United States
Environmental Protection
Agency
Office of Research and
Development
Washington DC 20460
EPA/600/R-98/157
December 1998
Sewer and Tank
Sediment Flushing
Case Studies
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EPA/600/R-98/157
December 1998
Sewer and Tank
Sediment Flushing:
Case Studies
by
William C. Pisano and James Barsanti
Montgomery Watson
40 Broad Street, Suite 800
Boston, MA 02109
and
James Joyce and Harvey Sorensen, Jr.
Odor & Corrosion Technology Consultants, Inc.
11250 West Road Building L
Houston, TX 77065
Contract No. 8C-R059-NTSX
Project Officer
Chi-Yuan Fan
Water Supply and 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, OH 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-R059-NTSX. It has been subjected to the
Agency's peer and administrative review for publication as an EPA document, but 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 risks 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; protectipn 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 innpyative, 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
Past studies have identified urban combined sewer overflow (CSO) and stormwater
runoff as major contributors to the degradation of many urban lakes, streams, and
rivers. Sewage solids deposited in combined sewer (CS) systems during dry weather
are major contributors to the CSO-pollution load. Innovative methods for cleaning
accumulated sludge and debris in CSO and stormwater conveyance systems and
storage tanks have emerged over the last 15 years by creating high speed flushing
waves to resuspend deposited sediments. Cleansing efficiency of periodic flush waves
depends on flush volume, flush discharge rate, sewer slope, sewer length, sewer flow
rate, sewer diameter and population density. Maximum flushing volumes at upstream
points are limited by available space, hydraulic limitations and costs. Maximum flushing
rates at the downstream point are limited by the regulator/interceptor capacities prior to
overflow. The relationship between cleaning efficiency and pipe length is important. The
aim of flushing is to wash the resuspended sediment to strategic locations, i.e., to a
point where the waste stream is flowing with sufficient velocity, to another point where
flushing will be initiated, to a storage sump which will allow later removal of the stored
contents, or to the wastewater treatment plant (WWTP). This reduces the amount of
solids resuspended during storm events, lessens the need for CSO treatment and
sludge removal at downstream storage facilities, and allows the conveyance of more
flow to the WWTP or to the drainage outlet. This report will demonstrate that sewer
system and storage tank flushing that reduces sediment deposition and accumulation is
of prime importance to optimizing performance, maintaining structural integrity, and
minimizing pollution of receiving waters.
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Contents
Notice ii
Forward iii
Abstract iv
Contents v
Tables ix
Figures x
Abbreviations and Acronyms xii
Acknowledgements xiii
Chapter 1 - Introduction 1
Project Scope 1
Background 2
Odor and Corrosion Perspective 2
Previous Research 3
Chapter 2- Conclusions and Recommendations 5
Chapter 3- Sewer Sediments and Sewage Gases Estimation Techniques 6
Introduction 6
Nature of Sewer Solids and Sediment in Sewers 6
Sewer Self Cleansing Criteria 7
Desktop Procedure 8
Chapter 4- Hydrogen Sulfide and Sulfuric Acid Estimation Techniques 11
Sulfide Generation in Sewers 11
Sulfide Ion (S=) 11
Bisulfide Ion (HS-) 11
Hydrogen Sulfide (Aqueous) 11
Hydrogen Sulfide (Gaseous) 12
Settleable Solids 12
Temperature 13
Sulfuric Acid Production 13
Turbulence Reduction 13
Concrete Corrosion 13
Metal Corrosion 14
Trends in Sulfide Production 15
Water Conservation Practices 15
Industrial Pretreatment 15
Design Deficiencies 16
Flushing Relationship to Hydrogen Sulfide Generation 16
Description of the Process 16
Odor Producing Conditions 17
Invert Erosion 17
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Contents
(continued)
Dissolved Sulfide Prediction Procedure 18
Sulfide Model Development 18
Dissolved Oxygen Effects 19
Chapter 5- Overview of Sewer Cleaning Flushing
Systems and CSO Tank Cleaning Technology 20
Conventional Sewer Cleaning Techniques 20
Power Roding 20
Balling 20
Jetting 20
Pigging 20
Power Bucket 21
Silt Traps 21
Sewer Flushing 21
Hydrass 21
Hydroself 22
Biogest Vacuum Systems 22
Storage Tank Cleaning Alternatives 23
Introduction 23
Primary Flushing Systems 24
Traveling Bridge 24
Traveling Bridge - Scraper 24
Traveling Bridge Suction Pickup 24
Traveling Bridge with Washdown Nozzles 24
Mechanical Mixers and Submerged Jets 25
Fixed Spray Nozzles and Headers 26
Tipping Flushers 27
Flushing Gates 27
Secondary Flushing Systems 29
Water Cannons 29
High Pressure Hoses 29
Novel Approach for Cleaning Circular Tanks 29
Chapter 6 - Case Evaluations 31
Case Studies: Combined Sewer Flushing Facilities using Flushing Gates 32
Case No. 1 Marht Wiesentheid 32
Case No. 2 Gemeinde Schauenburg 32
Case No. 3 Stadt Kirchhain 32
Case No. 4 Stadt Heidenheim 34
CaseNo.S-MarktGrossostheim 34
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Contents
(continued)
Case No. 6 - Osterbruch-Opperhausen 34
Case No.7- Geimeide Hettstadt 36
Hydraulic Analysis of Flushing Gate Performance for Sewers 36
Case Studies: CSO Storage Tank Flushing Facilities Using Flush Gates 37
Case No. 8 - Filterstadt-Bernhausen 37
Case No. 9 - Stadt-Essen 39
Case No. 10 - Markt-Wiesentheid 39
Case No. 11 - Stuttgart-Wangen 41
Case No. 12 - Heidenheim-Kleiner-Buhl 41
Case No. 13 - Cheboyan 42
Case No. 14 - Sarnia 44
Hydraulic Analysis of Flushing Gates for Rectangular Tanks 44
Evaluation of Flushing Gates for Tanks 48
Case Studies: CSO Storage Tanks Utilizing Tipping Flusher Technology 49
Case No. 15 - Port Colborne 49
Case No. 16 - Wheeler Avenue 52
