United States Office of Research and EPA/600/R-00/020
Environmental Protection Development January 2000
Agency Washington, DC 20460
Retrofitting Control Facilities For
Wet-Weather Flow Treatment
Research Report
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EPA#/600/R-00/020
January 2000
Retrofitting Control Facilities For
Wet-Weather Flow Treatment
by
Peter E. Moffa, Howard M. Goebel,
Daniel P. Davis and John J. LaGorga
Moffa & Associates Consulting Engineers
Syracuse, New York 13214
In Association with
Earth Tech, Inc.
South Portland, Maine 04106
Robert Pitt
Environmental Engineer
Birmingham, AL 35226
Contract No. 8C-R056NTSX
Project Officer
Thomas P. O'Connor
Technical Advisors
Richard Field & Mary K. Stinson
Urban Watershed Management Branch
Water Supply & Water Resources Division
National Risk Management Research Laboratory
Edison, NJ 08837
National Risk Management Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
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Notice
The information in this document has been funded by the United States Environmental
Protection Agency under Contract No. 8C-R056-NTSX. Although it has been subjected
to the Agency's peer and administrative review and has been approved for publication as
an EPA document, it does not necessarily reflect the views of the Agency and no official
endorsement should be inferred. Also, the mention of trade names or commercial
products does not imply endorsement by the United States government.
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Foreword
The U.S. Environmental Protection Agency is charged by Congress with protecting
the Nation's land, air, and water resources. Under a mandate of national environmental
laws, the Agency strives to formulate and implement actions leading to a compatible
balance between human activities and the ability of natural systems to support and
nurture life. To meet this mandate, EPA's research program is providing data and
technical support for solving environmental problems today and building a science
knowledge base necessary to manage our ecological resources wisely, understand how
pollutants affect our health, and prevent or reduce environmental risks in the future.
The National Risk Management Research Laboratory is the Agency's center for
investigation of technological and management approaches for reducing risk from threats
to human health and the environment. The focus of the Laboratory's research program is
on methods for the prevention and control of pollution to air, land, water and subsurface
resources; protection of water quality in public water systems; remediation of
contaminated sites and ground water; and prevention and control of indoor air pollution.
The goal of this research effort is to catalyze development and implementation of
innovative, cost-effective, environmental technologies; develop scientific and engineering
information needed by EPA to support regulatory and policy decisions; and provide
technical support and information transfer to ensure effective implementation of
environmental regulations and strategies.
This publication has been produced as part of the Laboratory's strategic long-term
research plan. It is published and made available by EPA's Office of Research and
Development to assist the user community and to link researchers with their clients.
E. Timothy Oppelt, Director
National Risk Management Research Laboratory
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Abstract
Available technologies were evaluated to demonstrate the technical feasibility and cost
effectiveness of retrofitting existing facilities to handle wet-weather flow. Cost/benefit
relationships were also compared to construction of new conventional control and
treatment facilities.
Desktop analyses of 13 separate retrofit examples were performed for 1) converting or
retrofitting primary settling tanks with dissolved air flotation and lamellae and/or
microsand-enhanced plate or tube settling units, 2) retrofitting existing wet-weather flow
storage tanks to provide enhanced settling/ treatment and post-storm solids removal, 3)
converting dry ponds to wet ponds for enhanced treatment, 4) retrofitting wet-weather
flow storage tanks for dry-weather flow augmentation, 5) using storage for sanitary sewer
overflow control , 6) retrofitting for industrial wastewater control in a combined sewer
system, and 7) bringing outdated/abandoned treatment plants back online as wet-weather
flow treatment facilities.
This analysis demonstrated that retrofitting existing wet-weather flow facilities can be
technically feasible in most cases and may be more cost effective than construction of
new conventional control and treatment facilities. The feasibility and cost effectiveness of
retrofitting was found to be a function of site-specific conditions and treatment
requirements. Retrofitting processes will better enable communities to meet EPA's
National CSO Policy and stormwater permitting program requirements.
IV
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Contents
Notice ii
Forward iii
Abstract iv
Tables vii
Figures ix
Acronyms and Abbreviations x
Acknowledgement xiii
Section 1 Introduction 1
Section 2 Conclusions 7
Section 3 Recommendations 10
Section 4 Methods and Material 11
4.1 Literature Search 11
4.2 UseofSWMM 19
4.3 Case Studies and Hypothetical Retrofit Examples 21
Section 5 Desktop Evaluation 22
5.1 Conversion of Primary Treatment Tanks 22
5.1.a. Augusta WWTF, Augusta, ME 22
5.1.b. Hypothetical Retrofit of Primary Treatment Facilities
Using the ACTIFLO System 37
5.2 Retrofitting Existing WWF Storage Tanks to Provide Enhanced
Settling/Treatment and Post-Storm Solids Removal;
Spring Creek AWPCP, New York, NY. 53
5.2.a. Cross-Flow Plate Settlers 56
5.2.b. Chemical Addition to AWPCP 63
5.2.c. Post-Storm Solids Removal 69
5.3 Converting Dry Ponds for Enhanced Treatment 107
5.3.a. Sunnyvale Detention Basin, Santa Clara County, CA 108
5.3.b. Monroe St. Detention Pond, Madison, Wl 121
5.3.c. Birmingham, AL Dry-Ponds 129
5.4 Retrofitting WWF Storage Tanks for DWF Augmentation 139
5.4.a. Erie Boulevard Storage System, Syracuse, NY. 139
5.4.b. Spring Creek AWPCP, New York, NY. 145
5.5 SSO Control Using Storage 149
5.5.a. Hypothetical Overflow Retention Facility Retrofit to an
Existing Sanitary Sewer System 149
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Contents, Continued
5.6 Retrofitting for Industrial Wastewater Control in a
Combined Sewer System 156
5.6.a. Rockland WWTF, Rockland, ME 156
5.7 Bringing Outdated/Abandoned POTWs Back Online as
Wet-Weather Flow Treatment Facilities 174
5.7.a. Auburn WWTF, Auburn, NY. 174
Section 6 References 185
Appendices
Appendix A: Bibliographic Databases 191
Appendix B: Selected Tables with English Equivalent Units 195
VI
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Tables
2-1. Summary of Retrofitting Desktop Analysis Projects 7
5.1 .a-1 Phase 1 CSO Abatement Program Wet-Weather Treatment Design
Criteria 30
5.1.a-2 Estimated O&M Costs, CSO Abatement Program-All Phases 35
5.1.a-3 Phase 1 CSO Abatement Program Project Costs 36
5.1.b-1 ACTIFLO Design Summary 37
5.1.b-2 Comparison of ACTIFLO and Existing Primary Treatment Systems 39
5.1 .b-3 Average Annual Overflow Volume From the Pre- and Post-Retrofit
Facilities (1962-1991) 44
5.1.b-4 Estimated O&M Costs 46
5.1.b-5 Estimated ACTIFLO Retrofit Capital Costs 47
5.1.b-6 Major-Operating Parameters of ACTIFLO Pilot Plant at Nominal Flow Rate
of150M3/H 47
5.1.b-7 Removal Efficiency From 20 Daily Samples in Primary Treatment
in Mexico City, Mexico 48
5.1.b-8 Effect of Coagulant Dosage on Removal Efficiencies in Galveston, Texas....50
5.1.b-9 Effect of Overflow Rate on Removal Efficiencies, Cincinnati, Ohio 51
5.2-1 Existing Dimensions of the AWPCP and Design Flow Rate 55
5.2.a-1 Design Parameters for the Cross-Flow Piste Separator 56
5.2.a-2 Cross-Flow Plate Settler Design Data 63
5.2.b-1 Coagulant Dosages and Performance Data for Six Treatment Plants
That Employ Chemical Coagulation for Advanced Primary Performance
In Sedimentation Tanks 66
5.2.b-2 Estimated Capital Costs Chemical Addition Retrofit 67
5.2.b-3 Estimated Chemical Addition O&M Costs 68
5.2.C-1 Work Plan Prototype Test Matrix 76
5.2.c-2 Summary of Prototype Testing Operation 82
5.2.c-3 Operating Data for Existing Traveling Bridge 89
5.2.c-4 Tipping Bucket Cleaning System Test Results at Representative
Solids Conditions 90
5.2.c-5 Spray Nozzle Cleaning System Test Results 93
5.2.c-6 Original Orifice Header Configuration Test Results 95
5.2.c-7 Modified Orifice Header Configuration Test Results at Representative
Solids Concentrations 96
5.2.c-8 Projected Cleaning Requirements for a Single Basin Cleaning Event 102
5.2.c-9 Cost Comparison of Full Scale Tipping Bucket Versus Orifice
Header Cleaning Systems 104
5.3.a-1 Comparison of Median Metal Concentrations at Inlet to Retrofitted Basin
to Other Santa Clara Valley Stormwater Monitoring Station Data 115
5.3.a-2 Inlet and Outlet Observed Concentrations and Pollutant Removals 116
5.3.a-3 Sediment Observations (mg/kg) 117
VII
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Tables, Continued
5.3.a-4 Comparison of Average Sediment Concentrations from Detention
Basins and Swales (mg/kg) 118
5.3.a-5 Estimated Mean Annual Load Reduction and Cost Effectiveness 119
5.3.a-6 Estimated Annualized Costs for Capital Expenditures and Operation 120
5.3.D-1 Summary of Observed Influent and Effluent Pollutant Concentrations
at Monroe St. Pond 124
5.3.D-2 Summary Table of Pollutant Control 127
5.3.b-3 Average Suspended Solids Removal for Variable n Values for the
Monroe St. Ponds 128
5.3.C-1 Outlet Device Descriptions 133
5.3.C-2 Subcatchment Summaries for Design Storms 133
5.3.C-3 Pond Results of HydroCAD Simulations 134
5.3.C-4 DETPOND Summary for Design Storms 135
5.3.C-5 Water Quality Output Summary for 1976 Rain File 135
5.3.C-6 Water Quality Output Summary for 1952-1989 Rain File 138
5.4.a-1 Average Annual Capture Volume From the EBSS Under Pre- and
Post-Retrofit Conditions (1962-19991) 143
5.4.a-2 Estimated O&M Costs 144
5.4.a-3 Estimated EBSS Retrofit Capital Costs 144
5.4.b-1 Wet-Weather Discharge Volume From the Spring Creek AWPCP
Under Pre-and Post-Retrofit Conditions for 1985 146
5.4.b-2 Estimated O&M Costs 147
5.4.b-3 Estimated Spring Creek DWF Retrofit Capital Costs 148
5.5.a-1 Average Annusl Overflow Volume from the Pre- and Post-Retrofit
Facilities (1971-1985) 151
5.5.a-2 ORF O&M Costs 153
5.5.a-3 Estimated ORF Construction Costs 154
5.6.a-1 SWMM Modeling Summary-Annusl Overflow Volumes 164
5.6.3-2 SWMM Modeling Summsry- Projected Peak Flows 164
5.6.3-3 SWMM Modeling Summsry- Projected Pesk Flows 165
5.6.3-4 Wet-Westher Trestment Design Criteris 167
5.6.3-5 Estimsted O&M Costs Phsse II CSO Improvements 172
5.6.3-6 Project Costs - Phsse II CSO Improvements 173
5.7.3-1 Bssis of Design of the Overflow Retention Fscilities 178
5.7.3-2 Average Annusl Overflow Volume From the Pre- snd Post-Retrofit
Fscilities (1971-1991) 181
5.7.3-3 ORF O&M Costs 183
5.7.3-4 ORF Chlorinstion/Dechlorinstion Cspitsl Costs 183
VIM
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Figures
5.1.a-1 Existing Site Layout Prior to Retrofit 24
5.1.a-2 Process Flow Schematic Prior to Retrofit 25
5.1.a-3 Site Layout Following Retrofit 28
5.1.a-4 Process Flow Schematic Following Retrofit 29
5.1 .b-1 Layout of Hypothetical WWTF Prior to Construction of ACTIFLO System 38
5.1.b-2 Layout of Proposed ACTIFLO System in Hypothetical Setting 40
5.1.b-3 ACTIFLO Process Flow Diagram 42
5.1.D-4 Helminth Eggs Removal in Primary Wastewater Treatment in Mexico
City, Mexico 49
5.1.D-5 SS Removal in Primary Wastewater Treatment in Mexico City, Mexico 49
5.1.b-6 Start-Up Efficiency of ACTIFLO Process 50
5.1.b-7 Twenty-Four Hour Continuous Demonstration Run, Cincinnati, Ohio 51
5.2-1 Spring Creek AWPCP Layout 54
5.2.a-1 Layout of Shell Co. Cross-Flow Plate Settler, Montreal Canada 57
5.2.a-2 Horizontal Projections of Inclined Cross-Flow Lamella Plates 60
5.2.a-3 Plan View of Aquarius Cross-Flow Plate Settler 61
5.2.a-4 Profile of Aquarius Cross-Flow Plate Settler 62
5.2.c-1 Wash Water Piping System Schematic 72
5.2.c-2 Tipping Buckets-Enlarged Plan 74
5.2.c-3 Tipping Buckets - Section A 75
5.2.c-4 Modified Plywood Training Wall Schematic 78
5.2.C-5 Flushing Gate-Section B 79
5.2.c-6 Spray Nozzle Header- Section 81
5.2.c-7 Original Orifice Header Configuration - Section A 85
5.2.c-8 Modified Orifice Header Configuration - Section A 86
5.3.a-1 Sunnyvale Pump Ststion No. 2 Showing Retrofit Structures 111
5.3.C-1 Stage v. Surface Area Curve 132
5.3.C-2 Pond Stage v. Particle Residue Control 136
5.3.C-3 Rain Depth v. Particle Residue Control 137
5.3.C-4 Rain Intensity v. Particle Residue Control 137
5.4.a-1 EBSS Layout 140
5.6.3-1 Existing Site Layout Prior to Retrofit 158
5.6.3-2 Process Flow schemstic Prior to Retrofit 159
5.6.3-3 Site Lsyout Following Retrofit 162
5.6.3-4 Process Flow Schemstic Following Retrofit 163
5.7.3-1 Existing Wsstewster Trestment Flow Disgram 176
5.7.3-2 Conceptusl Wsstewster snd Sludge Trestment Flow Disgram 180
IX
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Acronyms and Abbreviations
ac-ft
ASD
AWPCP
BATS
BMP
BOD
CCS
COM
CEAM
cfs
cm
CN
COD
COV
CPE
CSO
CSS
cyd
CWA
D
DAF
DEP
DWF
EA
EBSS
EPA
EMC
FDS
FeCI3
fps
ft
gal
gpd
gpm
ha
HP
h
IDF
In.
I/I
JSRS
kg
Acre-feet
Augusta Sanitary District
Auxiliary Water Pollution Control Plant
Burnet Avenue Trunk Sewer
Best Management Practice
Biochemical Oxygen Demand
Combined Collection System
Camp Dresser & McKee
Center for Exposure Assessment Modeling
Cubic Feet per Second
Centimeter
Curve Number
Chemical Oxygen Demand
Coefficient of Variation
Comprehensive Performance Evaluation
Combined Sewer Overflow
Combined Sewer System
Cubic Yards
Clean Water Act
Day
Dissolved Air Flotation
Department of Environmental Protection
Dry Weather Flow
Environmental Assessment
Erie Boulevard Storage System
U.S. Environmental Protection Agency
Event Mean Concentration
Flow Distribution Structure
Ferric Chloride
Feet per Second
Feet
Gallon
Gallons per day
Gallons per minute
Hectare
Horse power
Hour
Intensity Distribution Frequency
Inch
Infiltration and/or Inflow
James Street Relief Sewer
Kilogram
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Acronyms and Abbreviations, Continued
km
kW
I
Ibs
m
mA
mm
urn
mi
Metro
METRO
MG
MGD
mg
mg/L
jig
min
MIS
ml
msl
mt
MTP
NASA
NOAA
NPDES
NTIS
NURP
NWS
NYCDEP
NYSDEC
O&M
ORF
POTWs
ppm
PPS
PRF
psi
RCP
rpm
s
SCADA
scfm
Kilometer
Kilowatt
Liter
Pounds
Meter
Milliampere
Millimeter
Micrometer
Mile
Municipality of Toronto Metropolitan
Syracuse Metropolitan Wastewater Treatment Facility
Million gallons
Million gallons per day
Milligram
Milligrams per Liter
Microgram
Minute
Main Interceptor Sewer
Milliliter
Mean Sea Level
Metric ton
Main Treatment Plant
National Aeronautics and Space Administration
National Oceanic and Atmospheric Administration
National Pollution Discharge Elimination System
National Technical Information Services
Nationwide Urban Runoff Program
National Weather Service
New York City Department of Environmental Protection
New York State Department of Environmental Conservation
Operation and Maintenance
Overflow Retention Facility
Publicly Owned Treatment Plants
Parts per Million
Particulate Pollutant Strength
Peak Reduction Factors
Pounds Per Square Inch
Reinforced Concrete Pipe
Revolutions per Minute
Seconds
Supervisory Control and Data Acquisition
Standard Cubic Feet Per Minute
XI
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scs
SESD
SOR
STLC
SS
SSES
SSO
SW
SWMM
SYNOP
TB
TKN
TOO
TTLC
USGS
WET
WHO
WPCP
WQO
WWF
WWTF
Acronyms and Abbreviations, Continued
Soil Conservation Service
South Essex Sewerage District
Surface Overflow Rate
Soluble Threshold Limit Concentrations
Suspended Solids
Sewer System Evaluation Survey
Sanitary Sewer Overflow
Storm water
Stormwater Management Model
Synoptic Rainfall Analysis Program
Tipping Buckets
Total Kjeldahl Nitrogen
Total Organic Carbon
Total Threshold Limit Concentration
U.S. Geological Survey
Whole Effluent Toxicity
World Heath Organization
Water Pollution Control Plant
Water Quality Objective
Wet Weather Flow
Wastewater Treatment Facility
XII
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Acknowledgements
Moffa & Associates gratefully acknowledges the support of the project by the National
Risk Management Research Laboratory, Office of Research and Development, U.S.
Environmental Protection Agency, under the direction of Mr. Richard Field, Wet-Weather
Flow Research Program leader.
The cooperation of Thomas P. O'Connor, Project Officer, Urban Watershed Management
Branch, Water Supply and Water Resources Division, National Risk Management
Research Laboratory (NRMRL), Office of Research and Development (ORD), U.S.
Environmental Protection Agency (EPA), is acknowledged as are Dr. David Fischer and
Dr. Ronald Rovanesk, ORISE post graduates with EPA's UWMB, in Edison, New Jersey
and Dan Murray, Technology Transfer Division, NRMRL, ORD, EPA, Cincinnati, Ohio for
providing reviews for the report.
In addition, special thanks are given to the following consultants for their contributions to
this report:
Steve D. Freedman, P.E., Earth Tech Inc.
Sorin L. Goldstein, P.E., Camp Dresser & McKee, Inc., Woodbury, NY 11797
Dwight A. MacArthur, P.E., O'Brien & Gere Engineers, Inc., Syracuse, NY 13221
Robert Pitt, Environmental Engineer, Consultant
Robert B. Stallings, P.E., Earth Tech Inc.
The assistance of Gregory J. Hotaling with preparation of graphical materials presented is
hereby recognized, as is the assistance of Renee Davis for assistance with word
processing.
This report was prepared by Moffa & Associates Consulting Engineers, by Howard M.
Goebel, Daniel P. Davis, and John J. LaGorga under the direction of Peter E. Moffa,
Principal.
XIII
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Section 1
Introduction
Background
Wet-weather flow (WWF) is currently one of the leading causes of water-quality
impairment in the United States and improvement of controls is one of the two priority
water focus-areas cited by the U.S. Environmental Protection Agency's (EPA) Office of
Water in its National Agenda for the Future. Wet-weather pollution problems are
primarily manifested in three different ways: separate sewer overflows (SSO), combined
sewer overflows (CSO), and stormwater (SW) discharges. SSO are the result of the
unplanned relief of a sewer system intended only for sanitary sewage but also carrying
infiltration and inflow (I/I), illegal stormwater connections, and stormwater. CSO are the
result of the "designed" relief of a sewer system that intentionally carries stormwater and
sanitary sewage. Stormwater runoff discharges include flows that have been collected
in a separate storm sewer system or a surface drainage system.
WWF pollution is extensive throughout the country. There are approximately 15,000
CSO discharge points within 1,100 municipalities that have combined sewers. SSO
occur in more than 1,000 municipalities. SW discharges occur in over 6,500
municipalities with populations greater than 50,000 and at 1.2 million industrial,
commercial, institutional, and retail sources.
It was not until data from several comprehensive municipal CSO and SSO studies were
collected that the types of pollutants discharged by overflow systems and their relative
impacts on receiving waters were initially understood. The primary parameters of
importance are visible floatables and gross solids, bacteria and other pathogens,
suspended solids (SS), biochemical oxygen demand (BOD), nutrients, and toxic
organics and metals.
The projected costs for national CSO, SSO, and SW abatement are in the tens of
billions of dollars. Consequently, inexpensive abatement alternatives are required to
alleviate the high costs associated with conventional treatment. High-rate treatment
technologies can increase sewage treatment system capacity to allow treatment of a
significant portion of the WWF in sanitary and combined sewers. The primary benefits
of high-rate treatment technologies are their ability to handle higher flow rates and
compactness or small footprint compared to conventional treatment processes. High-
rate treatment technologies require less tankage and space and in urbanized areas are
often more cost effective than conventional treatment processes since storm flows are
significantly greater than dry-weather flows.
Wet-weather discharges are reflections of the watersheds or sewersheds from which
they are generated and the climatic patterns those areas experience. The impacts of
wet-weather discharges are determined by a variety of complex relationships between
the temporal and spatial distributions of discharges. These include the types and
magnitudes of pollutant loads being transported, the ecological and hydraulic nature of
the receiving water, as well as the designated beneficial uses, and the desires and
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expectations of stakeholders. The result is a wide-scale problem that requires very site-
specific solutions.
Solutions to WWF problems require investigation of a wide variety of management
options and the relationships between management costs and water quality benefits.
WWF must be controlled by storage-treatment systems since WWF are intermittent and
highly variable in both pollutant concentrations and flow rates. An optimized system
maximizes the use of the existing infrastructure prior to construction of new
treatment/storage devices.
Retrofitting existing sewerage systems to handle increased WWF that result in SSO,
CSO, and SW discharges may be a cost-effective alternative to the construction of new
treatment systems. Retrofitting existing sewerage systems and treatment plants to
convey and treat additional WWF can be accomplished by:
1. Increasing the hydraulic loadings at the control facilities (CSO and SSO);
2. Increasing the amount of storage throughout the conveyance system (All WWF);
and
3. Providing the additional function of pollution removal by settling at flood-control-
detention basins (SW).
Summary of Analyses
This report demonstrates real and hypothetical case studies. Retrofits of available
technologies for conventional sewer systems are presented in the form of desktop
analyses. The desktop analyses were conducted to determine the technical feasibility
and cost effectiveness of retrofitting existing sewage and WWF systems in lieu of
constructing new sewerage and conveyance systems. Site-specific analyses were
completed for WWF storage-treatment systems and their associated cost due to the
variability of wet-weather volume andtreatability (quantity and quality).
The desktop analyses including conceptual layouts and designs of systems to increase
treatment and storage of WWF through retrofits were performed for the following
scenarios:
1. Converting or retrofitting primary settling tanks at the publicly owned treatment
works (POTW) to alternative sedimentation or sedimentation enhancement
methods or methods of treatment that provide equivalent or higher levels of
treatment (SSO and CSO);
2. Retrofitting existing CSO storage tanks to provide enhanced settling/treatment
and post-storm solids removal;
3. Converting existing "dry-ponds" and/or other flood control devices to SW
detention ponds for separate SW systems to allow treatment through
sedimentation;
4. Retrofitting existing WWF storage tanks and ponds to serve a second function as
DWF equalization tanks;
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5. Retrofitting for industrial wastewater control in a combined sewer system;
6. SSO control using storage; and
7. Bringing outdated and/or abandoned POTWs back online as satellite WWF
treatment facilities (SSO and CSO).
The analyses were performed over a variety of orders of magnitude of flow and/or
volume for each scenario. The desktop analyses of the retrofitted systems addressed
the following design considerations, where appropriate:
1. Operations, maintenance and level of automation
2. Sludge handling requirements and disposal
3. Continuous operation versus storm flow event operations
4. Chemical storage and feed rate
5. Bypass requirements
Treatment values, pollutant and hydraulic loadings, and seasonal and diurnal variation
are also provided, where appropriate.
Simplified simulations were completed using historical rainfall data for an one-year
period using the EPA Stormwater Management Model (SWMM) with hourly time steps
or an appropriate alternative. The yearly rainfall data is provided including seasonal
variation, average antecedent duration, and storm intensity.
When feasible, the cost effectiveness of retrofitting versus construction of additional
conventional treatment works and/or storage basins was completed as part of each
desktop analysis. The effectiveness of each system, guidance on design and operation,
cost breakdowns (new construction and modification of existing structure, system
installation, and operation and maintenance [O&M]), and case studies are provided.
Regulatory Background
Since the 1970s, the U.S. Environmental Protection Agency's Construction Grant
Program financed the development and construction of hundreds of wastewater
treatment facilities (WWTF) around the nation. During this time the main focus was
regional WWTF designed to treat dry-weather sewage flows. Little or no money was
invested to abate wet-weather pollution sources. Since then, many of the POTWs have
exceeded their design flows, discharge standards have become more stringent, and
federal regulations and policies have begun to address WWF pollution. All of these
factors contribute to a need for more and better WWF treatment facilities. Today,
federal funding for new environmental facilities is scarce and taxpayers carry the
burden. Based on this climate, it becomes much more critical that any compliance
needs should first consider optimizing existing infrastructure through retrofitting.
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This report is intended to help municipalities who may be impacted by current policies
and regulations (i.e., National Pollutant Discharge Elimination System (NPDES) Phase I
permitting and the CSO Control Policy) and the newer regulations and policies that
address WWF (i.e., the upcoming NPDES (Phase II) permitting regulations and SSO
regulations). Also, this report can assist municipalities and others (e.g., industry and
commercial property owners) who are subject to local regulations.
Stormwater Regulations and Permits
Congress added Section 402(p) to the Clean Water Act in 1987 to require the
implementation of a comprehensive approach for addressing stormwater discharges.
Section 402(p)(4) required EPA to develop permit application regulations under the
NPDES, submission of NPDES permit applications, issuance of NPDES permits, and
compliance with NPDES permit conditions. Section 402(p)(6) requires EPA to designate
stormwater discharges to be regulated (within the statutory definitions provided in
section 402(p)(2)) and establish a comprehensive regulatory program, which may
include performance standards, guidelines, guidance, and management practices and
treatment requirements (EPA, 1999).
The first phase of NPDES stormwater permit application regulations ("Phase I") was
promulgated on November 16, 1990 (EPA, 1990). The provisions addressing municipal
separate-storm-sewer systems (MS4s) cover systems serving a population of 100,000
or more. This includes 173 cities, 47 counties and additional systems designated by
EPA or states. A total of 260 permits, covering approximately 880 operators (local
governments, state highway departments, etc.) were identified as subject to Phase I
permit application requirements. The CWA requires that MS4 permits effectively
prohibit non-stormwater discharges into the storm sewers as well as reduce the
discharge of pollutants to the maximum extent practicable (including management
practices, control techniques and system, design and engineering methods, and other
provisions appropriate for the control of such pollutants).
Phase I MS4 permittees were required to submit an application that included source-
identification information, precipitation data, existing data on the volume and quality of
stormwater discharges, a list of receiving water bodies and existing information on
impacts on receiving waters, a field screening analysis for illicit connections and illegal
dumping, and other information. The MS4 permittees were to gather and provide
additional information including:
• discharge characterization data based on quantitative data from 5 to 10 representative
locations in approved sampling plans; estimates of the annual pollutant load and event-
mean concentration of system discharges for selected conventional pollutants and
heavy metals; a proposed schedule to provide estimates of seasonal pollutant loads;
and the mean concentration for certain detected constituents in a representative storm
event; and
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• a proposed management program including descriptions of: structural and source
control measures that are to be implemented to reduce pollutants in runoff from
commercial and residential areas; a program to detect and remove illicit discharges; and
a program to control pollutants in construction site runoff.
The NPDES stormwater regulations for the second phase of stormwater discharge
control ("Phase II") was proposed on January 9, 1998 (US EPA, 1998c). EPA was
required to promulgate the Phase II rule in 1999 under a separate consent decree. The
proposal designated two classes of facilities to be automatically covered on a
nationwide basis under the NPDES program:
(1) small municipal separate storm sewer systems located in urbanized areas (about
3,500 municipalities would be included in the program); and
(2) construction activities (pollutants include sediments and erosion from these sites)
that disturb equal to or greater than one and less than five acres of land (about
110,000 sites per year will be included in the program).
Those facilities designated above would need to apply for NPDES stormwater permits
by 2002. EPA is anticipating that most permittees would be covered under general
permits.
Municipalities and others may be able to use retrofitting to comply with the new Phase II
rules by converting existing dry ponds (or a portion thereof), used typically for flood and
drainage control, to wet ponds. This is consistent the Phase I management program to
use structural and source control measures to reduce pollutants in runoff from
commercial and residential areas. Wet ponds, because of the permanent pool and
longer detention times are thought to remove more pollutants than dry ponds (e.g., SS
and BOD and COD removals) and reduce or eliminate discharges.
Some facilities that EPA is proposing to cover under the new Phase II rule are already
subject to state and/or local stormwater management requirements. The Rockland,
Maine case study demonstrates how retrofitting was incorporated into the Maine's CSO
Abatement Program permitting requirements. Among other planned retrofits a new
force main was commissioned to separate an industrial loading from the municipal
sewer and the wastewater treatment facility is being upgraded to handle additional
WWF.
CSO Control Policy
The need for high-rate treatment systems comes from the National CSO Policy that
requires treatment of additional WWF in existing POTW during high flow periods caused
by rainstorms. The recent EPA National CSO Control Policy (59 Federal Register
18688) resulted in the publication of several guidance documents. These two guidance
documents are 'Combined Sewer Overflow - Guidance for Nine Minimum Controls"
(EPA 832-B-95-003) and "Combined Sewer Overflow - Guidance for Long Term Control
Plan" (EPA 832-B-95-002). Required CSO abatement technologies that are addressed
in this report are:
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* Maxim ization of flow to the POTW for treatment;
* Evaluation of alternatives that enable the permittee, in consultation with the NPDES
permitting authority, Water Quality Standard (WQS) authority, and the public, to
select CSO controls that will meet CWA requirements;
* Cost/performance considerations to demonstrate the relationships among a
comprehensive set of reasonable control alternatives; and
* Maximization of WWF treatment at existing POTW treatment plants
Consensus Recommendation of SSO Federal Advisory Subcommittee
The EPA/State SSO work group distributed a set of draft NPDES permit regulations and
related guidance for municipal sanitary sewer collection systems and SSO to a small
municipal outreach group, the SSO Federal Advisory Subcommittee and other members
of the public. The SSO Federal Advisory Subcommittee made consensus
recommendations to EPA on:
• capacity, management, operation and maintenance (CMOM) program requirements;
• a permit prohibition for SSO;
• permit record keeping, reporting and public notification requirements; and
• peak excess flow treatment facilities.
The SSO subcommittee recommended to EPA the substance of the CMOM, Prohibition,
Record Keeping, Reporting and Public Notification, Remote Treatment Facilities
documents, and Satellite Collection Systems and watershed management agreed
principals on October 20, 1999. In addition, the Subcommittee recommended principles
for satellite collection systems and watershed management.
The protocols for the Federal Advisory Subcommittee provide that EPA is committed to
reflect the consensus reached by the SSO Subcommittee in an upcoming notice of
proposed rulemaking. While the Advisory Subcommittee made recommendation
addressing core issues, some outstanding issues remain. The consensus
recommendation of the SSO Federal Advisory Subcommittee clears the way for the
next step in the development of a proposed rule.
Retrofitting an existing treatment plant is often a more cost-effective way to control
additional flow than building new facilities. With the upcoming SSO regulations,
investigating the economics of retrofitting existing systems to treat more flow, may
become the first step in a compliance exercise.
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Section 2
Conclusions
Desktop analyses were performed for 13 separate retrofit examples. The analyses
demonstrated that in certain circumstances retrofitting existing WWF facilities is
technically feasible and may be more cost effective than construction of new
conventional control and treatment facilities. A summary of each retrofit is provided in
Table 2-1 and is discussed below.
Table 2-1. Summary of Retrofitting Desktop Analysis Projects
Project
Augusta WWTF
Hypothetical WWTF
Spring Creek AWPCP
Spring Creek AWPCP
Spring Creek AWPCP
Sunnyvale Detention Basin
Monroe St. Detention Pond
Birmingham Dry-Ponds
Erie Boulevard Storage System
Spring Creek AWPCP
Hypothetical Sanitary Sewer
System
Rockland WWTF
Retrofit
Headworks Upgrade - Enhanced
Primary Treatment
ACTIFLO System -Enhanced Primary
Treatment
Plate Settlers for Enhanced
Treatment in WWF Storage Facilities
Chemical Addition for Enhanced
Treatment in WWF Storage Facilities
Post-Storm Solids Removal for WWF
Storage Facilities
Retrofit for Enhanced Treatment
Retrofit for Enhanced Treatment
Retrofit for Enhanced Treatment
DWF Augmentation
DWF Augmentation
SSO Control Using Storage
Retrofitting for Industrial Wastewater
Control in a CSS
Treatment
Benefit
Yes
Yes
No
Marginal
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Cost
Benefit
Yes
No
No
Marginal
Not
Determined
Not
Determined
Not
Determined
Not
Determined
Marginal
Marginal
Yes
Yes
The headworks of the Augusta, ME WWTF were retrofitted to maximize flow to the
existing primary and secondary tanks and to minimize wet-weather bypass. This was a
very cost-effective alternative by maximizing existing plant capacity. This retrofit
allowed for minimizing the construction of new high-rate treatment facilities. However,
there are only limited experiences of such retrofitting documented in the literature.
Enhanced sedimentation methods employ flocculation with lamellar settling to speed up
the clarification process. One such system, the ACTIFLO, was hypothetically retrofitted
to a primary treatment facility. This technology provides enhanced SS removal with
short detention times and quick start-up as compared to conventional primary
clarification. One of the benefits of the ACTIFLO system is the extremely small footprint
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of the process. The downside is the high cost of chemicals and increased O&M
requirements.
Retrofitting existing WWF storage tanks for flow equalization and enhanced treatment
was not cost effective. The Spring Creek Auxiliary Wastewater Pollution Control Plant
(AWPCP) in New York, NY was used to evaluate this retrofit option. This is largely due
to the high variability of WWF and limitations of the technologies evaluated. Retrofitting
existing WWF storage tanks with cross-flow plate settlers was found to be technically
limiting due to the hydraulic loading rates of cross-flow plate settlers. This technology
may be more appropriate for primary clarifiers where hydraulic load rates are more
consistent than in WWF storage tanks.
Retrofitting existing WWF storage tanks with chemical addition to provide chemically
enhanced sedimentation of WWF is a feasible option. This technology could result in
60—90% SS removal, 40—70% BODS removal, and 30—60% chemical oxygen
demand (COD) removal. This technology does not affect the operation of the storage
tanks to serve their primary role of equalization.
The AWPCP was also used to evaluate several retrofitting options for post-storm solids
removal from WWF storage tanks including tipping buckets, flushing gates, and orifice
headers. The results of pilot testing and design considerations including capital and
O&M costs are provided for each technology.
Retrofitting existing dry-ponds to provide treatment through sedimentation was found to
be a relatively simple and viable treatment option. Three dry ponds were evaluated
from different regions of the country: 1) Santa Clara, CA, 2) Madison, Wl, and 3)
Birmingham, AL. The retrofits significantly reduced effluent flows and downstream
channel erosion and also provided a water quality benefit. This was done largely by
developing features to better control influent and effluent flows and to reduce short-
circuiting within the ponds.
The Erie Boulevard Storage System (EBSS) in Syracuse, NY and the AWPCP were
evaluated for the purpose of serving a dual role as wet-weather and dry-weather
equalization facilities. This analysis showed that this could be done without drastically
reducing the ability of the storage tanks to capture WWF. The primary benefit of
equalization provided by the DWF storage is the dampening of peak flow rates that
would otherwise result at the treatment plant, allowing the primary and secondary
treatment facilities to operate at a more uniform rate thereby provide more consistent
treatment efficiency throughout a typical day. However, this retrofit is only economically
viable for a treatment plant that is operating at or near design capacity where increased
O&M costs can be balanced against cost to increase capacity at the plant.
The retrofit of an unused aeration tank at the Rockland WWTF in Rockland, ME and the
construction of a separate force main from an industrial facility to the WWTF provided a
cost-effective alternative for the elimination of high-strength wastes from CSO
discharges. This retrofit conveyed high-strength waste from the industrial facility to the
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dry-weather headworks thereby eliminating the potential for CSO discharges of
industrial wastewater and consequent impact on the receiving water.
Retrofitting SSOs to use storage from overflow retention facilities provided a cost-
effective alternative to expansion of the sanitary collection system to convey the volume
of overflow to the treatment plant for primary and secondary treatment. However, this
alternative requires the availability of a sufficient amount of land to construct the storage
facility.
Retrofitting abandoned POTWs for use as WWF treatment facilities was shown to be an
effective treatment alternative. The Auburn WWTF in Auburn, NY was retrofitted using
the abandoned primary treatment tanks as WWF storage tanks to reduce the impact
associated with WWF in excess of the capacity of the existing treatment plant. This
retrofit was an extremely cost-effective alternative to other abatement options due to the
availability and condition of the existing facilities. The abandoned primary tanks were in
good condition and required only minor modifications. This retrofit is a cost-effective
alternative in cases where abandoned primary tanks are available in good condition for
retrofitting. This type of retrofit may be a less cost-effective solution for cases where the
existing tanks are in poor condition requiring a substantial cost to upgrade the tanks to
serve a functional role.
The feasibility and cost effectiveness of retrofitting is in large part controlled by site-
specific conditions and treatment requirements. The physical condition of existing
facilities, age, and level of maintenance, size, layout, hydraulic factors, water quality and
other site-specific factors all affect the suitability for retrofitting. As such, the opportunity
for retrofitting existing WWF facilities must be evaluated on a case by case basis.
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Section 3
Recommendations
The findings of this study indicate that retrofitting existing control facilities can be a cost-
effective alternative to the construction of conventional treatment facilities. The
appropriateness of retrofitting is however, a function of site-specific needs and
conditions. Therefore, it is recommended that retrofitting existing control facilities be
identified as an alternative for wet-weather abatement. Retrofitting should be evaluated
early on in the decision making process for wet-weather abatement.
The advanced sedimentation methods such as the ACTIFLO merit further study given
the results for high-rate treatment applications in Europe and due to the limited
experience in North America. This could include controlled, side-by-side pilot
demonstrations to assess the benefits and cost effectiveness of alternative advanced
sedimentation methods.
10
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Section 4
Methods and Materials
4.1 Literature Search
Introduction
Typically, urban infrastructure collects, conveys, and treats DWF. In the case of a CSO
community, a component of WWF is collected and conveyed in the same infrastructure
as the DWF, but points of relief are purposely provided before treatment because of
hydraulic constraints of the conveyance system and the capacity of the dry-weather
treatment facilities. In the case of an SSO community, the DWF (sanitary sewage) and
the stormwater are collected and conveyed in different systems. However, these
separated sanitary systems inadvertently receive I/I which result in overflows and
consequent pollutant concerns. Regardless of how WWF are manifested, the essential
infrastructure is made up of the collection, conveyance, and treatment systems. In each
of these systems, retrofitting specific structures can provide improved control and
treatment for WWF. Often surprisingly simple and inexpensive retrofits can provide
expanded capacity, enhanced treatment, extended facility lifespan, and reduced
operating costs.
This literature review focuses on retrofits made to collection, conveyance, and treatment
systems in order to better control and treat wet-weather pollution. A majority of the cited
literature was found through queries in such databases as,
• Environmental Sciences & Pollution Management Database
• Compendex*Plus
• Wilson Applied Science & Technology Abstracts
• ProCite Database,
• National Center for Environmental Publication and Information
Other literature was found through requests for information made to consulting
engineering firms. An emphasis was placed on national conference proceedings. The
referenced databases are described in Appendix A.
General Retrofitting Information
Martin (1988) developed a method to help government watershed planners and
managers to: (1) assess the pollutant potential of runoff from existing urban
development, (2) screen for feasible control measures, and (3) develop and implement
retrofit management strategies. This method strongly prompts the implementation of
watershed retrofits as remedial actions for stormwater pollution.
Walesh (1992) discusses the potential for retrofitting stormwater facilities. The paper
summarizes the history of urban stormwater detention facilities and suggests that many
of these existing facilities can be retrofitted to improve quantity control, include quality
11
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control, improve O&M, reduce safety hazards, enhance aesthetic attributes and have
recreational features. Beale (1992) and England (1995) also discuss retrofitting
stormwater facilities. Both authors address design considerations for stormwater
retrofitting techniques. Such techniques include detention ponds, retention ponds,
underground vaults, exfiltration pipes, and baffle boxes. Palhegyi, Driscoll, and
Mangarella (1991) evaluated the feasibility of retrofitting stormwater control facilities to
improve pollutant-removal performance. This paper presents the results of an analysis
that was performed to predict the removal efficiency of suspended sediments in storm
runoff under existing conditions and under several practical retrofit alternatives. A flood
risk analysis was performed for each condition. It was concluded that stormwater
quality could be improved by modifying operating strategies of existing facilities, but a
flood risk analysis must be considered.
Field and O'Connor (1997) discuss a strategy to optimize CSO control systems. The
optimization strategy maximizes the use of the existing system and sizes the storage
facility in accordance with the WWTF capacity to obtain the lowest-cost storage and
treatment system. Roesner and Burgess (1992) discuss CSO rehabilitation strategies
for urban areas. Roesner and Burgess address and cite examples for eliminating dry-
weather overflows, rehabilitating regulators, and optimizing existing interceptor capacity.
Collection System Retrofits
The practice of utilizing inlet control concepts to reduce the impacts of flooding from
urban runoff and pollution through catchbasin adjustments is common. Most
installations involve restriction of catchbasin outflow to cause surface inflow to be re-
directed by overland routes to more suitable discharge points or to underground
storage. The majority of installations involve retrofitting existing systems to mitigate
collection-system surcharging, reduce exfiltration and infiltration exchange within
adjacent sewer systems, and to cause reduction of downstream CSOs (Pisano, 1990).
The area known as the "Triangle" in Parma, Ohio, is a 12.1 ha (30 acre) low-relief area
in a 117.4 ha (290 acre) drainage system. In the "Triangle" storm drains are generally
inadequate and severe street flooding and basement flooding are common. Inlet
controls such as overland flow berms, vortex throttles, and shallow drains to de-water
street areas were implemented throughout the entire 117.4 ha (290 acre) drainage area.
The project mitigated surface-water ponding and provided basement-flooding protection
throughout the entire 117.4 ha (290 acre) area. Construction costs in 1984 were
$875,000 (Pisano, 1982).
Lake Quinsigamond, Worchester, Massachusetts, was becoming eutrophic from
stormwater discharges. Studies indicated that the dominant nutrient source was
contaminated DWF from the storm sewer. It was believed that this contamination
resulted from the mixing of the sanitary sewer with the storm sewer due to the
"over/under" sewer configuration. In this configuration the sanitary sewer was placed
over the stormwater sewer in the same trench. A system wide pipe replacement or
rehabilitation was rejected based on cost. Therefore, a number of system controls were
12
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installed; these were found to immediately eliminate much of the nutrient loading from
the storm sewer base flow. A vortex flow throttle was installed in the storm sewer to
divert the polluted base flow and the first flush to a nearby combined sewer. Inlet
controls were then installed in the upper reaches of the combined system to increase its
capacity to convey the polluted base flow and the first flush. Approximately 0.34 m3/s
(12 cfs) of stormwater (base flow and first-flush) were diverted to the combined system.
