed
the construction of trickling filters as high as 12 m (40
ft). Additional information on RBCs is included in
Section 4.3.6.
The additional second-stage process can be located
either upstream or downstream of the existing trickling
filter, depending on the physical constraints of the
plant and the physical condition of the existing filter. If
plugging or lack of ventilation is a problem with an
existing rock filter, consideration should be given ,to
locating the additional process in front of the rock
filter. The upstream location will serve to reduce the
organic load to the downstream trickling filter.
Information on design approaches for trickling filter
plastic media is included in Section 4.3.1.1.
4.3.1.4 Trickling Filter/Suspended Growth
Systems
Addition of a suspended growth system at an existing
trickling filter plant has been effectively used to
improve fixed-film facility performance. Examples of
such systems include the trickling filter/solids contact
(TF/SC), trickling filter/activated sludge (TF/AS),
roughing filter/activated sludge (RF/AS), and
biofilter/activated sludge (BF/AS) processes (12). The
basic difference between these processes is the
relative sizes of the trickling filter and aeration basin.
A trickling filter plant that is able to adequately remove
soluble BOD (i.e., effluent SBOD5 from the filter of
less than 15 mg/L) but is having problems with
dispersed solids in its effluent could benefit from a
conversion to a TF/SC system. The basic components
of the TF/SC process typically include one or two
small aeration tanks interposed between the filter,
secondary clarifier with an influent flocculating
structure and return activated sludge facilities. The
solids contact portion of the process provides for
improved solids capture and removal of particulate
BOD. The mechanism by which this improved
treatment occurs is the contact of unsettled trickling
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Figure 4-6. Flow schematics of two-stage fixed-film processes.
a. Two-Stage Without Intermediate Clarifier
Waste Sludge
b. Two-Stage With Intermediate Clarifier
filter effluent with well-flocculated trickling filter solids.
Possible configurations for the TF/SC process are
presented in Figure 4-7 (13). The selection of which
mode to utilize at a specific plant is dependent on the
particulate and soluble BOD removal requirements.
Mode A is the best choice if significant soluble BOD5
and particulate removal is required from a trickling
filter effluent. Mode B is recommended if particulate
removal is the main requirement, and Mode C works
best when particulate removal and a limited amount of
soluble BOD5 removal is necessary (13).
The hydraulic retention time (HRT) in the solids
contact basin should be 1 hr or less based on the
combined trickling filter and return sludge flows (13).
As such, the modification of a trickling filter plant to a
TF/SC system can be accomplished by constructing a
small aeration basin after the filter or by routing the
filter effluent to an abandoned basin that has been
provided with aeration. This latter approach was
successfully taken at the Springfield, Oregon
wastewater treatment plant where an abandoned grit
channel was converted into a biological contact
channel and a small sludge reaeration tank was
installed (12,91).
A TF/SC system can provide improved reduction in
soluble BOD5. At the Longmont, Colorado wastewater
treatment plant, 30-40 percent of the SBOD5 that
enters the solids contact unit is removed (9). The
solids contact system at Longmont includes an
aerated solids contact tank (HRT = 6 minutes), a
return sludge aeration tank (HRT =12 minutes), and
flocculating center wells in the secondary clarifiers
(HRT = 27 minutes). Soluble BOD5 enters the solids
contact process at a concentration of 8-15 mg/L and
exits the process at a concentration of 5-7 mg/L.
Though this effluent quality is better than that required
for secondary treatment, it indicates the potential
SBOD5 removal capability of the TF/SC process.
To benefit fully from the TF/SC modification it is
necessary that the aerated biological solids be
introduced into the secondary clarifier in a gently
stirred, flocculant condition. If effluent from the
aeration tank is agitated due to pumping conditions or
by being rapidly introduced into the secondary
clarifier, the SS may become redispersed. Original
TF/SC center well designs called for installation of a
flocculating mechanism that would gently stir the
mixed liquor as it entered the center well. Experience
with the process has shown that the action of
wastewater flowing into the center well can be
sufficient to create the gently stirred environment and
that the flocculating mechanism may not be needed.
In fact, at the Longmont wastewater treatment plant,
effluent SS were higher during periods when the
flocculating mechanism was activated than when the
mechanism was turned off (14). A more detailed
discussion of the factors to be considered when
modifying a suspended growth clarifier is provided in
Section 4.4.
In more severe organic overloading cases, the TF/SC
modification will not remove the required amount of
soluble BOD and an aeration basin with a longer HRT
and MCRT will be necessary. Depending on the
specific situation, the RF/AS, TF/AS, or BF/AS
process could provide the additional required
treatment. Research indicates that combined system
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designs should be based on reliably achieving a
specific effluent nonsettleable SS concentration (103).
This research suggests that effluent nonsettleable SS
are largely undegraded paniculate organics present in
the influent wastewater and that removal of
nonsettleable SS occurs by bioflocculation. System
nonsettleable SS removal is affected by both total
organic loading to the filter and MCRT in the
suspended growth system.
With the RF/AS process (see Figure 4-8), the aeration
basin is generally one-quarter to one-half the size
required for conventional activated sludge, but 3 to 7
times larger than the contact channel used in TF/SC
(12). Additional information on the RF/AS process is
presented in Section 4.4.4.2. In the BF/AS process
(see Figure 4-8), return activated sludge is recycled
over the filter rather than passing directly to the
aeration basin. Opinions differ as to the advantages of
operating a combined trickling filter and activated
sludge system as a RF/AS rather than a BF/AS
process. Many designers include the flexibility to
operate either way because the expense of adding the
necessary piping is usually minor (12). Additional
information, including a case history, on the BF/AS
process is included in Section 4.4.4.2.
In the TF/AS process, an intermediate clarifier is
located between the trickling filter and the aeration
basin. The need for removal of trickling filter solids in
an intermediate clarifier is not well established. On the
contrary, research has shown that the introduction of
these solids into the suspended growth stage provides
improved sludge settleability characteristics (31).
Because of these concerns, the TF/AS process has
been used less than the RF/AS or BF/AS process. A
case history of a TF/AS plant, is presented below.
Case History: Fort Collins, Colorado TF/AS
Wastewater Treatment Plant
The City of Fort Collins, Colorado modified their
existing wastewater treatment plant to a TF/AS facility
through the addition of a new short-term aeration
basin and secondary clarifier. The modified plant is
shown in Figure 4-9. Design parameters are included
in Table 4-5. The existing plant included headworks
facilities, primary clarifiers, a rock trickling filter, a final
clarifier, disinfection facilities, and anaerobic digesters.
The plant was modified to the TF/AS process by
Figure 4-7. Three modes of operation for the trickling filter/solids contact (TF/SC) Process.
Aerated Solids
Contact Tank
Trickling )
Filter 1 t
^ ^/
Waste Sludge
k
^
Secondary
Clarifier
Return Sludge
a.
Return Sludge
Aeration Tank
Trickling
Filter
V '
Return Sludge
Return Sludge
Aeration Tank
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Figure 4-8. Flow schematics for the roughing filter/activated sludge, and biofilter/activated sludge processes.
Waste Sludge
a. Roughing Filter/Activated Sludge
Waste Sludge
a. Biofilter/Activated Sludge
directing flow from the existing final clarifier (now an
intermediate clarifier) to a new aeration basin. Effluent
from the aeration basin flows to screw pumps where it
is lifted to a new secondary clarifier. The plant is base
loaded, and excess flow is diverted to a downstream
plant also owned by the City.
In 1987, the organic loading to the trickling filter
ranged from about 5 to 20 BOD5 kg/1,000m2/d (1-4
lb/d/1,000 sq ft) of media volume. During the same
period the organic loading to the aeration basin
averaged about 0.16 kg BOD5/m3/d (10 lb/d/1,000 cu
ft). Under these loading conditions, the system
performed well with effluent BOD5 and TSS generally
remaining below 10 mg/L (15).
4.3.1.5 Additional Recirculation Capacity
The benefits of trickling filter recycle are reportedly: 1)
enhanced removal of complex organic substrates, 2)
dilution of BOD5 concentrations, 3) increased DO
content of the wastewater, 4) uniform shear of
biological growth to minimize sudden sloughing during
seasonal changes, and 5) maintaining a minimum
wetting rate for the media during low flows. The
magnitude of these benefits in terms of treatment
capacity is difficult to quantify. A recently developed
model for plastic sheet media actually predicts an
increase in SBOD5 as the recycle ratio increases (28),
but if it is decided that additional recycle capacity
would enhance treatment capability, the following
factors should be considered:
• A recycle to influent flow ratio of 2:1 is generally
recommended.
• Modification to the distribution and underdrain
systems may be required due to the increased
hydraulic loading.
« The recirculation system should be configured so
that recycle flows do not pass through either the
primary or secondary clarifiers.
4.3.2 Inadequate Oxygen Transfer
When oxygen transfer throughout the trickling filter
media is inadequate, the filter may not be able to
achieve the desired performance. Some of the
conditions that may lead to inadequate oxygen
transfer include degraded rock media, broken
underdrains, partially submerged underdrains, and
high organic loadings.
Media degradation and"broken underdrains can result
in ponding and media plugging, both of which impact
oxygen transfer. If media degradation in rock trickling
filters is limited to the first several inches, or an
isolated area of the filter, it may be sufficient to
remove the fragments and replace them with new
rock. A smoke test is a helpful procedure for
identifying the degraded areas within the filter when
the extent and location of media degradation are not
known. If it is found that degradation is more
extensive than the first several inches, total media
replacement is warranted. Considerations for this
procedure are presented in Section 4.3.1.1.
For proper ventilation, it is recommended that at least
1 sq ft of ventilating area be provided for every 10 to
15 ft of filter periphery and a minimum of 1 ft be
provided for a plenum at the filter bottom (2). If this
condition is not present, the existence of oxygen
limiting conditions can be checked by installing a
temporary forced draft ventilation system and
assessing its effect on performance. This special
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Flguro 4-9. Fort Collins, Colorado RF/AS Wastewater Treatment Plant.
Secondary Treatment Capacities:*
Winter - 4.0 mgd (base loaded)
Summer - 6.0 mgd (base loaded)
"Some incidental warm weather nitrification
This line to Plant #2 Complex
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Table 4-5. Design Criteria for Fort Collins, Colorado TF/AS
Wastewater Treatment Plant
Parameter Design Value
Design Capacity, mgd 6
Existing Primary Clarifiers
Ngmber 2
Surface Overflow Rate, gpd/sq ft 530
Existing Trickling Filter
Number 1
Diameter, ft 130 ,
Depth, ft 5-75
Media Type Rock
Media Volume, cu ft 76,300
Existing Secondary Clarifier
(converted to intermediate clarifier)
Number 1
Surface Overflow Rate, gpd/sq ft 775
New Aeration Basin
Number 1
Volume, cu ft 42,200
Detention Time, hr 1.3
New Final Clarifier
Number 1
Surface Overflow Rate, gpd/sq ft 630
study could be accomplished by utilizing leased
portable ventilation fans and plastic sheeting to direct
air through the filter underdrain system. If performance
improves considerably, forced draft ventilation
equipment can be made a permanent part of the
trickling filter system. Typically, forced ventilation does
not work wel! with rock filters because of underdrain
constrictions. Structural modifications can be made to
improve ventilation, such as enlarging the inlet
openings into the underdrains. Also, if excessive
submergence is caused by filter effluent backing up in
the channel due to the absence of freefall conditions
at the outlet, structural modifications to the outlet
structure or installation of a low head lift station can
be considered.
Trickling filters that are oxygen limited due to organic
overloads should consider the modifications specified
for an undersized unit process, such as media
replacement or conversion to a two-stage system.
However, if the amount of on-line media is apparently
adequate for current organic loads, a forced-draft
ventilation system may improve performance. This
situation can arise in warm, humid climates where the
air density differences aren't sufficient for natural-draft
ventilation, or in trickling filters with tall, narrow
profiles. An example of a forced-draft installation is
found at the Stockton, California wastewater treatment
plant where rotary fans were installed to provide an
upward air flow through concrete ducts (4).
4.3.3 Inadequate Hydraulic Wetting for TF
The hydraulic wetting rate for a trickling filter is
determined by the combined flow of wastewater and
recirculated effluent that is directed to the filter. Solids
accumulation in the filter can occur in plants that have
inadequate hydraulic wetting capability, resulting in
decreased BOD removal efficiency and sudden
sloughing of solids to the final clarifier. If this
deficiency is suspected, various operational practices
can be initiated to simulate an increased wetting rate.
At plants with multiple filters, all of the wastewater
flow can be directed to one filter on a periodic basis
(e.g., weekly).
In some cases, increasing the flow to a filter may not
be adequate to control the growth of biomass in a
filter. Reducing the rotational speed of the distributor
may also be necessary. An operational practice called
"walking the filter" can be used on small filters to
temporarily slow the distributor speed.
One method of accomplishing this procedure involves
the plant operator tying a rope or cable to the
distributor arm and slowly "walking" the arm around
the filter on a periodic basis. If this practice results in
improved performance, modifications to the distributor
may be advantageous. Experience with flushing
intensities for various types of filter media and
methods of slowing distributor rotational speeds has
been reported by Albertson (110).
4.3.4 Lack of Cold Weather Protection for
Trickling Filter
Cold wastewater temperatures result in decreased
microbial activity and lower treatment efficiencies.
Trickling filters are susceptible to climatic changes
because the wastewater is exposed to the ambient air
as it percolates through the media. To achieve
adequate treatment, trickling filters operating in
freezing conditions should be loaded under 7.3 kg
BOD5/1,000 m.2/d (1.5 lb/d/1,000 sq ft) of media
surface area. Under non-freezing conditions, loads as
high as 1-1.2 BOD5/1,000 m2/d (2.3 lb/d/1,000 sq ft)
are appropriate (7).
An approach to increase winter treatment efficiency is
to reduce the organic load by modifying the trickling
filter as described in the previous section. However,
this approach will result in an oversized process
during the warmer months. Placing a cover over the
trickling filter protects the filter from wind, and retains
some of the heat contained within the wastewater.
The trickling filter's cold weather treatment capacity
can be increased by as much as 50 percent with this
modification. Fiberglass covers specifically designed
for trickling filters are available but can be expensive.
A low cost approach has been taken at several
treatment plants in northern Iowa by covering the
trickling filters with galvanized grain bins. These
prefabricated structures are available in a number of
different sizes and are relatively easy to construct and
inexpensive. Irrespective of the type of cover utilized,
consideration needs to be given to the ventilation
requirements for the media. If adequate ventilation is
not provided during warm weather, reduced treatment
efficiency will likely result.
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Other low cost measures for cold weather protection
include installing wind breaks around the filters,
earthen berms against the filter walls, or louvers or
doors on the ventilation ports. Although these
modifications will not result in any significant increase
in filter performance, minor expenditures toward these
types of modifications may be justified.
4.3.5 Undersized RBC System
To achieve consistent compliance with secondary
treatment standards, the total organic load to an RBC
process should be maintained under 2.9 kg
SBODs/1,000 m2/d (0.6 lb/d/1,000 sq ft) of media
surface (7). Measures that can be taken to control the
impact of organic loads from influent and sidestream
sources should be investigated before modifications to
increase the capacity are planned. If these changes
are not feasible, modifications will probably be
necessary. Modifications include increased media
surface area, preaeration and additional biological
processes.
4.3.5.1 Additional Media Surface Area
Where space is available, additional RBC trains can
be installed parallel to existing units. New or additional
pumping capacity should not be required. Since no
process changes are made, operational complexity for
the plant staff will be minimized.
In some RBC installations, it is possible to increase
the total surface area of media by replacing some of
the standard density media [9,300 m2 (100,000 sq ft
per shaft)] with high density media [13,900 m2
(150,000 sq ft or more per shaft)]. To avoid media
plugging, high density media should not be used in
the first two stages where the greatest biofilm growth
is occurring. For some units, it is possible to replace
media without removing the shaft from the basin. For
others, it will be necessary to lift the shaft out and
slide the media off the end of the shaft. RBC
manufacturers can provide procedures for these
modifications.
An RBC system with fewer than four stages should
consider adding shafts to increase the number of
stages. It is recommended that influent piping or
channels be installed during a retrofitting project to
provide the operator with a greater degree of process
flexibility (e.g., ability to step feed the various stages,
removable baffles between stages, etc.).
4.3.5.2 Preaeration
A recent modification to RBC facilities that are
experiencing high SBOD5 loadings or septic influent
conditions involves the use of preaeration of the
wastewater. At two locations - Woodburn, OR and
Granstran, SC - up to 70 percent SBOD5 conver-sion
in the influent was achieved in preaeration facilities
with detention times of 4-6 hr. In both cases, it was
believed that the presence of micro-organisms in the
plant influent contributed to the large SBOD5
conversion. Since limited information is available on
this modification, pilot studies should be completed
before pursuing this alternative (97).
4.3.5.3 Additional Biological Process
Additional biological processes can be used to
improve the capability of an RBC system. A trickling
filter can be installed ahead of the RBC to serve as a
roughing filter or an aeration basin, or a trickling filter
can be constructed to operate either parallel to or
after the existing RBC. The selection of a second
biological process should be site-specific and based
on the relative merits of each process. If an aeration
basin is added, the existing secondary clarifier that
was adequate for a fixed-film process may not be
adequate for suspended growth solids separation. An
advantage exists when a fixed-film process is followed
by a suspended growth process because of the
controllability that is associated with the latter process
(e.g., mass, dissolved oxygen, return sludge). For
further information, the reader is referred to the
discussion on two-stage systems in Section 4.3.1.
4.3.6 Undersized First Stage RBCs
Excessive organic loads to the first stage of an RBC
process have been identified as the cause of many
operational problems and, occasionally, compliance
problems. To prevent overload conditions from
occuring, the organic load to the first stage should be
maintained under 12.2 kg SBOD5/1,000 m2/d (2.5
lb/d/1,000 sq ft) of surface area and under 29.3 kg
TBOD5/1,000 m2/d (6.0 lb/d/1,000 sq ft) (7). Problems
that can arise from an overloaded first stage include:
• excessive growth of biofilm, hindering oxygen and
substrate transport
« DO deficit in the bulk liquid
• proliferation of Beggiatoa and other nuisance
microorganisms
• structural overload and failure of the media and
central shaft
• development of septic odors (16)
Several of the modifications presented to correct an
organic overload to the overall RBC system (e.g.,
influent and side-stream control and construction of
additional biological processes) will reduce the organic
load to the first stage and should be considered.
Since excessive biofilm growth and oxygen transfer
limitations are common in overloaded first stages,
conversion to high density media is not
recommended. Options that address an organic
overload in the first stage include:
• piping or structural changes for load redistribution
• positive control of flow distribution
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4.3.6.1 Organic Load Redistribution
When the organic load to the first stage exceeds 12.2
kg SBOD5/1,000 m2 of surface area/d (2.5 lb/d/1,000
sq ft) or 29.3 kg TBOD5/1,000 m.2/d (6.0 lb/d/1,000 sq
ft), modifications that allow redistribution of the
organic load should be considered. Options include
first-stage enlargement, step feeding to the first
several stages, and parallel feeding to form two
shorter trains from one long, multi-stage train. Flow
schematics for these modification options are
presented in Figure 4-10.
Figure 4-10. Flow schematics for RBC modification options
that provide for load redistribution.
a. Conventional
b. First Stage Enlargement
c. Step Feed
d. Parallel Feed
Since organic load redistribution involves decreasing
the load to the first stage by increasing the load to
other stages, it will generally be necessary to have at
least four stages to successfully implement these
modifications. POTWs that have fewer than four total
stages should consider the modifications described in
Section 4.3.4-
Organic loads can be redistributed by replacing
permanent first-stage divider walls with removable
barriers made of either wood or metal. Installing a
removable barrier allows the option of enlarging the
first stage (i.e., establishing more first stage area) by
removing the temporary barrier. It is also desirable to
install an influent header to redistribute the flow more
evenly across the modified first stage. This mod-
ification reduces the total number of stages available.
Converting an RBC system from series to parallel flow
operation increases the number of first stages and
decreases the total number of stages. Conver-sion to
the step feed mode of operation allows for the first
stage to be partially bypassed, thereby increasing the
organic load to the downstream stages. To convert a
series feed system to either a parallel or step feed
configuration requires .the installation of additional
piping and valves, or channels and gates. Provisions
should be made for positive flow control and splitting.
4.3.6.2 Positive Control of Flow Distribution
If nonuniform flow distribution exists between multiple
process trains, one first-stage unit may be overloaded.
Nonuniform flow distribution is common when several
trains are fed by one influent channel with slide gate
control, especially if the individual first-stage shafts
rotate in opposite directions. Uniform flow distribution
can be achieved at relatively low cost by installing a
splitter box or weirs ahead of each train.
4.3.7 Inadequate Oxygen Transfer for RBCs
Inadequate oxygen transfer is often associated with an
organic overload condition, especially in the first
stage. Organic overloads contribute to excessive
biofilm growth on the media, which in turn plugs air
passages and limits oxygen transfer. Facility
modifications that address the overload problem, such
as step feeding and construction of additional
biological processes, are recommended. However,
options exist to improve oxygen transfer rather than
reducing the organic loading rate. These options
should be considered when the cost to reduce the
organic load is prohibitive, or when the overload
condition occurs on an infrequent basis.
Oxygen transfer can be improved by providing
supplemental aeration and variable shaft rotational
speed capability. Supplemental aeration has been
employed extensively to control biofilm thickness and
improve oxygen transfer. In general, it will only be
necessary to provide supplemental aeration to those
biodiscs that are experiencing overload conditions. To
implement the modification, coarse bubble diffusers
are placed underneath the media and air is injected at
a rate sufficient to promote sloughing [50-100 L/s/m2
(100-200 scfm/100,000 sq ft of media surface area)].
If the supplemental aeration approach is selected, it is
recommended that mass load cells be installed on
each shaft. Load cells measure the total shaft weight
and can be used to provide an indication of when the
combined shaft-biofilm weight is approaching the
manufacturer's allowable maximum. In conjunction
with load cells, supplemental aeration can be used on
an intermittent basis to maintain shaft weights .(i.e.,
biofilm thickness).
Variable shaft rotational speed capability can be used
to maintain the thin biofilm that is necessary to allow
for good oxygen transfer. Increasing the shaft
rotational speed can be used to promote sloughing.
This measure will also raise the DO concentration as
83
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a result of the increased agitation. This capability only
needs to be installed in the first one or two stages.
Load cells, which permit monitoring of growth, are
recommended to provide additional process control.
In some treatment plants, an oxygen-deficient
environment may exist under the standard RBC
covers. Provisions should be made for adequate
ventilation for both performance and safety
considerations.
4.3.3 Inadequate Control With RBC Air Drive Units
Operational problems experienced with air drive RBCs
include slow rotational speeds, uneven biomass
thickness, "loping" (uneven rotation), and shaft
stoppage (17). Modifications that can be made include
the installation of larger air cups, provisions for
positively measuring and controlling air flow rates, and
the installation of hydraulic sprays to assist shaft
rotation.
Replacing 10-cm (4-in) air cups with 15-cm (6-in) air
cups has been effective at stabilizing rotational
speeds and reducing shaft loping and stalling (17).
However, excess biofilm growth was not controlled by
this measure. Most existing air drive RBCs are not
provided with equipment to measure and control air
flow rates to the individual shafts (18). Since biofilm
growth is different in each stage, installation of such
equipment is recommended to allow for better
operational control of shaft speeds. Hydraulic sprays
have been used to restart stopped shafts (17) and can
also be used to shear excess biofilm from the media.
The measures described above can be utilized to
obtain the best performance from an air drive system.
A more direct approach is to convert air drives to
mechanical drive units. This approach is more costly
but provides positive control of shaft rotation. If this
modification is pursued, consideration should be given
to the installation of variable speed drives. The air
drives should be retained to provide supplemental
aeration capability.
4.3.9 Undersized ABF
The Activated Bio-Filtration (ABF) process is a
proprietary system that consists of a fixed-film tower
utilizing redwood media and may or may not include a
separate aeration basin after the filter. Schematic
diagrams of these variations of the ABF system are
shown in Figure 4-11. A constant hydraulic loading to
the biofilter can be achieved by recycling biofilter
effluent or a combination of biofilter effluent and return
activated sludge. To achieve consistent compliance
with secondary treatment standards, it is recom-
mended that the ABF system include an aeration
basin.
Approaches that can be taken to address the
limitations of an undersized unit process are: 1)
reduce the organic load to the process or 2)
increase the effective size of the process. Measures
that can be taken to reduce the organic load, such as
influent and sidestream control, are described in
Section 4.3.1.
To increase the effective size, facility modifications
can be directed toward either the biofilter, the aeration
basin, or both. To consistently meet secondary
standards, organic loadings to the biofilter should be
maintained at or below 1.6 kg TBOD5/rn3 of media
volume/d (100 lb/d/1,000 cu ft), and the aeration basin
HRT, excluding sludge recirculation, should be 4 hr or
greater (7).
The treatment capacity of the biofilter can be
increased by either increasing the size of the tower so
that it can hold additional media or constructing an
additional biofilter. Typically, economic considerations
(e.g., pumping) and site limitations can be used to
determine the best approach.
The redwood media in some ABF treatment plants
has been replaced with plastic trickling filter media
because of operational problems (e.g., deterioration of
the media, poor wetting, and severe fungus growth)
(19). However, in the absence of these problems,
media replacement is not a generally accepted
method of increasing a biofilter's treatment capacity.
The capacity of a biofilter is typically based on
volumetric loadings (Ib BOD5/d/1,000 cu ft) rather than
areal loadings (Ib BOD5/d/1,000 sq ft) as with a
trickling filter. This difference in loading criteria
between biofilters and conventional trickling filters is
attributed to the effect that recycled sludge in a
biofilter has on BOD5 removal. Based on this criterion,
increasing the specific surface area of the media by
replacing redwood with plastic media will not
necessarily result in an increase in treatment capacity
above design levels.
Increasing the treatment capacity of the aeration basin
is the best approach for making a positive change in
overall treatment capability. An analysis of data from
several operating facilities has indicated that the
biofilter only satisfies 17-46 percent of the total
system oxygen demand. As such, the overall process
performance is directly related to aeration basin
capability (20). To increase the capacity of an aeration
basin, it is generally necessary to enlarge the existing
structure or to construct an additional basin.
Procedures and considerations for this type of
modification are presented in Section 4.4.
4.3.70 Inadequate Oxygen Transfer for an ABF
System
Oxygen-limiting conditions may occur in either the
biofilter or the aeration basin. Facility modifications
that improve oxygen transfer in a biofilter are generally
similar to those for trickling filters. One exception to
this similarity is mixed liquor recycle over the biofilter.
Since mixed liquor can have a significant oxygen
demand, its removal may improve oxygen transfer to
84
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Figure 4-11. Activated Biofilter (ABF) System.
uuuu
Biofilter
a. ASF System Without Aeration
Waste
Sludge
Primary
Clarifier
Waste Sludge
Biofilter
Wet Well
Biofilter Recycle
Return Sludge
Aeration
Basin
b. ABF System With Aeration
the biofilm. Media degradation is usually not a cause
of poor oxygen transfer unless severe redwood
deterioration has occurred. In such a situation,
however, the reduction in effective treatment volume
will probably become limiting before the reduction in
oxygen transfer. A possible cause of inadequate
oxygen transfer in a biofilter is insufficient ventilation.
Corrective measures include increasing the size or
number of ventilation ports, and providing forced draft
ventilation (see Section 4.3.3).
Oxygen requirements in the aeration basin are
dependent on the BOD5 removal in the biofilter as
well as the desired F/M loading to the basin. To allow
flexibility in plant operation, it is recommended that a
value of 0.8 kg (Vkg TBOD5 be utilized as the basis
for sizing aeration equipment for ABF systems (20).
Facility modifications that improve oxygen transfer
include: ,
• replacing coarse bubble diffusers with fine pore
diffusers
• replacing surface mechanical aerators with fine
pore diffusers
• supplementing surface mechanical aerators with
diffused air or induced draft aerators
• increasing blower capacity by replacing or
supplementing the existing blower(s)
Considerations for these modifications are presented
in Section 4.4.5.
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4.3.11 Inadequate Return/Waste Sludge
Capability for ABF System
It has not been established that strict control of return
flows is necessary to optimize the performance of an
ABF facility. Consistent poor sludge settling
characteristics are generally maintained because of
the positive effects that the discrete, fixed-film sludge
has on overall sludge settleability (21). Therefore, the
need for close control of the return sludge flow to
achieve desired performance is minimized.
Since the aeration basin is generally responsible for
removing the majority of the BOD5, adequate control
of sludge age and sludge wasting is necessary. As
such, modifications to provide sufficient wasting
capacity or flexibility are warranted. For information on
modifications to increase waste sludge flexibility, refer
to Section 4.4.
4.3.12 Inadequate Hydraulic Wetting for ABF
In ABF treatment plants that have inadequate
hydraulic wetting capability, solids accumulation in the
biofilter can occur, resulting in decreased BOD5
removal efficiency and sudden sloughing of solids
(see Section 4.3.3).
4.3.73 Undersized Clarifier
"Undersized Clarifier" represents a capacity limitation
when an excessive surface overflow rate results in the
loss of SS in the effluent. Secondary clarifiers utilized
in fixed-film systems are usually designed for surface
overflow rates of 40-48 m3/m2/d (1,000-1,200 gpd/sq
ft) at peak hydraulic flow rates. "Undersized Clarifier"
can also be a limitation when shallow clarifier depth
causes insufficient volume for solids separation,
thickening, and storage. In the past, many fixed-film,
secondary clarifiers were designed with side water
depths of 2.4-3 m (8-10 ft). Recent designs have
utilized depths of 3.7-4.9 m (12-16 ft). Some designs
incorporating the TF/SC process utilize clarifiers with
side water depths in the 6-6.6 m (18-20 ft) range and
flocculating center wells (22). If an existing fixed-film
facility is modified to include suspended growth
process, an existing secondary clarifier may not be
adequate because of the different sludge settling
characteristics.
In some cases, the performance of an undersized
clarifier can be improved through the addition of
internal baffles or polymers/coagulants to the clarifier
influent. The coagulants most commonly used in
wastewater treatment include: lime, alum, ferric
chloride, ferrous sulfate, ferric sulfate, and sodium
aluminate (2). Typically, chemical addition is not
considered a long-term solution because of increased
O&M costs and sludge production that result from the
practice.
4.3.13.1 Relocation of Reclrculatlon Flows
Facilities that recirculate flow following a secondary
clarifier can exert a significant hydraulic load on the
clarifier. Studies have shown that comparable
performance is achieved from trickling filter systems
that recirculate flow before and after the secondary
clarifier (2). Consequently, a modification to remove
the recirculation flow from the secondary clarifier can
be used to improve performance. To evaluate the
potential improved performance, the recirculation flow
can be stopped and resulting SS removal can be
assessed.
4.3.13.2 FJbw Equalization
The addition of flow equalization facilities to a
treatment plant can improve performance of a
secondary clarifier by allowing the unit to operate
more consistently at the daily average flow rate.
These facilities should always be added ahead of the
biological reactor and not between the reactor and its
clarifier. The pumping associated with equalization
facilities will break apart biological floe and affect its
capture in the clarifier. Information on flow equalization
can be found in Section 4.2.
4.3.13.3 Additional Clarifier
In treatment plants experiencing high clarifier surL.ce
overflow rates, as in facilities that may be used for
suspended growth systems, the construction of an
additional clarifier should be considered. Flocculator
clarifiers in conjunction with the TF/SC process have
been successfully used in upgrading trickling filter
facilities (see Section 4.3.1.3). Studies have shown
that the flocculator clarifier is, within limits, insensitive
to increases in overflow rate; consequently, they can
achieve excellent SS removal (13). The average
effluent TSS of the Couer d'Alene, Idaho, trickling
filter plant decreased from 25 mg/L to 16 mg/L when
the original 2.1-m (7-ft) deep clarifier was replaced
with a flocculator-clarifier (92).
Additional design information for flocculator-clarifiers
can be found in Section 4.4.7.1, and general design
information on clarifiers can be found in several
sources (2,93).
4.3.74 Excessive Clarifier Hydraulic Currents
Excessive currents within the clarifier can contribute
to loss of solids at the effluent weir. Currents exist in
all secondary clarifiers but typically have more pro-
nounced effects in suspended growth clarifiers than in
fixed-film facility clarifiers, but are present neverthe-
less. These currents develop in clarifiers as the result
of density and temperature differences between the
influent wastewater and clarifier contents. A
discussion on hydraulic currents can be found in
Section 4.4.
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4.4 Suspended Growth Treatment
Processes
A large number of secondary treatment facilities utilize
suspended growth processes for achieving effluent
compliance. Processes within this category include
conventional, complete mix, extended aeration,
contact stabilization, step feed, pure oxygen, and
oxidation ditch activated sludge systems. The
common components for these processes include:
return sludge equipment
waste sludge equipment
oxygen transfer equipment
aeration basin(s)
mixing equipment
solids separation equipment
The ability to adjust the mass of organisms in a
suspended growth facility, coupled with proper use of
the other process variables, offers a significant
resource in expanding the performance capability of
this type of plant. For this reason, modifications to
utilize this flexibility are warranted. Proper operation is
needed to support suspended growth facility
modifications.
Optimizing performance requires the development of
good sludge characteristics. For a given system, four
major controls are available: 1) sludge mass control,
2) return sludge control, 3) DO control, and 4)
aeration basin configuration. This relationship is
graphically shown in Figure 4-12. Although each of
these controls is important and must meet certain
minimum requirements, a priority in terms of impact
on performance can be established. Sludge mass
control, or the capacity to waste the desired sludge
mass, has frequently been identified as a major
performance limiting factor in suspended growth
systems (23). Facility modifications to address this
limitation offer significant potential for improved
performance. Aeration basin configuration flexibility
allows changes in operational modes to control the
development of good sludge characteristics. Improving
return sludge or DO control offers a lesser potential
for improving plant performance.
Figure 4-12. Relationship of variables for optimizing
suspended growth facility performance.
Effluent in Compliance
Good Sludge Characteristics
Basin Config.
DO
Sludge Mass
Return
Sludge
Table 4-6 summarizes design limitations/corrective
modifications that are prioritized based on their
potential for achieving improved performance.
Modifications with proven reliability and those that are
less construction intensive appear at the beginning of
the listing.
Table 4-6. Suspended Growth Design Limitations and
Potential Modifications
Design Limitation/Potential Modification Page. No.
4.4.1 Inadequate Waste Sludge Flexibility/Capacity 87
4.4.1.1 Sludge Disposal/Utilization Management 88
4.4.1.2 Sludge Treatment Facilities 88
4.4.1.3 Separate Waste Sludge Pumps 88
4.4.1.4 Sludge Wasting to Thickener 88
4.4.1.5 Separate Waste Sludge Hopper in Clarifier 88
4.4.1.6 Flow Measurement 88
4.4.2 Inadequate Process Flexibility 89
4.4.2.1 Step Feed 89
4.4.2.2 Contact Stabilization 91
4.4.2.3 Plug Flow 93
4.4.2.4 Chemical Addition 93
4.4.2.5 Selector Basin 94
4.4.3 Undersized Clarifier 94
4.4.3.1 Flow Equalization 94
4.4.3.2 Additional Clarifier(s) 95
4.4.3.3 Intrachannel Clarifier 95
4.4.4 Undersized Aeration Basin . 95
4.4.4.1 Flow Equalization 96
4.4.4.2 Fixed Film/Suspended Growth Systems 96
4.4.4.3 Additional Aeration Volume 97
4.4.4.4 Biological Aerated Filter 97
4.4.4.5 Two-Zone Process 98
4.4.4.6 Powdered Activated Carbon Treatment 99
4.4.4.7 Porous Biomass Support Systems 99
4.4.5 Inadequate Oxygen Transfer Equipment 100
4.4.5.1 Additional Blowers 100
4.4.5.2 Diffused Air System Upgrade 100
4.4.5.3 Mechanical Aerator Upgrade 101
4.4.5.4 Roughing Filters 101
4.4.6 Inadequate Return Sludge Flexibility 101
4.4.6.1 Flow Recycle Around Pump 101
4.4.6.2 Adjustable Speed Drives 102
4.4.6.3 Time Clocks 102
4.4.6.4 Multiple Pumps 102
4.4.6.5 Flow Measurement. 102
4.4.7 Excessive Clarifier Hydraulic Currents 102
4.4.7.1 Inlet Modifications 102
4.4.7.2 Baffle Addition 103
4.4.7.3 Weir Relocation/Addition 104
4.4.8 Inadequate Basin Mixing 105
4.4.9 Inadequate Scum Removal 105
4.4.1 Inadequate Waste Sludge Flexibility/Capacity
The ability to control the sludge mass is necessary to
maintain good sludge characteristics and, in turn,
desired effluent quality. Sludge mass is controlled by
the amount of sludge wasted from the system. A
systematic sludge wasting schedule, which is
supported by process control testing and data
collection and analysis, is the basis for providing this
control. The operation of the suspended growth
process should not have to be modified because of
limitations of the sludge wasting, treatment, and
disposal facilities.
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Once sludge is removed, it is typically directed to
some type of sludge treatment facility, such as
thickening, digestion, and dewatering. Inadequately
sized sludge treatment facilities necessarily limit the
ability to waste sludge from a suspended growth
system; however, they do affect sludge handling and
stabilization capability. In a similar manner, limited
sludge disposal alternatives can have a negative
impact on the capacity of sludge treatment facilities,
which in turn can affect sludge wasting from the
suspended growth system.
The following sections discuss alternatives for
improving the waste sludge flexibility/capacity.
4.4.1.1 Sludge Disposal/Utilization Management
Ultimately, waste sludge must be removed from the
plant and disposed in an acceptable manner, such as
landfilling or composting. Application of sludge on
agricultural land is currently a common method of
sludge utilization; however, this method by itself
frequently places significant limitations on treatment
facilities. Inclement weather conditions, high ground
water, odor generation, and controlled access to
farmland are limitations that affect this disposal
method. A good sludge management program
requires the availability of several disposal or
utilization methods at a treatment plant. For example,
a land application program could be augmented with
lagoons for long-term storage and composting as an
alternative utilization method. This type of flexibility
assures the removal of sludge from treatment
facilities, which, in turn, assures unlimited wasting
capability from the suspended growth process. Further
considerations for sludge disposal and utilization are
presented in Section 4.7.
4.4.1.2 Sludge Treatment Facilities
Sludge treatment and storage facilities should be
sized based on actual sludge production, including
consideration for periods of peak production that
occur due to changes in loading, temperature, and
operational mode. Information on estimating sludge
production can be found in Section 3.4. Information on
modifications to sludge treatment facilities is
presented in Section 4.7.
4.4.1.3 Separate Waste Sludge Pumps
Separate waste and return sludge pumps provide the
flexibility to optimize each function. In most plants that
use a common pump for these functions, the design
of this equipment was based on the return sludge
requirements. Typically, their sludge is wasted from
the return sludge flow stream, which can overload
subsequent sludge handling facilities. Selection of a
separate waste sludge pump should include the
following considerations:
• In small- to medium-sized plants, a positive
displacement pump, such as progressive cavity or
rotary lobe type, is typically the most appropriate
pump for these conditions. For larger plants,
centrifugal pumps are usually more cost effective.
• Include variable speed drives, time clocks, or a
combination of both to provide needed flexibility.
• Include a flow measurement device to optimize the
pumping rate (see Section 4.4.1.6).
4.4.1.4. Sludge Wasting to Thickener
Thickening equipment, such as dissolved air flotation,
can be used to increase the solids concentration of
waste sludge from less than 1 percent (weight basis)
to 1.5-3.0 percent. For short periods of time,
secondary waste sludge can be directed to primary
clarifiers for thickening with primary sludge. This
practice, over a long period of time, can dramatically
decrease the waste sludge concentration from the
primary clarifier. Additional information on sludge
thickening can be found in Section 4.7.
4.4.1.5 Separate Waste Sludge Hopper in Clarifier
The use of a designated hopper for the withdrawal of
waste sludge from clarifiers can be used to maximize
sludge concentration, thus minimizing sludge volumes
to the sludge treatment process. Modifying an existing
clarifier to include a separate waste sludge hopper is,
in most cases, not practical because of the demolition
and construction requirements. However, if the plant
modification includes a new clarifier,,this option should
be considered. A designated waste sludge hopper can
also be incorporated into clarifiers that utilize vacuum
sludge pickup for returning sludge.
4.4.1.6 Flow Measurement
Waste sludge mass is determined from the
concentration and volume of wasted sludge. Flow
measurement equipment is desired to determine
volume; however, where equipment is not present, the
waste sludge volume must be estimated.
Pump calibration can be used to estimate waste
sludge flow rate. For a centrifugal pump, the flow rate
is dependent on the pump speed and head. A pump
curve which describes this relationship can be
developed under field conditions by measuring a
timed volume displacement in a basin versus the
pumping condition (i.e., rpm, head). Once this curve is
established, it can be used to estimate the flow rate
for various speed settings. This method sometimes
can be unreliable because of the effects of variable
waste sludge concentrations and plugging on the flow
rate.
For positive displacement pumps, the flow rate can be
estimated from the pump speed (i.e., rpm) since
capacity is essentially constant over the typical head
conditions. The speed of the pump can be determined
by attachment of a tachometer.