Case No. 17 - 14th Street Pumping Station 52
Case No. 18 - Saginaw Township Center Road Storage Tank 58
Evaluation of Tipping Flushers 58
Other Flushing Case Studies 58
City of Essen, Germany 58
City of Konstanz, Germany 58
City of Augsburg, Germany 59
City of Elizabeth, New Jersey - Mechanical Flap Gate Flusher 59
Chapter 7- Cost Analysis 60
Case Study One: Fresh Pond Parkway Sewer Separation and Surface
Enhancement Project Storm and Sanitary Sewer Flushing 60
Description of Piping Systems to be Flushed 60
Description of Flushing Vaults 60
Life Cycle Costs Comparison: Automated Flushing versus Periodic 61
Details of the Automated Cleaning System 62
Details of the Manual Cleaning System 63
Case Study Two: Cost Effectiveness of Sewer Flushing Versus Conventional
Treatment 65
Description of Area 65
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Contents
(continued)
Case Study Three: Cost Effectiveness of Sewer
Flushing Versus Chemical Addition for Hydrogen Sulfide Control 67
General 67
Chemical Treatment of Dissolved Sulfide 68
Pure Oxygen 68
Hydrogen Peroxide 68
Chlorine 68
Potassium Permanganate 68
Nitrate Solution 68
Iron Salts 68
Flushing Gates for Sulfide Reduction 68
Case Study Description 69
Cost Analysis 69
Condition 1 - Sulfide Treatment with Ferrous Chloride 69
Condition 2 - Sulfide Treatment with Ferrous Chloride and
Flushing Gates 69
Case Studies: Cost Effectiveness Studies of CSO Tank Cleaning Methods 70
Case Study: Eastern Beaches, Toronto, Ontario, Canada 70
Operation and Maintenance 70
Flushing Spray 70
Manual Spray 71
Odor Control 71
Case Study: Sarnia Ontario 71
Cleaning Alternatives 72
References 73
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List of Tables
Table Page
3-1. Minimum Velocity Criteria 7
3-2. Minimum Shear Stress Criteria 7
3-3. Assumed Inorganic Material Composition 9
3-4. Distribution of Inorganic Material Settling Velocities 9
3-5. Assumed Organic Material Size, Composition and Settling Velocities 10
3-6. Assessment of 3.05 Meter Tunnel 10
4-1. Dissolved Sulfide and Total Metals Concentration
Trends at Hyperion WWTP, Los Angeles, CA 16
6-1. Overview of Case Studies 31
6-2. Marht Wiesentheid, Germany 32
6-3. Gemeinde Schauenburg, Germany 32
6-4. Stadt Kirchhain, Germany 33
6-5. Stadt Heidenheim, Germany 33
6-6. Markt Grossostheim, Germany 34
6-7. Osterbruch-Opperhausen, Germany 34
6-8. Gemeinde Hettstadt, Germany 36
6-9. Summary of Pipe Flushing Results 37
6-10. Filterstadt-Bernhausen, Germany 37
6-11. Stadt-Essen, Germany 39
6-12. Markt-Wiesentheid, Germany 39
6-13. Stuttgart-Wangen, Germany 41
6-14. Heidenheim-Kleiner-Buhl, Germany 41
6-15. Cheboygan, Michigan 42
6-16. Sarnia, Ontario Canada 44
6-17. Summary of Tank Flushing Results 48
6-18. Port Colborne, Ontario, Canada 49
6-19. Wheeler Avenue, Louisville, Kentucky 52
7-1. Flushing System Summary 63
7-2. Cost Effectiveness Analysis Flushing versus Manual Cleaning 64
7-3. Average Daily and Average Maximum Hourly Velocity and Shear Stress 65
7-4. Pertinent Overflow Characteristics 65
7-5. Comparison Satellite Treatment Versus Automatic Flushing 67
7-6. Present Worth Cost Comparison Flushing Gate Vs Satellite Treatment 67
7-7. Wastewater Characteristics 69
7-8. Chemical Treatment Costs 69
7-9. Capital and Operation and Maintenance Cost Comparison 71
7-10. Alternatives Capital and Operation and Maintenance Cost Assessment 72
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List of Figures
Figure Page
5-1. Hydrassฎ 21
5-2. Flushing Gate 23
5-3. (a) Traveling Bridge 25
(b) Submerged Jet 25
5-4. Fixed Spray Header and Nozzle Arrangement 26
5-5. Tipping Flusher 27
5-6. Plan View - Clough Creek OSO Treatment Facility, Cincinnati, OH 28
5-7. Section View - Clough Creek OSO Treatment Facility, Cincinnati, OH 28
5-8 Lincoln Park Schematic 30
6-1. Stadt Kirchhain - Off line Flushing Vault, Plan view 33
6-2. Stadt Kirchhain - Downstream Control Chamber, Plan View 33
6-3. Stadt Heidenheim - Plan View 34
6-4. Stadt Heidenheim - Section View 35
6-5. Osterbruch-Opperhausen - Sectional Views 35
6-6. Stadt-Essen - Plan View Tank Influent 38
6-7. Stadt-Essen - Plan View Tank Effluent 38
6-8. Markt-Wiesentheid - CSO Storage Tank, Section View 40
6-9. Cheboygan Circular - CSO Storage Tank Plan and Section Views 42
6-10. Cheboygan Circular - Photographs of Circular CSO Storage Tank 43
6-11. Sarnia - CSO Storage Tank, Plan View 45
6-12. Sarnia - CSO Storage Tank, Section View 46
6-13. Sarnia - Photographs of Hydraulic Opening Mechanism 47
6-14. (a) Port Colborne - CSO Storage Tank, Plan View 50
(b) Port Colborne - CSO Storage Tank, Section View 50
6-15. Port Colborne, Ontario Canada - CSO Storage Tank, Photographs 51
6-16. Wheeler Avenue - CSO Storage Tank, Plan View 53
6-17. (a) Wheeler Avenue - Photograph of Overall CSO Storage Tank 53
(b) Wheeler Avenue - Photograph of Filling Tipping Flushers 54
6-18. 14th Street Pumping Station -Tanks B1, B2 and B3 55
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List of Figures
(Continued)
6-19. 14th Street Pumping Station, Saginaw Michigan - B-4 56
6-20.(a) 14th Street Pumping Station - Flushing Sequence for Tank B4 56
(b) 14th Street Pumping Station - Flushing Sequence for Tank B4 57
(c) 14th Street Pumping Station - Flushing Sequence for Tank B4 57
7-1. Fresh Pond Parkway - Locations of Flushing Vaults 61
7-2. Fresh Pond Parkway - Flushing Gate Chamber 62
7-3. Conceptual Schematic for High Rate Treatment 66
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Acronyms and Abbreviations
ac
aq
BOD
ฐC
cfs
cm
CS
CSHG
CSO
DO
EDP
EPA
EXTRAN
fps
ft
gpd
gpm
HP(hp)
hr
in
kPa
I
Ips
m
mg
MG
MGD
mm
N
NRMRL
pkwy
POTW
ppm
psi
QAPP
R&D
sec
SS
SWMM
TF
US
WWTP
$
Acre
Aqueous
Biochemical oxygen demand
Degrees Celsius
Cubic feet per second
Centimeter
Combined sewer
Calcium Silicate Hydrate Gel
Combined sewer overflow
Dissolved oxygen
Environmental Design & Planning, Inc.