Total construction cost were estimated at $160,000 (Pisano, 1989).
Conveyance System Retrofits
The conveyance system is the infrastructure component designed to move water from
the site of origin to an acceptable receiving water body. In the case of dry-weather
sewage flow, the conveyance system carries wastewater to a treatment plant. In the
case of WWF, this water may be discharged into a water body without treatment. In
many stormwater instances, detention ponds are an integral part of the conveyance
system. In the past, these detention ponds were used primarily for the purpose of urban
flood control. In some instances, ponds are retrofitted for the capture and treatment of
stormwater, thereby improving the quality of the receiving water body.
Flow Equalization
Fairport Harbor, Connecticut, developed a unique solution to eliminate CSOs by
converting an abandoned industrial fuel oil tank into a cost-effective wet-weather
retention tank. The fuel oil tank was utilized for flow equalization after the original plan
to construct a new tank and pump station near the outfall met local resistance. The
conversion of the fuel oil tank has removed an environmental liability, saved the district
money, and reduced CSOs. An initial examination of the 36.6 m (120 ft) diameter fuel
oil tank indicated it had sufficient capacity 12,112 m3 (3.2 MG) for the intended capture
of the five-year intensity storm. However, of primary concern were structural integrity
and the presence of industrial sludge and asbestos. Rehabilitation of the tank began
with the removal of the crude oil sludge, interior piping, asbestos-covered exterior
piping, and lead-based paint. Welds that did not meet American Welding Society
standards were replaced. Utilization of the fuel oil tank also required building a pump
station and force main designed for a peak flow rate of 19,870 m3/d (5.25 million gallons
per day or MGD). The project total cost was $2.6 million (Shrout, 1994).
The Rohnert Park Demonstration Project included the design, construction, operation,
testing and evaluation of a surge facility designed to provide flow equalization and some
degree of treatment to all WWF and to provide rate control of wet-weather and dry-
weather wastewater flows to interceptor sewers. The principal feature of the surge
facility was a 2840 m3 (0.75 million gallon or MG) sedimentation-equalization basin. It
was tested under field conditions with influent flows varying from 2,270—20,800 rrfVd
(0.6—5.5 MGD). The ability of the surge facility to provide adequate hydraulic control to
diurnal flow variation was documented. Under WWF conditions, the surge facility was
able to remove approximately 45% of the influent SS. BOD removal under WWF
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conditions was not significant. The total construction cost for the demonstration facility
was $384,000 (Welbon, 1974).
Detention Basins
Santa Clara Valley, California is protected from flooding by a series of pump stations
and levees. Based on a preliminary evaluation of the feasibility of retrofitting detention
basins, a pilot study was implemented to retrofit a pump station and to conduct testing
to measure water quality benefits and costs. In the pilot study, structural and
operational retrofits were implemented at a pump station consisting of four primary
pumps rated at 1.1 m3/s (39 cfs) and one auxiliary pump rated at 0.25 m3/s (9 cfs). The
detention basin area is 1.8 ha (4.4 ac.) with a 37,005 m3 (30 ac-ft) capacity and receives
water from a 187.4 ha (463 acre) urbanized watershed. The structural retrofits included
installing a weir at the outlet of the detention pond, filling a channel that connected the
inlet and the outlet of the detention pond, and blocking off a drainage pipe that ran
under the channel. The purpose of these structural retrofits was to reduce short-
circuiting of the pond and provide better distribution of the flow into the outlet. The
operational retrofit consisted of modifying the pump schedule to create a two-foot
permanent pool in the pond. These retrofits provided greater retention time in the
detention pond and therefore enhanced gravitation settling of SS. Post-retrofit water
quality surveys showed removals of 29% for chromium, 42% for total copper, 53% for
lead, 51% for total nickel, 44% for total zinc, and 50% for SS. The metals removals
correlated well with the SS removal. The amortized cost over 20 years for retrofitting,
operation, and maintenance was $8,200/year (Woodward-Clyde, 1996).
Dye-tracing experiments performed in a stormwater pond in Kingston Township, Ontario
revealed short retention times, mainly due to the small size of the pond. Mathematical
modeling of this pond indicated irregular circulation patterns, resulting in short-circuiting
and dead zones. A study was conducted to retrofit the pond for the purpose of
improving pollutant-removal characteristics. A series of three baffles were installed
along the length of the pond, roughly perpendicular to the direction of flow, and
extending an average of two-thirds of the width across the pond. The installation of the
baffles increased the length-to-width ratio of the flow path in the pond from 1.5:1 to 4.5:1
and corrected the flow regime problems. In addition, an increase in the hydraulic
efficiency of the pond (defined as the ratio of measured to volumetric retention times)
from 0.65—0.86 was observed. An increase in pollutant removal through sedimentation
processes was inferred from a comparison of retention time measured before and after
baffle installation (Matthews et al., 1997).
Florida's Indian River Lagoon suffers from drainage problems such as increased volume
of freshwater runoff to the estuarine receiving water and deposition of organic
sediments, reduced water clarity because of increased discharge of SS, and
eutrophication caused by nutrient loadings. A project was initiated to create a
watershed control system for the Indian River Lagoon, develop management strategies
to relieve stresses resulting from runoff to the lagoon, and address the feasibility of
watershed retrofitting to reduce pollutant loads. The partially developed, 768.9 ha
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(1,900 acre), industrial area of NASA's Kennedy Space Center was selected as a
representative drainage catchment. After screening the retrofitting alternatives, it was
determined that constructing a weir in the main drainage channel, with discharge of
diverted flow to a wetland area, would provide the best system performance. The cause
and effect relationships between the catchment hydrology, channel hydraulics, and
pollutant loads were documented using a calibrated SWMM model. Model results
concluded that the retrofit could satisfactorily achieve flood control, increase SS
removal, and maximize water depth, but could have difficulty meeting the groundwater
discharge and water level-fluctuation criteria. Without periodic drawdown from the
wetland, water levels in the system would be nearly static due to the nearly flat relief.
Fixed cost of the proposed retrofit would be $28,000. The annual O&M costs were
estimated to be $17,100 (Bennett and Heaney, 1989).
Chemical Enhanced Treatment
As evidenced by the above-referenced literature, many structural retrofits have been
implemented to improve the treatment characteristics of wet-weather detention basins.
In addition to structural retrofits, some municipalities have experimented with chemically
enhanced wet-weather detention basins to improve the treatment characteristics of
these basins during wet-weather events. This low-structural retrofit requires small
amounts of chemicals and polymer to promote the coagulation and flocculation of
smaller, lighter particles into larger, heavier particles.
The Washington State Department of Transportation designed and constructed a scale
model detention basin to investigate contaminant-removal capabilities. Using typical
contaminants and concentrations, a simulated highway stormwater runoff was
formulated and applied to the model detention basin over a range of flow rates. Four
coagulants were evaluated for their ability to enhance removal of sediment and metals.
Coagulant addition resulted in significant increases in metals removal over the range of
stormwater flow rates studied. The greatest improvement was observed at the higher
flow rates. Further improvements in contaminant removal were observed following the
addition of an influent baffle. This baffle increased the hydraulic detention time by
reducing short-circuiting with an associated improvement in contaminant removal (Price
and Yonge, 1995).
A restoration project began in 1987 in an effort to improve water quality in Lake Ella,
Tallahassee, Florida. This effort constituted the first use of alum for treatment of
stormwater inputs into a receiving water body. Lake Ella is a shallow, 5.3 ha (13 acre)
lake that received stormwater runoff through 18 separate storm sewers from 64.8 ha
(160 acre) of mostly impervious urban watershed. With a volume of 113,550 rr? (30
MG), Lake Ella receives 518,550 m3 (137 MG) of runoff each year. The lake was highly
eutrophic and fish kills were common before installation of the alum stormwater
treatment system. Initially, common stormwater treatment technologies such as
retention basins and exfiltration trenches were considered. Alum treatment was
considered when it was determined that there was no available land surrounding the
lake that could be used for retention and the cost of purchasing homes to acquire land
15
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for construction of retention basins was cost prohibitive. The stormwater treatment
system was implemented using flow meters and variable speed injection pumps to
automatically inject liquid alum into the storm sewers. Mixing of the alum and
stormwater occurs as a result of turbulence in the storm sewer line, and the floe
produced settles on the lake bottom providing an added benefit of nutrient inactivation in
the sediments. In addition to the non-structural alum retrofit, bar type trash traps were
installed in underground structures upstream of the lake; the primary outfall lines were
extended into the lake about 30.5 m (100 ft). The total cost of the alum injection
system, excluding the extension of the storm sewer lines into the lake, was about
$200,400. The capital cost of this system was $3,140/ha ($1,270/acre) of watershed
area treated. The capital costs involved in constructing an alum treatment system are
relatively independent of watershed size and dependent primarily upon the number of
outfall locations treated (Harper and Herr, 1992).
Polymer injection has also been used for the control of sewer overflows in the city of
Dallas, Texas. An EPA-sponsored research program studied a non-structural retrofit
system wherein the capacity of a sewer was increased by the injection of certain water-
soluble chemicals to reduce turbulent friction. A polymer injection rate of 0.54 kg/min
(1.2 Ibs/min) into a 38 cm (15 in.), surcharged pipe conveying 18.9 m3/min (5,000
gallons per minute [gpm]) reduced head from 2.5 m (8 ft) above the pipe to 1.8 m (6 ft)
above the pipe. The capacity of a sewage lift station was also increased by injection of
a slurry of friction-reducing polymer into the pump intake. Polymer doses of 120 and
350 ppm were sufficient at increasing flow in a 15.2 cm (6 in.) force main by 35% and
64%, respectively (Chandler and Lewis, 1977).
Real-Time Control
Real-time control systems are gaining a renewed interest for the control of wet-weather
overflows. In 1967, the U.S. federal government funded demonstration projects to
investigate available technologies for control of CSOs. Demonstration grants were
awarded to sewer agencies in Cleveland, Ohio; Seattle, Washington; Minneapolis,
Minnesota; and Detroit, Michigan (Chantrill, 1990). These projects used computers for
the coordination of storage at different sites in a sewer collection system to optimize use
of storage on a system wide basis. All of the projects experienced problems getting the
central computers operational largely due to the limitation of computer technology at the
time. Today, municipalities have many more options for implementing computer-based
real-time control of wet-weather overflows.
Delattre (1990) outlined real-time control of combined sewer systems from the user's
perspective. Delattre's discussion included design objectives and operating functions,
implementation of supervision and control schemes, and evaluation of operation
performance thorough case studies. Chantrill (1990) discussed the history of real-time
control of combined sewers in the United States. Chantrill's discussion included the
control software developed in Seattle, Washington and Lima, Ohio, and the
development efforts by Cello Vitasovic and Wolfgang Schilling.
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Treatment Facility Retrofits
Increasing Capacity/Efficiency of Existing WWTF
Retrofitting dry-weather treatment plants to handle increased flows from wet-weather
events has proven to have merit in some communities. Typically it involves increasing
the capacity of the existing treatment plant by installing newer, more efficient
equipment.
The town of Dexter, New York, solved an infiltration problem by upgrading a treatment
plant with new aeration units rather than rehabilitating the collection system. Peak flows
of more than 2,271 m3/d (0.6 MGD) from wet-weather events and groundwater
infiltration were washing bacteria out of the 379 m3/d (0.1 MGD) activated sludge
WWTF. The town of Dexter decided to treat the excess infiltration, but did not have
enough space at the plant to build a larger mixed-liquor system to handle excess flows.
Installing prefabricated fixed-film treatment units that offered improved treatment
efficiency and quick bacterial growth (Myers and Wickham, 1994) solved this problem.
The city of Clatskanie, Oregon, conducted an infiltration/inflow (I/I) study and found that
the collection system was subjected to severe I/I. Additionally, deficiencies existed in
the trickling filter plant. A cost-effectiveness analysis indicated that the development of
a joint dry/wet-weather facility without major collection system improvements was the
best solution. The principal features of this alternative included a single primary clarifier
followed by an activated sludge secondary treatment process with a DWF capacity of
1,893 m3/d (0.5 MGD). The secondary process can be operated in a contact-
stabilization mode, and the primary clarifier can be operated in a dissolved air flotation
(DAF) mode. With these process modifications, flows up to 4,731 rrfVd (1.25 MGD)
could be effectively treated. SS and BOD removals during DWF were both 94%. WWF
removal efficiencies for flows ranging from 1,893—8,706 rrfVd (0.5—2.3 MGD) were
71% and 73% for SS and BOD respectively. The capital cost of the DAF-contact
stabilization capability was estimated to be 14% more than the cost of a standard dry-
weather plant (Whitney-Jacobsen and Associates, 1981).
Modifications to the clarification system at the main WWTF in New Orleans, Louisiana
have resolved a recurring hydraulic overloading problem caused by inflow and
infiltration. The modifications also have increased average waste activated sludge
concentrations by 50%. Sluice gates were installed to direct flows equally to the
clarifiers during normal flows. These gates are adjusted during high flows to allow for
optimal flows through the clarifiers, thereby eliminating sludge-blanket scouring that
occurred before the gates were installed. A polymer injection system was also added to
the thickener-clarifierto further enhance settling (Berry, 1994).
The Acheres WWTF in Paris, France was designed to treat the carbonaceous portion of
dry-weather sewage. The city is currently undergoing a major rehabilitation and
retrofitting plan to upgrade the plant to include nutrient removal and wet-weather
treatment. By the year 2001, the plant will be retrofitted with tertiary treatment for
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nutrient removal and physical-chemical treatment for wet-weather treatment. The wet-
weather treatment facility was designed to treat 0.76 rrfVmin (200 gpm), almost twice
the 0.42 m3/min (110 gpm) DWF (Gousailles, 1995).
Physical-Chemical Treatment
The physical-chemical treatment system at the Acheres wastewater plant will use the
ACTIFLO process. This process incorporates lamellar settling and weighted
flocculation by microsand. Guibelin, Delsalle and Binot (1994) presented operational
and performance results from the ACTIFLO process. They established that the process
allows an upflow velocity of 131 m/h (0.12 fps) over the lamella. Under this flow regime,
80% SS and 60% BOD removal is achieved using 60g/m3 of ferric chloride and 0.8g/m3
of polyelectrolyte. The process is also noted for its quick start up time, an important
criteria for treating WWF.
Another physical-chemical treatment system being studied in France is the Densadeg
unit (Le Poder and Binot, circa 1995; Rovel and Mitchell circa 1995). This unit
combines three principles: lamellar settling, coagulation-flocculation, and built-in
thickening. The Densadeg pilot unit has been studied at WWTF and at representative
sites along a combined sewer system. The researchers documented SS removals of
80% at a velocity of 50 m/h (0.5 fps) and noted that coagulant dose (FeCb) affected
treatment results more than velocity. Densadeg units are to be installed at the new
Colombes sewage treatment plant (240,000 m3/d [63.4 MGD]) in Paris, France. A key
feature of the Colombes is how it will be operated during wet-weather events. Rather
than sequential operation of the biological stage as during dry-weather, the stages will
switch to operate in parallel during periods of high flow (Hayward, 1996).
Nenov (1995) compared the effectiveness and costs of physical-chemical treatment
versus the activated sludge process. Laboratory experiments confirmed the high
efficiency of physical-chemical treatment for SS and BOD/COD removal. The data
obtained show that chemical treatment over a large range of surface flow rate provides
a reduction of SS and BOD/COD greater than 60% and 50%, respectively. Based on
capital and O&M costs from the Ravad WWTF it was shown that physical-chemical
treatment retrofitting could be done at a much lower cost than biological treatment
expansion.
Chemical Enhanced Treatment
As an alternative to physical-chemical treatment, other municipalities have
experimented with chemically enhanced treatment to increase plant capacity during
wet-weather events.
Sarnia, Ontario, studied the effectiveness of chemically enhanced primary treatment to
increase and sustain removal efficiencies during high plant flows for more than ten
years at an existing primary plant. Very little decrease in treatment efficiency was
observed at overflow rates as high as 114.1 m3/d/m2 (2,800 gpd/ft2). The study
18
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concluded that, with the addition of 14 ppm of ferric chloride and 0.3 ppm polymer, the
SS and BOD removals averaged 84 and 60%, respectively (Heinke, Tay, and Qazi,
1980).
In March 1990, a 22 day test of chemically enhanced treatment was conducted at the
South Essex Sewerage District (SESD) primary treatment plant in Salem,
Massachusetts. The testing conditions were unique in that the flow was split and the
plant was operated as two distinct and parallel treatment lines: half of the plant received
chemical additives while the other half operated as a conventional primary plant. BOD
removals on the conventional primary side averaged 17% but were 51% on the
chemically treated side (a 200% increase). SS, fats, oils and grease removals
increased by 60%. The test also demonstrated chemical addition can enable settling
tanks to handle maximum stormwater conditions without deterioration in removal
efficiency. Ferric chloride and polymer concentrations totaled approximately 30 ppm.
Average overflow rates of 32.6 m3/d/m2 (800 gpd/ft2) reached a peak of 77.4 m3/d/m2
(1,900 gpd/ft2) during wet-weather events. The SESD plant's average DWF of 94,625
m3/d (25 MGD) increased to over 340,650 m3/d (90 MGD) during a storm on the ninth
day of the test. SS and BOD removal rates did not decrease during or after this event
(Harleman, Morrissey, Murcott, 1991).
Converting Dry-Weather WWTF to a Wet-Weather Facility
Conversion of existing or abandoned dry-weather treatment plants into wet-weather
holding or treatment facilities is an option when communities build new dry-weather
treatment plants.
The Municipality of Toronto Metropolitan (Metro) has recently completed the Main
Treatment Plant (MTP) Environmental Assessment (EA). One of the goals of the EA
was to establish a preferred alternative for meeting CSO treatment needs. Metro
placed a high priority on source control as a means of reducing CSO's and associated
pollutant loadings. Sewer separation was not considered as the only solution to the
CSO problem because of concerns relating to increased stormwater discharges, the
legality of separation on private properties, and high costs. The preferred strategy
included, among other solutions, conversion of a WWTF to a CSO treatment plant
(CG&S, 1997).
4.2 UseofSWMM
SWMM is a large, complex model capable of simulating the movement of precipitation
and pollutants from the ground surface through pipe and channel networks,
storage/treatment networks, and finally to receiving waters. The model is capable of
simulating a single event or on a continuous basis for extended periods of time.
SWMM has been released under several different official versions (Metcalf and Eddy,
Inc., et al. 1971; Huber et al. 1975, 1984; Roesner et al. 1984; Huber and Dickinson
19
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1988; Roesner et al. 1988). The official versions were primarily designed for mainframe
computer use; however, later versions have been modified for use on personal
computers. The current "official" version, version 4, is available on disks or through the
Internet from the EPA at the following addressees:
Center for Exposure Assessment Modeling (CEAM)
U.S. Environmental Protection Agency
960 College Station Road
Athens, Georgia 30613
Phone: 706/355-8400
E-mail: ceam@epamail.epa.gov
WWW: ftp://ftp.epa. gov/epa_ceam/wwwhtml/ceam home, htm
The complete SWMM model documentation (Huber and Dickinson 1988; Roesner et al.
1988) can be obtained from the National Technical Information Services (NTIS) or
Oregon State University:
National Technical Information Service
5285 Port Royal Road
Springfield, Virginia 22161
Phone: 703/487-4650
NTIS no. PB88 236 641 - Huber and Dickinson (1988)
NTIS no. PB88 236 658 - Roesner et al. (1988)
Dr. Wayne C. Huber
Department of Civil Engineering
Oregon State University
Corvallis, Oregon 97331
Phone: 503/737-4934
E-mail: huberw@ccmail.orst.edu
The SWMM consists of five basic blocks (or components) that can be used together or
separately. The blocks are RUNOFF, TRANSPORT, EXTENDED TRANSPORT
(EXTRAN), STORAGE TREATMENT (S/T) and RECEIVING WATER (RECEIVE). The
RUNOFF Block simulates rainfall, the resulting surface runoff quantities, and
hydrographs for each drainage basin of the combined sewer collection system. The
EXTRAN Block routes the surface runoff hydrographs developed in the RUNOFF Block
through the collection system to the point of interception and overflow.
SWMM was used to perform simplified simulations using historical rainfall data for a
period of one year or the average of multiple years of hourly rainfall data with hourly
time steps or an appropriate equivalent. This was done using the RUNOFF and
TRANSPORT Blocks of SWMM. The RUNOFF Block generates surface runoff
hydrographs from a drainage area in response to precipitation based on watershed
characteristics including size, shape, imperviousness, and slope. The RUNOFF block
20
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uses the continuity equation that tracks the volume and depth of water on the ground
surface and Manning's equation that governs the rate of surface runoff.
These hydrographs were then used as input into the TRANSPORT Block that routes
flow through a sewer system that is depicted as a series of manholes, conduits, pump
stations, storage tanks, and overflow structures. TRANSPORT uses a simplified
version of the momentum equation where flow is a function of depth to route flow
through the collection system.
Unless otherwise noted, simplified simulations conducted for these analyses were done
using existing calibrated and validated RUNOFF and TRANSPORT models, modified to
estimate the pre- and post-retrofit conditions to assess the treatment/capture
effectiveness of the retrofit.
4.3 Case Studies and Hypothetical Retrofit Examples
The desktop analyses provided in this document are a mixture of case studies of actual
retrofit examples and hypothetical retrofit examples where actual retrofit examples were
not readily available. The case studies include site specific details of each retrofit
including description of facilities, operational parameters, flow rates, and costs. The
hypothetical examples are used to provide a conceptual overview of retrofitting
opportunities and allow comparison to other retrofitting examples. In some instances,
actual case studies were labeled as "hypothetical" to allow the use of information
without revealing the source to protect confidential information. In these instances,
actual information was used to the greatest extent possible to increase the accuracy of
the desktop analysis.
21
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Section 5
Desktop Evaluation
5.1 Conversion of Primary Treatment Tanks
Conversion or retrofitting primary settling tanks at POTWs to provide equivalent or higher
levels of treatment may be a technically feasible and economically attractive alternative to
the construction of new conventional primary settling tanks. The principal downside of
conventional primary treatment facilities is the large footprint required due to the low
surface overflow rate (SOR). Alternative sedimentation methods including micro carrier
enhancement, chemical precipitation, dissolved air floatation, and plate/tube settling may
provide an alternative to conventional primary treatment.
Two desktop analyses are presented to demonstrate the retrofit of primary treatment tanks
to provide equivalent or higher levels of treatment:
1. Retrofit of the Augusta Wastewater Treatment Facility, Augusta, ME
2. Hypothetical retrofit of the POTW using the ACTIFLO process
5.1.a. Augusta Wastewater Treatment Facility, Augusta, Maine
The Augusta, Maine Sanitary District (ASD) owns and operates a secondary wastewater
treatment facility (WWTF) that discharges to the Kennebec River, which is a Class C
waterbody for the state of Maine. The WWTF was originally constructed in 1962 with
upgrades in 1966, 1982 and 1997. The ASD WWTF provides secondary treatment using
pure-oxygen activated sludge. The WWTF was designed to handle a monthly-average flow
of 30,280 m3/d (8 MGD) and an instantaneous-peak flow of 60,560 m3/d (16 MGD). The
WWTF serves a partially combined collection system (CCS). ASD's existing collection
system services approximately 90% of the residential population, nearly all of the
commercial properties, and a portion of the industrial properties within the limits of the city
of Augusta. The system also serves four neighboring communities, the city of Hallowell and
the towns of Manchester, Wnthrop and Monmouth. The system contains three types of
sewers:
• combined sanitary and storms
• separate sanitary
• storm
The majority of the collection system within the limits of Augusta flows by gravity to the
WWTF. Ten pumping stations exist serving the lowlands and outlying areas. Currently,
there are over 167 km (104 miles) of gravity sewer ranging from 15—107 cm (6—42 in.) in
diameter. Pipe materials include stone and brick (dating back to the Civil War era) as well
as more recent installations of vitrified clay, asbestos-cement, concrete, cast iron, and
PVC pipe. According to the 1993 CSO Facilities Plan there are presently 38 diversion
22
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manholes and 29 permitted CSO located within the system. In an average year 219,530
m3 (58 MG) are discharged.
A majority of the wastewater currently entering the ASD sewer system is domestic sewage
from residential and commercial sources. The original WWTF design included an
allowance for a significant organic loading from local industry; however, many of the local
industries closed or relocated soon after Secondary Treatment System upgrade became
operational in the early 1980's. Consequently, the WWTF is operating with less than one-
half of its original design flow and organic loading. Currently, annual average wastewater
flows are 15,140 m3/d (4 MGD).
Prior to implementation of the CSO Abatement Program, the WWTF used the following unit
processes to provide preliminary, primary and secondary treatment:
• One mechanically-cleaned bar screen with 3.8 cm (11/4n.) clear spacing located in a
1.5 m (57 in.) wide channel
• One detritor-type grit chamber 4.3 m (14 ft) in diameter
• Two 91.5 cm (36 in.) comminutors with a manual bypass screen
• Three primary clarifiers, of which two are 16.8 m (55 ft) in diameter and one is 24.4
m (80 ft) in diameter, for a combined surface area of 910 m2 (9,800 ft2)
• Four main primary effluent lift pumps
• Two pure-oxygen activated sludge aeration tanks with a total volume of 2,725 m3
(0.72 MG)
• Three secondary clarifiers, each at 24.4 m (80 ft) diameter with a total surface area
of approximately 1400 m2 (15,100ft2.)
• One chlorine contact tank with a volume of 613 m3 (162,000 gal)
Figure 5.1.a-1 shows the existing site layout prior to implementing the CSO Abatement
Program. Figure 5.1.a-2 shows a schematic of the existing WWTF prior to retrofitting.
When influent peak flow approaches 34,065 m3/d (9 MGD), excess flow is bypassed to the
chlorine contact tank following degritting in the detritor. The bypass was automatically
activated by a side overflow weir located along the grit chamber outlet channel. The water
level was regulated by a 30.5 cm (12 in.) Parshall Flume located downstream of the weir.
The CSO Abatement Program approved by the Maine Department of Environmental
Protection recommended a four-phase plan to address CSO discharges. Phase I of the
CSO Abatement Program included improvements to provide treatment for WWTF bypass
that is the third largest CSO in the CCS based on projected peak flows. The following is a
summary of Phase I Improvements, which is a retrofit to the existing facilities:
23
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Figure 5.1 .a-1. Prior to
•*f
.' /'-t| \ /CT\ .--"
11; ___ xA-^./?"i|V
24
-------
Figure 5.1 .a-2. Flow Prior to
111 5; M
'-*i '•* •• '•*•• I" -• ""*:• 1
fc '•!.' -1 '-V ;l.:;s
II. i:i '' !
•-, (4 •- «r / i,
*,
.? ? >ii IS
-- - • -j j>|
4 * A
* i > * _"~" ~" r~—- ^* * *
25
-------
• A new headworks containing two mechanical screens with 2.5 cm (1 in.) clear
spacing located in a 1.5 m (60 in.) wide channels, two 167 m3 (44,000 gal) aerated
grit chambers, and one 0.9 m (36 in.) parshall flume. The mechanically cleaned bar
screens have screenings compactors to remove excess water. The grit is
removed from the aerated grit chambers using recessed impeller type pumps
which discharge to cyclone classifiers for washing and dewatering.
• A new flow distribution structure (FDS No. 1) for the primary clarifier influent. The
distribution structure utilizes motorized weir gates that will allow off-line tanks to be
activated during a wet weather event.
• A new primary effluent distribution structure (FDS No. 2) with motorized weir gate
to regulate flow receiving secondary treatment and flow to the CSO bypass.
• A new 250 m3 (66,000 gal) chlorine contact tank with mechanical mixers using
sodium hypochlorite for high-rate disinfection. A 24 m3 (6,400 gal) bulk storage
tank is provided for sodium hypochlorite storage. An 11 m3 (3,000 gal)
dechlorination contact chamber with mechanical flash mixer is provided. A 16 m3
(4,150 gal) bulk storage tank is provided for sodium bisulfite. Chemical metering
pumps are provided with flow pacing from an ultrasonic flow meter.
These improvements will allow the full 109,800 nf/d (29 MGD) of connected interceptor
capacity to receive preliminary and primary treatment. Daily-maximum flows of 45,400
m3/d (12 MGD) would receive secondary treatment while 64,300 m3/d (17 MGD) would be
diverted to the high-rate disinfection system after receiving primary treatment.
Later phases of the CSO abatement program (Phases II, III and IV) will allow additional
WWF from the Augusta CCS to be treated at the WWTF. This will be accomplished
through the construction of:
• Consolidation conduits to intercept CSO discharges throughout the system and
convey the flow to the WWTF;
• Three new vortex separators, each 10.7 m (35 ft) diameter, and 2.4 m (8 ft) side
water depth to provide the equivalent of primary treatment;
Additional high-rate disinfection and dechlorination tank capacity to increase the
tank volume to
dechlorination;
tank volume to 1,250 m3 (330,000 gal) for disinfection, and 62.5 m3 (16,500 gal) for
• A new wet-weather outfall from the WWTF to the Kennebec River.
After all phases are completed, the total influent peak flow capacity to the WWTF would be
increased from 109,800 m3/d (29 MGD) to 405,000 m3/d (107 MGD). Up to 295,200 m3/d
(78 MGD) will be treated in the vortex separators. The remaining 109,800 m3/d (29 MGD)
would be conveyed to the upgraded headworks. Approximately 268,700 m3/d (71 MGD) of
vortex separator effluent will be disinfected and dechlorinated in the expanded high-rate
26
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disinfection tank. Approximately 26,495 m3/d (7 MGD) of vortex separator underflow would
3/
be conveyed to the upgraded headworks. A total flow of 136,260 m /d (36 MGD) would be
3/
treated within the headworks and primary clarifiers (109,765 m /d or 29 MGD influent plus
26,495 m3/d or 7 MGD vortex underflow). Primary clarifier effluent flows of approximately
45,420 m3/d (12 MGD) would receive secondary treatment. The remaining 90,840 nf/d
(24 MGD) of primary clarifier effluent will receive high-rate disinfection and dechlorination.
The basis of design of these facilities is based upon SWMM results using a one-year, two-
hour storm event.
Figure 5.1.a-3 shows a site layout following completion of the CSO Abatement Program.
Figure 5.1.a-4 shows a schematic of the ASD WWTF following completion of the CSO
Abatement Program.
It should be noted that the 1993 CSO Facilities Plan is currently being updated. The
updated plan will re-assess the earlier recommendations for the later phases of the
program.
Stormwater Management Model Results
The EPA SWMM Version 4 was used to assess the net benefit of the retrofit by comparing
annual WWF volume treated at the WWTF under pre- and post-retrofit conditions. A
simplified model was developed to simulate hourly rainfall, based upon the previously
calibrated model developed for individual rainfall events.
The data used for these projections consists of 21 years of hourly rainfall data from the
National Weather Service (NWS) station in Portland, Maine for the years from 1971—
1991. These data were used as input to the simplified RUNOFF model that projected
flows that were used to simulate long-term overflow using the TRANSPORT block of
SWMM. The TRANSPORT model was constructed as a simplified network consisting of
overflows and regulator pipes. The simplified model was used to project the annual volume
of treated WWF at the WWTF.
Based on this analysis, the average annual WWF conveyed to the treatment plant was
found to be 582,890 rrf (154 MG). The layout of the pre-retrofit headworks allowed
484,480 m3 (128 MG) to receive treatment consisting of primary and secondary treatment
followed by disinfection while 98,410 m3 (26 MG) of WWF annually bypassed the treatment
plant and only received disinfection prior to discharge. The retrofitted headworks allowed
the entire 582,890 m3 (154 MG) to receive primary treatment and 484,480 m3 (128 MG)
received secondary treatment and disinfection. The remaining 98,410 m3 (26 MG) that did
not receive secondary treatment was bypassed to the high-rate disinfection system prior to
discharge to the Kennebec River.
27
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Figure 5.1 .a-3.
pv-.jt
a1 - ^
n
i '. IS1* ^ ^=* ***'' . "'\
: ^, f a f;:':l |l% r.';0 ** •
28
-------
Figure 5.1 .a-4.
J| Ji
£i*.» g : 4
A .»
|* li. 't1 ''i-' -i'*"
-------
Design Considerations
The retrofitting of the ASD WWTF to handle increased WWF required consideration of
several factors:
• Pollutant and Hydraulic Loadings
• Treatment Objectives
• Operational Requirements
• Chemical Storage and Feed Requirements
• Sludge Handling Requirements
• Operations and Maintenance Requirements
• Construction Sequencing and Site Constraints
Each of these considerations is described in this section.
Pollutant And Hydraulic Loadings - Pollutant and hydraulic loadings were developed for
the wet-weather treatment units at the ASD WWTF during the Facilities Planning stage.
These values were refined and updated as the project progressed through the preliminary
engineering and detailed design stages. The primary objective of the wet-weather
treatment units is to remove floatable and settleable solids to promote effective
disinfection.
Table 5.1.a-1 shows the design criteria for the wet-weather treatment processes at the
ASD WWTF. The primary focus of the Phase I improvements are to remove hydraulic
restrictions at the WWTF preventing flows of 109,800 nf/d (29 MGD) from reaching the
existing primary clarifiers. As noted previously, the completion of Phases II, III and IV will
provide equivalent primary treatment and disinfection for additional peak WWF of
approximately 295,200 m3/d (78 MGD).
Table 5.1 .a-1. Phase 1 CSO Abatement Program Wet-Weather Treatment Design
Criteria
Description and Criteria
Vortex Treatment
Number of Units (each)
Design Flow
Diameter Each
Loading Rate
Overflow to
High-Rate Disinfection
Underflow to Headworks
Phase I Phases II, I
0 3
0.30 m3/d
10.67m
1098m3/d/m2
268,735m3/d
26,495 m3/d
III & IV
78.0 MGD
35.0ft
27,000 gpd/ft2
71.0 MGD
7.0 MGD
Mechanical Screening
No. of Bar Screens (ea.) 2 2
Type Reciprocating Rake Reciprocating Rake
BarSize 0.95x5.1cm 3/8x2 in 0.95x5.1 cm 3/8x2 in
30
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Bar Spacing
2.54 cm
1 in
2.54cm
1 in
Table 5.1.a-1. Continued
Description and Criteria
Peak Influent Flow
Vortex Overflow
Total Peak Flow
Approach Velocity
Half Clogged
Screen Velocity
Half Clogged
Differential Head
Estimated Wet Screening
Quantity
Average De-watered
Screenings Volume
Screenings Container Size
Screening Storage
Time (days)
Phase I
109,765m3/d
0.0
109,765m3/d
0.76 m/s
2.07 m/s
27.43cm
3.7x1 0'5m3/m3
0.28 m3/d
.57 m3/d
29.0 MGD
29.0 MGD
2.5 fps
6.8fps
0.9ft
(5ft3/MG)
0.37cyd/d
0.75 cyd
Phases II, II
109,765m3/d
26,495 m3/d
1 36,260 m3/d
0.84 m/s
2.29 m/s
33.22 cm
3.7x1 0'5m3/m3
0.57 m3/d
0.57 m3/d
II&IV
29.0 MGD
7.0 MGD
36.0 MGD
2.75 fps
7.5 fps
1.09ft
(5ft3/MG)
0.74cyd/d
0.75 cyd
Grit Removal
No. of Tanks
Type
Average Flow
Peak Flow
Volume Each Tank
Detention Time -
Avg. Flow (min)
Detention Time -
Peak Flow (min)
Air Supply
No. of Air Blowers
Air Flow Each
Pressure
Blower Motor Hp
Grit De-watering & Storage
Type
No. of Classifiers
Flow
No. of Grit Pumps
Pump Head
Pump Speed (rpm)
Pump power
Grit Washer Diameter
Grit Washer Speed (rpm)
Estimated Grit Quantity
Grit Container Size
Grit Storage Time (days)
Description and Criteria
2
Aerated
I 4.0 MGD
d 29.0 MGD
44,000 gal
Aerated
30,280 m3/d
1 36,260 m3/d
167m3
2
8.0 MGD
36.0 MGD
44,000 gal
15,140m7d
109,765m3/d
167m3
31.7
4.4
0.27 to 3 to 8 cfm/ft
0.72 m3/min/m
2+1 standby
2.4 to 84 to 224 cfm
6.3 m3/min
3867 kg/m2 5.5 psig
5/10, two speed
Cyclone
2
1 m3/min 260 gpm
2+1 standby
8.5m 28ft
900
7.5kW(10HP)
30.5cm 12 in
18
45 cm3/m3 6.0 ft3/MG
4.6 m3 6 cyd
7
Phase I
16.8
3.5
0.27 to
0.72 m3/min/m
3 to 8 cfm/ft
2+1 standby
2.4 to 84 to 224 cfm
6.3 m /min
3867 kg/ m2
5.5 psig
5/10, two speed
Cyclone
2
1 m3/min 260 gpm
2+1 standby
8.5m 28ft
900
7.5 kW (10 HP)
30.5cm 12 in
18
45 cm3/m3 6.0 ft3/MG
4.6 rrr
6 cyd
Phases II, III & IV
Primary Clarifiers
Average Flow
15,140m3/d
4.0 MGD
30,280 m3/d
8.0 MGD
31
-------
Peak Flow
BOD Loading 1
SS Loading 1
No. Of Tanks (ea.)
109,765m3/d
3220 kg/d
5443 kg/d
29.0 MGD
7,100lbs/d
12,000 Ibs/d
136,260 m3/d
7258 kg/d
7348 kg/d
36.0 MGD
16,000 Ibs/d
16,200 Ibs/d
Table 5.1 .a-1 Continued
Description and Criteria
Diameter Each (ft)
Sidewater Depth
Total Volume (MG)
Average Hydraulic Loading
Peak Hydraulic Loading
Average Weir Loading
Peak Weir Loading
Detention Time-
Average Flow(h)
Detention Time-
PeakFlow(h)
Estimated BOD5 Removal1
Estimated SS Removal1
Estimated Sludge
Concentration (mg/L)
Estimated Sludge
Quantity @4.0%1
Primary Sludge/Scum
Pumps
Pump Type
Pump Flow Each
Total Head
Pump Speed (RPM)
Pump power
No. of Grinders
Phase I
Phases II, III & IV
2 @ 16.7m
1 @24.4 m
2.1 m
1938m3
16.7m3/d/m2
121 m3/d/m2
83.2 m3/d/m
603.4 m3/d/m
2 @ 55 ft
1 @ 80 ft
7.0ft
0.512
409 gpd/ft2
2,966 gpd/ft2
6,701 gpd/ft
48,584 gpd/ft
3.1
0.4
1352 kg/d 2,980 Ibs/d
2994 kg/d 6,600 Ibs/d
40,000
75m3/d 0.0198 MGD
5
Disc
0.3 m /min
85 gpm
4.6m 15ft
300
3.7 kW (5 HP)
5
2@16.7m
1 @24.4 m
2.1 m
1938m3
33.3 m3/d/ m2
150m3/d/m2
166.5m3/d/m
749.0m3/d/m
2 @ 55 ft
1 @ 80 ft
7.0ft
0.512
818 gpd/ft2
3,682 gpd/ft2
13,403 gpd/ft
60,311 gpd/ft
1.5
0.3
2205 kg/d 4,860 Ibs/d
3674 kg/d 8,100 Ibs/d
40,000
92 m3/d 0.0243 MGD
5
Disc
0.3 m3/min 85 gpm
4.6m 15ft
300
3.7 kW (5 HP)
5
CSO Disinfection
Chemical Type
Peak Flow
Dose Rate at Peak Flow
(mg/L)
Chemical Concentration
Sodium Hypochlorite
64,345m3/d 17.0 MGD
15.0
15.0
Sodium Hypochlorite
359,575m3/d 95.0 MGD
15.0
15.0
CI2 Feed Rate
No. of Pumps
Pump Capacity each
Contact Time (min)
Contact Chamber Volume
Length to Width Ratio
Description and Criteria
149.8 kg/m3
1.25lbs/gal
149.8 kg/m -
1.25lbs/gal
1 + 1 standby
0.27 m3/h 70.0 gph
5.5
250m3 66,000 gal
7:1
Phase I
2 + 1 standby
0.76 m3/h 200.0 gph
5.0
1250m3 330,000 gal
40:1
Phases II, III & IV
CSO Dechlorination
Chemical Type
Peak Flow
Chlorine Residual (mg/L)
NaHSO3Dose (mg/L)
Sodium Bisulfite
64,345m3/d 17.0 MGD
5.0
8.06
Sodium Bisulfite
359,575m3/d 95.0 MGD
5.0
8.0
32
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NaHSO3, Concentration (%)
38.0 38.0
Feed Rate NaHSO3 504.4 kg/m3 4.21 Ibs/gal 504.4 kg/m3 4.21 Ibs/gal
No. of Pumps 1 + 1 standby 2+1 standby
Pump Capacity each 0.05 m3/h 12.0gph 0.12m3/h 32.0 gph
Contact Time (s.) 15.0 15.0
Contact Chamber Volume 11.4m3 3,000 gal 62.5m3 16,500gal
Treatment Objectives. The state of Maine requires an Escherichia coliform (E-coli)
concentration of 949 colonies per 100 ml to comply with the maximum daily E-coli limit in a
Class C river. Sampling and monitoring performed during the development of the CSO
Abatement Program indicated that the Kennebec River does not currently meet water
quality criteria for bacteria during wet weather. The CSO Abatement Program
recommended a plan to treat CSO discharges to meet the current maximum-daily-water-
quality limit. Primary treatment and high-rate disinfection were selected as the most
economical methods to meet the treatment objectives for this project.
Operational Requirements. The Phase I Improvements to the Augusta WWTF designed
to operate on a continuous basis include:
• Mechanical Screening
• Grit Removal
• P ri m ary Treatm ent
The high-rate disinfection and dechlorination systems are intended to operate in an event-
based mode during periods of excessive WWF. Under current DWF conditions, the
WWTF operates with only one primary clarifier in service. The other two primary clarifiers
can be kept out of service and used as off-line storage during wet-weather events. If one
24.4 m diameter (80 ft) primary clarifier is in service, the remaining two 16.8 m (55 ft)
diameter primary clarifiers provide approximately 943 m3 (249,000 gal) of off-line storage.
The WWTF staff can activate the off-line tanks using the new Supervisory Control and Data
Acquisition (SCADA) System Controls. Motor-operated weir gates regulate the flow to the
primary clarifiers.
After the WWF subsides the primary clarifiers can be drained using a new pump with
variable speed controls. The primary clarifier tank drain pump discharge is returned to the
new Primary Clarifier Flow Distribution Structure (FDS No. 1).
The future improvements of Phases II, III and IV will use an event-based mode of operation
during wet weather periods. The existing overflow pipes located within the CSS will be
intercepted by new consolidation conduits that will convey the combined sewerage to the
WWTF. A series of vortex separators will be constructed to provide equivalent primary
treatment (removing floatable and settleable solids) to facilitate effective disinfection. The
high-rate disinfection and dechlorination tank will be expanded with a new parallel outfall
pipe provided to the Kennebec River from the WWTF. At the end of the wet-weather event
the vortex separators and high-rate disinfection tank will be drained using a pump with
variable speed control to the WWTF headworks.