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The recommended approach for determining sludge
flow rates is through the use of appropriate flow
measurement devices. Primary devices such as
flumes and weirs are recommended because of their
ease of calibration; installation of such devices in
existing plants usually requires some ingenuity.
Consideration can also be given to the installation of
secondary devices (e.g., magnetic meters and doppler
meters) if there is some method available for checking
their accuracy. A common method of checking the
accuracy of a flow meter is by pumping the flow to a
basin with a known volume.
Figure 4-13. Flow schematics of conventional and step feed
activated sludge processes.
Influent.
k
— w
*r
\ Clar
Aeration Tank
Return Sludge Waste Sludge
a. Conventional. Plug-Flow Mode
4.4.2 Inadequate Process Flexibility
The lack of flexibility in a suspended growth process
typically limits the plant's capability since most
POTWs are required to operate under a variety of
conditions. These conditions can include variable
hydraulic loadings caused by infiltration/inflow,
seasonal contributors and plant start-up, and variable
organic loading resulting from industrial sources and
plant start-up. Good plant performance under all of
these conditions sometimes can require different
aeration basin configurations.
Existing suspended growth plants sometimes can be
modified to operate under various conditions without
extensive modifications and capital cost. In this
section, methods of achieving process flexibility are
presented together with their associated advantages
and disadvantages. The complete mix operational
mode is commonly utilized at larger suspended growth
facilities; however, it is one of the most limited modes
in terms of available detention time and sludge mass
distribution capability. Possible alternatives for adding
flexibility to a complete mix system are discussed in
Section 4.4.2.3.
The development of filamentous sludge in a
suspended growth system frequently requires
corrective actions that also depend on process
flexibility. Additional aeration capacity or changes in
basin configuration are types of flexibility often
needed. Because of the time required to make these
types of changes, rapid methods of eliminating sludge
bulking symptoms, such as chlorination of the sludge
mass, may be necessary (88).
4.4.2.1 Step Feed
The impacts of flow and load variations can often be
addressed by providing the plant with the flexibility to
operate in the step feed mode. Flow schematics for
the conventional plug flow and step feed activated
sludge processes are presented in Figure 4-13. Step
feed differs from the conventional configuration in that
wastewater is fed in increments at several points
along the length of the basin. In both schematics,
return activated sludge enters the aeration basin at
the head end of the process.
Influent
Aeration Tank
Return Sludge
b. Step Feed Mode
The step feed process can handle organic shock
loads better than the conventional plug flow process.
At an equal solids load to the secondary clarifier, the
step feed process has a higher solids inventory under
aeration than the plug flow process, thereby reducing
the overall food-to-microorganism (F/M) loading.
Additionally, the organic load and, therefore, aeration
requirements, are more evenly distributed and locally
high F/M loadings are avoided.
These features are illustrated in Figure 4-14. Each
process receives an equal organic load [i.e., 11,350
kg (25,020 Ib) TBOD5/d] and the secondary clarifiers
receive equal solids loads. There is a significant
difference in the F/M loadings. The conventional plug
flow process (Figure 4-14a) has an overall F/M loading
of 1.0 kg TBOD5/kg MLVSS/d, with a loading of 3.0 in
the first basin. The step feed process (Figure 4-14b)
has an overall F/M loading of 0.73; and no individual
basin has an F/M loading greater than 1.0. Thus, the
treatment plant operating in the step feed mode is
less susceptible to process upset due to organic
shock loads.
Operating in the step feed mode also provides more
capability for handling hydraulic surges. Figure 4-15 is
a simplified presentation of a process control change
that can be made prior to a peak flow event to shelter
solids from washout. As shown, the first feed inlet is
valved off to create a sludge reaeration zone in the
first basin. In effect, this shelters some of the solids
from the mainstream of flow. The effect of this change
is highly dependent on sludge settling characteristics
89
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Figure 4-14. Advantages of the step feed process in treating
organic shock loads.
Total Volume Under Aeration - 1.5 million gallons
Volume of Each Aeration Basin = 500,000 gallons
Influent «• 6 mgd @ 500 mg/L BOD5
9 mgd @ 2,000 mg/L MLVSS
mg/L MLVSS
6 mgd
2,000
2,000
->• -
2,000
-*•
3 mgd @ 6,000 mg/L MLVSS
a. Conventional. Plug-Flow Mode
Waste Sludge
6 mgd
9 mgd @ 2,000 mg/L MLVSS
mgd ^2 mgd ^2 mgd
3,600
2,570
Effluent
2,000
mg/L MLVSS
3 mgd @ 6,000 mg/L MLVSS
b. Step Feed Mode
Figure 4-15. Managing hydraulic shock loads with the step
feed process.
6 mgd
9 mgd @ 2,800 mg/L MLVSS
= 210,170 Ib/d
t
v2 mgch
5,040
, 2 mgd L 2 mgc
-> -
3,600
->•
2,800
mg/L MLVSS
3 mgd @ 8,400 mg/L MLVSS
a. Before Hydraulic Surge
12 mgd
15 mgd @ 1,680 mg/L MLVSS
= 210,170 Ib/d
3 mgd @ 8,400 mg/L MLVSS
b. During Hydraulic Surge
and the duration of the hydraulic surge. Some
decrease in BOD5 and TSS removal should be
anticipated under these conditions; however,
performance should be considerably better than under
the plug flow mode. Sludge wasting and return rates
may need to be adjusted if the peak flow event lasts
over several days. After the peak flows have passed,
the process can be returned to the original step
loading configuration.
The step feed process also can be used to reduce the
solids loading rate to the secondary clarifiers.
Improved clarifier effluent quality often results from
reduced solids loading, especially if the solids settle
poorly. Rgure 4-16 illustrates two situations where
clarifier solids loading rates are decreased while
keeping the total inventory of solids under aeration at
a constant level. This is accomplished by valving off
feed inlets and creating sludge reaeration zones,
which leads to a redistribution of the aeration basin
solids. It may be necessary to temporarily adjust
return and waste sludge rates to achieve the desired
redistribution. A more complete description of the use
of step feed to lower solids loading rates can be found
in other sources (24, 109).
Modification of a conventional plug flow aeration tank
to step feed generally requires dividing the aeration
basins into separate compartments and changing the
piping system so that wastewater can be fed in
increments at any of several different points along the
aeration basin. Ideally, flow measurement should be
included for the flow entering each compartment. If
the flow stream to each compartment can be seen by
the plant operator, visual balance may be satisfactory
and flow measurement equipment can be avoided.
The aeration basin can be compartmentalized by
adding either "curtain" walls or simple plywood
partitions. The addition of load-bearing walls is quite
expensive, usually unnecessary, and should be
avoided At POTWs where multiple basins already
exist, it may be simply a matter of cutting holes in the
common walls to allow step loading flexibility. In most
cases, the conversion will not require modifications to
the aeration system unless tapered aeration is used.
Typically, modifications to provide step feed capability
are relatively inexpensive and can be carried out by
the plant staff or local contractors. Any modifications
that are made should maintain the capability of
operating in the plug flow mode.
Case History: Keokuk, Iowa Wastewater Treatment
Plant
A schematic of the Keokuk wastewater treatment plant
is shown in Figure 4-17. The plant was experiencing
performance problems due to a high strength, highly
90
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Figure 4-16. Use of step feed to reduce solids loading to the
secondary clarifier.
6 mgd
9 mgd @ 2,800 mg/L MLVSS
= 210,170 Ib/d
2 mgd
Secondary
5,040
— ir-
3,600
•^
2,800
mg/L MLVSS
1 Clarifier V
Waste Slue
3 mgd @ 8,400 mg/L MLVSS
a.
6 mgd
6,240
mgd
3,120
9 mgd @ 2,080 mg/L MLVSS
= 156,125 Ib/d"
3 mgd
2,080
mg/L MLVSS
3 mgd @ 6,240 mg/L MLVSS
b.
6 mgd
4,903
4,903
9 mgd @ 1,634 mg/L MLVSS
= 122,650 Ib/d
6 mgd
1,634
mg/L MLVSS
3 mgd @ 4,903 mg/L MLVSS
c.
variable industrial load and high flows due to inflow
during rainfall events. In addition, the industrial waste
encouraged a slow settling bulky sludge. During this
time period the average flow to the plant was 197 L/s
(4.5 mgd), and the aeration basin detention time was
about 29 hours. Numerous attempts to increase
sludge mass in the aeration basins were initiated while
the basins were operated, as indicated in Figure 4-17.
As the mass increased, solids loss would occur in the
final darifiers.
The plant staff modified the process by cutting a hole
in the wall between the two aeration basins and
modifying the return sludge piping as shown in Figure
Figure 4-17. Organic loading configuration for Keokuk, Iowa
Wastewater Treatment Plant.
Influent
X"
[ Primary ] I / Primary
I Clarifier I " \ Clarifier
\. y Waste Sludge .\.
to Stabilization
Waste Sludge
to Thickener
Secondary Effluent
4-18. With this modification, the plant was able to
operate either in the step feed or contact stabilization
mode, and the mass in the system was increased
dramatically. Despite the variable industrial loads, the
higher mass resulted in more stable operation and
allowed a method of protecting solids loss during
storm events.
Prior to the modification, the plant exceeded the
discharge requirements every month. After the
modification, the plant cycled into and out of
compliance on approximately a quarterly basis. The
key change, as the result of the modification, was the
fact that the microbiological population went from high
numbers of filamentous organisms to a more mixed
population, including some free swimming organisms.
Routine compliance was not achieved until one of the
major industrial contributors built their own facility and
went off line from the municipal system.
4.4.2.2 Contact Stabilization
Modifying an existing aeration basin to operate in the
contact stabilization mode can assist in alleviating
performance problems associated with high flows. A
91
-------
Figure 4-18. Modified loading configuration for Keokuk, Iowa
Wastewater Treatment Plant.
Influent
{ Primary \
V Cterifier I
± ( Primary )
~ V Clarifier I
Waste Sludge V ./
to Stabilization
Waste Sludge
to Thickener^
Relocated Return
Sludge Inlet
I Secondary Effluent
flow schematic of a typical contact stabilisation
process is shown in Figure 4-19. The process
involves a short term aeration phase (0.5-1.0 hr) in
which substrate is rapidly transferred from the
wastewater to the biological sludge and a long term
sludge reaeration phase (3-6 hr) in which the
Figure 4-19. Contact stabilization mode of the activated
sludge process.
Waste Sludge
Effluent
T
_^
Reaeration
Tank
Return Sludge
absorbed substrate is metabolized. It is important to
keep the detention time in the contact phase short to
ensure process stability. Longer detention times result
in poor sludge settling characteristics and undesirable
performance.
In comparing Figure 4-19 with Figure 4-16c, it is seen
that the flow distribution for contact stabilization is
essentially the same as that for step feed when only
the last basin is fed. Modifications needed for step
loading can also provide the capability to operate in
the contact stabilization mode. However, the biological
sludge found in an efficiently operating contact
stabilization process is different from that of a step
feed process in terms of oxygen uptake rate and
biokinetics. These different properties require time to
develop and stabilize and, therefore, it is
recommended that changes between the plug
flow/step loading mode and contact stabilization mode
not be performed on a frequent basis.
Contact stabilization plants are the least susceptible to
solids loss during hydraulic surge events. Solids in the
reaeration basin are protected from the mainstream of
flow, thereby minimizing solids washout. The effects
of hydraulic surges can be minimized even further by
building up the solids inventory in the reaeration basin
prior to increases in flow. This can be done by
decreasing the return sludge rate, thereby returning a
thicker sludge to the reaeration basin. Some
degradation in BOD5 removal will probably occur
during this time because of low MLSS concentration
in the contact basin; however, once the peak flow
event has passed, this concentration can be quickly
increased to improve BOD5 removal.
The contact stabilization mode may also be able to
handle organic shock loads better than a conventional
process. Contact stabilization plants are best suited
for treating wastewaters with a high percentage of
colloidal (non-soluble) organics, as is common for
most domestic wastes. If the shock load consists
mostly of colloidal organics, the contact stabilization
process is capable of providing effective treatment
without process upset. However, if the shock load
consists mostly of soluble matter, organics can "bleed
through" a contact stabilization process.
The total basin volume required for contact
stabilization is less than that required for conventional
systems. Therefore, conversion of a plug flow system
to contact stabilization can often be accomplished by
dividing the existing aeration basin into two separate
compartments with a division wall. A load-bearing
division wall allows for draining of each basin
separately for cleaning and maintenance. A curtain
wall is more economical to install but does not allow
for separate draining of each basin. The contact basin
should be sized to allow for 0.5-1.0 hr detention time
at daily average flow rates, with the remainder of the
basin being used as the reaeration tank. To be
effective, the volume of the reaeration basin should be
92
-------
approximately three to four times greater than the
volume of the contact basin. Piping and valves must
be installed to route return activated sludge to the
reaeration basin and both wastewater and reaerated
sludge to the contact basin.
Modifications to the air distribution system may not be
required. It is suggested that actual operating
experience be used to indicate if the contact basin is
oxygen limited before modifications are made. New
construction of contact stabilization plants calls for
about three to four times more oxygen per unit of
basin volume for the contact basin than for the
reaeration basin.
Since contact basins are typically designed for 0.5-1.0
hr detention time, they are relatively small and short-
circuiting can occur. When modifying an existing basin
to include contact stabilization capability, it is
important to locate the influent and effluent piping at
opposite locations in the contact tank. In some cases,
baffling within the contact tank will be required.
4.4.2.3 Plug Flow
Plug flow systems can often be used to develop a
stable sludge or to optimize treatment once a stable
sludge is achieved. Plug flow capability can be
provided by modifying basins to obtain high length-to-
width ratios or by subdividing a basin into
compartments. The following discussion presents
some considerations for achieving plug flow capability
in various types of suspended growth plants.
If influent organic loading is in a soluble form, a plug
flow system will provide better treatment than a
contact stabilization system. Typically, a contact
stabilization plant can be easily modified to
incorporate the plug flow mode by the addition of
piping or a channel to direct the wastewater influent to
the head of the reaeration basin. Flexibility should be
included to provide step feed capability since the
additional expense would be minimal.
Complete mix activated sludge plants are limited in
flexibility. Flexibility could be added by dividing the
aeration basin into smaller compartments and
modifying the influent, effluent, and return sludge
piping or channels. If several complete mix basins are
available, the piping could be modified to operate the
basins in series. When a single basin is available, a
compromise is typically made in the number of
compartments provided because of capital cost and
the decreasing improvement in performance with the
increasing number of compartments. A typical
modification would consist of dividing a complete mix
basin into four compartments of equal size. Another
alternative would include the addition of parallel walls
in the basin to form a serpentine flow pattern. These
configurations are shown in Figure 4-20.
A complete mix modification will also be dependent on
the type of aeration system being utilized. When a
basin is divided into smaller compartments, it is
important that adequate aeration is provided in each
compartment. This is especially true with the plug flow
configuration since the higher organic loading in the
beginning of the basin will result in a higher oxygen
demand. If a mechanical aeration system is utilized,
conversion to a plug flow system may not be
economical because of aeration modifications that
may be required. In these situations, it may be more
economical to install additional aerator volume.
Figure 4-20. Conversion of complete mix to plug flow.
Influent
#1
#2
Return Sludge
->
To Clarifier
#4
r
#3
New Basin
Walls
a. Cdnversion by segmenting basin into compartments
Return Sludge
Influent
New Basin
Walla
#1
#3
To Clarifier
b. Conversion by addition of baffle walls
4.4.2.4 Chemical Addition
Chemical addition can be utilized to improve a sludge
bulking problem caused by filamentous organisms by
enhancing the sludge settling rate or selectively killing
the organisms. Synthetic polymers or inorganic
coagulants can be used to enhance settling
characteristics. A wide variation in polymer type and
dosage have been reported; consequently, jar testing
is recommended. In general, the addition of polymer
for bulking control is more expensive than selective
chlorination of the sludge mass (88). Inorganic
coagulants, such as alum and ferric chloride, can also
be used to enhance sludge settling; however, these
chemicals produce voluminous precipitates which
increase the solids loading to subsequent sludge
handling facilities. Information on the design of
chemical feed facilities can be found in Reference 89.
93
-------
Both chlorine and hydrogen peroxide have been used
successfully in the selective destruction of filamentous
organisms. A variety of dosing points have been used,
including: 1) the aeration basin, 2) the flow channel
between the aeration basin and clarifier, and 3) the
return sludge stream. The most common dosing point
is the return sludge stream. If a long aeration basin
detention time exists, it is usually more advantageous
to dose directly into this unit. Design and operational
considerations for chlorine and hydrogen peroxide
addition are presented in Reference 88.
Chlorine addition has been successfully used at the
Missoula, Montana wastewater treatment plant.
Chlorine is added to the return sludge stream and to
the aeration basin. When filamentous sludge
characteristics develop, a chlorine dosage of 6-8 kg
Cla/t.OQO kg VSS is split equally between these two
dose points. This method of dosing has been
consistently effective in achieving a positive
improvement in sludge settling characteristics within
one day (98).
4.4.2.5 Selector Basin
Various types of filamentous organisms have been
found in suspended growth systems. The use of a
high to low F/M gradient has shown success in
controlling the low-F/M type filamentous bulking. This
approach can be produced in continuous flow systems
by utilizing selector or plug flow reactors. Strategies
include operation of the selector or initial compartment
of a plug flow reactor in a fully aerobic (i.e., DO>2
mg/L) environment or in an oxygen deficient (i.e.,
DOS0.2 mg/L) environment, both operating at high
F/M loadings (i.e., >3 g BOD5 applied/g MLSS/d)
(26). Under both conditions, the high F/M loading
encourages the growth of zoogleal colonies over the
growth of filamentous organisms.
Various approaches have been proposed for selecting
the size of aerobic selectors (99-101). The
mechanisms by which selectors function to prevent
filamentous orgamism growth and bulking at low F/M
conditions have been investigated (25). This
investigation showed that aerobic selector systems
were only successful in controlling bulking over a
narrow range of selector sizes. For the wastewater
and activated sludge studied, the successful selector
detention time and selector F/M ranges were 12-18
minutes and 20-30 g COD/g MLVSS/d, respectively.
An existing suspended growth system could be
modified to include an aerobic selector by partitioning
part of the aeration basin or by the addition of
separate comaprtment (s). It has been suggested that
selector systems can be designed on the basis of
experiments using fill-and-draw systems (25). It is
recommended that several compartments and bypass
capability be included in the selector design to allow
flexibility in loading and detention time. An example
layout of a complete mix suspended growth process
modified to include selector basins is shown in Figure
4-21.
Experience with the oxygen deficient selector
indicates that an OUR:DO greater than 250:1 should
be maintained to assure anaerobic conditions (i.e.,
zero DO and NO3-N) in the inner cell structure.
Factors such as mixing, floe size distribution, and
diffusional resistance will influence this relationship.
The selector should comprise at least two
compartments, and the volume of the first
compartment should maintain an F/M loading of at
least 2.3 g BOD5/g MLSS/d. The first compartment
will typically comprise 2-5 percent of the total
biological basin volume. Initial low DO zones can
comprise 10-40 percent of the total biological basing
volume depending on the design objectives (26,102).'
This strategy is also utilized in various biological
phosphorus removal processes (27).
4.4.3 Undersized Clarifier
A common misconception associated with the
performance of clarifiers in suspended growth
systems is that solids loss is the result of a clarifier
failure, when, in fact, it is often due to poor sludge
settling characteristics. Sludge settling characteristics
are established by the system environment (i.e.,
aeration basin and clarifier) and the process control
parameters (i.e., mass, DO, return rate, and basin
configuration). It is important that process control be
evaluated thoroughly before any decision is made to
correct a clarifier design deficiency (i.e., additional
capacity can be achieved from an existing clarifier by
changing to the step feed mode, as discussed in
Section 4.4.2.1).
A clarifier limitation may exist if the following criteria
are exceeded. Secondary clarifiers are usually
designed for surface overflow rates of 16-32 m3/m2/d
(400-800 gpd/sq ft). Depth is also an important
parameter that is necessary to provide solids
separation, thickening, and storage. Storage is
necessary because of changes in the solids
distribution between the aeration basin and elarifier
that occurs as a result of diurnal flow variations. A
sidewater depth of 3.6-4.3 m (12-14 ft) is commonly
used; however, some contemporary designers utilize
clarifiers with sidewater depths in the 4.9-6.1 m (16-20
ft) range (45).
4.4.3.1 Flow Equalization
A common method used to reduce the effects of peak
flows on clarifiers is flow equalization. Information on
flow equalization is presented in Section 4.2.1.5. In
some situations, sufficient flow equalization can be
achieved by utilizing the available freeboard in the
aeration tanks. This can be accomplished by: 1)
increasing the hydraulic resistance of the outlet weir
from the aeration basin with the corresponding
increase in water depth, or by 2) pumping from the
aeration basin to the clarifier. Additional information on
94
-------
Figure 4-21. Selector basin modification.
Aeration Basin
Influent
Selector
Basins
7\
Secondary Clarifier
Effluent
Return Sludge
Waste Sludge
to Stabilization
these techniques can be found in other sources
(86,87). Before this method of flow equalization is
pursued, the effect of varying water depths on the
existing aeration system must be evaluated.
4.4.3.2 Additional Clarifiers
If existing clarifier overflow rates are high under
average flow conditions, the construction of an
additional clarifier may be warranted. Before adding
clarifiers, however, investigate the effectiveness of
internal clarifier baffles (see Section 4.7). An important
consideration when adding new clarifiers, often of
different size and capacity, is flow splitting. When flow
splitting to multiple clarifiers is required, positive flow
control can be accomplished by construction of a flow
control structure with mechanically-adjusted control
gates. A staff gauge located near the control gate can
be used to measure the flow rate to each clarifier.
Information on the design of clarifiers can be found in
other sources (2,93).
4.4.3.3 Intrachannel Clarifiers
A recent innovation involves the use of an
intrachannel clarifier in the aeration channel (46).
These devices allow solids/liquid separation within the
aeration channel, thus eliminating the need for a
separate clarifier, as shown in Figure 4-22. This
alternative would be appropriate for an undersized
secondary clarifier utilized in an oxidation ditch
treatment facility. The volume reduction associated
with this modification is typically not significant, since
most oxidation ditches are designed with conservative
detention times of 20 hr or greater.
Influent flow to an intrachannel clarifier is typically
through the clarifier bottom; however, other designs
direct the influent through the end or side. A
quiescent zone is created by the walls of the clarifier
structure, and the settled water is collected by either
overflow or submerged weirs. The settled sludge exits
the clarifier through a baffled bottom section and is
carried away by the passing mixed liquor flow; Several
manufacturers currently market intrachannel clarifiers.
Information on the various designs is provided
elsewhere (46).
Figure 4-22. Conventional vs. intrachannel oxidation ditch
systems.
Influent
Return Sludge
Effluent
Waste Sludge
to Stabilization
a.- Conventional Oxidation Ditch
—K: -
Influent V
Intrachannel
Clarifier
Effluent
Waste Sludge
to Stabilization
b. Oxidation Ditch with I nlrachannel Clarifier
Intrachannel clarifier modifications provide options for
the use of the abandoned secondary clarifier. The unit
could be used in series with the intrachannel clarifier
or it could be used as a thickener for waste sludge
from the oxidation ditch. Since mixed liquor is
normally wasted from facilities utilizing an intrachannel
clarifier, availability of a thickener would benefit the
other sludge handling facilities.
4.4.4 Undersized Aeration Basin
Aeration basins for suspended growth systems are
sized based on hydraulic detention time and organic
and volumetric loading rates. The aeration basin
hydraulic detention time varies depending on the
operational mode being. Detention times can vary
from 2-4 hr for a complete mix process to 18 hr and
greater for extended aeration processes. Organic
loading (F/M) typically ranges from less than 0.10 kg
BOD5/kg MLSS/d for extended aeration processes to
over 0.4 kg BOD5/kg MLSS/d for complete mix
processes. Volumetric loading rates vary from a low of
95
-------
0.16-0.32 kg TBOD5/m3/d (10-20 lb/d/1,000 cu ft) for
extended aeration processes to a high of 0.8 kg
TBOD5/m3/d (50 lb/d/1,000 cu ft) or more for
complete mix processes.
Operational alternatives should be examined before
addressing aeration basin volume deficiencies.
Additionally, load reduction should be investigated.
Possible areas where load reduction could be
achieved include: 1) side stream control, 2)
pretreatment of industrial wastes, and 3) improved
efficiency of preliminary and primary treatment
processes. If it is determined that these solutions will
not provide adequate performance, physical
modifications will be necessary.
4.4.4.1 Flow Equalization
Flow equalization facilities can be effectively used to
reduce hydraulic and organic loading. Specific
information on the modification to include flow
equalization is provided in Section 4.2.
4.4.4.2 Fixed Film/Suspended Growth Systems
A film system can be added to provide a partial
reduction in the organic loading (i.e., roughing) before
final treatment in a suspended growth system. This
modification requires the use of primary treatment
prior to the fixed film system. If a filter is used,
pumping is usually required for loading and
recirculation. Various types of fixed film/suspended
growth combinations are available (30).
In the roughing filter/activated sludge (RF/AS)
process, a trickling filter is added in series with a
suspended growth system. A schematic view of the
process is shown in Figure 4-23. In this roughing
application, high organic and hydraulic loadings are
utilized. Typical organic loading rates are 1.6-3.2 kg
BOD/m3/d (100-200 lb/d/1,000 cu ft), and hydraulic
loading rates are 20-120 m3/m2/d (500 to 3,000
gpd/sq ft) (8).
The combination of the discrete biological solids from
the roughing filter and the zoogleal mass from the
suspended growth system results in a sludge with
excellent settling characteristics (31). Typically, this
process also requires less power consumption since
part of the organic load is treated by less energy
intensive trickling filters. The amount of power savings
depends on the share of the organic load that is
removed by the roughing filter, the required pumping
head to the filter, the amount of recirculation around
the filter, and the turndown capability of critical
process equipment such as blowers.
The rotating biological contactor/activated sludge
(RBC/AS) and biofilter/activated (BF/AS) sludge
processes also can be considered for modifying an
overloaded suspended growth system. If the RBC/AS
process is being considered, BOD5 loading to the first
stage of the RBC facility should be evaluated (see
Section 4.3.5). In a BF/AS system, the return sludge
from the suspended growth system is returned ahead
of the trickling filter. The following case history
illustrates a situation where an activated sludge plant
was expanded by conversion to a BF/AS facility.
Case History: City of Casper, Wyoming BF/AS Facility
(32)
The Sam Hobbs regional wastewater treatment facility
treats wastewater from the City of Casper and
surrounding entities. Prior to expansion, the facility
was utilizing an air-activated sludge system with a
capacity of 263 Us (6 mgd). In 1985 the facility's
capacity was increased to 631 Us (14.4 mgd) by
conversion to a biofilter/activated sludge system. A
schematic drawing of the modified plant is shown in
Figure 4-24, and design criteria for the BF/AS system
is shown in Table 4-7.
Primary effluent from four primary clarifiers flows by
gravity to the biofilter pumping system. The primary
effluent mixes with return activated sludge and a
portion of the biofilter effluent to be recycled over the
tower. The amount of biofilter effluent recycle is
dependent on the difference between the pumping
rate and the actual influent flow rate (primary effluent
and RAS) to the pump wet well. The maximum
biofilter effluent recycle is limited to 219 Us (5 mgd).
The combination primary effluent, RAS, and biofilter
effluent (mixed liquor) is discharged through a rotary
distributor at the top of the biofilter. The plastic media
used in the biofilter consists of rectangular PVC
modules that measure 0.6 m x 0.6 m x 1.2 m (2 ft x 2
ft x 4 ft). The remaining biofilter effluent flows by
gravity to four aeration basins operated in parallel.
Effluent from the aeration basins is split equally to four
secondary clarifiers. An RAS pumping system returns
sludge from the secondary clarifiers at a rate
proportional to the plant influent flow. A separate
waste activated sludge pumping system diverts a
portion of the activated sludge to a dissolved air
flotation thickening system.
The plant is required to meet secondary treatment
requirements, and a seasonal ammonia nitrogen limit
of 9-12 mg/L. Consistent effluent BOD5 concentrations
of 15-20 mg/L were achieved when operating at an
MCRT in the aeration basin ranging from 1.5 to 12
days. Corresponding effluent TSS levels ranged from
10 to 15 mg/L. Complete nitrification was achieved
with the system when the MCRT was maintained
above 8 days. SVIs ranged from 50 to 180 ml_/g.
Following the introduction of a routine biofilter flushing
procedure and a change in the RAS mixing location,
the SVI remained below 100 ml_/g at MCRT values
ranging from 1 to 4 days. The total secondary
treatment power requirements were 0.55 and 1.2
kWh/lb BOD5 at MCRT values of 1.5 and 8 days,
respectively. °'
96
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Figure 4-23. Roughing filter/activated sludge.
Recirculation
Influent
Waste Sludge
to Stabilization
Table 4-7. Design Criteria for Casper, WY BF/AS System
Parameter Design Criteria
Design Capacity, m3/d
Biofilter
Media Type
Surface Area, ma/m3
Diameter, m
Depth, m
Total Media Volume, m3
Aeration Basin
Volume (each), m3
Equipment
No. Aerators per Basin
54,500
Stacked PVC Modules
96
27.4
4.9
2,860
1,892
Surface Mechanical Aerators
2 @ 30 kW
4.4.4.3 Additional Aeration Volume
Additional aeration volume can be accomplished
through the construction of new tankage or often
through the utilization of existing abandoned tankage.
If this alternative is considered, the flexibility to allow
use of suspended growth modes (e.g., step feed and
contact stabilization) should be provided.
4.4.4.4 Biological Aerated Filter (BAF)
The BAF is a down flow, high rate, fixed film biological
wastewater treatment system. A schematic flow
diagram is shown in Figure 4-25. Primary effluent is
directed downward through a packed bed of granular
clay media. The media provides surface area for the
growth of microorganisms and also removes SS via
filtration. Air is introduced into the media bed to
provide oxygen for biological growth. Effluent exits the
reactor through a set of plastic nozzles located in an
underdrain plate.
Excess biological solids and trapped SS are normally
removed from the bed daily by backwashing the entire
media with treated effluent. A blower is used to force
air through the bottom of the bed during backwash to
scour solids from the media. Spent backwash water is
directed to the primary clarifier or plant headworks.
Figure 4-24. BF/AS system at Casper, Wyoming.
— j- -J Secondary
Clarifiers
Waste Sludge
to Stabilization
97
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Figure 4-25. Biological Aerated Filter (BAF).
Spent Backwash
Storage
Waste Sludge
to Stabilization
Influent
Treated Effluent
Air Scour
Plugging of the BAF reactor has been reported as a
potential problem when treating high strength organic
wastes. When clogging occurs in some part of the
reactor - such as the granular media, air distribution
piping, or water collection nozzles - backwashing
sometimes is not effective in correcting this situation.
It then becomes necessary to remove the media to
perform cleaning and repairs. Success in eliminating
reactor plugging by modifying the BAF understructure
has been reported (105). The first modification
included the replacement of the effluent collection
nozzles with a layer of gravel which contained
perforated pipe. The diameter of the perforations in
the collection pipe were above 10 mm (0.4 in). The
second modification involved the lowering of the
aeration diffuser pipes into the gravel layer.
The modification of an existing suspended growth
basin to a BAF is not compatible with the equipment
provided by the system's manufacturer. Presently,
modular plants complete with tankage, media,
controls, and miscellaneous components are provided.
Each module includes 46 m.2 (500 sq ft) of media
surface. Use of a BAF system would probably be
limited to treatment of in-plant or industrial
sidestreams, treatment of part of the plant loading at a
relatively constant rate (i.e., base load), or polishing of
plant effluent to meet stringent treatment
requirements.
The reported allowable soluble organic loading rate for
a BAF unit is 1-1.4 kg/m3/d (63-88 lb/d/1,000 cu ft)
depending on the influent wastewater temperature
(33). Reasonable run times between backwashes (i.e.,
24 hr and greater) can be achieved at hydraulic
loading rates of 0.68 L/m2/s (1 gpm/sq ft) and less.
With an effective media size of 4.4 mm, the amount of
water used for backwashing is 7-10 percent of the
influent flow. The system appears to have a higher
solids yield and lower oxygen requirement than
conventional activated sludge because of limited
degradation of the SS that are removed by the media.
Since the BAF process is relatively unproven
technology, a pilot study is recommended to establish
site-specific design criteria.
Advantages of the process are summarized below:
• The need for a secondary clarifier is eliminated.
• Nitrification can be achieved at lower loading rates.
• The process is marketed in modular units that
facilitates staged construction.
• Space requirements are significantly less than for a
suspended growth system.
Limitations are:
• A primary clarifier is required.
• Modifying an existing suspended growth basin is
limited since the process is marketed as a stand-
alone unit.
• Process performance is affected by peak loading
rates.
4.4.4.5 Two-Zone Process
The Two-Zone process was developed by Canadian
Liquid Air, Ltd. as an alternative to conventional
wastewater treatment systems. As shown in Figure 4-
26, the process is essentially an activated sludge
process that combines the aeration basin and
secondary clarifier functions into a single tank. An
external oxygenation device is used to super-saturate
the sludge recycle stream, which is returned to the
reactor to provide all of the oxygen requirements of
the process. The Two-Zone process is designed so
that it can be retrofitted into an existing aeration basin
or clarifier, provided that the tankage is rectangular or
square shaped.
98
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Figure 4-26. Flow diagram of the Two-Zone System.
Recycle Line From Oxygenator
Inf.
Biological
Reactor
Zone
Clarifier
Zone \ Sludge
.Rake
Effluent
02
Waste
Sludge
/
Oxygenator
To establish the capabilities of the Two-Zone process
to treat municipal wastewater in a retrofit situation,
Canadian Liquid Air, Ltd. and the U.S. EPA jointly
conducted a 22-L/s (0.5-mgd) demonstration project at
Norristown, Pennsylvania (34). The study found that
the process utilizes tank volume more efficiently than
conventional activated sludge processes. The Two-
Zone system was capable of achieving 82-87 percent
TBOD removal and 85-93 percent TSS removal at
F/M loadings of 0.33-0.95 kg TBOD/d/kg MLVSS, total
tank detention times of 3.3-7.0 hr, and MCRTs of 1.2-
3.7 days. These low MCRTs resulted in a relatively
low process oxygen requirement (0.5 kg Og/kg TBOD
removed), and a high sludge production value (1.54
kg MLSS/kg TBOD removed).
If all existing aeration tanks and secondary clarifiers at
a wastewater treatment facility are converted to the
Two-Zone process, the treatment capacity of that
facility will increase. If only the aeration basins are
converted, it is likely that there will be no net gain in
treatment capacity. Also, a higher demand will be
placed on the capacity of sludge stabilization
processes due to the high sludge production rate
associated with the process. The Two-Zone process
is not recommended for treatment facilities that must
achieve nitrification or for those that have influent
BOD5 concentrations greater than 200 mg/L. In the
demonstration project, the large quantity of oxygen
that had to be provided for these conditions resulted
in flotation of the biomass. The project report contains
more detailed information (34).
4.4.4.6 Powdered Activated Carbon Treatment
(PACT)
PACT is a waste treatment process marketed by
Zimpro that is based on the combined use of
powdered activated carbon and aerobic bacteria in the
activated sludge process (35). In a typical system,
carbon is added to the wastewater after the primary
clarifier at a dosage rate based on the degree of
treatment required. A small dose of liquid cationic
pojyelectrolyte is added prior to the final clarifier to
improve effluent clarity. Also, teriary filtration is
generally required to ensure the separation of PAC
prior to discharge. A wet air oxidation process has
generally been used to regenerate the spent carbon in
municipal applications. The decant from the wet air
oxidation unit can impose a high organic load on the
plant because of the production of acetic acid in this
unit operation.
Due to the adsorptive nature of the powdered
activated carbon, the PACT process may be capable
of removing many of the complex, high molecular
weight organics that are difficult to biodegrade in
conventional activated sludge processes. Included
among these compounds are many organic toxins,
priority pollutants, soluble dyes, and surfactants. As
such, the PACT process has been applied at many
industrial plants and at several municipal treatment
facilities that receive significant industrial loads. Its
applicability for municipal facilities should be
determined on a case-by-case basis using pilot-scale
studies. It is anticipated that the PACT process would
not be cost effective for a municipal activated sludge
system unless the reason for non-compliance is
directly due to heavy industrial loads.
Operational problems associated with the regeneration
system have been experienced at some municipal
PACT facilities. At both the South Burlington, North
Carolina and Bedford Heights, Ohio treatment plants,
inability to separate the inorganic particulate matter
from the regenerated carbon occured. As a result, the
volatile solids content in the mixed liquor was as low
as 34 percent, requiring excessively high MLSS
concentrations to be maintained. At the Bedford
Heights plant, the presence of heavy metals in the
recycled ash contributed to permit violations
associated with metals concentrations.
4.4.4.7 Porous Biomass Support Systems
Porous biomass support systems are a hybrid
technology that combine many of the features of
suspended growth and fixed-film processes. With
these systems, thousands of small plastic foam pads
are added to the mixed liquor in an aeration tank to
serve as a support matrix for biological growth. The
concept behind this approach is that the amount of
biomass under aeration can be considerably
increased, thereby lowering the F/M ratio and
increasing the treatment capacity of the basin. Some
systems, such as CAPTOR may not always require a
secondary clarifier. Other systems such as LINPOR
include a secondary clarifier and sludge recycle.
The Water Engineering Research Laboratory of the
U.S. EPA conducted a 2-year pilot-plant evaluation of
the CAPTOR process (36). The pilot plant was
hydraulically loaded at a rate of 1.6-1.9 L/s (25-30
gpm).
99
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The CAPTOR process utilizes polyurethane foam
pads with dimensions of 1"x 1" x 1/2". Approximately
1,000 pads are added for every cubic foot of aeration
tank volume (35,000 pads/m3). A screen is inserted
into the aeration tank's effluent discharge structure to
retain pads in the tank. To waste excess sludge, pads
are removed from the tank with air lift pumps and then
conveyed to pressure rollers that squeeze the loosely
entrapped and attached solids from the pads. The
pilot-plant study identified several problem areas,
including high concentrations of poorly set-tleable SS
in the effluent, and poor process economics. Contrary
to the manufacturer's claims, there was no evidence
of improved oxygen transfer efficiency. Use of the
pads did allow for operation with higher MLSS and
waste sludge concentrations.
Porous biomass support technology is still in the
developmental stages. Several full-scale LINPOR
systems are in operation in Germany. Other
processes, such as RING LACE, have been used in
some facilities in Japan. At this point, however, the
potential of porous biomass support processes for
retrofit purposes is not well defined.
4.4.5 Inadequate Oxygen Transfer Equipment
Oxygen transfer can be achieved by utilizing surface
mechanical aeration equipment, blowers in
combination with diffused air equipment, or a
combination of both. When inadequate oxygen is
transferred, deterioration of effluent quality typically
occurs because of insufficient oxygen to meet the
biological oxygen demand and the endogenous
oxygen demand of the biological mass. The
development of filamentous organisms is also
associated with low mixed liquor DO concentrations
(37). This problem can indirectly result in effluent
violations because of poor settling sludge
characteristics.
Chapter 2 discusses a method of estimating the
oxygen transfer capacity of an existing aeration
system. Under most conditions, this estimation should
provide sufficient information to determine whether
inadequate oxygen transfer is occuring in an aeration
basin. Following the modification or replacement of an
aeration system, the actual oxygen transfer efficiency
of the new system can be tested. The five methods
currently used to measure oxygen transfer efficiency
include: 1) steady-state analysis, 2) nonsteady-state
analysis, 3) off-gas analysis, 4) inert gas technique,
and 5) radioactive tracer technique. Information on
these methods is available in other sources (82-84).
Methods of reducing the organic loading should be
investigated before major modifications are made to
an inadequate oxygen transfer system. Also,
operational steps such as cleaning diffusers or
removing rag accumulation on surface mechanical
aerators should be pursued prior to modification
alternatives.
4.4.5.1 Additional Blowers
Addition of more blowers may be appropriate to
address an oxygen deficiency in a diffused aeration
system. The installation of additional blowers to an
existing system will increase the air velocity and
associated headloss in the piping system. This
additional headloss could overload the existing
blowers.
Increased air flow per diffuser may reduce oxygen
transfer efficiency. Oxygen transfer for dome and disc
fine bubble diffusers configured in a grid pattern
decreases significantly with increased air flow per
diffuser. A smaller decrease is seen with flexible tube
and porous plastic fine bubble diffusers. Typically, the
oxygen transfer efficiency for coarse bubble diffusers
is not affected by increased air flow per diffuser.
Reference 30 includes specific information on air flow
per diffuser and its effect on transfer efficiency for
various types of diffusers.
4.4.5.2 Diffused Air System Upgrade
Oxygen transfer can be improved by replacing a
mechanical system with a diffused air system or
replacing a low efficiency diffused air system with a
higher efficiency system. In some cases, replacing an
existing mechanical aeration system with a fine bubble
diffused aeration system can provide the needed
additional oxygen transfer required. If sufficient water
depth is available above the new diffusers [i.e., 3 m
(10 ft) or greater], a fine bubble diffuser system will
be more efficient than a mechanical aeration system.
With this modification, consideration must be given to
the space requirement for blowers and air piping, the
need for soundproofing, the need for increased O&M,
and special ventilation requirements for the blower
room.