United States Environmental Protection Agency
Extended Transport Block
Feet per second
Feet
Gallons per day
Gallons per minute
Horsepower
hour
Inch
kiloPascals
Liter
Liters per second
Meter
milligram
Million gallons
Million gallons per day
Millimeter
Newton
National Risk Management Research Laboratory
Parkway
Publicly Owned Treatment Works
Parts per million
Pounds per square inch
Quality Assurance Protection Plan
Research & Development
second
Suspended solids
Stormwater Management Model
Tipping flusher
United States
Wastewater treatment plant
Dollars
Percent
inch
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Acknowledgements
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 is acknowledged and appreciated.
William Pisano, Frank Ayotte, James Barsanti, Christopher Brown, Dennis Lai, David Slevin,
Joseph Uglevich III, and Brent Watts of Montgomery Watson, Boston, MA, and James Joyce
and Harvey Sorensen of Odor and Corrosion Technology Consultants, Inc., Houston, TX were
the principal authors of this report. The following individuals provided invaluable technical
assistance during the development of this report:
Nicholas Grande and Gabriel Novae, Grande, Novae & Associates, Inc., Montreal, Quebec,
Canada Mario Parente, CH2M Gore & Storrie Limited, North York, Ontario, Canada
The cooperation of Chi-Yuan Fan, Project Officer, Urban Watershed Management Branch,
Water Supply and Water Resources Division, National Risk Management Research Laboratory,
U.S. Environmental Protection Agency, is acknowledged.
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Chapter 1
Introduction
Past studies have identified urban combined sewer overflow (CSO) and stormwater runoff as
major contributors to the degradation of many urban lakes, streams, and rivers. Sewage solids
deposited in combined sewer (CS) systems during dry weather are major contributors to the CSO-
pollution load. In recent years, pollution caused by CSO has become a serious environmental
concern. Although requirements vary from state to state concerning allowable overflow
frequencies, compliance has resulted in the design and construction of storage facilities as well as
utilization of in-line storage or constructing deep tunnels. In the case of in-line storage, shallow
slopes and low mean velocities allow debris to settle along the invert of the pipe during storage
periods. Accumulation of sediments results in a loss of storage capacity that may cause
surcharge or local flooding and the establishment of septic conditions that create odor and
corrosion problems.
Some simple calculations illustrate the potential impacts of overflows on receiving waters.
Estimates of dry weather flow deposition in combined sewer systems have ranged from 5 to 30
percent of the daily pollution loading. If 25 percent of the daily pollution loading accumulates in
the collection system, an intense rainstorm lasting two hours after four days of antecedent dry
weather may wash the equivalent of a one-day's flow of raw wastewater overboard to the
receiving waters. The average antecedent dry period between storm events is about four days for
many areas of the U.S., especially along the eastern seaboard. Furthermore, a one-day
equivalent of raw wastewater discharged within a two-hour period, is twelve times the rate at
which raw wastewater enters the collection system.
This report will demonstrate that sewer system and storage tank flushing that reduces sediment
deposition and accumulation is of prime importance to optimizing performance, maintaining
structural integrity, and minimizing pollution of receiving waters.
Project Scope
The U.S. Environmental Protection Agency's (EPA's) National Risk Management Research
Laboratory's (NRMRL's) Office of Research and Development's Urban Watershed Management
Branch, Water Supply and Water Resources Division have supported the development of this
project report for the investigation of sewer and tank sediment flushing. The report is designed to
provide information and guidance to meet the following objectives:
1. Investigate the cost-effectiveness of combined sewer in-line and CSO storage tank flushing
systems for removing combined sewer sediments and CSO storage tank bottom deposits at
actual installation sites in urban watersheds.
2. Develop a long-term combined sewer hydrogen sulfide monitoring program.
To meet these objectives, the following tasks were developed:
1. Identification of 18 sites in North America and Europe for evaluation of in-line (10) and CSO
storage tank (8) flushing systems.
2. Desk Top analyses of published and unpublished information on the range of hydrogen
sulfide concentration combined sewers, the correlation between sediment characteristics and
hydrogen sulfide generation, and the effectiveness of combined sewer flushing systems to:
Decrease the rate of hydrogen sulfide generation during dry weather conditions by
removing an important and often significant contributory source of available microbial
food;
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Eliminate potential for creating highly unsafe transitory hydrogen sulfide levels
associated with rapid biological activity of resuspended sediments during high flow
conditions;
Lessen the potential for sewer decomposition associated with elevated hydrogen
sulfide generation; and
Maximize sewer flow carrying capacity by removing sediment/blockages.
3. Collect and analyze operational information on the 18 identified sites regarding:
The effectiveness of systems design in terms of sediment removal;
Capital and operation and maintenance costs;
Operational problems and lessons learned from these sites.
4. Perform cost-effective-benefit analyses of the 18 identified/selected in-line and storage tank
flushing systems.
5. Develop a generic Quality Assurance Protection Plan (QAPP) for conducting a field program
for monitoring and analyzing long term sediment characteristics and hydrogen sulfide
generation rates within a problem combined sewer system.
Background
Innovative methods for cleaning accumulated sludge and debris in CSO and stormwater
conveyance systems and storage tanks have emerged over the last 15 years by creating high
speed flushing waves to resuspend deposited sediments. In the last ten years, at least three new
passive hydraulic systems have been developed and installed in Europe to routinely flush sewer
deposits and wet weather storage tanks. In Europe, 77 installations have been in operation since
1985 to flush sewers, interceptors and tunnels ranging from 0.25 to 4.3 meters (10 inches to 14
feet) in diameter and flushing lengths of up to 335 meters (1100 feet) for large diameter pipes and
1000 meters (3300 feet) for smaller diameter.