33
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Chemical Storage and Feed Requirements. The CSO Abatement facilities will utilize two
chemicals:
• Liquid Sodium Hypochlorite for disinfection
• Liquid Sodium Bisulfite for dechlorination
The liquid sodium hypochlorite system design will use on commercial-grade liquid sodium
hypochlorite (15% solution) which contains the equivalent of 150 kg of chlorine/m3 (1.25 Ibs
of chlorine/gal of solution). The metering pumps are designed to deliver a dosage of 15
milligrams per liter (mg/L) at the peak design flow with one unit out of service. A bulk
storage tank with an effective working volume of 240.2 nf (6,400 gal) is provided. The
tank is used for both dry-weather, secondary effluent disinfection, and high-rate wet-
weather disinfection to minimize problems associated with product shelf life. The available
chlorine in sodium hypochlorite solution declines with age. The anticipated sodium
hypochlorite storage time will be 51 days initially, decreasing to 25 days at the future
design flow.
The storage system design is based on commercial grade liquid sodium bisulfite (38%
solution) which contains approximately 500 kg of sodium bisulfite per cubic meter of
solution (4.2 Ibs of sodium bisulfite/gal of solution). The metering pumps are designed to
deliver sodium bisulfite dosages necessary to dechlorinate effluent with a chlorine residual
of 5.0 mg/L. Approximately 1.6 kg of sodium bisulfite will be used per kg of chlorine
residual. A bulk storage tank with an effective working volume of 15.7 m3 (4,150 gal) is
provided.
The sodium hypochlorite and sodium bisulfite metering pumps are flow paced. An
ultrasonic level sensor is used for effluent flow measurement. A broad-crested rectangular
weir 3.4 m (11 ft) in length is the primary measuring device.
Sludge Handling Requirements. Sludge handling requirements will increase during
periods of WWF. The most pronounced increase is expected to occur during the initial
period of WWF (first flush). The configuration of FDS No. 1 will capture a significant
portion of the first flush solids in the off-line primary clarifiers.
A solids balance was performed to estimate the quantity of sludge produced during a
storm with a one-year recurrence interval. The daily sludge quantities increase as follows:
• Current Estimated Sludge Quantity (kg/d) 3,290 (7,250 Ibs/d)
• One-Year Storm Estimated Sludge Quantity (kg/d) 7,550 (16,650 Ibs/d)
The sludge quantity is estimated to increase 230% during a one-year storm event.
However, over the course of a week, the overall increase can be assimilated if sludge
pumping and processing schedules are adjusted. The following is a summary of weekly
sludge quantities during a period with a one-year storm:
• Initial Estimated Sludge Quantity (kg/wk) 23,010 (50,730 Ibs/wk)
34
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• One-Year Storm Projected Sludge Quantity (kg/wk) 27,270 (60,140 Ibs/wk)
The estimated sludge quantity would only increase by 19% during a week with a one-year
storm event.
Operation and Maintenance Requirements. The implementation of the CSO Abatement
Program will result in increased O&M costs. The following list summarizes the areas of
expected O&M cost increases at the WWTF:
• Labor to perform sampling, laboratory analyses, reporting, wet-weather process
adjustments, and clean-up following an event.
• Power for in-plant pumping, sludge processing, and chemical feed systems.
• Chemicals for disinfection, dechlorination, and polymer for additional sludge
quantities.
• Sludge disposal tipping and transport fees.
Table 5.1.a-2 summarizes the expected increase in annual O&M costs at the WWTF
resulting from the initial Phase I and future Phases II, III and IV of the CSO Abatement
Program.
Table 5.1 .a-2. Estimated O&M Costs, CSO Abatement Program - All Phases
Description Estimated Annual Cost
Sampling $800
Laboratory Analyses $3,200
Report Preparation $1,500
Process Control Adjustments $1,200
Clean-Up of Wet-Weather Treatment Structures $6,000
Power $1,000
Sodium Hypochlorite $4,100
Sodium Bisulfite $2,000
Polymer for Sludge Dewatering $1,000
Sludge Disposal Transport and Tipping Fees $11,500
Total Estimated Cost $32,300
Construction Sequencing and Site Constraints. The existing ASD WWTF site is
crowded, with little room for expansion. Therefore, it was necessary to construct some of
the CSO Abatement Facilities in locations previously used for other purposes. The
following is a listing of special sequencing and site considerations related to the CSO
Abatement Program at the WWTF:
• Portions of the existing screen structure and detritor grit chamber were demolished
to facilitate the construction of the new headworks.
• A temporary junction structure and CSO bypass was needed.
35
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• The existing primary clarifiers were taken out of service in sequence, to facilitate
new interconnecting piping and mechanical refurbishment.
• The existing comminutor structure required temporary piping connections to keep it
in operation during the new headworks construction.
• After the new headworks are built and placed into service, the existing comminutor
structure will require demolition and removal to facilitate installation of new
underground piping.
A completion time of 540 calendar days was specified for the Phase I CSO Abatement
Program.
Project Costs
The bids for the Phase I CSO Abatement Program were opened on January 23, 1997.
Costs for the CSO Abatement components are listed in Table 5.1.a-3. Costs included
construction requirements and project services. Project services included design
engineering, construction engineering, legal, and fiscal expenses but did not include
contingencies, right-of-way expenses, or other miscellaneous items.
Table 5.1 .a-3. Phase 1 CSO Abatement Program Project Costs
Description Cost1
General Conditions $200,000
Sitework and Yard Piping $344,000
Headworks Building and Chemical Feed Systems $1,688,000
CSO Disinfection and Dechlorination Structure $536,000
Primary Pump Room Renovations $80,000
Primary Clarifier Renovations $40,000
Flow Distribution Structure No. 1 $117,000
Flow Distribution Structure No. 2 $122,000
Operations Building Pump Room Renovations $60,000
Yard Electrical Ductbanks and Lighting $88,000
Instrumentation and SCADA System $167,000
Total Construction Cost $3,422,000
Project Services2 $761,000
Total Project Cost $4,183,000
1 Based on January 23, 1997 Bid Prices
2Includes Design Engineering, Construction Engineering, Legal, and Fiscal
Expenses; does not include contingencies and right-of-way costs.
Conclusions
This retrofit will utilize the entire capacity of the existing primary clarifiers to provide primary
treatment for the maximum rate of influent flow, thereby increasing the portion of the WWF
that will receive primary treatment from 60,560 m3/d (16 MGD) to 109,765 m3/d (29 MGD).
This will result in net 20% increase: 484,480—582,890 m3 (128—154 MG) in the annual
36
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WWF that receives primary treatment. Only a few examples of retrofitting WWTF have
been published. Often these examples are strongly influenced by site-specific constraints
and therefore do not lend themselves to comparison with other retrofit situations. The
retrofit solution described here has been selected based on the cost effectiveness. For
comparison purposes, the alternative treatment scheme of off-line storage at the WWTF for
the 13 MGD of WWF was estimated to be $8,336,000 versus the $4,183,000 for the
selected retrofit.
5.1.b. Hypothetical Retrofit of Primary Treatment Facilities Using the ACTIFLO
System
ACTIFLO System
This desktop analysis involves the retrofit of hypothetical primary treatment tanks for
enhanced treatment of WWF in excess of the 908,400 m3/d (240 MGD) peak wet-weather
capacity of the existing WWTF. The treatment method selected for this retrofit scenario is
the ACTIFLO process that combines microsand-enhanced flocculation with lamellar plate
settling. The process has been shown to be effective in treating CSO/SSO and primary
wastewater flows in installations in Paris, France (Le Poder and Binot 1994). Typical
performance results are 85-95% reduction in SS, 50—80% reduction in Total BOD, 85—
95% reduction of total phosphorus and 10—20% reduction in total kjeldahl nitrogen (TKN).
The hypothetical WWTF has eight primary clarifiers that are 41.2 m (135 ft) in diameter
with a 3 m (10 ft) side water depth. The surface-settling rate of the existing units at the
design peak flow of 908,400 rrf/d (240 MGD) is 85.6 rrf/d/m2 (2,100 gpd/ft2) with a
hydraulic detention time of 48 min. Figure 5.1.b-1 shows the existing layout of the
hypothetical WWTF prior to construction of the Phase I.
Due to the limited side water depth, retrofit of the existing primary clarifiers is not practical.
A more economical approach is the demolition of one of the existing clarifiers and
replacing it with a 908,400 m3/d (240 MGD) ACTIFLO system which has a depth of 6.7 m
(22 ft). The seven remaining clarifiers would be retained for storage of WWF during storm
events or for future plant modifications.
The proposed 908,400 m3/d (240 MGD) ACTIFLO system design consists of six 151,400
m3/d (40 MGD) process trains. Each of the process trains is designed to operate at a
nominal overflow rate of 1.2 m3/min/m2 (30 gpm/ft2) with a total hydraulic retention time of
5.6 min.
Table5.1.b-1. ACTIFLO Design Summary
Design Parameter Quantity
Number of Process Trains 6
Nominal Train Capacity 151,400 m3/d 40 MGD
Total System Capacity 908,400 m3/d 240 MGD
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Hydraulic Retention Time
5.6 min
I 35'tT(°E.!*
' ', • css^yv^ it r "•
..'f"8*:.. ,.i ; • typi ^ .. j *.,—j ; .
»•*-!,.!»• «"- J "\ .•'{'•_.' 5
»TKi55ifn:.
[ *^tf*?r ; \ } : "w^ . •*«
\ :-*- M'T^S .' \ "^=Jei^4''1!!5 .' 3 I **
; ••"" - f -•' ', , .-\ i '•
5.1 .b-1. of WWTF to of ACTIFLO
System
38
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The high overflow rate and short retention time offered by the ACTIFLO process result in an
extremely compact system. Overall, the proposed ACTILFO design will meet the treatment
objectives of the project in approximately 5% of the space currently occupied by the
existing primary clarifiers. A brief comparison of the existing facilities and the ACTIFLO
design are shown in Table 5.1.b-2.
Table 5.1 .b-2. Comparison of ACTIFLO and Existing Primary Treatment Systems
Net Savings with
Parameter Existing System ACTIFLO ACTIFLO
Total Capacity 908,400 nf/d 908,400 nf/d
Number of Units 8 6
Unit Capacity 113,550 nf/d 151,400 nf/d
Area Required 1,329.1 m2 (per unit) 87.8 m2 (per train) 526.7 94% per unit
10,632.7m2 (total) m2 (total) 95% total
Process Volume 3,987 m3 (per unit) 3,533 m3 (per unit) 85% per unit
31,898 m3 (total) 3,588.3 m3 (total) 89% total
Hydraulic Retention 51 min 5.6 min 89%
Time
A layout drawing of the hypothetical WWTF showing the location of the proposed ACTIFLO
system is provided in Figure 5.1 .b-2.
ACTIFLO Process Theory
Fundamentally, the ACTIFLO process for wastewater treatment is very similar to
conventional water treatment technology of coagulation, flocculation and sedimentation.
Although influent characteristics vary for different wastewater applications and influent
streams, the removal of suspended materials is accomplished by the same mechanisms in
each application. Therefore, for the purposes of this discussion, the term wastewater
(WW) collectively refers to any domestic wastewater stream including primary sewage,
CSO, SSO and/or inflow and infiltration (I/I).
As in conventional water treatment technology, a coagulant is used for the destabilization of
suspended materials entering the process and a flocculent aid polymer is added to
aggregate the destabilized solids into larger masses. The resulting floes are then
subsequently removed by settling for disposal. The primary advance made in the
ACTIFLO process is the addition of very fine sand as a "seed" for the development of high-
density floe. The resulting floes, ballasted by the microsand, are more easily removed by
settling. It is in this step that ACTIFLO differs significantly from conventional treatment
processes. A brief overview of the physiochemical principals involved in conventional
coagulation and flocculation is presented in "Section 5.2.b. Chemical Addition to AWPCP"
to help understand the advantages of ACTIFLO over chemical coagulation alone.
39
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'Cn,.
-^ .. ; r
%- ',' v *5 : / -•' 'M v.vrr^ 3 pi ,
Cfl3' •.• Xw^.,.y _„ c™ r *: \ •*? < : ™ !
-x^ st-vawt
H ; S ?*
•_' \
5.1 .b-2. of ACTIFLO in
40
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The primary advance made in the ACTIFLO process is the addition of a very fine (60—140
jam mean diameter) clean silica sand or microsand in the flocculation process step. With
its addition, the microsand serves several key roles in the ACTIFLO process:
• The high specific surface area to volume ratio of the microsand particles serves as a
seed for the development of floe.
• Together, the microsand and polymer seed promote inter-particle contact and
enmeshment of suspended materials resulting in development of large stable floe.
• The high microsand concentration within the ACTIFLO process effectively dampens
the effects of fluctuating influent quality and provides for stable operation and
treatment throughout changes in the raw water quality.
• The relatively high specific gravity of the microsand (~2.65) serves as a ballasting
agent creating heavier floe that settle very quickly.
• The chemically inert microsand does not react with the process chemistry and allows
the microsand to be efficiently separated from the sludge via a hydrocyclone and
reused in the process.
The advantages of microsand ballasted flocculation allow ACTIFLO to offer surface loading
or overflow rates many times higher than competing processes. Overflow rates as high as
3.3 m3/min/m2 (80 gpm/ft2) are claimed in typical wastewater applications of ACTIFLO.
The high loading rates result in a process that is extremely compact compared to
conventional or competing processes of similar capacity. The high overflow rates and size
savings provided by ACTIFLO translate directly into significant savings in project cost.
The use of microsand for ballasted flocculation provides ACTIFLO with several operational
advantages in wastewater applications. The high solids content within the ACTIFLO
process provides for effective treatment of influent with varying SS concentrations. In such
applications, the high microsand concentration within the process allows for SS removal by
particle enmeshment in the microsand-polymer complex. This performance is also
extremely beneficial in chemical precipitation reactions such as tertiary phosphorus
removal/effluent polishing or the treatment of dilute waste streams.
Microsand ballasted flocculation is also effective in the treatment of extremely dirty
wastewater (high turbidity and SS). Here, the high microsand concentration within the
process allows it to provide efficient removal of high-suspended solids concentrations
without difficulty. For similar reasons, the process is virtually unaffected by sudden
fluctuations in influent quality or volume. Minor adjustments to chemical dosage are all that
are needed to provide effective treatment under such conditions.
Overall, the use of microsand results in the development of chemical floe that is significantly
denser and more durable than floe from conventional clarification processes. The
41
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ACTIFLO floes have considerably higher settling velocities than conventional floe allowing
much higher clarifier loading rates. Ballasted flocculation also provides for a process that
is rapidly started-up and optimized, as well as stable in treating with variations in raw water
quality. These characteristics make the ACITLFO process ideal for treating the difficult
conditions posed by wastewater/CSO treatment.
ACTIFLO Process Description
The ACTIFLO® process is a compact high performance water clarification system that
combines the advantages of microsand enhanced flocculation with lamellar settling.
Wastewater treatment is accomplished through a series of consecutive process steps that
consist of influent screening, coagulant addition, microsand and polymer injection
(injection), floe maturation (maturation), settling and sand recirculation. Each step in the
process is discussed in greater detail in the paragraphs that follow. A flow
diagram/process schematic of the ACTIFLO system is provided in Figure 5.1 .b-3.
TO
Prior to entering the ACTIFLO process, raw wastewater is directed through fine screens to
remove large debris and prevent equipment fouling in the process. Debris removed by the
fine screening process is typically sent directly to a sanitary landfill for disposal. Chemical
coagulant is then added to the screened raw water in the influent line via an in-line
mechanical or jet mixing system to provide for thorough and instantaneous dispersion of
the coagulant into the influent stream. The coagulant, typically ferric
42
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chloride (FeCI3) in wastewater applications, serves to destabilize suspended materials as
they enter the process.
The coagulated water enters the ACTIFLO system in the injection tank. Here, flocculent aid
polymer (polymer) and microsand are added to initiate floe formation. Microsand and
polymer are incorporated into the coagulated water via mixing over 1 min of retention time.
Together, the microsand and polymer form ballasted pin-floe that is allowed to develop
further in the next process step.
Treatment continues as water passes through the underflow passage from the injection
tank into the maturation tank. Although chemical floe formation actually begins with the
addition of polymer and microsand in the injection tank, the majority of ballasted floe
formation occurs during the maturation process step. Gentle mixing and increased
hydraulic retention time of approximately 3 min provide ideal conditions for the formation of
polymer bridges between the microsand and the destabilized SS. This process is further
augmented by the large specific surface area of the microsand that provides enhanced
opportunity for polymer bridging and enmeshment of suspended materials.
Fully formed ballasted floe leave the maturation tank and enter the settling tank where they
rapidly settle and are removed from the treated water via lamellar settling. Laminar up flow
through the lamellar settling zone provides effective removal of smaller floe and suspended
materials in the treated water. Clarified water exits the ACTIFLO system by a series of
collection troughs for subsequent treatment.
Sludge Handling. The ballasted floe sand-sludge mixture is collected at the bottom of the
settling tank and withdrawn using a rubber-lined centrifugal slurry pump. The sand-sludge
mixture is then pumped to the hydrocyclone for separation. Energy from pumping is
effectively converted to centrifugal forces within the body of the hydrocyclone causing
chemical sludge to be separated from the higher density microsand. Once separated, the
microsand is concentrated and discharged from the bottom of the hydrocyclone and re-
injected into the process for re-use. The lighter density sludge is discharged from the top
of the hydrocyclone and sent for thickening and/or final disposal.
Coagulant Process Data. As previously discussed, chemical coagulant is added to the
raw water to destabilize suspended materials as they enter the ACTIFLO system. To
provide effective treatment, coagulant should be injected in the influent line upstream of the
system. In-line mechanical or jet injection mixing, depending on flow rate should be
provided to ensure nearly instantaneous incorporation of the coagulant into the influent
stream.
Although any number of different coagulants could be used, FeCb is typically used in
wastewater and CSO treatment applications since it is less expensive, and thus more
economical in high capacity treatment applications than other available coagulants.
FeCI3 also offers the advantages of being less sensitive to pH and more aggressive in the
removal of organics. The only disadvantage to FeCI3 use is the need for corrosion
43
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resistant coagulant dosing and mixing equipment in the influent line. Once incorporated
into the raw water, FeCI3 does not pose a serious corrosion threat to process equipment
within the ACTIFLO system.
Stormwater Management Model (SWMM) Results
The EPA SWMM Version 4 was used to assess the benefit of the retrofit by comparing
annual WWF volume treated at the WWTF under pre- and post-retrofit conditions for the
hypothetical retrofit. A simplified model was developed to simulate hourly rainfall for the
hypothetical collection system.
The data used for these projections consists of 30 years of hourly rainfall data for the years
from 1962—1991. These data were used as input to the simplified RUNOFF model that
projected flows that were used to simulate long-term overflow using the TRANSPORT
block of SWMM. The TRANSPORT model was constructed as a simplified network
consisting of hypothetical overflows and regulator pipes. The simplified model was used to
project the annual volume of treated WWF at the WWTF.
Table 5.1.b-3 demonstrates that the hypothetical ACTIFLO system would provide
considerable annual capture of WWF that is presently discharged without treatment. This
is due to the use of the remaining seven primary treatment tanks as wet-weather storage
devices with the ACTIFLO system. These tanks have a combined storage capacity of
28,388 m3 (7.5 MG).
Table 5.1 .b-3. Average Annual Overflow Volume from the Pre- and Post-Retrofit Facilities
(1962-1991)
Average Annual Overflow Volume
Pre-Retrofit WWTF Overflow
Post-Retrofit
439,060 m3/d
34,065 m3/d
116MG
9MG
Percent Capture 92%
The proposed ACTIFLO system in combination with the remaining 7 primary clarifiers
modified for storage of WWF would provide a system to treat 92% of the WWF that is
presently discharged at the WWTF.
Project Costs
Operational Costs. The primary costs associated with the operation of the ACTIFLO
system are electrical power and process chemicals. The major power components in the
system are the mixers, scraper drive and microsand recirculation pumps.
44
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Additionally, a small amount of electrical power is required for the PLC control panels,
polymer mix and feed system, and coagulant feed system. Wherever possible, energy
efficient equipment is selected for use in the ACTIFLO system to help reduce overall
operating costs. In wastewater/CSO treatment applications, the ACTIFLO system is
typically operated at less than full capacity, further reducing electrical costs.
Chemical costs are the other major expense associated with operation of the ACTIFLO
process. The chemical costs consist of primarily of coagulant and flocculent aid polymer.
Depending on the raw water characteristics, pH adjustment chemical costs might also be
required in certain applications. As with any water treatment process, chemical dosages
are difficult to predict without pilot or full-scale operating data from the site. Furthermore,
chemical dosages are subject to vary with changes in the influent conditions. Therefore, a
range of typical or anticipated chemical dosages is used to calculate operating costs.
Although chemicals are a major component in the overall operating costs of the ACTIFLO
process, the chemicals are used very efficiently due to the mixing provided within the
system. Overall, ACTIFLO is typically capable of producing similar or better treatment
results with 20—50% reductions in chemical usage as compared to conventional physical-
chemical treatment systems of similar capacity. The estimated annual O&M costs are
provided in Table 5.1 .b-4.
Capital Costs. Capital costs estimates for retrofitting the proposed ACTIFLO system into
the existing WWTF that were provided by the ACTIFLO manufacturer (Kruger Inc.) are
listed in Table 5.1.b-5. The costs include construction requirements and project services
including design engineering, construction management, legal and fiscal expenses but do
not include contingencies or other miscellaneous items.
The conventional abatement alternative for the hypothetical retrofit would be to construct
wet-weather storage facilities adjacent to the existing primary treatment facilities to
equalize WWF for ultimate treatment at the WWTF. This would require the construction of
a 28,388 m3 (7.5 MG) overflow retention facility (ORF). The estimated cost to construct this
facility based upon EPA cost guidelines for CSO control technologies (EPA 1992) is $18.4
million derived from the following equation:
Cost = 3.577V0812 (1)
where,
Cost = Average construction cost, millions of dollars
V = Storage volume, MG
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Table 5. 1. b-4. Estimated O&M Costs
DESCRIPTION
Estimated Annual Cost
SAND COMSUMPTION
Est. Sand Loss (g/m 3 water
produced)
Design Flow Rate (m 3/d)
Sand Loss (kg/day)
Sand Loss (mt/y)
Sand Cost ($/ton)
Typical Operation Cost ($/y)
1
908,400 (240 MOD)
793(1, 748 Ibs/d)
263 (290 tons/y)
$88 ($80/ton)
$9,253
POLYMER COMSUMPTION
Est. Polymer Dosage (ppm)
Polymer Consumption (kg/d)
Polymer Consumption (mt/y)
Polymer Cost ($/mt)
Typical Operation Cost ($/y)
0.8
205 (452 Ibs/d)
68 (75 tons/y)
$3,858 ($3,500/ton)
$104,694
COAGULANT COMSUMPTION
Coagulant Type
Est. Coagulant Dosage (ppm)
Coagulant Consumption (kg/d)
Coagulant Consumption (mt/y)
Coagulant Cost ($/mt)
Typical Operation Cost ($/y)
Ferric Chloride
65
26,008 (57,501 Ibs/d)
8,650 (8,540 tons/y)
$220 ($200/ton)
$760,978
ENERGY COMSUMPTION
Injection Mixer kW per Train
Maturation Mixer kW per Train
Scraper kW per Train
Sand Pump kW per Train
kW per Train
Total kW
kW/1,OOOm3
KW/d
Typical Operation Cost ($/y)
Total Typical Operation Cost
($/y)
11 (15 HP)
15 (20 HP)
3.7 (5.0 HP)
37 (50 HP)
67 (90 HP)
403 (540 HP)
0.44 (2.25 HP/MGD)
7,725 (9,688 HP/d)
$107,090
$982,015
(1) Typical annual O&M costs are based on operation at 33% capacity 302,800 m3/d (80 MOD)
for 365 days per year with the provision for 75 wet-weather events lasting 12 h each requiring
operation at 100% capacity. 75 wet-weather events per year are based upon long-term
continuous simulation of 30 years of data through a hypothetical collection system using the
TRANSPORT block of SWMM.
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Table 5.1.b-5. Estimated ACTIFLO Retrofit Capital Costs
Description Estimated Cost(1)
Equipment Cost $12,000,000
Installation Cost $960,000
Concrete Cost $700,000
Total Project Cost $13,660,000
Includes Design Engineering, Construction Engineering, Legal, and
Fiscal Expenses based on information provided by ACTIFLO
manufacturer, Kruger, Inc. Does not include contingencies and right-of-
way costs
In comparison, the capital cost for the ACTIFLO process is less costly than the
conventional storage facility, $14.5 million versus $18.4 million; however, the projected
O&M cost of the ACTIFLO facility at almost $1 million per year make it a less cost-effective
alternative to storage. This is a result of high chemical costs associated the ACTIFLO
process.
Typical ACTIFLO Performance in WW/CSO Treatment
ACTIFLO Pilot Testing Results. A pilot plant with a nominal capacity of 150 nf/h and a
maximum capacity of 200 rrf/h was used by Kruger to conduct the pilot testing of the
ACTIFLO system. The pilot plant consists of three mixing tanks with a total volume of 12.5
m3 and two parallel hopper settling tanks each having a surface area of 1.2 rrf. Each
mixing tank is equipped with an axial flow mixer designed to allow for optimal coagulation
and flocculation. A rubber lined centrifugal pump extracts the sand/sludge slurry from the
bottom of the hopper settling tanks. The major operating parameters of the pilot plant at
the nominal flow rate of 150 m3/h are summarized in Table 5.1 .b-6.
Table 5.1 .b-6. Major-operating parameters of ACTIFLO pilot plant at nominal flow rate of
150m3/h
Parameters
Coagulation Tank Detention Time
Injection Tank Detention Time
Maturation Tank Detention Time
Settler Overflow Rate
Microsand Recirculation Rate
Microsand Concentration
Velocity Gradient (G) in Coagulation Tank
Velocity Gradient (G) in Injection Tank
Velocity Gradient (G) in Maturation Tank
Values
1
1
1
150
8
3—6
200—300
200—300
150—200
Units
min
min
min
m/h
%
kg/m3
1/s
1/s
1/s
47
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Case Studies
Primary Wastewater Treatment Application - Wastewater Reuse for Agricultural Irrigation
in Mexico City, Mexico. Wastewater reuse for agricultural irrigation must be treated for the
removal of Helminth eggs and protozoa cysts, and elimination of pathogenic bacteria and
inactivation of enteric viruses. Of great importance is the removal of Helminth eggs. They
are extremely persistent, surviving in harsh environmental conditions, and due to their
latency period, are transmitted primarily through uncooked food. The Helminth disease
transmission has been identified as the top-priority heath problem of the wastewater reuse
in developing country. The World Heath Organization (WHO) has recommended that the
Helminth egg concentration be under 1 Helminth egg per liter for all kinds of agriculture
(WHO, 1989).
After several studies, the Mexican authorities had determined that the best and most
economical way to reuse the Mexico City wastewater for agriculture irrigation was to use
advanced primary treatment. Due to the existing situation and the extremely large flow of
74.5 m3/s (1,700 MGD), the Mexican authorities decided to evaluate new treatment
technologies and organized pilot studies to see their performances. A six-month pilot test
was conducted to investigate the performance of the ACTIFLO process in 1997 (Le Poder
1997).
The results are summarized in Table 5.1.b-7 and in Figure 5.1.b-4 and 5.1.b-5 and show
that it was possible to obtain very low SS (25 mg/l in average) and Helminth eggs residual
concentration (1 egg per test in average) even at very high overflow (up to 200 m/h) and
low chemical dosage (<60 ppm of alum). It confirms that the ACTIFLO process, even
without the downstream filtration process, is able to produce an effluent water with less than
1 egg/liter in daily average samples the majority of the time, and less than 5 eggs/liter in all
cases, which is the maximum amount permitted for certain kinds of agriculture.
Table 5.1 .b-7. Removal efficiency from 20 daily samples in primary treatment in Mexico
City, Mexico
Influent
Effluent
Removal (%)
SS
mg/l
302
24.7
91.7
Helminth
Eggs
Eggs/I
24.3
0.99
96
Fecal
Coliform
/100ml
8x1 08
1.4x108
82.8
COD
mg/l
442
172
61.1
Total N
mg/l
18.4
15.5
15.8
Total P
mg/l
10.3
1.9
81.6
Sulfurs
mg/l
7.1
1.3
81.7
48
-------
1 nn -f orrt
.2
S -in -
ti
in
D)
D)
LU
CO
-^ 1 -
c
E
0)
X
.1 n
/
s | i i Rise Rate = Influent TSS S Effluent TSS
^**"" "'- . ™ ™
.«_
—
•t-
••-
—
•*
/\
A
\
\
—
V
n
^ —
•&-
-^
—*»«„
„— — -~
-^.
--_.--
""---*»,
^K
*-"-"*"
-*
/
3F
^ /////////// /
Date
•200
£~
• 150 £
QJ
• 100 a
cc
•50
Figure 5.1.b-4.
Mexico
Helminth eggs removal in primary wastewater treatment in Mexico City,
700
600 -
500 -
400 -
300 -
200 -
100 -
0
Rate
Influent TSS
Effluent TSS
\
X
X
250
• 200
• 150
100
50
(0
OL
0)
2
Date
Figure 5.1 .b-5. SS removal in primary wastewater treatment in Mexico City, Mexico
CSO Treatment Application in Galveston, Texas, Pilot Study. A pilot study was
conducted by COM (1998) in Galveston, TX to evaluate the effectiveness of the ACTIFLO
process under simulated CSO conditions. In order to simulate CSO conditions raw
wastewater was pumped through a screen into a holding basin where it was subsequently
blended with the secondary effluent from the existing WWTF prior to entering the pilot unit.
The effect of coagulant dosage on SS, COD and BOD5 removal efficiencies is shown in
Table 5.1 .b-8. The pilot unit was operated at an overflow of 75 m/h, a polymer dosage of
1.0 mg/l and a pH of coagulation of 6.0 while the coagulant dosage (FeCb) was varied
from 75—125 mg/l. As shown in the figure, the SS and COD removal efficiencies
improved as the FeCI3 was increased from 75 mg/l to 100 mg/l. The removal efficiencies
of SS and BOD5 remained constant as the FeCb was increased from 100 mg/l to 125
mg/l. Therefore, the optimum coagulant dosage was approximately 100 mg/l.
49
-------
Table 5.1 .b-8. Effect of Coagulant Dosage on Removal Efficiencies in Galveston, TX
FeCI3 SS COD BODS
Dose Raw Settled % Raw Settled % Raw Settled %
Mg/l Mg/l Mg/l Rem. Mg/l Mg/l Rem. Mg/l Mg/l Rem.
75
100
125
123
147
135
16.9
13.4
10.9
86%
91%
92%
500
600
300
112
105
105
78%
92%
65%
105
123
195
39
35
52
63%
72%
73%
In the event of CSO conditions a process that is primarily used during overflow periods will
have to start up and achieve steady state very quickly. Figure 5.1.D-6 illustrates the rapid
start up capabilities of ACTIFLO process. The first SS and COD samples were taken after
two hydraulic retention times (18 min), the time needed to achieve steady state. After the
process reached steady state the SS and COD removal efficiencies were 88% and 75%
respectively, and the average SS and COD removal efficiencies over the testing period
were 91% and 73%, respectively. The short overall retention time of the process allows the
rapid start up capabilities of ACTIFLO process. During this experiment the pilot unit was
operated at an overflow rate of 75 m/h, a FeCI3 dosage of 100 mg/l and a polymer dosage
of 1.0mg/l.
100
90--
o
0)
ce
0)
0.
0)
D)
ro
80
70-
60
The process was started up at 15:00
50-
15:00
15:10
15:20
15:30
15:40 15:50
Time
16:00
16:10
16:20
16:30
Figure 5.1 .b-6. Start-Up Efficiency of ACTIFLO Process
CSO Treatment Application in Cincinnati, Ohio Pilot Study. Kruger Inc conducted a pilot
study using the ACITFLO® process for the Metropolitan Sewer District at the Mill Creek
WWTF in Cincinnati, OH (Kruger 1998). The testing period was from April 14, 1998
through April 29, 1998. This pilot study was conducted to demonstrate the effect of
overflow rate on the percent removal of turbidity, BOD5, and total phosphorous during CSO
conditions. On April 17, 1998 the pilot unit was operated at a polymer dosage of
50
-------
1.0 mg/l, a FeCI3 dosage of 35 mg/l while the overflow was varied from 75 m/h to 150 m/h.
Table 5.1 .b-9 depicts the effect of overflow rate on the removal efficiencies.
Table 5.1 .b-9. Effect of overflow rate on removal efficiencies, Cincinnati, OH
Rise Turbidity BOD5 Total Phosphorous
Rate Raw Settled % Raw Settled % Raw Settled %
m/h NTU NTU Rem. mg/l mg/l Rem. mg/l mg/l Rem.
75
100
150
64
72
80
2.06
2.35
4.65
96.6%
96.8%
94.2%
125
139
71
23
20
20
81 .6%
85.6%
71.8%
0.60
1.03
2.99
ND1
ND
ND
>99%
>99%
>99%
ND = None Detected
ACTIFLO process achieved greater than 94% removal of turbidity and > 70% removal of
BOD5 and >99% removal of total phosphorous. The process performance was not
compromised at the higher overflow rates in regards to total phosphorous removal. By
comparing the settled water results, higher overflow rates do not adversely effect the
process performance.
An overnight run was performed to demonstrate the stability and the performance of the
process during CSO conditions. The results of this overnight run are shown in Figure
5.1 .b-7. During the night of April 22 and the morning of April 23' the process was operated
at an overflow rate of 100 m/h, a polymer dosage of 1.0 mg/l to 1.3 mg/l and a FeCb
dosage of 45 mg/l to 100 mg/l. The SS removal varied from 60% to 97.2% with an
average percent removal of 78.8% and a standard deviation of 7%. The BOD5 percent
removal varied from 17.1% to 71.6% with an average percent removal of 45.4% and a
standard deviation of 18.2%.
120
10
Hour
15
20
25
Figure 5.1 .b-7. Twenty-four Hour Continuous Demonstration Run, Cincinnati, Ohio
51
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Conclusions
As a primary clarification process, the ACTIFLO process can achieve high removal
efficiencies of pollutants including SS and Total Phosphorous. Additionally, the quick start-
up and short detention time of ACTIFLO process make it a viable alternative for CSO
treatment.
This desktop analysis demonstrates that while the ACTIFLO process may be a viable
treatment process, the capital and O&M costs are high versus conventional alternatives.
One of the primary benefits of the ACTIFLO system is the extremely small footprint of the
process. This can make the ACTIFLO system a more cost-effective alternative for sites
where space for large storage facilities is limited.
This process can essentially be adapted and retrofitted into existing primary treatment
facilities and for CSO control.
52
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5.2 Retrofitting Existing WWF Storage Tanks to Provide Enhanced Settling/
Treatment and Post-Storm Solids Removal. Spring Creek AWPCP, New York, NY
Background
The Spring Creek Auxiliary Water Pollution Control Plant (AWPCP) is a wet-weather
combined sewage detention facility with an existing maximum storage volume of
approximately 49,200 m3 (13 MG): 37,900 m3 (10 MG) in basin storage and 11,400 m3
(3 MG) in influent barrel storage. The AWPCP facility is located on Spring Creek in
Queens, New York, approximately 1 mile east of the 26 Ward Water Pollution Control
Plant (WPCP), near the Brooklyn-Queens border. The function of the AWPCP is to
capture combined sewage, above elevation -7.00, from tributary drainage areas in
Brooklyn and Queens. Flow is conveyed to the plant by the four overflow barrels from
the Autumn Avenue regulator located in Brooklyn, and by two overflow barrels from the
157th Avenue regulator located in Queens. The plant has six basins that fill with
combined sewage to a depth of 4.6 m (15 ft) and up to a maximum elevation of +1.00,
depending on Jamaica Bay tide elevations. Once the basins fill to elevations of -0.50 or
greater depending on tides, CSO is released into the bay through the flap gates at the
south end of the basins. As flow recedes in the sewers, combined sewage retained
within the basins and influent barrels above elevation -7.00 flow by gravity back into the
sewer system for treatment at the 26 Ward WPCP. Retained combined sewage below
elevation -7.00 is screened and pumped back into the sewer system through the 0.6 m
(24 in.) diameter plant effluent sewer. The layout of AWPCP is shown in Figure 5.2-1.
Overview
This desktop analysis evaluates the concept of retrofitting the AWPCP to provide
enhanced treatment of stored WWF and removal of solids accumulated in the basin
following storm events. WWF is presently retained in the AWPCP to minimize wet-
weather overflow through equalization, however, no provision for treatment is provided.
The residual solids and debris deposited within the storage basins are currently cleaned
by a system of traveling bridges equipped with a horizontal spray header and a water
cannon. Each bridge, one per basin, is operated in multiple passes over the basin
length and uses spray water (brackish bay water) to clean the basin walls and floor.
Two alternatives, cross-flow plate settlers and chemical addition, were evaluated to
determine the dual effectiveness of retrofitting AWPCP to serve as an equalization
system and to enhance the removal of solids prior to discharge to Jamaica Bay.
The criteria for retrofitting the AWPCP for enhanced treatment were selected to allow
conceptual design of each retrofit. The primary design factor of these alternatives is the
peak flow rate that must be conveyed and treated. The one-year frequency, two-hour
duration design storm was selected for this analysis. The peak flow rate associated
with the one-year design storm is 66 m3/d (2,330 cfs) based on SWMM modeling of the
combined sewer system tributary to Spring Creek and the 26 Ward WPCP.
53
-------
i-«-^=- r^»*vi' ^ si -'
V
_^ jj_» ^^^.sp,^ . y~OT__»| rnTi'jTrTri in S
JiJi^Ju,; ...... ^ J™L;i-i31i ! ,U '
n- ! ' -,
"I :. '
• > , r i i >
\.\ lilM<4
''•\\ \ \
ViiU
54
-------
The existing dimensions of the AWPCP and the design flow rate for the proposed
treatment facilities are shown in Table 5.2-1.
Table 5.2-1. Existing Dimensions of the AWPCP and Design Flow Rate
Design Data
Quantity
Design Storm Flow (one-year storm)
Number of Retention Basins
Flow / Basin
Length of Basin
Width of Basin
Depth of Basin
Basin Volume
Total Volume
Basin Surface Area
Total Surface Area
Cross Sectional Area / Basin
Surface Loading Rate
Hydraulic Detention Time
63.2 m3/s (2,230 cfs)
6
11.0 m/s (388 cfs)
152.4m (500 ft)
15.2m (50 ft)
3.7m (12.2 ft)
8,670 m3 (306,145 ft3)
52,000m3 (1,836,158 ft3)
2,322m2 (25,000 ft2)
13,935 m2(150,000 ft2)
56.7m2 (61 On2)
410 m3/d/m2 (10,067 gpd/ft2)
15 min
5.2.a. Cross-Flow Plate Settlers
Cross-flow plate settlers use lamellar clarification to remove solids from wastewater.
This process takes advantage of the decantation principle for SS that are denser than
water for clarification. Cross-flow plate settlers have performed as well as conventional
clarifiers, with a smaller footprint to achieve a similar level of settling.
Cross-flow lamellar clarification are principally used for the removal of oils and grease
present in residual liquids emanating from industrial activities in the petrochemical,
chemical, mechanical, metallurgical, paper-processing and food-processing sectors.
The cross-flow plate settler technology has also been successfully utilized in
conventional primary clarifiers and for gravity settling of surface water runoff.
Case History: Shell Montreal Canada
The Shell Company in Montreal, Canada retrofitted their existing storage basin with
cross-flow plate settlers to improve the water quality of the effluent prior to discharge.
This concrete storage basin collects surface runoff from the refinery area. The
55
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parameters that were used for design of the cross-flow plate settlers are shown in Table
5.2.a-1.
Table 5.2.a-1. Design Parameters for the Cross-Flow Plate Separator
Parameter Quantity
Treatment Capacity (peak flow) 7,570 L/min (2,000 gpm)
Influent TSS Concentration 200 mg/L
Average TSS Density 1,159 mg/L
Temperature 20 deg. C
pH 8 to 9
Overflow Rate 404 m3/d/m2 (9,930 gpd/ft2)
The concrete tanks were modified to include cross-flow plate settlers that provided a 27
m2 (290 ft2) surface area for a total projected surface of 223 rr2 (2,400 ft2). The
installation included:
• Inlet deflectors
• Inlet screening
• Two cross-flow plate cells
• A V-notch weir
• An adjustable skimmer
• An effluent weir
The layout of the retrofitted settling tanks is shown on Figure 5.2.a-1.
Limited evaluation of this retrofit has shown that SS concentrations have been reduced
to between 100—125 mg/L throughout a range of influent flow rates and water qualities.
This retrofit would improve the treatment capacity of the existing settling tank without
the requirement for costly expansion or the construction of new settling tanks to achieve
comparable levels of SS removal. Cost information for this retrofit is unavailable.
Proposed Retrofit of AWPCP with Cross Flow Settlers
This retrofit consists of converting the existing Spring Creek basins to include cross-flow
lamellar plate settlers to provide treatment through gravity settling of dense particles.
The use of inclined plates significantly increases the ratio of settling surface to footprint
that would be required for conventional primary clarifiers. The capacity of primary
clarifiers is primarily a function of the surface area rather than depth. The settling area
is a function of the surface area. Theoretically, the settling area of a given spatial area
could be increased by stacking horizontal plates to achieve a settling area equal to the
surface area times the number of plates. However, this arrangement does not allow for
removal of the accumulated sludge.
56
-------
5.2.a-1 of Co,
-,-.-.,,,-B* ;' ' • *
:
57
-------
Inclining the plates would make sludge removal possible. Each plate would have an
effective settling area equal to its horizontal projection. The projections of inclined
plates packed close together (e.g. 4 in. apart) overlap. The total settling area is
determined by adding the horizontal projections. This can increase the theoretical
settling area by 10 times the settling area compared to a conventional clarifier (Huebner
1979).
There are two major force vectors acting on SS contained within wastewater in the
cross-flow separator. The force of gravity acts to pull the SS down while the velocity
vector pushes the SS up along the length of the plates. The resultant of these forces
directs the SS to the plate where particles impinges on the plate and come in contact
with other particles sliding downward along the plate. The flow between the plates is
maintained in the laminar range to promote settling.
The treatment process begins with the screened influent entering the lamellar zone that
consists of cross-flow cells. Particle settling occurs by gravity according to Stoke's Law
that describes the velocity of a particle falling through a fluid. Stoke's Law states that
the velocity of a particle is equal to the square of the particle radius.
In the design of sedimentation basins, the usual procedure is to select a particle with a
terminal velocity Vc and to design the basin so particles that have a terminal velocity
equal to or greater than Vc will be removed.
The lamellar cell design is a practical application of Hazen's theory on settling that
provides a large equivalent surface of separation (as noted by the projected horizontal
area for each plate). Hazen's theory is stated by Huebner (1979) where the velocity is:
V = average linear velocity =—— (2)
d -w
q = volumetric flow rate
cf = distance between plates
w = width of plates
0 = angle of elevation to the horizontal
Vs = particle settling velocity
ds = maximum settling distance
L = length of plates
Particles are removed from the flow stream when its settling time is less than its flow
through time.
t, * *q (3)
t, = — (4)
Vs
58
-------
L L L-d-w .._,
t = — = —, = (5)
1 rr I 1 V '
V q/d-w q
The maximum settling distance, cfs can be calculated as a function of the distance
between the plates, d and the angle of inclination of the plate, 0.
^
(6)
,
Cos®
Which equates to:
d/Cos® L-d-w
Vs " q
and further simplifies to:
< VsCos® (8)
L-w
Hazen's theory provides the surface loading rate (Y) for a conventional clarifier, as:
7 = -JL (9)
L-w
which is equivalent to the simplified inclined plate formula derived above in that q is the
total flow divided by the number of plates. The conventional surface loading rate occurs
when the angle 0 goes to zero, resulting in a Cos 0 of 1 .
The principal design parameters of cross-flow plate settlers are:
Wastewater flow rate - Settling systems are designed to accommodate the peak flow
rate, but the maximum removal efficiency is normally achieved at a lower design flow
rate.