In many older suspended growth plants, coarse
bubble aeration systems are utilized. Additional
oxygen transfer can be achieved by replacing these
systems with medium or fine bubble diffusers. In most
cases, the existing blowers can be utilized since the
higher efficiency diffusers require significantly less air
volume.
The 3,945-L/s (90-mgd) Central Wastewater System
Plant, a major wastewater treatment facility serving
the Dallas, TX area, is a case where coarse bubble
diffusers have been replaced with fine bubble diffusers
(104). Effluent discharge limits are 10 mg/L BOD5, 15
mg/L TSS, and 3-5 mg/L NH3-N. Equipment age and
other problems required the replacement of the
existing "snap cap" coarse bubble diffusers, which
had been operating for nine years. Over 50,000
ceramic disc fine bubble diffusers were installed in six
30 m x 90 m (100 ft x 300 ft) aeration tanks at a total
project cost of $1.75 million, plus engineering fees.
Construction was completed in June, 1988. Off-gas
testing of the new aeration system showed an in-
waste oxygen transfer of 16-17 percent. Following
100
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completion of the plant modification, the plant staff
determined that only three of the existing four blowers
would be needed to meet the secondary process air
requirements. Projections show that the new fine:
bubble system will pay for itself in less than four
years.
When designing fine bubble aeration systems, it is
important to consider the potential for diffuser fouling.
Air-side diffuser fouling is caused by dust, oil, rust, or
scale in the air distribution system and from
wastewater that enters the diffuser following an
interruption in power. By exercising care during
construction and by installing air filtration equipment,
air-side fouling can be minimized. Liquid-side diffuser
fouling is caused by oil and grease deposits, growth of
a biofilm on the diffusers, and chemical precipitates.
A variety of cleaning techniques to address diffuser
fouling are available. Some techniques require
interruption of aeration system operation, while others
do not. An additional distinction can be made between
those cleaning techniques that require diffuser
removal (ex-situ) and those that do not (in-situ).
Common in-situ cleaning methods are water hosing,
steam cleaning, and acid cleaning, all of which are
process interruptive. Gas cleaning is an in-situ method
that is process noninterruptive. Both hosing and
steam cleaning effectively remove loosely adherent
liquid-side biological growths. The application of 14-
percent HCI to each ceramic diffuser followed by
hosing or steam cleaning can be effective in removing
both organic and inorganic foulants. Gas cleaning
involves the injection of an aggressive gas (e.g., HGI)
into the air feeder lines. The gas is effective in
removing most foulants with the exception of
gelatinous slimes adhering to the liquid-side of the
diffuser.
Refiring ceramic diffusers, an ex-situ technique, is the
most expensive cleaning method. It involves removal
of the diffusers and heating them in a kiln. The result
is removal of most foulants both inside and outside
the diffuser, and restoration to almost original
condition. Additional information on diffuser cleaning is
available in other sources (38,79). Given the
increased O&M due to fouling of fine bubble diffusers,
small treatment facilities may not find this conversion
to be operationally attractive.
4.4.5.3 Mechanical Aerator Upgrade
The oxygen transfer capacity of some mechanical
aeration systems can be increased through a
combination of operational changes and equipment
modification. Tests conducted at the Kitchener,
Ontario, wastewater treatment plant demonstrated that
the oxygen transfer capacity of existing surface
mechanical aerators could be increased by
approximately 40 percent by refurbishing the old
aerator cones, increasing aerator submergence, and
operating all aerators at a higher rotational speed (85).
Oversubmergence should be avoided since this
condition can result in decreased oxygen transfer.
Other considerations for providing supplemental
aeration to an existing mechanical aeration system
include the use of aspirating type aerators or diffused
aeration around the periphery of the aeration tank.
Aspirating aerators consist of a hollow tube with an
electric motor on one end and a propeller at the other.
The propeller draws air from the atmosphere at high
velocity and injects it underwater where both velocity
and propeller action create turbulence and diffuse the
air into the water. These relatively low-efficiency
aerators are portable and are typically mounted on
floats where they can be positioned at various angles
depending on basin depth and mixing and circulation
requirements.
4.4.5.4 Roughing Filters
Oxygen deficiencies can be corrected by the addition
of roughing filters. This modification is discussed in
Section 4.4.3.2.
4.4.6 Inadequate Return Sludge Flexibility
Return sludge equipment is utilized to control the
distribution of sludge between the aeration basin and
the clarifier. Problems that can result in the violation of
effluent standards are :
• unbalanced solids distribution and excessive
hydraulic loading to the final clarifiers due to
constant speed pumps
• variable flow rates due to plugging of control valves
on the discharge side of constant speed pumps
• variable flow rates due to plugging and creep with
the air flow control, on air lift pumps
• improper control adjustments due to lack of flow
measurement
Typically, modifications required to correct a problem
with a return sludge system are not extensive and can
be completed by plant staff or local contractors.
4.4.6.1 Flow Recycle Around Pump
The use of a flow recycle system around a constant
speed pump can be a simple method of achieving
variable flow control for some return sludge
applications. This concept is presented in Section
4.2.1.1.
4.4.6.2 Adjustable Speed Drives
The most effective method of providing a wide range
of return sludge flow rates is by utilizing pumps with
adjustable speed drives. Existing constant speed
pumps can be retrofitted with equipment to achieve
adjustable speed control, including:
• mechanical adjustable speed drive
101
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adjustable frequency drive
adjustable voltage drive
wound-rotor induction motor with liquid rheostat
eddy current coupling
hydraulic coupling
The selection of the appropriate type of speed control
Is dependent on speed range, cost, equipment
mounting, compatibility with existing motor, space
requirements, and efficiency. Other sources provide
detailed information on selection of speed control
equipment (2,39).
4.4.6.3 Time Clocks
AJr-lift pumps are often used in package plants for
returning sludge; however, control is often difficult.
Frequently, high return sludge rates are maintained to
avoid plugging. Excessively high return sludge rates
commonly result in poor sludge settling conditions in
the final clarifier and violation of SS permit limits.
Improved control has been achieved with the addition
of a solenoid valve on the air line to the air-lift pump
with a time clock to control valve operation. This
modification was successfully implemented at three
package plants in Montana. Clarifier blanket levels
decreased over 1.2 m (4 ft) at each plant, allowing
each facility to meet their permit requirements.
A time clock can also be used to adjust the run time
of oversized constant speed pumps. The motor
manufacturer should be contacted to determine the
maximum allowable motor starts per hour before this
type of operation is initiated.
The use of time clocks to control the operation of
return sludge pumps will be most beneficial when the
associated clarifiers are being operated at low surface
overflow rates. The use of intermittent recycle
pumping introduces hydraulic transients in the final
clarifier, and these transients can degrade the
performance of a clarifier being operated at high
surface overflow rates.
4.4.6.5 Flow Measurement
Continuous flow measurement capability is necessary
to optimize return sludge control. The guidelines for
return sludge flow measurement are similar to those
discussed in Section 4.4.1.6. for waste sludge flow
measurement.
4.4.7 Excessive Clarifier Hydraulic Currents
Extreme currents can develop as the result of density
differences and the hydraulic characteristics of the
clarifier inlets and outlets (40). The typical
characteristics of currents in center-feed, peripheral
overflow clarifiers is graphically shown in Figure 4-27.
In this situation, the influent experiences a "water fall
effect" whereby the flow drops to the clarifier bottom
and moves along the top of the sludge blanket and up
along the far end of the basin wall to the effluent
weirs. As shown in Figure 4-28, this situation can also
occur in rectangular clarifiers where the overflow weirs
are located near the far end of the basin. Excessive
currents can contribute to loss of solids at the effluent
weir.
Figure 4-27. Typical flow pattern for center-feed, peripheral
overflow clarifier.
Center Line
Figure 4-28. Typical flow pattern in rectangular clarifier.
l_
V
^"••••^7r->- — — ».
U-jU^U
4.4.6.4 Multiple Pumps for Return Sludge
Pumping
A common problem with plants that have a single,
constant speed pump for returning sludge is that the
pump is designed for a flow rate approaching design
capacity. Start-up conditions and variable sludge
characteristics require a return sludge pumping range
of 25-125 percent of the influent flow rate. Typically,
a single, constant speed pump is adjusted by throttling
a discharge valve. This practice is not energy efficient
and frequently results in plugging of the throttling
valve. The use of multiple pumps of variable
capacities can provide more flexible return sludge flow
rates. This arrangement, when used in combination
with some discharge valve throttling, results in
reasonable variable return rate capability.
Dye and SS testing can be used to identify excessive
currents. These testing procedures have been utilized
in field tests (41-43). Modifications to correct identified
problems have produced mixed results. Modifications
that have been utilized include changes to inlet
structures and effluent weirs, and installation of
baffles.
4.4.7.1 Inlet Modifications
Inlet configurations are important to the proper
performance of a clarifier. The ideal inlet should
achieve both horizontal and vertical distribution of the
incoming flow across the entire cross-sectional area,
while minimizing short-circuiting and turbulence
conditions (2). Inlets for circular clarifiers can be either
a center or peripheral configuration. With the center
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feed configuration, the influent is introduced through
either the side or bottom of a center feedwell. Center
feed clarifiers with inlet pipes that enter the feedwell
from the side can create asymetrical flow distribution.
The influent to peripheral feed clarifiers is distributed
through orifices located in an inlet raceway that
extends around the periphery. Inlet configurations for
rectangular clarifiers usually consist of baffled inlet
ports or gates.
A research project at the Stamford, Connecticut
wastewater treatment plant tested a full scale
"distributed inlet" in one of the facility's center feed
secondary clarifiers (44). The distributed inlet
consisted of a full-depth feed well with baffled outlet
ports spaced evenly from top to bottom. The special
inlet design failed to produce a uniform flow pattern
throughout the clarifier depth and actually resulted in a
negative effect on performance.
Field experience with the modification of inlet
structures to rectangular clarifiers has also resulted in
little improvement. At the Herkimer, New York
wastewater treatment plant, the vertical inlet baffles
were installed in front of the inlet slots to reduce
turbulence; however, the modification did not result in
any detectable change in clarifier performance (43).
Some improvement in performance was achieved
from a small rectangular clarifier when an L-shaped
reaction baffle was located in front of each inlet slot
(41). The improved performance in this case was
attributed to a reduction in scouring of the sludge
sump.
A recent trend in secondary clarifier inlet design has
included the enlargement of the center feedwell to
form a flocculation chamber (40,45). Advantages
claimed for the chamber are enhanced flocculation of
incoming solids and dissipation of inlet energy. The
concept of the flocculator-clarifer is shown in Figure 4-
29.
Figure 4-29. Secondary clarifier with flocculating center well
Influent
adjustable side ports into the flocculator center well.
By 'varying the gate openings in the diffusion well, the
flocculating energy can be increased or decreased in
the larger flocculator center well. Typically, the
diameter of the flocculator center well is 25-35
percent of the tank diameter and results in a 20-30
minute detention time. The depth of this center well
can be approximated as one-half the tank depth near
its center for clarifiers with depths of 4.6 m (15 ft) or
greater (40).
Performance data from both suspended growth and
TF/SC plants utilizing flocculator-clarifiers show that
effluents with average TSS concentrations of 10 mg/L
or less can be achieved at overflow rates up to 48
m3/m.2/d (1,200 gpd/sq ft) (45). The flocculator-clarifier
concept can be incorporated into an existing
secondary clarifier. However, to realize the full
benefits, a minimum depth of 4.6 m (15 ft) is
recommended.
4.4.7.2 Baffle Addition
Recent field studies have shown that installation of
mid-tank baffles can achieve some success in
reducing the bottom currents and subsequently
improving clarifier performance. Bender and Crosby
(41) modified a 24.4-m (80-ft) diameter center feed,
circular clarifier by installing a mid-radius baffle that
extended from mid-depth to just off the clarifier
bottom. The baffle rotated with the sludge collector
mechanism. The mid-radius baffle configuration is
shown in Figure 4-30. The effect of this baffle
configuration was the creation of a floe center well
and the breaking of the interface curents. Comparative
studies showed a 37.5 percent reduction in TSS in
the baffled clarifier.
Figure 4-30. Mid-radius baffle for circular clarifier.
Center Line
Influent enters the diffusion well, which is typically 10-
13 percent of the tank diameter and closed at the
bottom. Flow exits this compartment through
Esler and Miller installed a mid-tank baffle in a 47.8-m
(157-ft) long rectangular clarifier being used in a high-
purity oxygen activated sludge plant (43). Dye studies
conducted on the clarifier indicated a bottom current
of 3.6 m (12 ft)/min and a reverse current at the
surface of 1.5 m (5 ft)/min. The baffle was located
from just below the water surface to slightly above the
bottom sludge collection mech.-anisms and consisted
of 15-cm (6-in) plywood strips spaced 15 cm (6 in)
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apart for the bottom half of the baffle and 20 cm (8 in)
apart for the top half. A schematic diagram is shown
in Rgure 4-31. Currents were reduced prior to the
baffle, and little current detection could be identified
downstream of the baffle. Comparative TSS tests
showed a 20-39 percent improvement in removal
efficiency. The baffle was relocated to the first third of
the clarifier length, and continued to demonstrate
improved performance. However, because of higher
reverse currents in the downstream section of the
clarifier, the mid-tank baffle was recommended over
the "first third" baffle.
Figure 4-31. Mid-tank baffle for rectangular clarifier.
Mid-Tank i
Baffle |
u u u
The use of baffles near effluent weirs has also proven
successful. Crosby (40) installed a baffle to direct flow
away from a peripheral weir located on a 39.6-m (130-
ft) diameter center-feed clarifier. This baffle
configuration is shown in Figure 4-32a. A 38.3 percent
decrease in SS was achieved over the effluent SS
from a parallel clarifier without a baffle. Crosby
recommends that the baffle be located no lower than
1/3 the distance from the weir to the bottom of the
clarifier (42).
A horizontal weir baffle was modeled and studied by
the University of Kansas (40). This configuration is
shown in Figure 4-32b. The baffle extends horizontally
from the edge of the effluent trough a distance of
approximately two feet. As discussed in the following
case history, sludge accumulation on the horizontal
baffle may occur, which, in turn, could affect clarifier
performance.
Case History: City of Helena, Montana Wastewater
Treatment Facility
The Helena, Montana, wastewater treatment facility
includes two 22.9-m (75-ft) diameter secondary
clarifiers with a 3.2-m (10.5-ft) sidewater depth. The
plant staff installed a deflection baffle along the
periphery of the units to try and improve TSS
removals. The horizontal fiberglass baffle was located
approximately three feet below the water surface, and
it extended approximately two feet out from the
clarifier wall. The baffle was similar to the design
shown in Rgure 4-32a with the exception that the
Helena baffle was installed perpendicular to the
clarifier wall.
An improvement in effluent turbidity was observed;
however, when the clarifiers were dewatered for
inspection, approximately 15 cm (6 in) of sludge was
detected on top of the horizontal baffles. The plant
Figure 4-32. Baffled weir configurations.
Center
Line
a. Baffled Weir Demonstrated by Crosby (42)
b. University of Kansas Horizontal Weir Baffle (40)
staff recommended that it would be advantageous to
direct the baffle slightly downward to minimize
deposition.
4.4.7.3 Weir Relocation/Addition
The relocation or addition of weirs can be used to
improve clarifier performance. Rectangular clarifier
weir location is usually dependent on the sludge
collection mechanism. Figure 4-33 shows a cross-
section of a typical rectangular clarifier with a sludge
collector mechanism that travels from the back to the
front of the clarifier. On the return trip, the mech-
anism travels along the surface to collect scum and
then drops below the water surface to allow space for
the weirs. This design limits the available space for
overflow weirs. Solids loss in this type of clarifier
typically occurs as the flow moves along the top of
the sludge blanket, resuspending settled solids, which
are carried up the end wall toward the weirs (43). This
condition is shown graphically in Figure 4-28.
Providing additional weirs in rectangular clarifiers is
usually not practical. In some situations, performance
has been improved by blocking weirs that are located
near the end walls. Research indicates that the
velocity of the flow current along the top of the sludge
blanket is considerably higher than near the overflow
weirs; consequently, greater performance improve-
ment may be achieved by taking steps to reduce the
velocity of flow above the sludge blanket (42).
104
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Figure 4-33. Typical rectangular clarifier.
Influent
Channel
I
t
_ U U U
1 \
; \
; " x . •
; xx.
i x'~..
O *. _. ._._.--D
The addition of weirs may be appropriate for center
feed, peripheral overflow clarifiers that are
experiencing solids loss. The flow pattern for this type
of clarifier is shown in Figure 4-27. The flow radiates
outward along the top of the sludge blanket and up
the outside wall, frequently carrying solids over the
weirs. One method of reducing the wall effect is to
use cantilevered double or multiple weirs, as shown in
Figure 4-34. Cantilevered weirs located a distance of
at least 20 percent of the clarifier radius from the
outside wall have reportedly worked well (40). The
addition of a cantilevered weir could be an expensive
modification because of the required new weir,
structural supports, and changes to the scum
collection mechanism. This modification does not
allow mechanical scum removal for the annular area
between the weirs and the side of the tank.
Figure 4-34. Center-feed clarifier with cantilevered weir.
4.4.8 Basin Mixing
Proper mixing is necessary to assure that mixed liquor
solids are kept in suspension. Mixing is provided by
diffused aeration equipment, mechanical aerators, or a
combination of mechanical and diffused aeration
equipment (i.e., submerged turbines). Inadequate
mixing is seldom found to be a significant
performance limiting factor even in cases where minor
deposition of slids occurs. All other factors that could
be affecting plant performance should be evaluated
before any major modifications are made to improve
mixing in an aeration basin.
Aeration system design must consider the energy
required to supply oxygen and to provide mixing. By
meeting the requirements shown in Table 4-8,
satisfactory mixing should be available in most
aeration basins (2).
Table 4-8. Aeration Basin Mixing Requirements
Type of Equipment Minimum Mixing Requirement
0.12 scfm/sq ft floor coverage
20 scfm/1,000 cu ft
0.5 hp/1,000cu ft
Fine Bubble Diffused Aeration
Coarse Bubble Diffused Aeration
Mechanical
Correcting problems associated with inadequate
mixing is limited to the addition of energy into the
water or, in some cases, the use of baffles to
redistribute energy in an existing basin. If additional
energy is required in an aeration basin, the use of
submerged or floating mechanical mixers should be
investigated. Section 4.5.2.1 contains information on
mechanical mixers.
4.4.9 Inadequate Scum Removal
Under some, conditions significant scum can develop
on the surface of both primary and secondary
clarifiers. Typically, primary clarifiers are fitted with
adequate scum handling equipment. However, scum
handling equipment is frequently missing or
undersized on secondary clarifiers. Scum
accumulations can significantly contribute to effluent
SS concentrations, especially in systems that operate
at high mean cell residence times and do not have
primary elarifiers (i.e., package plants).
Scum can be removed in aeration basins, channels,
and clarifiers by the use of mechanical skimmers. The
skimmers located in aeration basins, channels, and
rectangular clarifiers typically consist of slotted pipe
that can be rotated to collect scum from the water
surface. Most scum collection designs for circular
clarifiers include a rotating collection arm, a collection
beach that is located near the periphery, and a scum
baffle located adjacent to the weir. Scum moves to
the outside of the clarifier surface where it is collected
by the beach. If heavy scum conditions exist, much of
the collected scum can pass under the collection
beach and the scum baffle. Properly designed scum
baffles should allow for a minimum of 15 cm (6 in) of
baffle above and below the water surface, especially
near the collection beach. The use of a full radius
scum collection device is most effective since scum is
removed across the full radius of the clarifier once per
each revolution of the collector mechanism.
To be effective, scum should not be reintroduced to
the system once it is removed. Removal can be
accomplished by directing the scum to a solids
handling process; however, scum frequently includes
large volumes of water which can have a detrimental
effect on solids handling. Scum volume can be
105
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reduced by installing a scum storage basin with
decanting capability.
4.5 Wastewater Stabilization Ponds
Wastewater stabilization ponds are a commonly used
treatment process for many small communities. Ponds
are popular because of their ability to handle a wide
range of hydraulic and organic loads with minimal
effect on effluent quality, their relatively low capital
cost, and their low operation and maintenance
requirements. Types of wastewater stabilization ponds
vary based on the method of mixing and aeration, the
detention time, and the number and arrangement of
individual cells. The two main types are facultative and
aerated ponds.
Facultative ponds consist of medium depth [0.9-2.1 m
(3-7 ft)] treatment cells that rely on photosynthesis
and surface contact with the atmosphere for their
source of oxygen. In aerated ponds, the depth of
individual cells is 1.5-6 m (5-20 ft), and aeration is
provided by a combination of mechanical aeration and
natural processes. Detention time and organic loading
for stabilization ponds is typically established by using
empirical and rational design models. Additional
design information is available from several sources
(2,47,48).
Wastewater stabilization ponds utilize a variety of
natural processes to treat the wastewater and,
therefore, limited process controls are available to
adjust system performance. Consequently, the ability
of wastewater stabilization ponds to meet discharge
limitations is largely dependent on the flexibility
provided in the facility design.
Table 4-9 summarizes design limitations/corrective
modifications that are prioritized based on their
potential for achieving improved performance.
Modifications with proven reliability and those that are
less construction intensive appear at the beginning of
the listing.
4.5.7 Undersized Ponds
Undersized ponds may be contributing to poor
performance if a unit is operating at an organic loading
rate higher than design for the type of facility in that
climatic zone. Unfortunately, exact organic loading
rates to meet performance requirements vary
considerably. Table 4-10 summarizes typical design
organic loading rates for wastewater stabilization
ponds. In addition to these values, other factors such
as aeration capacity, short-circuiting, and waste-water
temperature must be carefully analyzed for their
impact on facility performance.
In a community with industrial contributors,
consideration should be given to pretreatment options
before modifications are made. The following case
history presents an example of the impact that an
Table 4-9. Stabilization Pond Design Limitations and
Potential Modifications
Design Limitation/Potential Modification
Page No.
4.5.1 Undersized Ponds
4.5.1.1 Improved Utilization of Existing Volume
4.5.1 .2 Additional Treatment Cell Volume
4.5.1.3 Hydrograph Controlled Release
4.5.1.4 Aquaculture
4.5.2 Inadequate Oxygen Transfer Capability
4.5.2.1 Mechanical Mixers
4.5.2.2 Additional Aeration Equipment
4.5.3 Short-Circuiting
4.5.3.1 Baffles
4.5.3.2 Mixing Equipment
4.5.3.3 Multiple Inlets and Outlets
4.5.4 Inadequate Suspended Solids Control
4.5.4.1 Controlled Discharge
4.5.4.2 Variable Level Draw-Off
4.5.4.3 Aquaculture
4.5.5 Inflexible Mode of Operation
4.5.5.1 Series/Parallel Operation
4.5.5.2 Recycle Capability
4.5.6 Low Pond Temperature
4.5.6.1 Short-Term Diffused Aeration Cells
106
108
108
109
109
110
110
110
110
110
111
111
111
112
112
112
113
113
113
• 114
114
Table 4-10. Design Organic Loading Rates for Wastewater
Stabilization Ponds
Design BOD5
Source Loading Rate
kg/ha/d (Ib/ac/d)
Facultative Ponds
EPA Design Manual (47)
Ave. Winter Air Temp. > 15°C (59°F) 45-90 (40-80)
Max. first cell loading 101(90)
Ave. Winter Air Temp. 0-15°C (32-59°F) 22-45 (20-40)
Ave. Winter Air Temp. <0°C (32°F) 11-22 (10-20)
Recommend detention time - first cell 120-180 days
Max. BOD5 loading - first cell 37 (35)
Recommended Stds. for Sewage Works (50)
Primary pond(s) 17-39 (15-35)
Aerated Ponds
Metcalf & Eddy (49)
Typical values (higher values also used) 34-112 (30-100)
EPA Design Manual (47)
Loadings for case histories 86-184 (77-164)
industrial waste can have on stabilization pond
performance.
Case History: City of Sterling, Colorado Aerated
Stabilization Pond Facility (51)
The City of Sterling, Colorado provides wastewater
treatment by the use of an aerated pond facility. The
facility schematic is shown in Figure 4-35. Treatment
is provided by two aerated cells and one settling cell.
Aeration is provided by a diffused air system and
supplemental surface mechanical aerators. The City
has the flexibility to discharge treated water to
recharge ponds or to a receiving stream. The design
BOD5 loading to the facility is 6,170 kg (13,600 lb)/d
[560 kg/ha/d (500 Ib/ac/d)]. A local meat packing
106
-------
Figure 4-35. Schematic of Sterling, Colorado wastewater stabilization pond system.
Influent
Control
Bldg.
Cell #2
Level Control
Structure
Outfall
Headline
Microscreen
Bldg.
To Recharge Site
industry accounts for up to 80 percent of the BOD5
loading. The industry has on-site pretreatment
facilities (e.g., air flotation with chemical feed
equipment) which are capable of removing 70 percent
of the BOD5 load.
Prior to 1986, minimal efforts were made by the
industry to operate their pretreatment facilities, and, as
a result, design BOD5 loadings to the city's facility
were frequently exceeded. During the end of 1986
and all of 1987, the industry placed a higher priority
on operation of the pretreatment equipment to reduce
their surcharge costs to the City. As shown in Table
4-11, a 39 percent decrease in BOD5 loading was
achieved in 1987 versus 1986 loadings. The TSS
loading decreased by 27 percent during this same
period.
Prior to improved operation of the pretreatment
equipment, aeration capacity at the city facility was
frequently inadequate. During the high loading period
in 1986 (January-July), the DO level in the first
aerated cell was near zero. With improved
pretreatment, the aeration equipment at the facility
was capable of achieving a residual DO of about 2
mg/L in the first cell.
Performance of the city's facility has improved. In
1986 the average BOD5 and TSS concentrations from
the settling cell were 37 mg/L and 34 mg/L,
Table 4-11.
Month
January
February
March
April
May
June
July
August
September
October
November
December
Average
Loading to
Stabilization
1986
TSS
16,589
20,201
20,894
18,954
17927
15,013
13,870
9,491
13,324
1 1 ,685
10,839
9,085
14,822
the Sterling, Colorado
Treatment Facility
(Ib/d) 1987
BOD5
14,091
17,554
20,566
15,755
• 14,610
11,976
10,870
7,188
10,927
10,647
9,099
6,815
12,508
TSS
1 0,644
12,643
12,893
5,100
2,607
5,861
8,774
9,583
9,459
10,390
10,090
9,915
8,997
Wastewater
(Ib/d) -
BODg
7,554
10,452
12,003
6,231
3,809
7,175
1 1 ,064
11,739
9,543
9,531
11,102
8,842
9,087
respectively. In 1987, these values decreased to 31
mg/L and 20 mg/L. Periodic performance problems
still occur because of significant sludge deposits
located in the first and second treatment ponds. The
operations staff is currently experimenting with mixing
equipment to reduce the effects of these sludge
deposits on performance.
If administrative solutions to correct a loading
problem, such as the previous pretreatment case
history, are not feasible, modifications to the
107
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wastewater stabilization facility will probably be
required. The following sections present information
on modifications to correct undersized ponds.
4.5.1.1 Improved Utilization of Existing Volume
Short-circuiting is a common problem that occurs with
wastewater stabilization ponds. This topic is discussed
in Section 4.5.3; however, it is mentioned here since,
in many cases, it is more cost effective to correct
short-circuiting than to construct additional ponds.
When short-circuiting occurs in wastewater
stabilization ponds, the actual total volume in the
ponds is not used effectively. Short-circuiting can
exist because of pond configuration and temperature
stratification. An example of poor pond configuration is
shown in Figure 4-36. The design BOD5 loading for
this facility is 168 kg/ha/d (150 Ib/ac/d); however,
because of the location of inlet pipes and the square
shape of the cells, the effective volume of the ponds
is substantially reduced. In this example, the BOD5
loading could conceivably be as high as 526 kg/ha/d
(470 Ib/ac/d), or about three times the design loading
rate
Figure 4-36. Effect of short-circuiting on organic loading.
Flow » 22 Us (0.5 mgd)
BODS « 250 mg/L
Design Organic Loading - 168 kg/ha/d (150 Ib/ac/d)
_ . ... Effective Area
Surface Aerator
Influent
Flow
Utilized Area=0.52 ha (1.3 ac) Utilized Area=0.36 ha (0.9 ac)
Probable BODS Load @ Design Conditions = 526 kg/ha/d
(470 Ib/ac/d)
Short-circuiting effects shown in Figure 4-36 could be
improved through modification to the cell inlets and
installation of baffles to form three or more treatment
cells. Section 4.5.3 provides information on installing
baffles in existing ponds.
Temperature stratification in facultative ponds occurs
during the spring and fall of the year and can result in
ineffective utilization of treatment volume. During
these periods, improved mixing can be used to
maintain desired system performance. Section 4.5.3.2
includes information on the use of mechanical mixers
in ponds. h
In older stabilization ponds or in heavily loaded
facilities, the volume available for treatment can be
reduced significantly because of solids deposition.
This situation typically occurs in the initial treatment
cells. Solids removal can be accomplished through
dewatering the cells and removing the dry solids, or
by pumping the solids from the cell with the use of
dredging equipment.
4.5.1.2 Additional Treatment Cell Volume
Additional cell volume will be required to improve
performance if addressing short-circuiting is
ineffective. Several design approaches are available
for sizing pond systems, including: 1) Areal loading
rate, 2) Gloyna equation, 3) Marais & Shaw equation
(complete mix model), 4) Plug flow model, and 5)
Wehner-Wilhelm equation. Descriptions of these
approaches are presented elsewhere (47).
Additional cell volume can be achieved by expanding
the volume of existing cells or by adding new cells.
The performance of existing systems and the design
equations indicate that overall performance improves
with the number of cells in series; consequently, it is
recommended that additional cells be added. The
following design example, using the complete mix
model, demonstrates the approach for adding cell
volume:
Design Example:
Description of Existing Pond System:
Design Flow = 22 Us (0.5 mgd)
Influent BOD5 = 200 mg/L
Effluent BOD5 Limit = 30 mg/L
First Cell Volume (aerated) =
22,710 m3 (6,000,000 gal)
Second Cell Volume (aerated) =
22,71 Om3 (6,000,000 gal)
Critical Cell Temperature = 1°C (34°F)
Reaction Coefficient k (base 10) @ 20°C = 0.2 d-1
Reaction Coefficient k (base 10) @ 1°C = 0.042 d-1
Detention Time (t) 1st & 2nd Cells = 12 d
Expected Performance from Existing System per
Complete Mix Equation:
Ce= C,-*(1 + 2.3kt)
First Cell Effluent BOD5 = 93 mg/L
Second Cell Effluent BOD5 = 43 mg/L
Alternative 1: Increase the system volume by adding
15,140 m3 (4,000,000 gal) of capacity
to the first cell and 7,570 m3
(2,000,000 gal) to the second cell.
First Cell Volume (aerated) =
37,850 m3 (10,000,000 gal)
First Cell Detention Time (t) = 20 d
Second Cell Volume (aerated) =
30,280 m3 (8,000,000 gal)
108
-------
Second Cell Detention Time (t)= 16 d
First Cell Effluent BOD5 = 68 mg/L
Second Cell Effluent BOD5 = 27 mg/L
Alternative 2: Increase the system volume by adding
a third cell with a volume of 22,710
m3 (6,000,000 gal). No changes to the
first and second cells.
First Cell Volume (aerated) =
22,710 m3 (6,000,000 gal)
Second Cell Volume (aerated) =
22,710 m3 (6,000,000 gal)
Third Cell Volume (aerated) =
22,710 m3 (6,000,000 gal)
First Cell Effluent BOD5 = 93 mg/L
Second Cell Effluent BOD5 = 43 mg/L
Third Cell Effluent BOD5 = 20 mg/L
The model indicates better performance can be
expected from the three-cell modification (i.e.,
Alternative 2) than from the expanded two-cell
modification (i.e., Alternative 1) even though both
alternatives increase total cell volume by the same
amount.
4.5.1.3 Hydrograph Controlled Release
Hydrograph-controlled release (HCR) systems can be
used to allow pond facilities to meet discharge
requirements by managing the discharge of
wastewater in accordance with the receiving stream's
assimilative capacity (80,107). An HCR facility
restricts discharge when flow in the receiving stream
is low and stream assimilative capacity is limited.
Conversely, as flow increases, previously stored flow
is released. HCR system components include an HCR
lagoon, stream flow monitoring system, and effluent
discharge system.
A schematic of a pond facility incorporating the HCR
concept is shown in Figure 4-37. HCR ponds are
designed for variable depth operation (i.e., minimum
depth of 0.6 m (2 ft), maximum depth of 2.4 m (8 ft)).
Pond size is related to both stream and plant flows.
Detention times in the pond can typically range from
30 days to in excess of 120 days, based on the
average plant flow (80).
Existing facilities can be modified to operate with HCR
flexibility if they have sufficient storage volume.
Modifications to the effluent control structure will
probably be required. If sufficient storage volume does
not exist, expansion of existing ponds or the addition
of a new pond would be required.
In some cases, local regulatory agencies may not
allow plant discharges to be varied in accordance with
stream flow. The capability of obtaining a flow-
Figure 4-37. Wastewater stabilization pond system with HCR
flexibility.
Treatment Ponds
Influent
Receiving
Stream ,
Vs/NX^ ™
HCR Pond
1 1 HWL I \
\ ' LWL J ^—
regulated permit must be established prior to
consideration of the HCR alternative.
4.5.1.4 Aquaculture
Aquaculture can be used to improve performance of
existing wastewater stabilization ponds. The Brazilian
water hyacinth, which is commonly used in
aquaculture systems located in sub-tropical and
tropical regions, is very sensitive to temperature and
does not grow in water with a temperature of 10 °C
(50°F) or lower (53). The American hyacinth,
however, is more cold-tolerant (106). Use of water
hyacinths requires consideration of loading rate, pond
depth, plant harvesting, and disposal. A wastewater
stabilization pond facility could utilize water hyacinths
in an existing cell, or a new cell(s) could be
constructed.
Water hyacinth treatment systems have produced
effluents meeting secondary standards when loaded at
31-197 kg BOD/ha/d (28-176 Ib/ac/d) (52). In cases
where frequent harvesting is practical and good
contact between plants and nutrients is attained,
significant nitrogen and phosphorous removal can also
be achieved. Depths of hyacinth ponds vary from 0.3
to 1.8 m (1 to 6 ft), with 0.9 m (3 ft) being commonly
recommended. Good practice calls for the root
system of the water plants to penetrate the entire
depth.
The main disadvantage of utilizing water hyacinths is
the harvesting and disposal requirements for the
excess plants. These plants are very prolific, and
harvesting is required at least once per month during
rapid growing seasons and more frequently if
significant nutrient removal is a treatment objective.
For small communities, the use of solids handling
processes such as digestion and composting to treat
the excess hyacinths may not be feasible. Storage or
disposal of the excess hyacinths on site frequently
results in the production of odors and unsightly
conditions. Unless a reliable method of excess
hyacinth disposal is available, other modifications to
correct the undersized ponds should be investigated.
109
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The use of the duckweed plant in stabilization ponds
has been proposed for removal of TSS and
associated BOD5 nutrients, and heavy metals (53).
Duckweed plants form a mat over the pond surface,
and, under the right conditions, this mat can be very
effective in blocking sunlight from penetrating the
pond, thus controlling the growth of algae. The
presence of a duckweed results in anaerobic
conditions due to the lack of surface reaeration and
the low oxygen production from the duckweed plants.
If an existing pond system is to be retrofitted to
include duckweed for solids control, consideration
should be given to the reduced volume available for
BODS removal. Consequently, if soluble BOD5
removal is part of the performance problem, the use
of duckweed may not be appropriate. Duckweed
appears to be most applicable for situations where
algal solids present TSS problems (54). The use of
duckweed for the control of algae is discussed in
Section 4.5.4. Other innovations in the use of
aquaculture can be found in Design Manual:
Constructed Wetlands and Aquatic Plant Systems for
Municipal Wastewater Treatment (108).
4.5,2 Inadequate Oxygen Transfer Capability
Facultative pond systems utilize photosynthesis and
surface contact with the atmosphere to provide the
necessary oxygen for the treatment process. If
facultative ponds become deficient in oxygen due to
overloading, supplemental aeration can be added by
the use of surface mechanical aerators or diffused
aeration systems. Caution is advised when trying to
incorporate mechanical aeration equipment into
facultative pond systems. The shallow depth found in
most ponds is not suitable for diffused aeration
systems because of the low transfer efficiency, and
the installation of surface mechanical aerators can
result in erosion damage to the pond bottom. Because
of these disadvantages it may be more appropriate to
investigate additional pond volume.
Aerated pond systems typically rely on mechanical
aeration equipment to supply oxygen and mixing.
Aeration requirements are usually determined by
multiplying the expected BOD5 removal per treatment
cell by an oxygen demand factor (e.g., 1.5 kg O2/kg
BOD5 removed is a commonly used value).
Horsepower required is determined by converting
standard oxygenation rates for specific equipment to
actual oxygenation rates. This procedure is presented
in Appendix F.
4.5.2.1 Mechanical Mixers
Frequently, the horsepower included in an aerated cell
is provided to meet the oxygen requirement with little
attention given to mixing. As such, the horsepower
provided can be considerably less than that required
for complete or even partial mixing. Limited mixing
capability results in inadequate dispersion of available
oxygen throughout the entire cell volume.
Mechanical mixers can be used to enhance oxygen
transfer in a poorly mixed aerated cell. Typically, with
surface mechanical aerators, only 40-50 percent of
the shaft energy is actually applied to mixing. In
contrast, surface mechanical mixers apply almost all
of the shaft energy for mixing. It is estimated that the
mixing capability of surface mechanical mixers is two
to three times greater than the mixing capability for
surface mechanical aerators of the same horsepower
(56).
4.5.2.2 Additional Aeration Equipment
Types of aeration equipment utilized in aerated pond
systems can be described as either surface
mechanical or diffused aeration. Detailed information
on available aeration equipment can be obtained from
other sources (2,48). Selection of supplemental
aeration equipment is dependent on the existing
system, energy requirements, cold weather operation,
installation requirements, and O&M requirements.
Surface mechanical aerators can be added while the
facility remains in operation. Installation requires an
electrical power connection and mooring of the
aerator to the banks. In cold climate areas, ice build-
up can result in equipment failure; however, heaters
can be installed to limit this condition. Those aerators
that utilize submerged motors or that inject air into the
water (aspirator type) typically do not experience
significant cold weather problems.
Diffused aeration systems can be considered where
sufficient pond depth is available to provide oxygen
transfer (e.g., depths greater than eight feet).
Installation of a diffused aeration system typically
requires dewatering; consequently, flexibility is needed
to bypass during installation. Occasionally, diffuser
breakage will occur; therefore, some method of
repairing and monitoring the difference must be
provided. Diffused aeration systems have an
advantage over most surface mechanical systems in
conserving wastewater temperature. This advantage is
discussed in more detail in Section 4.5.5.
4.5.3 Short-Circuiting
Short-circuiting is a common problem in wastewater
stabilization ponds. Impact of this problem on effective
utilization of pond volume was discussed in Section
4.5.1.1. Other effects include ineffective use of
aeration equipment, promotion of algae growth in
quiescent areas, and scum accumulation. In many
wastewater stabilization ponds, existing performance
can be improved by minimizing short-circuiting.
4.5.3.1 Baffles
The most effective method of correcting short-
circuiting is the use of baffles. Various materials have
been used for baffle walls including metal, fiberglass,
and plastic. Recently, several manufacturers have
introduced baffles made of reinforced plastic material
that are supported by a continuous float at the water
110
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surface and a chain weight at the bottom. Each baffle
wall is held in place by anchors located at the pond
dikes. Accurate field measurements must be taken to
determine the correct length and depth of individual
baffle walls since the depth of water may vary slightly
because of sludge deposits. Designs should also
include consideration for icing conditions that can
cause damage to the baffle walls.
Numerous flow configurations can be achieved with
baffles. Two possible configurations are shown in
Figure 4-38. In the parallel pattern, baffle walls are
used to divide an existing cell and thus the influent
load into three separate cells. This arrangement can
be used when additional dilution of a high strength
waste is desirable at the beginning of the first
treatment cell. In the serpentine arrangement, baffle
walls are used to form a plug flow pattern through the
existing cell. This arrangement does not require flow
splitting and often better performance can be
expected because of the multiple cells in series.
Organic loading is highest at the beginning of the first
cell, and decreases gradually through the system.
Aeration equipment must be proportioned accordingly
to meet the anticipated oxygen demand.
Figure 4-38. Modification of ponds with baffle walls.
Baffle Walls
contributing to short-circuiting, as shown in Figure 4-
36. Multiple inlets and outlets can improve utilization
of cell volume. Figure 4-39 illustrates four inlet and
outlet points provided along the full width of the cell.
More positive control of flow distribution could be
accomplished by including valves on the individual
inlets and outlets. Dispersion could then be optimized
by utilizing a dye study to indicate appropriate valve
adjustments. Figure 4-39b illus-trates the use of
distribution pipes with orifices for the inlet and outlet
structures. This approach would provide more even
distribution than the multiple inlet/outlet alternatives.