Cleansing efficiency of periodic flush waves depends on flush volume, flush discharge rate, sewer
slope, sewer length, sewer flow rate, sewer diameter and population density. Maximum flushing
volumes at upstream points are limited by available space, hydraulic limitations and costs.
Maximum flushing rates at the downstream point are limited by the regulator/interceptor capacities
prior to overflow. The relationship between cleaning efficiency and pipe length is important. The
aim of flushing is to wash the resuspended sediment to strategic locations, i.e., to a point where
the waste stream is flowing with sufficient velocity, to another point where flushing will be initiated,
to a storage sump which will allow later removal of the stored contents, or to the wastewater
treatment plant (WWTP). This reduces the amount of solids resuspended during storm events,
lessens the need for CSO treatment and sludge removal at downstream storage facilities, and
allows the conveyance of more flow to the WWTP or to the drainage outlet.
Flushing gates and tipping flushers for cleaning accumulated sludge and debris in CSO and
stormwater storage tanks have emerged in Germany and Switzerland. Both methods create high
speed flushing waves to resuspend sediments on the tank floor and sweep these materials to a
disposal channel at the end of the tank. The flushing gate system has been used extensively in
Europe with 209 installations and 436 units in operation since 1985. Approximately 60 percent of
the installations are for cleaning sediments from CSO tanks. The tipping flushers were initially
developed in Switzerland, and were optimized to present design in Germany. Presently there are
several thousand CSO tanks throughout Europe using the tipping gate technology.
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Odor and Corrosion Perspective
Sewer sediments create odor and sewer decomposition problems in addition to CSO pollution.
The production and release of hydrogen sulfide gas in municipal wastewater collection systems is
responsible for numerous odor complaints and the destruction of sewer pipes and other
wastewater facilities. Sulfates are released from organic substances contained in the sewer
sediments by bacteria under anaerobic conditions. In the absence of dissolved oxygen and
nitrates, sulfates serve as electron acceptors and are chemically reduced to sulfides and to
hydrogen sulfide by bacteria. The hydrogen sulfide is then converted to sulfuric acid, which
disintegrates the sewer pipes.
The process begins with the biological reduction of sulfate to sulfide by the anaerobic slime layer
residing below the water surface in wastewater collection systems. The anaerobic bacteria utilize
the oxygen in the sulfate ion as an electron acceptor in their metabolic processes. The resulting
sulfide ion is transformed into hydrogen sulfide gas after picking up two hydrogen ions from
wastewater.
Once released to the sewer atmosphere, aerobic bacteria (Thiobacillus) which reside on sewer
walls and surfaces above the water line consume the hydrogen sulfide gas and secrete sulfuric
acid. In severe instances, the pH of the pipe can reach 0.5. This causes severe damage to
unprotected collection system surfaces and may eventually result in the total failure of the sewer
piping and the uncontrolled release of raw wastewater to the environment.
For obvious reason, the control and reduction of hydrogen sulfide in wastewater systems is of vital
importance. While, the biological and chemical processes resulting in sulfide production in
wastewater are well understood, there are significant contributing factors are not understood.
Even the well known Pomeroy-Parkhurst equations contains an empirical "M Factor" to account
for the unknown biological and chemical transformations which occur with sulfide in wastewater
(USEPA, 1985).
Settled solids and other debris in sanitary sewers and wastewater collection systems can provide
a greatly increased surface area upon which anaerobic sulfate reducing bacterial slime can grow,
thereby increasing the incremental (per foot) sulfide production potential of sewers. Methods need
to be developed to measure the sulfide production in a sewer with moderate to heavy settled
solids and debris and sample and to characterize the solids in the sewer.
It is important to develop a method to measure in-situ dissolved sulfide concentrations inside the
interstitial spaces of a typical debris pile in the sewer. By measuring the pore space dissolved
sulfide concentration in a typical debris pile and by removing the debris pile and characterizing the
debris by sieve size and mineralogy, the additional surface area provided by the pile can be
calculated. From knowledge of the practical pore space volume and the surface area, the specific
sulfide production rate can be determined. This would allow calculation of the mass of sulfide that
could be prevented by cleaning the upstream sewers.
The method analyzed to clean the pipes in this study is passive flushing. This technology holds
great potential as an economical way to maintain sewers in a clean and free flowing condition.
Clean sewers provide maximum wastewater carrying capacity thereby preventing sewer overflows
and protecting the environment. There is another benefit to be gained by maintaining sewers in a
clean and free flowing condition, namely, sulfide odor and corrosion reduction. However, the
flushing event that disturbs the settled debris and solids and carries them downstream will
generate a significant turbulent wave. This hydraulic wave will release the sulfide in the debris
pore spaces and subject it to turbulence. The turbulence associated with the hydraulic wave will
strip dissolved sulfide from the wastewater and release it as hydrogen sulfide gas. Although the
event is short-lived, the concentrations produced may cause a short burst of odor release from the
sewer that could result in odor complaints. Identification of the potential for odor release and
complaints as a result of sewer flushing may require measurement of the instantaneous spike in
sewer headspace hydrogen sulfide concentrations caused by the hydraulic wave. Sewers that
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have not been cleaned in many years may have accumulated significant debris piles that may
cause a large spike in hydrogen sulfide release when first flushed. However, following one or two
flush events, the debris piles should be smaller or totally removed, thus reducing the hydrogen
sulfide spikes for the second, third and subsequent flushing events.
Previous Research
In 1966, EPA through its federal water program initiated research to demonstrate the feasibility of
periodic flushing during dry weather. The first phase of work performed by FMC Corporation
included a study of the overall flushing concept, small-scale hydraulic modeling, and design and
development of cost estimates for constructing test equipment (FMC, 1966). The second phase
produced a flushing evaluation facility consisting of 0.30 meter and 0.45 meter (12 and 18 inch)
diameter test sewers about 488 meters (1600) feet long, supported above ground (thus allowing
for slope adjustment), including holding tanks at three points along the test sewer for the flushing
experiments (FMC, 1972). Limited periodic flushing of simulated combined sewer laterals was
accomplished. The report documenting this research recommended a third phase be made for
flushing larger sizes of pipe, flush wave sequencing, and determination of solids buildup over long
periods of time.