Settling rate of solids - The settling rate of solids is measured in laboratory tests. A
design velocity is then selected according to the required removal efficiency. The
required settling area is calculated by dividing the design flow rate by the design
velocity.
Angle/spacing of the plate - The angle of the plates typically varies from 55 — 65
degrees and the spacing varies from 2.5 — 10.2 cm (1 — 4 in.) depending upon the
nature and the concentration of solids and space limitations. The 10.2 cm (4 in.)
spacing is commonly used in wastewater applications.
A depiction of the cross-flow plate is shown on Figure 5.2.a-2.
59
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Figure 5.2.a-2. Horizontal Projections of Inclined Cross-Flow Lamellar Plates
Cross Flow Plate Settler Equipment
The cross-flow plate settlers selected for this retrofit are from Aquarius Services &
Technologies Inc., Quebec. The major equipment of the cross-flow lamellar plate
settlers is shown in Figures 5.2.a-3 and 5.2.a-4, and include the following:
1. Flow Deflectors - Flow deflectors at the inlet of each basin dissipate the energy
generated by the influent flow. This encourages flocculation and distributes the flow
evenly along the width of the basin and wet perimeter of the lamellar cell.
2. Inlet Bar Screen - Floating material that may clog the lamellar cells is removed by a
inlet bar screen with 5.1 cm (2 in.) square openings
3. and 4. Cross Flow Cells - Cross-flow type lamellar cells consist of inclined plates at a
60° angle and are constructed of either aluminum, stainless steel or synthetic materials.
Spacing between plates may vary according to the design requirements. For the
purpose of this retrofit, stainless steel plates with 10.2 cm (4 in.) spacing were selected
based on the manufacturer's suggestions for this installation.
5. Skimming Device - Oils and hydrocarbons are skimmed from the surface through an
adjustable skimmer.
6. Scum Baffle - A scum baffle is installed in back of the skimmer to retain the floatables
for collection and disposal through the skimmer.
7. Weir-A V-notch weir is installed at the discharge end of the basin to regulate the
liquid level in the basin.
8. Sludge Vacuum Collector- Each sludge silo is equipped with a sludge vacuum
collector to transfer and dispose of collected sludge.
60
-------
5.2.a-3. Plan View of Cross-Flow
-Sit"
Si
A
wl
"1:
MS"'
I
:S
»/
0
61
-------
Figure 5.2.a-4. Profile of Aquarius Cross-Flow
^)r£^^ctb^^"^iSn^
*S™"™"™T™""""'****T! "T"* "™""'""™"
,.?: ....-—„,,„ , .
0,
jf
a) ? i;
D ^ •'•
s
t
62
-------
Following the completion of the storm, the basins would be automatically washed and
the sludge will be transferred for disposal.
The hydraulic design criteria for the cross-flow plate settler technology is shown on
Table 5.2.a-2.
Table 5.2.a-2. Cross-Flow Plate Settler Design Data
Design Data Quantity
Hydraulic Loading Rate* 0.15 m3/s/100 m2 (0.5 cfs/100 sq. ft)
Overflow Rate 132 m3/day/m2 (3,250 gpd/sq. ft.)
* Loading Rate furnished by Aquarius Services & Technologies Inc. Quebec
Under the above loading rates and available basin surface area, the one-year design
flow rate of 63 m3/s (2,230 cfs) would not be accommodated by a cross flow plate settler
system. The proposed system is capable of treating a maximum flow rate of 1,835,725
m3/d (485 MGD) or 32% of the design flow rate based on the physical size of the Spring
Creek basins and the limited hydraulic loading rates of the cross-flow plate settlers. The
proposed facilities would provide treatment under a small percentage of wet-weather
events observed at Spring Creek and would create a hydraulic limitation to WWF
entering the facility. Therefore, retrofit with cross-flow plate settlers is not considered
viable for this site.
5.2.b. Chemical Addition to A WPCP
This desktop analysis involves retrofitting the AWPCP by chemical addition to provide
enhanced settling/treatment. Chemical addition describes a method of treatment where
chemicals are added to enhance settling of particles present in the wastewater. Treated
water is then decanted and conveyed for further treatment or discharged. The resultant
sludge is collected for disposal and can be de-watered to reduce the volume of solids.
Chemical precipitation can be used to remove metals, fats, oils and greases (FOG), SS
and some organics. It can also to be used to remove phosphorus, fluoride, ferrocyanide
and other inorganics.
Conventional wastewater clarification processes primarily involve the destabilization and
subsequent removal of colloidal SS materials that are not readily removed by gravitation
alone. These suspended materials can be natural or synthetic organic or inorganic
compounds, microorganisms, and/or viruses that typically range in size from 10~4//m to
1,000//m. In most natural systems, the stability of colloidal suspended materials is
attributed to a net negative surface charge that causes individual particles to repel each
other and remain in suspension. To counteract these repulsive forces a highly charged
ionic chemical, such as alum (Ab (804)3), FeCb, ferric sulfate (Fe2(S04)s), poly-
aluminum chloride (PACI), is added to bring about a net reduction in the repulsive force
between the suspended materials. This process, termed coagulation, results in the
63
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destabilization and/or attraction of the SS to form chemical floe. Although destabilized,
the chemical floe may remain in solution due to its extremely small size and low mass.
These floe, often called "pin floe," are difficult to remove from the treated water without
first aggregating the smaller particles together into larger, heavier, more settleable floe
by flocculation.
Flocculation is the term used to describe the aggregation of pin floe into larger, heavier,
more settleable floe. This is most readily accomplished with the addition of a chemical
(flocculant aid) polymer. Such polymers are available from many sources in both liquid
and dry form with varying molecular weights and net surface charges. In most cases
selection of the best polymer for a specific application and set of conditions is largely a
matter of trial and error. Larger, more settleable floes form by inter-particle
polyelectrolyte bridges between floe and destabilized solids. Although the formation of
inter-particle bridges can take many forms, the end result is the enmeshment of large
volumes of particles to form large dense floe that are more readily removed by
gravitational settling.
The use of chemical addition would enhance the settling of discrete particles and
suspended matter thereby increasing the overall removal of BODs and SS. This would
be accomplished by retrofitting the existing basins with chemical feed, mixing and
contact facilities. The retrofit would also include dedicated sludge collection and
removal facilities. Chemical coagulation of raw wastewater before sedimentation
promotes flocculation of finely divided solids into more readily settleable floes, thereby
increasing SS, BODs, and phosphorous removal efficiencies. Sedimentation with
coagulation may remove 60—90% of the SS, 40—70% of the BOD5, 70—90% of the
phosphorous, and 80—90% of the bacterial loadings. In comparison, sedimentation
without coagulation may remove only 40—70% of the SS, 25—40% of the BOD5, 5—
10% of the phosphorous loadings and 50—60% of the bacteria loading.
Chemical Coagulants
Advantages of coagulation include greater removal efficiencies, the ability to treat higher
flow rates, and more consistent performance. Disadvantages of coagulation include an
increased mass of primary sludge that is often difficult to thicken and de-water, and
increased in operational cost and operator attention (WEF 1992). Iron salts, particularly
FeCb, are the most common coagulant used for primary treatment. WWTF using lime
for primary treatment have found that it produces more sludge than do metal salts and
is more difficult to store, handle and introduce into the waste stream. Coagulants
should be selected for performance, reliability and cost and evaluations made to
determine dosages and effectiveness.
64
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Case Histories
Table 5.2.b-1 lists coagulant dosages and performance data for six (6) treatment plants
that employ chemical coagulation for advanced primary performance in sedimentation
tanks. The Joint Water Pollution Control Plant (JWPCP) at Carson, California evaluated
30 different polymers and found anionic polymers to be the most effective type for
enhancing primary sedimentation. The Hyperion plant in Los Angeles, California uses
to great success FeCb and an anionic polymer at the headworks before primary
sedimentation. This process increased primary sludge production 45% through
improved SS and colloidal matter removal. This increase was partially offset by a
decrease in waste activated sludge production.
The Los Angeles and San Diego sedimentation tanks with chemical coagulation operate
with overflow rates between 69.3—81.5 m3/d/m2 (I700—2000 gpd/ft2) as compared to
conventional overflow rates of 32.6 m3/d/m2 (800 gpd/ft2) for primary settling. Research
in Sarnia and Windsor, Ontario indicated that overflow rates up to 97.8 m3/d/m2 (2,400
gpd/ft2) did not significantly affect effluent quality.
Design Considerations
The design flow rate for this retrofit is 66 m3/s (2,330 cfs) as shown in Table 5.2-1 with a
corresponding overflow rate of 410.1 m3/d/m2 (10,067 gpd/ft2) based on the dimensions
of AWPCP. This high surface SOR is greater than what would typically be applied for a
chemical coagulation system; however, for the purposes of this retrofit it is assumed
that during the higher SOR's, a reduced efficiency will result. It is important to note that
the frequency at which these high SOR's are likely to occur are expected to be relatively
infrequent (e.g., once per year).
The proposed retrofit consists of equipping the existing influent barrels with a chemical
feed and mixing system. These facilities would provide the necessary mixing and
contact for the rapid mix and flocculation steps, which include the addition of FeCb and
an anionic polymer. An important element in this analysis is an evaluation of the
hydraulic and physical parameters dictated by the existing basins as depicted on Table
5.2-1.
This retrofit would include the specific elements outlined below. Based on a ratio of 452
kW/ m3/d (0.16 HP/MGD) at a peak flow of 953,820 m3/d (252 MGD) per barrel, a total
of 30 kW (40 HP) is necessary to achieve the mixing required for the FeCb per barrel.
There will also be 2 mixers per basin for mixing the polymer. Therefore, 24—15 kW (20
HP) mixers (4 /barrel) are necessary for the 6 basins. Chemical storage requirements
are based on the need to treat approximately 2,500,000 m3 (660 MG) of CSO per year.
Storage of FeCb (37% solution) and polymer (0.25 mg/l) to treat a month (on average)
of CSO would require approximately 16 m3 (4,200 gal) of chemical storage.
65
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Table 5.2.b-1. Coagulant Dosages and Performance Data for Six Treatment Plants That Employ Chemical Coagulation
for Advanced Primary Performance in Sedimentation Tanks
Advanced Primary Performance
Location
Point Loma
City of San Diego
Orange County
Plant No. 1
Orange County
Plant No. 2
JWPCP
Los Angeles County
Hyperion
Los Angeles City
Sarnia
Ontario, Canada
Flow
(m3/day)
722,935
227,100
696,440
1,438,300
1,400,450
37,850
(MGD)
191
60
184
380
370
10
Influent
mg/L
276
263
248
365
300
98
BOD
Effluent
mg/L
119
162
134
210
145
49
Rem.
%
57
38
46
42
52
50
Influent
mg/L
305
229
232
475
270
124
TSS
Effluent
mg/L
60
81
71
105
45
25
Chemical Addition
Rem.
%
80.3
64.6
69.4
77.9
83.3
79.8
Type
FeCI3
Anionic Poly
FeCI3
Anionic Poly
FeCI3
Anionic Poly
Anionic Poly
FeCI3
Anionic Poly
FeCI3
Anionic Poly
Cone.
mg/L
35
0.26
20
0.25
30
0.14
0.15
20
0.25
17
0.30
Duration
Continuous
8 hours
Peak Flow
12 hours
Peak Flow
Continuous
Continuous
Continuous
66
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Chemical feed pumps will be necessary to deliver the correct dosing of chemical during
wet-weather events. The FeCb solution will require a feed rate of approximately 0.05
m3/min (13 gpm) per barrel at peak flow; resulting in 2 metering pumps per barrel for a
total of 24. The pumps would be hose (or peristaltic type) pumps. Half as many feed
pumps would be required to deliver the polymer to the influent barrels.
Operators could control the chemical feed system using a new control panel and
SCADA system housed in the existing control room. The capital costs for this retrofit
are shown on Table 5.2.b-2.
Table 5.2.b-2. Estimated Capital Costs Chemical Addition Retrofit
Description Cost
General Conditions $25,000
Sitework $350,000
Chemical Storage $25,000
Chemical Feed System $75,000
Chemical Mixers (4 in each of 6 basins) $720,000
Instrumentation and SCADA System $25,000
Total Construction Cost $1,220,000
Project Services1 $240,000
Total Project Cost $1,460,000
1 Includes Design & Construction Engineering, Legal, and Fiscal Expenses
Does not include contingencies and right-of-way costs
Operational Requirements
The existing AWPCP operates intermittently based on wet-weather events. Following
CSO events, the facility is de-watered by gravity and pumping. These operations would
not change under the proposed retrofit. The following list summarizes the areas of
expected O&M cost increases at the facility:
• Increased labor to perform wet-weather process adjustments and clean-up following
a wet-weather event.
• Power for chemical feed and mixing systems , and
• Chemicals
The costs are based on the facility treating approximately 2,498,100 m3 (660 MG) of
CSO per year and maintaining a chemical storage of approximately one month. The
annual volume of CSO is based on modeling performed for a typical year using the
Storm Water Management Model (SWMM). Hourly rainfall data were used from a local
NWS Weather Station as input to the model. It is estimated that approximately 100 wet-
weather events occur per year and that each event is approximately 6 h in duration.
The O&M costs are shown in Table 5.2.b-3.
67
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Table 5.2.b-3. Estimated Chemical Addition O&M Costs
Description Estimated Annual Cost
Coagulant Consumption
Coagulant Type Ferric Chloride
Coagulant Dosage (ppm) 20
Design Flow Rate (MGD) 1,510
Coagulant Consumption (Ibs./day) at Peak Flow 252,000
Coagulant Consumption (Ibs/month) 9,200
Coagulant Consumption (tons/year) 44
Coagulant Cost ($/ton) 200
Coagulant Cost ($/vear) $8.800
Polymer Consumption
Est. Polymer Dosage (ppm) 0.25
Design Flow Rate (MGD) at peak flow 1,510
Polymer Consumption (Ibs./day) at Peak Flow 3,100
Polymer Consumption (Ibs./month) 110
Polymer Consumption (tons/year) 0.53
Polymer Cost ($/ton) 3,500
Polymer Cost @ 100% 724/365 ($/vear) $1.848
Labor
Cleanup/Operations $21,000
Energy Consumption
Rapid Mixer HP per Basin 30
Chemical Feed HP per Basin 2
Total HP (for 6 basin compartments) 192
kWh/year 115,200
Total Annual Energy Cost ($/vear) $6,912
Total Annual Operation Cost ($/year) $38.600
6 hours per event
Conclusions
This retrofit utilizes the existing storage facilities to provide chemically enhanced
sedimentation of WWF. This could result in the removal of 60—90% of the SS, 40—
70% of the BOD5, and 30—60% of the COD from the effluent to the receiving water.
This is an expected 58% increase over current performance without chemical addition.
At the one-year design flow, the facility will exceed typical SORs by a factor of 4—5,
however, these would occur relatively infrequently.
68
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5.2.c. Retrofitting Existing WWF Storage Tanks For Post-Storm Solids Removal,
Spring Creek AWPCP, New York, NY
In 1997, COM conducted a basin-cleaning prototype testing program at the Spring
Creek AWPCP in New York City (COM 1997). The prototype testing was performed in
response to the New York City Department of Environmental Protection's (NYCDEP)
desire to investigate alternatives to the existing traveling bridge-cleaning system. This
analysis was performed to evaluate the results of the prototype testing and provide
recommendations for basin cleaning system design.
Background
The residual solids and debris deposited within the storage basins are actively cleaned.
Currently, for cleaning of solids and debris within the basins, the facility uses a system
of traveling bridges equipped with a horizontal spray header and a water cannon. Each
bridge, one per basin, is operated in multiple passes over the basin length and uses
spray water (brackish bay water) to clean the basin walls and floor.
The basin cleaning alternatives evaluated through prototype installation and testing at
the facility were:
• Tipping buckets
• Fixed spray nozzles
• Orifice headers
• Flushing gate (Hydroself Fluid Flap)
• Re-suspension/mixing (used to enhance any of the above)
Objectives and Scope
The effectiveness of the cleaning systems at the Spring Creek AWPCP were evaluated
to establish design criteria for feasible alternatives for full implementation and to
develop operational requirements and operational and capital costs associated with
each of the systems. The data collected from the prototype study was used to either
determine or indicate the following:
• Effectiveness of floor cleaning
• Effectiveness of wall cleaning
• Water usage
• Power usage
• Overall cleaning time required
• Special operating requirements
• Safety concerns (if any) associated with each system
• Cost
Each of the systems listed above was installed in test areas within three of the six
basins at Spring Creek. As discussed below, each system was arranged in different
69
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configurations and operated during actual basin cleaning events for a total of 16
cleaning test events, over the period from July 1 to December 16, 1997. The cleaning
systems were tested under conditions where solids accumulation varied widely. Each
system was operated under a variety of cleaning water volumes, flow rates, and
pressures. As the testing proceeded, it became apparent which technologies were not
providing effective cleaning. As a result, the original test protocol was modified to focus
on, and optimize the performance of the most effective systems.
The actual test conditions and the equipment used in the study are summarized below.
Basin Cleaning Prototype Equipment and Operation. The prototype testing consisted of
16 test runs, from July 1, 1997 through December 17, 1997. The prototype fieldwork
was performed in conjunction with NYCDEP's routine basin cleaning operations to
benchmark the existing cleaning operations at the facility.
The following parameters were monitored and recorded during the prototype testing:
• Solids depth within basins
• Equipment test parameters (flow rate, total wash-water volume, pressure, etc.)
• System cleaning effectiveness
• Cleaning duration
The overall cleaning effectiveness was based on a qualitative assessment of the degree
of cleaning and the approximate percentage of the floor area cleaned. The test area
was judged to be clean when approximately 99% of the test area was without solids.
Water consumption and operating pressures were measured using flow meters and
pressure gauges set up throughout the system. Cleaning time was measured for the
prototype systems and then estimated regarding full-scale implementation of each
system.
An evaluation of the existing basin cleaning operation was performed and provided a
benchmark for the prototype cleaning systems. This benchmarking was performed by
observing the cleaning operations, documenting the cleaning time and using the rated
pump capacity to calculate the flow rate and water used by the existing traveling
bridges.
Using the traveling bridges, the basins were generally cleaned one side at a time.
Typically, the traveling bridge requires a minimum of two passes per side. The first
pass loosens the solids and provides some cleaning, while the latter passes provide for
a more thorough cleaning. A manually controlled water cannon breaks up large heavy
solids. In addition to cleaning the basin floor, the water cannon cleans the sidewalls of
the basin and the walkways of the basin area.
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General Prototype Systems Installation and Operation. Five prototype cleaning
systems were tested: tipping buckets, flushing gate, fixed spray headers with nozzles,
fixed spray headers with orifices, and a submersible mixer. The prototype systems
were located in basins 1, 2 and 3.
Bay water was used as the source of cleaning water for the prototype systems.
Cleaning water was supplied from a single, 37.3 kW (50 HP), three stage vertical
turbine wash-water pump, installed in the existing spray water channel between basins
1 and 2. The pump was sized for a flow rate of 3 m3/min (800 gpm) against a head of
50.3 m (165 ft). A PVC piping system distributed wash-water to the various cleaning
systems. A flow schematic of the wash-water equipment and piping is provided in
Figure 5.2.C-1. Flow meters, pressure gages, and butterfly valves were installed
throughout the cleaning-water piping-system to provide a positive means of regulating
the system flow and pressure. Pressure control valves were installed within the piping
system to regulate the delivery pressure of the cleaning water to the required level. A
pressure sustaining valve was installed on the main piping header to relieve excess flow
and to maintain system pressure during times the full 3 rrfVmin (800 gpm) flowed but
was not required.
Prototype testing occurred in conjunction with NYCDEP's normal basin cleaning
operations. The basins were pumped and drained by plant staff prior to the normal
cleaning operations. Plant staff cleaned the length of the basins, except for the 6.1 m
(20 ft) test sections located in basins 1, 2, and 3 using the existing traveling bridge
system. On occasion, some test sections were not available for testing either due to
cleaning of the test section by plant staff or by inflow of the tide through leaking tide
gates. Generally after the plant staff completed cleaning, the prototype system testing
began. Sediment depth in the prototype system test sections was measured and
recorded by entering the basins. The distribution of solids within each test section was
documented. The tipping buckets, spray nozzles and orifice headers at the influent end
of the basins were generally tested first. After the testing at the influent end of the
basins was finished, the tipping buckets, flushing gate, spray nozzles and orifice
headers at the effluent end of the basins were then tested. During several test runs, the
prototype testing was halted early due to overloading of solids in the AWPCP's cross
collector channel and the chamber screen channels. As discussed later, sudden slugs
of cleaning water, carrying large quantities of solids, overloaded the channels causing
the testing to be ended prematurely.
71
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5.2.c-1.
'"••—-\«,Arf-^-*--_-\.,A,A A,,A,,
5 »
IJ
,
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t -, f • .if
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72
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As the results of the prototype testing were analyzed, the operating conditions were
adjusted and the actual prototype system was modified or the entire prototype system
was removed from testing when deemed ineffective.
Tipping Buckets. Grande, Novae & Associates, Inc., of Montreal, Canada supplied the
units tipping buckets. Four tipping buckets were installed in basin 1. Two buckets, one
each on opposite sidewalls, were installed at elevation +1.5 m (+ 5.00 ft) at both the
influent (TB-1A and TB-2A) and overflow (TB-1B and TB-2B) test sections of the basin,
as shown in Figures 5.2.C-2 and 5.2.C-3. The buckets were installed at either end of the
basin to test the cleaning effectiveness of the highest and lowest solids loading in the
basin. The overflow end buckets were used to determine the ability of the buckets to
convey the solids down the full length of the center velocity channel.
Each bucket was 6.1 m (20 ft) long and had a liquid capacity of 0.15 m3 per linear meter
(11 gal per linear foot [gal/ft]) for a total volume of 0.8 m3 (220 gal) per bucket. During
testing, a constant supply of cleaning water was fed into the buckets. When the
maximum water level was reached in the buckets, the buckets would automatically tip
due to the change in the bucket's center of gravity, a 0.8 m3 (220 gal) wave of water
down the basin wall and along the floor, to the velocity channel.
A training wall, stretching the width of the basin, from the velocity channel to the basin
sidewall, was originally installed along the edge of the test area. The purpose of the
training wall was to prevent the wave of cleaning water from dispersing laterally, thereby
maintaining the full force of the wave within the test section. In addition, curved wall
fillets were installed at the bottom of the sidewall to allow for a smooth transition of
water flow from the wall to the floor. Figure 5.2.C-3 shows the approximate size and
location of the training walls and wall fillets.
Once the NYCDEP's cleaning operations were completed and the sediment in the
basins measured, filling of the tipping buckets commenced. The two buckets at the
influent end of the basins were filled simultaneously and then discharged automatically.
The process was repeated as necessary until the test section was considered clean.
Table 5.2.C-1 summarizes the test conditions for the tipping buckets along with the other
prototype systems tested.
After the installation of the equipment it was found that bucket TB-2B did not tip
automatically. A counter weight was added to correct bucket mounting/misalignment
and to permit the bucket to tip. Periodically, bucket TB-1B also experienced hesitation
in tipping.
Following the initial test runs, modifications were made to the training walls and fillets to
improve their effectiveness at eliminating dead zones within the basins. The original
training walls, made of steel angle approximately 15—30 cm (0.5—1 ft) in height, were
bolted to the basin floor. The training walls extended from the walls at the column line
to the velocity channel. During testing it was observed that the solids along the training
73
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5.2,c-2. Tipping -
*"!?"""'" *'"*!*' IsSKSi !^nS|KJit VMl'^SW^W™' ^^rf% ,
/ I'"' > »-- !
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11
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74
-------
5.2.c-3. Tipping - A
w
75
-------
5.2.01.
ft.
76
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wall were the most difficult to clean. It was believed that this was due to a dead zone
between the edge of the bucket and the basin wall pilaster. A modified plywood training
wall was installed along the width of the pilaster to eliminate this dead zone as shown in
Figure 5.2.c-4.
In separate subsequent tests, the training wall from bucket TB-2A and the wall fillets
from buckets TB-1B and TB-2B were removed. The training wall was removed to
determine what effect its removal would have on the solids located in the dead zones.
The wall fillets were removed for use in the modified orifice header configuration.
Flushing Gate. The flushing gate system was tested for possible installation in the
facility's influent barrels, and/or cross collector channels. Currently, accumulated solids
in the influent barrels must be removed manually. The wave produced by a flushing
gate can produce a high velocity flow of water over a relatively long distance. The basin
velocity channel offered an opportunity to test the effectiveness of the flushing gate
without actually entering the influent barrels. Grande, Novae & Associates, Inc. of
Montreal, Canada supplied the flushing gate. A single flushing gate, 1.22 m (4 ft) wide
with two 2.74 m (9 ft) long steel subwalls was installed at the overflow end of basin 1.
The flushing gate was centered at the head of the existing velocity channel, while the
two subwalls were located on either side of the flushing gate. The purpose of the
subwalls was to accommodate the installation of the gate and to provide a storage
volume of approximately 15.1 m3 (4,000 gal) of flushing water. This system utilized both
CSO and bay water as its source of flushing water. Cross sectional views of the
flushing gate are shown in Figure 5.2.C-5.
When the desired volume of water behind the flushing gate was obtained, a remote
hydraulic actuator opened the flushing gate and sent a wave of cleaning water down the
length of the velocity channel. The time of travel for the leading edge of the wave
produced by the gate was measured at two points in the center velocity channel, 30.5 m
(100 ft) and 76.2 m (250 ft) downstream of the gate. The velocity versus time of the
flow following the wave was determined by measuring the time of travel of objects
floating in the channel.
There was no active means of closure for the flushing gate. After the stored wash-water
volume had discharged, the flap portion of the gate would close under its own weight.
The flap was required to sit flush against the gate frame in order for the hydraulic unit to
lock the flap in the closed position. As a result, any debris or leaking tide water flow
would interfere with the gate closure. When the gate closed, debris caught between the
flap and frame would often cause leakage of the stored volume. Several times during
the testing, a person was required to enter the basin in order to clear debris from the
gate and allow it to be closed. On one occasion, an automobile tire became wedged in
the gate opening.
77
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5.2.c-4. Training
i.
r ffft, ~v^ f ^* at p
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Fioure S.2.C-* Mo-difi'SO Pfywocxs T-'aifiirifl- Waji Sc
78
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5.2.C-5. - B
a
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79
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Fixed Header with Spray Nozzles. Two separate spray header arrangements
(Alternative-1 and Alternative-2) were installed in basin 2. Spraying Systems Inc. of
Wheaton, Illinois provided spray nozzles installed on these headers. Alternative-1 used
the model HH30100 nozzle that delivered a full cone spray pattern of 0.05
m3/min/nozzle (12.3 gpm/nozzle) at approximately 42,190 kg/m2 (60 psi). Alternative-2
used the model PSS35200 nozzle that delivered a flat spray pattern of 0.1
m3/min/nozzle (24 gpm/nozzle) at approximately 42,190 kg/m2 (60 psi). Both
arrangements also utilized a flat-spray-pattern-nozzle-model 50200 [delivering
approximately 0.1 m3/min/nozzle (24 gpm/nozzle) at 42,190 kg/m2 (60 psi) to clean the
sidewalls. Each spray manifold was installed with 3 separate 10.2 cm (4 in.) header
pipes.
Two pipes were dedicated to clean the floor, and one pipe was dedicated for wall
cleaning as shown in Figure 5.2.C-6. The spray water manifolds were located on
opposite walls at both the influent and overflow ends of the basins at elevation +1.5 m
(+5.00 ft). At this elevation the nozzles were approximately 4.3 m (14 ft) from the basin
floor. This maintained the nozzles above the basin flood line and reduced the potential
for nozzle damage and clogging.
The flow rate and pressure of the cleaning water supplied to the nozzles were held
constant throughout each test run. Provisions to adjust the flow and pressure, using
throttling valves located in each header, were made so that these parameters could be
varied from run to run. The cleaning system was allowed to run for up to 15 min. The
flow rate and operating pressure were recorded for both nozzle arrangements. Table
5.2.c-2 provides the test conditions for the fixed spray headers with nozzles.
Even at the highest attainable pressures and flows, the nozzles proved ineffective at
cleaning the basin floor and were removed from service. The nozzles were also subject
to clogging from debris found in the cleaning water supplied from the spray water
channel.
Fixed Spray Header with Orifices. Initially, four headers were installed in two
arrangements in the influent and effluent ends of basin 3. Two orifice headers were
located on opposite walls at the influent end of the basin, and two orifice headers were
located on opposite walls at the overflow end of the basin. Each orifice header was 6.1
m (20 ft) long and equipped with 1.6 cm (5/8 in.) orifices spaced on approximately 30.5
cm (1 ft) centers. The spacing and size of the holes were selected to provide a water
flow rate of 0.5 m3/min/m (40 gpm/ft) at a discharge pressure of 7,030 kg/m2 (10 psi).
One pair of headers was installed at a centerline elevation of +1.5 m (+5.00 ft) (influent
header 1A and overflow header 2B) with the orifices aimed directly at the sidewalk
These headers were designed to send cleaning water down the wall in a continuous
sheet, thereby washing the basin floor and carrying solids to the velocity channel. The
second pair of headers (influent header 2A and overflow header 2B) was installed at a
centerline of elevation -8.50 with the header aimed directly at bottom of the wall.
80
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5.2.C-6. -
81
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Jable 5.2.C-2. Summary of Prototype Testing Operation
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82
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Table 5.2.C-2 page 2
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83
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This system was also designed to provide a sheet flow of cleaning water along the floor.
Since no nozzles were utilized under this alternative, there was less concern for
clogging while the headers were submerged. Cross sectional views of this original
orifice header configuration are shown in Figure 5.2.C-7.
Training walls stretching the width of the basin from the velocity channel to the basin
sidewalls were installed at the outside end of the basin test section. Similar to the
tipping buckets, the purpose of the training walls was to prevent dispersion of the
cleaning water and maintain the flow within the basin test section.
After the initial six test runs, the original orifice header configuration was determined to
be ineffective at cleaning. Subsequently, modifications to the header configuration were
made including the following:
• Removal of the overflow end orifice headers (for reuse in the modified configuration
at the basin influent)
• Rotation of the influent orifice header 1A (elevation +1.5 m [+ 5.00 ft]) to orient the
water jets nearly tangential to the sidewalk
• Rotation of the influent orifice header 2A to orient the spray downward and towards
the velocity channel at a spray angle of approximately 30°s with the floor.
• Installation of two new influent orifice headers (2B and 3) on the eastern side of the
test section. Header 2B was installed on the basin floor at elevation -2.6 m [-8.5 ft]
and header 3 was installed on the sidewall at elevation +1.5 m (+5.00 ft). The spray
on header 2B was oriented similar to header 2A while header 3 was oriented similar
to header 1A.
• Installation of wall fillets at the wall-floor joints beneath influent headers 1A and 3.
Figure 5.2.C-8 provides a section showing the modified header configuration.
Flow rate, operating time and pressure was recorded for each orifice header. Table
5.2.c-2 provides the operating conditions and performance for this system.
Few operational and mechanical problems were encountered while operating this
system. Occasionally rags and other debris became entangled around the supports of
the orifice headers; however, this debris did not effect the cleaning operation.
Submersible Mixer. A single submersible 18.6 kW (25 HP) jet mixer was supplied by
Davis EW of Thomasville, Georgia (now a division of US Filter). The jet mixer was
installed at the influent end of Basin No. 2 using steel guide rails and lifting cables
similar to those typically associated with submersible pumping equipment. The mixer
was provided with an integral diffuser and air supply piping system to achieve a roll
mixing effect within the basin.
84
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Figure 5.2.C-7. Original Orifice Header Configuration - Section A
85
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5.2.c-8. - A
86
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The jet-mixer was placed into continuous service after test run no. 3. The mixer was
controlled using automatic level control floats to turn on while the basins were filling
during a CSO event and off when the basins were almost completely drained. The
mixer remained in operation sometimes for more than one day until the basin was
drained.
The purpose of the mixer was to provide re-suspension of the solids, such that solids
would be removed with the wastewater during the de-watering operation, minimizing
solids deposition within the basins. During testing the mixer proved ineffective at re-
suspending the solids in the test area and reducing solids deposition. The mixer was
removed from service after test run no. 7. Following removal of the jet mixer from
service, it was found that the diffuser and discharge piping of the mixer were almost
completely clogged with leaves, rags and other debris.
Prototype Test Results
Solids Composition and Distribution. As would be expected for a CSO facility, the
composition and distribution of residual solids in the detention basins varied significantly
between events. The measured depth of solids ranged from a trace coating (less than
1.5 cm) up to 45.7 cm (1.50 ft) at individual piles. The solids ranged in consistency from
a thin watery mixture, to a viscous "clay like" consistency, to heavy grit. Test events
with the greatest depth of solids and heavier grit generally followed rainstorms of high
intensity. Test events with fewer solids and solids with a thinner consistency generally
followed rainstorms with low intensity or low total precipitation.
In general, the influent ends of the basins contained the heavier solids, with the overflow
ends characterized primarily by floatables, leaves, and a thin coating of solids. The
solids depth was greater at the influent end than at the overflow end. Dead zones at the
influent headwall of each basin contributed to the formation of piles or long mounds of
solids in the influent test sections. The largest piles were observed on the basin side of
the influent headwall columns and along the sidewalk Solids appeared to build up
behind these obstructions, as scouring of solids during CSO inflow was prevented.
These and other piles of solids often posed the most resistance to cleaning. Within
basins no. 1 and 3, a ridge of deeper solids was observed several times along the
western wall of the basin influent. At the influent end of basin No. 1, the solids depth
was generally greater on the western side of the test section than the eastern side. This
likely contributed to the formation of piles along the western sidewalk The difference in
solids depth between the two sides of the basin No. 1 influent may have been due to a
partial blockage of the influent barrel feeding the eastern side of this basin, resulting in
unequal flow distribution.
Based on subjective observations, it appeared that the consistency of the residual solids
was generally thicker and more cohesive when more days had transpired between the
inflow event and the cleaning test as compared to when cleaning was performed
immediately following an inflow. The nature of the solids consistency affected cleaning
system performance, with the thicker/ cohesive solids being more resistant to cleaning.
87
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Benchmark of Existing Cleaning Operations. As discussed previously, the existing
traveling bridge cleaning system operations were observed to document general
operations, cleaning time, and water consumption. The cleaning time, water
consumption and estimated electrical cost for this system are presented in Table 5.2.c-
3.
The bridges provide very effective cleaning of the basin walls and floor. Generally,
several complete passes along the basin length were required to clean each basin.
With the bridge mounted water cannon, solids or objects which might have been
unaffected by the spray nozzles were readily moved into the velocity channel and
cleaned from the basin floor. The bridges readily achieved complete basin cleaning.
The primary advantage of the traveling bridge system is that it is very effective at
cleaning the basins. The degree of cleaning is only limited by the amount of time an
operator expends in cleaning. Disadvantages observed with the bridge system included
the following:
• Cleaning with the bridges was labor intensive, requiring staffing of each bridge
during operation
• Because there is no separation from the basin airspace, the operator had the
potential to be exposed to H2S levels exceeding standards
• The bridges appear to require frequent maintenance. Often during the test runs, one
of the six bridges was observed to be temporarily out of service, due to problems
with the bridge controls or with the electrical bus bar supplying each bridge
Tipping Buckets. The test results for the tipping bucket prototype cleaning system are
presented in Table 5.2.c-4. This table presents tabulated results for what are
considered representative solids conditions. For several test events, the preceding
inflow did not result in any significant deposition of solids, or the test section may have
been cleaned by tidal inflow or by plant personnel. The data for such events was not
considered representative and is not included in Table 5.2.c-4. Data for the influent and
overflow test sections are presented separately, with averages for the number of bucket
tips per test section and projected water usage per basin calculated over each test run,
each test section (influent, overflow), and over the entire basin. The average over the
basin length assumes a linear distribution of solids (and associated cleaning water
consumption) between the solids depth measured at the influent test section, and that
measured at the overflow test section.
The tipping buckets provided very effective cleaning of the sidewalls and the basin floor.
The high velocity wave produced by each bucket tip easily cleaned solids and debris
from the basin floor and conveyed them to the center velocity channel. With greater
depths of solids or for heavier solids or grit, more tips were required as compared to test
conditions with fewer solids or with looser solids.
88
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Jable 5.2.C-3. Operating Data for Existing Traveling Bridge
Tali!? 5.2.C-3 Operating fur fxisling Travelling Clean ing System
'* ; i
-------
Table 5.2.C-4. Tipping Bucket Cleaning System Test Results at Representative Solids
Conditions
tliHwi'h.r
•»f*
90
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Averaged over the basin length, 7 tips are estimated to be required per bucket, resulting
in a projected water usage of 291.4 m3 (77,000 gal) per basin. The range in projected
water usage is from a low of 41.6 m3 (11,000 gal) per basin, to greater than 1249 m3
(330,000 gal) per basin. The highest projected water consumption was required to
clean a pile of matted leaves mixed with solids in the influent test section during test run
no. 16. In this case, the leaves seemed to bind the solids together, resulting in a single
large mass that was difficult to clean. This pile was slowly eroded with each tip. After
30 tips the test was terminated while the pile still remained. Difficulty cleaning leaves
was also observed in the overflow test section of run no. 10. There was a pile of matted
leaves in the test section for bucket TB-1B. With each tip, the entire mass of leaves
was lifted, carried a short distance, and redeposited. Ultimately, 12 tips were required
to wash this pile into the center velocity channel.
The solids that had collected along the training wall proved difficult to clean and were
often the limiting factor in determining when the basin was 99% clean. Elsewhere on
the test section floor, the wave of wash-water was able to mix and resuspend the solids
in the wash-water. However, along the training wall the solids would generally erode
slowly with each tip. In the prototype installation, the wall-floor fillet on the eastern tank
wall was installed flush against the sidewall in the recess between the column pilasters.
As a result, over the pilaster width, there was no smooth transition for the wave between
the wall and floor, and the wave would lose energy in this dead zone when it impacted
the floor. The modified plywood training-wall improved the situation somewhat by
eliminating the dead zone created by the sidewall pilaster. The plywood training-wall
worked by preventing the deposition of solids in the area blocked by the pilaster. Solids
along the modified training wall still eroded slowly with each tip. However, the amount
of solids collected along the training wall was less than for the original vertical training-
wall installation.
When the training-wall was removed from the test area for bucket TB-2A, the cleaning
effectiveness was the same if not more effective than with the training-wall. Without the
training-wall the wave could penetrate laterally into the solids, resulting in more effective
resuspension of the solids at this location.
Operationally, the buckets were very simple to operate, requiring only that the water be
turned on; however, difficulty in tipping first one bucket (TB-2B) then another bucket
(TB-1B) was observed. Due to possible misalignment of TB-2B during installation,
counterweights were added to the bucket so that it would tip automatically during filling.
This worked for a time. However, later in the test program the bucket had to again be
tipped manually. Periodically, the same problem was observed with bucket TB-1B.
Flushing Gate. The flushing gate produced a high velocity flow in the center velocity
channel that extended at least 76.2 m (250 ft) down the length of the tank. The velocity
of the leading edge of the wave is a function of the slope of the channel and the steep
hydraulic grade created by the wave surge. The center velocity channel slopes
approximately 0.63 percent towards the cross channel. The flow velocity was
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calculated using Mannings Equation ranged between 1.3—1.9 m/s (4.4—6.2 ft/s) for
uniform flow at typical wash-water depths of 0.15 and 0.30 m (6 and 12 in). During
operation of the flushing gate, the leading wave velocity was measured at 3.4—4.3 m/s.
(11-14 ft/s). The water velocity in the channel later dissipated to 1.2 m/s (4 ft/s) after
approximately 40 s and 100 s at the 30.5 m and 76.2 m (100 ft and 250 ft) measuring
stations, respectively. Although there were generally no significant solids within the
velocity channel, this high initial flow rate would be expected to scour most if not all
solids and objects from the channel.
Based on the size and configuration of the gate supplied, which was primarily designed
to flush the velocity channel, the lateral extent of the high velocity flow was limited to the
velocity channel. In the immediate vicinity of the gate, the high velocity flow extended
laterally approximately 61—152.4 cm (2—5 ft) from the center channel. However, the
high rate of discharge produced backwater within the basin effluent test area, which did
not result in any effective cleaning of the test area outside of the velocity channel.
As indicated previously, significant operating problems were encountered with the
flushing gate. These problems were primarily related to floatables and debris that
caused problems with the closure and sealing of the gate.
Fixed Header with Spray Nozzles. The test data for the spray nozzle prototype system
are presented in Table 5.2.C-5. The spray nozzles did not provide effective cleaning.
The nozzles only cleaned the immediate area impacted by the central portion of the
spray jet. Outside the central portion or cone of the spray jet, the nozzles produced a
fine mist, which was ineffective at cleaning. Away from this immediate area the wash-
water would form channels in the deposited solids and would not provide any further
cleaning. Overall, approximately only 25% of the floor test-area was cleaned by either
nozzle configuration. This degree of cleaning was achieved within the first minute of
operation. The system was generally allowed to operate for 15 min. However, the
added operating time did not result in any further cleaning. Since the spray nozzles did
not provide complete cleaning, the cleaning times and volumes presented in Table
5.2.c-5 have been qualified to indicate that greater cleaning time and volumes would be
required. The flat spray nozzles directed at the sidewall provided an adequate job of
cleaning the wall only.
Based upon field observations, it appears that in order to provide adequate cleaning
with this spray nozzle configuration, the headers would need to be swept back and forth
across the floor during operation, similar to the existing traveling bridge operation.
Fixed Header with Orifices. The test data for the original and modified orifice header
configurations are presented in Tables 5.2.C-6 and 5.2.c-7, respectively. The original
header configurations provided only partial cleaning of the floor test area. An area
extending approximately 1.5—3 m (5—10 ft) from the sidewall was completely cleaned
within the first minute of operation. These solids were usually cleaned with the initial
surge of wash-water.
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Table 5.2.C-5. Spray Nozzle Cleaning System Test Results (2 pg.)
£ • ; 5 • 1l i " ~ ?
"^ '* '', '- \ *| .?" "
L i : i ~ S I
•- ! = b i £ « : «
~ i ^' i ? *i -
93
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Table 5.2.C-5. Spray Nozzle Cleaning System Test Results (2nd pg)
I!
94
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Table 5.2.C-6. Original Orifice Header Configuration Test Results
Q«fr3c-».;tfc:nJsr St'i. ! i
&«.».! (hrrftu-*, itittiin Xo. 23-* Ei -S.
;.;-,;
95
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Table 5.2.C-7. Modified Orifice Header Configuration Test Results at Representative
Solids Concentrations (3pg.)
•%» """-
1C i
tl
•5 iS
a i,
l:iil€ :
96
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Table 5.2.C-7. Modified Orifice Header Configuration Test Results at Representative
Solids Concentrations 2nd page
97
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Table 5.2.C-7. Modified Orifice Header Configuration Test Results at Representative
Solids Concentrations (3rd page)
'1 I'.
98
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Further away from the wall, the wash-water would form channels and the remaining
floor area would be cleaned only very slowly, as the flow eroded the edges of the
channel. Based upon field observations, it appeared that a higher velocity of wash-
water was required to completely resuspend the solids over the full width of the test
section and resist the formation of channels. The headers installed at elevation +1.5 m
(+5.00 ft) provided adequate cleaning of the sidewalk The system was generally
allowed to operate for 15 min, by which time only 30—75% of the floor test area was
effectively cleaned. Since the cleaning was incomplete, the cleaning times and volumes
shown in the table are qualified to indicate that actual values are greater than those
shown.