The inlet and outlet orifice pipes must be hydraulically
designed to equally distribute and collect the
wastewater. Primary ponds as large as 36 ha (90 ac)
have successfully used this type of flow distribution
for over 20 years (47).
Figure 4-39. Inlet/outlet configurations.
a. Multiple Inlet/Outlet
a. Parallel Flow Pattern
b. Inlet and Outlet Pipes w/Orifices/Eftluent Baffle
Baffle Walls
b. Serpentine Flow Pattern
4.5.3.2 Mixing Equipment
Short-circuiting can be reduced by the addition of
more mixing energy. Guidelines for power
requirements for mixing can be found in several
sources (2,47,56). A minimum power level of 0.0004
KW/m3 (2 hp/Mgal) is recommended for oxygen
dispersion in aerated ponds, and a minimum power
level of 0.003 W/m3 (15 hp/Mgal) is recommended to
maintain solids in suspension (56). Equipment
manufacturers should be contacted for specific
information.
4.5.3.3 Multiple Inlets and Outlets
The existence of only one inlet and one outlet in a
square cell configuration can be a major factor
4.5.4 Inadequate Suspended Solids Control
High concentrations of-algae and the resulting high
SS levels in wastewater stabilization ponds
periodically affect the performance of these facilities.
This situation typically occurs during warm weather
conditions which promote the growth of algae, and it
is also cyclical during this time as the predominance
of various organisms in the system changes. A
variance is allowed in the Federal TSS effluent
requirements for pond systems less than 44 L/s (2
mgd) in capacity. However, even with this variance,
effluent TSS levels can occasionally exceed discharge
permit limits.
Alternatives available for the control of TSS in pond
effluents include:
Chemical addition
Controlled discharge
Aquaculture
Variable level draw-off
Intermittent sand filtration
Land application (except overland flow)
111
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A comprehensive review of available technology can
be found in Design Manual: Wastewater Stabilization
Ponds (47). The following approaches can be
implemented before more capital intensive alternatives
are recommended.
4.5.4.1 Controlled Discharge
Controlled discharge of pond effluent can be an
effective means of meeting effluent TSS limitations for
facilities that have sufficient volume to store
wastewater for an extended period of time (e.g., one
to three months). Studies of controlled discharge from
facilities in Michigan and Minnesota have shown that
effluent TSS concentrations of less than 30 mg/L can
be achieved (47). Section 4.5.13 includes information
on hydrograph-controlled release facilities.
For wastewater stabilization ponds that are not
designed to allow controlled discharge, the selective
release of wastewater from multiple treatment ponds
frequently can be an effective approach to controlling
effluent TSS. In multiple pond systems, it is not
uncommon for effluent quality in a middle pond to be
superior to that in the final pond. By drawing down the
"clean pond," it is possible to obtain storage volume.
This approach requires the flexibility to discharge from
multiple ponds and the ability to draw these ponds
down to the desired levels.
4.5.4.2 Variable Level Draw-Off
In stabilization ponds with depths greater than six feet,
variable level effluent draw-off can be effective in
controlling TSS. Algae growth decreases at increasing
pond depths because of the limited light penetration.
A schematic view of a variable level draw-off structure
is shown in Rgure 4-40. A pipe connected to a swivel
joint provides the flexibility to remove effluent from
various depths while an overflow weir located in the
control structure establishes the pond operating level.
Figure 4-40. Variable level draw-off structure.
Winch and,
Cable
Draw Off
Range
Swivel
Joint
V
Drain
Discharge
4.5.4.3 Aquaculture
Aquatic plants can be utilized to control algae growth
because of their ability to block sunlight penetration.
Both water hyacinths and duckweed have been
reported as being effective in controlling the growth of
algae (52-54). The practice of aquaculture requires
consistent attention to the growth of the aquatic
plants, and significant effort is required to harvest,
process, and dispose of excess plants. Because of
these operational-intensive requirements, aqua-culture
is typically not recommended as a modification for
small pond systems. This practice may be viable at
larger stabilization ponds that have the needed staff.
Refer to Section 4.5.1.3 for information on the use of
hyacinths for wastewater treatment.
Duckweed (Lemna sp.) is a small, green freshwater
plant with a leaflike frond that is a few millimeters in
width, and with a short root that is less than one
centimeter in length. Lemna sp. grown in wastewater
effluent at 17°C (63°F) double in frond numbers every
four days. Duckweed is more tolerant of colder
temperatures than water hyacinths. A minimum
temperature of 7°C (45°F) has been suggested as the
practical lower limit for considering the use of
duckweed. During freezing conditions, duckweed lies
dormant on the pond bottom. The high protein and fat
content of duckweed make the partially dried plants
an attractive food source for animals (54).
A patented treatment process called the LEMNA
system can utilize existing pond structures in
conjunction with floating surface barriers and hydraulic
membranes to add aquaculture capability to a facility
(53). With this process the duckweed are contained
within confined surface areas by floating barriers
which limit the turbulent effects of wind. The hydraulic
membranes are attached to the floating barriers and
are weighted to sink to the pond bottom. The
membranes are used to distribute the flow evenly
throughout the pond. The duckweed mat significantly
reduces sunlight penetration into the pond, which
limits algae growth and thus TSS concentrations.
Duckweed was used in an existing wastewater
stabilization pond in Gilcrest, Colorado. Small, multiple
cells are utilized for infiltration of the treated effluent
rather than settling. Partial plugging at the soil/water
interface results in a water level of 0.3-0.6 m (1-2 ft)
in the cells at all times. During summer months
duckweed is grown on the surface of the infiltration
cells. When the duckweed cover is present, TSS
concentrations of the treated effluent in these cells
generally remain below 10 mg/L. Direct discharge
from these cells would meet a 30 mg/L TSS standard.
The following design considerations are taken from
successfully operating systems (54):
• For algae control, a minimum detention time of 20
days should be used for the duckweed-covered
portion of the ponds to allow sufficient time for
existing algae to die and settle.
112
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• A baffled compartment with a length to width ratio
of 10:1 or more is recommended to encourage plug
flow conditions.
« Post aeration may be required since anaerobic
conditions will develop under the duckweed mat.
• Discharge from the duckweed-covered portion of
the pond should be located below the water surface
to minimize the removal of duckweed plants.
• Regular harvesting (e.g., monthly for secondary
treatment) is recommended to control the
decomposition of dying plants. Specialized
harvesting equipment is commercially available.
• A backup system, such as intermittent sand
filtration, may be necessary to provide consistent
(tertiary) effluent quality.
A schematic of a pond system utilizing duckweed for
algae control is shown in Figure 4-41.
Figure 4-41. Stabilization pond system utilizing duckweed
cover.
Surface,
Aerator
Settling Cell
with Baffles and
Duckweed Cover
\
28C
Influent
Effluent
¥
the organic loading to the first treatment cell can be
reduced by 50 percent.
Figure 4-42. Series and parallel operational flexibility.
Flow = 22 Us (0.5 mgd)
BOD5 = 200 mg/L
Area = 0.8 ha (2 ac)
Effluent
Influent
nt
->• #1 Area = 1 .6 ha (4 ac)
1
VI.
#3
A
1st Cell Organic Loading '= 233.5 kg/ha/d (208.5 Ib/ac/d)
a. Series Operation
Area = 0.8 ha (2 ac)
Effluent
Influent
-> #1 Area = 1.6 ha (4 ac)
— > #2 Area = 1 .6 ha (4 ac)
—
-
V
#3
1st Cell Organic Loading = 116.8 kg/ha/d (104.2 Ib/ac/d)
a Parallel Operation
4.5.5 Inflexible Mode of Operation
For facilities that receive periodic high organic loading,
it is sometimes advantageous to split incoming
wastewater flow to more than one treatment cell.
Recirculation of a treatment cell effluent back, to a
previous cell can have a similar effect in that the
influent organic strength is diluted by the recirculation
flow.
4.5.5.1 Series/Parallel Operation
Under most circumstances it is more advantageous to
operate a stabilization pond system in a series pattern
instead of in a parallel pattern. However, at facilities
that have limited aeration capacity, the first treatment
cell in a series operation can become organically
overloaded, and anaerobic conditions can develop.
The ability to split the incoming wastewater flow to
more than one treatment cell is shown in Figure 4-42.
With the modification shown in Figure 4-42b, the first
two treatment cells can be operated in either a series
or parallel mode. By operating in the parallel mode,
4.5.5.2 Recycle Capability
Recirculation is utilized in stabilization ponds that
periodically or seasonally receive high strength
organic wastes. Advantages include: 1) dilution of the
incoming wastewater strength, 2) return of active algal
cells to the primary treatment pond to provide
photosynthetic oxygen, and 3) mixing of the pond
contents to reduce the effects of stratification. A
flexible recirculation system is shown in Figure 4-43.
The first two treatment cells can be operated using
either series or parallel flow patterns, and the
recirculation flow can be taken from either the first or
second cells or from both cells. Typically, high-
volume, low-head propeller pumps are used.
Information on recirculation pump stations can be
found elsewhere (47).
A disadvantage of pond recirculation is the energy
cost. Therefore, other modifications, such as
additional aeration, mixing, or additional pond volume
may be more cost effective.
113
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Figure 4-43. Stabilization pond system with recirculation
flexibility.
Effluent
->• #1
\
Recirculation System ^
t
/
r
L
\
I ^ '
* X
•> #2 I
/
^
s
s
^
I
#3
t k
4.5.6 Low Pond Temperature
Continuous discharge pond systems that are subject
to extended periods of cold weather often violate their
BOD5 and TSS limits since biological activity
decreases significantly during these periods. Aeration
equipment can also contribute to cold weather
performance problems. Some types of surface
mechanical aerators and brush-type aerators are
subject to ice build-up. Other aeration equipment may
contribute to low pond temperatures through heat loss
from the aeration process.
4.5.6.1 Short-Term Diffused Aeration Cells
Performance could be improved in pond systems if
the influent wastewater temperature could be
conserved through the facility. Short-term, diffused
aeration cells can be used to partially achieve this
objective. The short-term cells should be designed to
maintain the wastewater temperature above freezing.
Two to three short-term cells are recommended to
minimize short-circuiting and maximize BOD5 removal.
A diffused aeration system avoids the contact of the
treatment cell contents with cold air, and actually
provides a small amount of heat to the water from the
compressed air.
Figure 4-44. Modifications to existing two-cell pond.
Air Supply
Inf.
New Cell #1
New Cell #2
New Cell #3
3 Short-Term Diffused Air Cells
Old First Cell
New Cell #4
Old Second Cell
New Cell #5
Surface
Aerator
To Disinfection
diffusers. The existing surface mechanical aerators
were fitted with heaters and relocated in the large
ponds. All modifications to the existing system were
made while the facility remained in operation. At the
time of the modification, the wastewater flow to the
facility ranged from 7.9 to 11.0 L/s (0.18-0.25 mgd).
An aerated pond facility located in Woodland Park,
CO, was modified to include three, short-term cells,
as shown in Rgure 4-44. The short-term cells were
formed by installing baffle walls in the primary cell.
Each cell has a wastewater depth of about 1.8 m (5.5
ft) and was designed for a detention time of two days.
Aeration is provided by two positive displacement
blowers, air piping, and rubber-membrane, fine bubble
During the winter season following the modification,
some improvement in performance over the previous
winter occured; however, the facility still, exceeded
their 30-mg/L BOD5 effluent limit during January and
February. Following this period the facility staff
discovered that a flap gate between treatment cell 1
and cell 4 had remained open, thus allowing some of
the influent to escape the short-term treatment cells.
Total effectiveness of the modification therefore has
not been determined.
114
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4.6 Sludge Treatment and Disposal
4.6.1 Background
Sludge handling limitations have a significant impact
on achieving desired performance from existing
wastewater treatment facilities (57). This section
defines alternatives for addressing the common
deficiencies observed. Guidance for addressing
proposed new sludge regulations being developed by
the U.S. EPA and State's government agencies are
not included. These regulations probably will require
more stringent levels of treatment than presently exist
plus provide additional restrictions for ultimate disposal
practices.
Data from operating facilities have indicated that an
individual plant's sludge handling problem was often
multifaceted (i.e., more than one aspect of the total
sludge handling capability would be identified as a
deficiency). Two broad areas of deficiencies were
identified: 1) inappropriate assessment of sludge
production (i.e., mass, volume and variability), and 2)
lack of process flexibility.
4.6.1.1 Sludge Production
Much information is written concerning sludge
production in terms of mass and volume and, to some
extent, on the variable nature of sludge produced
(48,49,59,62,94,95). Ranges in published information
are believed to contribute to observed limitations. The
designer has the option to select criteria on the low
end of published values resulting in economically
sized facilities at the expense of limiting capability to
handle variations from the design values. Additionally,
sludge handling limitations are often masked by a
plant's capability to store quantities of sludge within
the wastewater treatment unit processes (e.g.,
clarifiers, thickeners, digesters, and aeration basins)
and by the fact that new facilities often receive
loadings less than design. A preliminary assessment
of the capability of the sludge handling facilities can
be achieved by completing a sludge accountability
analysis. (See Section 2.3.5.1.)
In order for a plant to meet effluent performance
criteria on a consistent basis, the sludge handling
facilities must be provided with adequate capacity to
handle the mass, volume, and variability of sludge
produced. The following case study illustrates the
effects of limited capability.
Case Study: The Effects of Limited Sludge Handling
Capability on the Stevensville, Montana Wastewater
Treatment Plant (58)
The Stevensville, Montana wastewater treatment plant
is an oxidation ditch facility that was designed for an
average daily flow of 13.1 Us (0.3 mgd). Secondary
effluent from the oxidation ditch process is directed to
two infiltration/percolation (I/P) ponds for direct
discharge to ground water. A layout of the treatment
plant is shown in Figure 4-45.
Figure 4-45. Schematic of Stevensville, Montana WWTP.
Screen
Secondary
Clarifier
Parshall
Return Flume
Sludge
Discharge to
Bitterroot River
Infiltration/Percolation Cells
Discharge to Ground Water
Major components include:
• Comminutor (not used because of high
maintenance)
• Hand-cleaned bar screen
• 1,000-m3 (264,000-gal) oxidation ditch
• 79.1 m diameter x 30.5 m deep (26 ft x 10 ft)
peripheral feed clarifier
« 123.6 m x 123.6 m (40 ft x 40 ft) I/P ponds (2)
• 223-m3 (59,000-gal) aerobic digester
• 76 m x 76 m (25 ft x 56 ft) sludge drying beds (2)
A facultative lagoon was replaced by the mechanical
plant; however, because of plugging problems
associated with the infiltration/percolation (I/P) ponds,
the lagoon was placed back in operation. Effluent from
the lagoon discharges to a receiving stream.
115
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A CPE was conducted at the plant because of
suspected performance problems. The performance
potential graph for the facility is shown in Figure 4-46.
The major design limitations include the aerobic
digester and the sludge drying beds. These
limitations, plus the presence of "Operator Application
of Concepts and Testing to Process Control" as a
factor limiting performance, had contributed to
insufficient sludge wasting and solids loss from the
secondary clarifier. Most of these solids had been
captured in either the I/P ponds or the facultative
lagoon; consequently, plant effluent generally met the
discharge standards. However, solids loss was
contributing to plugging of the I/P cells, and thus non-
disinfected effluent was discharged from the old
lagoon. A sludge accountability analysis projected that
an annual average effluent TSS of 56 mg/L and BOD5
of 32 mg/L could have been expected from the
secondary clarifier discharge given the existing
operational practices.
The CPE results led to initiation of a CCP at the plant.
Emphasis was placed on the implementation of a
mass control program resulting in routine sludge
wasting from the oxidation ditch system. Because of
the limitations of the aerobic digester and the sludge
drying beds, a land application program had to be
developed. The town purchased a 5.7-m3 (1,500-gal)
truck for hauling sludge. Arrangements were made
with local farmers to use their land for application of
the digested sludge. Preparation for the land
application program included evaluation of the sites
(e.g., soil, slope, ground water) and testing of the
sludge for required parameters (e.g., metals, nitrogen,
phosphorus).
Since completion of the CCP, solids loss from the
secondary clarifier has been eliminated and the sludge
handling limitation has been addressed through the
implementation of the land application program. In
this study, inaccurate projections of waste sludge
production (mass and volume) resulted in significant
limitations in sludge handling capabilities.
Even when the correct mass of sludge produced is
established, poor estimation of sludge volume can
result in severely overloaded treatment processes. For
example, poor settling and accompanying poor
compaction characteristics often occur in activated
sludge processes. These conditions result in the need
to waste higher sludge volumes in order to remove
the desired mass of sludge. Variability in sludge
production (e.g., peak as well as average data) is
shown by the following.
Case History: Tuscaloosa, Alabama Wastewater
Treatment Plant
The Tuscaloosa wastewater treatment plant is an
activated sludge facility with anaerobic digestion. The
design flow of the plant is 679.4 L/s (15.5 mgd). The
primary treatment portion was built in 1958, along with
three anaerobic digesters. The secondary treatment
portion and one additional digester was built in 1974.
In 1982, a dissolved air flotation unit was constructed
to thicken sludge from the activated sludge process
prior to introducing the sludge to the anaerobic
digesters. Two sludge lagoons, adjacent to the plant,
are used for disposal of all solids from the plant.
Supernatant from these lagoons is returned to the
headworks.
Sludge production data for 1986 are summarized in
Figure 4-47 and Table 4-12. Primary, secondary, and
total sludge production are shown.
During the year the plant treated an average flow of
539.1 L/s (12.3 mgd), and influent BOD5 and TSS
concentrations averaged 205 mg/L and 161 mg/L,
respectively. The average sludge production for the
year was about 10,440 kg (23,000 lb)/d. The
combined total maximum monthly and weekly sludge
production amounts were 11,723 kg (25,820 lb)/d and
12,967 kg (28,561 |b)d, respectively.
The data show that the peak monthly sludge
production was 12 percent higher than the average
yearly value. The maximum weekly sludge production
occurred in December, and was 24 percent higher
than the yearly average value.
The peak monthly and weekly sludge production for
the primary clarifiers was 30 percent and 54 percent,
respectively, higher than the yearly average value. For
the activated sludge (secondary) portion of the plant,
the peak monthly and weekly sludge production was
27 percent and 46 percent, respectively, higher than
the yearly average value.
4.6.1.1.1 Variability Considerations
Variability in the mass and volume of sludge produced
occurs for a variety of reasons including:
• Normal influent loading variations. Reliable data
bases on POTW influent wastewater charac-
teristics give preliminary definition to the variability
of primary and/or waste secondary sludge solids.
The variability of concentrations of BOD5 and TSS
plus the variability in peak day/period flows must
be considered to provide an adequate sludge
management system. Table 4-13 summarizes
variations for selected periods based on reported
minimum and maximum deviations encountered
with sustained mass influent loadings for BOD5
and TSS (49).
• Operational changes (e.g., increased solids
processing requirements resulting from reducing
system SRT, changes in return sludge flow rates).
• Normal variations in biological unit process
performance (e.g., changes in activated sludge
116
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Figure 4-46. Stevensville, Montana performance potential graph.
Wastewater Flow, mgd*
Unit Process Q1 Q15 Q2 Q25 03 0.35 Comments
Oxidation Ditch
O2 Supply, Ib O2/lb BOD Applied
Clarifier
Overflow Rate, gpd/sq ft
I/P Ponds
Sludge Handling and Disposal
Digester
Detention Time, days
Drying Beds
Summer, percent capacity
Winter, percent capacity
Ultimate Disposal
I Rated at 24 hr
Rated at 1 0 lb/d/1 ,000 cu ft
Rated at 2.5 Ib O2/lb BOD App.
Rated at 500 gpd/sq ft
Capacity not projected
0.65 kg TSS prod./kg BODRi
7 500 mg/L waste rated at 1 5
days
Inadequate-Capacity <0.1
mgd
Inadequate-Capacity <0.1
mgd
Inadequate (none available)
Current Flow Des gn Flow
(0.238 mgd) (0.30 mgd)
* Assumed base infiltration rate of 110,600 gpd.
Figure 4-47. Tuscaloosa, Alabama 1986 sludge production.
Avg. Sludge Produced, Ib/d
28,000
Avg. Annual Sludge Prod. Secondary Sludge
= 23,000 Ib/dC—, F—,rn /
Jan.
Primary Sludge
Dec.
settling characteristics, sloughing of solids from
fixed film facilities).
• Normal variations in unit process performance
(e.g., thickening, dewatering).
• Impact of downtime of sludge processing
equipment.
« Impact of climatic factors (e.g., winter
performance of sand drying beds, weather
impacts on ultimate disposal options).
Because of these factors, variability is never
eliminated and must be considered in all phases of
sludge management system modification design.
Design based on "average" conditions is inadequate.
Good design requires adequate projection of mass,
volume (e.g., concentration), and the variability of
each of these individual parameters.
4.6.1.1.2 Mass Considerations
Inaccurate projections of the amount of sludge that a
process will produce is frequently a limitation that
117
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Tablo 4-12. Sludge Production Data for Tuscaloosa, Alabama WWTP
Process
Primary Clarification
Secondary
(Activated Sludge)
Combined
Primary/Secondary
Parameter
Primary Sludge Produced, Ib/d
Ratio of Maximum Period to Average Year
Unit Sludge Production Ratio, Ib solids/lb BOD removed
Sludge Produced, Ib/d
Ratio of Maximum Period to Average Year
Unit Sludge Production Ratio, Ib solids/lb BOD removed
Total Sludge Produced, Ib/d
Ratio of Maximum Period to Average Year
Avg. Year
11752
1.00
1.82
11,285
1.00
0.86
23,037
1.00
1986 Data
Max. Month
15,253
1.30
2.84
14,331
1.27
1.01
25,820
1.12
Max. Week
18,071
1.54
3.12
16,310
1.46
1.32
28,561
1.24
Table 4-13. Summary of Reported Minimum and
Maximum Variations with a Sustained Mass
Loading for Selected Periods (49)
Percent of Average Loading
Period
Daily
Weekly
15 Days
Monthly
BODg
15-250
60-160
65-125
75-120
TSS
15-270
67-167
75-145
80-125
impacts plant performance. Following are
considerations that should be included in sludge mass
projections.
Primary Sludge Accumulation
Primary clarifiers are normally assumed to remove
between 50 to 70 percent of the influent TSS and 25
to 35 percent of the influent BOD5. However, these
removals are a function of, among others, influent
wastewater characteristics, clarifier overflow rates,
clarifier depth, and operations variables (e.g., sludge
pumping rates and blanket depths).
To adequately project TSS and BOD5 removals from
primary clarifiers, and thus project impact on sludge
management system components, requires that
minimum and maximum influent mass loading days be
considered. For example, from Table 4-13 the primary
sludge pumps should be sized to accept between 0.2
to 2.7 times the mass of TSS anticipated on an
average day. Typically, capacity to handle this total
range of diurnal fluctuation is not required since
operational changes in pumping rate and in blanket
depth (e.g., in-plant storage) can be used to
accommodate a portion of these variations. If no
additional intermediate storage facilities are available,
solids loading from a primary clarifier to a high rate
anaerobic digester (e.g., 15-day hydraulic detention
time) could be based on a 15-day sustained loading.
From Table 4-13 this would be approximately 145
percent of the long-term average value, based on
influent suspended solids loadings.
Secondary System Sludge Production
TSS and BOD5 not removed by primary clarifiers are
treated in secondary facilities. Inaccurate projections
of the mass of waste secondary sludge solids
produced can have significant impact of a POTW's
performance capability. Several approaches to make
these projections are discussed.
Actual Data
Use of actual plant data is the most reliable method of
developing accurate sludge mass projections. This
method requires extensive data and detailed
information on the plant's operational practices.
Accurate data is required on flow, volumes, and waste
concentrations from all unit processes, including
recycle streams. This data is necessary over long
time periods to allow for assessment of the variability
Tablo 4-14. Guidelines for Modifying Existing Sludge Handling Facilities
Process/Facilities
Basis for Modification
Waste Sludge Pumping (Primary and/or Secondary)
Waste Sludge Thickening (Primary and/or Secondary)
Stabilization
Dawatering
Transportation
Storage
Utilization/Disposal
Maximum Daily Sludge Production
Maximum Daily Sludge Production
Maximum Monthly Sludge Production*
Maximum Weekly Sludge Production
Maximum Weekly Sludge Production
Maximum Sludge Production Rate for Storage Period Being Considered
Average Annual Sludge Production for Establishing Area Requirements
Maximum Sludge Production for Equipment Sizing and Daily Operating
Requirements to Allow for Downtime, Weather Conditions, etc. (e.g., Monthly)
Maximum weekly sludge production if the hydraulic residence time in the stabilization process is 10 days or less.
118
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that occurs. Additionally, extensive awareness of
operations is necessary to assure that undocumented
effluent solids loss does not exist and that plant
storage of sludge, which masks true sludge
production, is not occurring. Facilities that have
documented a need for modifications to their sludge
handling processes, following the steps in this manual,
should have this kind of data available. Table 4-12 is
an example of a good summary of one year of actual
plant sludge production and variability data.
Other Similar Processes
In the absence of information to utilize actual
information, data from other similar processes can be
used. Tables 2-7 and 2-17 summarize data based on
full scale operating facilities for average total sludge
production rates for a variety of secondary processes.
Sludge Mass Production Variability
Independent of the procedure utilized to project the
average secondary system sludge production,
variability effects must also be projected. If actual data
are not available, variability based on the minimum
and maximum sustained loads, as shown in Table 4-
13, can be used for estimation purposes. For
example, if a dissolved air flotation (DAF) unit is used
to thicken waste secondary sludge from a conven-
tionally loaded suspended growth process, this unit
should be designed to process the estimated max-
imum day loading. However, if the waste activated
sludge is pumped from the DAF to an aerobic digester
(e.g., 20-day hydraulic detention time), the design of
the DAF unit could be based on the 20-day sustained
loading projections. From Table 4-13 the variability of
TSS would be 80 to 130 percent of average and 75 to
120 percent for BOD5. These values could be applied
to the anticipated average secondary system loadings
to include the necessary variability projection in the
design of facility modifications.
4.6.1.1.3 Concentrations Considerations
Coincident with establishing masses of sludge
accumulation, adequate sludge management system
design must accurately project sludge stream
concentrations and thus volume. Tables 2-8 and 2-17
present typical sludge stream concentrations from a
variety of wastewater treatment processes. Values
shown can be significantly varied based on plant
operational factors. For example, return activated
sludge concentrations are functions of sludge settling
characteristics, recycle rates, and other operating- and
equipment-related parameters. Changes in sludge
characteristics, such as the growth of filamentous
organisms, can produce significantly lower return
sludge concentrations than those listed. Additionally,
return sludge concentrations can vary due to changes
in diurnal flow rate and/or operational changes in
return sludge flow rate.
In addition to variations in concentrations from the
mainstream treatment processes, variations occur in
the concentrations from sludge management system
unit processes. The effect of these variations on other
interactive sluge management system processes must
be considered.
In many plants with observed sludge management
system limitations, operational adjustments (e.g., in-
plant storage of solids in suspended growth
processes) had been used to compensate for the
sludge unit process limitations, at the expense of
plant effluent quality. Operational adjustments
significantly impact sludge concentration and volume
projections. Because of the complex interrelationship
of unit process capability and operational controls, it is
proposed that design proceed by addressing
concentration and resulting volume variations on a
worst case basis, taking the plant's operational
flexibility into consideration.
4.6.1.2 Flexibility
Ideally, adequate flexibility exists when any mass of
sludge, as required by the process control strategy for
the wastewater treatment processes, can be removed
from the treatment process at any time of the year. To
achieve this flexibility requires that multiple equipment
or operational options be available to the plant
operators. Facilities thus equipped will be able to
reduce the impact on plant performance attributable to
sludge handling deficiencies.
Figure 4-48 illustrates many of the current options for
sludge handling facilities. When evaluating
modifications to existing sludge handling processes, it
is important to be aware of the multiple process
options available. Another important aspect of sludge
facility modifications is the interelation of processes
(e.g., improved thickening can dramatically improve
stabilization capabilities and improved stabilization can
improve dewatering, etc.). Therefore, the most
effective solution for a. sludge handling problem may
often be the improvement of an adjacent interactive
process.
One of the most effective additions to sludge handling
capability is storage. Sludge storage can be after
stabilization (e.g., second stage digester), after
dewatering, or, off-site near the disposal/reuse
location. Even multiple storage points in the process
diagram may be of value. When storage occurs
sooner in the process flow diagram, more downstream
processes are benefited, however, greater sludge
volumes are involved.
To ensure adequate flexibility, facility modifications of
sludge treatment and disposal processes should be
based on sludge production (e.g., mass, volume, and
variability) for the peak design period loading. In the
absence of actual operating data, guidelines have
been developed and are presented in Table 4-14.
Flexibility can be assessed by developing a mass
balance schematic for the unit processes to be
119
-------
Figure 4-48. Typical sludge handling, treatment, and disposal processes (96).
• Sldestream Treatment
• Flow Equalization
• Biological Treatment
• Chemical Treatment
• Additional Solids/Liquid Separation
Sldestream
Rotum
• Step Feed
Rotum
• Multiple Rajii
Locations
Raw
Wastowateri
Primary
Sludge Jr
Wastewater — ^
Treatment
Faciliti0s S^ndary
Sludge
Other
Sludges
combined
or separate
Plant
Eflluent
t
:
^*
Sidestream Recycles
Decant,
Supernatant,
Odor Control Scrubbers,
Other Recycles
2c .......... f !
J:
*
r
/
[ Subnatant,
j Supernatant,
i Filtrate, etc.
k i
'lit
/
_. s . :
i / i i
i^.' Centrate, i Free Water j
i Filtrate, j J
i etc. j j
» ' fc ' k '
lit ill II
Free
j Free Water,
Water. : Runoff.
Condensate, j etc.
etc. |
Sludge Transport
r
nir^
Incineration,
Scrubbers,
Landfill,
Compost Site
Runoff, etc.
T
Thickening
• Gravity
• OAF
• Centrifuges
• Gravity Belts
• Rotary Screens
• Other
Stabilization
• Aerobic
Digestion
• Anaerobic
Digestion
• Lime
• Heat Treatment
• Wet Air
Oxidation
• Sludge Fixation
• Other
Dewatering
• Centrifuges
• Belt Processes
• Vacuum Filters
• Filter Presses
• Centripress
• Other
Stabilization
• Lime
• Sludge Fixation
• Other
Drying
• Rotary Drums
• Rotary Discs
• Multiple Effect
Evaporators
• Flash Dryers
• Sand Beds
• Lagoons
• Other
Storage Disposal or
• On-Site Reuse
• Off-Site • Combustion
and Ash
Disposal
• Composting
• Landfilling
• Lagooning
• Other
incorporated into the plant's design. This schematic
can be used to depict the "worst case" scenarios for
the operation of the sludge handling processes.
Variable values for flow, concentration, and mass can
be added for the anticipated worst case operating
conditions to "test" the flexibility of the sludge
handling capability provided.
4.6.2 Selecting Modification Approaches
Table 4-15 summarizes design/corrective
modifications that are prioritized based on their
potential for achieving improved performance.
Modifications with proven reliability and those that are
less construction intensive appear at the beginning of
the listing.
4.6.3 Inadequate Sludge Thickening Capability
The flexibility to thicken waste sludge from a
treatment process, typically a suspended growth
process, provides the following advantages: 1)
increased detention time in stabilization processes, 2)
increased solids concentration to enhance digestion,
and 3) reduced sludge volume to ultimate disposal or
beneficial use. Common processes used for
thickening include gravity, flotation, and mechanical
thickening with centrifuges or presses.
Table 4-15.
Sludge Handling Design
Potential Modifications
Design Limitation/Potential Modification
Limitations and
Page. No.
Thickening
4.6.3 Inadequate Sludge Thickening Capability 120
4.6.3.1 Chemical Addition to Existing Thickener 121
4.6.3.2 Additional or Alternate Thickening Process 121
Stabilization
4.6.4 Inadequate Sludge Stabilization Capability 121
4.6.4.1 Improved Thickening/Dewatering 122
4.6.4.2 Additional Blowers (aerobic digestion) 122
4.6.4.3 Lime Stabilization 122
4.6.4.4 Additional Heat Exchange Capacity 122
4.6.4.5 Improved Mixing Capability 122
4.6.4.6 Additional or Alternate Stabilization Process 123
Dewatering
4.6.5 Inadequate Sludge Dewatering Capability 123
4.6.5.1 Additional or Alternate Dewatering Process 123
4.6.6 Sidestream Return to Wastewater 123
4.6.6.1 Pretreatment 123
4.6.7 Inadequate Sludge Storage Capability 123
4.6.7.1 Mechanical Thickening 124
4.6.7.2 Additional or Alternate Storage Facilities 124
4.6.8 Inadequate Sludge Transportation Capability 124
4.6.9 Inadequate Sludge Utilization/Disposal Capability 124
4.6.9.1 Additional or Alternate Utilization/Disposal
Facilities 124
120
-------
In some cases, minor operational or design related
changes can be implemented to increase waste
sludge concentration. For example, the addition of
chemicals to primary or secondary clarifiers can
produce thicker sludges. The use of positive
displacement pumps will allow thicker sludge
withdrawals than non-clog centrifugal types. Treatment
plants with dedicated waste sludge pumps can use a
time clock with the pump to waste small quantities of
sludge several times each day instead of wasting the
total volume once per day. Since sludge concentration
typically decreases with increasing duration of the
pumping cycle, the pump operation can be stopped
for a period of time to increase the sludge
concentration at suspended growth plants that utilize
the same pump for the return and waste sludge
functions. Comparing pump "off times" versus
corresponding sludge concentrations can be used to
optimize this approach. Often waste sludge
concentrations of 8,000-15,000 mg/L can be achieved
when the pump is off for a period of 15-60 minutes.
This procedure may require similar around-the-clock
operation of some downstream processes.
4.6.3.1 Chemical Addition to Existing Thickener
Generally, the capacity and solids capture of sludge
thickening processes can be increased through the
addition of a polyelectrolyte to the sludge stream
before the process. The thickened sludge
concentration will not typically increase. To determine
the feasibility of this modification, it is recommended
that jar testing utilizing different polymers be
completed. If tests indicate that improved performance
is possible, then full-scale polymer addition should be
tested before large capital expenditures are made for
polymer storage and feed equipment.
The optimum location for polymer addition depends on
differences in polymer charge densities, required
polymer reaction times, and sludge characteristics.
Recommended locations include:
• Directly before the inlet side of the sludge feed
pump
• Immediately downstream of the sludge feed pump
• Prior to introduction to the sludge thickening
process (59)
Polymer addition to a gravity thickener may allow a
solids loading rate increase of two to four times the
rate for a unit not receiving polymer (59). Typical
polymer dosages are from one to five milligrams per
liter (61). In the case of dissolved flotation thickeners,
polymers are usually essential to achieve effective co-
flocculation of the air bubbles and sludge particles
(59). Polymer requirements are usually 2.5-7.5 g
polymer/kg dry solids (5-15 Ib/ton) (61). Similar
polymer dosage rates can be expected when using
centrifuges, gravity belts, or rotary drums for
thickening.
4.6.3.2 Additional or Alternate Thickening Process
The addition of a sludge thickening process can be
used to address a sludge handling limitation. Other
sources provide design, operation, and maintenance
information on sludge thickening (59,61,62).
When evaluating thickening alternatives, con-
sideration should be given to the following factors:
• Compatibility of the thickening equipment with the
existing sludge
» Process flexibility
• Worst case performance conditions.
Often the thickened sludge output concentration given
for different thickening processes is optimistic. To
provide adequate flexibility, worst case concentration
values should be used during design.
If a new process or new application of an existing
process is being evaluated, completion of a pilot study
is usually advantageous. A pilot study, utilizing existing
sludge, can provide valuable information on solids
loading rates, solids capture, chemical dosages, and
operational requirements. The pilot study should be of
sufficient duration to test the proposed system under
a variety of sludge conditions. Pilot scale thickening
equipment is frequently available from manufacturers.
If centrifuges or other mechanical thickening
processes are being evaluated, consideration should
be given to providing flexibility to thicken sludge from
several sources. For example, with a suspended
growth process utilizing aerobic digestion, the
thickening equipment could be used to: 1) thicken
waste activated sludge prior to digestion, 2) thicken
aerobic digester sludge to increase digester solids
detention time, or 3) thicken digester sludge prior to
disposal or utilization.
4.6.4 Inadequate Sludge Stabilization Capability
Sludge stabilization processes provide: 1) volatile
solids reduction, 2) pathogen destruction, and 3)
reduction of the odor potential and putrescibility of the
sludge. If inadequate sludge stabilization capability
exists, problems typically occur with disposal or
utilization of the sludge. Secondary treatment process
are affected if insufficient sludge can be wasted or if
supernatant returned from a digester adversely affects
the secondary process.
Before modifying sludge stabilization facilities,
operational changes should be investigated:
121
-------
• Eliminate supernatant return (e.g., apply to
agricultural land)
• Pretreat supernatant
• Improve solids capture from thickening or
dewatering processes
• Thicken aerobic digester sludge (e.g., add chemical
coagulants to the digester, allow settling, and
remove supernatant)
4.6.4.1 Improved Thickening/Dewatering
The capacity and performance of sludge stabilization
processes can sometimes be improved through
additional thickening or dewatering of the feed sludge
to the process. With aerobic digestion, additional
thickening of the feed sludge will increase the solids
residence time in the basins and subsequently the
volatile solids reduction. With anaerobic digestion,
thickening of primary sludge can usually be regulated
through the operation of the primary sludge pumps.
However, low concentration sludge from a suspended
growth process can sometimes have a negative
impact on the optimum temperature of the anaerobic
digestion process. Under these circumstances, it is
advantageous to direct the secondary sludge to a
thickening process prior to anaerobic digestion. Other
stabilization processes, such as composting, require a
dry sludge cake to optimize the process performance.
Information on sludge thickening is presented in
Section 4.6.2.
4.6.4.2 Additional Blowers or Diffuser
Replacement (Aerobic Digestion)
The aerobic digestion process requires 1.7-3.7 kg of
oxygen to oxidize 1 kg of cell mass (59,63). If
inadequate oxygen supply is available volatile solids
reduction will not be optimized. The addition of
blowers or the replacement of existing diffusers can
provide increased oxygen as discussed in Section
4.4.4. When considering diffuser replacement, the
impact of diffuser fouling should be carefully
evaluated. Diffuser fouling potential is greater in
aerobic digesters because of high sludge
concentrations, accumulation of rags, and the practice
of stopping the aeration equipment during periods of
digester settling and supernatant removal. Fine bubble
ceramic diffusers are least suitable in this application
because the actual oxygen transfer values are found
to be so low in the high solids environment that the
diffusers are not cost effective.
4.6.4.3 Lime Stabilization
Lime addition to sludge reduces odors and pathogens
by the creation of a high pH environment that limits
biological activity. Lime addition also affects the
chemical and physical characteristics of sludge.
Changes include reduced fertilizer value, improved
dewaterability, and changes in the character of liquid
sidestreams. Parameters used in sizing lime
stabilization facilities include: pH, contact time, and
lime dosage. The process objective is to maintain the
sludge at a pH above 12 for about two hours to
ensure pathogen destruction and to provide sufficient
residual alkalinity to maintain the pH above 11 for
several days to allow time for reuse or disposal
without renewed biological activity (59). Experience
has shown that lime dosages of 10-50 percent of the
total dry solids in the sludge are required to achieve
the conditions described above.
Lime stabilization is a relatively simple process to
implement; consequently, it may be applicable as a
backup process for existing stabilization facilities or as
an interim sludge handling process. Disadvantages of
this process include high chemical and labor
requirements and a substantial increase in the amount
of dry solids that require disposal. Design information
can be found in other sources (59).
4.6.4.4 Additional Heat Exchanger Capacity For
Anaerobic Digesters
Thickening of the feed sludge can be used to reduce
heating requirements for anaerobic digesters. This
option should be investigated before major
modifications are made. The capacity of anaerobic
digesters can be increased by replacing an existing
heat exchanger or adding a new heat exchanger to a
non-heated or under-heated digester. Heating is an
important component of the anaerobic digestion
process. By increasing the temperature of the
digesting sludge, the metabolic rate is increased and
the required digestion time is reduced. Temperature
control is also important. To achieve process stability,
the temperature of the digester contents is maintained
within ±0.56°C (1°F).
The most common heating method is the use of
external heat exchangers. Three common types
include: water bath, jacketed pipe, and spiral (59).
Design information on these systems can be found in
other sources (2,59).
4.6.4.5 Improved Mixing Capability
Adding mixing capability to a non-mixed anaerobic
digester will improve process stability and increase
digester capacity. Replacing existing mixing equipment
with more effective mixing equipment may also
improve process stability or increase capacity. Mixing
benefits include: 1) distribution of the feed sludge with
the active biomass; 2) consistent physical, chemical,
and biological conditions; 3) rapid dispersion of any
toxic materials, thus minimizing their effect on
microbial activity; 4) uniform temperature; and 5)
minimization of sand, grit, and scum accumulation
(59).
Methods used for mixing include external pumped
circulation, internal mechanical mixing, internal gas
122
-------
mixing, and direct steam injection. Design information
can be found in other sources (2,59).
4.6.4.6 Additional or Alternate Stabilization
Process
Where major sludge stabilization limitations exist,
either lime stabilization of a portion of the sludge flow
should be considered or additional processes must be
added. Discussion of commonly used stabilization
processes can be found in other sources (2,57).