In 1974, a combined sewer management study performed by Process Research, Inc., Cambridge,
Massachusetts, focused on assessing alternative strategies for abating CSO discharges to
portions of Boston Harbor was completed (Process Research, 1974). As part of the research work
conducted during this study a number of theoretical formulas for prediction of dry weather
deposition and flushing criteria were developed. The development of the deposition analysis
techniques stemmed from critical fluid shear stress considerations. The theoretical formulas were
roughly field checked to ascertain solids accumulations. Although the model was crude, the
agreement with visual field observations was reasonable. The model was then used to analyze
deposition problem segments within a service area of 1200 hectares (3000 acres) entailing
roughly 152,500 meters (0.5 million feet) of sewer. Roughly 3000 manhole-to-manhole segments
were analyzed for deposition and it was determined that roughly 17 percent of the segments
contained about 75 percent of the estimated dry weather wastewater depositions. It turned out
that most of these segments were small-diameter combined sewer laterals. Flushing criteria were
empirically developed using data generated during the earlier FMC research to estimate required
flushing volumes.
In 1979, a three year research and development (R&D) program sponsored by EPA was
conducted by Environmental Design & Planning, Inc. (EDP) in the Dorchester area of Boston to
determine the pollution reduction potential of flushing combined sewer laterals. It was concluded
that small volume flushing would transport organics/nutrients and heavy metals sufficient
distances to make the option feasible and attractive (EPA, 1979). Relevant conclusions were as
follows:
Approximately 20 to 40 percent of heavy metal (cadmium, chromium, copper, lead, nickel,
and zinc) associated with particles entrained by flush waves will not settle within a two hour
settling period.
An automated sewer flushing module using a simple hydraulic gate powered by an air
cylinder, and time clock triggered, operated without intervention for a 5.5-month period to
back up wastewater and retract and induce flush waves. Flushed pollutant loads were
comparable to removals noted by manual flush tanker means.
An empirical methodology was prepared for providing first-cut deposition solids, nutrient and
organic daily collection system estimates simply by knowing the total length of pipe, service
area, average pipe slope or ground slope, and per capita wastewater flow rate. Methods were
also developed to determine which segments in collection systems should be flushed to
remove specified portions of deposits.
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Based on the prototype success of the flushing R&D project, a full-scale demonstration project
was recommended to obtain full-scale experience in Boston. Clinton Bogert & Associates
completed a detailed cost-effectiveness study in 1980 with support from EDP focusing on the
Dorchester neighborhood in Boston. It was concluded from the cost-effectiveness analysis that
sewer flushing can be an adjunct to, but cannot substitute for, structural alternatives and that the
use of storage treatment available in large combined sewers in conjunction with sewer flushing
could reduce the cost of large stand-alone satellite storage facilities.
Sewer flushing of large-diameter combined sewers was investigated in the CSO Facility Plan for
the City of Elizabeth, NJ by Clinton Bogert & Associates. It was concluded that daily flushing of
troublesome deposition section within seven sub-areas using 12 automatic flushing systems was
estimated to reduce about 28 percent of the first flush overflow pollutant loading from the service
area. Control is by level sensing to centralized computer control. Combined sewers to be flushed
ranged from 0.45 to 1.4 meters (18 to 54 inches). Construction of 12 flushing modules was
completed in 1990. Estimated construction costs for complete modules (structural, mechanical,
electrical, and site work excluding computer control) ranged from $175,000 for small diameter
lines up to $275,000 (costs based on ENR cost index of 5000). No evaluation data has been
reported regarding effectiveness.
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Chapter 2
Conclusions
The following conclusions were derived from the evaluations of the 18 sewer and tank sediment
flushing facilities and the case studies developed in Section 5.
1. The tipping flusher and flushing gate technology appear to be the most cost-effective means
for flushing solids and debris from tanks. The most efficient method for flushing large
diameter flat depositing sewers is the flush gate technology.
2. In general, the performance of both types of flushing equipment for tanks and flush gates for
sewers was rated as good to excellent. Based on calculations for most of the facilities using
flushing gates, the terminal velocities at the end of the flushing wave exceeded 1 m/second.
This terminal velocity was adequate for cleansing.
3. Cost effectiveness analysis comparing flushing gate technology versus conventional large
pipe cleaning operations using bucketing methods was conducted for an actual project
undergoing construction. A system of flushing gates to flush 1500 m of large diameter
sanitary sewer and storm drains was examined. Present worth savings of at least $500,000
is expected using the flush gate technology in lieu of periodic cleaning using conventional
means.
4. A desktop analysis was conducted comparing the use of the flushing gate technology to
minimize potentially overflowing pollutant loads as an alternative to downstream high rate
satellite treatment. The desktop analysis showed that periodic flushing of long flat depositing
reach of a conduit prior to an overflow was extremely cost effective and superior to a satellite
treatment facility.
5. A desktop analysis was conducted to explore the use of flushing gate technology for
minimizing sediments in a long flat depositing sewer carrying warm sewage with high organic
loadings. Dissolved hydrogen sulfide levels attributable to both the slime layer and to
accumulated sediments were estimated. The present worth costs for treating excessive and
dangerous levels of hydrogen sulfide with chemicals (iron salts) were estimated. Flushing
gate technology was explored to reduce sediments and thereby reduce incremental hydrogen
sulfide loadings. The cost effectiveness of chemical treatment versus flushing with reduced
chemical treatment indicated that flushing as an adjunct to chemical treatment is cost
effective.
Recommendations
1. A demonstration project using flushing gate system to minimize sewer deposits within CSO
impacting a receiving water should be conducted. The project should involve long term pre
and post construction overflow flow and pollutant characterization. The pre project
relationship between dissolved sulfide and sediment deposits should also be understood.
Instantaneous hydrogen sulfide levels during post construction flushing conditions should be
monitored to note nuisance level potential of instantaneous hydrogen sulfide spikes. A cost
effective analysis demonstrating the benefits of this technology using actual field experience.
Operation and Maintenance requirements should be prepared to document this work.
2. Experimental laboratory work to document the kinetics of sediment generation of hydrogen
sulfide is recommended followed by full-scale demonstration work.