Overall, the modified orifice header configuration showed significant improvement over
the original orifice header configuration. The modified orifice header no. 1A provided
the same degree of cleaning, approximately 30—75% of the floor test area, as the
original configuration. However, the area appeared to be cleaned sooner than the
original configuration. Additionally, the wall-floor fillet minimized the quantity of solids
deposited along the sidewalk While no significant solids/grease buildup was observed
on the basin sidewalls, the spray from modified header no. 1A provided better cleaning
action than original configuration. Because of the tangential flow spray of the wash-
water onto the wall, all of the water flow washed down the wall in a sheet of water. In
the original configuration in which the orifices were directed more at the wall, a large
portion of the flow sprayed off the wall and rained down on the basin floor.
The combination of modified headers no. 2A, 2B, and 3 provided complete and effective
cleaning of the test area floor and sidewall within a short period of time. The
combination of the two lower headers (2A and 2B) spraying towards the velocity
channel provided a high scouring velocity and prevented the formation of channels in
the flow path that would otherwise limit cleaning effectiveness. With all three headers
operating, the test area was generally 95% clean within the first minute of operation. As
shown in Table 5.2.C-7, the average required cleaning time for the test section was 2.68
min with a projected basin wash-water usage of 530 rrrVbasin (140,000 gal/basin).
During the prototype testing, the header operating sequence and flow rates were varied
to determine the optimum operating conditions. Simultaneous operation of headers 2A
and 3, followed by header 2B alone, appeared to minimize the consumption of wash-
water. Simultaneous operation of all three headers was also tested, as this was most
representative of the future full-scale operating scenario. This provided cleaning
effectiveness equal to sequential operation in a shorter time period, but at higher
projected water consumption. A wash-water flow rate of 2.3 rrr/min (600 gpm) per
header (30 gpm/lf /header) was sufficient to clean the majority of the solids. However,
in test run no. 13, a partially full trash bag was present in the test area and a flow rate of
2.8 m3/min (750 gpm) per header (37.5 gpm/lf /header) was required to move the trash
bag into the velocity channel. The purpose of header no. 3 was to clean the sidewall
and the area behind header no. 2A. The flow rate on header number 3 was varied
between 0.8—1.5 m3/min (200—400 gpm or 10—20 gpm/ft). A flow rate of between
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0.8—1.2 rrrVmin (200—300 gpm) appeared to be sufficient to meet these cleaning
objectives.
Operation of the orifice headers was simple. The biggest difficulty was modulating the
flow at startup to minimize the occurrence of an initial surge of water. The steps taken
to minimize this effect included slowly opening the valves at startup and tapping the
feed pipe to provide air relief points. During operation, some rags and debris became
entangled on the orifice header pipe supports. The amount of material caught on the
supports was minor and did not affect cleaning performance. It is important to note that
after a total of 6 months of testing the lower headers, no clogging of orifices was
observed. This was originally identified as a concern for submerged orifices.
Submersible Mixer. There is no specific test data on the submersible mixer other than
solids depth measurements and qualitative observations. From observations of the
residual solids distribution following basin de-watering, the mixer did not appear to
effectively minimize the deposition of solids. Typically, solids were scoured from a small
area approximately 1.5—3.0 m (5—10 ft) in diameter immediately at the discharge of
each of the mixer nozzles. However, the solids depth immediately surrounding the
scoured areas was deeper than for other basins, indicating that the scoured solids were
immediately redeposited away from the nozzle discharge. Often there was a deep pile
or ridge of solids formed around the scoured area. Elsewhere on the test area floor, the
solids were unaffected by the mixer.
The mixer did not have any effect on floatables and leaves, which remained in the test
area following basin de-watering.
The discharge nozzles would periodically become clogged as indicated by decreased
mixing turbulence in the tank during mixer operation. Following removal of the
submersible mixer prototype, the flow distribution box and several of the nozzles and
feed lines were found to be clogged with leaves and debris. This material apparently
became so compacted in the distribution box that the top weld of the box cracked open.
Comparison of Technologies and Recommendations for Design
Summary of Cleaning Effectiveness. The results of CDM's field testing program
demonstrated that either the tipping buckets or the modified orifice header system
would provide satisfactory cleaning of the basin floor, equivalent in performance to the
existing traveling bridge system. Each of these two systems achieved complete
cleaning of the test areas.
The spray headers equipped with nozzles and the original orifice header configuration
provided only partial cleaning of the test areas. This configuration was considered to be
unsuitable for application at Spring Creek given the existing basin geometry. The spray
headers only provided cleaning of the immediate area impacted by the nozzle spray.
This resulted in cleaning only 25% of the test area. The original orifice header
configuration was designed to provide sheet flow of water over the basin floor. With this
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system, channeling of the flow occurred rapidly, which limited the ability of the system to
clean the entire test area. Under the best of conditions, this system cleaned only up to
75% of the test area. These two systems are, therefore, not considered further. The
submersible mixer did not minimize the deposition of solids in the test area and created
deep piles and ridges of solids adjacent to scoured areas. Also, the mixer became
clogged with leaves and debris. Therefore, the submersible mixer is not considered
further.
The flushing gate was effective at producing a high velocity wave with initial velocity of
3.4—4.3 m (11—14ft) per second in the center velocity channel at 30.5—76.2 m (100—
250 ft) downstream of the gate. This velocity would be more than sufficient to scour
solids from the velocity channel or the influent barrels. It was not intended to evaluate
the flushing gate for cleaning of the basin floor. The gate was demonstrated on the
center velocity channel as a surrogate for possible application to the influent barrels.
Operational problems were encountered with floatables and debris interfering with gate
closure and sealing. For full-scale application, some means of positive closure would
be recommended. Also, if an alternate method of sealing without significant leakage
could not be developed, an alternate water supply would be required to fill the storage
volume prior to a flushing event.
The remainder of the discussion in this section will address the two successful prototype
systems for basin cleaning, tipping buckets and orifice headers.
The average cleaning requirements determined from the prototype testing for the tipping
bucket and modified orifice header systems are summarized in Table 5.2.C-8 and are
compared against the existing traveling bridge system. The cleaning requirements for
the tipping buckets and orifice headers are based upon cleaning a single 6.1 m (2 ft)
length of the basin at one time, which was the length of the test areas. The cleaning
time indicated for both the buckets and orifice headers could be reduced from that
shown in the table by increasing the size of the cleaning zone, and consequently,
increasing wash-water pumping and conveying requirements. However, for the purpose
of comparing the prototype test data, the cleaning requirements for the two systems are
presented on the same 6.1 m (20 ft) basis.
As shown in the Table 5.2.C-8, the average basin cleaning time for the tipping buckets is
more than twice that for either the orifice headers or traveling bridges. This is primarily
due to the multiple tips required to clean each zone. For full-scale implementation of
either the tipping buckets or orifice headers, the actual time to clean all six basins would
not be six times the average basin cleaning time, but rather some fraction of this
quantity. The total cleaning time would be dependent upon a number of factors
including:
• Length and number of cleaning zones in each basin
• Number of basins to be cleaned simultaneously
• The maximum allowable or cost-effective wash-water flow rate
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• The maximum allowable solids loading rate to the cross channel and climber
screens
The last two factors would limit the number of cleaning zones in operation at any one
time. The flow and solids loading rates selected would impact the sizing and cost of the
cleaning and de-watering screening equipment. Therefore, while it is difficult to assess
the minimum full-scale cleaning time without considering all the above factors, it is
believed that the use of the tipping buckets will result in a significantly greater total
cleaning time than either orifice headers or traveling bridges.
Table 5.2.C-8. Projected Cleaning Requirements for a Single Basin Cleaning Event
(Based on 20-ft. Cleaning Zone).
Cleaning Parameter
Average Time (min/basin)
No. of Cycles/Zone (averaged
Tipping
Buckets
175
7 tips
Mod. Orifice
Headers (2 A, 2B,
and 3)
\
67
1
Traveling
Bridge
72
2-6 passes
over basin length)
Water Consumption
- Influent Test Area (mVbasin) 457.99 530.77 NA
(gal/basin) 121,000 140,229 NA
- Average Entire Basin (m3/basin) 291.45 NA 272.05
(gal/basin) 77,000 NA 71,875
Estimated Electrical Consumption
(kW-hr/basin) 40 40 40
Level of Staffing Required for Automated Automated Fully Manned
Operation
Note: NA denotes data not available.
The performance of the tipping buckets and the modified orifice headers compare
favorably with each other and with the traveling bridge system in that all three systems
generally are capable of achieving complete cleaning of the basin floor and walls. In
one instance, run no. 16, the tipping buckets could not completely clean a pile of matted
leaves mixed with solids. After 30 tips the test was terminated as it was determined that
no further cleaning could be achieved. Similarly, difficulty with the tipping buckets
moving leaves was also noted in the overflow test area of run no. 10, where 12 bucket
tips were required to move a pile of leaves. The orifice headers exhibited similar
difficulty moving a trash bag. However, once the lower header flow rate was increased
to 2.8 m3/min (750 gpm) per header, the bag was swept into the velocity channel. With
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the proper design flow rate, the orifice headers should be capable of providing complete
cleaning.
The average wash-water consumption for the tipping buckets and the traveling bridges
is similar at 270—290 m3 (71,875—77,000 gal) per basin. There is no average over the
entire basin length for the modified orifice headers as this system was only tested at the
basin influent end. This represents the worst case solids cleaning condition. For the
influent test areas, projected basin water consumption values are presented for the
tipping buckets and modified orifice headers. Although, based on this test area the
tipping buckets use approximately 14% less water than the orifice headers. The wash-
water volumes ranged from 100 m3 to greater than 1,250 m3 (27,500 to >330,000 gal)
per basin for the tipping buckets and 175—880 m3 (46,050—232,900 gal) per basin for
the modified orifice headers. This data variability would need to be accounted for in full-
scale design by assuming conservative design values. Therefore, the difference in
wash-water consumption between these two systems is not considered significant.
Estimates of power consumption for the three systems are presented primarily for
informational purposes. Because the cleaning systems are operated only intermittently,
on the order of 8 h/week, the differences in electrical costs are insignificant. As shown
in the table, the tipping buckets are estimated to use the least amount of electricity while
the orifice headers use the most. This is because the orifice headers are a high flow
rate, moderate head system, while the buckets are a low flow and low head system.
Both the tipping buckets and orifice header system would be designed for automatic
operation in full-scale application. This is an advantage over the traveling bridges that
are continually manned during operation.
Cost Comparison
The estimated cost of implementing either tipping buckets or orifice headers at the
Spring Creek facility are presented in Table 5.2.c-9. These estimates are based upon
average solids conditions and water consumption. For the tipping buckets, water
consumption was averaged over the length of each basin based upon the influent and
overflow test area results. The modified orifice header system was only tested on the
basin influent end. Similar to the tipping buckets, the water consumption for the
overflow end of the basin would be expected to be less than the influent end, due to the
distribution of solids. For this analysis, this was not taken into account. It is likely that
the operating time for the orifice headers, and associated electrical cost, could be than
the calculated electrical cost.
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Table 5.2.C-9. Cost Comparison of Full Scale Tipping Bucket Vs. Orifice Header
Cleaning Systems
Item
Direct Capital Costs
Piping and Valves
Mechanical Equipment
Instrumentation and Controls
Electrical Equipment
Indirect Capital Costs
Contractor OH&P (1)
Design / Contingency (2)
SUBTOTAL CAPITAL COSTS
Annual O&M Costs
Electrical Costs
SUBTOTAL O&M COSTS
O&M PRESENT WORTH (3)
TOTAL
Tipping
Buckets
$ 781,000
$ 4,956,000
$ 10,000
$ 362,000
$ 1,283,000
$ 916,000
$ 8,308,000
$ 1,500
$ 1,500
$ 17,205
$ 8,325,205
Orifice Headers
Piping Alt 1
Pipe Gallery
$ 10,255,107
$ 120,000
$ 16,000
$ 300,000
$ 2,245,000
$ 1,069,000
$ 14,005,107
$ 3,000
$ 3,000
$ 34,410
$ 14,039,517
Piping Alt 2
Pipe on Roof
Deck
$ 5,840,000
$ 120,000
$ 16,000
$ 300,000
$ 1,318,000
$ 941,000
$ 8,535,000
$ 3,000
$ 3,000
$ 34,410
$ 8,569,410
Notes:
1. Contractor's overhead and profit are assumed to be 21 % of the direct capital cost.
2. Contingency is assumed to be 15% of the direct capital cost for the Tipping Buckets
and Orifice Headers Alt 2, and 10% for Orifice Headers Alt 1.
3. Present worth analysis assumes a 20-year design life and an interest rate of 6.0%.
The design assumptions used in this cost analysis were as follows:
Tipping Bucket Design Assumptions
Bucket unit volume
Bucket length
Number of buckets/basin
Average water consumption
Material of construction
Orifice Header Design Assumptions
Number of headers/zone
Length of cleaning zone
Orifice header diameter
Orifice pressure drop
2.7 m3/m (220 gal/ft)
6.1 m(20ft)
50
290 m3 (77,000 gal)/basin
316L Stainless steel
24.4 m (80 ft)
25.4 cm (10 in.)
7030kg/m2(10psi)
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Cleaning flow rate (for each side of basin)
Wall header 0.2 m3/min/m (15 gpm/lin ft)
Floor headers 0.5 m3/min/m (40 gpm/lin ft) /header
Total cleaning flow rate 1.2 m3/min/m (95 gpm/lin ft)
Average water consumption 530 m3 (140,229 gal) /basin
Conceptual level capital and electrical operating costs are presented. The total net
present worth cost is calculated based upon design life of 20 years and an interest rate
of 6.0%. Costs are presented for two different feed pipe layouts for the orifice header
system. Alternative-l is based upon installation of the main wash-water feed headers in
pipe galleries located in the former spray water channels. This alternative has been
included in the 30% design phase. Due to the space constraints in the gallery and
obstructions by support columns, this alternative involved significantly more fittings and
pipe flanges than the second alternative. Alternative-2, the preferred alternative for the
orifice headers, is based on installation of the main wash-water feed headers on top of
the basin cover system. Without the obstructions of the pipe gallery, the feed headers
could be installed in straight pipe runs with pre-fabricated sections and with branches
running 90° off the main header. This significantly reduced the number of fittings and
pipe flanges. For comparison to the tipping buckets, only the cost for alternative-2 will
be considered.
As shown in Table 5.2.c-9, the estimated present worth costs to implement either the
tipping bucket cleaning system or the orifice header cleaning system are equivalent.
There is only approximately a 3% difference in these estimates, with the tipping bucket
cost being slightly less. The operating electrical costs are not significant compared to
the total capital cost. The cost for regular maintenance of the equipment is not included
in the estimate. The primary maintenance items could include the following:
• Maintenance and periodic replacement of pumps and valves
• Maintenance and periodic replacement of tipping bucket bearings
• Repair/replacement of orifice headers
• Entry into basins to remove large debris items
Currently long-term (e.g., 10 or 20 year) maintenance data is not available on tipping
bucket or orifice header installations in CSO facilities. It would be difficult to assign
maintenance/ replacement costs for servicing of the bucket bearings or the orifice
header pipes. Also, the cost for staffing during cleaning operations has not been
included in the cost comparison. Both the tipping bucket and orifice header systems
would be automated for full-scale application. Some minimal staffing would be required
to monitor and troubleshoot the cleaning operations. However, this could largely be
combined with other facility staffing responsibilities. A staffing labor estimate for either
of these two systems would be dependent upon operating philosophy and configuration
(e.g., sequential vs. concurrent basin cleaning, staffing level). It is expected that the
staffing cost for these two alternatives would be similar and would be significantly less
than the costs for the existing traveling bridges. Assumptions on the extent of O&M
costs required could significantly affect the total costs. Given this uncertainty on the
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O&M costs, only the electrical costs which are well defined have been included in the
total present worth cost.
Implementability and Operability
Both cleaning technologies, tipping buckets and orifice headers, are readily
implementable. There are no known constructability issues that would limit the
applicability of one technology over the other. Both technologies provide effective
cleaning. However, matted leaves posed a cleaning problem to the tipping buckets
while this was not observed to be an issue with the orifice headers. Because of the
higher velocity imparted by the orifice headers, this technology appears to be better
suited at cleaning matted leaves, piles, and large objects.
There were two operability issues noted with the tipping buckets, including potential
bucket misalignment and transient overloading of the cross channel and climber
screens by waves carrying high solids loads. Misalignment of one of the test buckets
(TB-2B) caused it not to tip automatically. This was retrofitted in the field by adding
counterweights. Later a second bucket (TB-1B) developed a hesitation in tipping
automatically. With up to 300 buckets planned to be installed in a full-scale facility, this
would present a significant concern. The second issue was a concern whenever the
facility received a high inflow of grit. During testing, high quantities of grit were washed
into the cross channel with the initial few tips of the buckets. Because the buckets
produced discontinuous flow surges separated by zero flow conditions, the grit would
often settle out in the channel and immediately before the climber screen when the
water velocity decreased. This caused the channel and climber screen to become
clogged with grit, which then had to be removed manually.
With the orifice headers there are two operability concerns, entanglement of debris on
the pipe supports and potential clogging of the orifices. Regarding the first issue, some
rags/debris became entangled on the pipe supports of the prototype system during
testing. However, the spray from the orifices was sufficient to prevent a buildup of
solids with the rags. With proper design, the number of pipe supports and their
configuration can be selected so as to minimize this effect. Periodic entry of the basins
will likely be required to remove entangled material. As for potential clogging of the
orifices, no clogging of the prototype system was observed during the 6 months the
headers were in the basins. If clogging does occur, the orifices in the PVC headers
may be easily cleaned by plant staff.
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5.3 Converting Dry Ponds for Enhanced Treatment
Dry ponds are extensively used throughout the U.S. and other countries (EPA 1983).
These ponds have been constructed to reduce peak runoff rates (peak shaving),
typically with little consideration to runoff quality improvement. Dry ponds have been
widely used in flood control projects to detain runoff and thus reduce peak flows and
water elevations in the receiving waters. These flow reductions can also improve the
aquatic habitat by reducing flushing of fish and other organisms from urban creeks (Pitt
and Bissonnette 1984). Flow reductions also reduce downstream channel bank erosion
and bottom scour. The use of many dry ponds in a watershed, without regard to their
cumulative effect, can actually increase downstream flooding or channel scour problems
(McCuen, et a/. 1984). The delayed discharge of a mass of water from a dry pond may
be superimposed on a more critical portion of the receiving water hydrograph.
Because these ponds are normally dry and only contain water for relatively short
periods of time, they can be constructed as part of parking lots, athletic fields, tennis
courts and other multi-use areas. The outlets are designed to transmit all flows up to a
specific design flow rate, after which excess flows are temporarily backed-up. In many
cases, the ponds only contain water during a few rains each year.
Several dry detention ponds were examined as part of the National Urban Runoff
Program (NURP), with monitored pollutant removals ranging from insignificant to quite
poor (EPA 1983). Sedimentation may occur in dry ponds, but only during the major
storms when flows are retained in the pond. The deposited material must be removed
after each treated rain, or it can easily be re-suspended by later rains and washed into
the receiving waters. Adler (1981) found that new sediment deposits have little cohesion
and without removal as part of a maintenance program, or without several feet of
overlaying water, bottom scour is probable. Because of the poorly documented
stormwater pollutant control effectiveness of dry detention ponds, they cannot, by
themselves, be recommended as viable water quality control measures. However, they
can be very effective when used in conjunction with other stormwater control practices
(e.g., between a wet detention pond and an infiltration or grass filter area).
Stanley (1996) examined the pollution removal performance at a dry detention pond in
Greenville, NC, during eight storms. The pond was 0.7 ha in size and the watershed
was 81 ha of mostly medium density single family residential homes, with some
multifamily units, and a short commercial strip. The observed reductions were low to
moderate for SS (42—83%), phosphate (-5—36%), nitrate nitrogen (-52—21%),
ammonia nitrogen (-66—43%), copper (11—54%), lead (2—79%), and zinc (6—38%).
Stanley (1996) also summarized the median concentration reductions at dry detention
ponds studied by others. In all cases, the removal of the stormwater pollutants was
substantially less than would occur at well designed and operated wet detention ponds.
The re-suspension of previously deposited sediment during subsequent rains was
typically noted as the likely cause of these low removals. The conditions at one of the
ponds in Greenville, NC were observed three years after its construction. The most
notable change was that the pond bottom and interior banks of the perimeter dike were
107
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covered with weeds and many sapling trees (mostly willows), which indicated that the
interior areas were too wet to be mowed. The perforated riser was also partially
clogged and some pooling was occurring near the pond outlet. It appeared that the dry
pond was evolving into a wetland. The monitoring activity had been conducted a few
months after the pond was constructed and was not affected by these changes. Stanley
felt that the wetlands environment, with the woody vegetation, if allowed to spread,
could actually increase the pollutant trapping performance of the facility. With continued
lack of maintenance, the dry pond will eventually turn into a wet pond, with a significant
permanent pool. The pollutant retention capability would increase, at the expense of
decreased hydraulic benefits and less flood protection than originally planned.
Nix and Durrans (1996) examined the benefits of off-line stormwater detention ponds.
Off-line ponds (side-stream ponds) are designed so that only the peak portion of a
stream flow is diverted to the pond (by an in-stream diversion structure). They are
designed to reduce the peak flows from developed areas, with no direct water quality
benefits, and are typically dry ponds. Off-line ponds are smaller (by as much as 20—
50%) than on-line ponds (where the complete storm flow passes through the pond) for
the same peak flow reductions. However, the outflow hydrographs from the two types of
ponds are substantially different. The off-line ponds produce peak outflows earlier and
the peak flows do not occur for as long a period of time as on-line ponds. If located in
the upper portion of a watershed, off-line ponds may worsen flooding problems further
downstream, whereas downstream on-line ponds tend to worsen basin outlet area
flooding. Off-line dry ponds can be used in conjunction with on-line wet ponds to provide
both water quality and flood prevention benefits. Off-line ponds have an advantage in
that they do not interfere with the passage of fish and other wildlife and they do not
dramatically affect the physical character of the by-passed stream itself. On-line dry
ponds may substantially degrade the steam habitat by removing cover and radically
changing the channel dimensions. The peak flow rate created by online dry ponds
reductions can also have significant bank erosion benefits in the vicinity of the pond,
although these benefits would be decreased further downstream.
Case Studies
The retrofitting detention pond case studies presented in this section cover a broad
range of conditions, from underground facilities for very small paved drainage areas, to
large detention facilities for major drainage areas. The problems encountered in each
case history are described, along with the costs and benefits, to the extent that
information was available.
5.3.a. Sunnyvale Detention Basin, Santa Clara County, California
South San Francisco Bay has a serious heavy metal pollution problem, especially for
copper, and numerous methods are being investigated to reduce the discharges of
metals to the Bay. Woodward Clyde Consultants (1994) conducted a retrofit project for
the Santa Clara Valley Nonpoint Source Pollution Control Program to demonstrate the
108
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benefits of modifying an existing dry detention pond for enhanced water quality benefits.
The discussion in this section is mostly taken from that report.
According to a survey conducted in 1990 by Woodward-Clyde (1990), there are
seventeen municipally owned and operated pump stations in Santa Clara Valley. These
pump stations generally consist of pumps, storage units such as a sump or a detention
basin, and inlet and outlet works. The sumps and detention basins reduce the required
pump capacity needed to pass the peak flood flows. The pump stations provide flood
protection to low lying areas that have historically subsided and are now protected by
levees. The pump stations have generally operated as single-purpose flood control
facilities. The pumps are designed to go on as soon as water begins to fill the basin
with the goal of emptying the basin as soon as possible after the event. Therefore, no
net pollutant removals are assumed to be occurring.
One retrofitting option to achieve water quality benefits is to change the pump-operating
schedule in order to increase detention time and to provide for a seasonal wet pond. A
preliminary evaluation of retrofitting detention basins was encouraging and a pilot study
to retrofit a facility and perform testing to measure water quality benefits and costs was
August 1990 through July 1993.
The following tasks were conducted as part of this study:
• Retrofit the pump station and modify pump schedules to increase water
retention and thus improve SW pollutant removal,
• Conduct water quality sampling to estimate the pollutant removal effectiveness
of the retrofitted detention basin, and
• Measure sediment concentrations in the basin in order to determine whether
sediments are classified as hazardous waste.
The detention basin has a channel between the inlet and outlet that, prior to the
modifications, encouraged short-circuiting. A gabion baffle was installed at the outlet to
reduce short-circuiting and to provide better distribution of flow into the outlet. Rock
was dumped into the channel leading from the inlet, and a drainage pipe that ran below
the channel was blocked off. Operational changes consisted of modifying the pump
schedule to create a 0.61 m (2 ft) permanent pool at the outlet and to provide temporary
storage and slow release of water over the depth range of 0.61—0.73 m (2—2.4 ft).
Site Description. The northern portion of Santa Clara Valley has a history of subsidence
caused by groundwater pumping. In order to protect subsiding areas from flooding, a
system of levees and pump stations was built. The pump stations collect and pump SW
runoff from these low-lying areas through the levees into the flood control channels. In
order to accommodate large flows and reduce the number and pumping capacity of the
pumps required, some pump stations include relatively large sumps or detention basins.
An inventory of the pump stations indicated that there are nine such facilities in the
Valley with relatively large detention basins (Woodward Clyde Consultants 1990). The
design and operating philosophy of these systems is to: 1) attenuate the peak flow to
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reduce required pump size, and 2) drain the basins as soon as possible following the
storm so that flood capacity is available for subsequent storms.
An example of one of these pump stations is Sunnyvale Pump Station No. 2, located at
the junction of the Milpitas/Alviso Road (Route 237) and Calabazas Creek. The Pump
Station consists of four primary pumps, each rated at 1.1 m3/s (39 cfs) capacity, and
one auxiliary electric pump with a capacity 0.25 m3/s (9 cfs). The detention basin area is
approximately 1.8 ha (4.4 ac.) in area and has a capacity of approximately 37,005 m3
(30 ac-ft) as shown in Figure 5.3.a-1. It receives runoff from a 187.4 ha (463 acre)
catchment consisting of the following land uses: industrial park (30%), commercial
(10%), and residential (60%). There is a 2.13 m (7 ft) diameter concrete reinforced pipe
that drains into the basin. A second 0.91 m (3 ft) diameter line drains a 101.2 ha (250
acre) catchment (primarily open space) and bypasses the basin to the north and directly
enters the pump house.
Treatment Concepts and Retrofitting Objectives. The pump stations could provide an
opportunity to reduce nonpoint source loads entering the South Bay if they can be cost-
effectively retrofitted and maintained. The primary treatment is settling of particles.
Settling can be an effective treatment for some pollutants that are mostly in the
particulate fraction in stormwater. The particulate fraction of locally collected stormwater
exhibited a range of 36—94% (mean of 69%) for copper and 24—97% (mean of 66%)
for lead. These high particulate fractions allow sedimentation to be an effective control
practice.
The retrofitting scheme would increase the detention time to allow more particulates to
settle out into the basin while not significantly increasing the flood risk. A goal of this
retrofit was to prevent high flows from re-suspending previously settled sediments in the
detention pond. Scour protection was provided by having at least a two-foot deep
permanent wet pool during the wet weather season.
A 24—40 h hydraulic detention time for a pool several feet deep was found to be
necessary to effectively settle out most of the suspended sediment in the local
stormwater.
In all cases, the basin must maintain a relatively large flood control capacity and
associated outlet works and pumps in order to provide the necessary flood control
benefits.
Description of Sunnyvale Retrofit Activities.
Change Pump Operational Rules to Create a Permanent Pool and Temporary Storage.
In order to create the permanent pool in the pond, the pumps were set to turn off when
water levels in the basin (as measured at the outlet) dropped below 0.61 m (2 ft) In
order to create temporary storage, pump settings were adjusted to phase in (and out)
very slowly for depths between 0.61—0.73 m (2—2.4 ft). These operational conditions
created a temporary storage depth above the permanent pool of 0.12 m (0.4 ft) with a
capacity of 2158.6 m3 (1.75 ac-ft). Because this is an existing flood control facility, the
110
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5.3.a-1 No, 2
i |
!L
;J4!
~~ ;t"-\-.
111
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temporary storage depth was determined primarily based on flood control and
secondarily on water quality considerations. The temporary storage depth was the
maximum depth that would still allow the basin to pass the 100-year flood.
Prevent Short Circuiting. The pond contains a trapezoidal open pilot channel (2.44 m (8
ft) bottom width, 5.2 m (17 ft) top width, and 1.4 m (4.5 ft) depth) between the inlet and
outlet. In addition to this open channel, a 0.76 m (2.5 ft) reinforced concrete pipe (RCP)
is situated below the channel to convey low flows between the inlet and outlet. These
conveyances effectively "short circuited" flows between the inlet and outlet, a condition
that is highly unsuitable for water quality control.
In order to limit this short-circuiting, three modifications were made. At the outlet weir
near the pump house, a gabion baffle was constructed around the original outlet weir to
prevent short-circuiting of flows along the channel and to promote a better distribution of
flow from the basin into the outlet weir. A second modification involved placing rocks
into the channel near the inlet. The third modification involved covering the entrance of
the 0.76 m (2.5 ft) RCP with a steel plate and vertical riser that reduced the rate at
which flow would enter the drain below the trapezoidal channel.
Plug Storm Drain that Directly Entered Pump House. A 0.91 m (3 ft) RCP drained a
101.2 ha (250 acre) undeveloped area west of the detention basin directly to the pump
house sump. This pipe was plugged with sandbags in one of the manholes upstream of
the sump to prevent the runoff from this drainage area from mixing with outflow from the
detention basin in the sample collection area.
Problems Encountered. No significant problems were encountered during the structural
retrofitting of the detention basin. However, the pump control system needed major
repairs in order to operate the basin within the water level tolerances required for the
study. Specific problems were encountered with the liquid level sensors and transmitter
(inaccurate flow monitoring because of the very low flow rates) voltage instabilities
caused when certain pumps came on line, and fluctuations in the power supply.
Therefore, an important aspect in evaluating the feasibility of retrofitting pump stations is
the design and condition of the pump control system and the possible need for repairs
and upgrading.
Monitoring Program. The goal of this study was to measure the total runoff and collect
flow-weighted composite water samples at both the inlet and outlet of the detention
basin during and after storms in order to estimate pollutant removal performance.
Sediment samples were also taken to characterize basin sediments.
Station Design and Equipment. Automated flow and water quality monitoring stations
were located at the inlet and the outlet of the basin. The inlet pipe was a 2.13 m (7 ft)
diameter RCP that was quite low, placed at a level below the basin floor, and tended to
be full of water during most of the wet weather season. The inlet sampling station was
located 10.7 m (35 ft) upstream of the end of the pipe and consisted of a Druck
pressure transducer, velocity meter, ISCO Model 3700 automatic water quality sampler
and Campbell Scientific CR-10 data logger/controller. Flow volumes were estimated
112
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using the measured velocity times the wetted area of the pipe and the flow-weighted
composite samples were to be collected at the inlet based on these estimates. Initially,
a Montadero-Whitney electromagnetic velocity meter was used. However, the velocities
in the pipe were too low to measure with this instrument and it was replaced in March
1992 by a Detectonics I.S. Surveylogger, which relies on the Doppler effect and
suspended sediment passing the instrument. When compared with estimates of
anticipated runoff volumes, neither instrument appears to have measured flow velocities
in a consistent and accurate manner. The primary cause appears to be the relatively
low velocities in the large pipe.
The outlet sampling was conducted in the pump house where a Druck pressure
transducer, an ISCO Model 2700 automatic sampler, and CR-10 datalogger/controller
were installed. Sampling based on estimated flow through the pumps was planned.
These estimates were based on the pump run times and pump characteristic curves
(which show the relationship between flow and head for the design rpm of 700). To
achieve this, the data logger was connected to the pump house control panel to
determine pump run times and calculate discharge from the sump. Field visits during
the 1991—92 season revealed that the pumps did not operate at the design rpm,
especially during the warm-up period, resulting in inaccurate flow estimates much of the
time.
Sampling Methods. At the inlet, a pressure sensor was used to estimate the water level
in the detention basin. During each sampling event, flow was calculated as a product of
velocity and area by the CR-10 microprocessor. Based on the flow estimate (which was
generally poor), the CR-10 initiated and continued water quality sampling at pre-
specified flow intervals. During a sampling event, instantaneous velocity and pressure
were recorded each time a water quality sample was taken. Based on anticipated
rainfall, the sampling algorithm in the CR-10 was designed to instruct the water quality
sampler to collect twenty 500 ml sub-samples in a 10 liter borosilicate bottle over the
duration of the storm event. Following the sampling event, the pressure sensor was also
used to measure water level drops in the pond.
At the outlet, average hourly flowrate was estimated based on the pump run times and
the pump characteristic curves (also inaccurate), and was recorded over the duration of
the wet-weather season. To begin an event, field crews manually initiated the
automated samplers based on anticipated flow volumes for that storm. As with the inlet,
the automated samplers recorded instantaneous flow measurements when each
sample was collected.
Stations were visited prior to, during, and after monitored events to ice samples,
exchange sample bottles, and ensure proper equipment operation. Conductivity, pH,
and temperature were measured during the site visits.
Data Collected. Eight storm events were sampled. For six of these events, flow-
weighted composite water quality samples and hydrologic measurements were taken at
both inlet and outlet stations. In most cases, only partial flow measurements were made
113
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because equipment malfunctioned, threshold velocities were not achieved, or there
were problems with the pump control system.
Due to the uncertainty in flow volume measurements, pollutant loads were not used to
estimate treatment effectiveness. Instead, effectiveness was estimated based on the
flow composite water quality concentration data, assuming that the inlet and outlet
volumes for an event are equal.
Sediment samples were taken at three locations: in the center of the basin, near the
inlet, and near the outlet. Three sets of sediment samples were collected during dry
periods when the basin was empty, or nearly empty, of water (June 15, 1990, May 14,
1992, and on July 12, 1993). The first samples were obtained using a 10.2 cm (4 in)
stainless steel hand auger, while the other samples were collected by scraping the top
1.27 cm (0.5 in) of sediment with a Teflon™-lined scraper.
Flooding Analysis, Storm Hydrology, Water Quality and Sediment Monitoring Results
Flooding Analysis. Woodward Clyde used a reservoir routing model to estimate water
levels in the basin for the 100-year inflow event and for two pump operating scenarios.
The first pumping scenario corresponded to the original pumping schedule used for
flood control. The second scenario corresponded to the revised pumping schedule
appropriate for a multipurpose flood control and water quality control facility. Based on
the results of the model, the maximum water level in the basin for the 100-year flood did
not change by modifying the pump operation schedule.
Precipitation. Rainfall was measured with a tipping bucket, which registered the time
when the bucket collected 0.25 cm (0.1 in) of rainfall. The ranges of storm volumes
during the sampling period were from 1—5.6 cm (0.4—2.2 in) and the storm durations
ranged from 6—60 h. The Synoptic Rainfall Analysis Program (SYNOP) examined the
long period characteristics of the local historical rainfall data that was collected by the
NWS at the San Jose Airport (Gage No. 7821). The median event rainfall volume for the
San Jose Airport gage for the period from 1948—1989 was 1.27 cm (0.5 in).
Runoff. Flow measurements collected at the inlet and outlet for various events were
compared with rainfall to calculate the volumetric runoff coefficients. The flow
measurements at both the inlet and outlet stations were not considered very reliable, as
the measured runoff coefficients ranged from 0.1—1.89. Woodward Clyde estimated
that the actual values would be about 0.5—0.8 for these rains and watershed
characteristics.
Comparison of Inlet Water Quality to Other Santa Clara SW Monitoring Station Data.
Laboratory chemical analyses were conducted on the water samples collected at the
basin inlet and outlet stations during the six storm events. The median flow-weighted
composite concentrations of total metals (cadmium, chromium, copper, lead, nickel, and
zinc) from the inlet station are summarized in Table 5.3.a-1. The table shows median
concentrations obtained from other Santa Clara Valley SW monitoring stations
representing residential-commercial, industrial, and open land uses. The inlet
concentrations of copper, lead, nickel, and zinc are higher than concentrations from
114
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open land use, but lower than concentrations at residential-commercial and industrial
land use stations. The cadmium concentration appears to be very similar to the
residential-commercial land use, while the chromium concentration is closer to the open
land use.
Table 5.3.a-1. Comparison of Median Metal Concentrations at Inlet to Retrofitted Basin
to other Santa Clara Valley Stormwater Monitoring Station Data
Inlet to Retrofit Residential/Commercia Industrial Land Open Space Land
Metal Basin I Land Use Station Use Station Use Station
(n=6) (n=21) (n=25) (n=4)
(mg/l) (mg/l) (mg/l) (mg/l)
Cadmium
Chromium
Copper
Lead
Nickel
Zinc
1.1
12
24
38
21
180
1.0
16
33
45
30
240
3.9
24
51
91
46
1,150
0.3
11
11
2.0
5.0
5.0
*n= number of storm events
Pollutant Removal Effectiveness. Table 5.3.a-2 summarizes inlet and outlet
concentrations for total and dissolved metals (cadmium, chromium, copper, lead, nickel,
and zinc), SS, hardness and total oil and grease for several storm events. Based on
these data, pollutant reductions were estimated as the outlet minus inlet concentration
divided by the inlet concentration. The average pollutant removal effectiveness for the
metals ranged from about 30—50%. The metals removal data indicated that the
removal of total chromium, copper, lead, nickel and zinc were well correlated with SS
removal.
Comparison to Water Quality Objectives (WQOs). Of these metals, total and dissolved
chromium, lead and nickel did not exceed the acute WQOs. Total and dissolved
cadmium exceeded the WQO in only one storm out of six monitored storms. Total
copper at the inlet station exceeded WQOs in four out of six storm events. However,
concentrations at the outlet station never exceeded WQOs (though the outlet
concentration was essentially equal to the WQO for one event). None of the dissolved
copper concentrations exceeded the acute WQOs. Total zinc concentrations at the inlet
and outlet stations exceeded the acute WQOs for all six storms. Dissolved
concentrations of zinc at the outlet station exceeded the WQOs in three of the six
events.
Sediment Quality. The objectives of the sediment sampling was to characterize
sediment quality in the detention basin and to compare the sediment concentrations to
hazardous waste criteria. Results of these sediment samples are summarized in Table
5.3.a-3.
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Table 5.3.a-2. Inlet and Outlet Observed Concentrations and Pollutant Removals
SE17
Inlet
Outlet
Reduction
SE20
Inlet
Outlet
Reduction
SE21b
Inlet
Outlet
Reduction
SE23
Inlet
Outlet
Reduction
SE24
Inlet
Outlet
Reduction
SE27
Inlet
Outlet
Reduction
Average
Reduction
Cadmium
(l-ig/L)
nf | f
0.4 <0.2
0.2 <0.2
--
6.6 1.3
4.8 2.5
--
1.1 0.2
1.5 <0.2
--
1 0.2
0.6 <0.2
--
1.6 <0.2
1.3 0.2
--
1 0.5
0.6 0.4
--
--
Chromium
(l-ig/L)
nf | f
3.6 1.8
2.7 1.1
25% --
12 1
6 1
50% --
18 1
14 1
22% --
11 <1
8.3 1.4
25% --
21 1.1
15 8.6
29% --
6.3 1.4
4.9 1.7
22% --
29% --
Copper
(l-ig/L)
Nf
8.7
6.8
22%
24
9
63%
24
16
33%
27
12
56%
40
24
40%
14
8.9
36%
f
5.
4
4.
7
--
3
3
--
2
2
--
5
4.
7
--
2.
1
5
--
5.
4
4.
5
--
42% --
Lead
(^g/L)
nf | f
6.4 2.
2
3.4 1
47%
45 1
10 1
78%
53 <1
35 <1
34%
30 1
12 <1
60% --
76 <1
40 1.
4
47% --
13 <1
6.6 <1
49%
53%
Nickel
(l-ig/L)
nf | f
1.7 <2
1.7 <2
0%
16 1
4 1
75% --
25 <1
19 <1
24% --
13 3.
9
5.8 2.
2
55% --
42 9.
6
29 15
31% --
83 63
25 20
70% --
51% --
Zinc
(l-ig/L)
nf f
46 28
26 19
43% --
180 19
73 22
59% --
180 5
120 7
33% --
190 41
82 45
57% --
270 22
160 31
41% --
70 35
47 26
33% --
44% --
ss
(mg/L)
12
73
39%
90
24
73%
140
93
34%
74
31
58%
180
96
47%
30
15
50%
50%
TH
(mg/L)
97
120
--
110
63
--
--
--
--
100
90
--
140
140
--
110
220
--
--
O&G
1.5
1.4
7%
0.2
<0.2
--
--
--
--
0.7
0.5
--
0.6
3.5
--
1.6
1.3
--
--
nf: non-filtered (total)
f: filtered ("dissolved")
Removals are only given
SS: suspended solids
TH: total hardness, as CaCOs
if most observations were >PQL
O&G: oil and grease
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Table 5.3.a-3. Sediment Observations (mg/kg)
%TOC Cadmium Chromium Copper Iron Lead Manganese Nickel Zinc
6/15/90 -- 2.2 -- 92 -- 36 -- 61 320
core
5/14/92
SWface 3.8 23 200 150 49,000 280 610 94 750
Inlet
Middle 5.5 17 220 140 38,600 350 640 87 570
Outlet 1.9 35 140 47 47,700 18 680 76 260
7/12/93
SWface 2.4 1.0 170 110 34,000 260 560 96 220
Inlet
Middle 0.65 0.2 120 37 36,000 12 700 75 85
Outlet 0.93 0.3 110 43 30,000 24 570 73 63
TILC 100 2,500 2,500 -- 1,000 -- 2,000 5,00
0
TTLC: Total Threshold Limit Concentration
In the second and third rounds of sampling, the highest concentrations for copper,
nickel and zinc were found at the inlet station. Cadmium, chromium and lead were also
highest at the inlet station for the July 12, 1993 sampling round. The high concentration
of the majority of the metals near the inlet is consistent with other studies.
Average sediment concentrations observed in Pump Station No. 2 are compared in
Table 5.3.a-4 with sediment data collected from other detention basins in the Valley and
elsewhere. Results from the various basins differ substantially and indicate that
sediment quality is highly site specific and varies depending on soils, catchment land
use, and other factors, especially time when the samples were analyzed (for lead).
To evaluate whether the sediments were hazardous, concentrations were compared to
standards established in the California Administrative Code, Title 22. Under Title 22,
there are two criteria for designating solids as hazardous waste. The first criterion is that
the sediment concentrations not exceed the Total Threshold Limit Concentrations
(TTLC). The second criterion is that the extract obtained from the whole effluent toxicity
(WET) extraction method not exceed the Soluble Threshold Limit Concentrations
(STLC). For this pilot scale screening level of analysis, it was considered adequate to
compare with the TTLC only. In situations where disposal is being considered, the WET
extraction test should also be conducted.
None of the sediment sample concentrations collected in the Sunnyvale Pump Station
basin exceeded the TTLC. The highest concentrations of cadmium, lead, and zinc were
4, 3, and 7 times lower than the TTLC, respectively. The highest concentrations
reported for chromium, copper, and nickel were 11, 17 and 21 times lower than the
TTLC, respectively. Based on these concentrations, sediments are not considered
hazardous.
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Table 5.3.a-4. Comparison of Average Sediment Concentrations from Detention Basins
and Swales (mg/kg)
Detention Basin
This Retrofit Basin
Other Santa Clara County
Eastside Basin A
Easts! de Basin B
Eastside Basin C
River Oaks
Fresno NURP
Recharge F
Recharge G
Recharge M
Recharge EE
Recharge MM
Wigington (1983)
Bulk Mail Basin
Kmart Basin
Nightingale (1975)
Detention Basin
Special Pit
Wigington (1986)
Fairidge Swales
Stratton Woods Swales
Rte. 234 Rd. Swales
Cadmium
11.2
0.37
0.37
1
Nd
--
--
--
--
--
2.8
0.8
--
--
0.26
0.18
0.82
Copper
88
32
36
71
24
37
25
55
25
9.5
19
13
20
23
4.2
10
23
Nickel
80
36
40
100
72
32
37
53
22
11
--
--
--
--
--
--
-.