4.6.5 Inadequate Sludge Dewatering Capability
The objective of dewatering is the removal of water
and thus volume from stabilized sludge prior to reuse
or ultimate disposal. A secondary objective is to
minimize the quantity of solids recycled to the main
process in the filtrates or centrates. The process
reduces the subsequent cost of sludge transportation
and disposal. The desired percent solids content of
dewatered sludge is established by the disposal or
reuse requirements. Inadequate sludge dewatering
occurs if throughput from existing equipment exceeds
capacity, or if the solids content of dewatered sludge
is less than the requirement for disposal or reuse.
Inadequate dewatering capability can limit the
performance of a secondary treatment process if it
inhibits sludge wasting.
4.6.5.1 Additional or Alternate Dewatering
Process
Dewatering alternatives include air drying and
mechanical processes. Air drying processes include
conventional sand beds, paved beds, reed beds,
sludge lagoons, vacuum-assisted beds, wedgewire
beds, and several other emerging technologies.
Mechanical dewatering processes include belt filter
presses, centrifuges, filter presses, vacuum filters,
and some emerging technologies. Specific information
on these various processes is summarized in recent
EPA publications (62,81).
4.6.6 Sidestream Return to Wastewater
With nearly all types of sludge handling processes, a
sidestream flow, such as supernatant from a digester,
is produced that is typically returned to the treatment
plant for processing. These sidestreams are typically
low in volume when compared to the plant flow;
however, they can contain high concentrations of
solids, organics, and other compounds, such as
nitrogen and phosphorus, which can exert an oxygen
demand on the system. A design limitation exists
when the sidestream flow produces a negative effect
on the secondary process. Sidestreams that can
potentially be returned but often are overlooked are:
odor scrubbers, incineration scrubbers, collected
landfill leachate, free water from storage facilities, and
stormwater runoff from sludge compost sites.
Before major modifications are planned, operations-
related solutions should be investigated. Changing the
operational strategy for anaerobic digesters can result
in an improvement in supernatant quality. Possible
control strategies include: 1) controlled release of
supernatant at night or over a 24-hour period, 2)
shutdown of mixing equipment for quiescent
decantation, and 3) two-stage operation with
supernatant from the second stage. Also, where and
how the flows are returned and distributed in the plant
can have a major impact on operations. Properly
distributed recycles can help to minimize impacts.
Filtrate quality from sludge presses can sometimes be
improved by changing the sludge conditioning or
operation of the dewatering equipment (e.g., type of
chemical, dose, sludge feed rate, roller compression,
etc.). Off-site disposal of sidestreams should also be
investigated although it may not always be feasible
because of site limitations and regulatory require-
ments.
4.6.6.1 Pretreatment
Pretreatment can be effective in reducing sidestream
strength; however, the practice is not common
because of the additional operational requirements
and cost. Possible pretreatment alternatives include
fixed-film and suspended growth processes. Chemical
coagulation and sedimentation may also provide
additional liquid/solids separation. (See Sections 4.3
and 4.4.)
The return of sidestreams to a flow equalization basin
can be effective in diluting waste strength.
Consideration should be given to: 1) odor potential, 2)
equalization basin volume needed for storage of the
sidestream flow, 3) basin aeration requirements, 4)
basin mixing requirements, 5) wash down facilities,
and 6) solids collection and removal equipment.
4.6.7 Inadequate Sludge Storage Capability
Sludge storage facilities provide the flexibility to retain
stabilized sludge in a contained basin or pond for an
: extended period of time, typically during periods when
access to a disposal or reuse alternatives is not
possible. The case history that was presented in
Section 4.6.1 discussed the variability in sludge
production that occurs throughout a typical year. This
variability requires that sludge handling facilities,
including storage facilities, be sized to handle peak
loads (periods of down time, bad weather, seasons,
etc.).
A common practice in plants that have sludge
handling limitations is to increase the sludge inventory
in the secondary process as a means of storing
excess solids. Although sometimes effective on a
short-term basis, this practice can have negative
effects on sludge settling characteristics and
eventually system performance.
Another common practice for increasing sludge
storage capacity in a treatment.plant is the removal of
supernatant from digesters. The development of
123
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supernatant in digesters can sometimes be enhanced
by the addition of a coagulant. Portable submersible
pumps have been successfully used to enhance
supernatant removal in aerobic digesters.
4.6.7.1 Mechanical Thickening
Mechanical thickening can be an effective means of
increasing the detention time of solids in an aerobic
digester or storage basin. If conventional gravity
thickening is practiced, the solids concentration of
sludge in these units is limited to two to three percent
by weight. By mechanically thickening the contents of
the digester/storage basin, this concentration can be
increased to four to six percent by weight. This
modification may be limited by the aeration capacity of
the aerobic digester. Section 4.6.3 provides
equipment and information references.
4.6.7.2 Additional or Alternate Storage Facilities
Various alternatives are available for the storage of
liquid, thickened, and dewatered sludges. The most
appropriate alternative will be dependent on the
existing sludge handling limitations, the eventual use
for the sludge, the length of time that storage is
required, and the available resources (i.e., land,
existing basins).
Where land application is the ultimate disposal
method, the application may be halted for long periods
due to weather conditions. In this case, storage
facilities for stabilized sludge significantly increase the
flexibility of this practice. Information on design
considerations for sludge storage facilities can be
found in Design Manual on Sludge Treatment and
Disposal (59).
4.6.8 Inadequate Sludge Transportation Capability
The relocation of wastewater sludge to the disposal or
reuse site requires some method of transportation.
Conventional transportation methods include trucks
and pipelines. This limitation should rarely affect the
performance of secondary treatment processes since
contract haulers and/or leasing options are typically
available. Additional information can be found in
Design Manual on Sludge Treatment and Disposal
(59).
4.6.9 Inadequate Sludge Utilization/Disposal
Capability
Wastewater sludge is disposed of through such
means as landfilling or for soil conditioning and
fertilization. Inadequate utilization or disposal facilities
can have a major effect on secondary treatment
process performance. As such, it is essential to have
several alternatives available.
4.6.9.1 Additional or Alternate Sludge
Utilization/Disposal Facilities
The development of additional or alternate sludge
utilization/disposal facilities is one of the most
effective methods of providing plant flexibility.
Numerous alternatives are available. Often
development of alternatives does not require large
capital expenditures but does require planning,
coordination, and public relations. Specific information
can be found in other sources (59,64,65).
4.7 Additional Facility Modifications '
This section discusses performance-related facility
modifications not covered previously. Included are
modifications of the disinfection system, the
wastewater stream, and emergency or non-routine
operational procedures. Commonly identified design
limitations associated with these categories are
presented in Table 4-16.
Table 4-16. Additional Design Limitations
Design Limitation/Potential Modification
Page. No.
Disinfection Systems
4.7.1.1 Modifications to Chlorine Disinfection Systems- 125
- Short-Circuiting
- Inadequate Contact Time
- Inadequate Chlorine Feed Capacity
- Inadequate Mixing
4.7.1.2 Modifications to Ozone Disinfection Systems 127
- Inadequate Ozone Transfer Efficiency
- Inadequate Feed Gas Preparation
- Short-Circuiting
- Inadequate Contact Time
4.7.1.3 UV Radiation Disinfection Systems 128
- Inadequate Cleaning System
- Short-Circuiting
Miscellaneous Systems
4.7.2.1 Modifications to Wastewater Characteristics 129
4.7.2.2 Modifications to Provide Instrumentation 129
Emergency Systems
4.7.3.1 Modifications to Respond to Non-Routine 130
Operation
4.7.3.2 Modifications to Respond to Emergency 130
Conditions
4.7.1 Disinfection System Modifications
Disinfection is used to destroy or inactivate
pathogenic (disease-causing) microorganisms in a
treatment plant's effluent. Chlorine, ozone, and
ultraviolet radiation have been the primary
disinfectants used to accomplish this task. Before
modifications are proposed, the performance of
upstream processes should be optimized operation-
ally to ensure that the highest quality effluent is being
achieved prior to disinfection. Modifications to
upstream processes to improve disinfection perfor-
mance are generally not cost effective unless effluent
BOD5 and TSS standards are also being violated.
Operational efforts to reduce organic compounds and
SS can improve disinfection performance. Organic
compounds and ammonia exert a chlorine demand;
consequently, chlorine dosage to an effluent with
significant quantities of these constituents must be
124
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higher to achieve desired bacterial kill. SS impact
chlorine efficiency by sheltering the bacteria from the
chlorine. Wastewater quality affects ozone transfer
efficiency and thus the performance of existing
contact basins. SS also inhibit the effectiveness of
ozone disinfection. SS affect performance of UV
systems to a greater extent than either chlorine or
ozone systems. Chlorine and ozone can penetrate
solid particles to provide disinfection. However, UV
radiation must make direct contact with the
microorganism to inactivate it. Suspended solids will
deflect UV radiation, thereby protecting entrapped
microorganisms. Organic and inorganic compounds in
the wastewater absorb UV energy, reducing the
intensity of the radiation, and thus disinfection
capability.
Facility modifications directed toward improving the
efficiency of disinfection processes are presented in
the following sections.
4.7.1.1 Chlorine Disinfection Systems
Chlorine disinfection systems maintain a chlorine
residual in the wastewater for an extended period of
time. Design-related factors that can impact the
effectiveness of the process include: short-circuiting,
initial mixing of chlorine with wastewater, applied
chlorine dosage, and length of contact time.
a. Short-Circuiting
Short-circuiting most often can cause poor disinfection
efficiency, as illustrated in Table 4-17. As shown, a
minimal amount of short-circuiting has a significant
impact on disinfection efficiency.
Table 4-17. Effect of Short-Circuiting on Disinfection
Performance (67)
Influent
Coliform
(#/100 mL)
100,000
100,000
100,000
100,000
Effluent
Coliform
Target
(#/100 mL)
200
200
200
200
Short-
Circuiting
(%)
0.00
0.10
1.00
2.00
Resulting
Effluent
Coliform
(#/100 mL)
200
300
1,200
2,200
If short-circuiting is suspected, it is recommended that
a dye tracer study of the contact basin be conducted.
Such a study provides data to evaluate the extent of
short-circuiting, such as the effectiveness of initial
mixing, and the ratio of actual to theoretical contact
time (66). Ideally, the contact basin should exhibit plug
flow characteristics (e.g., no wastewater passes
through the basin in less than the theoretical detention
time). The best method of approximating plug flow is
to use a long, full-flowing pipeline as the chlorine
contact unit. This may be possible at plants that have
an outfall sewer line with a sufficiently long time of
travel. Otherwise, the chlorine contact basin can be
modified to approximate plug flow with the addition of
baffles and deflecting vanes.
Occasionally, an existing circular basin (i.e,
abandoned clarifier) will be used as a chlorine contact
basin. If the basin isn't baffled, serious short-circuiting
will occur. Figure 4-49 illustrates a method for baffling
a circular basin. The angled extensions at the end of
some of the baffles provide for more efficient use of
the tank volume.
Figure 4-49. Addition of baffles to existing circular contact
basin.
Inlet
Effluent
Some older rectangular contact basin designs utilized
a cross-baffled configuration. This configuration is not
as efficient as a longitudinal configuration because the
cross-baffled layout usually has more turns for a given
length to width ratio (68). Two potential modifications
to a cross-baffled basin are shown in Figure 4-50. The
first modification involves the installation of diffusion-
wall baffles near the inlet, outlet, and at the turns. The
second modification involves the installation of turning
vanes. Both approaches have proved effective in
increasing the efficiency of cross-baffled contact
basins (68). '
b. Initial Mixing of Chlorine With Wastewater
The initial mix of chlorine with wastewater should be
very rapid and thorough to ensure that all pathogens
are brought into close contact with the disinfectant.
This is the most important factor affecting chlorine
disinfection efficiency. This initial mixing should be
completed 5-15 seconds after chlorine comes in
contact with the wastewater (66). Common mixing
systems include: in-line diffuser, mechanical mixer,
hydraulic jump, and jet mixer. This is the most
important factor affecting chlorine disinfection
efficiency.
To improve mixing for systems utilizing in-line
diffusers, relocation of the diffuser to a location with
more turbulence can sometimes be effective. In cases
where a pump is used to transport secondary clarifier
effluent to a contact chamber, chlorine can be added
prior to the pump suction. If typical chlorine dosages
are used (i.e., 2-10 mg/L), damage to the pump
125
-------
Figure 4-50. Modifications for cross-baffled contact basins.
Effluent.
Inlet
Plan View
Front View
Baffle
Inlet
Plan View
impeller due to reaction with the chlorine should not
be a problem. For cases where the clarifier effluent
flows by gravity through pipes and channels to the
contact chamber, potential sites for a chlorine diffuser
include transition points (e.g., change from channel to
pipe), flow measurement stations (e.g., Parshall
flume), and locations where a sudden change occurs
in grade. The following case history presents an
example of diffusion relocation.
Case History: Modification of the Chlorine Feed Point
at the Great Falls, Montana Wastewater Treatment
Plant (69)
The Great Falls wastewater treatment plant has two
chlorine contact basins with a combined volume of
2,460 m3 (650,000 gal). In 1981, the plant was
treating an average dry weather flow of about 438 Us
(10 mgd), and flow during the day varied from 131 to
789 Us (3-18 mgd). The theoretical basin contact time
at the peak flow of 789 Us (18 mgd) was 52 minutes.
Dye studies indicated that a large portion of the dye
was leaving the basins in less than 30 minutes.
The discharge permit for the plant included a 200
fecal coliforms/100 mL monthly average limit and a
maximum chlorine residual limit of 0.5 mg/L. Since
dechlorination facilities were not available, the staff
were required to balance the chlorine feed rate to
achieve the desired bacterial kill while not exceeding
the maximum residual chlorine concentration. The
plant was consistently exceeding their coliform permit
limit
In an effort to reduce short-circuiting, four redwood
baffles were installed, thereby creating an over-under
flow pattern. The effluent fecal coliform count
decreased but was still higher than the permit limit. A
modification was implemented to introduce chlorine
solution into the wastewater flow stream before it
entered the clarifier effluent launders. The
modification, shown in Figure 4-51, consisted of 1/2-in
flexible PVC tubing which was attached to the
clarifiers' scum baffle below the water level. 1/16-inch
holes were drilled into the tubing at 30-cm (12-in)
intervals.
Figure 4-51. Plan view of secondary clarifier with chlorine
solution diffuser.
Launder
Chlorine Solution Pipe
Following the installation of the modification, a 50
percent increase in the chlorine feed rate was
required to meet the fecal coliform limit; however, the
residual chlorine limit was achieved. Table 4-18
summarizes fecal coliform counts and chlorine resid-
ual concentrations before and after the modification.
The presence of chlorine in the clarifier effluent has
significantly limited the growth of algae on the
launders and walls; consequently, the amount of time
required for clarifier housekeeping has been reduced.
Direct injection of chlorine gas into wastewater is an
alternative approach to the conventional technique of
injecting strong aqueous chlorine solutions. The direct
gaseous injection technique, referred to as jet
disinfection, has been reputed to provide superior
bacterial kills per pound of chlorine at reduced
detention times. Such qualities would make jet
disinfection a good candidate for retrofitting a chlorine
disinfection process that has either poor initial mixing,
inadequate contact time, or insufficient dosage
capabilities. However, results from studies by
independent investigators did not substantiate these
claims (70,71). Onsite pilot testing of such a process
is recommended before a full-scale installation is
pursued.
c. Applied Chlorine Dosage
Typically, the chlorine dosage required to disinfect
secondary treatment plant effluents ranges from 2 to
10 mg/L (72). The required dosage will vary
126
-------
Table 4-18.
Fecal Coliform Counts Before and
Installation of Diffuser
After
Month
April 1978
May
June
July
August
September
October
November
December1
January 1 979
February
March
April
May2
June
July
August
September
October
November
December
Geometric Fecal
Coliform Mean
(#/100 mL)
800
2,000
1 1 ,600
1 1 ,700
6,100
174,000
2,300
46,000
511
1,000
83
63
32
282
58
175
138
147
28
26
34
Average CI2
Residual
(mg/L)
0.3
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.3
0.3
0.3
0.3
0.3
0.3
0.4
0.3
0.3
0.3
0.3
0.3
0.3
1 Installation of diffuser rings completed in December 1978.'
2 During May 1979 the ring diffusers were taken out of service for
maintenance, resulting in high coliform counts on two days.
depending on water quality, mixing conditions,
temperature, pH, contact time, and the level of
disinfection required. Additional feed capability can be
added by increasing the capacity of the chlorine
delivery system (e.g., chlorine regulators,
evaporators). Depending on the size of the existing
regulator, the capacity of an existing system can
sometimes be increased by exchanging rotometers.
The equipment supplier can provide information on
the expandability of existing chlorine feed equipment.
The potential for increasing the chlorine feed rate is
sometimes limited due to residual chlorine discharge
standards. In these situations, the installation of a
dechlorination system may be required. Recent
regulatory changes are also increasing the need for
dechlorination capability at treatment plants.
d. Length of Contact Time
Modifications that address short-circuiting are
intended to make the actual contact time nearly equal
to the theoretical contact time. However, these
measures may not be sufficient at treatment plants
where the theoretical contact time is too short. As a
general guideline, the wastewater should be in contact
with the chlorine residual for at least 15 and preferably
30 minutes at peak flow. The most direct method of
increasing contact time is to enlarge the basin. If this
modification is pursued, the expansion should be in
terms of the length of the flow path rather than
channel depth or width. Design information can be
found in other sources (66,68,72).
4.7.1.2 Ozone Disinfection Systems
Ozone disinfection systems perform by maintaining a
disinfectant residual in the wastewater over a given
period of time. Performance-limiting factors include:
inadequate ozone transfer efficiency, inadequate feed
gas preparation, short-circuiting, and inadequate
contact time.
a. Inadequate Ozone Transfer Efficiency
Factors that affect transfer efficiency include location
and type of diffusers, depth of contactor, applied
ozone dosage, and water quality (66). A summary of
important design considerations related to diffusers is
listed below:
• Diffusers should be at least 4.9 m (16 ft) deep for
systems located near sea level, and deeper at
higher plant elevations.
• All diffusers should be installed at the same
elevation to ensure an even gas flow distribution to
each diffuser.
• Gas flow rates should be 0.24-1.18 Us (0.5-2.5
cfm)/diffuser. High gas flow rates may result in
coarse bubbles; low flow rates may result in
unequal gas flow distribution.
• Diffusers should be located in the wastewater's
downflow stages of the contactor to maximize
transfer efficiency. Figure 4-52 shows an ozone
contactor utilizing this configuration.
Figure 4-52. Schematic of a 3-stage ozone contact basin
(66).
Control
Ozflne__
Flow
Meter
Stage
1
\J
n_r
Stage
2
Stage
3
\J>
Fine L-T
Bubble
Diffusers
More diffusers should be located in the first stage
than in subsequent stages to satisfy the initial
ozone demand of the wastewater. For example, a
127
-------
three-stage contactor may contain 50 percent of
the diffusers in the first stage (66).
however, it is recommended that, at a minimum,
three stages be provided.
In many cases, the above conditions can be met by
relocating and redistributing diffusers within an existing
contactor. Gas flow rates outside the recommended
range can generally be corrected by adding or
plugging off diffusers. When doing so, the existing
diffusers should be inspected for defects and replaced
as necessary. New diffusers should be identical to
those being replaced to maintain equal gas flow
distribution. If the maximum water depth above the
diffusers is less than 4.9 m (16 ft), modifications to
the ozone generator capacity or contactor may be
required. Increasing the depth of an existing contactor
is typically not cost effective; consequently, if this
approach is being considered, a new contactor should
be investigated.
b. Inadequate Feed-Gas Preparation
If either air or recycled oxygen is used as feed-gas for
the ozone generator, considerable attention should be
paid to the feed-gas preparation system. The two
parameters of most concern are the dew point
temperature and the particulate content of the feed-
gas. The dew point temperature, which is a measure
of the feed-gas moisture content, should never be
higher than -50°C (-58°F), and should generally be
less than -60°C (-76°F) to maximize ozone generator
output. An inability to maintain the dew point
temperature below these values is an indication of
needed modifications to the dessicant dryer process.
If existing towers can hold additional dessicant, they
should be topped off. Otherwise, it will be necessary
to provide additional towers.
Post-dessicant filters should be capable of removing
particulates less than 0.3-0.4 microns in diameter from
the feed-gas in order to optimize generator production
efficiency. To achieve this degree of removal may
require two-stage filtration. The first stage filter should
remove particulates greater than 1 micron, and the
second stage filter should remove particulates greater
than 0.3-0.4 microns. Further details on these and
other modifications is described in Design Manual:
Municipal Wastewater Disinfection (66).
c. Short-Circuiting
Poor disinfection has been noted at some facilities
due to excessive short-circuiting (66). Short-circuiting
has the potential of being a serious problem in many
ozone contactors for the following reasons:
• Extensive mixing and backflow occurs in every
stage of an ozone contactor due to continuous
diffusion of gas into the wastewater. This feature
makes it difficult to approximate plug flow.
• Contactors are often designed with an inadequate
number of positively isolated stages. The optimum
number of stages for a contactor is not known;
• The hydraulic detention time in an ozone contactor
is relatively short, approximately one-third that of a
chlorine contact basin, which compounds the
impact of short-circuiting.
It is often possible to increase the number of stages
within an existing ozone contactor by rearranging the
internal baffles and adding new ones. The newly
installed baffles should be made of either stainless
steel or fiberglass and should provide a serpentine
flow pattern that directs downward flow toward the
diffusers. Changing the staging in a contactor will also
necessitate rearrangement of the diffusers. To
minimize short-circuiting between stages, any drain
holes in the baffles between the stages should be
closed off and replaced with individual drain pits in
each stage.
d. Inadequate Contact Time
To reduce the fecal coliform density to 200 per 100
ml_, the minimum liquid contact time should be six
minutes, and preferably at least ten minutes at design
flow rates (66). In most cases, ozone contactors are
designed with theoretical contact times greater than
these values.
4.7.1.3 Ultraviolet (UV) Radiation Disinfection
Systems
UV disinfection systems inactivate bacteria by
damaging their genetic code with ultraviolet radiation,
thereby preventing replication. Two basic reactor
designs are currently used for UV disinfection
systems. In the first design, the lamps are encased in
quartz sleeves which are submerged in the
wastewater at all times (i.e, quartz system). In the
second design, wastewater flows through teflon tubes,
and the lamps are located outside and parallel to the
teflon tubes (i.e., teflon system). Inadequate
maintenance and cleaning can reduce UV system
performance. UV tubes and lamps must be cleaned
frequently and lamps replaced on a regular basis to
maintain a high level of radiation intensity transferred
to the liquid.
a. Inadequate Cleaning System
If a UV system was installed without an adequate
cleaning system, one should be provided. Cleaning is
generally accomplished by either mechanical or
chemical systems or a combination of the two. Teflon
systems can be modified for cleaning by adding a
high pressure nozzle system.
At the Rock Springs, Wyoming wastewater treatment
plant, a high-pressure nozzle system was able to
remove significant portions of the fouling material from
the inside surface of the teflon tubes; however, the
degree of cleaning was not the same for all tubes. At
the Chinook, Montana wastewater treatment plant, the
128
-------
high-pressure nozzle system was effective in
removing almost all of the fouling material from the
tubes (66,73). These results indicate that, even with a
high-pressure cleaning system, internal swabbing of
teflon tubes with a soft rag and detergent will
occassionally be necessary.
For quartz UV systems, either mechanical wipers or
ultrasonics can be provided as adjuncts to chemical
cleaning. Wipers appear to have greater potential for
maintaining quartz surfaces clean than do ultrasonics.
To facilitate chemical cleaning, a recirculation system
can be installed to pump a cleaning solution through
the UV disinfection unit. Additional information on
cleaning UV disinfection systems can be found in
other sources (66,73,74).
b. Short-Circuiting
Maximum use of the reactor volume is of great
importance in UV systems. A system that does not
provide the desired level of disinfection may have
significant dead zones or short-circuited areas. These
conditions can be identified by analyzing data from
dye tracer studies. The manner in which wastewater
enters and exits the reactor or channel strongly
influences the effective volume (66,73). With proper
design, the velocity should be equivalent at all points
upon entering and exiting. These conditions can be
ensured by installing weirs or perforated baffle plates
at the inlet and outlet. Figure 4-53 illustrates the use
of weirs and baffles for distributing the flow evenly
through UV reactors. Additional information can be
found in other sources (66,73,74).
4.7.2 Modifications Directed Toward the
Wastewater Stream
There are several physical modifications that can be
made at a wastewater treatment plant that do not
pertain to a particular unit process (e.g., modification
of wastewater characteristics and analytical and
process instrumentation).
Figure 4-53. Inlet and outlet considerations for quartz UV
systems (66).
to
Wastewater
4.7.2.1 Modifications
Characteristics
Treatment plants that receive a significant fraction of
their wastewater from industrial sources may
experience problems with biological treatability. In
most cases, the preferred remedy is implementation
and enforcement of an industrial pretreatment
program that can help to prevent pH extremes, toxic
compounds, and compounds resistant to biological
degradation from entering the POTW. In some cases,
it may be necessary to modify the composition of the
wastewater at the POTW. This may involve adding
caustic or acid to adjust the pH or adding a
coagulation process to precipitate heavy metals.
Before implementing any of these approaches, pilot-
scale studies should be performed to obtain design
information and to establish reliability of the process.
Weir.
. Perforated
Plates
Effluent
Chamber
/
/
f
S2
R-*
T-»
;::::::£::::T!H;::::
^ /
+r /
*• s
;-
s
\
Influent
Chamber
Lamps are Parallel or
Perpendicular to Flowpath
Lamp
Battery
a. Open Channel Type Configuration
Lamp Battery (Generally)
Parallel to Flowpath
Potential
Dead Zones
Perforated
Baffles ~
b. Sealed Cylindrical Reactor Configuration
4.7.2.2 Modifications to Provide Instrumentation
To gain an added measure of control over the
treatment process, many plants have installed various
types of in-basin analytical instruments that allow for
remote monitoring. Commonly used measurements
include pH, chlorine residual, dissolved oxygen, and
suspended solids. Considering the fact that this type
of instrumentation is on continuous duty in a relatively
unforgiving environment, many of these instruments
are prone to fouling and failure. If the instruments
require excessive recalibration to maintain a level of
reliability, many of the benefits are lost.
In general, remote monitoring systems for chlorine
residual are fairly reliable as are some pH monitoring
systems. Systems for continuous in-basin monitoring
of dissolved oxygen and suspended solids do not
have as good a performance record, though there
have been successful applications (67). If any of these
systems are to be installed, full-scale trial runs should
be conducted using instruments from several different
manufacturers. A review of considerations for
selecting and using in-line analytical instruments is
contained in Wastewater Treatment Plant
Instrumentation Handbook (75).
129
-------
Another facility modification designed to optimize
operator time and improve performance is the
installation of programmable controllers. These
devices automatically control such parameters as
operating sequences, operating speeds, and on-off
times of mechanical equipment. A time clock is an
example of a simple programmable controller. More
advanced controllers regulate the equipment based on
analog signals that provide feedback on a given
condition. An example of this would be a
programmable controller that regulates pumping on-off
times based on wastewater levels in a wet well.
Programmable controllers generally perform reliably
and give the POTW an added measure of
controllability.
4.7.3 Modifications Directed Toward Non-
Routine or Emergency Operation
It is necessary to provide operational flexibility that
allows for continued compliance during and after
periods of non-routine and emergency operation. A
facility that otherwise has adequate treatment capacity
may periodically experience permit violations due to
such events as power outages or equipment
downtime for maintenance purposes.
4.7.3.1 Modifications to Respond to Non-Routine
Operation
Flexibility in the form of unit process bypasses and
standby equipment is recommended for every major
unit process. A general guideline is to provide a firm
capacity for equipment such that, with any given piece
of equipment out of service, full capacity is still
available. For those processes not provided with
standby equipment, critical spare parts should be
either kept on-hand or readily available. Such
provisions will allow for minimal disruption in plant
performance due to equipment downtime.
4.7.3.2 Modifications to Respond to Emergency
Conditions
In terms of emergency operation, installation of an
alarm system that indicates equipment failures and
other abnormal conditions is recommended. The
alarm system should include an automatic dialer that
notifies someone in responsible charge when critical
components fail. For plants that have a history of
frequent power outages, a backup power generator
may be needed. If the duration of power outages is
not significant, a more economical approach for
facilities that experience only occasional outages
would be auto-restart devices that enable a piece of
equipment to automatically resume operation once
power is regained.
4.8 References
When an NTIS number is cited in a reference, that
reference is available from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
(703) 487-4650
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11. Lewis, R.S. Upgrading Mineral Media Trickling
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Wastewater Treatment Plants. Paper presented at
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47. Design Manual: Municipal Wastewater
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Facilities for Sewered Small Communities. EPA-
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Agency, Cincinnati, OH, 1977.
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State Sanitary Engineers, Health Education
Services, Albany, NY, 1978.
51. Personal communication with Richard
Montemayor, Wastewater Treatment Plant
Superintendent, City of Sterling, Colorado, May 5,
1988.
52. Reed, S.C. and R.K. Bastian. Aquaculture
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1980.
53. Ngo, V. and W. Poole. Boosting Treatment Pond
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54. Zirschky, J. and S.C. Reed. The Use of
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55. Wolverton, B.C. and R.C. McDonald. Upgrading
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56. Christie, S.E. Aerator-Mixer Combination - A New
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57. Hegg, B.A., K.L. Rakness, J.R. Schultz, and L.D.
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Wastewater Treatment Plant Performance - Phase
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OH, 1980.
58. Pederson, D. and B.A. Hegg. Results of the
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Northern Montana College, August 1986.
59. Process Design Manual - Sludge Treatment and
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60. Schultz, J.R., B.A. Hegg, K.L. Rakness. Realistic
Sludge Production For Activated Sludge Plants
Without Primary Clarifiers. JWPCF 54:1355,
1982.
61. Ettlich, W.F., D.J. Hinrichs, and T.S. Lineck.
Operations Manual Sludge Handling and
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Environmental Protection Agency, Washington,
DC, 1978.
62. Design Manual: Dewatering Municipal
Wastewater Sludges. EPA/625/1-87/014, U.S.
Environmental Protection Agency, Cincinnati, OH,
1987.
63. Cohen, D.B. and J.L. Puntenney. Metro Denver's
Experience With Large Scale Aerobic Digestion of
Waste Activated Sludge. Paper presented at 46th
WPCF Conference, Cleveland, OH, 1973.
64. Process Design Manual: Municipal Sludge
Landfills. EPA-625/1-78-010, U.S. Environmental
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65. Composting Municipal Wastewater Sludge.
EPA/625/4-85/014, U.S. Environmental Protection
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132
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66. Stover, E.L, C.N. Haas, K.L. Rakness, and O.K.
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Disinfection. EPA-625/1-86/021, U.S.
Environmental Protection Agency, Cincinnati, OH,
1986.,
67. Winter, F.J., and D.F. Ogle. On-Line Monitoring
Data Replaces Analytical Data. JWPCF 56, 1099,
1984.
68. Trussell, R.R. and T. Pollock. Design of
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Presented at Preconference Workshop at 56th
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1983.
69. Brown, D.F. C/2 Ring System. Water Engineering
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70. Warren, M.E., T.J. Heinemann, and R.D. Knight.
A Full-Scale Comparison of Two Chlorination
Systems, Jet Versus Conventional Aqueous
. Solution. Presented at the 55th Annual
Conference of the WPCF, St. Louis, MO, 1982.
71. Warriner, R., J.Y.C. Huang, and N. Sun-Nain Ni.
Methods of Chlorine Application for Secondary
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Conference of the WPCF, New Orleans, LA,
October 1974.
72. Wastewater Disinfection - Manual of Practice FD-
10. WPCF, 1986.
73. Kreft, P., O.K. Scheible, and A. Venosa. Hydraulic
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,1985.
74. White, S.C., E.B. Jernigan, and A. D. Venosa. A
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75. Manross, R.C. Wastewater Treatment Plant
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Water Engineering Research Laboratory, U.S.
Environmental Protection Agency, Cincinnati, OH,
1985.
76. Londong, Jorg. Dimensioning of Aerated Grit
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77. Evaluation of Flow Equalization in Municipal
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1979.
78. Melbinger, N.R., A.R. Lopez, A.M. Borowiec, and
J.J. Chack. The Bar Screens of New York.
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79. Marx, J.J., L. Ewing, W.C. Boyle, and P.E.
Thormodsgard. Full Scale Comparison of Ceramic
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80. A Practical Technology - Hydrograph Controlled
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1983.
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86. Olsson, G. J. Stephenson, and D. Chapman.
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1986.
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1988.
133
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FD-8, 1985.
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1988.
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1988.
99. Patoczka, Jerzy and W.W. Eckenfelder.
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100. Linne, S.R. and S.C. Chiesa. Operational
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Water/Engineering Management 135:26,
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134
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Appendix A
CPE Classification System, Factors Checklist, and Definitions
for Assessing Performance-Limiting Factors
Classification System for Prioritizing Performance-Limiting Factors*
Rating
A
B
C
NR
Adverse Effect of Factor on Plant Performance
Major effect on a long-term repetitive basis
Minimum effect on a routine basis or major
effect on a periodic basis
Minor effect
No Rating - factor has no adverse effect on
plant performance (i.e., satisfactory
assessment of this potentially performance-
limiting item)
* Factors are assessed based on their adverse effect on achieving desired
effluent quality.
135
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Checklist of Performance-Limitina Factors
Factor
A. Administration
1. Plant Administrators
a. Policies
b. Familiarity with Plant Needs
c. Supervision
d. Planning
2. Plant Staff
a. Manpower
1) Number
2) Plant Coverage
3) Workload Distribution
4) Personnel Turnover
b. Morale
1) Motivation
2) Pay
3) Work Environment
4) Working Conditions
c. Staff Qualification
1) Aptitude
2) Level of Education
3) Certification
d. Productivity
3. Financial
a. Insufficient Funding
b. Unnecessary Expenditures
c. Bond Indebtedness
Rating*
xxxxx
xxxxx
xxxxx
xxxxx
xxxxx
xxxxx
Comments
•
,
-
i
A - Major effect on a long-term repetitive basis.
B - Minimum effect on a routine basis or major effect on a periodic basis.
C - Minor effect.
NR - No rating.
136
-------
Checklist of Performance-Limitina Factors (continued)
Factor >
B. Maintenance
1. Preventive
a. Effective/Formal Program
b. Spare Parts Inventory
2. Corrective
a. Procedures
b. Critical Parts Procurement
3. General
a. Housekeeping
b. References Available
c. Staff Expertise
d. Technical Guidance (Maintenance)
e. Equipment Age - . . . .
C. Design
1 . Plant Loading
a. Organic
b. Hydraulic
c. Industrial
d. Toxic
e. Seasonal Variation
f. Infiltration/Inflow
g. Return Process Streams
Rating*
xxxxx
xxxxx
xxxxx
xxxxx
xxxxx
xxxxx
Comments
A - Major effect on a long-term repetitive basis.
B - Minimum effect on a routine basis or major effect on a periodic basis.
C - Minor effect.
NR - No rating.
137
-------
Checklist of Performance-Limitinq Factors (continued)
Factor
2. Unit Design Adequacy
a. Preliminary
b. Primary
c. Secondary
1) Process Flexibility
2) Process Controllability
3) "Aerator"
4) Clarifier .
d. Advanced Waste Treatment
1)
2)
3)
e. Disinfection
f. Sludge Wasting Capability
g. Sludge Thickening
h. Sludge Treatment
i. Ultimate Sludge Disposal
Rating*
xxxxx
xxxxx
xxxxx
Comments
' A - Major effect on a long-term repetitive basis.
B - Minimum effect on a routine basis or major effect on a periodic basis.
C - Minor effect.
NR - No rating.
138
-------
Checklist of Performance-Limitina Factors (continued)
Factor
3. Miscellaneous
a. Plant Location
b. Unit Process Layout
c. Lack of Unit Bypass
d. Flow Proportioning to Units
e. Alarm Systems
f. Alternate Power Sources
g. Process Automation
h. Lack of Standy Units for Key
Equipment
i. Laboratory Space and Equipment
j. Process Accessibility for Sampling
k. Equipment Accessibility for
Maintenance
1. Plant Inoperability Due to Weather
m. Equipment Malfunction
D. Operation
1. Testing
a. Performance Monitoring
b. Process Control Testing
Rating*
xxxxx
-
xxxxxx
xxxxxx
Comments
• • •- --
A - Major effect on a long-term repetitive basis.
B - Minimum effect on a routine basis or major effect on a periodic basis.
C - Minor effect.
NR - No rating.
139
-------
Checklist of Performance-Limiting Factors (continued)
Factor
3. Process Control Adjustments
a. Wastewater Treatment
Understanding
b. Application of Concepts and
Testing to Process Control
c. Technical Guidance (Operations)
d. Training
e. Insufficient Time on Job
4. O&M Manual/Procedures
a. Adequacy
b. Use
E. Miscellaneous
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
Rating*
xxxxxx
xxxxxx
xxxxxx
•
Comments
•
' A - Major effect on a long-term repetitive basis.
B - Minimum effect on a routine basis or major effect on a periodic basis.
C - Minor effect.
NR • No rating.
140
-------
Definitions for Assessing Performance-Limiting Factors
Category Explanation
A. Administration
1. Plant Administrators
a. Policies
b. Familiarity With Plant Needs
c. Supervision
d. Planning
2. Plant Staff
a. Manpower
1) Number
2) Plant Coverage
3) Workload Distribution
4) Personnel Turnover
b. Morale
1) Motivation
2) Pay
3) Work Environment
Do staff members have authority to make required operation (e.g., adjust
valve), maintenance (e.g., hire electrician), and/or administrative (e.g.,
purchase critical piece of equipment) decisions, or do policies require a
strict adherence to a "chain of command" (which has caused critical
decisions to be delayed and in turn has affected plant performance and
reliability)? Does any established administrative policy limit plant
performance (e.g., non-support of training; industrial contributions not
being controlled; or plant funding too low because of campaign to avoid
rate increases)? .
Do administrators have a first-hand knowledge of plant needs through
plant visits or discussions with operators? If not, has this been a cause
of poor plant performance and reliability through poor budget decisions,
poor staff morale, poor operation and maintenance procedures, poor
design decisions, etc.?
Do the management styles, organizational capabilities, motivational skills,
budgeting skills, or communication practices at any management level
adversely impact the plant to the extent that performance is affected?
Does lack of long range plans for facility replacement, emergency
response, etc., adversely impact plant performance?
Does a limited number of people employed have a detrimental effect on
plant operations or maintenance (e.g., not getting the necessary work
done)?
Is plant coverage adequate such that necessary operational activities are
accomplished? Can appropriate adjustments be made during the
evenings, weekends or holidays?
Does the improper distribution of adequate manpower (i.e., a higher
priority on maintenance tasks) prevent process adjustments from being
made or cause them to be made at inappropriate times, resulting in poor
plant performance?
Does a high personnel turnover rate cause operation and/or maintenance
problems that affect process performance or reliability?
Does the plant staff want to do a good job because they are motivated
by self-satisfaction?
Does a low pay scale or benefits package discourage more highly
qualified persons from applying for operator positions or cause operators
to leave after they are trained?
Does a poor work environment create a condition for more "sloppy work
habits" and lower operator morale?
141
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c. Staff Qualifications
1) Aptitude
2) Level of Education
3) Certification
d. Productivity
3. Financial
a. Funding
b. Expenditures
c. Bond Indebtedness
B. Maintenance
1. Preventive
a, Effective/Formal Program
b. Spare Parts Inventory
2. Corrective
a. Procedures
b. Critical Parts Procurement
c. Technical Guidance
(Maintenance)
3. General
a. Housekeeping
b. References Available
Does the lack of capacity for learning or understanding new ideas by
critical staff members cause improper O&M decisions leading to poor
plant performance or reliability?
Does a low level of education result in poor O&M decisions? Does a
high level of education cause needed training to be felt unnecessary?
Does the lack of adequately certified personnel result in poor O&M
decisions?
Does the plant staff conduct the daily operation and maintenance tasks
in an efficient manner? Is time used efficiently?
Does the lack of available funds (e.g., inadequate rate structure) cause
poor salary schedules, insufficient spare parts that results in delays in
equipment repair, insufficient money for capital outlays for improvements
or replacement, etc.?
Does the manner in which available funds are used cause problems in
obtaining needed equipment, staff, etc.? Are funds spent on lower
priority items while needed, higher priority items are unfunded?
Does the annual bond debt payment limit the amount of funds available
for other needed items such as equipment, staff, etc.?
Does the absence or lack of an effective scheduling and recording
procedure cause unnecessary equipment failures or excessive downtime
that results in plant performance or reliability problems?
Does a critically low or nonexistent spare parts inventory cause
unnecessary long delays in equipment repairs that result in degraded
process performance?
Are procedures available to initiate maintenance activities on observed
equipment operating irregularities (e.g., work order system)? Does the
lack of emergency response procedures result in activities that fail to
protect process needs during breakdowns of critical equipment (e.g.,
maintaining oxygen supply to organisms during blower breakdowns)?
Do delays in getting replacement parts caused by procurement
procedures result in extended periods of equipment downtime?