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Chapter 3
Sewer Sediments and Sewage Gases
Estimation Techniques
Introduction
As noted in the Chapter 1, solids deposition in flat sewerage systems can be significant, in some
situations on the order of 15 to 20 percent of the total daily solids input. In combined sewer
systems, these accumulations can be scoured during wet events and can contribute to first flush
conditions. During overflow events, high concentrations of these accumulated pipe deposits can
be discharged into the environment. Satellite CSO treatment facilities have been used worldwide
to handle such problems. In Chapter 6 the cost effectiveness of using automated sewer flushing
technology is conducted.
To perform these calculations, a desktop procedure is utilized to estimate the amount of
depositing solids in a long pipe segment during low flow average condition, and then to estimate
during high flow conditions the amount of solids scoured from the sediment beds, re-suspended,
and carried toward the overflow location. Important concepts underlying this crude desktop
procedure have been developed using results from recent European research, which focused on
sewer self, cleansing criteria.
Nature of Sewer Solids and Sediment Movement in Sewers
The generic term sewer sediment is used to describe any type of settleable particulate material
that is found in storm water or sewage and is able to form bed deposits in sewers and associated
hydraulic structures. Some particles of very small size or low density may remain in suspension
under all normal flow conditions and would be transported through a sewerage system as
washload. Such particles have a negligible effect on the hydraulic capacity of sewerage systems,
but can have an important influence on pollutant loading in the flow and at points of discharge
such as treatment works and sewer overflows.
By contrast, larger (inorganic and organic) and denser particles (inorganic with a specific gravity in
the range of 1.5 to 2.5) having settling velocities in the range 0.2 centimeters/second to 30
centimeters/second that are constantly inputted into sanitary systems, but at low rates (5-15 mg/l
typical), may only be transported by peak flows that occur relatively infrequently. In some cases,
they may form permanent stationary deposits at the point of entry to the sewer system.
If liquid flows over a sediment bed in a sewer running full or partially full, hydrodynamic lift and
drag forces are exerted on the deposited particles. If two combined forces do not exceed the
restoring force, then entertainment occurs, resulting in the movement of the particles at the
flow/sediment boundary. Not all the particles of a given size at the flow/sediment boundary
dislodge and move at the same time because the flow is turbulent and contains short term
fluctuations in velocity. The limiting condition, below which sediment movement is negligible,
known as the threshold of movement, is usually defined in terms of either the critical bed shear
stress or the critical erosion velocity.
Once sediment is entrained, it may travel down the sewer in one of two general ways. Finer,
lighter material tends to travel in suspension, while heavier material travels in a rolling, sliding
mode as bedload. In the transport of suspended sediment, there is a continuous exchange
between particles settling out and those being entrained upwards into the flow. Under certain
conditions, fine grained and organic particles can form a highly concentrated mobile layer of 'fluid
mud'near the invert (Ashley, 1992).
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If the flow velocity or turbulence level decreases, there will be a net reduction in the amount of
sediment held in suspension. The material accumulated at the bed may continue to be
transported as a stream of particles without deposition. However, below a certain limit, the
sediment will form a deposited bed, with transport occurring only in the surface layer (the limit of
deposition). If the flow velocity is further reduced, sediment transport will cease completely. The
flow conditions necessary to prevent deposition depend on the pipe size and on properties of the
sediment, such as particle size and specific gravity. Flocculation of fine particles can also be
important. The flow velocities needed to entrain sediment tend to be higher than those at which
deposition occurs.
Sewer Self Cleansing Criteria
An important parameter in the criteria for sewer self-cleansing is average shear stress. Average
shear stress is the amount of force the fluid exerts on the wetted perimeter of the pipe. Another
important parameter is bed shear stress which is the amount of force the fluid exerts on the bed of
sediment in the pipe. Bed shear stress is related to bed load scour and movement.
Historically, in respect to the sediments found in sewerage systems, the design of systems has
been based on a set of conditions that prevent the deposition of sediments in pipes. These
conditions are based on a minimum velocity of flow or a minimum shear stress that the flow
should exert on the walls of the pipe to maintain self-cleansing conditions. The minimum velocity
of flow or minimum shear stress corresponds to a particular depth of flow or with a particular
frequency of occurrence. Available design criteria were reported by the Construction Industry
Research and Information Association (CIRIA, 1987), and a summary of these, outlined by Nalluri
and Ab Ghani (1996), are recorded in Table 3-1 and Table 3-2.
Table 3-1. Minimum Velocity Criteria
Source
Country Sewer Type Minimum Pipe
Velocity (mis) Conditions
American Society of Civil
Engineers (1970)
British Standard (1987)
Bielecki(1982)
USA
UK
Germany
Sanitary
Storm
Sanitary
Combined
Not noted
0.6
0.9
1.0
1.0
1.5
Full/Half-full
Full/Half-full
Full
Full
Full
Table 3-2. Minimum Shear Stress Criteria
Source
Lysne(1969)
ASCE(1970)
Yao(1974)
Mag u ire
Bischof(1976)
Country
USA
USA
USA
UK
Germany
Sewer Type
Not noted
Not noted
Storm
Sanitary
Not noted
Not noted
Minimum Shear Pipe Conditions
Stress (N/m )
2-4
1.3-12.6
3.0-4.0
1.0-2.0
6.2
2.5
Not noted
Not noted
Not noted
Not noted
Full/Half-full
Not noted
-------
These criteria take no account of the characteristics of the sediment, of the suspended sediment
concentration, the bed load or of any cohesion between the sediment particles. Nonetheless,
inspection of Table 3-1 and Table 3-2 indicates that as stand alone criteria taken individually
and/or jointly, minimum velocity and minimum shear stress levels of 1 meter/second and 2 N/m
be realized on a regular basis. Recent work by many researchers has shown that a single value of
minimum velocity or shear stress cannot adequately describe the self cleansing conditions in all
pipes of different size, roughness and gradient for a range of sediment characteristics and flow
conditions.
In practice, sewer pipes will not be maintained self-cleansing at all times. The diurnal pattern of
the dry weather flow and the temporal distribution and nature of sediments found in sewer flows
may result in the deposition of some sediments at times of low flow and the subsequent erosion
and transport of these sediments, either as suspended load or bed-load, at times of higher flow.
The deposited sediments will exhibit additional strength due to cohesion and provided that the
peak dry weather flow velocity or bed shear stress is of sufficient magnitude to erode these
sediments, the sewer will maintain self cleansing operation at times of dry weather. May, et. al.