Lead
140
17
6
11
14
713
487
1333
297
93
112
368
224
801
42
18
936
Zinc
324
68
73
330
84
--
--
--
--
--
224
114
107
236
102
70
106
Cost Effectiveness Evaluation
The mean annual runoff volume of 433,000 m3 (351 ac-ft) was estimated based on
mean annual rainfall of 33 cm (13 in.) in the vicinity of the basin, an assumed runoff
coefficient of 0.7, and the 187.4 ha (463 ac.) catchment area. Mean concentrations and
removal efficiencies are averages of observed data. For the metals, annual load
reductions ranged from 272 grams (0.6 Ibs) for cadmium to 29.5 kg (65 Ibs) for zinc. For
copper, the annual load reduction is estimated at 4.1 kg (9 Ibs), which represents
approximately 40% of the total copper that enters the basin. Table 5.3.a-5 summarizes
the estimated cost effectiveness for the removal of heavy metals from the pond.
118
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Table 5.3.a-5. Estimated Mean Annual Load reduction and Cost Effectiveness*
METAL
Cadmium
Chromium
Copper
Lead
Nickel
Zinc
SS
Mean Inlet
Concentration
(mg/L)
0.002
0.012
0.023
0.037
0.038
0.156
87
Average Annual
Removal
Efficiency (%)
35
29
42
53
51
0.44
0.50
Load
Reduction
(kg/yr.)
0.3
1.5
4.2
8.2
8.2
29.5
18,600
Cost
Effectiveness
(kg/$1,000)
0.03
0.18
0.5
1.0
1.0
3.6
2,270
*Assuming an annual runoff volume of 433,000 m3 (351 ac-ft)
Solids Accumulation and Removal. About 18,600 kg (41,000 Ibs) of SS would be
collected annually in the retrofitted detention basin, which represents about one-half of
the annual input of solids. Assuming a specific gravity of about 1.5 for saturated
sediment collected in the basin, this would correspond to about 12.2 m3 (16 cyd) of
material annually. If uniformly distributed over the 1.78 ha (4.4 acre) basin, the mean
annual accumulation rate would be 0.08 cm per year (0.03 in. per year). Sediments are
expected to accumulate near the inlet and, in this specific case, in the pilot channel. In
ten years, this accumulation rate would equal about 123.3 m3 (0.1 ac-ft) compared with
the capacity of the basin that is 37,000 m3 (30 ac-ft). Therefore, this accumulation of
sediments does not pose a risk to reducing the flood control capacity of the basin.
Accumulation of at least 15.24 cm (6 in.) of sediment is required before removal is
practical. This amount of sediment may take as long as 10 or 20 years to accumulate.
Capital, Operation and Maintenance Costs. Capital and O&M costs were estimated for
the retrofitted pump station and are shown in Table 5.3.a-6. Costs were classified as
capital expenditures, O&M, and disposal. Capital costs for the structural retrofitting were
based on actual costs; whereas the costs for repair of the pump electronic control
systems were estimated. Operations and maintenance assumes 100 hours per year
labor in addition to that already being conducted to operate and maintain the facility for
flood control. Disposal costs assume disposal is conducted every 10 years and include
estimated future costs for landfill fees, trucking, and excavation. The total annualized
cost is therefore estimated to be $8,200 for the 187.4 ha (463 acre) watershed, or about
$7.3 per ha ($18 per acre) of watershed per year. The removal costs for copper were
estimated to be about 0.5 kg per $1,000 (1.1 Ibs per $1,000) that compares very
favorably with other stormwater control alternatives, e.g. copper removal via street
cleaning is approximately 0.7 kg per $1,000 (1.5 Ibs per $1,000) (Pitt 1979, 1985, and
1987).
119
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Table 5.3.a-6. Estimated Annualized Costs for Capital Expenditures and Operation
CATEGORY COST
1. Capital Expenditures
Structural retrofitting = $15,000
Amortized over 20 years at 8% $1,500
2. Operations and Maintenance
Inspection and repair (100 h @ $50/h) $5,000
3. Disposal (every 10 years)
Landfill (122 m3 @ $38/m3 $8,000
Trucking (16 trips @$75/h x 2 h/trip $2,400
Excavation (122 cyd @ $7.7/m3 $1,600
Amortized over 10 years at 8% $1,700
4. Total Cost per Year $20,200
Conclusions
Implications for Other Facilities. According to an inventory conducted by Woodward-
Clyde in 1990, nine of the existing 17 municipal pump stations in Santa Clara Valley are
designed with detention basins (rather than sumps) and are, therefore, suitable for
comparison with the pilot project. The detention basins range from 0.6—5.7 ha (1.5—14
ac.), with capacities of 5,550—182,556 m3 (4.5—148 ac-ft). The watershed area for
each pump station ranges from 10.1—405 ha (25—1,000 ac.) and the total watershed
area served by all nine stations is 1,725 ha (4,260 ac.) or 17 km2 (6.6 mi2). This is about
2% of the 900 km2 (350 mi2) area of the Santa Clara Valley below the upland reservoirs.
If we assume that other similar facilities could be retrofitted to achieve a performance
comparable to that measured at Pump Station No. 2, the net reduction in copper load to
the Bay would be about 45 kg (100 Ibs). This is only about 1% of the estimated mean
annual load of 6,350 kg (14,000 Ibs) of copper entering San Francisco Bay.
• A 100-year flood analysis indicated that modification of the pump schedule to
achieve water quality benefits did not increase the maximum 100-year elevation in
the pond.
• Based on measured inlet and outlet flow composite concentrations from 6 storm
events, the average pollutant removal efficiencies were: total chromium, 29%; total
copper, 42%; total lead, 53%; total nickel, 51%; total zinc, 44%; and total SS, 50%.
• The removal efficiencies for chromium, copper, lead, nickel and zinc correlated
well with SS removal, indicating that SS may be used as a surrogate parameter to
monitor effectiveness of metals removal in detention basins.
• Metal concentrations of basin sediments were generally highest at the inlet
location.
120
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• None of the sediment concentrations exceeded the TTLC standard, indicating that
the sediments are not hazardous.
• The estimated mean annual load reduction of metals ranged between 2.7—29.5
kg (0.6—65 Ibs), depending on the metal. The mean annual load reduction for
copper was 4.1 kg (9 Ibs)
• The amortized annual capital and O&M cost for retrofitting the Sunnyvale Pump
Station No. 2 is estimated at $8,200. The cost-effectiveness removal rate for
copper is 0.5 kg/$1,000 (1.1 lb/$1,000).
• Solids accumulation rates are very low and are estimated to be approximately 123.3
m3 (0.1 ac-ft) over 10 years. Given that the basin has a capacity of 37,000 m3 (30
ac-ft), increased deposition caused by retrofitting does not increase flood risk.
Implications for Management.
• The total watershed area in Santa Clara Valley served by the nine pump stations
with retention basins is approximately or 17 km2 (6.6 mi2) (only 2% of the total area
of the Valley downstream of the reservoirs). Thus, even if an improved treatment
performance could be obtained from these basins, the total load reduction to the Bay
would be minimal. For example, the load reduction of copper would only be 45.4 kg
(100 Ibs), which is less than 1% of the estimated mean copper load to the Bay.
• Since pump stations are relatively easy to retrofit, water quality benefits could be
achieved by simply changing the pumping schedule.
• If a retrofitting program is to be pursued, it would be important to ensure that the
pump control equipment is operational and well maintained, and that staff are well
trained in its use.
5.3.b. Monroe Street Detention Pond, Madison, Wisconsin
Introduction and Site Description
This case study considers the benefits of retrofitting the outlet works of an existing wet
detention pond. The original pond was creating severe downstream erosion in the
channels between the pond and the receiving water, and the pond storage volume was
not effectively being used for either flood control or water quality benefits. The outlets
were modified and the pond has undergone extensive monitoring to confirm the water
quality benefits of the retrofit.
The US Geological Survey (USGS), in cooperation with the Wisconsin Department of
Natural Resources (WDNR) investigated the Monroe St. wet detention pond located in
Madison, Wl (House, et a/. 1993). The University of Wisconsin originally constructed the
pond to protect the water quality and ecology of Lake Wingra and surrounding wetlands
from stormwater. The pond is located on the downstream side of Monroe Street at the
121
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outlet of a storm sewer that drains a 0.96 km2 (237 acre) urbanized area. Land use in
the watershed area consists mostly of single-family residences and commercial strip
development, with some institutional uses (i.e., schools and churches). The average
basin slope is 2.2%.
The Monroe Street pond has a surface area of 5,670 m2 (1.42 acre), a maximum depth
of 2.3 m (7.5 ft) and an average depth of 1.1 m (3.6 ft) at normal pool elevation. The
shape of the pond is round to oval with a small island. The inlet side is nearest to
Monroe Street and the two outlets are on the far side away from Monroe Street. The
pond has a surcharge storage volume above the normal pool elevation that is capable
of holding the 10-year, 24 h storm-runoff volume without overtopping the containment
berm around the pond. The pond has two outlets, each controlled by 90° V-notch weirs
that drain to channels leading to Lake Wingra. The weirs are located in 24 m (8 ft)
diameter concrete vaults, with 0.8 m (2.5 ft) concrete pipes leading to the pond. The
initial primary outlet configuration consisted of two 2.4 m (8 ft) long rectangular weirs
located in the vaults, made with concrete block walls. The original flow capacity of these
two weirs was enormous, being about 1.4 m3/s (50 cfs) at 0.3 m (1 ft) head and 7 m3/s
(250 cfs) at 0.9 m (3 ft) of head. The outlets in the pond are submerged and the bottom
of the pond consists of a clay layer that inhibits infiltration of water from or into the pond.
As noted above, the discharges from the pond were little attenuated from the inflow
velocities and severe channel erosion was occurring in the wetlands, negating the
sediment trapping benefits of the pond. The emergency spillway appeared to never
have been used since construction, even with several massive storms. In fact, the pond
elevation barely fluctuated. The water quality benefits of the existing dry pond were
assumed to be negligible.
The outlets were therefore modified in 1995 to reduce the downstream erosion
problems by removing several courses of concrete blocks and installing 90° V-notch
weirs made of plate steel in each vault. The pond normal water level was dropped
about 15.3 cm (6 in.) with a lowered invert. The new primary outlets have total flow
capacities of about 0.14 m3/s (5 cfs) at 0.3 m (1 ft) head and 2.3 m3/s (80 cfs) at 0.9 m
(3 ft) head. The pond surface fluctuates more now, and the emergency spillway has
been active every few years. Most significantly, the downstream channels are now
stable.
The pond was designed for an expected 90% event mean concentration (EMC) removal
for SS (particulate residue). The ratio of pond to drainage area is 0.6%. This
percentage is close to the value (0.4—0.8%) required for 5 micron control, which
generally corresponds to a 90% reduction of SS.
The Monroe Street pond may have more water quality monitoring data than any pond in
the country. A total of 64 events were extensively monitored between February 1987
and April 1988. The monitored rains varied from 2 to more than 82 mm during this
period. Periodic water quality and flow monitoring has also continued at this pond since
1988.
122
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Method of Investigation
Water-quality data were collected by the U.S. Geological Survey (House, et al. 1993)
using programmable automatic refrigerated water samplers installed at the inflow and
outflow sites of the pond. The outflow data was collected at two locations, east and
west. The samplers were programmed to obtain flow-proportional samples for each
storm. These samples represent the flow-averaged constituent concentrations during a
runoff event. These samples were removed from the samplers, preserved, and shipped
to the Denver USGS laboratory for analysis within 24 h of being collected. The samples
were analyzed for SS, volatile total solids, total and dissolved chemical oxygen demand
(COD), total chloride, total and dissolved phosphorus, phosphate total and dissolved
forms of TKN, nitrates, and total and dissolved forms of copper, zinc, and lead.
Precipitation data were also recorded at 5 min intervals during the storm events using a
recording rain gage located at the pond site. Storm runoff (pond inflow) was monitored
at the box culvert that was the terminus of the 0.96 km2 drainage area. Discharge rates
and flow volumes passing through the culvert were determined by use of a flow velocity
sensor and water level indicator installed inside the culvert. The velocity and depth
sensors were connected to a data logger that recorded the water level and velocity data
and computed discharge rates based on the culvert geometry.
Data Analysis and Observations
The pond inlet and outlet pollutant concentrations were analyzed to determine the
pollutant reduction. Various relationships between inlet and outlet concentrations were
investigated by statistical analysis, including Particulate Pollutant Strength (PPS) and
percent controls. Each statistical process is described in the following paragraphs. The
basic data are contained in the USGS report. (House, et al. 1993).
Hydrograph/Flow Calibration. An important part of the Monroe Street project was
validating the DETPOND wet detention pond water quality model that was used to
design the retrofit of the outlet structures (Pitt and Voorhees 1995). The first step in the
validation was to check flow volumes and peak flow rates, and the complete
hydrographs.
Fifteen storm events were used to validate the flow portions of the DETPOND program.
The program predicted outflow flow values from the inflow hydrographs using the
modified pulse routing method. The outfall predictions (at five-minute intervals) were
compared to the observed outfall flow values. The predicted outflow hydrographs very
closely matched the corresponding observed outflow hydrographs. In addition to
comparing the general shape of the discharge hydrographs, the outflow total discharge
volume, peak discharge flow rate, SS removal, and outflow particle size distribution
were also compared for validation. The predicted outflow volumes and peak discharges
very closely matched the observed outflow conditions.
Observed Influent and Effluent Pollutant Concentrations. Table 5.3.b-1 lists the influent
and effluent conditions observed at the pond. The number of observations, and the
mean, maximum, and average concentrations are shown. The standard deviation (Std.
123
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Dev.) and coefficient of variation (COV) are given. The Mann Whitney a values, which
indicates whether the influent and effluent values are statistically significantly different,
are also summarized.
Table 5.3.D-1. Summary of Observed Influent and Effluent Pollutant Concentrations at
Monroe St. Pond
RESIDUE
Number of
Observations
Minimum
Max.
Mean
Std.
Dev.
COV
*
Mann
Whitney
Total Solid (mg/L)
IN
OUT
64
57
76
63
5810
1180
526
276
816
259
1.55
0.94
0.0003
Suspended Solid (mg/L)
IN
OUT
64
57
21
4
1330
140
230
32
257
28
1.11
0.87
0.0000
Filtered Residue (mg/L)
IN
OUT
64
57
303
244
789
244
2.6
1.0
0.0758
Volatile Residue (mg/L)
IN
OUT
62
55
28
0
376
232
92
48
63
30
0.68
0.63
0.0000
Total Chlorides
IN
OUT
Number of
Observations
65
57
Minimum
0.8
2.1
Max.
3100
570
Mean
120
90.9
Std.
Dev.
430
135.5
COV
3.58
1.49
Mann
Whitney
.0032
COD
Total COD, (mg/L)
IN
OUT
63
57
21
16
370
350
90
37
63.4
44
0.70
1.2
0.000
Particulate COD, (mg/L)
IN
OUT
56
52
8
-3
327
281
53
16
53
38.5
1.0
2.4
0.000
Filtered COD, (mg/L)
IN
OUT
56
52
10
9
160
69
38
21.4
32.3
11
0.85
0.51
0.0019
Coefficient of Variation - standard deviation divided by the mean.
124
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Table 5.3.b-1. Summary of Observed Influent and Effluent Pollutant Concentrations at
Monroe St. Pond, Continued
RESIDUE
Number of
Observations
Minimum
Max.
Mean
Std.
Dev.
COV
Mann
Whitney
PHOSPHORUS
Total Phosphorus, (mg/L)
IN
OUT
65
59
0.16
0.13
1.7
1.6
0.56
0.26
0.39
0.19
0.69
0.73
0.000
Particulate Phosphorus, (mg/L)
IN
OUT
60
53
0.08
0.04
1.53
1.52
0.33
0.16
0.27
0.2
0.82
1.25
0.000
Filtered Phosphorus, (mg/L)
IN
OUT
60
53
0.06
0.02
1.33
0.27
0.23
0.1
0.28
0.06
1.2
0.6
0.0001
PHOSPHATES
Total Phosphates, (mg/L)
IN
OUT
56
51
0.026
0.004
1.22
0.25
0.2
0.08
0.27
0.06
1.35
0.75
0.0002
COPPER
Total Copper, (i-ig/L)
IN
OUT
64
57
10
3.5
130
60
50
46
14
12.43
0.28
0.27
0.592
Particulate Copper, (ug/L)
IN
OUT
60
52
7
.5
120
49
43
42.3
13.93
12.02
0.32
0.28
0.0652
Filtered Copper, (ug/L)
IN
OUT
60
53
3
3
16
13.5
6.35
4.5
3.32
2.17
0.52
0.48
0.0035
LEAD
Total Lead, (ug/L)
IN
OUT
64
57
18
4
420
100
85
68
52.4
39
0.61
0.57
0.100
Particulate Lead, (ug/L)
IN
OUT
59
52
15
1
417
97
83
66
54
39
0.65
0.59
0.1800
125
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Table 5.3.b-1. Summary of Observed Influent and Effluent Pollutant Concentrations at
Monroe St. Pond, Continued
RESIDUE
Number of
Observations
Minimum
Max.
Mean
Std.
Dev.
COV
Mann
Whitney
Filtered Lead, (ug/L)
IN
OUT
59
53
3
3
12
4
3.5
3.0
1.7
0.17
0.49
0.06
0.54
ZINC
Total Zinc, (ug/L)
IN
OUT
64
57
35
30
1100
1400
152
191
136
265
0.89
1.39
0.0450
Particulate Zinc, (ug/L)
IN
OUT
58
47
-3.0
-15.5
980
630
103.5
60.3
131
101
1.26
1.68
0.0000
Filtered Zinc, (ug/L)
IN
OUT
57
48
9.0
20.0
160.0
1218
51
150
34
215
0.66
1.43
0.0020
The coefficient of variation (COV) is calculated by dividing the standard deviation by the
mean and normalizes the standard deviation. A high value indicates that the data
spread is wide, requiring many data observations to obtain a precise estimate of the
EMC.
COV varied from 3.58 for inlet chlorides, which may not be significant due to seasonal
variations, to 0.06 for outlet filtered lead. All filtered lead had lower COV values which
indicated small concentration variations for samples.
The Mann-Whitney test is a non-parametric analysis comparing two sets of data. The
null hypothesis used in the Mann-Whitney is inlet pollutant concentration minus outlet
pollutant concentration equals zero. Generally, an a value < 0.05 indicates that the
sample sets are from two different populations (significantly different at the 95%
confidence level). The following constituents had significant a values indicating that the
pond significantly affected the concentrations:
Total solids
Suspended solids
Volatile solids
Chlorides
COD (all forms)
Phosphorus (all forms)
Phosphate
TKN (total and particulate)
Nitrate
Copper (filtered)
Zinc (all forms)
126
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Particulate Pollutant Strength. PPS is the ratio of a participate pollutant concentration
to the suspended solid concentration expressed in mg/kg. PPS was calculated for each
pollutant with a particulate form. The small particles in stormwater have greater PPS
values than the large particles. Therefore, all pollutants had higher outlet than inlet PPS
values due to preferential removals of large particles in the detention pond, leaving
relatively more small particles in the discharge water. A wide difference between influent
and effluent PPS values indicates that the main components of the contaminant (such
as for TKN and phosphorus) are associated with fine particles.
Control of Pollutants. The reduction of pollutants was calculated from the difference in
pollutant concentrations in the inlet and outlet water for each event, as shown on Table
5.3.b-2. As expected, control was higher for all particulate forms of the constituents
than for filtered forms, except for copper. Filtered constituents (<0.45 |j,m) have
characteristics like colloidal particles and will tend to be transported through the wet
detention pond relatively unchanged. A well designed wet detention pond will remove
70—90% of SS, 70% of COD, 60—70% of nutrients and 60—95% of the particulate
forms of the heavy metals.
Table 5.3.b-2. Summary Table of Pollutant Control*
Parameter 10% 50% 90%
Suspended solids 35 87 97
Total Residue <0 52 86
Volatile Residue <0 41 76
Filtered Residue <0 <0 56
Particulate COD 15 80 95
Total COD 29 60 84
Filtered COD <0 24 80
Particulate -20 60 80
Phosphorus
Total Phosphorus <0 47 81
Filtered Phosphorus <0 43 83
Particulate TKN -40 40 80
Total TKN <0 45 75
Filtered TKN <0 12 68
Particulate Zinc -117 70 95
Total Zinc <0 31 69
Filtered Zinc <0 <0 59
* Copper and Lead observations were mostly below the detection limits
and are therefore not shown.
127
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Particle Size Distributions and Short-Circuiting
Seven events were studied to find the short-circuiting "n" factors using observed and
predicted particle size distributions in effluent water. Particle size distributions were
measured using the Sedigraph method at the USGS Denver laboratory. This technique
measures settling rates of different size suspended solid particulates down to 2 microns.
The value of n is calculated using the concentrations of large particles that are found in
the effluent. In ideal settling, no particles greater than the theoretical critical size (about
5 |j,m for Monroe Street) should appear in the effluent. However, there is always a small
number of these larger particles in the effluent. It is generally assumed that short-
circuiting is responsible for these large particles. The measured values for n were one,
or less, indicating a high degree of short-circuiting in the pond. However, these
observations were possibly affected by scour of bottom deposits near the subsurface
effluent pipes. The maximum effect of short-circuiting on pond performance is shown in
Table 5.3.b-3, showing the average reduction in SS removals for different n values,
compared to the best performance (n value equal to 8):
Table 5.3.b-3. Average Suspended Solids Removal for variable n values for the
Monroe
St. Ponds
n value % SS removal Reduction in % SS
(average) removal compared to n=8
8
3
1
0.5
0.2
85
84
80.7
78.5
59
1
4.3
6.5
26
The median value of n observed was about 0.35, indicating a reduction in SS capture
efficiency of no more than about 10%. The effects of this short-circuiting, has a minimal
effect on the SS percentage removals. The Monroe Street pond provided an average
SS reduction of 87%, compared to the design goal of 90%. The small difference (3%)
between actual and design SS reduction indicates that the short-circuiting has a
negligible effect on actual performance.
Monroe St. Pond Retrofitting Conclusions
The retrofitting of the Monroe Street wet detention pond was very successful. Changing
the outlet structures from large rectangular weirs to V-notch weirs significantly reduced
effluent flows, reduced downstream channel erosion and improved water quality
benefits of the pond.
All effluent concentrations were lower than associated influent concentrations, except
for chlorides. The measured short-circuiting factor indicated a severe short-circuiting
problem, but that could be a false indication due to minor scour near the effluent works
128
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in the pond. Data suggest that short circuiting does not adversely impact treatment
efficiency. The Monroe Street pond is meeting all reasonable expectations in both
downstream channel protection and in contaminant capture.
5.3.c. Birmingham, Alabama Dry-Ponds
This retrofit case study examines a dry detention pond at a new apartment complex in
Birmingham, Alabama. This is an example of a typical small pond installed to meet local
regulations for peak flow control. The pond is not even expected to operate satisfactory
even as a peak flow rate pond based on review of the current hydrological assessment.
This analysis makes suggestions for retrofitting this pond to a wet pond and to improve
the overall benefits of the device. It also demonstrates how a combination of water
quality and flow analysis computer programs can be used together to evaluate and
design effective multiple benefit SW controls. The water quality benefits of the existing
dry pond were assumed to be negligible for this analysis.
Redesign of Dry Pond for Water Quality Benefits
There are a multitude of options for retrofitting the existing dry pond: excavation below
the outlet, raising the outlet, or some combination of the two. However, since relocating
and/or reconstructing the existing outlet structures to gain additional water volume
below the outlet would be quite expensive we will only consider excavating below the
existing outlet bcated at 219.5 m (720 ft) mean sea level (msl). This redesign will only
require excavation within the existing space with a minimum of re-grading.
There are several factors to consider when designing a wet pond, among these are
depth, safety, and storage criteria. Pitt (1998) recommends 0.9—1.8 m (3—6 ft) of
storage below the invert of the lowest outlet, 219.5 m (720 ft). In addition, safety
requires a shallow ledge, submerged 0.3 m (1 ft) below the permanent storage area.
The redesign presented here will include the preferred depth guideline of 1.8 m (6 ft),
requiring excavation from 219.5—217.6 m (720—714 ft), and a minimal three-foot ledge
at 219.2 m (719 ft) to match existing grades. The side slope will be 1:1 from 219.5 m
(720 ft) to the ledge at 219.2 m (719 ft), and also 1:1 from the interior of the ledge to
217.6 m (714 ft). Assuming a prismatic cross-section, the additional wet storage to be
constructed below the 219.5 m (720 ft) of elevation is approximately 250 m3 (0.204 ac-
ft).
Safety Criteria. Pitt (1998) recommends a gradual slope, 1:1 to 1:4, on the approach to
the pond's normal high water mark, 221.6 m (727 ft) in this case. The existing pond
may be dangerous, as the side slope is approximately 1:2 and quite long about 23 m
(75 ft). As an additional safeguard, a barrier should be constructed to prevent access to
the pond, preferably some type of barrier vegetation. This apartment complex is a new
development with the majority of the construction occurring in 1997. Therefore, some of
the landscaping is not complete as yet. There are some shrubs and small trees planted
around the perimeter, however these do not completely surround the pond. In fact, the
western slope, which is the longest and steepest approach to the pond, is unfortunately
the area that does not have barrier vegetation. Since the stormwater runoff is not
129
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planned for water contact activities because of high levels of pollutants, e.g. bacteria,
some proactive effort should be made to discourage the public from making contact with
the water.
Peak Reduction Factors (PRF). The dry pond barely reduces the peak outflow rates
from the contributing area. The 100-year storm's runoff rate is expected to be reduced
from 4.3 m3/s (153 cfs) to about 4.1 m3/s (145 cfs) by constriction at the pond's outlet
structures and by storage in the pond.
PRF = 1-Qo/Qi. (10)
Where: Q0 = effluent
Qi = influent
The PRF for this event is only 0.05, corresponding to a 5% reduction of the inflow
hydrograph in the pond. For the 50-year and 25-year storms, the PRFs are 0.06 and
0.07, respectively. This pond is likely inadequate for any measurable peak flow
reduction. The pond also likely exceeds the maximum stage of 233.4 m (733 ft). This is
especially a problem because the water coming through the spillway flows directly onto
a busy street with high speed traffic.
Upflow and Critical Settling Velocities. The most important rain events to control for
water quality improvement are the small events that occur with great frequency, or
those that produce less than approximately 3.18 cm (1.25 in.) of runoff. For the smaller
rain events, the DETPOND simulations demonstrated that the retrofitted pond would
satisfy the maximum up flow velocity (or critical settling velocity) maximum of 0.0004
mm/s (0.00013 fps) which is necessary for 5 micron particle control. Analyses also
indicated that removal of 90% of SS was easily obtained on all but the largest rain
events. The analyses using DETPOND and the 1976 Birmingham rain file (a typical
year) showed that the annual average suspended control was 86%.
Pond's Water Quality Storage. A reasonable goal is to have the pond's water quality
storage be equal to the runoff associated with 3.18 cm (1.25 in.) of rain for the land use,
development, and cover of the watershed served by the pond (Pitt 1998). The
composite CN for the watershed was estimated to be 75. This corresponds to
approximately 1 cm (0.40 in) of runoff, from a storm of 3.18 cm (1.25 in.) (Pitt 1987).
Therefore the minimum active pond storage (between the invert elevation of the lowest
outlet and the secondary outlet discharge devices) required should be a least 1727 m3
(1.4 ac-ft). Since the retrofitted pond's water quality storage is about 826 m3 (0.67 ac-
ft), this requirement is not met.
Pond's Surface Area Requirements. A pond's surface area should be sized as a percent
of watershed's area based on land use, development, cover, and the particle size
control desired. The apartment site has residential and woodland land uses. Pitt (1998)
recommends 0.8% of the watershed area for 5 micron particle control in a commercial
area (based on NURP, National Urban Runoff Program, studies). Therefore the
130
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minimum surface area of the pond should be at least 0.13 ha (0.33 ac.), which is close
to the normal water surface area of the retrofitted pond of .125 ha (0.31 ac.).
Possible Problems. Experience and observations of similar ponds in this region of the
country have shown that the shallow areas near the shore of the retrofitted pond might
become a breeding area for mosquitoes. The shallow shoreline might allow too much
aquatic vegetation to grow into the pond's interior area causing congestion and
periodically dying and subsequently increasing the BOD. In addition, the aquatic plants
located on the shallow shelf could grow in and around the outlet and reduce its output
capacity. Reptiles (including snakes), waterfowl, rodents (i.e., muskrats) might become
too plentiful in the pond area and become nuisances. Decaying vegetation may result in
strong odors in the summer, and these odors might have a negative impact on
residents.
Background Information Related to Site Evaluation
The peak inflow hydrograph values were determined by HydroCAD's soil conservation
service (SCS) TR-20 methodology. For the site, a SCS Type III rainfall intensity
distribution frequency (IDF) curve was selected. Rainfall depths for the 100-year, 50-
year, and 25-year storms were approximately 22 cm (8.6 in.), 20 cm (7.8 in.), and 18 cm
(7.1 in.) respectively. Time of concentration for the watershed was also calculated
using HydroCAD's built-in TR-20 methods; Tc = 24.3 min for the apartment complex
area, and Tc = 33.8 for the Woodland area.
Land Use, Development, Cover, Soils Type, and CNs. SCS soil maps for the area were
examined, and it was determined that the site consisted of Nauvoo-Sunlight complex,
with 15—25% slopes, and Townley silt loam, with 12—18% slopes. The SCS
Hydrologic Soil Groups for these soils are type B and type C respectively. Research
conducted at the University of Alabama Birmingham has shown that development, due
to construction disturbances, compaction, and soil mixing, can significantly reduce the
actual infiltration rates from those assumed. Therefore, the CN assigned to the
developed area was for the worst case, type D soil. However, the undeveloped
woodland area, mostly Nauvoo soil, was assigned a CN based upon the type B soil.
Therefore for the developed area of 10.5 ha (26.4 ac.), a composite CN of 87 was
assigned. This composite CN of 87 is a weighted average at the sub-basin CNs. The
basins include 6.5 ha (16 ac.) of residential land use with one-eighth-acre lots, SCS soil
type D, and 65% impervious cover (CN = 92), plus the remaining 4.2 ha (10.4 ac.) of
open lawns with good grass cover (> 75%), and type D soil (CN = 80). For the
woodland area of 6.1 ha (15 ac.), a CN of 55 was assigned, corresponding to woods
with good hydrologic condition and type B soils.
Analysis of Design Storms
HydroCAD™. The HydroCAD Stormwater Modeling System (version 4.53) was used to
analyze the pond for hydraulic characteristics. This computer software allows for
calculating hydrographs, based upon design storm and watershed characteristics, and
then routing these through a drainage system composed of subcatchments, reaches,
131
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and ponds. The program does not consider the dead storage below the first outlet,
assuming that this is always full of water, therefore the flow characteristics of both the
existing pond and the retrofitted pond are identical.
The subcatchment component is used to model a given drainage area or watershed. In
this case, there are two subcatchments: subcatchment 1 refers to the 10.5 ha (26.4 ac.)
of the apartment complex, and subcatchment 2 consists of the 6.1 ha (15 ac.) of
woodland area that drains to the complex. The program uses built-in SCS TR-20
hydrology methods for determining the hydrograph characteristics. Next, the
hydrograph is routed through a series of defined reaches and/or ponds. In this case,
there are two hydrographs, one from each of the two subcatchments, which are routed
through a single pond.
The pond component of this model is described using a stage v. surface area curve,
shown in Figure 5.3.C-1 and Table 5.3.c-1. In addition, the model requires descriptions
of the outlet structures.
Elevation v. Surface Area
Stonecrest Dry Detention Pond
0.32
0.28
0.08
0.04
720 721 722 723 724 725 726 727 728 729 730 731 732 733
Water Surface Elevation (ft)
Figure 5.3.C-1. Stage v. Surface Area Curve.
132
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Table 5.3.C-1. Outlet Device Descriptions
#
Route
Invert
Outlet Device
1
Primary 219.5 m (720ft)
2
3
to#1
to#1
219.5m (720 ft)
221.6 m (727 ft)
Secondary 223.1 m (732ft)
61 cm (24 in) culvert
n=0.013, length=56.4 m (185ft),
slope=0.02 m/m, Ke=0.5, Cc=0.9
30.5 cm (12 in) orifice
sharp-crested rectangular weir
length = 3.7m (12 ft)
height = 20.3 cm (8 in)
(square concrete box with cap)
3 m (10 ft) broad-crested
rectangular weir emergency
spillway
Ke = 0.5 represents the entrance loss for a flush entrance
Cc = 0.9 represents the coefficient of discharge for the culvert
The HydroCAD simulations were run for three, 24 h, SCS Type III design storm
frequencies: 25-year 18 cm (7.1 in.), 50-year 20 cm (7.8 in.), and 100-year 22 cm (8.6
in.). Table 5.3.C-2 summarizes the model's output for these three storms, showing the
hydrographs peaks and volumes. As expected, an examination of the last two columns
shows that the most significant contribution to the hydrograph flowing into the pond
comes from the apartment complex area. The surface area of the complex is larger
than that of the woodlands and land use, soil characteristics, and other hydrologic
characteristics of the complex all contribute to generating much more and rapid runoff
per unit area.
Table 5.3.C-2. Subcatchment Summaries for Design Storms
Subcatch- Description Design Storm Rainfall Peak Flow
ment No. Frequency (yr) (cm) (in) (m3/s) (cfs)
1
2
1
2
1
2
apartment
complex
woodland
apartment
complex
woodland
apartment
complex
woodland
25
25
50
50
100
100
18
18
20
20
22
22
7.1
7.1
7.8
7.8
8.6
8.6
2.91
0.54
3.24
0.67
3.62
0.82
102.6
19.13
114.4
23.59
127.9
28.94
Volume
(m3) (ac-ft)
13,310
3,035
14,850
3,690
16,615
4,490
10.79
2.46
12.04
2.99
13.47
3.64
Next the hydrographs were routed through the pond. Table 5.3.C-3 summarizes the
results. As previously mentioned, the peak reductions are quite low (5—7%), and the
peak discharge lag is only a few minutes. The predicted peak water surface elevation in
the pond is higher than the maximum elevation in the pond, 223 m (733 ft) (column 3),
133
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indicating flooding. This is a dangerous situation because it means that the water is
flowing uncontrollably over the dam. This could damage the emergency spill way and
cause erosion of the dam itself. These events occur even at the lowest storm frequency
modeled 25-year. It would appear that the pond is inadequate for the amount of runoff
generated by these storms.
Table 5.3.C-3. Pond Results of HydroCAD simulations
Design Peak Elev.** Peak Storage Peak Q jn Peak Q out Peak Q out|ett Peak Q ^gmt Atten. Lag
Event* Time
(m) (ft) rn3 ac_ft. m3/s cfs m 3/s cfs m3/s cfs m 3/s cfs % (min)
25-yr. 233.5 733.4 2,603 2.11 36.1 118.3 33.6 110.2 15.4 50.5 18.2 59.7 7 8.1
50-yr. 233.7 733.8 2,726 2.21
134.2
38.4
126.1
51
75.2
100-yr. 223.£
734.1 2,850 2.31
* Design storms are type III 24 h for Shelby County (SCS methods).
** Flood elevation is at 223.4 m (733 ft).
f Peak flow through the first and second outlets to a 0.6 m (2 ft) diameter culvert.
t Peak flow in the emergency spillway (flowing onto Bowling Drive).
DETPOND Modeling. DETPOND generally uses a simplified triangular hydrograph for
individual storms in long-term simulations (several decades) for water quality
evaluations, so this model is not suitable for modeling synthetic rainfall distributions,
such as those appropriate for design storms. However, HydroCAD is capable of these
types of simulations, and one can input the hydrograph generated by HydroCAD into
DETPOND using the "user defined hydrograph" option. Therefore, the 25-year, 50-year,
and 100-year hydrographs generated using HydroCAD's TR-20 methods were input,
and these results are presented in the Table 5.3.C-4. A comparison of the hydraulic
results from HydroCAD (Table 5.3.C-4) shows that these results are similar. For
instance during the 25-year event, HydroCAD calculated a maximum flow rate into the
pond of 36.1 rrfVs (118.3 cfs), while DETPOND's calculations resulted in 36.0 m3/s
(118.0 cfs). Even under these worst-case conditions, the pond is removing
approximately 50% of the SS.
134
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Table 5.3.C-4. DETPOND Summary for Design Storms
Design
Event
25 yr
50 yr
100 yr
Max.
(m)
223.6
223.7
223.9
Stage
(ft)
733.5
734.0
734.5
Max. Inflow
(m3/s) (cfs)
36.0
40.9
46.5
118.0
134.2
152.6
Max. Outflow Max particle
(m3/s) (cfs) size discharged
(Mm)
30.5
36.0
41.6
100.1
118.0
136.6
95
95
95
Avg. Min Particle
Size Controlled
(Mm)
26.
28.
29.
,5
,1
,7
%SS
Removed
50,
48,
46,
.8
.6
.7
Analysis Using True Rainfall Characteristics
DETPOND evaluates long-term continuous simulations using many years of actual
rainfall data. Rain files are created by determining the rain depth, rain duration, and
interevent duration for multiple storms. DETPOND then routes a simple, triangular
hydrograph for each event, and routes it through the pond to evaluate the SS removal.
DETPOND simulations were conducted using rain files created from the 1976
monitoring data, and also the 1952 through 1989 rain record. The 1976 file is
representative of a typical year, and was used to enable more model runs because of
shorter run time. The larger file was then run to continuously evaluate the pond based
on almost forty years of actual storm data. There are only 23 events, out of a total of
4,107 in the large 40 year rain file in which the pond stage rises to the level of the
second outlet. All of the DETPOND simulations were conducted using the retrofitted
pond, as water quality benefits in the existing dry pond were assumed to be negligible.
The results of the simulations using the 1976 file are presented in Table 5.3.c-5.
Table 5.3.c-5. Water Quality Output Summary for 1976 Rain File
Inter- Max Pond Flow- Approx.
Rain Depth Rain nEvent Rain Intensity Stage wnei^d Pnart' Peak
Duration DuratlO ,m> lm PaitlCle ReS. D-Hnrtinn
Statistic (cm) (in) Duaton n (ds) (cm/h) (m) (ft> size Control* ^f
(h) (in/h) Factor
Mean 1.27 0.50 12.01 1.81 0.10 0.04 1.94 6.30 4.26 86.3% 0.07
Std.Dv. 1.91 0.75 10.77 2.36 0.15 0.06 0.16 0.51 4.23 13.5% 0.07
COV 3.84 1.51 0.90 1.30 3.76 1.48 0.03 0.08 0.99 0.2% 1.00
Minimu 0.03 0.01 1.00 0.00 0.00 0.00 1.84 6.00 0.00 57.3% 0.01
Event
Flushing
Ratio**
1.75
2.41
1.37
0.00
Max. 9.75 3.84 45 11.68 0.79 0.31 2.73 8.84 15.70 100% 0.31 9.34
* Approximate Particle Residue Control (SS).
* * Event flushing ratio - the storm runoff volume divided by the watervolume contained in the pond at the beginning
of the event.
The control for the 5//m particles corresponds to approximately 90% SS control based
on the mean values of the columns of Flow Weighted Particle Size and Approximate
135
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Particle Control. On average, in a typical year, the pond will collect particle sizes 4.26
microns and greater in size. The average rain depth is 1.27 cm (0.5 in.), and the
average duration is 12 h. For the smallest storms, the pond is achieving greater than
95% control, and for the largest storm in 1976, the SS control is still 57%.
Figure 5.3.C-2 shows the maximum pond stage. Axis labels denote the elevation above
the pond bottom (1.8 m [6 ft] corresponds to 219.5 m [720 ft] msl elevation, the invert of
the first outlet device) versus the percent particle control. There is an expected trend;
the control decreases with increasing stage.
Redesigned Pond, Bham76
o
1:
o
O
a>
u
uu m •• •
<
/b
1
/U
I
• V
•
1 £
1 £
•
•_
* •
»
•
6.5
7 7.5 8
Maximum Pond Stage (ft above bottom)
8.5
Figure 5.3.C-2. Pond Stage v. Particle Residue Control
Figure 5.3.C-3 shows the water quality performance of the retrofitted pond (percent
particulate control) versus the rain depth in inches. There is considerably more scatter
in this plot, but one can still observe a trend. Generally, control decreases as rain depth
increases, as expected. The results are similar for Figure 5.3.C-4 depicting control
versus rain intensity.
136
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Redesigned Pond, Bham76
IUUW -
r
95
90 "
5= 85 •
"o
0
o <
0) _c .
o 75
1 .
65 "
• •
•• •
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'.
, v"v
• •
•
•
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•
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• •
• •
•
f
• •
• '
•
•
, •
1
•
•
•
• •.
• •
• :
• •
0.5
1.5 2 2.5
Rain Depth (in)
Figure 5.3.c-3. Rain Depth v. Particle Residue Control
Redesigned Pond, Bham76
3.5
1001
95 '
IS 85~
"o
o
0 <
«
ra «
60 '
1
>:
• *
: -
i,;
• •
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. I
<
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<
•
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"l.
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•
i
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•
•
•
0.05 0.1 0.15 0.2 0.25 0.3 0.35
Rain Intensity (in/hr)
Figure 5.3.c-4. Rain Intensity v. Particle Residue Control
137
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Long-term Simulation of 1952-1989 Rains
Table 5.3.C-6 shows the statistics for the 4,107 rain events that occurred in Birmingham
from 1952-1989. Notice that the minimum and maximum values are different than those
from the 1976 simulations, but the mean values are quite similar. The most important
value in this table is the mean particle control, 80%. For 1976, the control was
approximately 86%. This is not significantly different. With close to forty years of rains,
the redesigned pond still averages 80% SS removal.
Table 5.3.C-6. Water Quality Output Summary for 1952—1989 Rain File
Inter- Max Pond Flow- Approx.
Statistic
Mean
Std. Dv.
COV
Minimu
m
Max.
Rain Depth
(cm) (in)
1.27
1.91
3.84
0.03
34.4
9
0.50
0.75
1.50
0.01
13.5
8
Rain
Duration
(h)
6.31
6.88
1.09
1.00
93
tveni
Duration
(ds)
2.57
3.54
1.38
0.00
44.31
Rain Intensity
(cm/h)(in/h)
0.10
0.15
3.76
0.00
0.79
0.04
0.06
1.48
0.00
0.31
die
(m)
1.94
0.16
0.03
1.84
2.73
ige
(ft)
6.30
0.51
0.08
6.00
8.84
weigniea
Particle
Size
6.43
4.91
0.76
0.20
23.2
rare. Kes.
Control*
(%)
80.13
14.36
0.18
48.50
99.8
Peak
Reductio
n Factor
0.13
0.10
0.76
0.00
0.57
Event
Flushing
Ratio**
1.91
2.44
1.28
0.00
9.96
* Approximate Particle Residue Control (SS).
** Event flushing ratio-the storm runoff volume divided by the water volume contained in the pond atthe beginning of the
event.
Conclusions
As described above, retrofitting a dry pond to a wet pond can provide improvements to
the overall benefits of the device. Average SS removal for particles of 5 |j,m or greater
is expected to approach 80% removal annually. Additionally, a combination of water
quality and flow analysis computer programs can be used together to evaluate and
design effective stormwater controls and SW control structures can provide both water
quality protection and flood control.