Is technical guidance for repairing or installing equipment necessary to
decrease equipment downtime, is it available and retained?
Does a lack of good housekeeping procedures (e.g., grit channel
cleaning; bar screen cleaning; unkempt, untidy, or cluttered working
environment) cause an excessive equipment failure rate?
Does the absence or lack of good equipment reference sources result in
unnecessary equipment failure and/or downtime for repairs (includes
maintenance portion of O&M Manual, equipment catalogs, pump curves,
etc.)?
142
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c. Staff Expertise
d. Technical Guidance
(Maintenance)
e. Equipment Age
C. Design
1. Plant Loading
a. Organic
b. Hydraulic
c. Industrial
d. Toxic
e. Seasonal Variation
f. Infiltration/Inflow
g. Return Process Streams
2. Unit Design Adequacy
a. Preliminary
b. Primary
c. Secondary
1) Process Flexibility
2) Process Controllability
Does the plant staff have the necessary expertise to keep the equipment
operating and to make equipment repairs when necessary?
Does inappropriate guidance for repairing, maintaining or installing
equipment from a technical resource (e.g., equipment supplier or
contract service) result in equipment downtime that adversely affects
performance?
Does the age or outdatedness of critical pieces of equipment cause
excessive equipment downtime and/or inefficient process performance
and reliability (due to unavailability of replacement parts)?
Does the presence of "shock" loading characteristics over and above
what the plant was designed for, or over and above what is thought to
be tolerable, cause degraded process performance by one or more of
the loadings (a-e) listed below?
(e.g., high-volume on-off lift station pumps)
(e.g., winter flows at ski resort)
Does excessive infiltration or inflow cause degraded process
performance because the plant cannot handle the extra flow?
Does excessive volume and/or a highly organic or toxic return process
flow stream cause adverse effects on process performance, equipment
problems, etc.?
Do the design features of any preliminary treatment unit cause problems
in downstream equipment or processes that have led to degraded plant
performance?
Does the performance of the primary treatment unit contribute to
problems in downstream equipment or processes that have degraded
plant performance? Do the units have any design problem areas that
have caused less than required performance to meet overall treatment
objectives?
Does the unavailability of adequate valves, piping, etc. limit plant
performance and reliability when other modes of operation of the existing
plant can be utilized to improve performance (e.g., operate activated
sludge plant in plug, step, or contact stabilization mode; operate RBCs in
step loading mode)?
Do the existing process control features provide adequate adjustment
and measurement over the appropriate flows (e.g., return sludge) in the
range necessary to optimize process performance; or is the flow difficult
to adjust, variable once adjusted, not measured and recorded, not easily
measurable, etc.?
143
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3) "Aerator"
4) Clarifier
d. Advanced Waste Treatment
e. Disinfection
f. Sludge Wasting Capability*
g. Sludge
Thickening/Dewatering*
h. Sludge Treatment*
i. Ultimate Sludge Disposal*
Does the type, size, shape, or location of the "aerator" (aeration basin,
trickling filter, RBC, etc.) hinder its ability to adequately treat the sewage
and provide for stable operation? Is oxygen transfer capacity
inadequate?
Does a deficient design cause poor sedimentation due to the size, type,
or depth of the clarifier; placement or length of the weirs; or does
inadequate scum removal adversely affect performance?
Advanced waste treatment is any process of wastewater treatment that
upgrades water quality to meet specific effluent limits that cannot be met
by conventional primary and secondary treatment processes (i.e.,
nitrification towers, chemical treatment, multimedia filters). (Space is
available in the Checklist to accommodate advanced processes
encountered during the CPE.)
Does the unit have any design limitations that contribute to its inability to
accomplish disinfection (i.e., proper mixing, detention time, feed rates,
feeding rates proportional to flow, etc.)?
Does the inability to waste sludge adversely affect plant performance?
Can desired volume of sludge be wasted? Can sludge wasting be
adequately controlled? Can sludge wasted be sampled without extreme
difficulty?
Does the type or size of the sludge thickening/dewatering process
hinder sludge wasting capability or sludge treatment such that plant
performance is adversely affected?
Does the type or size of the sludge treatment process hinder sludge
stabilization (once sludge has been removed from the wastewater
treatment system), thereby causing process operation problems (e.g.,
odors, limited sludge wasting, poor quality recycle streams, etc.)?
Is the ultimate sludge disposal program, including facilities and disposal
area, of sufficient size and type to adequately handle the sludge
production from the plant? Are there any specific areas that limit ultimate
sludge disposal such as seasonal weather variations or crop harvesting?
For the purposes of this manual, these factors are assessed on their impact on a plant to achieve final
effluent requirements and are not assessed relative to meeting sludge regulation criteria.
3. Miscellaneous
a. Plant Location
b. Unit Process Layout
c. Lack of Unit Bypass
The design "miscellaneous" category covers areas of design inadequacy
not specified in the previous design categories. (Space is available in the
Checklist to accommodate additional items not listed.)
Does a poor plant location or poor roads leading into the plant cause it
to be inaccessible during certain periods of the year (e.g., winter) for
chemical or equipment delivery or for routine operation?
Does the arrangement of the unit processes cause inefficient utilization
of operator's time for checking various processes, collecting samples,
making adjustments, etc.?
Does the lack of a unit bypass cause plant upset and long-term poor
treatment when a short-term bypass could have minimized pollutional
load to the receiving waters; cause necessary preventive or emergency
maintenance items to be cancelled or delayed; or cause more than one
unit to be out of service when maintaining only one unit?
144
-------
Does inadequate flow proportioning or flow splitting to duplicate units
cause problems or partial unit overloads that degrade effluent quality or
hinder achievement of optimum process performance?
Does the absence or inadequacy of an alarm system for critical pieces
of equipment or processes cause degraded process performance?
Does the absence of an alternate power source cause problems in
reliability of plant operation leading to degraded plant performance?
Does the breakdown or improper workings of automatic process
monitoring or control features cause degradation of process
performance? Could the availability of automatic monitoring or control
devices enhance process control and improve plant performance?
h. Lack of Standby Units for Key Does the lack of standby units for key equipment cause degraded
Equipment process performance during breakdown or during necessary preventive
maintenance activities?
d. Flow Proportioning Units
e. Alarm Systems
f. Alternate Power Source
g. Process Automation
i. Laboratory Space and
Equipment
j. Process Accessibility for
Sampling
k. Equipment Accessibility for
Maintenance
I. Plant Inoperability Due to
Weather
m. Equipment Malfunction
D. Operation
1. Testing
a. Performance Monitoring
b. Process Control Testing
2. Process Control Adjustments
a. Wastewater Treatment
Understanding
b. Application of Concepts and
Testing to Process Control
c. Technical Guidance
(Operations)
d. Training
Does the absence of an adequately equipped analytyical and/or process
control laboratory limit plant performance?
Does the inaccessibility of various process flow streams (e.g., recycle
streams) for sampling prevent needed information from being obtained?
Does the inaccessibility of various pieces of equipment cause extensive
downtime or difficulty in making needed repairs or adjustments?
Are certain units in the plant extremely vulnerable to weather changes
(e.g., cold temperatures) and, as such, do not operate at all or do not
operate as efficiently as necessary to achieve required performance?
Does malfunctioning equipment (i.e., not functioning in accordance with
design) cause deteriorated process performance?
Are the monitoring tests truly representative of plant performance (e.g.,
does a sludge accountability analysis support plant performance
records)?
Does the absence or wrong type of process control testing cause
improper operational control decisions to be made?
Is the operator's lack of a basic understanding of wastewater treatment
(e.g., limited exposure to terminology, lack of understanding of the
function of unit processes, etc.) a factor in poor operational decisions
and poor plant performance or reliability?
Is the staff deficient in the application of their knowledge of wastewater
treatment and interpretation of process control testing such that
improper process control adjustments are made?
Does inappropriate operational information received from a technical
resource (e.g., design engineer, equipment representative, State trainer
Does inattendance at available training programs result in poor process
control decisions by the plant staff or administrators?
145
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e. Plant Familiarity
4. O&M Manual/Procedures
a. Adequacy
b. Use
E. Miscellaneous
Does the short time on the job and associated unfamiliarity with plant
needs result in the absence of process control adjustments or in
improper process control adjustments being made (e.g., opening or
closing a wrong valve, turning on or off a wrong pump, etc.)?
Does inappropriate guidance provided by the O&M Manual/Procedures
result in poor or improper operational decisions?
Does a good O&M Manual/Procedures, not used by the operator, cause
poor process control and poor treatment that could have been avoided?
The "miscellaneous" category allows addition of factors not covered by
the above definitions. Space is available in the Checklist to
accommodate these additional items.
146
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Appendix B
CPE Summary Sheet for Ranking Performance-Limiting Factors
147
-------
CPE Summary Sheet for Ranking Performance-Limiting Factors
Plant Name/Location -
CPE Performed by '
Date
Plant Type:
Design Flow:
Actual Row:
Plant Performance Summary:
RANKING TABLE
Ranking
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Rating
A
A
A
A
B
B
Performance-Limiting Factor
148
-------
CPE Summary Sheet Terms
PLANT TYPE
DESIGN FLOW
ACTUAL FLOW
PLANT PERFORMANCE SUMMARY
RANKING TABLE
RANKING
RATING PERFORMANCE-LIMITING FACTORS
Brief but specific description of type of plant (e.g., two-
stage trickling filter with anaerobic digester or activated
sludge with aerobic sludge digestion and drying beds).
Daily average plant design flow rate as of most recent
upgrade.
Daily average wastewater flow rate for current operating
condition (e.g., for past year).
Brief description of plant performance as related to
discharge permit requirements (e.g., for past year).
A list of the major causes of decreased plant performance
and reliability.
Causes of decreased plant performance and reliability, with
the most critical factors listed first. (Typically only "A" and
"B" factors are listed.)
Items identified from the Checklist (Appendix A).
149
-------
-------
Appendix C
Example CPE Report
Results of a CPE at the Springfield,
Kansas POTW
Facility Background
The Springfield POTW is a rotating biological
contactor (RBC) type of treatment plant designed for
an average daily wastewater flow of 87.6 L/s (2.0
mgd). The plant was originally completed as a primary
treatment plant in 1965 and upgraded for secondary
treatment in 1980. The plant serves the City of
Springfield with an estimated population of 9,000. A
railcar washing operation is thought to be the source
of the only significant industrial waste, but sampling
has not confirmed this.
Plant records indicated that wastewater flows had
been averaging close to design flow; however, flow
calibration during the CPE indicated that flow was
actually about 52.6 L/s (1.2 mgd), or only 60 percent
of design flow. Current organic loading was estimated
to also be about 60 percent of design.
The Springfield plant consists of the following unit
processes:
• Vortex grit chamber
• Mechanical bar screen
• 23-cm (9-in) Parshall flume
• Lift station
• One 18-m (60-ft) diameter primary clarifier
• Four-stage RBCs (12 shafts)
• Two 6 m x 33 m (20 ft x 110 ft) secondary clarifiers
• Two-cell chlorine contact chamber
• Two 11-m (35-ft) diameter anaerobic digesters
(Note: During the CPE, the second-stage digester
was not in service.)
• 2,800 m2 (30,000 sq ft) drying beds
The Springfield plant is required to meet standard
secondary treatment effluent requirements with
20,000/40,000 fecal coliform limits. Monthly average
NPDES monitoring data indicate performance of the
plant has generally been within standards, but
individual daily analyses indicated violation of effluent
requirements. During the CPE, solids loss from both
the primary and secondary clarifiers was observed.
Major Unit Process Evaluation
Major plant processes were evaluated for their
capacity to adequately treat current loadings and the
general applicability of a CCP to improve
performance. Current hydraulic loadings were based
on historical plant flow data records. During the CPE,
the totalizer and flow recording devices were
evaluated by comparing depth measurements taken in
the Parshall flume to the instrument recordings.
Measured flows were within 8 percent of indicated
flows. Organic loading on the RBC as measured by
soluble BOD5 was estimated using 30 percent total
BOD removal in the primary clarifier and 50 percent of
the primary effluent total BOD as soluble BOD.
A sludge accountability analysis indicated that
reported effluent quality was probably representative
of actual effluent quality.
The ability to handle current loads was assessed
using a numerical point system, which resulted in the
plant being categorized Type 1, 2, or 3 as described
below:
• Type 1. Facilities are adequate. Performance
problems could be alleviated with training and/or
minor facility modifications.
• Type 2. Facilities are marginal. Improved
performance from existing facilities may be possible
by addressing performance-limiting factors.
« Type 3. Facilities are inadequate. To consistently
meet effluent requirements, major facility
modifications are necessary.
The results of the major unit process evaluation are
shown in Table C-1.
151
-------
Tablo C-1. Springfield, Kansas POTW Major Unit Process
Evaluation.
"Assessed Type ,
Aerator
Secondary Clarifier
Sludge Handling Capability
Total of Major Processes
Plant Type
1
1
2
1
2
As shown in Table C-1, the aerator, secondary
clarifier, and the total of major processes all received
sufficient points to receive a Type 1 classification.
Sludge handling capability received a Type 2
classification. The major limitation regarding sludge
handling was ultimate sludge disposal capability in
winter.
The evaluation of major unit processes indicates that
sludge handling will likely require supplemental
capacity, but the other major processes have
adequate capacity. In general, the CCP approach
appears applicable if the ultimate sludge disposal
capacity can be addressed.
The potential capacity of major unit processes in the
Springfield plant is illustrated in Table C-2. The
horizontal bar graph associated with each major
process depicts the potential capacity of that process.
Primary clarifiers, secondary clarifiers, and anaerobic
digesters were all projected to have capacities slightly
greater than design. The chlorine contact basin was
rated at design capacity. The RBCs were not
projected to have enough capacity to adequately treat
design loadings; however, the units were rated to
have adequate capacity to treat current flows.
The ultimate sludge disposal capability as represented
by the sludge drying beds was assessed to be
inadequate at current and design flows. The limitation
in ultimate sludge disposal is during the winter when
the drying beds freeze and do not dry.
Performance-Limiting Factors
During the CPE, the plant's performance-limiting
factors in the areas of design, administration,
operation, and maintenance were identified. These
factors are listed below and the most significant ones
briefly discussed.
1. Sewage Treatment Understanding (Operation). A
lack of understanding of biological treatment
process fundamentals and operational
requirements and goals significantly limit plant
performance. This limitation could be addressed
with onsite training over a period of months or by
periodic attendance at seminars, schools, etc.,
over a period of many years.
2. Ultimate Sludge Disposal (Design). Existing drying
beds are inadequate for needed year-round
sludge disposal. Additional beds may be a long-
term solution. Liquid sludge haul to farmland may
be a workable interim solution. Documentation for
the need and .administrative training are
necessary.
3. Process Control Testing (Operation). An almost
complete lack of process control testing existed
. prior to the CPE. Base-level testing was initiated
during the CPE. Onsite training is required to
optimize process control testing and to teach the
operational staff to properly apply the test results
to process controls.
4. Return Process Streams (Design). Secondary
sludge being returned to the primary clarifier was
causing primary clarifier "bulking." Anaerobic
digester supernatant return was also adversely .
impacting plant performance. The capability to
minimize the adverse impact of return process
streams can be acquired through long-term, onsite
traning.
5. Administrative Familiarity With Plant Needs
(Administration). Through past poor
communication and improper operation, the plant
administrators have been misled regarding plant
needs. Increased familiarity in the areas of
treatment fundamentals, operation and
maintenance requirements, funding needs, and
safety concerns is needed.
6. Process Controllability (Design). Existing flow-
splitting capability to the RBCs is inadequate.
Correction of this deficiency will likely require
minor design modifications.
Other factors that contributed to limited performance,
but in a less significant way, include: an inadequate
spare parts inventory, limited staff expertise, in
handling emergency maintenance, a lack of an
alternate power source, and a lack of needed sample
taps.
Performance Improvement Activities
Improved effluent quality, to consistently meet NPDES
permit limits, is expected if a CCP is implemented.
Costs
Costs associated with a 12-month CCP at Springfield
would be for the CCP facilitator and for laboratory
equipment to allow process control testing, minor
modifications (i.e., sample taps and flow splitting), and
supplemental winter sludge disposal (Table C-3).
Estimated costs for conducting a CCP at Springfield
are listed in Table C-3:
152
-------
Table C-2. Springfield, Kansas POTW Capacity Potential.
Major Unit Process
Flow. m3/d (mgd)
Comments
3,000 3,800 4,500 5,300 6,100 6,800 7,600 8,300 9,100
(0.8) (1.0) (1.2) (1.4) (1.6) (1.8) (2.0) (2.2) (2.4)
Primary Clarifier
RBCs
Secondary'Clarifier
Chlorine Contact
Digesters
Ultimate Disposal
Ultimate Disposal Liquid Haul
to Land
Rated at 33 m3/ma/d (800 gpd/sq ft).
Estimated 30% BOD5 removed.
Rated at 3.9 kg SBOD/m2/d (0.8
lb/d/1,000 sq ft) lower than 4.9 kg
SBOD/m2/d (1.0 lb/d/1,000 sq ft)
because of sidestream return.
Rated at 20 m3/m2/d (500 gpd/sq ft
because of configuration.
30 minute average hydraulic detention
time.
Rated at 30 days detention time.
Assume 4.5% combined sludge
wasted to digester.
42 days turnover time at current flow;
inadequate in winter.
Farm land is available. Will require
truck or contract haul.
Estimated Current
Flow
Design
Flow
Table C-3. Springfield, Kansas Estimated CCP Costs.
Item Cost($)
CCP Facilitator
City Costs
Minor Modifications
Lab Equipment
Supplementary Sludge Disposal
Total CCP Costs
9,500 (one-time)
7,000 (one-time)
400 (one-time)
18,000 (annual)
25,400
34,900
153
-------
-------
Form
D-1
D-2
D-3
D-4
D-5
D-6
D-7
Appendix D
Data Collection Forms Used In Conducting CPEs
Title
Preliminary Plant Information
Administration Data
Design Data
Operations Data
Maintenance Data
Performance Data
Interview Data
155
-------
Form D-1
Preliminary Plant Information to Collect by Telephone
Plant Name _
Phone Contact
Position
Phone. No.
Design Flow _
Date
Current Flow
Service Population
Year Plant Built
Most Recent Upgrade
Directions to Plant
Major Processes (type and size):
Preliminary Treatment
Primary Treatment
Secondary Treatment
Aeration Basin
Trickling Rlter
Clarifier
Disinfection
Unusual Processes or Equipment
Any processes or equipment currently not operational
Preliminary Data
156
-------
Who does performance monitoring tests?
Who does process control test?
What process control and laboratory test equipment is available?
Plant coverage (8 am - 5 pm, 24 hr, etc.)
Work hours of key individuals
Known conflicts with scheduling fjeldwork
Contact for scheduling fieldwork
Administrator or owner (responsible official)
Who has records on the budget? ..
Who is consultant? . ,_.
Information resources (availability):
As-built construction plans
O&M Manual
Monitoring records.
Equipment literature.
Process control records
157
Preliminary Data
-------
A, Name and Location
Name of Facility
Type of Facility
Owner
Form D-2
Administration Data
Administrative Office:
Mailing Address
Primary Contact
Title
Telephone No.
Treatment Plant:
Mailing Address
Primary Contact
Title
Telephone No.
Administration Data
158
-------
B. Kickoff Meeting
Purpose of CPE
• Background
• Assess plant potential for achieving compliance
• Identify current factors limiting performance
• Outline follow-up activities
Schedule of Events
a. Kickoff Meeting
b. Plant Tour
c. Review Budget/User Charge Ordinance/Revenues
d. On-site Data Collection/Review O&M Manual/
NPDES Permit/Design Data/Operating Records
e. Conduct Personnel Interviews
f. Exit Meeting
Day
Time
159
Administration Data
-------
B. Kickoff Meeting (continued)
Personnel Interviews Scheduling Sheets (includes off-site administrators/owners, budgeting personnel,
laboratory, maintenance personnel, plant administrators, shift personnel, operators, etc.)
Person
Title
Day
Time
Administration Data
160
-------
B. Kickoff Meeting (continued)
Attendance List
Municipality
Name
Date
Title/Dept.
Telephone No.
161
Administration Data
-------
C. Permit Information (attach copies of pertinent pages from actual permit if available)
Permit Number
Dale Permit Issued
Date Permit Expires
Receiving Stream
Effluent Limits and Monitoring Requirements:
Parameter*
Flow, mgd
BOD5, mg/L
BOD5, Ib/d
TSS, mg/L
TSS, Ib/d
Fecal Coliform, No./100 ml
Chlorine Residual, mg/L
pH, units
Phosphorus, mg/L
Phosphorus, Ib/d
Ammonia, mg/L
Ammonia, Ib/d
Oil & Grease, mg/L
Others
Maximum
Monthly
Average
Maximum
Weekly
Average
Monitoring
Frequency
Reguired
Sample
Type
Reguired
' Note if seasonal limit.
Administration Data
162
-------
O. Organization
Governing Body (name and scheduled meetings)
Structure
From Governing Body to POTW
POTW Staff
Staff Meetings (formal/informal)
163
Administration Data
-------
D. Organization (continued)
Reporting Requirements (formal/informal)
Service Area (general description - residential, industrial, seasonal influences, etc.)
Compliance Schedule
Observations (openess, awareness of plant needs, management style, etc.)
Administration Data
,164
-------
£. Personnel
No.
Plant
Title/Name
Certification
Pay Scale
% of Time
atPOTW
Off-Site
No,
Title/Name
Pay Scale
% of Time
Allocated to POTW
165
Administration Data
-------
F. Training
Operator Training Budget
Training Incentives
Training Over Last Year,
G. Plant Coverage
Weekdays
Shifts (timesloverlap?/number per shift)
Weekends & Holidays
Alarms (on what processes? dialer?)
Administration Data
166
-------
H. Plant Budget/Expenditures (attach copy of actual budget and/or expenditures if available)
Budget Year: • - - •- • to ' -
Expenditure Period: to
Category
Administrative Salaries (including fringes)
Plant Staff Salaries (including fringes)
Utilities
Electric (see Part I)
Gas (see Part I)
Chemicals
Vehicles
Training
Budget
Amount
Expenditure
Amount
Operations Subtotal
Capital Outlay (see Part J)
Bond Debt Retirement
Reserve
Capital Improvement Subtotal
Plant Total
Observations
Budget Prepared By:
167
Administration Data
-------
/. Energy Consumption
Electricity
Source of Information
Base Cost 0/kWh (attach rate schedule if available)
Demand Cost
Month/Year
Days in
Billing Period
Energy Energy
Usage (kWh) Demand Cost
EGA Cost*
Base Cost Total Cost
Total
"EGA = Energy Cost Adjustment, which is the pass-through cost allowed to public service companies for
increased fuel cost to generate electricity.
Administration Data
168
-------
/. Energy Consumption (continued)
Natural Gas/Fuel Oil/LPG
Source of Information
Unit Cost
Month/Year
(attach rate schedule if available)
Days in
Billing Period
Usaqe
Cost
Total
Average
Miscellaneous (Fuel Oil, Digester Gas, etc.):
169
Administration Data
-------
J. Capital Outlays
Bond Retirement
Bond Type Year Issued
Duration
Interest Rate
Annual
Payment
Description of
Project Financed
Capital Improvement Reserve (Self sustaining utility? Exceed bond requirements? Replacement philosophy?)
Capital Replacement Plan (Available? Items scheduled for replacement? Attach if available.)
Observations
Administration Data
170
-------
K. Revenue
User Charges
Type of Connection
Residential
Non-Residential (Industry, Commercial,
Government Institutions)
Other
Fee
User Fee Revenue
Connection Fee
Type of Connection
Residential
Non-Residential
Fee
Connection Fee Revenue
Other Sources of Revenue
Total Revenue for Evaluation Period (see Part H)
Miscellaneous
Are rates and budgets reviewed annually?
When was last rate increase? (How much?)
Proposed increases?
171
Administration Data
-------
L Exit Meeting
Attendance List
Municipality
Name
Date
Attach Copy of Exit Meeting Handouts
Title/Dept.
Telephone No.
Administration Data
172
-------
Form D:3
Design Data
A. Plant Flow Diagram (attach if available)
B. Plant Solids Handling Diagram (attach if available)
C. Upgrading and/or Expansion History (historical studies, current evaluations, proposed modifications, etc.)
173
Design Data
-------
D. Influent Characteristics
Average Daily Flow: Design
Current
Maximum Daily Flow: Design
Current
Maximum Hourly Flow: Design
Current
Average Daily BOD5:
Average Daily TSS:
Infiltration Inflow
Seasonal Variations
Major Industrial Wastes
Name
Design
Current
Design
Current
Collection System
Lift Stations
Number of Residential Taps Served
Population Served
Flow
mgd x 3,785 =
mgd x 3,785 =
mgd x 3,785 =
mgd x 3,785 =
mgd x 3,785 =
mgd x 3,785 =
Ib x 0.454 =
Ib x 0.454 =
Ib x 0.454 =
Ib x 0.454 =
m3/d
m3/d
m3/d
m3/d
m3/d
m3/d
kg
kg
kg
kg
BOD.; Load TSS Load
Other
Design Data
174
-------
£. Unit Processes
Flow Stream Measured
Control Section:
Type and Size
Flow Measurement
(Form for each flow measuring device)
Location
Comments (operational problems, maintenance problems, unique features, preventive maintenance procedures,
etc.):
Recorder
Name
Model
Flow Range
Calibration Frequency
Date of Last Calibration
Totalizer
Comments (operation and design problems, unique features, etc.
Accuracy Check During CPE
Method of Check:
Results:
175
Design Data
-------
£. Unit Processes (continued)
Pumping
(Complete as many forms as necessary)
Flow Stream No. of
Pumped Type Pumps Name Model hp_
Capacity Head
Comments: (flow control, suitability of installed equipment, results of capacity check, etc.)
Comments:
Comments:
Design Data
176
-------
E. Unit Processes (continued)
Mechanical Bar Screen
Name ' -
Model
Preliminary Treatment
Horsepower
Bar Screen Width
Bar Spacing
Within Building?
Description of Operation:
Hand-Cleaned Bar Screen
Bar Screen Width
Bar Spacing
Cleaning Frequency
Within Building?
Description of Operation:
Screenings Volume:
Normal
Peak
Screenings Disposal
Comments
inch x 2.54 =
inch, O.C. x 2.54 =
Heated?
inch x 2.54 =
inch, O.C. x 2.54 =
Heated
cu yd x 0.75 =
cu yd x 0.75 :
cm
cm
cm
cm
m3/d
m3/d
177
Design Data
-------
E. Unit Processes (continued)
Comminutor
Name
Model
Within Building?
Maintenance:
Preliminary Treatment
Horsepower
Heated?
Comments:
Grit Removal
Description of Unit:
Grit Volume:
Normal
Peak
Disposal of Grit:
Comments:
cu yd x 0.75 =
_ cu yd x 0.75 =
m3/d
m3/d
Design Data
178
-------
E. Unit Processes (continued)
Primary Clarifier(s)
Number
Water Depth (Shallowest)
Water Depth (Deepest)
Weir Location
Weir Length
Total Surface Area
Total Volume
Flow (Design)
(Operating)
Weir Overflow Rate
(Design)
(Operating)
Surface Settling Rate
(Design)
(Operating)
Collector Mechanism Name
Model
Scum Collection and Treatment:
Scum Volume:
Scum Treatment/Disposal:
Primary Treatment
Surface Dimensions
ft x 0.3 =
ft x 0.3 =
m
m
ft x 0.3 =
m
sq ft x 0.093 =
cu ft x 0.028 =
mgd x 3,785 =
mgd x 3,785 =
gpd/ftxO.012 =
gpd/ft x 0.012 =
gpd/sq ft x 0.04 =
gpd/sq ft x 0.04 =
Horsepower
m3/d
m3/d
_ m3/m/d
m3/m/d
m3/m2/d
m3/m2/d
179
Design Data
-------
£. Unit Processes (continued)
Aeration Basin(s)
Number
Secondary Treatment (Activated Sludge)
Surface Dimensions
Water Depth
Total Volume
Flow (Design)
(Operating)
Wastewater Detention Time (Design)
(Operating)
BOD5 Loading
(Design)
(Operating)
ft x 0.3 = m
cuft X7.48 = gal
mgd x 3,785 = , m3/d
mgd x 3,785 = m3/d
hr
hr
Ib/d/1,000cuftx0.16 =
Ib/d/1,000cuftx0.16 =
kg/m3//d
kg/m3/d
Covered?
Comments:
Modes of Operation (current and other options; i.e., complete mix, plug flow, step loading, tapered aeration
sketch options):
Design Data
180
-------
£. Unit Processes (continued)
Contact Basin
Surface Dimensions (ea unit)_
Water Depth '
Volume (ea)
Secondary Treatment (Contact Stabilization)
Flow (Design)
(Operating)
Sewage Detention Time (Design)
(Operating)
Covered?
Comments:
Reaeration Basin
Surface Dimensions (ea unit).
Water Depth
Volume (ea)
ft x 0.3 =
cu ft x 7.48 = _
mgd x 3,785 =
mgd x 3,785 =
ft x 0.3 =
cu ft x 7.48 =
Hydraulic Detention Time at 100 Percent Return
(Design)
(Operating)
Flexibility to Operate as Conventional or Step Feed:
Covered?
Comments:
mm
min
No. of Units
m
_gal
_m3/d
m3/d
No. of Units
m
gal
hr
hr
181
Design Data
-------
£. Unit Processes (continued)
Surface Mechanical Aeration
No. of Aerators
SecondaryTreatment (Oxygen Supply)
Name
Model
Rated Capacity
Speed Control:
Submergence Control:
Diffused Aeration
Blowers
No. of Blowers
Model
Capacity
Minimum Inlet Air Temperature
Horsepower
Ib/hr x 0.454 =
Name
Horsepower
cfm x 0.028 =
Diffusers
Types of Diffusers (coarse, fine, ceramic, stainless steel, etc.):
kg/hr
m3/min
Manufacturer
Water Depth.
Rated Standard Transfer Efficiency
Water Temperature (maximum)
Plant Elevation
Jet Aeration
No. of Aerators
Model
Rated Capacity
Controls:
Comments
Model
Name
Horsepower
Ib/hr x 0.454 =
kg/hr
NOTE: See Appendix F for procedure for converting standard oxygenation rates to actual oxygenation rates.
Design Data
182
-------
E. Unit Processes (continued)
Filter(s)
Number
SecondaryTreatment (Trickling Filter)
Covered? .
Surface Dimensions
Media Type
Specific Surface Area
Media Depth
Surface Area
Media Volume
sq ft/cu ft x 32.8 =
ft x 0.3 =
m
Flow
(Design)
(Operating)
Organic Loading
(Design)
(Operating)
Hydraulic Loading
(Design)
(Operating)
sq ft x 0.093 =
cu ft x 0.028 =
mgd x 3,785 =
mgd x 3,785 =
m2
m3/d
m3/d
Ib/d/1,000cuftx0.016 =
Ib/d/1,000cuftx0.016 =
gpd/sq ft x 0.04 =
gpd/sq ft x 0.04 =
kg/m3/d
kg/m3/d
m3/m2/d
Recirculation (description, ranges, current operation, sketch relative to clarifiers)
Mode of Operation
Climate (freezing in winter?)
Comments
183
Design Data
-------
£. Unit Processes (continued)
SecondaryTreatment (Rotating Biological Contactor - RBC)
RBC
Manufacturer L
No. of Shafts
Length of Shafts
Disk Diameter
ft x 0.3 =
No. of Stages
m
ft x 0.3 =
m
Total Media Surface Area
sq ft x 0.093 =
rr.2
Type of Drive (air, mechanical):
Covered?
Heated?
Flow
(Design)
(Operating)
Hydraulic Loading
(Design)
mgd x 3,785 =
mgd x 3,785 =
gpd/sq ft x 0.04 =
m3/d
m3/d
m3/m2/d
(Operating)
Temperature (Design)
gpd/sq ft x 0.04 =
(Operating)
First Stage Organic Loading
(Design)
(Operating)
Total System Organic Loading
(Design)
(Operating)
Ib SBOD/d/1,000 sq ft x 4.88 =
Ib SBOD/d/1,000 sq ft x 4.88 =
Ib SBOD/d/1,000 sq ft x 4.88 =
Ib SBOD/d/1,000 sq ft x 4.88 =
g SBOD/m2/d
g SBOD/m2/d
g SBOD/m2/d
g SBOD/m2/d
Flexibility to Distribute Load to Stages (sketch)
Load Cells Available?
Comments
Design Data
184
-------
E. Unit Processes (continued)
SecondaryTreatment (Activated Biofilter - ABF)
Biocell
Manufacturer
Surface Dimensions
Total Surface Area
Media Depth
Total Media Volume
Media Type
No. of Cells
sq ft x 0.093 =
ft x 0.3 =
m
cu ft x 0.028 =
Specific Surface Area
BOD5 Loading
(Design)
(Operating)
sq ft/cu ft x 32.8 =
Ib/d/1,000cuftx0.016 =
Ib/d/1,000cuftx0.016 =
m2/m3
kg/m3/d
kg/m3/d
Recirculation Tank
Dimensions (LxWxD)
Volume
ft x 0.3 =
cu ft x 7.48 =
gal
m
Aeration Basin
Surface Dimensions
Depth
Volume
Hydraulic Detention Time (Design)
Comments
ft x 0.3
cu ft x 7.48 =
m
gal
min (Operating)
mm
185
Design Data
-------
£. Unit Processes (continued)
Number
SecondaryTreatment (Secondary Clarifiers)
Surface Dimensions
Water Depth (Shallowest)
Water Depth (Deepest)
Weir/Launder Location(s)
ft x 0.3 =
ft x 0.3 =
m
m
Percent of Clarification Developed by Launders
Weir Length
ft x 0.3 =
m
Weir Overflow Rate
(Design)
(Operating)
Total Surface Area
Total Volume
Flow (Design)
(Operating)
Surface Settling Rate
(Design)
gpd/ftx 0.012 =
gpd/ftxO.012 =
sq ft x 0.093 = _
cu ft x 0.028 = _
mgd x 3,785 = _
mgd x 3,785 =
m3/m/d
m3/m/d
m3/d
m3/d
(Operating)
Hydraulic Detention Time (Design )
gpd/sq ft x 0.04 =
gpd/sq ft x 0.04 =
hr (Operating)
hr
(Actual From Dye Test)
hr
Collector Mechanism Name
Model
Horsepower
Return Sludge Collector Mechanism Type
Mechanical Seal Location (center well?/collector arm?):
Scum Collection and Removal:
Scum Volume:
Normal
cu yd x 0.75 =
Peak
cu yd x 0.75 =
m3/d
m3/d
Scum Treatment/Disposal:
Design Data
186
-------
E. Unit Processes (continued)
Number
SecondaryTreatment (Stabilization Ponds)
Surface Dimensions
Water Depth #1
Water Depth #2
Water Depth #3
Flow (Design)
(Operating)
Hydraulic Detention Time
BOD5 Loading
(Design)
(Operating)
Type of Aeration
Type of Aeration
Type of Aeration
Name
No. of Blowers
Horsepower
Mixing Pond #1
Mixing Pond #2
Mixing Pond #3
Recirculation Description
ft x 0.3 - m Length: Width Ratio
ft x 0.3 - m LengthrWidth Ratio
ft x 0.3 - m Length:Width Ratio
mgdx 3,785 = m3/d
mgd x 3,785 = m3/d
(Design ) days (Operating) days
Ib/ac/d x 1.12 = kg/ha/d
Ib/ac/d x 1.12 = kg/ha/d
No. of Aerators/Pond #1
No. of Aerators/Pond #2
No. of Aerators/Pond #3
Model Horsepower
Name Model
Air Capacity
hp/1 06 gal x 0.0002 = kW/m3
hp/1 06 gal x 0.0002 = kW/m3
hp/1 06 gal x 0.0002 = kW/m3
Ratio of Recirculation to Flow: (Design)
Flexibility (series/parallel operation; discharge structure)
(Operating)
Comments (short-circuiting?, etc.)
187
Design Data
-------
£. Unit Processes (continued)
Contact Basin(s)
Number
Chlorine Disinfection
Surface Dimensions
Channel Length-to-Width Ratio No. of Bends
Water Depth ft x 0.3 = m
Total Volume
cu ft x 7.48 =
gal
Detention Time: (Design)
Drain Capability:
min (Operating)
mm
Scum Removal Capability:
Comments:
Chlorinator(s)
Name
Capacity
Type of Injection:
Flow Proportioned?
Number
Ib/d x 0.454 =
kg/d
Feed Rate (Operating)
Dosage (Operating) _
Comments:
Ib/d x 0.454 =
mg/l
kg/d
Design Data
188
-------
£. Unit Processes (continued)
Ultra-Violet Disinfection
Number
Length of Flow Path
Lamp Type
ft x 0.3 =
m
Number of Lamps per Unit
Lamp Length ft x 0.3 = m
Effective Arc Length
Nominal UV Output
ft x 0.3 =
m
(W/ft arc) x 3.281 =
W/m arc
Lamp Horizontal Spacing (center-to-center)
Lamp Vertical Spacing (center-to-center) _
Flow (Design)
(Operating)
in x 2.54 =
cm
in x 2.54 =
cm
mgd x 3,785 =
mgd x 3,785 =
m3/d
m3/d
Maximum Influent Bacterial Density
Average Influent TSS mg/l
Comments:
No.7100 mL
189
Design Data
-------
£. Unit Processes (continued)
Sludge Handling Facilities
Primary Sludge
Description of Pumping Procedure (time clocks; variable speed pumps; etc.)
Method of Sludge Volume Measurement
Sampling Location
Sampling Procedure
Comments
Return Sludge
Description of Sludge Movement (scrape to clarifier hopper; pump to aeration basin inlet channels; etc.)
Controllable Capacity Ranges
(Low) _ mgd x 3,785 =
(High)
mgd x 3,785 =
m3/d
Method of Control
Method of RAS Volume Measurement
Sampling Location
Sampling Procedure
Comments
Waste Sludge
Description of Waste Procedure (variable-speed pump wastes from separate clarifier hopper; continuous or by
time clock; etc.)
Method of Waste Volume Measurement
Sampling Location
Sampling Procedure
Comments
Design Data
190
-------
E. Unit Processes (continued)
Digesters
Number of Basins
Treatment (Aerobic Digestion)
Surface Dimensions
Water Depth
Total Volume
Covered?
ft x 0.3 =
m
cu ft x 7.48 =
gal
Heated?
Mode of Operation
Oxygen Supply [Complete Form "Secondary Treatment (Oxygen Supply/1/
Decanting Procedure
Scum Removal
Comments
191
Design Data
-------
£. Unit Processes (continued)
Treatment (Anaerobic Digestion)
Primary Digesters
Number of Digesters Diameter ft x 0.3 =
m
Sidewall Depth
Center Depth
Total Volume
ft x 0.3 =
m
ft x 0.3 =
m
cu ft x 0.028 =
Floating Cover?
Flow (Design)
(Operating)
Detention Time (Design)
mgd x 3,785 = • • •
mgd x 3,785 = '
days (Operating)
m3/d
m3/d
days
Volatile Solids Loading (Design) _
(Operating)
Heating
Manufacturer
Ib/cuftx 16 =
Ib/cuftx 16 =
kg/m3
kg/m3
Capacity
Mixing
Manufacturer
Number of Units
Sampling Ports
Model Number
106 Btu/hrx 0.29 =
106 W
Type
Mode of Operation
Gas System
Comments
Design Data
192
-------
E. Unit Processes (continued)
Secondary Digesters
Number of Digesters
Treatment (Anaerobic Digestion)
Diameter
ft x 0.3 =
m
Sidewall Depth
Center Depth
Total Volume
ft x 0.3 =
m
ft x 0.3 =
m
cu ft x 0.028 =
Floating Cover?
Flow (Design)
(Operating)
mgd x 3,785 =
mgd x 3,785 =
m3/d
Detention Time (Design)
Volatile Solids Loading (Design)
days :(Operating)
Ib/cuftx 16 =
days
(Operating)
Ib/cuftx 16 =
kg/m3
kg/m3
Heating
Manufacturer
Capacity
Mixing
Manufacturer
Number of Units
Sampling Ports
Model Number
106 Btu/hrxO.29 =,
106 W
Type
Mode of Operation
Gas System
Comments
193
Design Data
-------
£. Unit Processes (continued)
Sludge Drying Beds
Number of Beds Dimensions (ea)
Covered? (Glass?) (Plastic?)
Dewatered Sludge Removal:
Dewatering
_Surface Area (Total)_
Subnatant Drain To
Mode of Operation (depth of sludge draw; seasonal operation; etc.):
Comments:
Other Dewatering Unit(s)
Type(s) of Unit(s)
Number of Units
Model
Loading Rate (Design)
(Operating)
Polymer Used
Manufacturer
Horsepower
Ib/hr x 0.454 =
Ib/hr x 0.454 =
Ib/dry ton
Cake Solids (Design)
xO.5 =
.g/kg
(Operating)
Hours of Operation (Design) _
percent solids
percent solids
(Operating)
hr/wk
hr/wk
Comments:
kg/hr
kg/hr
Design Data
194
-------
£. Unit Processes (continued)
Describe Procedure
Options
Seasonal Operation
Comments
Ultimate Disposal
195
Design Data
-------
E. Unit Processes (continued)
Summary of Plant .Horsepower
Item
HP
Usage, % Weighted HP
TOTAL
Design Data
196
-------
F. Other Design Information •
Standby Power (description of unit; automatic activation? capacity for which processes? frequency of use; etc.)