(1996), presented a definition to describe a self cleansing sewer as "an efficient self-cleansing
sewer is one having a sediment-transporting capacity that is sufficient to maintain a balance
between the amounts of deposition and erosion, with a time-averaged depth of sediment deposit
that minimizes the combined costs of construction, operation and maintenance." To achieve such
self-cleansing performance, the following criteria apply:
1. Flows equaling or exceeding a limit appropriate to the sewer should have the capacity to
transport a minimum concentration of fine-grain particles in suspension (applicable for all
types of sewerage systems).
2. The capacity of flows to transport coarser granular material as bed-load should be sufficient to
limit the depth of deposition to a specified proportion of the pipe diameter. This criteria
generally relates to combined and storm water systems. Limit of deposition considerations,
i.e., "no deposition" generally applies to sanitary sewer designs. In this context, there must be
sufficient shear in sanitary systems to avoid deposition of large particles.
3. Flows with a specified frequency of occurrence should have the ability to erode bed particles
from a deposited granular bed that may have developed a certain degree of cohesive strength
(applicable to all systems).
To meet these criteria, new guidelines have recently been developed (CIRIA, 1996), and are
currently being adopted throughout Europe for the design of sewers to control sediment problems.
Design criteria for the transport of fine grained material in suspension, the transport of coarser
sediments as bed load and the erosion of cohesive sediment deposits and guidelines on the
minimum flow velocity and pipe gradient for different types and sizes of sewer are outlined. To
account for the effects of cohesion (Criterion 3, above), the design flow condition should produce
a minimum value of bed shear stress of 2.0 N/m2 on a flat bed with a Colebrook White roughness
of 1.2 millimeters (CIRIA, 1996).
The third criterion is of specific interest to the problem of cleansing accumulated mature sediment
beds. Various researchers have studied the flow conditions required to release particles from a
deposited bed, which has developed a degree of cohesion. Summaries of investigations forming
much of the basis for Criterion 3 are as follows:
Nalluri and Alvarez (1992), whose laboratory studies used synthetic cohesive sediments,
concluded that there were two ranges of bed shear stress at which erosion occurred: 2.5
N/m2 applying for the weakest material, comprising a surface layer of fluid sediment: and
6 to 7 N/m2 for the more granular and consolidated material below. It was found that, after
erosion, the synthetic cohesive sediments behaved very much like non-cohesive material.
-------
Ristenpart and Uhl (1993) found in field tests that during dry weather an average bed
shear stress of 0.7 N/m2 was required to initiate erosion, increasing to an average of
about 2.3 N/m2 during wet weather, or to 3.3 N/m2 after a prolonged period of dry weather
and presumably, consolidation of the deposited bed.
Ashley (1993) has suggested that the bonds between particles at the surface of a
deposited bed are weakened by the presence of the water, so that surface layers can be
successively stripped away by the flow. Measurements in the Dundee, Scotland sewers
indicated that it began to move at a fluid shear stress of about 1 N/m2, with significant
erosion of a deposited bed occurring at bed shear of 2 to 3 N/m2. Taking account of a
review of work by other researchers, Ashley concluded that most deposits should be
eroded at a shear stress exceeding 6 to 7 N/m2.
Desktop Procedure
A desktop procedure, developed for a recent sediment deposition investigation of the sediment
accumulation problems in the South Ottawa Tunnel, Ottawa, Canada (Montgomery Watson,
1998) was used. The entire conduit is considered as a single "tank" with no differentiation of
heavy particles settling at the front end of the tunnel in contrast to finer particles settling in mid-
sections. The desktop procedure is a single cell mass balance model for large diameter sewer
lines. Inputs are: daily flow rates; average daily minimum hourly, and maximum hourly discharge.
A calendar year of flow data is used.
The desktop procedure analyzes deposition on a daily basis noting accumulations of inorganic
and organic deposits as well as scour and erosion of prior deposits over the course of the input
time series. Inorganic and organic solids are input on a daily basis as an invariant concentration
basis, i.e., and independent of flow quantity.
The procedure investigates the settling and scouring behavior of material loosely categorized as
inorganic "grit" and or large organic particles having fall settling velocities ranging between 0.1 and
30 cm/sec.
Table 3-3. Assumed Inorganic Material Composition
Sieve
Size#
4
6
10
16
20
30
40
50
70
100
200
Sum
Mesh Relative Assumed Composition By Sum
Opening Mass Specific Gravity
Fraction
(mm)
4.76
3.36
2.00
1.25
0.84
0.60
0.42
0.30
0.20
0.15
0.025
(%)
1
2
11
10
15
10
12
12
16
8
3
100
1.7
40
35
35
30
30
30
20
20
15
10
5
1.9
40
35
35
30
30
30
20
20
15
10
5
2.1
10
15
15
20
20
20
30
25
30
30
40
2.5
10
15
15
20
20
20
30
35
40
50
50
100
100
100
100
100
100
100
100
100
100
100
As presented in Table 3-3, material is noted by sieve size varying from # 4 sieve size (3/8" gravel)
to #200 sieve (0.075 mm- fine sand). Relative fractions per sieve size are noted along with density
-------
breakdown per sieve ranging from 1.7 (soil) to 2.5 (sand ). The inorganic material distribution
shown in Table 3-3 represents average value conditions.
Table 3-4 notes fall velocities for each of the sieve sizes and specific gravity subcategories. Fall
velocities are computed using the worn angular assumption and sewage temperature of 10ฐC,
which is very common. Table 3-5 depicts a similar assumed distribution of organic particles and
estimated fall velocities. This data was developed by US EPA in the early 1980's to establish the
basis of design for the hydraulic model studies leading to the design of the Swirl Concentrator
(EPA, 1982).