138
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5.4 Retrofitting WWF Storage Tanks for DWF Augmentation
The operation of a WWTF requires the accommodation of wide fluctuations in flow due
to naturally occurring variations in the generation of wastewater and the associated
effects of I/I. Diurnal variations in flow rates are typically observed with higher flow rates
occurring during the daytime when water use is high and lower flow rates at night when
water use is low.
Wastewater equalization can dampen these diurnal variations to achieve a relatively
constant hydraulic loading to the WWTF. Equalization basins are normally located
within the WWTF following grit and screening facilities; however, equalization is
sometimes located in the collection system to relieve overloaded trunk sewers.
Equalization can relieve overloaded treatment facilities in existing treatment plants and
can improve the efficiency, reliability, and operability of these facilities. Equalization
may also be a more cost-effective solution to modifying existing facilities. Retrofitting
existing WWF storage tanks to serve a second function as DWF equalization tanks may
prove to be a cost-effective alternative to the modifying existing treatment facilities or
the construction of new equalization tanks at the treatment plant. Two desktop
analyses will be evaluated to demonstrate this:
• Retrofit of the existing Erie Boulevard Storage System (EBSS) in Syracuse, NY
• Retrofit of the existing Spring Creek Auxiliary Wastewater Pollution Control Plant in
New York, New York.
5.4.a. Erie Boulevard Storage System, Syracuse, New York
Facility Description
The EBSS was constructed as part of the Best Management Practice (BMP)
improvements in 1982. The existing large diameter storm sewer running underneath
Erie Boulevard 2x3 m (7.5x10.5 ft) was retrofitted with automated sluice gates and level
sensors to entrap CSOs that discharge to this structure. These discharges are
temporarily stored in the EBSS until the Syracuse Metropolitan Wastewater Treatment
Facility (METRO) has capacity to accept the WWF. The 18,950 m3 (5 MG) volume of
the EBSS can adequately contain the discharge from the 90 percentile storm in its
tributary area. Operational problems with control equipment prevented the system from
functioning as intended. Reactivation of the EBSS as a WWF storage facility is targeted
for completion by July 2002.
As shown in Figure 5.4.a-1, the EBSS receives flow from three major sources:
1. Burnet Avenue Trunk Sewer (BATS) overflows, including the James Street Relief
Sewer (JSRS).
2. Fayette Street Trunk Sewer (FSTS) overflows.
3. Storm Sewers at the upstream end of the system as part of the natural drainage
basin.
139
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Figure 5.4.a-1.
i: J|
T Z.
II'
•
15
-
s
,,.<-- I
il fr-1
_ SI
^--
fa**,,/ 11: i I-
•'---" «f ,:-
'
;•**; ,;*fc
.?*-;-
3«i^
li I
140
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The EBSS will be automatically controlled through the use of state-of-the-art SCADA
control system. This system consists of aboveground sluice gate control structures for
each of the four control gates that operate based on level sensors in the influent
sewers. The SCADA system also controls operation of the inflatable dam that regulates
flow from the BATS, the major source of flow to the EBSS.
Three sluice gates (Gates 1, 3, and 4) on the EBSS establish maximum water surface
elevations as shown on Figure 5.4.a-l Gate Chamber 2 is located on the regulator
sewer that connects the EBSS to the main interceptor sewer (MIS) that conveys flow to
METRO. This control gate operates based on the water level in the MIS.
Upon the initiation of WWF into the EBSS, it is filled sequentially through the closure of
Gate Chambers 1, 3, and 4. Flow fills each basin sequentially until the 18,950 m3 (5
MG) volume is exceeded, at which time, flow exits the EBSS through the high-water
overflow at Gate Chamber 1. Gate Chamber 2 is closed except for dewatering of the
EBSS following a wet-weather event as described below.
Stored volume behind Gate Chamber 1 is dewatered first by opening Gate Chamber 2
to convey flow to the MIS. Gate Chamber 3 is dewatered following dewatering of Gate
Chamber 1 by opening Gate Chamber 3, and then Gate Chamber 4 is opened allowing
the stored volume to flow into the lower chambers to the MIS for ultimate treatment at
METRO. Gate Chamber 1 remains closed during this phase. Gate Chamber 1 is
opened upon final dewatering of the system and EBSS is returned to standby mode.
This facility in its intended wet-weather operational mode will be used to evaluate the
hypothetical potential for retrofitting the EBSS to serve a second function for DWF
equalization. Equalization of DWF in the EBSS could prove to be beneficial to the
treatment plant by eliminating the diurnal flow variability, providing METRO a relatively
constant DWF component from the EBSS tributary area.
Proposed Retrofit
A portion of the DWFwill be directed from the BATS to the EBSS through the JSRS by
the use of regulator structure located in the BATS immediately downstream of the
JSRS. This structure is proposed as part of the EBSS reactivation to direct WWF into
the EBSS. This structure contains a bottom-outlet orifice device to allow the DWF to be
conveyed downstream through the BATS. The proposed retrofit will modify this orifice
to allow only a portion of the dry-weather to pass downstream while regulating the
higher DWF into the EBSS. DWF in excess of the orifice capacity will be directed into
the EBSS where it will be stored and regulated to the MIS using a new vortex valve that
would be retrofitted into Gate Chamber 2, for ultimate treatment at METRO.
The storage volume between Gate Chambers 1 and 3 targeted for use for DWF
equalization is 8,680 m3 (1.5 MG) based on the storage volume below the invert
elevation of the JSRS. This will allow the remaining 13,250 m3 (3.5 MG) of the 18,950
m3 (5 MG) EBSS capacity (2,650 m3 (0.7 MG) remaining capacity between Gate
141
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Chambers 1 and 3 and 10,600 m3 (2.8 MG) upstream of Gate Chamber 3) to be
available for storage of WWF.
Monitoring of the BATS downstream of the JSRS conducted from July 29, 1998 through
August 6, 1998 found the DWF to vary between 0.16-0.69 m3/s (5.7-12.8 cfs) with an
average of 0.22 m3/s (7.6 cfs). It is proposed to divert DWF in excess of 0.15 m3/s (5.2
cfs) to the EBSS for equalization to provide, on average, 8,680 m3 (1.5 MG) of flow to
be equalized per day utilizing the capacity of the previously described DWF volume.
Stored DWF will be routed to METRO through a new vortex valve that will be retrofitted
into the existing Gate Chamber 2 with a capacity of 0.065 rrrYs (2.3 cfs). This low
release rate will allow effective equalization of DWF and utilization of the proposed
8,680 m3 (1.5 MG) dry-weather storage volume. Gate Chamber 2 is directly connected
to the MIS and will serve as the drain of the entire EBSS during wet-weather operation.
Stormwater Management Model Results
The EPA SWMM Version 4 assessed the impact of the retrofit by comparing the use of
the EBSS under pre- and post-retrofit conditions. A simplified model was simulated
hourly rainfall, based upon a previously calibrated model.
The data consisted of 30 years of hourly rainfall data form the NWS station at Hancock
Field in Syracuse, NY for the years from 1962-1991 and were used as input to the
simplified RUNOFF model. The projected flows were used to simulate long-term
overflow using the TRANSPORT block of SWMM. The TRANSPORT model was
constructed as a simplified network consisting of overflows and regulator pipes. The
simplified model was used to project the annual wet-weather capture from the EBSS
tributary area.
Table 5.4.a-1 demonstrates that the EBSS operated in a wet-weather mode provides a
considerable annual capture of WWF. The retrofitting of the EBSS for DWF
equalization reduces the ability to capture and retain WWF. Table 5.4.a-1 shows that
this retrofit reduces the overall WWF capture from 80% to 75%.
142
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Table 5.4.a-1. Average Annual Capture Volume from the EBSS Under Pre- and Post-
Retrofit Conditions (1962 - 1991)
Average Annual Total
WWF in the Combined Average Annual percent
Sewer System WWF Captured Volume
EBSS CONDITION (m3) (MG) (m3) (MG) CaPture
Existing Wet-Weather 1,071,465 283 859,450 227 80%
Operation
Dry-Weather Retrofit 1,071,465 283 802,650 212 75%
Operation*1}
(1) Assumes dry-weather portion of storage is fully utilized during wet-weather events to
represent worst case
Design Considerations
Retrofitting the EBSS to provide equalization of a portion of the DWF from the BATS
required careful consideration of several key design factors, as follows:
• Hydraulic Loadings
• Flushing Requirements
• Operational and Maintenance Requirements
These design factors are described in further detail in this section.
Hydraulic Loadings. It is proposed to divert DWF in excess of 0.15 m3/s (5.2 cfs) to the
EBSS for equalization. This would result in an average of 8,680 m3 (1.5 MG) of flow to
be equalized per day. DWF in excess of the available 8,680 rr? (1.5 MG) storage
volume will be directed to METRO through the existing BATS.
Operational Requirements. The only adjustable control point necessary for this retrofit
is a pinch valve located on the dry-weather orifice device. The dry-weather orifice will
be automatically adjusted when flow in the BATS exceeds the maximum DWF of 0.69
m3/s (12.8 cfs) or when the dry-weather storage volume is fully utilized. This will be
done by fully opening the pinch valve on the dry-weather orifice to allow the entire DWF
to be conveyed downstream through the BATS. This will be controlled through the
EBSS SCADA system. The pinch valve will be engaged in the normal operational mode
to only allow passage of the lower portion of the DWF.
Flushing Requirements. Accumulated solids deposition will increase during equalization
of DWF, however, the layout of the EBSS serves to provide considerable flushing
following wet-weather events. This is accomplished by staged release of Gate
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Chambers 3 and 4 to provide a wave of water to direct accumulated solids downstream
for conveyance to METRO through the MIS. This system will be an effective method for
the removal of accumulated solids from the DWF equalization.
Operation and Maintenance Requirements. The use of EBSS for DWF equalization will
result in increased O&M costs. The following list summarizes the areas of expected
O&M cost increases:
• Labor to perform inspection of the facilities
• Power for the control system
Table 5.4.a-2 summarizes the expected increase in annual O&M costs resulting from
the use of the EBSS for DWF equalization.
Table 5.4.a-2. Estimated O&M Costs
Estimated Annual
Description Cost
Process Control Adjustments $3,000
Clean-Up of Wet-Weather Treatment $10,000
Structures
Power $2,000
Total Estimated O&M Cost $15,000
Project Costs
The projected capital costs for retrofitting the EBSS for DWF equalization are provided
in Table 5.4.a-3.
Table 5.4.a-3. Estimated EBSS Retrofit Capital Costs
Description Estimated Cost(1)
Flow Diversion Structure Retrofit $10,000
Flow Regulator Structure Retrofit $15,000
Electrical Controls $35,000
Instrumentation and SCADA System $55,000
Total Project Cost $140,000
(1) Includes Design Engineering, Construction Engineering, Legal, and Fiscal
Expenses, does not include contingencies and right-of-way costs
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5.4.b. Spring Creek AWCP, New York, New York
The Spring Creek Auxiliary Water Pollution Control Plant (AWPCP) is a CSO wet-
weather detention facility with approximately 49,200 m3 (13 MG): 37,900 m3 (10 MG) in
basin storage and 11,400 m3 (3 MG) in influent barrel storage. The details of the Spring
Creek facility are provided in Section 5.2 of this document.
Proposed Retrofit
A portion of the DWF will be directed from the Autumn Avenue regulator in Brooklyn to
basin 6 to minimize the diurnal fluctuations of DWF from Autumn Ave. regulator
drainage area that is tributary to the 26 Ward WPCP. The DWF will be conveyed to
AWPCP to allow a specified level of DWF to be conveyed to the 26 Ward WPCP.
The storage volume in basin 6 targeted for use for DWF equalization is 3,790 m3 (1.0
MG) or approximately one-half of the storage volume. This will allow the remaining
34,000 m3 (9.0 MG) of the 37,900 m3 (10 MG) AWPCP capacity to be available for
storage of WWF.
O'Brien & Gere Engineers previously performed an extensive evaluation of the 26 Ward
WPCP throughout the past 15 years. Their work has shown the average DWF from the
Autumn Avenue regulator to vary between 1.06-2.07 m3/s (37-73 cfs) with an average of
1.5 mP/s (53 cfs). It is proposed to divert DWF in excess of 1.33 rrrVs (47 cfs) to
AWPCP Basin No. 6 for equalization to provide, on average, 3,790 m3 (1.0 MG) of flow
to be equalized per day. Stored DWF will be routed to the 26 Ward WPCP through a
new dry-weather pump station with a capacity of 0.28 m3/s (10 cfs). This low release
rate will allow effective equalization of DWF and utilization of the proposed 3,790 m3
(1.0 MG) dry-weather storage volume.
The proposed retrofit will consist of a physical modification to the existing diversion
structure by installation of sluice gate on the 1.5 m (60 in.) diameter dry-weather outlet
of the regulator structure. This sluice gate will be controlled by a flow meter located
downstream of the sluice gate in the 1.5 m (60 in.) diameter interceptor sewer to limit
flow to 1.33 m3/s (47 cfs). The regulator chamber will also be modified to include a weir
structure to convey DWF in excess of 47 cfs to Bay Number 6 for equalization. Level
sensors in the dry-weather equalization tank will also be used to control the dry-weather
sluice gate when the 1 MG volume is utilized.
The dry-weather sluice gate will also be designed to open fully when sewage overflows
the Autumn Ave. regulator structure into Spring Creek to maximize flow to the treatment
plant by the used of a level sensor located at the invert elevation of the overflow weir.
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Stormwater Management Model (SWMM) Results
The SWMM Version 4 assessed the impact of the retrofit by comparing the use of the
Spring Creek AWPCP under pre- and post-retrofit conditions. A simplified model was
use to simulate hourly rainfall, based upon a previously calibrated model.
The data consisted of hourly rainfall data from the National Oceanographic and
Atmospheric Administration rain gauge No. 305803 at J.K.F. International Airport in New
York, NY for the 1985. This data was used as input to the simplified RUNOFF model.
The projected flows were used to simulate long-term continuous simulation using the
TRANSPORT block of SWMM. The TRANSPORT model was constructed as a
simplified network consisting of the overflow and regulator pipes. The simplified model
was used to project the annual wet-weather capture from the Spring Creek AWPCP
under pre- and post-retrofit conditions.
Table 5.4.b-1 demonstrates that the AWPCP presently discharges a considerable
amount of WWF to Spring Creek. The retrofitting of the AWPCP for DWF equalization
reduces the ability to capture and retain WWF only to a small extent. Table 5.4.b-1
shows that this retrofit increases the annual WWF by 12% or approximately 50,000 m3
per year (13.2 MG per year).
Table 5.4.b-1. Wet-Weather Discharge Volume from the Spring Creek AWPCP Under
Pre- and Post-Retrofit Conditions for 1985
Wet-Weather
Discharge for 1985
Spring Creek Storage Volume (m3) (MG)
Existing (37,850 m3 [10 MG])
Retrofitted (34,065 m3 [9 MG]) (1)
Increase In Wet- Weather Discharge
Percent Increase
433,005
482,967
49,962
12%
114.4
127.6
13.2
12%
(1) Assumes dry-weather portion of storage is fully utilized during wet-weather
events
Design Considerations
Retrofitting the AWPCP to provide equalization of a portion of the DWF from the
Autumn Avenue regulator required several design factors:
• Hydraulic Loadings
• Operational Requirements
• Flushing Requirements
• Operational and Maintenance Requirements
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These design factors are described in further detail in this section.
Hydraulic Loadings - It is proposed to divert DWF in excess of 1.33 rrrYs (47 cfs) to
AWPCP for equalization. This would result in an average of 8,680 m3 (1 MG) of flow to
be equalized per day. DWF in excess of the available 8,680 m3 (1 MG) storage volume
will be directed to the 26 Ward WPCP.
Operational Requirements - The two primary control points for this retrofit are the new
DWF sluice gate that will be installed in the Autumn Ave. regulator and the new DWF
pump station that will be used to convey equalized flow to the 26 Ward WPCP.
The dry-weather sluice gate will be automatically adjusted when flow in the 1.5 m (60
in.) diameter interceptor exceeds 1.33 rrfVs (47 cfs) or when the 1 MG dry-weather
storage volume is fully utilized.
New level sensors will be used to control operation of the DWF pump. These controls
will be set to energize the pump station when flow is sensed in the storage tank and to
turn the pump off when the tank is empty.
Flushing Requirements - Accumulated solids deposition will increase during
equalization of DWF, however, a flushing system will remove accumulated solids
following wet-weather events. The design of new flushing system for the Spring Creek
AWPCP is presently being evaluated to provide more efficient removal of accumulated
solids. The selected system will be an effective method for the removal of accumulated
solids from the DWF equalization.
Operation and Maintenance Requirements - An increased O&M burden will be created
for the use of Spring Creek for DWF equalization. The following list summarizes the
areas of expected O&M cost increases:
• Electrical Power to operate the dry-weather pump station and the orifice control
• Labor to perform inspection of the facilities
• Labor to perform flushing of accumulated solids
Table 5.4.b-2 summarizes the expected increase in annual O&M costs resulting from
the use of Spring Creek for DWF equalization.
Table 5.4.b-2. Estimated O&M Costs
Description Estimated Annual Cost
Electrical Power $3,000
Labor (Maintenance of Facilities) $10,000
Labor (Tank Flushing) $25,000
Total Estimated O&M Cost $38,000
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Project Costs
The projected capital costs for retrofitting Spring Creek for DWF equalization are
provided in Table 5.4.D-3.
Table 5.4.b-3. Estimated Spring Creek DWF Retrofit Capital Costs
Description Estimated Cost1
Regulator Structure Retrofit $205,000
DWF Pump Station and Force Main $170,000
Process Control System $45,000
Instrumentation and SCADA System $65,000
Total Project Cost $485,000
11ncludes Design Engineering, Construction Engineering, Legal, and
Fiscal Expenses, Does not include contingencies and right-of-way costs
Conclusions
Retrofitting the EBSS and Spring Creek AWPCP were evaluated for the purpose of
serving a dual role as wet-weather and dry-weather equalization facilities without
drastically reducing the ability of the storage tank to capture WWF. The primary benefit
of equalization provided by the DWF storage is the dampening of peak flow rates that
would otherwise result at the treatment plant. This retrofit will increase the treatment
capacity for a treatment plant that is operating at or near design capacity and will also
allow the primary and secondary treatment facilities to operate at a more uniform rate to
provide consistent treatment efficiency throughout a typical day. However, there is
adequate hydraulic and biological capacity the Syracuse METRO and 26 Ward
treatment facilities.
This retrofit option may also provide a reduction in operation of high flow pumps, where
appropriate, as a result of eliminating the high diurnal flows. This could result in a
reduction in O&M at the treatment plant; however, the O&M required at the equalization
facility may offset any benefit at the treatment plant.
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5.5 SSO Control Using Storage
5.5.a. Hypothetical Overflow Retention Facility Retrofit to an Existing Sanitary
Sewer System
This desktop analysis involves a hypothetical retrofit to a sanitary sewer system to
eliminate SSOs from an existing regulator structure. This desktop analysis was
developed from a real life project from a confidential source. Overflow from this
regulator occurs during and following rainfall events due to a well-established I/I
problem. The I/I problem was assessed as part of a sewer system evaluation survey
(SSES) that identified and quantified various sources of I/I with the collection system. A
rehabilitation program was implemented in accordance with the findings of the SSES to
provide removal of the primary I/I sources. This program also requires the elimination of
the existing regulator overflow.
Sewer system modeling was performed using the Environmental Protection Agency's
SWMM Version 4. The previously calibrated model was expanded to identify flow
conditions at the regulator following the implementation of the I/I rehabilitation program.
The model was used to identify peak overflow rates, volumes and hydraulic grade lines
for various overflow abatement alternatives.
Project Approach
A 0.9 m (36 in.) diameter interceptor and an 0.5 m (18 in.) diameter interceptor enter the
regulator chamber. A 0.9 m (36 in.) diameter changes to a 1 m (42 in.) diameter
interceptor then exits the chamber and continues to the WWTF. The existing
interceptor and other downstream piping configurations restrict the flow to the treatment
plant. An 0.5 m (18 in.) diameter overflow also exits the chamber that discharges to the
receiving water body when wet-weather events cause the chamber to overflow.
The regulator overflow was likely developed to relieve a persistent flooding problem in
the vicinity of the regulator. The interceptor network in the vicinity of the regulator
presently surcharges during the two-year design storm resulting in approximately
12,100 m3 (3.2 MG) of surface flooding. The existing overflow relieves surcharge and
minimizes flooding. The peak hydraulic grade line cannot increase beyond surcharge
with the overflow closed. Closing the overflow would result in pronounced basement
and street flooding in the areas upstream of the existing overflows and would not
increase the peak flow to the treatment plant from the interceptor. The high head losses
are associated with the overflow and limitations of the interceptor network that includes
two double-barrel siphons downstream of the regulator overflow. The alternative of
constructing a new sewer parallel to the existing sewer to convey flow to the treatment
plant was not considered feasible given long length of sewer, deep rock excavation, to
long siphons, and a shoreline construction.
The two-year frequency, two-hour duration design storm was selected in order to
determine what control was necessary in terms of treatment, transportation or storage
of the regulator overflows. Based on the SWMM model, during a two-year storm, a
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peak rate of approximately 28,000 m3/d (7.4 MGD) overflows to the receiving water with
a total overflow volume of 6,400 m3 (1.7 MG). One recommended abatement
alternative is to use a 6,400 m3 (1.7 MG) ORF to eliminate overflows up to the two-year
design storm and to treat overflows in excess of the storage capacity with high-rate
disinfection.
The WWTF is presently permitted to treat 64,000 m3/d (17 MGD). The monthly average
flow during a two-year monitoring period of the SSES was 76,000 m3/d (20 MGD). As
the plant already exceeds its flow limit, recommendations to increase the peak WWF to
the plant were not made, but to store the WWF during wet events with a slow release to
the WWTF following the storm.
Overflow Retention Facilities
The proposed retrofit consists of a 6,400 m3 (1.7 MG) ORF located between the
regulator chamber and the existing outfall to capture the flow associated with the two-
year design storm. Overflow events producing less than 6,400 m3 (1.7 MG) are
retained and returned to the treatment plant when influent flows subside. Overflow
events producing in excess of the available storage volume are disinfected using
chlorine and dechlorinated before being discharged to the receiving water. The retained
volume of 6,400 m3 (1.7 MG) is returned to the treatment plant through the interceptor
sewer utilizing an overflow return pump.
The overflow retention facilities include the following components:
ORF Tanks. The ORF tanks will consist of an in-ground reinforced concrete tank with a
dimension of 30.5 m (100 ft) wide by 73.2 m (240 ft) long with a 3 m (10 ft) depth to
provide a useable retention volume of 6,400 m3 (1.7 MG).
Overflow Return. A 10.2 cm (4 in.) overflow return pump will be used to return the
captured volume to the WWTF through the existing interceptor sewer within 48 h after
the occurrence of an overflow event.
Disinfection. The disinfection system will utilize sodium hypochlorite introduced into a
contact tank sized for 5 min detention time at peak flow rate of 28,000 m3/d (7.4 MGD)
and requires the use of mechanical mixers to induce energy to provide effective
disinfection. The mixing velocity gradient and chlorine dose were calculated on the
basis of estimated influent and effluent fecal coliform levels for sanitary sewer
overflows. The high-rate disinfection system was sized by estimating disinfectant
dosage and mixing energy required to assure the inactivation of fecal coliform. Based
on the optimum velocity gradient calculated, horsepower requirements for mixers were
determined. From this analysis, it was determined that an optimum chlorine dosage of
20 mg/l and approximately 1.1 kW(1.5 HP) of total mixing energy are required to insure
proper disinfection. A 5 cm (2 in.) disinfection tank pump will be used to return the
disinfection tank volume to the WWTF through the existing interceptor sewer within 48 h
after the occurrence of an overflow event.
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Dechlonnation. Dechlorination, which reduces the chlorine residual prior to discharge,
will be accomplished by injecting sodium bisulfite in a 10 m3 (2,600 gal) tank with a
high-rate mixer to provide detention time of 30 s at a peak flow of 28,000 rrfVd (7.4
MGD). The dechlorination facilities are metered automatically as a function of overflow
flow rate to minimize chlorine residual.
Overflow Metering. Overflows will be metered following discharged from the
dechlorination tank through an open channel-type metering device located in the outfall
pipe.
Control Building. A 63.2 m2 (680 ft2) structure will be provided as the control building for
this facility. This structure will include process control equipment and be used to house
the chemical storage tanks.
Stormwater Management Model Results
The EPA SWMM Version 4 was used to assess the benefit of the retrofit by comparing
annual WWF volume presently discharged to the volume captured by the retention tank.
A simplified model was developed to simulate hourly rainfall for the hypothetical
collection system
The data used for these projections consisted of 15 years of hourly rainfall data for the
years from 1961-1985. These data were used as input to the simplified RUNOFF model
that projected flows that were used to simulate long-term overflow using the
TRANSPORT block of SWMM. The TRANSPORT model was constructed as a
simplified network consisting of hypothetical overflows and regulator pipes. The
simplified model was used to project the annual volume presently discharged and the
percent of the annual volume captured by the proposed storage facility.
Table 5.5.a-1 demonstrates that the 6400 m3 (1.7 MG) storage facilities provide
considerable annual capture of WWF compared to that was presently discharged
without treatment.
Table 5.5.a-1. Average Annual Overflow Volume from the Pre- and Post-Retrofit
Facilities (1971 -1985)
Average Annual Overflow Volume
m3 (MG)
Pre-Retrofit WWTF Overflow
Post-Retrofit (1)
Percent Capture
70,400
18,000
75%
18.6
4.7
Although the ORF was sized on the two-year design storm, the system annually
captures only 75% of the WWF that is presently discharged. This occurs as a result of
the regulator chamber modification required to reduce the hydraulic grade line at the
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regulator and relieve surcharge and street flooding in the vicinity of the regulator
chamber. Therefore, the ORF will be active more frequently than overflow presently
occurs from the existing overflow.
Design Considerations
The retrofitting of a sanitary sewered collection system to eliminate overflows through
storage and disinfection required the following design considerations:
• Hydraulic Loadings
• Treatment Objectives
• Operational Requirements
• Chemical Storage and Feed Requirements
• Sludge Handling Requirements
• Operations and Maintenance Requirements
Hydraulic Loadings - Hydraulic loadings were developed for the ORF to provide
retention of WWF for subsequent treatment at the WWTF following a wet-weather event
and to provide disinfection of WWF beyond the 6,400 m3 (1.7 MG) ORF capacity.
Treatment Objectives - The objective of this retrofit is to eliminate overflow from the
sanitary sewer system up to the two-year design storm and to provide disinfection of
WWF beyond the two-year design storm level.
Operational Requirements - The ORF are designed to operate on a continuous basis
include:
• WWF retention
• High-rate disinfection of flow in excess of ORF capacity
• Chemical Storage and Feed Requirements - The ORF utilize two chemicals:
• Liquid Sodium Hypochlorite for disinfection
• Liquid Sodium Bisulfite for dechlorination
The liquid sodium hypochlorite system is based on commercial-grade liquid sodium
hypochlorite (15% solution) which contains the equivalent of 150 kg of chlorine/m3 of
solution (1.25 Ibs of Cb/gal of solution). Two metering pumps are designed to deliver a
dosage of 20 mg/L at the peak design flow with one unit as a backup. The liquid
sodium hypochlorite is stored in a bulk storage tank with an effective working volume of
11.4 m3 (3,000 gal). The available chlorine in sodium hypochlorite solution declines
with age. The anticipated sodium hypochlorite storage time will be 50 days.
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The liquid sodium bisulfite storage system is based on commercial grade liquid sodium
bisulfite (38% solution) that contains appropriately 503.2 kg of sodium bisulfite/m3 of
solution (4.2 Ibs of sodium bisulfite/gal of solution). Two metering pumps are designed
to deliver sodium bisulfite dosages necessary to dechlorinate effluent with 1.5 mg/L of
sodium bisulfite per mg/L of chlorine residual. Approximately 1.6 kg of sodium bisulfite
will be used per kg of chlorine residual. A bulk storage tank with an effective working
volume of 5.7 m3 (1,500 gal) will be used for sodium bisulfite storage.
The sodium hypochlorite and sodium bisulfite metering pumps are flow paced. An
ultrasonic type level sensor is used for effluent flow measurement.
Sludge Handling Requirements - Accumulated sludge will be returned to the WWTF
during the pump back of retained WWF and the associated tank flushing following tank
draining. Tank flushing will be accomplished manually using a 7.6 cm (3 in.) municipal
water service.
Operation and Maintenance Requirements - The implementation of the ORF will create
an O&M cost. The areas of expected O&M burden include:
• Chemicals for disinfection and dechlorination
• Power for tank draining and chemical feed systems
• Water for tank flushing.
The estimated annual O&M costs are provided in Table 5.5.a-2.
Table 5.5.a-2. ORF O&M Costs
Annual Chemical Costs Cost
Sodium Hypochlorite $ 8,200
Sodium Bisulfite $ 3,800
O&M Labor $10,000
TOTAL COST $22,000
Project Costs
The estimated construction costs for the ORF are provided in Table 5.5.a-3.
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Table 5.5.a-3. Estimated ORF Construction Costs
Item Description
Site Work
Common Excavation
Rock Excavation-Blasting Method
Select fill
Sheeting
Site Dewatering
New Pavement & Restoration
Fencing Restoration
(General Site & Bldg Restoration
Structural & Architectural
CIP Concrete-Base
CIP Concrete-Walls
CIP Concrete-Roof
Control Builriinp Brink & Block
Mechanical
Chemical Metering Pumps
Submersible Mixers-1.1 kW
Oder Control Blowers-1 1 .2 kW
ORF Tank Dewatering Pump-10.2 cm
Disinfection Tank Dewatering Pump-5.1 cm
Back Flow Preventer-7.6 cm
Ra^k\«rator Valvo-^T T r-m
Special Construction
Carbon Absorption Unit
Chemical Storage Tank (Chlorination)
Chemical Storage Tank (Dechlorination)
Flow Meter
SCADA Tansmitter
Jib Crane
Misc Safety Fquipment
Site Piping & Utilities
106.7 cm Interceptor Relocation
91 .4 cm Interceptor Relocation
61 cm ORF Inf/Effl. Sewer
45.7 cm Interceptor Relocation
182.9 cm Storm Sewer Relocation
Bypass Pumping Interceptor
Diversion Structure
Storm Sewer Junction Manhole
Special Manhloe for Sewer Crossing
10.2 cm Force Main
7.6 cm Facility Water Service
Wet Tap for Water Main Service
Temporary Overflow Bypass
Construction Cost Subtotal
Other Contracts
Electrical
Heating & Ventilating
Plnmhing
Subtotal Constuction Costs
Estimating Contingency
Total Construction Cost
Pnninoorinri & 1 Anal
Quantitv
1,529
574
459
743
1
785
61
1
214
191
199
63
2
5
1
1
1
1
1
1
1
1
1
1
2
1
18.3
55
9.1
4.6
67
1
1
1
1
6
67
1
9 1
15%
7%
9%
25%
9HOA
Unit
3
m
3
m
3
m
2
m
Ea
2
m
m
Fa
3
m
3
m
3
m
2
m
Ea
Ea
Ea
Ea
Ea
Ea
Fa
Ea
Ea
Ea
Ea
Ea
Ea
Fa
m
m
m
m
m
Ea
Ea
Ea
Ea
m
m
Ea
m
PCT
PCT
PP.T
PCT
PP.T
Unit Cost
$26
$131
$29
$377
$20,000
$24
$98
SIP nnn
$523
$1 ,046
$1,177
S1 507
$3,500
$3,700
$4,500
$3,500
$2,200
$2,000
-------
Conclusions
Retrofitting SSOs with ORF can provide a cost-effective alternative in lieu of expansion
of the sanitary collection system to convey the volume of overflow to the treatment plant
for primary and secondary treatment. This alternative requires the availability of a
sufficient amount of land to build such a facility.
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5.6 Retrofitting for Industrial Wastewater Control in a Combined Sewer System
EPA has identified maximizing flow to the POTW in combined sewer systems as one of
the nine minimum controls for CSO abatement (EPA 1995). The objective of this
control is to reduce the magnitude, frequency, and duration of CSOs that flow untreated
into receiving waters. Industrial wastewater is often characterized by high-strength
pollutants that can be detrimental to receiving waters. The control of industrial
wastewater in a combined sewer system can reduce the impact of the CSO discharges
on receiving waters. A desktop analysis of the Rockland WWTF in Rockland, ME, was
evaluated to demonstrate the technique.
5.6.a. Rockland Wastewater Treatment Facility, Rockland, Maine
The city of Rockland, Maine owns and operates a secondary WWTF that discharges to
Rockland Harbor, a Class SC water body. The WWTF was originally constructed in
1978 with upgrades in 1994 and 1998. The Rockland WWTF provides secondary
treatment using conventional activated sludge. The WWTF was designed to handle a
monthly-average flow of 11,000 rrfVd (2.9 MGD) and an instantaneous-peak flow of
22,000 mP/d (5.7 MGD). The WWTF serves a CCS. Rockland's existing collection
system services most of the residential population, commercial properties, and industrial
properties within the limits of the city of Rockland. The system also serves the
neighboring community of Rockport and the Samoset Resort. The system contains
three types of sewers:
• combined sanitary and storm
• separate sanitary
• storm
Nine pumping stations exist serving various areas of the collection system. At present
there are approximately 21 km (13 miles) of gravity sewer ranging from 15 cm (6 in.) to
76 cm (30 in.) in diameter. Pipe materials include stone and brick (dating back to the
Civil War era) as well as more recent installations of vitrified clay, asbestos-cement,
concrete, cast iron, and PVC pipe. According to the 1998 CSO Facilities Plan, there are
presently two permitted CSOs located within the system. In addition, five other CSOs
were located during the Facility Planning effort that began in 1993. In an average year,
187,000 m3 (49.3 MG) are discharged from these seven CSOs.
A majority of the wastewater currently entering the Rockland sewer system is domestic
sewage from residential and commercial sources. The WWTF also receives a
significant flow and loading from the FMC Corporation Food Ingredients Division facility.
This facility manufacturers carrageenan, a seaweed based food additive. The WWTF is
designed to receive approximately 67% of the organic loading and approximately 22%
of the DWF from the FMC Facility. Annual average wastewater flows to the WWTF are
7,570 m3/d (2.0 MGD) and average daily BOD5 loadings are 1,565 kg/d (3,445 Ibs/d).
Prior to implementation of the CSO Abatement Program, the WWTF used the following
unit processes to provide preliminary, primary and secondary treatment:
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• Three raw sewage pumps with a capacity of 10 m3 (2,600 gal) per min each,
• One 92 cm (36 in.) comminutor with a manual bypass screen,
• Two primary clarifiers (each at 14 m [45 ft] with a total surface area of
approximately 295 m2 [3,180 ft2]). Primary sludge is conveyed to two cyclone
classifiers to remove grit prior to thickening,
• Six mechanically aerated activated sludge aeration tanks with a total volume
of6,590m3(1.74MG),
• Two secondary clarifiers (each at 17 m [55 ft] with a total surface area of
approximately 440 m2 [4,750 ft2 ]),
• A chlorine contact tank with a volume of approximately 345 m3 (91,000 gal),
• A dechlorination chamber with a total volume of approximately 15 m3 (4,000
gal).
Figure 5.6.a-1 shows the existing site layout prior to implementing the CSO Abatement
Program. Figure 5.6.a-2 shows a schematic of the existing WWTF prior to retrofitting
with a description of the numbered overflow points. When influent peak flows would
reach 18,930 m3 (5 MGD), the WWTF influent pumps would be throttled and excess
flow bypassed to the outfall downstream of the chlorine contact tank resulting in
untreated, undisinfected discharges.
These overflows are discharged to Rockland Harbor via permitted CSO #002 (located in
the wetwell of the WWTF influent pump station Figure 5.6.a-2). The wetwell receives
the FMC Industrial Wastewater plus WWTF sidestream flows from the primary sludge
gravity thickener overflow, waste activated sludge DAF thickener overflow, and sludge
dewatering system vacuum filter filtrate. The industrial wastewater and WWTF
sidestream flows are mixed with the combined sewage upstream of the influent pump
station wetwell. Consequently, these high-strength wastes are included in the overflow
discharged via CSO #002.
In March 1998, a CSO Facilities Plan was completed. The CSO Facilities Plan was
approved by the State of Maine Department of Environmental Protection (DEP) and
EPA. The approved plan recommended a three-phase program to address CSO
discharges.
Phase I of the CSO Abatement Program included rehabilitation of the Lindsey Brook
Sewer. This sewer is located within the streambed for Lindsey Brook and has been
determined to be a considerable source of I/I during wet weather or high groundwater
periods. The Lindsey Brook Sewer is also suspected to be a source of exfiltration
during periods of dry weather.
157
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5.6.a-1. Prior to
158
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Fioure F> 6 a-? Flow Schematic Prior to Retrofit
tiff
» S < '
= i- = - *
159
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Phase II of the CSO Abatement Program included upgrading the WWTF to handle
additional WWF. Phase II also includes new consolidation conduit to intercept
overflows from the second and third largest CSO's (#004 and #005) that are located
approximately 92 m (300 ft) upstream of the WWTF on Main Street (Route 1) in
downtown Rockland. Phase II also includes a new force main to separate industrial
wastewater produced at the FMC Food Ingredients Division Facility from the City
interceptor sewer and provide a direct connection to the Rockland WWTF. Finally,
Phase II will redirect WWTF sidestream flows to the primary and secondary treatment
units downstream of the influent pumping station.
Phase III of the CSO Abatement Program includes selected sewer separation in the
south portion of the Rockland CCS to address the remaining CSO discharges.
This case study will focus on Phase II Improvements at the Rockland WWTF. Phase II
Improvements include:
• A new 76 cm (30 in) diameter consolidation conduit from CSO #004 and CSO
#005 to the WWTF.
• A new flow diversion structure (SS#1) at the WWTF.
• Conversion of the existing headworks into a submersible pumping station to
handle DWF and WWF. Two dry weather pumps are provided. Each dry
weather pump has a capacity of 15 rr? (4,000 gal) per min. Three wet
weather pumps are provided. Each wet weather pump has a capacity of 30
m3 (7,800 gal) per min.
• Construction of a new headworks with mechanically cleaned bar screen,
screenings compactor, manually cleaned bar rack, grit chamber, recessed
impeller grit pump, cyclone grit classifier, and 31 cm (12 in.) parshall flume.
• Construction of two new upflow style flow distribution structures for the
existing primary and secondary clarifiers to improve hydraulic flow splitting.
• Construction of a new vortex separator to provide high-rate primary treatment
for projected peak wet weather CSO flows of 127,560 m3/d (33.7 MGD).
• Construction of a new CSO Disinfection and Dechlorination Structure to
provide high-rate disinfection and dechlorination for projected peak WWFs of
114,690 nf/d (30.3 MGD); 127,560 m3/d (33.7 MGD) peak WWF minus
12,870 m3/d (3.4 MGD) vortex underflow.
• Constructions of a new 20 cm (8 in.) diameter force main for industrial
wastewater. This force main would convey industrial wastewater to the
existing primary clarifiers, existing aeration tanks, or a new 1,100 m3 (0.29
MG) equalization tank. The equalization tank was created by refurbishing
one (1) unused existing aeration tank. The equalization tank utilizes diffused
aeration for mixing. The equalization tank contains two submersible pumps
(1 primary and 1 standby). Each pump has a variable speed drive to allow
output to be adjusted.
160
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• Construction of new plant drain piping to eliminate discharge of WWTF
sidestreams to the influent sewer. The gravity thickeners for primary sludge
are eliminated by replacing the recessed impeller primary sludge pumps with
new positive displacement units designed to remove thickened sludge directly
from the primary clarifiers. The waste activated sludge filtrate drain was
redirected to the existing aeration tank inlet channel. The sludge dewatering
filtrate drain was redirected to the primary clarifier influent.
Figure 5.6.a-3 shows a site layout of the Rockland WWTF following construction of the
Phase II CSO Abatement Program. Figure 5.6.a-4 shows a process flow schematic of
the Rockland WWTF following Phase II of the CSO Abatement program. These
improvements will allow high-rate primary treatment and disinfection for peak wet-
weather CSO flows at 128,000 rrfVd (33.7 MGD). This provides treatment for CSO
#002, #004, and #005 that represent approximately 90% of the total annual overflow
volume from the Rockland CCS.
Stormwater Management Model Results
The EPA SWMM Version 4 was used in the facility plan to predict overflow rates and
volumes at all of Rockland's CSOs. The SWMM is a relatively complex mathematical
model. To model the various physical processes associated with stormwater runoff and
CSO discharge, a large amount of input data is required. Drainage basin characteristics
(such as size, average ground slope, width, percent imperviousness, roughness,
surface storage, soil infiltration and gutter or pipe flow) must be supplied to the model.
The RUNOFF and EXTRAN blocks of SWMM were used in this study. The model was
constructed using records of the physical system: pipe sizes, slopes, materials, lengths
and elevations. Each CSO diversion structure was modeled. The existing Park Street
Pumping Station was also modeled. Then drainage area characteristics were modeled
for such factors as total acres, percent imperviousness, slope and dimensions. Once
the SWMM model was mathematically constructed, it was calibrated using the extensive
rainfall and CSO quantity data gathered during 1993, 1994 and 1995.
Unlike the calibration process, where actual "real-time" rainfall data are used, SWMM
projections are made using two types of rainfall data:
• Synthetic rainfall hyetographs (or designs storms) for determining peak rates
of overflow.
• Long-term rainfall records from NOAA for determining statistically based
overflow volumes.
The former data are used in the evaluation of pipes and conduits, pumps and CSO
treatment facilities where peak rates of discharge are used as the basis of design. The
latter data are used to design storage facilities and to test the long-term effectiveness of
overall CSO abatement programs.
161
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5.6.a-3.
__i^
» : i
162
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5.6.a-4. Flow
i
l»
1 .--"--, P|| : JS^i;
*-* -* " '. t 1 J: 4 -, i
163
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For the city of Rockland CCS, the predictions were made using design storms with
return frequencies of 0.5, 1, 2 and 5 years. A two-hour storm duration was selected for
the design to reflect an average of time of concentration of the Rockland system. The
SWMM Modeling identified several important hydraulic characteristics of Rockland's
CCS:
• CSO #005 has the highest projected peak overflow rate, CSO #002 (WWTF
Bypass) has the second highest peak overflow rate.
• CSO #002 (WWTF Bypass) discharges the greatest overflow volume with the
North Subsystem overflows (CSO #004 and #005) discharging the second
greatest volume.
Table 5.6.a-1 shows the predicted overflow activity in terms of average annual volume
of overflow per year. These data were developed using the SWMM model and
continuous simulation. Tables 5.6.a-2 and 5.6.a-3 show the projected peak overflow
rates for each CSO using the design storms.
Table 5.6.a-1. SWMM Modeling Summary - Annual Overflow Volumes
Drainage Area
Description
WWTF Bypass
North/Lindsey/Downtown
South
CSO
ID Number
002
004 and 005
003, 006, 007
and 008
Estimated
Annual
CSO Discharge
(m3/yr.)(1)
110,900
54,000
21,200
Estimated
Annual
CSO Discharge
(MG/yr.)(1)
29.3
14.4
5.6
TOTAL 186,100 49.3
(1)Using long-term simulation based upon 21 years of hourly rainfall data form the
NWS station in Portland, ME for the years from 1971 to 1991.
Table 5.6.a-2. SWMM Modeling Summary - Projected Peak Flows
Estimated Peak Flow (m3/d)
CSO
ID Number
002
003
004
005
006
007
008
0.5 Year
Storm(1)
23,850
5,300
0.0
47,700
380
6,440
1,900
1 year
Storm(1)
31,800
5,300
10,100
53,800
6,440
6,440
3,400
2 year
Storm(1)
33,000
5,300
23,100
54,900
14,000
6,440
5,300
2 year
Storm(1)
34,100
5,300
37,100
56,000
21,200
6,440
7,600
(1)
Using 2 h duration.