Alarm Systems (description of system; units covered; etc.)
Plant Automation (description of any plant automation not covered under more specific topics)
Miscellaneous (see miscellaneous disgn factors list in Appendix A)
197
Design Data
-------
Form D-4
Operations Data
A. Process Control Strategy and Direction
Who Sets Major Process Control Strategies?
Who Sets Dally Adjustments?
Where Is Help Sought When Desired Performance Is Not Achieved?
How is Staff Input Utilized (Opinions, etc.)?
B. Specific Process Control Procedures
Sampling and Testing
Automatic Sampling
Sampling Locations
Composites (flow porportioned?)
Flow Measurement and Recording
Meter Calibration
Influent
Prim. Sludge RAS WAS To Digesters
Readings Taken
Onsite Accuracy Check (describe)
198
Operations Data
-------
S. Specific Process Control Procedures (continued)
Primary Clarification
Sludge Removal (method of control/adjustment?)
Performance Monitoring (tests used? solids balance?)
Other
A. S. Secondary Systems
Sludge Mass Control (tests used? method of control/adjustment?)
Return Sludge Control (tests used? method of settling rates?)
Microscopy Used?
Filament Identification Capability?
DO Control (monitoring locations; frequency; control/adjustment?)
Mode Changes (capability; changed?)
Other
Fixed Film Secondary Systems
Secondary Clarifier Sludge Removal (adjustments?)
Soluble BOD Removal (monitored? recycle adjustments?)
Sidestream Returns (monitored? options?)
Other
Operations Data
199
-------
fl. Specific Process Control Procedures (continued)
Sludge Handling and Disposal
Purpose Relative to Other Processes
Sludge Thickening (monitored? process control/optimization?)
Sludge Stabilization (monitored? process control/optimization?)
Sludge Dewatering (monitored? process control/optimization?)
Sludge Disposal (meet requirements? monitoring? options?)
Miscellaneous
Unit Process Monitoring
Data Development/Interpretation
Trend Charts
C. Process Control References (Specifically note references that are the source of poor process control
decisions or strategies, suspected or definitely identified)
200
Operations Data
-------
D. Operations and Maintenance Manual
Manual Contents
Manual Located at Treatment Plant? Yes
No
Adequate Coverage? Yes
No
Operator/Manager Responsibilities Defined? Yes
Design Criteria Listed? Yes No
Process Control Covered? Yes No '. •. •
Manual Used? Yes No
Operating Staff
Specified in Manual? Yes No
No
Present Staffing Meets/Exceeds Specified Staffing Requirement? Yes
No
Operator Qualifications
Specified in O&M Manual? Yes
No
Operator Possesses Specified Qualifications? Yes
Certification? Yes No
Experience? Yes No
Comments:
No
Operations Data
201
-------
E Pretreatment Program
Is There An Active Pretreatment Program? Yes
Significant Industries
Problems
Staffing
Sampling Frequency (announced? unannounced?)
Comments
No
F. Sewer Use Ordinance
Are There Problems At The Plant Related to Wastewater Influent?
Does The Ordinance Contain Provisions Necessary To Solve The Problem?
Is The Sewer Use Ordinance Being Enforced?
Comments:
202
Operations Data
-------
G. Laboratory Capability
Bench Space
Storage Space
Floor Area
Lighting
Electricity
Potable Water Supply
Compressed Air
Vacuum
Chemical Fume Hood
Air Conditioning
Desk
Records Storage
Facilities
Adequate
(Yes/No)
Comments
Equipment and Instruments
Available
(Yes/No)
Comments
Portable DO Meter
Settleometer
Graduated Cylinder
Turbidimeter
Core Sampler (Sludge Depth)
pH Meter
Centrifuge
Distilled Water (Source)
BOD Incubator
TSS Drying Oven
FC Water Bath Incubator
Hot Air Oven
Refrigerator
Autoclave
Analytical Balance
Microscope
Desiccator
Automatic Samplers
Operations Data
203
-------
G. Laboratory Capability (continued)
Total Solids
TSS
VSS
Volatile Acids
Alkalinity
Temperature
PH
Turbidity
BOD
COD
TKN
Ammonia
NO2-NO3-N
Total Phosphorus
Total Coliform
Fecal Coliform
Oxygen Uptake Rate (OUR)
Specific OUR
Analytical Capabilities
Available
(Yes/No)
Method/Comments
Miscellaneous
Quality Control
EPA Reference Samples
Duplicate Tests (schedule, records, etc.)
Other
Standard Procedures/References
Standard Methods
Site-Specific Procedures
Training
204
, Operations Data
-------
Form D-5
Maintenance Data
A. Preventive Maintenance Program
Program Description
Method of Scheduling
Method of Documenting Work Completed
Small Spare Parts (fuses, belts, bearings, packing diffusers, etc.)
Major Spare Parts (large motors, gear boxes, blowers, flowmeter, etc.)
References
O&M Manual
Accurate As-Builts
Manufacturers' Literature
Adequacy of Resources Available
Outside Support
Tools/Lubricants
Work Area
Maintenance Data
205
-------
S. Emergency Maintenance Program
Priority Setting (relationship to process control decisions)
Expertise
On-Sile
Technical Support
Method of Initiating Work Activities (work order?)
Method of Documenting Work Completed
Control Parts Procurement (policy restrictions? sources?)
Comments
206
Maintenance Data
-------
C. General
Housekeeping
Method Of Factoring Costs For Parts/Equipment Into Budgeting Process
Equipment Age
Equipment Accessibility for Maintenance
Equipment or Processes Out of Service Due to Breakdowns (identify equipment or process, description of
problem, length of time out of service, what has been done, what remains to be done, estimated time before
repair, how it affects performance)
During the CPE (list and explain)
During the last 12 months (list and explain)
Maintenance Data
207
-------
Form D-6
Performance Data
A. Source of Data
B. Reported Monitoring Data for Previous 12 Months (flows in mgd; others in mg/l, except as noted)
Mo/Yr
AVG
Raw
Flow BOD5 TSS
Primary Effluent
BOD5 TSS
' ,
208
Performance Data
-------
S. Reported Monitoring Data for Previous 12 Months (flows in mgd; others in mg/l, except as noted)
Mo/Yr
AVta
Final Effluent
Flow BOD5 TSS
.'•••.'• •
Other
BOD5 TSS
Influent Quality Control Checks
Population:
Per Capita BOD5 Contribution (0.17-0.22 Ib/capita typical):
Service Taps:
Persons per Tap (2-4 typical):
Performance Data
209
-------
C. Selected Plant Data for Previous 12 Months
Parameter
Units
Mo/Yr
AVG
210
Performance Data
-------
D. Permit Performance Violations Within Last 12 Months (30-day averages, 7-day averages, instantaneous
violations, effluent mass violations, percent removal violations)
E. Reasons (if any) Reported Monitoring Data Are Not Believed to Represent Actual Effluent Quality
(unrepresentative sampling, improper lab analyses, selective reporting, unaccounted-for sludge loss [see
attached sludge accountability evaluation], etc.)
Performance Data
211
-------
F. Performance Monitoring/Sludge Accountability Summary Sheet
1. Sludge Accountability
• Anticipated Sludge Production (see Table 2-7; Note: unit production
values include solids lost in plant effluent)
• Accounted-For Sludge
- wasted intentionally
- effluent sludge
Total: 2 + 3
• Unaccounted-For Sludge: 1 - 4
5 v 365
• Unaccounted-For Sludge Percentage: 100 x 5 -r 1
if -15 < 7 < 15 then not possible to conclude that a problem with
sludge wasting exists.
if 7 > 15 then problem with effluent monitoring indicated.
if 7 < -15 then may indicate organic loading greater than
typical domestic (i.e., industrial loading).
2. Performance Monitoring Assessment
• Projected Acutual Effluent TSS
- recorded effluent TSS
- projected increase in effluent TSS: 6 * (8.34 x flow in mgd)
- estimated actual effluent TSS: 8 + 9
• Projected Actual Effluent BOD5
- recorded effluent BODs
- projected increase in effluent BOD5: 0.5 x 9
- estimated actual effluent BOD5: 10 + 11
Jb/yr
_mg/l
_mg/l
mg/l
_mg/l
_mg/l
mg/l
Item
1
Ib/yr
Ib/yr
Ib/yr
Ib/yr
Ib/d
%
2
3
4
5
6
7
8
9
10
11
212
Performance Data
-------
Form D-7
Interview Data
A. Interview Concerns
Interviews are used to obtain feedback in the four categories of administration, design, operation,
maintenance. The following items are presented to assist the interviewers in obtaining this feedback.
1. Administration
Owner Responsibility?
Attitude toward staff?
Attitude toward regulatory agency?
Self-sustaining facility attitude?
Attitude toward consultants?
Policies?
Communications (Formal/Informal)?
Performance Goal
Is plant in compliance?
If yes, what is making it that way?
If no, why not?
Is regulatory pressure felt for performance?
What are performance requirements?
What is stated performance goal?
Administrative Support
Budget
Within range of other plants?
"Drained" to general fund?
Unnecessary expenditures?
Sufficient?
Attitude toward rates?
Sewer Use Ordinance/Pretreatment Programs .
Available?
Enforced?
Personnel
Within range of other plants?
Allows adequate time?
Motivation, pay/comparison with other munbicipal units, supervision, working conditions?
Productivity?
Turnover?
Training Support?
Involvement
Visits to treatment plant?
Awareness of facility?
Request status reports (performance and cost-related)?
Familiarity with plant needs?
and
Interview Data
213
-------
2. Design
Influent problems?
Equipment problems?
Status of warranties?
Return process streams?
Preliminary treatment?
Secondary treatment?
Disinfection?
Advanced waste treatment?
Sludge handling and disposal?
Flow measurement?
Flow splitting?
Alarms or alternate power?
3. Operation
Communication of decisions?
Key control parameters?
Involvement of staff?
Laboratory quality?
Administrative support?
Staffing?
Performance problems?
Unit process optimization?
External support?
Process control testing/adjustments?
O & M Manual/references?
4. Maintenance
How are priorities set?
Attitude toward program?
Emergency versus preventive?
Reliability? (spare parts or critical part procurement)
Staffing?
Equipment assessibility?
214
Interview Data
-------
B. Personnel Interviews
Name
Title
Certification
Years at Plant
_Years Experience
Area of Responsibility
Education
Training
Concerns/Recommendations (Administration, Design, Operation, Maintenance):
Interview Data
215
-------
-------
Appendix E
Procedures for Converting Oxygenation Rates and Flows
Procedures for Converting Standard Oxygenation Rates to Actual Oxygenation Rates
AOTR = SOTR(a)
sw
.(T-20)
or
AOTE = SOTE(a)
PC -CT
* cvtr I
JT-20)
Where:
AOTR = actual oxygen transfer rate, Ib O2/hp-hr.
AOTE = actual oxygen transfer efficiency, percent.
SOTR = standard oxygen transfer rate, Ib O2/hp-hr (from Table 2-3).
SOTE = standard oxygen transfer efficiency, percent (from Table 2-3).
a = relative rate of oxygen transfer in wastewater compared to water. Estimate from Table E-1.
]} = relative oxygen saturation value in wastewater compared to water. Estimate 0 = 0.95 for mixed
liquor.
0 = temperature correction constant, 9 = 1.024.
Cs = Oxygen saturation value of clean water at standard conditions, Cs = 9.17 mg/L.
Csw = oxygen saturation value of clean water at site conditions of temperature and pressure, mg/L.
p — p
sw ^14.7 [ 14 7
Ct = mixed liquor DO concentration, mg/L.
T = temperature of the liquid, °C.
C14 7 = oxygen saturation value of clean water at standard pressure of 14.7 psi and actual water temperature
(see Table E-2).
217
-------
= actual pressure at oxygen transfer point.
a) For surface aerators, use atmospheric pressure (see Figure E-1).
b) For others, use atmospheric pressure from Figure E-1, plus the pressure at one-third depth of
the tank from the surface to the diffusers (i.e., diffuser depth in feet x 0.33 x 0.434 psi/ft).
[NOTE: The one-third depth is an approximation. Actual pressure at oxygen transfer point may
vary depending on such factors as type of diffusers and tank geometry. To be more
conservative, utilize atmospheric pressure.]
Procedure for Converting Actual (or Inlet) CFM to Standard CFM
rT
I a
ACFM = SCFM —
IT
Where:
ACFM = volume of air measured at actual site conditions of elevation and maximum summertime air
temperature.
SCFM = volume of air measured at standard conditions of 14.7 psia and 20°C.
Ta = maximum summertime inlet temperature, °R [NOTE: °R = °F + 460].
Ts = standard inlet temperature, °R.
Pa = inlet pressure to blower at site conditions, psia (see Figure E-1).
Ps s inlet pressure to blower at standard conditions, 14.7 psia.
218
-------
Table E-1. Typical Values of Alpha (a) Used For Estimating AOTR/SOTR
Aeration Device Typical a
Coarse Bubble Diffusers
Fine Bubble Diffusers
Jet Aeration
Surface Mechanical Aerators
Submerged Turbines
0.85
0.50
0.75
0.90
0.85
Table E-2. Oxygen Saturation at Standard Pressure and Actual Water Temperature
Temperature
°C
0
1
2
3
4
5
6
7
8
g
10
11
12
13
14
15
16
17
18
19
• , 20
21
22
23
24
25
26
27
28
29
30
Dissolved Oxygen
Saturation Level
mg/l
14.62
14.23
13.84
13.48
13.13
12.80
12.48
12.17
11.87
11.59
11.33
11.09
10.83
10.60
10.37
10.15
9.95
9.74
9.54
9.35
9.17
8.99
8.83
8.68
8.53
8.38
8.22
8.07
7.92
7.77
7.63
219
-------
Figure E-1. Atmospheric pressure at various altitudes.
Atmospheric pressure,
psia
15
14
13
12
11
10
i—r
-I—I—1-
T—i—i—i—i—i—r
Read to left
for pressure
i—I—i—i—i—I—'
Read to right
for vacuum
i—r
Barometer,
inches Hg absolute
3456
Altitude above sea level, 1,000 feet
i—r
30
28
26
24
22
8 g
220
-------
Appendix F
Guidelines for Field Estimating Equipment Power Usage
The power that a particular piece of equipment is drawing can be estimated in the field by measuring the
current being drawn by the motor. The measured power being drawn by a motor (inductive user) is "apparent
power" and must be multiplied by the power factor (PF) to calculate actual power. Four methods are available
to arrive at a suitable power factor:
1. Assume a power factor:
Use 0.9 for recently constructed plants that likely included use of capacitors to adjust the power factor
toward 1.0. Use 0.75 for old and small plants where it is unlikelythiat capacitors have been added.
s
2. Measure the "plant power factor" using an ammeter and the plant kilowatthour meter and assume the
power factor applies for larger pieces of equipment. See Table F-1 for calculation worksheet. (WARNING:
DO NOT USE THIS METHOD UNLESS QUALIFIED.)
3. Ask the electric company to measure the power factor or actual power usage of specific equipment.
4. Rent an appropriate instrument and measure power factor or actual power usage. (WARNING: DO NOT
USE THIS METHOD UNLESS QUALIFIED.)
Once the PF has been determined, the following calculations can be used to estimate power drawn by a
particular piece of equipment:
Amps
Measure:
Average Voltage (line-to-line) =
Average Amperage = •
Calculate:
kVA = Vx Ax (3)1/2 ^ 1,000(3-phasepower)
kW = kVA x PF
whp = kW -r 0.746
Volts
221
-------
Table F-1. Worksheet for Calculation of Power Factor
Apparent Power
Line-to-Line Voltage on Incoming Power:
V!.2 = _ Volts
V2-3 « _ VoltS
Vvs = _ Volts
Vavg = _ Volts
Amperage for Each Phase on Incoming Power:
IT = _ _ Amps
I2 = _ Amps
la = _ Amps
Amps
avg =
kVA = Volts x Amps x (3)1/2 ^ 1,000 =
Actual Power
watthours/revolution (from meter)
CTRa =
PTRb =
CTRx PTR =
Disc Speed =
Seconds/
Revolutions(s)
kW = Kh x TR x (Disc Rev/sec) x (3,600 sec/hour) x (1 kW/1,000 Watts) =
Power Factor
PF = kW •=• kVA -r
aCTR (Current Transformer Ratio) - ratio of primary to secondary current. For current transformer rated
200:5, ratio is 200/5 or 40/1.
*>PTR (Potential Transformer Ratio) - ratio of primary to secondary voltage. For potential transformer
rated 480:120, ratio is 480/120 or 4/1.
CTR (Tranformer Ratio) - total ratio of current and potential transformers. For CTR = 200:5 and PTR =
480:120, TR = 40x4 = 160.
222
-------
Appendix G
Example Forms for Establishing a Preventive Maintenance Program for Small POTWs
Form
G-1
G-2
G-3
G-4
Title
Equipment Information Sheet
Daily Preventive Maintenance
Weekly Preventive Maintenance
Monthly Preventive Maintenance
223
-------
Equipment:
Location
Manufacture^
Type
Additional Data
Drive
Type_
Description
Form G-1
Equipment Information Sheet
Plant Equipment Number
Model
Original installation Date
Serial No.
Rated Capacity
Rated Pressure or Head
Manufacturer
Motor
Manufacturer
Frame
Type
HP
RPM
Enclosure Type
S.F.
Suppliers
Company Name & Address
Rated Amperage
Rated Voltage
Contact Person
Telephone No.
Additional Information and Comments
224
-------
Form G-1 (continued)
Equipment Information Sheet
Recommended Preventive Maintenance
Frequency
Recommended Lubricants
Part
Lubricant Name/Description
Source
Recommended Spare Parts
Part Description
Number
Quantity
225
-------
Form G-2
Daily Preventive Maintenance
Intel Building
• Check operation of grit pump, cyclone, grit bin, pump seal water
pressure
(psi), leakage (drops/min)
• Check grit collector for unusual noise or torque.
• Check flow meter operation, chain, float, stilling well.
• Check auto sampler operation and bottle installation.
Grit Separator #2 Building
• Check for unusual noise or vibration in collector or conveyer.
Primary Clarifier
• Check for unusual noise or vibration in drive unit.
Aeration Building
• Check blowers for unusual noise or vibration.
Temperature
#1 Inlet °C Outlet
#2 Inlet °C
°C
#3 Inlet
Outlet
Outlet
Check auto sampler operation and bottle installation
a.m.
p.m.
a.m.
p.m.
a.m.
p.m.
a.m.
p.m.
a.m.
p.m.
a.m.
p.m.
a.m.
p.m.
a.m.
p.m.
Time
Initials
(Form continued to include all process units and buildings requiring daily maintenance.)
226
-------
Form G-3
Weekly Preventive Maintenance
Date
Initials
Inlet Building
• Grit #1 Collector Drive: Apply grease to upper and lower bearings in worm
gear housing.
• Grit #1 Collector Drive: Check oil level in gear housing; remove condensate
in gear drive.
• Grit #1 Collector Drive: Lubricate chain between drive unit and motor gear. ;
• Grit #1 Collector Drive: Check torque overload alarm for proper operation. ,
• Comminutor: Check oil level in main gear box (lower).
• Comminutor: Check oil level in motor gear unit.
• Automatic Sampler: Remove and clean sampling tube and strainer. •
Grit Separator #2 Building
• Grit #2 Drive Unit: Check oil level in Philadelphia gear reducer.
• Grit #2 Conveyor Unit: Apply grease to all bearings of chain drive and
support sprockets.
• Grit #2 Conveyor Unit: Check oil level in conveyor drive reducer.
Aeration Building
• Automatic Sampler: Remove and clean sampling tube and strainer.
• Aeration Blowers: Check oil level - 3 points (gears, two bearings). '
» Aeration Blowers: Operate blower(s) (10-min each) not in service. Check oil
level and temperature.
(Form continued to include all process units and buildings requiring daily maintenance.)
227
-------
Form G-4
Monthly Preventive Maintenance
Date
Initials
Inlet Building
• Automatic Sampler: Check pump tubing for signs of failure. Remove from
pump housing to inspect.
Grit Separator #2 Building
• Grit Reducer: Apply grease to upper and lower bearings.
Primary Clarifier
• Drive Mechanism: Check gear lubrication (dipstick). Check base plate
lubrication (oil cap).
• Gear Reducer: Apply grease to upper, lower, and two side bearings.
(Form continued to include all process units and buildings requiring daily maintenance.)
Provide similar forms for:
• Quarterly Preventive Maintenance
* Semiannual Preventive Maintenance
• Annual Preventive Maintenance
228
-------
Appendix H
Example Wastewater Treatment Plant Administration and Management Audit
229
-------
PLANT:
IMMEDIATE SUPERVISOR
DATE
DEPARTMENT (circle one)
Administration (off-site)
Administration (on-site)
Operation
Maintenance
Laboratory
Instructions
Listed below are several questions that will be used to help assess existing conditions. By answering these
questions you can provide helpful feedback to the management and administration and indicate areas where you
feel changes could improve conditions, areas that you feel need enhancement, or areas where you want to
support present conditions. Your answers are confidential. Please circle the number you think best describes
your opinion. If you want to clarify a point, please do so in the space immediately below each question. If you
need more room, reference the question number and use another sheet of paper or the back of the page. Thank
you for your cooperation and support.
1. In your present position, how adequate is your pay (compensation including benefits)? (1 is very
poor, 5 is average, and 10 is excellent)
1
8
10
2. Relative to your immediate supervisor, please rate the following factors. (1 is poor, 5 is average, and
10 is high)
A. Technical capability
B. Ability to recognize problems and set priorities
C. Ability to conduct staff meetings
D. Ability to discuss job-related ideas or problems
E. Ability to follow through with decisions on tasks
F. Use of staff suggestions
G. Ability to motivate staff
H. Provide recognition for a job well done
I. Fairness
J. Communication skills
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
6
6
6
6
6
6
6
6
6
7
7
7
7
7
7
7
7
7
7
8
8
8
8
8
8
8
8
8
8
9
9
9
9
9
9
9
9
9
9
10
10
10
10
10
10
10
10
10
10
230
-------
3. What is your potential for continuing employment with the City for more than 3 years? (1 is definitely
no potential, 5 is possible potential, and 10 is definite potential)
•123456 7- -8 '9 10
4. What is the adequacy of the number of staff members in your department? (1 is very inadequate, 5 is
adequate, and 10 is more than adequate)
8
10
5. How adequate are the benefits (i.e., sick leave, vacation, insurance, etc.) provided by the City? (1 is
very poor, 5 is average, and 10 is very good)
10
6. What is the level of productivity (efficiently getting things done) on the staff? Please rate each
"department" that you are familiar with. (1 is low, 5 is average, and 10 is high)
A. Laboratory
B. Maintenance
C. Administration (off-site)
D. Administration (on-site)
E. Operations
1
1
1
1
1
2
2
2
2
2
3
3
3
3
3
7. Relative to supervisory personnel other than your immediate
following factors? (1 is poor, 5 is average, and 10 is high)
A. Technical capability
B. Ability to recognize problems and set priorities
C. Ability to conduct staff meetings
D. Ability to discuss job-related ideas or problems
E. Ability to follow through with decisions or tasks
F. Use of staff suggestions
G. Ability to motivate staff
H. Provide recognition for a job well done
I. Fairness
J. Communication skills
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
5
5
5
5
5
supervisor,
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
6
6
6
6
how
6
6
6
6
6
6
6
6
6
6
7
7
7
7
7
would
7
7
7
7
7
7
7
7
7
7
8
8
8
8
8
you
8
8
8
8
8
8
8
8
8
8
9
g
9
9
9
rate
9
9
9
9
9
9
9
9
9
9
10
10 ,
10
10
10
the
10
10
10
10
10
10
10
10
10
10
231
-------
8. Is it clear to you as to what is expected of you in your current position? (i is very unclear, 5 is clear,
and 10 is very clear)
8
10
9. What is the level of the staff morale? Please rate your "department" and any other "departments" that
you have an opinion of. (1 is low, 5 is medium, and 10 is high)
A. Laboratory
B. Maintenance
C. Administration (off-site)
D. Administration (on-site)
E. Operations
2
2
2
2
2
3
3
3
3
3
4
4
4
4
4
5
5
5
5
5
6
6
6
6
6
7
7
7
7
7
8
8
8
8
8
9
9
g
g
9
10
10
10
10
10
10. How consistently do fellow staff/shift personnel implement daily routines, tasks, or tests (e.g., does
previous shift perform all required duties)? (1 is never consistent, 5 is consistent most of the time, and
10 is always consistent)
1
8
10
11. How adequate is the equipment (i.e., tools, trucks, typewriters, computers, etc.)? (1 is very
inadequate, 5 is satisfactory, and 10 is very adequate)
1 234 5 6789 10
12. What Is the qualification level (i.e., education or training) of the staff members you directly work
with? (1 is poorly qualified, 5 is average, and 10 is highly qualified)
1234 5678910
13. To what degree is there a spirit of teamwork or cooperation between staff members in your
"department"? (1 is very poor, 5 is average, and 10 is a high degree of cooperation)
1
8
10
14. What is the adequacy of the physical environment you work in (i.e., appearance, safety, cleanliness,
lighting, etc.)? (1 is very poor, 5 is average, and 10 is very good)
1
8
10
15. Do you use a data-based (i.e., use results of analyses, calculations, trend charts, etc.) decision-
making process in optimizing treatment plant performance? (1 is never used, 5 is used for some
decisions, and 10 is always used)
8
10
232
-------
16. How well does the management or City administration keep the staff informed of existing policies,
new policies, or other current events that you feel should be disseminated? (1 is very poor, 5 is
average, and 10 is very well)
1
89 10
17. To what degree is there a spirit of cooperation between different "departments"? (1 is very poor, 5 is
average, and 10 is a high degree of cooperation)
1 2 3 4 5 6 7 8 9 10
18. How do you rate the treatment plant staff in terms of being well organized? (1 is very low, 5 is
average, and 10 is very high)
1
8 9 10
19. How do you rate the treatment plant staff in terms of communications? (1 is very low, 5 is average,
and 10 is very high)
1
8 9 10
20. Are you challenged in your present position? (1 is not challenged, 5 is average, and 10 is very
challenged)
1 23 4 5
89 10
List any suggestions you have that would improve overall performance of the treatment plant. Please be specific.
What are your goals? '
233
-------
-------
Appendix I
Example Process Monitoring Summary for an Activated Sludge POTW
235
-------
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236
-------
Column Information for Daily Control Calculation Sheet
Column
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
Symbol
DATE
ATC
RSC
CSC
DOB
TURB
FLOW
RSF
RFP
RSP
ASU
CSU
SDR
TSU
ESU
XSU
WSU
MCRT
RSU
SDTA !
SDTC
Symbol Meaning
Self explanatory
Aeration Tank Concentration
Return Sludge Concentration
Clarifier Sludge Concentration
Depth of (Sludge) Blanket
Turbidity
Daily Wastewater Flow
Return Sludge Flow
Return Sludge Flow Percentage
Return Sludge Percentage
Aerator Sludge Units
Clarifier Sludge Units
Sludge Distribution Ratio
Total Sludge Units
Effluent Sludge Units
Intentionally Wasted Sludge
Units
Total Waste Sludge Units
Mean Cell Residence Time
Return Sludge Units
Sludge Detention Time in the
Aerator
Sludge Detention Time in the
Clarifier
Explanation
Average of values recorded during the day on the Daily Data Sheet.
Average of values recorded during the day on the Daily Data Sheet.
Average concentration of sludge within the Clarifier as determined from a core
sample (Method 1) or by calculation (Method 2).
Method 1 . Core Sample - average of values recorded during the day on the
Daily Data Sheet.
Method 2. Calculation - average of ATC and RSC.
CSC = (ATC + RSC) * 2
Average of values recorded during the day on the Daily Data Sheet.
Average of values recorded during the day on the Daily Data Sheet.
Total wastewater flow for a given 24-hour time period (e.g., 8:00 a.m. to 8:00
a.m.).
Total daily return sludge flow. [Note: Time period for determining this rate should
be the same as the time period used for determining daily wastewater flow rate
(e.g., 8:00 a.m. to 8:00 a.m.)].
Return sludge flow divided by average daily wastewater flow.
RFP = (RSF * FLOW) x 100
Return sludge flow percentage based on mass balance.
RSP = 100 x [(ATC) * (RSC - ATC)]
Total aeration tank volume (AV) times ATC.
Method 1 . Core Sample - clarifier sludge concentration times the clarifier volume
(CV).
CSU '= CSC x'CV
Method 2. Calculation - clarifier sludge concentration times the fraction of the
clarifier filled with sludge, times the clarifier volume (CV).
CSU = CSC x [(DOB)/(CD)] x CV
Ratio of the quanitty of solids under aeration vs. the quantity of solids in the
clarifier.
SDR = ASU -s- CSU
TSU = ASU + CSU
Quantity of sludge lost in the effluent each day. ESU = Effluent suspended
solids (TSS) times FLOW divided by ratio of MLSS to percent solids by
centrifuge (e.g., ratio = MLSS divided by ATC).
ESU = (TSS x FLOW) + Ratio
Quantity of sludge intentionally wasted from the system each day = average
concentration of wasted sludge times the volume of sludge wasted (from the
Daily Data Sheet).
WSU = ESU + XSU
Average number of days a given quantity of sludge remains in the system.
MCRT = TSU * WSU
Return sludge flow rate times the return sludge concentration.
RSU = RSF x RSC
Average number of hours a given quantity of sludge remains in the aerator.
SDTA = (AV x 24 hr/d) ,* (FLOW x RSF)
Average number of hours a given quantity of sludge remains in the clarifier.
SDTC = (CSU x 24 hr/d) * RSU
237
-------
Column Information for Daily Control Calculation Sheet (continued)
Column
22
23
24
25
26
Symbol
SDTAx
ATC
CSL
OFR
sscs
SSC60
Symbol Meaning
See meanings above.
Clarifier Solids Loading
Clarifier Surface Overflow Rate
Settled Sludge Concentration in
5 Minutes
Settled Sludge Concentration in
60 Minutes
Explanation
Indication of the treatment pressure in the system.
Average daily mass of sludge to the Clarifier divided by the Clarifier surface area
(CSA).
CSL = [(FLOW + RSF) (ATC)] + CSA
Upward velocity of the treated wastewater in the secondary Clarifier.
OFR = FLOW * CSA
Average of values recorded during the day on the Daily Data Sheet.
Average of values recorded during the day on the Daily Data Sheet.
238
-------
Appendix J
Example Process Monitoring Summary for an RBC POTW
239
-------
Example RBC Process Monitoring Summary
Plant Name:
Week of
19
Day
Date
Flow, m3/d
Influent
BODS, mg/L
SBOOS, mg/L
TSS, mg/L
pH, units
Temperature, °C
Wot Well
SBODS, mg/L
Primary Clarifier
DOB', m
BOD5, mg/L
SBOD5, mg/L
TSS, mg/L
pH, units
Temperature, °C
Secondary Claritiers
DOB), m
DOB2, m
BOD5, mg/L
SBODS, mg/L
Chlorine Contact Effluent
C\2 residua), mg/L
Fecal Coliform, MPN/liter
pH, units
TSS, mg/L
O & G*. mg/L
Secondary Sludge
min/d pumped
liter/s
Sun
Mon Tues Wed
Thur
Weekly
Fri Sat Average
spin*, percent
ratio"
mg/L
kg/a
Primary Sludge
Start time
End time
Minutes
spin., percent
ratio
mg/L
kg/d
GM"
240
-------
Stage 1
Stage 2
Stage 3
Stage 4
DO
(Date
Example RBC Process Monitoring Summary (continued)
RBC Dissolved Oxygen RBC Train Performance
) (Date Time )
Time
Train 1 Train 2 Train 3
mg/l
(Date
RBC Effluent
Time
SBOD5
Train 1 mg/l
Train 2 mg/l
Train 3 mg/l
TSS
mg/l
PH.
Temp
20
18
16
14
12
Imhoff Cone Sludge
Interface, ml/l
10
15
Time, min
20
25
30
1DTB Depth of (sludge) Blanket
GM Geometric Mean
Splkf CoraentratfonTpercent of sample volume the compacted sludge occupies after a 15-minute laboratory centrifuge spin
RATIO MLSS divided by ATC (see Appendix I, Column Information for Daily Control Calculation Sheet)
241
-------
-------
Appendix K
Parameters Used to Monitor the ABF Treatment Process
243
-------
Parameters Used to Monitor the ABF Treatment Process
Column
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
Symbol
DATE
ATC
RSC
CSC
DOB
TURB
FLOW
RSF
RFP
BCRF
BCRFP
TFBC
BCHL
ASU
CSU
TVBC
BCSU
TSU
SDR
ESU
XSU
WSU
MCRT
RSU
Symbol Meaning
Self Explanatory
Aeration Tank Concentration
Return Sludge Concentration
Clarifier Sludge Concentration
Depth to (Sludge) Blanket
Turbidity
Daily Wastewater Flow
Return Sludge Flow
Return Sludge Flow Percentage
Biocell Direct Recirculation Flow
Biocell Direct Recirculation Flow
Percentage
Total Flow to the Biocell
Biocell Hydraulic Load
Aerator Sludge Units
Clarifier Sludge Units
Total Volume in the Biocell
Biocell Sludge Units
Total Sludge Units
Sludge Distribution Ratio
Effluent Sludge Units
Intentionally Wasted Sludge
Units
Total Waste Sludge Units
Mean Cell Residence Time
Return Sludge Units
Explanation
Average of values recorded during the day on the Daily Data Sheet.
Average of values recorded during the day on the Daily Data Sheet.
Average concentration of sludge within the Clarifier. Average of values recorded
during the day on the Daily Data Sheet.
Average of values recorded during the day on the Daily Data Sheet.
Average of values recorded during the day on the Daily Data Sheet.
Total wastewater flow for a given 24-hour time period (e.g., 8:00 a.m. to 8:00
a.m.).
Total daily return sludge flow. [Note: Time period for determining this rate should
be the same as the time period used for determining daily wastewater flow rate
(e.g., 8:00 am. to 8:00 a.m.)].
Return sludge flow divided by average daily wastewater flow.
RFP = (RSF -r FLOW) x 100
Daily total flow from the biocell under drain directly to the recirculation wet well.
(Note: Time period for determining this flow should be the same as for
determining the average daily wastewater flow.)
Percentage expression of the ratio of the volume of direct biocell recirculation to
the volume of raw wastewater.
BCRFP = (BCRF* FLOW) x 100
Total volume of liquid pumped to the biocell.
TFBC = FLOW + RSF + BCRF
Volume of liquid pumped to the biocellper unit area of biocell in operation.
(TFBC x 695) * Biocell Surface Area
Total aerator volume (AV) times ATC.
Clarifier volume (CV) times Clarifier sludge concentration.
CSU = CV x CSC
Volume of mixed liquor in the biocell and associated appurtenances. TVBC =
volume in tower, volume in underdrain, volume in recirculation, and volume in
tower piping.
BCSU = TVBC x ATC
TSU = ASU + BCSU + CSU
Ratio of the mass of solids in the aeration tank + mass of sludge in the biocell to
the mass of sludge in the final Clarifier.
SDR = (ASU + BCSU) •*• CSU
Quantity of sludge lost in the effluent each day.
ESU = (TSS x FLOW) * Ratio
Ratio = MLSS * ATC
Quantity of sludge intentionally wasted from the system each day = average
concentration of wasted sludge times the volume of sludge wasted (from the
Daily Data Sheet).
WSU = ESU + XSU
Average of number of days a given quantity of sludge remains in the system.
MCRT = TSU * WSU
Return sludge flow rate times the return sludge concentration.
RSU = RSF x RSC
A Daily Control Calculation Sheet similar to the one presented for activated sludge in Appendix I can be used to
present these parameters in tabular form.
244
-------
Parameters Used to Monitor the ABF Treatment Process (continued)
Column
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
Symbol
;SDTA
SDTBC
SDTC
TDT
CYCLES
SDTAx
ATC
CSL
OFR
SSC5
SSCso
SSCgo
RSP
OUR
WASTE
RECIRC
AERATION
WASTE
Symbol Meaning
Sludge Detention Time in the
Aerator
Sludge Detention Time in the
Biocell
Sludge Detention Time in the
Biocell
Total Detention Time
Sludge Cycles per Day
See meanings above.
Clarifier Solids Loading
Clarifier Surface Overflow Rate
Settled Sludge Concentration in
5 Minutes
Settled Sludge Concentration in
30 Minutes
Settled Sludge Concentration in
60 Minutes
Return Sludge Percentage
Oxygen Uptake Rate
Waste Volume
Recirculation Power
Aeration Blower Power
Waste Power
Explanation
Average number of hours a given quantity of sludge remains in the aerator.
SDTA = (AV x 24 hr/d) * (TFBC - BCRF)
Average number of hours a given quantity of sludge remains in the Clarifier.
SDTBC = TVBC -s- TFBC
Average number of hours a given quantity of sludge remains in the Clarifier.
SDTC = CSU * RSU
TDT = SDTA + SDTBC + SDTC
Number of times the sludge cycles through the entire system during the day.
CYCLES = (24 hr/d) * TDT
Indication of the treatment pressure in the system.
Average daily mass of sludge to the Clarifier divided by the Clarifier surface area
(CSA).
CSL = [(TFBC - BCRF) (ATC)] * CSA
Upward velocity of the treated wastewater in the secondary Clarifier.
OFR = FLOW * CSA
Average of values recorded during the day on the Daily Data Sheet.
Average of values recorded during the day on the Daily Data Sheet.
Average of values recorded during the day on the Daily Data Sheet.
Indication of return flow percentage thst is based on a solids balance.
RSP = 100 x [(ATC) -s- (RSC - ATC)]
Average of values from the Daily Data Sheet.
Average of values from the Daily Data Sheet.
Average of values from the Daily Data Sheet.
Average of values from the Daily Data Sheet.
Average of values from the Daily Data Sheet.
245
-------
-------
Appendix L
Suspended Growth Major Unit Process Evaluation Worksheet
This worksheet is used to evaluate the capability of existing major unit processes, i.e., aerator, secondary
clarifier, and sludge handling system. Key loading and process parameters are compared with standard values
and point scores are assigned. These points are subsequently compared with expected point scores for Type 1,
Type 2, and Type 3 facilities, and a determination of the plant Type is made.
Instructions for use:
• Proceed through the steps contained in this worksheet in order.
• Use actual values in lieu of calculations if such data are available and appear reliable, e.g., waste sludge
volume.
• When assigning points, interpolate and use the nearest whole number.
• Minimum and maximum point values are indicated - do not exceed the range illustrated.
Aeration
Basin
Calculate Hydraulic Detention Time (HOT):
HOT =
HOT =
Aeration Basin Volume
Peak Month Average Daily Flow
( cuft)
( 9Pd)
x180 =
hr
Determine HDT Point Score:
3 5
HDT (hr)
10
24
-6
I I I
6
Points
HDT Point Score =
10
0
Calculate BOD5 Loading:
Average Daily BOD5 Loading
BOD5 Loading =
BOD5 Loading =
Aeration Basin Volume
Ib/d)
cuft)
xlOOO =
lbBOD5/d/1000 cuft
247
Suspended Growth (Rev. 1/89)
-------
Determine BOD5 Loading Point Score:
80 50
BOD5 Loading'
(lb/d/1,000cuft)
_J
25
15
-6
0 Points 6
BOD5 Loading Point Score =
10
©
Calculate Oxygen Availability:
Mechanical Aeration Systems
If data are not available on oxygen transfer capacity, calculate it as Wire Horsepower (Appendix F)
times Actual Oxygen Transfer Rate (Appendix E).
_hp) x
_lb/hp-hr) x 24 =
Jb O2/d
Diffused Air Aeration Systems
If data are not available on oxygen transfer capacity, calculate an actual oxygenation rate (Section
2.3.3.1).
Jb O2/d
Oxygen Availability =
Oxygen Availability =
Oxygen Transfer Capacity
BOD5 Loading to Aerator
( Ib/d)
( Ib/d)
Determine Oxygen Availability Point Score:
Ib O2/lb BOD5
0.8
Oxygen Availability (Ib O2/lb BOD5)
1.0 1.2 1.5
I
2.0
1 1 1 1
1 1 1 1
1 1 1 1
1 1 I 1
-10
-5
0
with Nitrification
10
0.6
0.8
1.0
1.2
1.5
-10
-5
without Nitrification
Points
Oxygen Availability Point Score =
10
©
Suspended Growth (Rev. 1/89)
248
-------
Secondary.
Clarifier
Add Scores 1, 2, and 3 to Obtain Subtotal for Aeration Basin:
Aeration Basin Subtotal
Determine Clarifier Configuration Point Score:
Configuration Points
Circular with "donut" or interior launders
Circular with weirs on walls
Rectangular with 33% covered with launders
Rectangular with 20% covered with launders
Rectangular with launder at or near end
10
5
5
0
-10
Clarifier Configuration Point Score =
©
Calculate Clarifier Surface Overflow Rate (SOR):
Peak Month Average Daily Flow
SOR =
SOR =
Clarifier Surface Area
gpd)
( sqft)
Determine SOR Point Score:
gpd/sq ft
SOR (gpd/sq ft)
1,200
1,000
800
650
500
300
-15
-10
0 5
Points
10
15
SOR Point Score =
©
Determine Depth at Weirs Point Score:
Depth at Weirs (ft)
7 8 10
12
15
-10
-5
0 4
Points
Depth at Weirs Point Score =
10
©
249
Suspended Growth (Rev. 1/89)
-------
Determine Return Activated Sludge Point Score:
RAS Removal
Circular, rapid withdrawal
Circular, scraper to hopper
Rectangular, co-current scraper
Rectangular, counter-current scraper
No mechanical removal
Points
10
8
2
0
-5
RAS Removal Point Score =
©
Determine Typical RAS Rate from Following:
Process Type
RAS Rate, % of Average
Daily Wastewater Flow
Conventional (plug flow or complete mix)
Extended Aeration (including oxidation ditches)
Contact Stabilization
Minimum Typical RAS Rate =
MaximumTypical RAS Rate =
Minimum
25
50
50
percent
percent
Maximum
100
100
125
Calculate Typical RAS Flow Range:
Min. Typical RAS Rate x Min. Month POTW Flow = Min. Recommended RAS Flow
( %) x ( god) x (0.01) = gpd
Max. Typical RAS Rate x Max. Month POTW Flow = Max. Recommended RAS Flow
( %) x ( god) x (0.01) = • • gpd
Determine Possible RAS Flow Range for Existing Facilities:
Minimum Possible RAS Flow = gpd
Maximum Possible RAS Flow = gpd
Suspended Growth (Rev. 1/89)
250
-------
Determine RAS Control Point Score:
RAS Control
Points
The possible RAS flow range is completely within the typical RAS 10
flow range and the capability to measure RAS flow exists
The possible RAS flow range is completely within the typical RAS 7
flow range but the capability to measure RAS flow does not exist
50% of the typical RAS flow range is covered by the possible RAS 5
flow range and the capability to measure RAS flow exists
50% of the typical RAS flow range is covered by the possible RAS 0
flow range but the capability to measure RAS flow does not exist
The possible RAS flow range is completely outside the typical RAS -5
flow range
RAS Control Point Score =
©
Add Scores 5, 6, 7, 8, and 9 to Obtain Subtotal for Secondary Clarifier:
Secondary Clarilier Subtotal =
Sludge Handling Determine Sludge Controllability Point Score:
Capability
Controllability
Points
Automated sampling and volume control
Metered volume and hand sampling
Hand measured volume and hand sampling
Sampling or volume measurement by hand not practical
Sludge Controllability Point Score =
Calculate Expected BOD5 Mass to be Removed (in the following calculations, 1.25 is a
variability factor, described in Chapter 2):
POTW w/Primary Clarification:
Primary BOD5jn - Primary BOD5out = Primary BOD5 Concentration Removed
( mg/L) - ( ___mg/L) = mg/L
Primary BOD5out - POTW Effluent BOD5 = Secondary BOD5 Concentration Removed
( mg/L) - ( mg/L) = mg/L
Prim. BOD5 Cone. Rem. x Avg. Annual POTW Flow x 1.25 = Prim. BOD5 Mass Removed
( mg/L) x ( gpd) x (8.34 x 10-6) x 1.25 = Ib/d
Sec. BOD5 Cone. Rem. x Avg. Annual POTW Flow x 1.25 = Sec. BOD5 Mass Removed
( mg/L) x ( gpd) x (8.34 x 10-6) x 1.25 = Ib/d
251
Suspended Growth (Rev. 1/89)
-------
PQTW w/o Primary Clarification:
BOD5jn - POTW Effluent BOD5 = Total BOD5 Concentration Removed
( mg/L) - ( mg/L) = mg/L
Total BOD5 Cone. Rem. x Avg. Annual POTW Flow x 1.25 = Total BOD5 Mass Removed
( mg/L) x ( gpd) x (8.34 x 10-6) x 1.25= Ib/d
Determine Typical Unit Sludge Production From Following:
Process Type
Ib TSS (sludge)/lb
BOD5 Removed
Primary Clarification
Activated Sludge w/Primary Clarification
Activated Sludge w/o Primary Clarification
Conventional1
Extended Aeration2
Contact Stabilization
1.7
0.7
0.85
0.65
1.0
1 Includes tapered aeration, step feed, plug flow, and complete mix with wastewater times
<10hr.
2 Includes oxidation ditch.
If plant records include actual sludge production data, the actual unit sludge production value
should be compared to the typical value, if a discrepancy of more than 15 percent exists
between the two values, further evaluation is needed. If not, use the actual unit sludge
production value.
Calculate Expected Sludge Mass:
POTW w/Primary Clarification:
Unit Sludge Production x Primary BOD5 Mass Removed = Primary Sludge Mass
( Ib/lb) - ( Ib/d) = Ib/d
Unit Sludge Production x Secondary BOD5 Mass Removed = Secondary Sludge Mass
( Ib/lb) - ( Ib/d) = Ib/d
Total Sludge Mass = _lb/d
POTW w/o Primary Clarification:
Unit Sludge Production x Total BOD5 Mass Removed = Total Sludge Mass
( Ib/lb) - ( Ib/d) = Ib/d
Suspended Growth (Rev. 1/89)
252
-------
Determine Typical Unit Sludge Production From Following:
Sludge Type
Waste Concentration, mg/l
Primary
Activated
Return Sludge/Conventional
Return Sludge/Extended Aeration
Return Sludge/Contact Stabilization
Return Sludge/small plant with low SOR1
Separate waste hopper in secondary clarifier
50,009
6,000
7,500
8,000
10,000
12,000
1 Returns can often be shut off for short periods to thicken waste sludge in clarifiers with surface
overflow rates < 500 gpd/sq ft.
Calculate Expected Sludge Volume:
POTW w/Primary Clarification:
Sludge Volume =
Primary Sludge Mass
Primary Sludge Concentration
Sludge Volume =
50,000
Ib/d)
mg/l)
-.x (120,000) =
Sludge Volume =
Sludge Volume =
Secondary Sludge Mass
Secondary Sludge Concentration
mg/l)
x (120,000) =
.9Pd
Total Sludge Volume =
POTW w/o Primary Clarification:
Total Sludge Volume =
Total Sludge Volume =
Total Sludge Mass
Waste Sludge Concentration
Ib/d)
. mg/l)
x (120,000) =
.gpd
253
Suspended Growth (Rev. 1/89)
-------
Calculate Capability of Sludge Handling Unit Processes:
1. Establish capability of each existing sludge handling process (treatment and disposal).
The most common unit processes for which this calculation will have to be performed
are:
Aerobic digestion
Anaerobic digestion
Gravity thickening
Mechanical dewatering
Drying beds
Liquid haul
For example, the capability of a gravity thickener is based on the maximum sludge loading it
must handle:
Thickener Loading =
Total Sludge Mass
Thickener Surface Area
Thickener Loading =
Ib/d)
sqft)
Ib/d/sq ft
2. Determine percentage of the expected sludge production that each process can handle.
Assume the sludge being thickened by the gravity thickener above is mixed primary and
activated. From Table 2-9, 10 Ib/d/sq ft is considered typical loading for the thickener. Its
capability would therefore be calculated as:
10 Ib/d/sq ft)
( Actual Loading Ib/d/sq ft
x 100 =
percent
List Each Process and its Associated Sludge Handling Capability and Identify the
Lowest Percentage Capability:
Process
Percentage
Lowest Capability =
Suspended Growth (Rev. 1/89)
254
-------
Determine Sludge Handling Capability Point Score:
% of Calculated Sludge Production
50
75
100
125
150
I
I I
-10
15
Points
20
Sludge Handling Capability Point Score =
25
Add Scores 11 and 12 to Obtain Subtotal for Sludge Handling Capability:
Sludge Handling Capability Subtotal =
Compare Subtotals and Total Score with Following to Determine Whether POTW is
Type 1, Type 2, or Type 3:
Points Required
Aeration Basin
Secondary Clarifier
Sludge Handling Capability
Total
Aeration Basin
Secondary Clarifier
Sludge Handling Capability
Total
Score Type 1
(4) 13-30
(10) 25-55
(13) 10-30
60-115
Type
Type 2
0-12
0-24
0-9
20-59
TypeS
<0
<0
<0
<20
Select the Worst Case: POTW is Type
255
Suspended Growth (Rev. 1/89)
-------
-------
Appendix M
Trickling Filter Major Unit Process Evaluation Worksheet
This worksheet is used to evaluate the capability of existing major unit processes, i.e., aerator, secondary
clarifier, and sludge handling system. Key loading and process parameters are compared with standard values
and point scores are assigned. These points are subsequently compared with expected point scores for Type 1,
Type 2, and Type 3 facilities, and a determination of the plant Type is made.
Instructions for use:
• Proceed through the steps contained in this worksheet in order.
• Use actual values in lieu of calculations if such data are available and appear reliable, e.g., waste sludge
volume.
• When assigning points, interpolate and use the nearest whole number.
• Minimum and maximum point values are indicated - do not exceed the range illustrated.
"Aerator"
Calculate Equivalent Filter Media Volume:
Actual Filter Media Specific Surface Area
Equivalent Filter
Media Volume
Rock Filter Media Specific Surface Area
x Actual Media Volume
Equivalent Filter _ _(
Media Volume
sq ft/cu ft)
( 13 sq ft/cu ft)
Calculate BOD5 Loading:
Primary Effluent BOD5
x(
BOD5 Loading =
BOD5 Loading =
Equivalent Filter Media Volume
( Ib/d)
x 1,000 =
( cu ft)
Determine BOD5 Loading Point Score:
BOD5 Loading (lb/d/1,000 cu ft)
70
-20
50
30
_cu ft) =
cuft
,lbBOD5/d/1,OOOcuft
20
-10
15
Freezing Temperatures
10
I
I I I I
I
I I I I
I
I II II! II Ml II I
I I
I
I II
I I
20
I I I I
-10
-5 10
Covered Filter/Nonfreezing Temperatures
Points
20
BOD5 Loading Point Score =
©
257
Trickling Filter (Rev. 1/89)
-------
Calculate Recirculation Ratio:
Recirculation Ratio = •
Return Flow
Recirculation Ratio = •
,'eak Month Avg. Daily Wastewater Flow
( 9Pd)
gpd)
: 1
Determine Recirculation Ratio Point Score:
None
Recirculation
1:1
2:1
2
Points
Recirculation Ratio Point Score =
Determine Anaerobic Side Streams Point Score:
Anaerobic Side Streams1 Points
Not returned to plant 0
Returned to the wastewater stream ahead of TF
returned to flow equalization tank or prim. clar. -6
returned directly ahead of trickling filter -10
1 Supernatant from anaerobic digesters or filtrate/concentrate from
the dewatering processes following anaerobic digesters.
Anaerobic Side Streams Point Score =
Add Scores 1, 2, and 3 to Obtain Subtotal for "Aerator":
"Aerator" Subtotal =
©
Secondary
Clarifler
Determine Clarifier Configuration Point Score:
Configuration
Circular with "donut" or interior launders
Circular with weirs on walls
Rectangular with 33% covered with launders
Rectangular with 20% covered with launders
Rectangular with launder at or near end
Points
10
5
5
0
-10
Clarifier Configuration Point Score =
Trickling Filter (Rev. 1/89)
258
-------
Calculate Clarifier Surface Overflow Rate (SOR):
Peak Month Average Daily Flow
SOR =
SOR =
Clarifier Surface Area
9Pd)
sqft)
gpd/sq ft
Determine SOR Point Score:
1,200
1,000
SOR (gpd/ sq ft)
800 650
500
-15
-10
0 5
Points
10
SOR Point Score =
Determine Depth at Weirs Point Score:
Depth at Weirs (ft)
7 10
300
15
©
12
3
Points
Depth at Weirs Point Score =
©
Add Scores 5, 6, and 7 to Obtain Subtotal for Secondary Clarifier:
Secondary Clarifier Subtotal =
Sludge Handling Determine Sludge Controllability Point Score:
Capability
Controllability Points
Automated sampling and volume control
Metered volume and hand sampling
Hand measured volume and hand sampling
Sampling or volume measurement by hand not practical
Sludge Controllability Point Score =
©
259
Trickling Filter (Rev. 1/89)
-------
Calculate Expected BOD5 Mass to be Removed (in the following calculations, 1.25 is a
variability factor described in Chapter 2):
Primary BOD5jn - Primary BOD5out = Primary BOD5 Concentration Removed
( mg/l) - ( mg/l) = mg/l
Primary BOD5out - POTW Effluent BOD5 = Secondary BOD5 Concentration Removed
( mg/l) - (_. mg/l) = mg/l
Prim. BOD5 Cone. Rem. x Avg. Annual POTW Flow x 1.25 = Prim. BOD5 Mass Removed
( mg/l) x ( gpd) x (8.34 x 10-6) x 1.25 = Ib/d
Sec. BOD5 Cone. Rem. x Avg. Annual POTW Flow x 1.25 = Sec. BOD5 Mass Removed
( mg/l) x ( gpd) x (8.34 x 10-6) x 1.25= Ib/d
Determine Typical Unit Sludge Production From Following:
Process Type
Ib TSS (sludge)/lb
BODS Removed
Primary Clarification
1.7
Trickling Filter
0.9
If plant records include actual sludge production data, the actual unit sludge production value
should be compared to the typical value. If a discrepancy of more than 15 percent exists
between the two values, further evaluation is needed. If not, use the actual unit sludge
production value.
Calculate Expected Sludge Mass:
Unit Sludge Production x Primary BOD5 Mass Removed = Primary Sludge Mass
( Ib/lb) - ( Ib/d) = Ib/d
Unit Sludge Production x Secondary BOD5 Mass Removed = Secondary Sludge Mass
( Ib/lb) - ( _lb/d) = Ib/d
Total Sludge Mass = Ib/d
Trickling RIter (Rev. 1/89)
260
-------
Calculate Expected Sludge Volume:
Method 1 (Primary and Secondary Sludge Wasted Separately):
Sludge Volume =
Sludge Volume =
Primary Sludge Mass
Primary Sludge Concentration
50,000
Ib/d)
mg/l)
x (120,000) =
Sludge Volume =
Sludge Volume =
Secondary Sludge Mass
Secondary Sludge Concentration
Ib/d)
20,000
mg/l)
x (120,000) =
Total Sludge Volume =
.gpd
Method 2 (Secondary Sludge Wasted to Primary):
Total Sludge Volume =
Total Sludge Volume =
Total Sludge Mass
Combined Sludge Concentration
( Ib/d)
35,000
mg/l)
x (120,000) =
Calculate Capability Sludge Handling Unit Processes:
1. Establish capability of each existing sludge handling process (treatment and disposal).
The most common unit processes for which this calculation will have to be performed
are:
Aerobic digestion
Anaerobic digestion
Gravity thickening
Mechanical dewatering
Drying beds
Liquid haul
For example, the capability of a gravity thickener is based on the maximum sludge loading it
must handle:
Thickener Loading =
Thickener Loading =
Total Sludge Mass
Thickener Surface Area
Ib/d)
sqft)
Ib/d/sq ft
261
Trickling Filter (Rev. 1/89)
-------
2. Determine percentage of the expected sludge production that each process can handle.
Assume the sludge being thickened by the gravity thickener above is from a trickling filter.
From Table 2-9, 8 Ib/d/sq ft is considered typical loading for the thickener. Its capability
would therefore be calculated as:
8
Ib/d/sq ft)
( Actual Loading Ib/d/sq ft)
x 100 =
percent
List Each Process and its Associated Sludge Handling Capability and Identify the
Lowest Percentage Capability:
Process
Percentage
Lowest Percent Capability =
Determine Sludge Handling Capability Point Score:
% of CalculatedSludge Production
50
-10
75
100
125
5 15
Points
Sludge Handling Capability Point Score =
20
Add Scores 9 and 10 to Obtain Subtotal for Sludge Handling Capability :
Sludge Handling Capability Subtotal =
Trickling Filter (Rev. 1/89)
262
-------
Compare Subtotals and Total Score With Following to Determine Whether POTW is
Type 1, Type 2, or Type 3:
Points Required
Score
Type 1
Type 2
Types
"Aerator"
Secondary Clarifier
Sludge Handling Capability
Total
(4)
. (8).
(11)
17-23
17-30
10-30
45-83
0-16
0-16
0-9
15-44
-------
-------
Appendix N
RBC Major Unit Process Evaluation Worksheet
This worksheet is used to evaluate the capability of existing major unit processes, i.e., aerator, secondary
clarifier, and sludge handling system. Key loading and process parameters are compared with standard values
and point scores are assigned. These points are subsequently compared with expected point scores for Type 1,
2, and 3 facilities, and a determination of the plant Type is made.
Instructions for use:
• Proceed through the steps contained in this worksheet in order.
• Use actual values in lieu of calculations if such data are available and appear reliable, e.g., waste sludge
volume.
• When assigning points, interpolate and use the nearest whole number.
• Minimum and maximum point values are indicated - do not exceed the range illustrated.
"Aerator"
Calculate First-Stage Loading:
Avg. Daily RBC SBOD5 Load
BOD5 Loading =
BOD5 Loading =
First-Stage Media Surface Area
or
Ib/d)
sqft)
-x 1,000 =
0.5 Avg. Daily RBC BOD5 Load
First-Stage Media Surface Area
Ib SBOD5/d/1,000 sq ft
Determine First-Stage Loading Point Score:
First-Stage Loading (Ib SBOD5/d/1,000 sq ft)
6.0 4.0
2.5
-6
0
Points
10
First-Stage Loading Point Score =
Calculate System Loading:
Avg. Daily RBC SBOD5 Load
BOD= Loading = ~
Total Media Surface Area
or
BOD5 Loading =
Ib/d)
sqft)
x 1,000 =
0.5 Avg. Daily RBC BOD5 Load
Total Media Surface Area
Ib SBOD5/d/1,000 sq ft
265
RBC (Rev. 1/89)
-------
Determine System Loading Point Score:
System Loading (Ib SBOD5/d/1,000 sq ft)
1.5 1.0
System Loading Point Score =
0.6
111!
I I I
I I I II
I I I I I I I I I
-6 0 1
Points
Determine Number of Stages Point Score:
Number of Stages
2 3
7
Points
Recirculation Ratio Point Score =
10
©
Determine Anaerobic Side Streams Point Score:
Anaerobic Side Streams1 '_ Points
Not returned to plant 0
Returned to the wastewater stream ahead of the RBC
returned to flow equalization tank or prim. clar. -6
returned directly ahead of RBC -10
1 Supernatant from anaerobic digesters or filtrate/concentrate from
the dewatering processes following anaerobic digesters.
Anaerobic Side Streams Point Score =
Add Scores 1, 2, 3 and 4 to Obtain Subtotal for "Aerator":
"Aerator" Subtotal =
0
0
RBC (Rev. 1/89)
266
-------
Secondary
Clarifier
Determine Clarifier Configuration Point Score:
Configuration Points
Circular with "donut" or interior launders
Circular with weirs on walls
Rectangular with 33% covered with launders
Rectangular with 20% covered with launders
Rectangular with launder at or near end
10
5
5
0
-10
Clarifier Configuration Point Score =
Calculate Clarifier Surface Overflow Rate (SOR):
Peak Month Average Daily Flow
SOR =
SOR =
Clarifier Surface Area
9Pd)
sqft)
gpd/sq ft
Determine SOR Point Score:
1,200
1,000
SOR (gpd/ sq ft)
800 650
500
-15
-10
0 5
Points
10
SOR Point Score =
Determine Depth at Weirs Point Score:
Depth at Weirs (ft)
7 10
3
Points
Depth at Weirs Point Score =
Add Scores 5, 6, and 7 to Obtain Subtotal for Secondary Clarifier:
Secondary Clarifier Subtotal =
©
300
15
©
12
>
267
RBC (Rev. 1/89)
-------
Sludge Handling Determine Sludge Controllability Point Score:
Capability
Controllability Points
Automated sampling and volume control 5
Metered volume and hand sampling 3
Hand measured volume and hand sampling 2
Sampling or volume measurement by hand not practical 0
Sludge Controllability Point Score =
Calculate Expected BOD5 Mass to be Removed (in the following calculations, 1.25 is a
variability factor discussed in Chapter 2):
Primary BODsjn - Primary BOD5out = Primary BOD5 Concentration Removed
( mg/l) - ( mg/l) = mg/l
Primary BOD5out - POTW Effluent BOD5 = Secondary BOD5 Concentration Removed
( mg/l) - (_ mg/l) = mg/l
Prim. BODs Cone. Rem. x Avg. Annual POTW Flow x 1.25 = Prim. BOD5 Mass Removed
( mg/l) x ( gpd) x (8.34 x 10-6) x 1.25 = _lb/d
Sec. BOD5 Cone. Rem. x Avg. Annual POTW Flow x 1.25 = Sec. BOD5 Mass Removed
( mg/l) x ( gpd) x (8.34 x 10-6) x 1.25= Ib/d
Determine Typical Unit Sludge Production From Following:
Process Type
Ib TSS (sludge)/lb
BODS Removed
Primary Clarification
RBC
1.7
1.0
If plant records include actual sludge production data, the actual unit sludge production value
should be compared to the typical value. If a discrepancy of more than 15 percent exists
between the two values, further evaluation is needed. If not, use the actual unit sludge
production value.
Calculate Expected Sludge Mass:
Unit Sludge Production x Primary BOD5 Mass Removed = Primary Sludge Mass
( Ib/lb) x ( Ib/d) = Ib/d
Unit Sludge Production x Secondary BODg Mass Removed = Secondary Sludge Mass
( Ib/lb) x( Jb/d) = Ib/d
Total Sludge Mass = Ib/d
RBC (Rev. 1/89)
268
-------
Calculate Expected Sludge Volume:
Method 1 (Primary and Secondary Sludge Wasted Separately):
Sludge Volume =
Sludge Volume =
Primary Sludge Mass
Primary Sludge Concentration
50,000
Ib/d)
mg/l)
x (120,000) =
.9Pd
Sludge Volume =
Sludge Volume =
Secondary Sludge Mass
Secondary Sludge Concentration
Ib/d)
20,000
mg/l)
x (120,000) =
Total Sludge Volume =
.gpd
Method 2 (Secondary Sludge Wasted to Primary):
Total Sludge Volume =
Total Sludge Volume =
Total Sludge Mass
Combined Sludge Concentration
35,000
Ib/d)
mg/l)
x (120,000) =
Calculate Capability Sludge Handling Unit Processes:
1. Establish capability of each existing sludge handling process (treatment and disposal).
The most common unit processes for which this calculation will have to be performed
are:
Aerobic digestion
Anaerobic digestion
Gravity thickening
Mechanical dewatering
Drying beds
Liquid haul
For example, the capability of a gravity thickener is based on the maximum sludge loading it
must handle:
Thickener Loading =
Thickener Loading =
Total Sludge Mass
Thickener Surface Area
Ib/d)
sqft)
Ib/d/sq ft
269
RBC (Rev. 1/89)
-------
2. Determine percentage of the expected sludge production that each process can handle.
Assume the sludge being thickened by the gravity thickener above is mixed primary and
RBC. From Table 2-9, 15 Ib/d/sq ft is considered typical loading for the thickener. Its
capacity would therefore be calculated as:
15 Ib/d/sqft)
( Actual Loading Ib/d/sq ft)
x100 =
percent
List Each Process and Its Associated Sludge Handling Capability and Identify the
Lowest Percentage Capability:
Process
Percentage
Lowest Capability =
Determine Sludge Handling Capability Point Score:
% of CalculatedSludge Production
50
-10
75
100
125
I I
I
5 15
Points
Sludge Handling Capability Point Score
20
Add Scores 10 and 11 to Obtain Subtotal for Sludge Handling Capability:
Sludge Handling Capability Subtotal =
r ">
RBC (Rev. 1/89)
270
-------
Compare Subtotals and Total Score With Following to Determine Whether POTW is
Type 1, 2, or 3:
Points Required
Score
Type 1
Type 2
Type 3
"Aerator"
Secondary Clarifier
Sludge Handling Capability
Total
(5)
(9)
(12)
14-30
17-30
10-30
48-90
0-13
0-16
0-9
15-47
<0
<0
<0
Type
"Aerator"
Secondary Clarifier
Sludge Handling Capability
Total
Select the Worst Case: POTW is Type
271
RBC (Rev. 1/89)
-------
-------
Appendix O
ABF Major Unit Process Evaluation Worksheet
This worksheet is used to evaluate the capability of existing major unit processes, i.e., aerator, secondary
clarifier, and sludge handling system. Key loading and process parameters are compared with standard values
and point scores are assigned. These points are subsequently compared with expected point scores for Type 1,
2, and 3 facilities and a determination of the plant Type is made.
Instructions for use:
• Proceed through the steps contained in this worksheet in order.
• Use actual values in lieu of calculations if such data are available and appear reasonable, e.g., waste sludge
volume.
• When assigning points, interpolate and use the nearest whole number.
• Minimum and maximum point values are indicated - do not exceed the range illustrated.
"Aerator"
Calculate BOD5 Loading:
Avg. Daily Biocell BOD5 Loading
BOD5 Loading =
BOD5 Loading =
Biocell Media Volume
Ib/d)
cuft)
'xlOOO =
lbBODg/d/1000 cuft
Determine BOD5 Loading Point Score:
BOD5 Loading (Ib BOD5/d/1,000 cu ft)
300 250 200 175
150
-10
-5
0 5
Points
10
100
15
BOD5 Loading Point Score =
©
Calculate Aeration Basin Detention Time:
Aeration Basin Volume
Aeration Basin _
Detention Time
Peak Month Average Daily Flow
Aeration Basin _ _(_
Detention Time ~ .
cuft)
gpd)
x180 =
hr
273
ABF (Rev. 1/89)
-------
Determine Aeration Basin Detention Time Point Score:
Aeration Basin Detention Time (hr)
0.5 0.75 -1 2 3
-10
5 12
Points
15
Aeration Basin Detention Time Point Score -
20
Calculate Oxygen Availability:
Mechanical Aeration Systems
If data are not available on oxygen transfer capacity, calculate it as Wire Horsepower (Appendix F)
times Actual Oxygen Transfer Rate (Appendix E).
_hp) x
Jb/hp-hr) x 24 =
Jb O2/d
Diffused Air Aeration Systems
If data are not available on oxygen transfer capacity, calculate an actual oxyqenation rate (Section
2.3.3.1).
Jb O2/d
Oxygen Availability =
Oxygen Availability =
Oxygen Transfer Capacity
BOD5 Loading to Aerator
( Ib/d)
( Ib/d)
Ib O2/lb BOD5
Determine Oxygen Availability Point Score:
Oxygen Availability (Ib O2/lb BOD5)
0.3 0.4 0.5 0.75
1.0
-15
3 7
Points
Oxygen Availability Point Score =
10
©
ABF (Rev. 1/89)
274
-------
Calculate Recirculation Ratio:
Recirculation Ratio =
Return Flow
Recirculation Ratio =
Peak Month Average Daily Wastewater Flow
( gpd)
gpd)
: 1
Determine Recirculation Ratio Point Score:
Recirculation
None 0.5:1
1:1
Points
Recirculation Ratio Point Score =
©
Secondary
Clarifier
Add Scores 1, 2, 3 and 4 to Obtain Subtotal for "Aerator":
"Aerator" Subtotal =
Determine Clarifier Configuration Point Score:
Configuration
Points
Circular with "donut" or interior launders
Circular with weirs on walls
Rectangular with 33% covered with launders
Rectangular with 20% covered with launders
Rectangular with launder at or near end
10
5
5
0
-10
Clarifier Configuration Point Score =
(s.
©
Calculate Clarifier Surface Overflow Rate (SOR):
Peak Month Average Daily Flow
SOR =
SOR =
Clarifier Surface Area
gpd)
sqft)
gpd/sq ft
275
ABF(Rev. 1/89)
-------
Determine SOR Point Score:
SOR (gpd/ sq ft)
1,200
1,000
800
650
500
-15
-10
0 5
Points
10
SOR Point Score =
Determine Depth at Weirs Point Score:
Depth at Weirs (ft)
7 10
300
15
©
12
3 5
Points
Depth at Weirs Point Score =
©
Calculate Desired RAS Flow Range:
Min. Typical RAS Rate x Min. Month POTW Flow = Min. Recommended RAS Flow
x ( gpd) x (0.01) = gpd
Max. Typical RAS Rate x Max. Month POTW Flow = Max. Recommended RAS Flow
(100%) x ( gpd) x (0.01) = gpd
Determine Possible RAS Flow Range:
Minimum Possible RAS Flow = gpd
Maximum Possible RAS Flow = gpd
ABF (Rev. 1/89)
276
-------
Determine RAS Control Point Score:
RAS Control
Points
The possible RAS flow range is completely within the typical RAS 10
flow range and the capability to measure RAS flow exists
The possible RAS flow range is completely within the typical RAS 7
flow range but the capability to measure RAS flow does not exist
50% of the typical RAS flow range is covered by the possible RAS 5
flow range and the capability to measure RAS flow exists
50% of the typical RAS flow range is covered by the possible RAS 0
flow range but the capability to measure RAS flow does not exist
The possible RAS flow range is completely outside the typical RAS -5
flow range
RAS Control Point Score =
©
Add Scores 5, 6, 7, 8, and 9 to Obtain Subtotal for Secondary Clarifier:
Secondary Clarifier Subtotal =
Sludge Handling Determine Sludge Controllability Point Score:
Capability
Controllability Points
Automated sampling and volume control
Metered volume and hand sampling
Hand measured volume and hand sampling
Sampling or volume measurement by hand not practical
Sludge Controllability Point Score =
Calculate Expected BOD5 Mass to be Removed (in the following calculations, 1.25 is a
variability factor discussed in Chapter 2):
Primary BODsjn - Primary BOD5out = Primary BOD5 Concentration Removed
( ___mg/l) - ( mg/l) = mg/l
Primary BOD5out - POTW Effluent BOD5 = Secondary BOD5 Concentration Removed
( mg/l) - ( _mg/l) = mg/l
Prim. BOD5Conc. Rem. x Avg. Annual POTW Flow x 1.25 = Prim. BOD5 Mass Removed
( mg/l) x ( gpd) x (8.34 x 10-6) x 1.25 = Ib/d
Sec. BOD5 Cone. Rem. x Avg. Annual POTW Flow x 1.25 = Sec. BOD5 Mass Removed
(_ mg/l) x ( gpd) x (8.34 x 10-6) x 1.25= Ib/d
277
ABF(Rev. 1/89)
-------
Determine Typical Unit Sludge Production From Following:
Process Type
Ib TSS (sludge)/lb
BODS Removed
ABF
1.0
If plant records include actual sludge production data, the actual unit sludge production value
should be compared to the typical value. If a discrepancy of more than 15 percent exists
between the two values, further evaluation is needed. If not, use the actual unit sludge
production value.
Calculate Expected Sludge Mass:
Unit Sludge Production x Primary BOD5 Mass Removed = Primary Sludge Mass
( Mb) - ( Ib/d) = Ib/d
Unit Sludge Production x Secondary BOD5 Mass Removed = Secondary Sludge Mass
( Mb) - ( Ib/d) = Ib/d
Total Sludge Mass = Ib/d
Calculate Expected Sludge Volume:
Method 1 (Primary and Secondary Sludge Wasted Separately):
'Primary Sludge Mass
Sludge Volume =
Sludge Volume =
Primary Sludge Concentration
50,000
Ib/d)
mg/l)
x (120,000) =
Sludge Volume =
Sludge Volume =
Secondary Sludge Mass
Secondary Sludge Concentration
Ib/d)
10,000
mg/l)
•x (120,000) =
Total Sludge Volume =
,9Pd
.gpd
Method 2 (Secondary Sludge Wasted to Primary):
Total Sludge Volume =
Total Sludge Volume =
Total Sludge Mass
Combined Sludge Concentration
( Ib/d)
( 30,000 mg/l)
Calculate Capability Sludge Handling Unit Processes:
x (120,000)
.gpd
ABF (Rev. 1/89)
278
-------
1. Establish capability of each existing sludge handling process (treatment and disposal).
The most common unit processes for which this calculation will have to be performed
are:
Aerobic digestion
Anaerobic digestion
Gravity thickening
Mechanical dewatering
Drying beds
Liquid haul
For example, the capability of a gravity thickener is based on the maximum sludge loading it
must handle:
Thickener Loading =
Total Sludge Mass
Thickener Surface Area
Thickener Loading =
Ib/d)
sqtt)
Ib/d/sq ft
2. Determine percentage of the expected sludge production that each process can handle.
Assume the sludge being thickened by the gravity thickener above is ABF. From Table 2-9,
4 Ib/d/sq ft is considered typical loading for the thickener. Its capacity would therefore be
calculated as:
Ib/d/sq ft)
( Actual Loading Ib/d/sq ft)
x100 =
percent
List Each Process and its Associated Sludge Handling Capability and Identify the
Lowest Percentage Capability:
Process
Percentage
Lowest Capability =
279
ABF (Rev. 1/89)
-------
Determine Sludge Handling Capability Point Score:
% of CalculatedSludge Production
50
-10
75
100
15
Points
Sludge Handling Capability Point Score =
125
20
Add Scores 11 and 12 to Obtain Subtotal for Sludge Handling Capability:
Sludge Handling Capability Subtotal =
Compare Subtotals and Total Score With Following to Determine Whether POTW is
Type 1, 2, or 3:
"Aerator"
Secondary Clarifier
Sludge Handling Capability
Total
"Aerator"
Secondary Clarifier
Sludge Handling Capability
Total
Select the Worst Case: POTW is Type
Points Required
Score Type 1 Type 2
(5) 15-48 0-14
(10) 20-55 0-19
(13) 10-30 0-9
50-133 15-49
Type
Type 3
<0
<0
<0
<15
ABF (Rev. 1/89)
280
-------
Appendix P
Stabilization Pond Major Unit Process Evaluation Worksheet
This worksheet is used to evaluate the capability of existing major unit processes, i.e., aerator, secondary
clarifier, and sludge handling system. Key loading and process parameters are compared with standard values
and point scores are assigned. These points are subsequently compared with expected point scores for Type 1,
Type 2, and Type 3 facilities, and a determination of the plant Type is made.
Instructions for use:
• Proceed through the steps contained in this worksheet in order.
• Use actual values in lieu of calculations if such data are available and appear reliable, e.g., waste sludge
volume.
• When assigning points, interpolate and use the nearest whole number.
• Minimum and maximum point values are indicated - do not exceed the range illustrated.
Facultative
Pond Facilities
Calculate Total BOD5 Loading:
Total BOD5 Loading =
Total BOD5 Loading =
Average Daily BOD5 Loading
Total Surface Area of All Ponds
(_ Ib/d)
( ac)
Ib BODg/ac/d
281
Stabilization Pond (Rev. 1/89)
-------
Determine Total BOD5 Loading Point Score:
Total BOD5 Loading (Ib/ac/d) , Average Winter Air Temperature >15°C
120 100 80 60
40
I I I I
-10
-5
10
Total BOD5 Loading (Ib/ac/d)
80 60
Average Winter Air Temperature 0-15°C
40 20
10
-10
-5
10
Total BOD5 Loading (Ib/ac/d)
40 30
Average Winter Air Temperature <0°C
20 15
10
-10
-5
0
Points
10
Total BOD5 Loading Point Score =
Calculate 1st Pond BOD5 Loading:
1st Pond BOD5 Loading = Average Daily BOD5 Loading
1 st Pond Surface Area
( Ib/d)
ac)
Ib BOD5/ac/d
Stabilization Pond (Rev. 1/89)
282
-------
Determine 1st Pond Loading Point Score:
Average Winter Air Temperature 1 st Pond BODS Loading
°C Ib/ac/d
Points
0-15
<0
>73
<73
>39
<39
-3
0
-3
0
-3
0
Calculate Detention Time:
Detention Time =.
1st Pond BOD5 Loading Point Score =
Total Pond Volume
;'.. Average Daily Flow
_ J gift)
( gpd)
Determine Detention Time Point Score:
Detention time (days)
15 20
x 7.5 =
©
days
30
-5
20
Average Winter Air Temperature > 15°C
30
80
-5
Average Winter Air Temperature 0-15°C
40
50
100
-5
0 5
Average Winter Air Temperature <0°C
Points
Detention Time Point Score =
©
283
Stabilization Pond
-------
Determine Number of Ponds in Series Point Score:
Number of Ponds in Lines
1
-5
n i
0 5
Number of Ponds in Series Point Score
= -©
Determine Length-to-Width Ratio Point Score:
Length-to-Width Ratio
, >2
1.5-2
<1.5
Points
2
0
-2
Length-to-Width Ratio Point Score =
Determine Short Circuiting Ratio Point Score:
Pond #1
Pond #2
Inlet to Outlet Distance
Maximum Pond Dimension
Inlet to Outlet Distance
Maximum Pond Dimer
Average Short
Circuiting Ratio
>0.75
0.5 - 0.75
<0.5
ision
Points
2
0
-2
( ft)
( ft)
( ft)
( ft)
:1
Average =
Short Circuiting Point Score =
©
©
Determine Operational Flexibility Point Score:
Flexibility to Operate in Series and Parallel
No Flexibility to Operate in Series and Parallel
Points
Operational Flexibility Point Score =
©
Stabilization Pond (Rev. 1/89)
284
-------
Determine Variable Level Draw-Off Point Score:
Variable Level Draw-Off Available
Variable Level Draw-Off Not Available
Points
3
-3
Variable Level Draw-Off Point Score =
Add Scores 1 through 8 to Obtain Total Score
'• Facultative Pond Facilities Total Score =
Determine Whether POTW is Type 1, Type 2, or Type 3:
Points Required _^
Score Type 1 Type 2 Type 3
Facultative Pond Facilities (9) >15 0-15 <0
POTW is Type
©
©
Aerobic
Pond Facilities
Calculate Total BOD5 Loading:
Total BOD5 Loading =
Total BOD5 Loading =
Average Daily BOD5 Loading
Total Surface Area of All Ponds
( Ib/d)
( ac)
Ib BODg/ac/d
Determine Total BOD5 Loading Point Score:
' Total BOD5 Loading (Ib/ac/d)
250
200
150
100
50
-10
-508
Points
Total BODs Loading Point Score =
16
285
Stabilization Pond (Rev. 1/89)
-------
Determine Detention Time Point Score:
Detention Time (days)
10
15
40
-5 0
Points
Detention Time Point Score_
Determine Number of Ponds in Series Point Score:
Number of Ponds in Lines
1
-10
0 10
Number of Ponds in Series Point Score =
Determine Length-to-Width Ratio Point Score:
Length-to-Width Ratio
>2
, 1.5 - 2
Points
2
0
-2
Length-to-Width Ratio Point Score =
Determine Short Circuiting Ratio Point Score:
Pond #1
Pond #2
Inlet to Outlet Distance
Maximum Pond Dimension
Inlet to Outlet Distance
Maximum Pond Dimer
Average Short
Circuiting Ratio
>0.75
0.5 - 0.75
<0.5
ision
Points
2
0
-2
( ft)
( ft)
( ft)
( ft)
Average =
Short Circuiting Point Score =••
Stabilization Pond (Rev. 1/89)
286
-------
Calculate Oxygen Availability:
Mechanical Aeration Systems
If data are not available on oxygen transfer capacity, calculate it as Wire Horsepower (Appendix F)
times Actual Oxygen Transfer Rate (Appendix E).
hp) x (_
_lb/hp-hr) x 24 =
Ib O2/d
Diffused Air Aeration Systems
If data are not available on oxygen transfer capacity, calculate an actual oxygenation rate (Section
2.3.3.1).
Jb 02/d
Oxygen Availability =
Oxygen Availability =
Oxygen Transfer Capacity
BOD5 Loading to Aerator
J Ib/d)
( Ib/d)
Ib O2/lb BOD5
Determine Oxygen Availability Point Score:
Oxygen Availability (Ib 02/lb BOD5)
0.8
1.0
1.2
1.5
2.0
-10
-5
0
Points
Oxygen Availability Point Score =
Calculate Mixing Energy:
Total Energy in Primary Pond*
Mixing Energy =
Mixing Energy =
Primary Pond Volume
"Total energy includes energy used for aeration and mixing
( hp) _
10s gal)
hp/10bgal
10
287
Stabilization Pond (Rev. 1/89)
-------
Determine Mixing Energy Point Score:
Mixing Energy (hp/106 gal)
5 10
3
Points
Mixing Energy Point Score
Determine Operational Flexibility Point Score:
Flexibility to Operate in Series and Parallel
No Flexibility to Operate in Series and Parallel
Points
Operational Flexibility Point Score =
Determine Variable Level Draw-Off Point Score:
Variable Level Draw-Off Available
Variable Level Draw-Off Not Available
Points
3
-3
Variable Level Draw-Off Point Score =
15
0
Add Scores 10 through 18 to Obtain Total Score
Aerobic Pond Facilities Total Score =
Determine Whether POTW is Type 1, Type 2, or Type 3:
Points Required
Score Type 1 Type 2 Type 3
Aerobic Pond Facilities
POTW is Type
>20
5-20
<5
*U'.S. GOVERNMENT PRINTING OFFICE 1993.750- 00*60125
Stabilization Pond (Rev. 1/89)
288
-------
-------
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