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Table 5.6.a-3. SWMM Modeling Summary - Projected Peak Flows
Estimated Peak Flow (MGD)
CSO
ID Number
002
003
004
005
006
007
008
0.5 Year
Storm(1)
6.3
1.4
0.0
12.6
0.1
1.7
0.5
1 year
Storm(1)
8.4
1.4
2.9
14.2
1.7
1.7
0.9
2 year
Storm(1)
8.7
1.4
6.1
14.5
3.7
1.7
1.4
2 year
Storm(1)
9.0
1.4
9.8
14.8
5.6
1.7
2.0
(1)Using2 h duration.
Design Considerations
The following is a list of key design considerations for the Rockland WWTF retrofit:
• Impacts of High Strength Wastes
• Hydraulic Loadings
• Treatment Objectives
• Operational Requirements
• Chemical Storage and Feed Requirements
• Sludge Handling Requirements
• Operations and Maintenance Requirements
• Construction Sequencing and Site Constraints
Each of these considerations is described in this section.
Impacts of High-Strength Wastes. One of the nine minimum CSO controls is the review
and modification of pretreatment programs to assure that CSO impacts are minimized.
The elimination of the industrial wastewater connection from FMC Food Ingredients
Division to the city of Rockland CCS, and construction of a separate force main to the
WWTF will satisfy this objective.
The industrial wastewater design loading to the Rockland WWTF is approximately 2,990
kg (6,583 Ibs) per day. This is approximately 67% of the total organic loading of 4480
kg (9,871 Ibs) that the WWTF receives daily. Following the implementation of Phase II
improvements, this industrial wastewater can be directly discharged to three locations at
the Rockland WWTF:
• Existing Primary Clarifier Influent at New Flow Distribution Structure No. 1
• Existing Aeration Tank Inlet Channel
• Equalization Tank
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The industrial wastewater enters the WWTF via a 20 cm (8 in.) diameter force main
connected to a new pre-cast concrete valve vault (SS #2). SS #2 contains valves to
direct flow to the three discharge locations. A 20 cm (8 in.) magnetic flow meter
measures flow. If the flow is directed to the equalization tank, then it is mixed using a
fine bubble diffused air system. The design airflow is 30 scfm per thousand cubic feet.
The air blower has a variable frequency drive allowing adjustments to the airflow. The
WWTF staff is able to mix the equalization basin at a lower rate, if desired. A total of
324 membrane diffusers each with a maximum airflow of 3.5 scfm are provided in the
equalization basin.
Two submersible pumps (1 primary and 1 standby) convey industrial wastewater from
the equalization basin to the WWTF. Each pump has a rated capacity of 2.3 m3 (600
gal) per min at a total dynamic head of 4.5 m (14 ft). The pumps are driven with 3.7 kW
(5 HP) motors controlled with variable frequency drives that allow the WWTF staff to
adjust the flow of industrial wastewater to the process. The discharge from the
submersible pumps can be directed to two locations:
• Existing Primary Clarifiers via new Flow Distribution Structure No. 1
• Existing Aeration Tank Inlet Channel
A 15 cm (6 in.) diameter magnetic flow meter is provided on the submersible pump
discharge allowing the rate of industrial wastewater flow to be monitored by the WWTF
staff.
Other high-strength wastes eliminated from the CSO discharge with the Phase II
Improvements are the sidestream flows from the sludge processing system. The
sidestream flows represent a relatively small portion of the daily load. Approximately
127 kg (279 Ibs) per day are discharged (approximately 3% of the overall design
organic loading of 4480 kg (9,871 Ibs) per day). However, these loadings are
concentrated within a few hours during the day, which magnifies their effects.
The sludge thickener filtrate is produced in a 3.5-4 h period and represents
approximately 8% of the hourly organic loading while the sludge de-watering filtrate is
produced in a 3.5-4 h period and represents approximately 12% of the hourly organic
loading. Overall, the sidestreams represent approximately 20% of the hourly organic
loading at the WWTF when the units are in service. The sludge thickening sidestream
was redirected to the existing Aeration Tank Inlet Channel while the sludge dewatering
sidestream was redirected to the existing Primary Clarifier influent at new Flow
Distribution Structure No. 1 with the Phase II Improvements.
Hydraulic Loadings. Pollutant and hydraulic loadings were developed for the wet-
weather treatment units at the Rockland WWTF during the Facilities Planning and
Design stages. The primary objective of the wet-weather treatment units is to remove
floatable and settleable solids to promote effective disinfection. The primary objective of
the industrial wastewater equalization basin is to remove high-strength industrial
wastewater from the CCS to allow full secondary treatment of these wastes at all times.
166
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Table 5.6.a-4 shows the design criteria for the wet-weather treatment processes at the
Rockland WWTF.
Table 5.6.a-4. Wet-Weather Treatment Design Criteria
Description and Criteria
A. Flows and Loadings
WWTF Peak Monthly Avg. Flow
WWTF Peak Daily Flow
WWTF Instantaneous Maximum
Total Annual CSO Volume
CSO Annual Volume at WWTF
CSO North/West Subsystem Peak (5 yr., 2 h)
CSO North/West Subsystem Peak (2 yr., 2 h)
Phase II Design Value
10,100m3/d
1 2,500 m3/d
21,600m3/d
186,600m3
188,100m3
1 27,550 m3/d
11 0,900 m3/d
2.90 MOD
3.30 MOD
5.70 MOD
49.3 MG
43.7 MG
33.7 MGD
29.3 MGD
CSO North/West Subsystem Peak (1 yr. 2 h)
CSO North/West Subsystem Peak (1/2 yr. 2 h)
B. Influent Pumping - Dry Weather
No. of Pumps (each)
Type
Flow
TDH
Power (each)
C. Screenings Handling System
Type
No. of units (each)
Channel Width
Bar Screen Clear Spacing
Bar Size
Headless, half clogged
D. Grit Removal
Type
No. of Units (each)
Diameter
Hopper Diameter
Type of pump
No. of pumps (ea.)
Flow
TDH
Horsepower (each)
Type of De-watering
Pressure
Screw Diameter
E. Wet Weather Pumping
No. of pumps (each)
Type
96,500 m3/d 25.5 MGD
71,500m3/d 18.9 MGD
Non-clog Submersible
15m3/min 4,000 gpm
9m SOn
37.3 kW 50 HP
Mechanical Screen
1
36 in.
1.0 in.
2-1/2x3/8 in.
9.0 in.
91.0cm
2.5cm
6.5x1.Ocm
22.9 cm
Pista
1
3.7m 12.0ft
1.5m 5.0 ft
Recessed Impeller
1
1 m3/min 260 gpm
9m 30n
10
Cyclone
4,922 kg/m2 7.0 psig
30.5cm 12.0 in.
Non-Clog Submersible
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Table 5.6.a-4. Wet-Weather Treatment Design Criteria, continued
Description and Criteria
Phase II Design Value
Flow each
TDH
Power each
Maximum Speed (rpm)
Minimum Efficiency (%)
F. Vortex Separator
No. of Units (each)
Type
Diameter each
Sidewater Depth each
5 year Hydraulic Loading
2 year Hydraulic Loading
1 year Hydraulic Loading
V* year Hydraulic Loading
Influent Pipe Size
Effluent Pipe Size
Underflow Pipe Size
Underflow Rate, Minimum
Underflow Rate, Maximum
30 m3/min
8.5m
74.6 kW
10.7m
2.7m
1,430m3/d/m
1,240m3/d/m
1,080m3/d/m
780 m3/d/m2
107cm
107cm
35.5 cm
6435 m3/d
1 2,870 m3/d
7,800 gpm
28 ft
100 HP
720
76
1
EPA
35 ft
8.75 ft
2 35,000 gpd/ft2
2 30,400 gpd/ft2
2 26,500 gpd/ft2
1 9,600 gpd/ft2
42 in.
42 in.
14 in.
1.7 MOD
3.4 MOD
No. of Underflow pumps
Type of pump
Pump Flow (each)
TDH
Motor power each
Minimum Efficiency (%)
Maximum Pump Speed (rpm)
G. CSO Disinfection And Dechlorination
No. of Units (each)
Volume
Length to Width Ratio
Chlorine Detention Time - 5 yr.
Q = 114,685.5 m3/d (30.3 MOD) (min)
Chlorine Detention Time - 2 yr.
Q = 98,031.5 m3/d (25.9 MOD) (min)
Chlorine Detention Time - 1 yr.
Q = 83,648.5 m3/d (22.1 MOD) (min)
Chlorine Detention Time - !4 yr.
Q = 58,667.5 m3/d (15.5 MOD) (min)
Dechlorination Tank Volume
Dechlorination Detention Time - 5 yr.
Q = 114,685.5 m3/d (30.3 MOD) (s)
No. of NaHSO3 pumps (each)
Capacity of NaHSO3 Pumps (each)
No. of Sodium Bisulfite Pumps (each)
Capacity of NaHSO3 Pump (each)
Self Priming Non-Clog
4.5 m3/min
4.3m
7.5 kW
1,170 gpm
14ft
10 HP
400m3
55
860
1
40:1
5.0
5.8
6.7
9.2
105,300 gal
19m'
5,100 gal
15
2+1 standby
0.4m3/h 105gph
1+1 standby
0.1 m3/h 25gph
168
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Treatment Objectives. Sampling and monitoring performed during the development of
the CSO Abatement Program indicated that Rockland Harbor does not meet current
water quality criteria for enterococcus bacteria during wet weather. Currently, the State
of Maine allows a maximum geometric mean enterococcus concentration of 14 colonies
per 100 ml and a daily maximum enterococcus concentration of 94 per 100 ml to
comply with Standards established for Class SC waters. The primary focus of the
phase II Improvements are:
• Increasing the quantity of CSO conveyed to the WWTF for treatment and
disinfection.
• Segregating high-strength industrial wastewater and WWTF sidestreams from
the overflow at the influent pump station.
• Providing high-rate primary treatment to remove floatable and settleable
solids from the CSO to facilitate disinfection.
The CSO Abatement Program recommended a plan to treat CSO discharges to meet
the current maximum daily water quality limits. The high-rate primary treatment and
disinfection processes were selected as the most economical method to meet the
treatment objectives to meet water quality objectives for this project.
Operational Requirements. The Phase II Improvements to the Rockland WWTF which
are designed to operate on a continuous basis include:
• Dry Weather Pumping Station
• Industrial Wastewater Equalization
• Mechanical Screening
• Grit Removal
• Primary Treatment
The wet-weather pumping system, high-rate primary treatment (vortex separator) high-
rate disinfection, and dechlorination systems are intended to operate in an event-based
mode during periods of excessive WWF. Under normal dry weather conditions flow
from the north and south CCS enter a new diversion structure (SS #1) upstream of the
influent pumping station. SS #1 contains a side overflow weir separating the overflow
from the normal sewage flow through the structure. As sewage flows increase during a
wet-weather event, the new submersible dry weather pumps will be throttled to control
flow to the secondary WWTF. As water levels rise in the dry-weather pump wetwell flow
will automatically be diverted to the overflow chamber. These overflows are conveyed
to the wetwell of the wet-weather pumping system. When levels in the wet-weather
wetwell rise, the wet-weather pumps will automatically start and convey flow to the new
vortex separator via a 1.0 m (42 in.) force main. Once flow through the vortex separator
has been established, a level switch located at the inlet to the high-rate CSO
Disinfection and Dechlorination Structure will start the vortex underflow pumping
system. The vortex underflow pumps are rated to deliver 4.50 m3 (1,170 gal) per min at
169
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4.5 m (14ft) total dynamic head. Each pump has a 7.5 kW(10 HP) electric motor with a
variable frequency drive allowing the WWTF staff to adjust the rate of vortex underflow.
The level switch located at the inlet of the CSO Disinfection and Dechlorination
Structure will also start the chemical metering pumps (sodium hypochlorite for
disinfection and sodium bisulfite for dechlorination) and the tank mixers.
An ultrasonic flow meter is provided at the 160 cm (64 in.) broad crested rectangular
weir at the inlet of the CSO Disinfection and Dechlorination Structure. The flow meter
records CSO flows, and provides a 4-20 mA signal to control the speed of the sodium
hypochlorite and sodium bisulfite chemical metering pumps. Flow pacing of the
chemical feed rate will improve disinfection and dechlorination efficiency, and reduce
overall chemical consumption.
At the end of the wet weather event automatic level controls will stop the submersible
wet-weather pumps. When the influent flow stops, the level in the vortex separator will
fall as it is drained with the vortex underflow pumping system. The chemical metering
pumps and tank mixers automatically stop when the level in the CSO Disinfection and
Dechlorination Structure falls below the pre-selected set point. The WWTF staff would
then drain the CSO Disinfection and Dechlorination Structure and wash down the
structures to prepare for the next event.
Chemical Storage and Feed Requirements. The CSO Abatement facilities will utilize
two chemicals:
• Liquid Sodium Hypochlorite for disinfection
• Liquid Sodium Bisulfite for dechlorination
The liquid sodium hypochlorite system is designed for commercial-grade liquid sodium
hypochlorite (15% solution) which contains the equivalent of 150 kg of chlorine/m3 (1.25
Ibs Cb/gal) of solution. The metering pumps can deliver a dosage of 25 mg/L at the
peak design flow with one unit out of service. The bulk storage tank has an effective
working volume of 4 m3 (1,000 gal). This tank is interconnected with the existing 15 m3
(4,000 gal) sodium hypochlorite bulk storage tanks used for secondary effluent
disinfection, giving the WWTF a total of 19 m3 (5,000 gal) of hypochlorite storage. The
available chlorine in sodium hypochlorite solution declines with age, the anticipated
sodium
MGD).
sodium hypochlorite storage time will be 52 days at the design flow of 10,980 m3/d (2.9
The liquid sodium bisulfite storage system is designed for commercial grade liquid
sodium bisulfite (38% solution) which contains appropriately 500 kg sodium bisulfite/m3
(4.2 Ibs NaHSOs/gal) of solution. The metering pumps can deliver sodium bisulfite
dosages necessary to dechlorinate effluent with a chlorine residual of 5.0 mg/L.
Approximately 1.6 kg of sodium bisulfite will be used per kg of chlorine residual. The
bulk storage tank has an effective working volume of 4 m3 (1,000 gal). This tank is
interconnected with the existing 8 m3 (2,000 gal) sodium bisulfite tank used for
170
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secondary effluent dechlorination, giving the WWTF a total of 12 rr? (3,000 gal) of
sodium bisulfite storage.
Sludge Handling Requirements. Sludge handling requirements increase during periods
of WWF. The most pronounced increase occurs during the initial period of WWF (first
flush).
A solids balance was performed to estimate the quantity of sludge produced during a
one yr frequency, two hr duration design storm. The daily sludge quantities would
increase as follows:
• Estimated Sludge Quantity 2,870 kg (6,326 Ibs) per d
• One-Year Storm Estimated Sludge Quantity 3,640 kg (8,029 Ibs) per d
The sludge quantity is estimated to increase by approximately 27% during a one-year
storm event. The overall increase can be assimilated if sludge pumping and processing
schedules are adjusted during the remainder of the week. The following is a summary
of weekly sludge quantities during a period with a one-year duration, two-hour
frequency design storm:
• Initial Estimated Sludge Quantity (kg/week) 20,090
• Projected Sludge with One-Year Design Storm (kg/week) 20,860
The estimated weekly sludge quantity would increase by approximately 4% during a
week with a one-year storm event.
Operation and Maintenance Requirements. The implementation of the Phase II
Improvements will result in increased O&M costs. The following list summarizes the
areas of expected O&M cost increases at the WWTF:
• Labor to perform sampling, laboratory analyses, reporting, wet-weather
process adjustments, and clean-up following an event.
• Power for in-plant pumping, sludge processing and chemical feed systems.
• Chemicals for disinfection, dechlorination, and polymer for additional sludge
quantities.
• Sludge disposal tipping and transport fees.
Table 5.6.a-5 summarizes the expected increase in O&M costs at the WWTF resulting
from the Phase II Improvements.
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Table 5.6.a-5. Estimated O&M Costs Phase II CSO Improvements
Estimated Annual
Description Cost
Sampling $600
Laboratory Analyses 2,500
Report Preparation 1,200
Process Control Adjustments 1,000
Clean-Up of Wet-Weather Treatment Structures 4,500
Power 700
Sodium Hypochlorite 3,100
Sodium Bisulfite 1,600
Polymer for Sludge De-watering 300
Sludge Disposal Transport and Tipping Fees 4,000
Total Estimated Cost $19,500
Construction Sequencing and Site Constraints. The existing Rockland WWTF site is
crowded with little room for new facilities in the area where the influent sewers enter the
WWTF. Therefore, it was necessary to construct some of the CSO Abatement Facilities
in locations previously used for other purposes. The following is a listing of special
sequencing and site considerations related to the CSO Abatement Program at the
WWTF:
• A storage garage was demolished to facilitate construction of the vortex
separator.
• There was limited space available for wet-weather pumping. The existing
preliminary treatment building (which contained the influent pumps) was
converted into a submersible pumping station for DWF and WWF. The dry
weather pumps are installed in the existing wetwell. The wet-weather pumps
are installed in the existing pump room that is converted into a wetwell for the
submersible wet-weather pumps. The modifications will require temporary
bypass pumping of flow into the WWTF during construction.
• New junction structures are required on the existing outfall pipe (SS #4 and
SS #5) to increase the hydraulic capacity. An emergency relief discharging to
Lermond Cove (adjacent to the WWTF site) will be preserved.
A substantial completion time of 510 calendar days was specified for the Phase II
Improvements.
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Project Costs
The bids for the Phase II Improvements were opened on July 8, 1998. Costs for the
CSO related components of the project are listed in Table 5.6.a-6. Costs include
construction requirements and project services. Project services include design
engineering, construction engineering, legal, and fiscal expenses. Project services
costs do not include contingencies, right-of-way expenses, or other miscellaneous
items.
Table 5.6.a-6. Project Costs -Phase II CSO Improvements
Description Cost1
General Conditions $175,000
Site Work and Yard Piping 375,000
Consolidation Conduit 140,000
Special Structure No. 1 90,000
Dry Weather/Wet Weather Pumping Station Renovation 180,000
Headworks/CSO Disinfection and Dechlorination 1,080,000
Structure
Vortex Separator 240,000
Special Structure No. 2 40,000
Equalization Tank Renovation 58,000
Industrial Wastewater Force Main 22,000
Flow Distribution Structure No. 1 60,000
Flow Distribution Structure No. 2 65,000
Special Structure No. 4 60,000
Special Structure No. 5 55,000
Yard Electrical Ductbanks and Lighting 90,000
Instrumentation and SCADA System 70,000
Total Construction Cost 2,800,000
Project Services2 600,000
Total Project Cost 3,400,000
1 Based on July 8, 1998 Bid Prices
Includes Design Engineering, Construction Engineering, Legal, and Fiscal
Expenses. Does not include contingencies and right-of-way costs.
Conclusions
The retrofit of an unused aeration tank and the construction of a separate force main
from the FMC Corporation Food Ingredients Division facility to the WWTF provides a
cost-effective alternative for the elimination of a established portion of high-strength
wastes from CSO discharges at the Rockland WWTF. This retrofit, combined with other
WWTF improvements directed at CSO abatement, will direct high-strength waste from
the FMC facility to the dry-weather headworks, eliminating the potential for CSO
discharge of industrial wastewater. This retrofit was part of an overall CSO Abatement
Program necessary to address system-wide CSO discharges.
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5.7 Bringing Outdated/Abandoned POTWs Back Online as Wet-Weather Flow
Treatment Facilities
The wastewater treatment facilities remaining following upgrades or construction of new
POTWs are often abandoned or demolished. These outdated or abandoned facilities
could possibly serve a useful function as part of the new treatment facilities depending
on their condition and location with respect to the new treatment facilities and the
combined sewer system. These facilities may be utilized as wet-weather storage or
flow equalization facilities.
5.7.a. Auburn WWTF, Auburn, New York
Background
The city of Auburn Water Pollution Control Plant was initially constructed during the
1930s and provided primary treatment. In 1970, the original primary treatment facilities
were upgraded to provide secondary treatment. Since that time, several improvements
have been completed.
The WWTF includes preliminary treatment (screening and grit removal), primary
sedimentation, trickling filtration, final sedimentation, and chlorination. Sludge treatment
facilities include gravity sludge thickeners, belt filter press for sludge dewatering, and
incinerators for sludge disposal. The ash from the incineration process is landfilled.
The Auburn WWTF services the city of Auburn and the surrounding Towns of Aurelius,
Fleming, Sennett, and Owasco. Portions of the existing sewer system consist of
combined sewers that carry both sanitary sewage flow and storm sewer flow in the
same pipe.
In December 1989, the city of Auburn entered into a consent order agreement with the
New York State Department of Environmental Conservation (NYSDEC). The consent
order required the city of Auburn to complete an Infiltration/Inflow Analysis, a CSO
Analysis, BMP Program, and a Comprehensive Performance Evaluation (CPE) for the
city of Auburn Water Pollution Control Plant.
Phase I improvements to the city of Auburn WWTF were performed between
1993-1995, as summarized below:
1. New preliminary treatment facilities consisting of mechanical bar screens
housed in a new influent building, aerated grit channels, and new influent flow
metering.
2. New primary treatment facilities, including new primary settling tanks and
sludge handling pumps and rehabilitation of the existing- settled sewage
pumps and controls.
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3. A new ORF utilizing the abandoned primary settling tanks, with chlorination
(using sodium hypochlorite) and dechlorination (using sodium bisulfite) facilities
and flow metering equipment for the ORF overflow.
4. Trickling filter rehabilitation, including replacement of rotary distribution arms,
rehabilitation of existing recirculation pumps and controls, and installation of
recirculation flow metering equipment.
5. New septage receiving facilities, including an enclosed septage receiving
station, mechanical septage screening, underground septage storage tank, and
septage handling equipment.
6. Odor control facilities to treat ventilated air from influent building, screenings
and grit loading area, septage receiving station and septage storage tank, and
belt press room of the existing administration building.
Construction costs for the Phase I improvements were estimated to be $9,600,000.
Figure 5.7.a-1 shows the WWTF flow schematic prior to construction of the Phase I
improvements
The following summarizes the retrofit of the abandoned primary treatment tanks for use
as overflow retention facilities for the treatment of WWF in excess of the treatment plant
capacity.
Previous Studies
As a requirement of the consent order with New York State, the city of Auburn has
completed studies on I/I and CSO abatement.
Infiltration and Inflow Study. In January 1991, the city of Auburn completed an I/I study
of the wastewater collection system. The purpose of this study was to develop a
recommended plan of corrective action to eliminate excessive I/I within the city's
sanitary sewer system. The study recommended the removal of sewer system
conveyance restrictions and the elimination of direct inflow sources. The results of the
I/I study indicated that infiltration was not excessive and that it was more cost effective
to transport and treat, than to eliminate. The study also suggested that approximately
31,420 m3/d (8.3 MGD) of peak inflow could cost effectively be removed.
CSO Abatement. In July 1993, the city of Auburn completed a CSO abatement
study/comprehensive water collection system plan. The CSO abatement study included
the evaluation of alternatives for the elimination, storage, and/or treatment of CSOs that
discharge to the Owasco Outlet within the city of Auburn. The CSO study identified the
alternative of centralized high rate treatment as the most cost effective and
environmentally sound alternative for the city of Auburn's collection system. This
alternative includes the construction of a new interceptor to increase the capacity of the
existing North Owasco and South Interceptors, and the construction of a high rate
treatment facility and diversion structure at the existing WWTF. The CSO study
recommended the implementation of the North Interceptor improvements as described
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5.7.a-1
176
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in the North Interceptor Replacement Sewer Basis of Design Report dated April 1993.
Also included in the CSO study was an estimate for the future (year 2010) total peak
flow to the WWTF of 191,000 m3/d (50.46 MGD). Of this total flow, 44,970 m3/d (11.88
MGD) was estimated for total base flow plus infiltration.
Comprehensive Performance Evaluation. In June 1992, an interim CPE report for the
city of Auburn was completed. This interim report developed the conceptual WWTF
improvements required to meet the anticipated reclassification of Owasco Outlet and
revised effluent limits.
Project Approach
The CSO report recommended the construction of peak flow storage facility on the
North Interceptor and the construction of parallel interceptors to convey peak flows to
the WWTF. This report had also recommended the construction of new overflow
facilities, such as a swirl concentrator, to treat flows above 78,400 nf/d (20.7 MGD).
These facilities were completed and operation began in July 1996.
In lieu of the swirl concentrators to treat peak flows at the WWTF, an ORF was
evaluated. The ORF would treat flows above the 95,000 m3/d (25 MGD) WWTF
capacity and be constructed from the abandoned primary settling tanks, which became
available after the treatment plant was upgraded. The reuse of this tankage represented
a significant cost saving to the city of Auburn
Overflow Retention Facilities
The abandoned primary settling tanks were rehabilitated to function as an ORF. The
basis of design for the ORF is summarized in Table 5.7.a-1 as described below.
Table 5.7.a-1. Basis of Design of the Overflow Retention Facilities
Overflow Retention Tanks Peak hourly flow 96,500 m3/d (25.5 MGD)
Number of Units 5 (abandoned primary settling tanks)
Total Surface Area 782 m2 (8,416 ft2)
Maximum SOR 123 m3/d/m2 (3,025 gpd/ft2)
Equipment
Overflow Return Pumps
Number of Units 4
Type Centrifugal
Return Duration 24 h
Overflow Influent Metering
Type Open Channel
Flow Element Weir
Flow Meter Ultrasonic
Range 0—11,355 m3/d ( 0—3 MGD)
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Table 5.7.a-1. Basis of Design of the Overflow Retention Facilities, continued
High-Rate Disinfection
Number of Units
Contact Tank Volume
Detention Time
Tank Dimensions (existing)
(Width x Length x Side Water Depth)
Mixing Energy
Chlohnation Chemical Feed Equipment
1 (abandoned primary settling tank #6)
318m3 (11,220 ft3)
4.7 min at peak hourly flowrate
5.9m x 20.9m x 2.6 m
(19.5 ft x 68.5 ft x 8.4 ft)
22.4 kW (30 HP)
Type
Dosage
Dechlorination
Liquid Sodium Hypochlorite
25 mg/l
Detention Time
Contact Tank Volume
Tank Dimensions
(Width x Length x Side Water Depth)
Equipment
Dechlorination Chemical Feed
Type
Dosage per mg/l of chlorine residual
Mixers
Number of Units
Type
Tank Flushing System
12 s at peak hourly flow
13m3 (458 ft3)
1.5 mx 3.6 mx2.4m
(5 ft x 11.85 ft x 7.74 ft)
Liquid Sodium Bisulfite
1.5 mg/l
1 per tank
Constant Speed
Source
Overflow Effluent Metering
Chlorinated Plant Effluent
Type
Flow Element
Flow Element
Flow Meter
Range
Open Channel
Open Channel
Weir
Ultrasonic
0—113,550 m3/d (30 MGD)
Influent flows in excess of the 95,000 m3/d (25 MGD) WWTF capacity are directed to
the retention facility through a controlled diversion structure. The maximum SOR
provided by using five of the six abandoned primary settling tanks as an overflow
retention tank(s), is 123 m3/d/m2 (3,025 gal/d/fr) at the estimated peak hourly flow of
96,500 m3/d (25.5 MGD). Overflow events producing less than approximately 4,100 m3
(1.083 MG), which includes volume disinfection contact chamber, are retained and
returned to the treatment process when influent flows subside. Overflow events
producing in excess of the available storage volume are disinfected using chlorine and
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dechlorinated before being discharged to the Owasco Lake Outlet. The retained
volume is returned to the aerated grit chambers utilizing four overflow return pumps.
Figure 5.7.a-2 shows the flow schematic of the Auburn WWTF following construction of
the ORF.
The overflow retention facilities include the following:
Modifications to the Abandoned Primary Settling Tanks. The primary sludge longitudinal
and cross collectors were removed to use the abandoned primary settling tanks as an
ORF. Deteriorated concrete surfaces were repaired and weirs were installed to allow
flow between tanks. A high volume tank flushing system was provided that uses
chlorinated plant effluent to wash the tank walls and bottom following an overflow event.
Overflow Return. The old primary sludge pump was replaced with overflow return
pumps. The new pumps are suitable for pumping grit and primary sludge. In order to
prevent odors, the entire overflow retention tank volume is returned to the treatment
process within 24 h after the occurrence of an overflow event provided influent flow
rates have subsided.
Disinfection. Overflow events producing stormwater in excess of the available storage
volume are disinfected using a high-rate disinfection system. The system consists of a
contact tank sized for a 4 min detention time at peak hourly flows of 96,520 m3/d (25.5
MGD) and requires the use of mechanical mixers to induce energy to provide effective
disinfection. The mixing velocity gradient and chlorine dosage were calculated on the
basis of estimated influent and effluent fecal coliform levels for CSOs, detention time,
concentration of TKN, BOD, and an estimated value for pH. The high-rate disinfection
system was sized by estimating disinfectant dosage and mixing energy required to
assure the inactivation of total coliform. Based on the optimum velocity gradient
calculated, horsepower requirements for mixers were determined. From this analysis, it
was determined that an optimum chlorine dosage of 25 mg/l and approximately 22.4 kW
(30 HP) of total mixing energy are required to insure proper disinfection.
The abandoned Primary Settling Tank #6 was modified for use as the high-rate
disinfection contact tank. The chlorination facilities are housed in a new structure
constructed over a portion of the new overflow retention tank. This structure houses the
chlorination and dechlorination equipment. The chlorination equipment was designed to
be automatically flow-paced according to the overflow flow rate.
Dechlorination. Dechlorination facilities were installed to reduce the chlorine residual
before discharge to the Owasco Outlet. The dechlorination facilities are metered
automatically as a function of overflow flow rate and will be designed to minimize
chlorine residual. The dechlorination tank contains a high-rate mixer designed for a
detention time of 30 s at peak hourly flows of 96,500 m3/d (25.5 MGD). This tank was
constructed within a portion of abandoned Primary Settling Tank #6.
Overflow Metering. Overflows discharged to the Owasco Lake Outlet are metered after
being discharged from the dechlorination tank through an open channel-type metering
device located in a separate concrete structure.
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5.7.a-2. and
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Stormwater Management Model Results
The EPA SWMM Version 4 was used to predict the annual capture of WWF that
previously discharged at the WWTF and is now treated through the ORF. A simplified
model of the WWTF drainage area was used to determine the impact of pre- and post-
retrofit. This analysis is based on 21 years of hourly rainfall from the NWS Station at
Hancock Field in Syracuse, NY. Table 5.7.a-2 demonstrates that use of the ORF
resulted in a considerable annual capture of WWF that was previously discharged
without treatment.
Table 5.7.a-2. Average Annual Overflow Volume from the Pre- and Post-Retrofit
Facilities (1971 -1991)
Average Annual Average Annual
Overflow Volume Overflow Volume
Condition (^) (MG)
Pre-Retrofit WWTF Overflow 29,000 7.65
Post-Retrofit Overflow(1) 7,000 1.85
Percent Capture 76%
(1)0verflows receive preliminary treatment and disinfection prior to discharge
The ORF annually captures 22,000 m3 (5.8 MG) of wet-weather overflow at the WWTF
that is returned to the WWTF influent for treatment following storm events.
Design Considerations
The retrofitting of the abandoned primary settling tank to provide enhanced wet-weather
treatment through storage and disinfection required careful consideration of several
factors. The following is a list of key design considerations for this retrofit:
• Pollutant and Hydraulic Loadings
• Treatment Objectives
• Operational Requirements
• Chemical Storage and Feed Requirements
• Sludge Handling Requirements
• Operations and Maintenance Requirements
Each of these considerations will be described in this section.
Pollutant and Hydraulic Loadings. Pollutant and hydraulic loadings were developed for
the ORF during the Facilities Planning stage. These values were refined and updated
as the project progressed through the preliminary engineering and detailed design
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stages. The primary objective of the ORF is to provide preliminary treatment and
disinfection of WWF beyond the 95,000 m3/d (25 MGD) WWTF capacity. The basis of
design for the ORF is shown on Table 5.7.a-1.
Treatment Objectives. The rationale for this retrofit was meeting the revised effluent
limits from the WWTF in the Owasco Lake outlet. Preliminary treatment and high-rate
disinfection were selected as the most economical method to meet the treatment
objectives for this project.
Operational Requirements. The ORF are designed to operate on a continuous basis
include:
• Preliminary Treatment
• High-Rate Disinfection
Each of the five ORF tanks fill uniformly during an overflow event. After the wet-
weather flows subside, the flow retained in the ORF is pumped to the WWTF
headworks for treatment. WWTF staff manually controls this operation. The tanks are
flushed by a manually controlled system using chlorinated plant effluent.
Chemical Storage and Feed Requirements. The ORF utilize two chemicals:
• Liquid Sodium Hypochlorite for disinfection
• Liquid Sodium Bisulfite for dechlorination
The liquid sodium hypochlorite system design is based on commercial-grade liquid
sodium hypochlorite (15% solution) which contains the equivalent of 160 kg of
chlorine/m3 (1.25 Ibs Cb/gal) of solution. The metering pumps are designed to deliver a
dosage of 25 mg/L at the peak design flow with one unit out of service. The liquid
sodium hypochlorite is stored in a bulk storage tank with an effective working volume of
10 m3 (2,500 gal). The available chlorine in sodium hypochlorite solution declines with
age. The anticipated sodium hypochlorite storage time will be 51 days initially,
decreasing to 25 days at the future design flow.
The liquid sodium bisulfite storage system design is based on commercial grade liquid
sodium bisulfite (38% solution) that contains appropriately 503 kg of sodium bisulfite/m3
(4.2 Ibs NaHSOs/gal) of solution. The metering pumps are designed to deliver sodium
bisulfite dosages necessary to dechlorinate effluent with 1.5 mg/L of sodium bisulfite per
mg/L of chlorine residual. Approximately 1.6 kg of sodium bisulfite will be used per kg
of chlorine residual. A bulk storage tank with an effective working volume of 3.8 rrr
(1,000 gal) is provided.
The sodium hypochlorite and sodium bisulfite metering pumps are flow paced. An
ultrasonic type level sensor is used for effluent flow measurement. A broad-crested
rectangular weir 3 m (10 ft) in length is provided as the primary measuring device.
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Sludge Handling Requirements. Accumulated sludge will be handled through the pump
back of retained WWF and the associated tank flushing following tank draining.
Operation and Maintenance Requirements. The implementation of the ORF will result
in increased O&M costs. The following list summarizes the areas of expected O&M
cost increases at the WWTF:
• Chemicals for disinfection and dechlorination
• Power for tank draining and flushing, and chemical feed systems.
The estimated annual O&M costs are provided in Table 5.7.a-3.
Table 5.7.a-3. ORF O&M Costs
Annual Chemical Costs Cost
Sodium Hypochlorite $7,000
Sodium Bisulfite $3,000
O&M $8,000
Total $18,000
Project Costs
Project costs for the capital costs for the ORF are provided in Table 5.7.a-4.
Table 5.7.a-4. ORF Chlorination/dechlorination Capital Costs
Chlorination Cost
Chemical Feed Pumps and Controls $20,000
Storage Facilities $14,000
Installation $21,000
Subtotal $55,000
Dechlorination
Chemical Feed Pumps and Controls $20,000
Storage Facilities $16,000
Residual Analyzer System $10,000
Installation $19,000
Subtotal $65,000
Total Cost $120,000
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The conventional treatment alternative for this retrofit would be to construct wet-weather
storage facilities to equalize WWF for ultimate treatment at the WWTF. This alternative
would require the construction of a 4,100 m3 (1.083 MG) ORF. The estimated cost to
construct this facility based upon EPA cost guidelines for CSO control technologies
(EPA 1992) is $3.816 million derived from the following equation:
Cost = 3.577V0812 (1)
where,
Cost = Average construction cost, millions of dollars
V = Storage volume, MG
The ORF retrofit is a simple solution, requiring minimal O&M cost, resulting in a total
cost that is a fraction of the cost of constructing new wet-weather storage facilities. The
ORF retrofit provides an average annual 76% wet-weather volume capture. The cost for
this retrofit is $120,000 dollars compared to the $3,816,000 cost for conventional
storage facilities.
Conclusions
Retrofitting the abandoned primary treatment tanks for use as ORF was a success, in
large part due to the availability and condition of the existing facilities. The abandoned
primary tanks were in good condition and required only minor modifications for use as
an ORF. The ORF retrofit is a cost-effective alternative in cases where abandoned
primary tanks are available in good condition for retrofitting. This type of retrofit may be
a less cost-effective solution for cases in where the existing tanks are in poor condition
requiring a substantial cost to upgrade the tanks to serve a functional role as an ORF.
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Section 6 - References
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95-002, Washington, D.C., May 1995.
U.S. Environmental Protection Agency (EPA) (1998), "National Pollutant Discharge
Elimination System-Proposed Regulations for Revision of the Water Pollution Control
Program Addressing Storm Water Discharges." (Phase II Proposed Rule.) Federal
Register. 63 FR 1536, January 8, 1998.
U.S. Environmental Protection Agency (EPA) (1999), Preliminary Data Summary of
Urban Storm Water Best Management Practices. Office of Water, EPA-821-R-99-012,
Washington, DC, August 1999.
Walesh, S.G. (1992) Retrofitting Storm Water Facilities for Quantity and Quality
Control. In Water Resource Planning and Management. American Society of Civil
Engineers, NY, NY. 786-791
Welborn, H.L. (1974) Surge Facility for Wet and Dry Weather Flow Control. EPA-
670/2-74-075, U.S. Environmental Protection Agency, Cincinnati, OH.
Whitney-Jacobsen. (1981) Joint Dry/Wet Weather Treatment of Municipal Wastewater
at Clatskanie, Oregon. U.S. Environmental Protection Agency, Municipal Environmental
Research Lab., Cincinnati OH. Whitney-Jacobsen and Associates, Portland, OR.
189
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Woodward-Clyde Consultants. (1990) Evaluation of Existing Stormwater Pump Stations
in Santa Clara County for Pollution Control Potential. Prepared for Santa Clara Valley
Water District, July, 1990.
Woodward-Clyde Consultants. (1994) Sunnyvale Detention Basin Demonstration
Project. Prepared for Santa Clara Valley Water District, July, 1994.
Woodward-Clyde Consultants. (1996) Sunnyvale Detention Basin Demonstration
Project. Santa Clara Valley Nonpoint Source Pollution Control Program. Woodward-
Clyde Consultants, Oakland, CA.
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190
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APPENDIX A
BIBLIOGRAPHIC DATABASES
Wilson Applied Science & Technology Abstracts (WBA)
H.W. Wilson Company Phone: (718) 588-8400
950 University Ave. 800-367-6770
Bronx, NY 10452 Fax (718) 538-2716
Contact: Technical Support Department.
E-mail: techmailinformationo.hwwiison.com (Internet).
Type: Bibliographic,
Content: Contains abstracts to all articles cited in the Applied Science Technology
Index (described in a separate entry) from March 1993 to date. Includes an index of
non-English-language periodicals English abstracts are provided. Sources include 400
English language scientific and technical publications.
Subject Coverage. Applied science and technology, including such areas as
aeronautics and space science, chemistry, computer technology, construction, data
processing, energy, engineering, the environment, fire prevention, food, geology,
machinery, marine technology and oceanography, mathematical metallurgy,
meteorology, mineralogy, optics, petroleum and gas, physics, robotics, solid state
technology, telecommunications, textiles, transportation, and water and waste.
Language: English.
Geographic Coverage: International.
Time Span: October 1983 to date (indexing coverage); March 1993 to date (Abstracts).
Updating: Approximately 1,000 records per week.
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Compendex*Plus
Engineering Information, Inc. (Ei) Phone: (201) 216-8500,
Castle Point on the Hudson 800-221-1044
Hoboken, NJ 07030 Fax: (201) 216-8532
Type: Bibliographic.
Content: Contains more than 3.63 million citations, with abstracts, from more than
2,500 journals, reports, books, and conference proceedings covering fields of
engineering and technology. Sources include approximately 2500 journals, reports,
books, and conference proceedings. Conference review records contain conference
code numbers that are links to complete coverage of all individual papers. Corresponds
in part to The Engineering Index Monthly and Engineering Index Annual.
Subject Coverage: All fields of engineering, including related areas of app science,
management, and energy. Specific fields covered include civil engineering,
construction and materials, transportation, waterworks, sanitary, engineering,
bioengineering, ocean engineering, mining, petroleum, fuel technology, metallurgical
engineering, mechanical engineering, aerospace, engineering, automotive engineering,
electronics and electrical engineering, control engineering, chemical engineering,
nuclear engineering, and computers and robotics engineering.
Language: English.
Geographic Coverage: International.
Time Span: 1970 to date.
Updating: Weekly; 13,000 records a month.
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Environmental Sciences & Pollution Management Database
Cambridge Scientific Abstracts (CSA) Phone: (301 )961 -6750
7200 Wisconsin Ave., Ste. 601 800-843-7751
Bethesda, MD 20814-4823 Fax: (301 )961 -6720
Type: Bibliographic.
Content: Contains more than 64,000 citations, with abstracts, to literature covering the
environmental sciences. Covers some 1498 core journals, monographs, and
conference proceedings, as well as 5500 secondary sources, drawn from the primary
sources of abstracts journals such as: Ecology, Pollution Digest of Environmental
Impact Statements, toxicology, Health & Safety Science, Bacteriology, Environmental
Engineering, and others.
Subject Coverage: Environmental sciences, including agricultural biotechnology,
aquatic environments, bacteriology, ecology, environmental biotechnology,
environmental engineering, environmental impact statements, industrial hygiene,
microbiology applied to industrial and environmental issues, pollution (land, air, water,
noise, and solid waste), risk assessment, safety science, toxicology, water quality, and
water resource issues.
Language: English.
Geographic Coverage: United States.
Time Span: 1983 to date.
Updating: Monthly. 76,000 records/year.
National Center for Environmental Publications and Information
United States Environmental Protection Agency
Washington, D.C.
http://www.epa.gov/ncepihom/index.html
Type: National Publications Catalog
Content: 5,500 titles in paper and/or electronic.
Subject Coverage: Repository for all EPA documents.
Language: English.
Time Span:
Updating: Yearly.
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ProCite
Robert E. Pitt, P.E., Ph.D., DEE
University of Alabama at Birmingham
Hoehn Engineering Building
1075-13th St. So.
Birmingham. AL 35294
Phone: (205)934-8434
Fax: (205) 975-9042
e-mail: RPITT@ENG.UAB.EDU
Type: Bibliographic.
Content: 3,700 wet-weather references.
Subject Coverage: The database was prepared by Robert Pitt for the Wet Weather
Floe Design for the Future project under contract by the United States Environmental
Protection Agency.
Language: English.
Time Span: 1899-1997
Updating: Database constructed in 1997.
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APPENDIX B
SELECTED TABLES WITH ENGLISH EQUIVALENT UNITS
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Table 5.1 .b-2. Comparison of ACTIFLO and Existing Primary Treatment Systems
Net Savings with
Parameter
Total Capacity
Number of Units
Unit Capacity
Area Required
Existing System
240 MG
8
30MGD
14,307 ft2 (per unit)
ACTIFLO
240 MG
6
40MGD
945 ft2 (per train)
ACTIFLO
94% per unit
114,453 IT (total) 5,670 fT (total) 95% total
Process Volume 143.066 ft3 (per unit) 20,790 ft3 (per unit) 85% per unit
1,114,530 ft3 (total) 124,740 ft3 (total) 89% per total
Hydraulic Retention 51 min 5.6 min 89%
Time
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