Table 3-4. Distribution of Inorganic Material Settling Velocities
Sieve Mesh Fall Velocity* (cm/s)
Size Opening
# (mm)
density=1.7 Density=1.9 density=2.1 density=2.5
4
6
10
16
20
30
40
50
70
100
200
4.76
3.36
2.00
1.25
0.84
0.60
0.42
0.30
0.20
0.15
0.075
15.5
9.4
7.0
5.6
4.7
3.3
2.5
1.8
1.0
0.61
0.24
20.0
12.0
9.0
7.2
6.0
4.9
3.2
2.3
1.3
0.78
0.30
24.0
14.6
11.0
8.8
7.3
5.9
4.0
2.8
1.5
0.95
0.37
33.0
20.0
15.0
12.0
10.0
8.1
5.4
3.8
2.1
1.3
0.5
*Worn Angular Particles, Temperature = 10ฐC
Table 3-5. Assumed Organic Material Size, Composition and Settling Velocities
Sieve #
4
6
10
20
30
40
50
70
Mesh
Opening
(mm)
4.76
3.36
2.00
0.84
0.60
0.42
0.30
0.20
Mass
Fraction
(%)
0.5
25
23
21
14
3
4
9.5
Fall Velocity*
(cm/s)
9.0
7.5
6.0
3.3
1.5
0.9
0.5
0.1
* Temperature = 10ฐC
Specific Gravity = 1.1
The daily mass input into the desktop procedure is assumed to be in the range of 3 to 15 mg/l
inorganic and 5 to 15 mg/l organic. The desktop procedure uses flow inputs to develop daily flow
velocities (average, maximum, and minimum) and shear stress (average, maximum, and
minimum). The shears are used to determine deposition and erosion potential. If the average
shear stress is smaller than the critical shear stress for that sieve size and density, as determined
by Macke (1982) and Brombach (1993), deposition will occur.
-------
Erosion of the bed load is based on maximum hourly shear stress. Maximum shear stress is
compared to 2 N/m2 as determined by CIRIA standards. When the shear stress is above 2 N/m2,
erosion of the bed load occurs.
The degree of erosion, as a function of average fluid shear stress, used in the Desktop procedure
generally follows the empirical work of Nalluri and Avarez (1992), Ristenpart and Uhl (1993), and
Ashley (1993) noted above. Table 3-6 demonstrates the full range of deposition and erosion
potential as a function of discharge, velocity, and shear stress for a 3.05 meter tunnel. These
results were prepared using the full annual range of expected flows into the new 7km Interlsland
Tunnel connecting the Nut Island Headworks Facility with the new Deer Island Wastewater
Treatment Plant in Boston.
Table 3-6. Assessment of 3.05 Meter Tunnel
Discharge
(m 3/sec)
2.6
3.5
4.4
5.3
6.1
7.0
8.8
10.5
13.2
15.8
17.5
Note: Kb=1 .2
Velocity
(mis)
0.36
0.47
0.60
0.72
0.84
1.06
1.20
1.44
1.80
2.17
2.41
millimeters = ro
Fluid Shear
(N/m2)
0.27
0.48
0.74
1.04
1.43
1.86
2.85
4.15
6.47
9.3
11.5
ugh concrete
Deposition or Erosion Potential
Severe deposition
Moderate to severe deposition
Mild to moderate deposition
Slight to mild deposition
Skims juvenile sediments
None to slight erosion top layer
Slight to mild erosion of consolidated beds
(2-5%)
Mild erosion of consolidated beds (5-15%)
Moderate erosion of consolidated beds (15-
25%)
Substantial erosion (25-35%)
Substantial erosion (35-50%)
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Chapter 4
Hydrogen Sulfide and Sulfuric Acid
Estimation Techniques
SULFIDE GENERATION IN SEWERS
Sulfide generation is a bacterially mediated process occurring in the submerged portion of
sanitary sewers and force mains. Fresh domestic sewage entering a wastewater collection system
is usually free of sulfide. However , a dissolved form of sulfide soon appears as a result of the
following conditions:
Low dissolved oxygen content
Long detention time in the collection system
Elevated wastewater temperature
The root cause of odor and corrosion in collection systems is sulfide, which is produced from
sulfate by bacteria residing in a slime layer on the submerged portion of sewer pipes and
structures. Once released from the wastewater as hydrogen sulfide gas, odor and corrosion
problems begin. Another type of bacteria utilizes hydrogen sulfide gas to produce sulfuric acid that
causes the destruction of wastewater piping and facilities. Operation and maintenance
expenditures are required to correct the resulting damage caused by this sulfuric acid. In severe
instances, pipe failure, disruption of service and uncontrolled releases of sewage can occur.
Fresh domestic sewage entering a wastewater collection system is usually free of sulfide. When
certain conditions exist within the collection system, dissolved sulfide soon begins to appear.
These sulfide producing conditions are low dissolved oxygen content, high-strength wastewater,
long detention times, extensive pumping and high wastewater temperatures. The first step in this
bacterially mediated process is the establishment of a slime layer below the water level in a sewer
pipe or force main. This slime layer is composed of bacteria and inert solids held together by a
biologically secreted protein "glue" called zooglea. When this biofilm becomes thick enough to
prevent dissolved oxygen from penetrating it, an anoxic zone develops within it. Approximately two
weeks is required to establish a fully productive slime layer in pipes. Within this slime layer, sulfate
reducing bacteria use the sulfate ion (SO4~), a common component of wastewater, as an oxygen
source for the assimilation of organic matter in the same way dissolved oxygen is used by aerobic
bacteria. Sulfate concentrations are almost never limiting in normal domestic wastewaters. When
sulfate is utilized by these bacteria, sulfide (S=) is the by-product. The rate at which sulfide is
produced by the slime layer depends on a variety of environmental conditions including the
concentration of organic food source (BOD), dissolved oxygen concentration, temperature,
wastewater velocity, and the area of the normally wetted surface of the pipe.
As sulfate is consumed, the sulfide by-product is released back into the wastewater stream where
it immediately establishes a dynamic chemical equilibrium between four forms of sulfide; the
sulfide ion (S=), the bisulfide or hydrosulfide ion (HS-), aqueous hydrogen sulfide (H2S(aq)), and
hydrogen sulfide gas (H2S(g)).
Sulfide Ion (S=)
The sulfide ion carries a double negative charge indicating that it reacts primarily by giving up two
electrons in the outer shell. It is a colorless ion in solution and cannot leave wastewater in this
form. It does not contribute to odors in the ionic form.
-------
Bisulfide Ion (HS-)
The bisulfide (or hydrosulfide) ion carries a single negative charge. This is because one of the
negative charges of the sulfide ion is taken up by a positively charged hydrogen ion. It is a
colorless, odorless ion which can only exist in solution. It also does not contribute to odors.
Hydrogen Sulfide (Aqueous)
Hydrogen sulfide can exist as a gas dissolved in water. The polar nature of the hydrogen sulfide
molecule makes it soluble in water. In the aqueous form, hydrogen sulfide does not cause odor;
however, this is the only sulfide specie that can leave the aqueous phase to exist as a free gas.
The rate at which hydrogen sulfide leaves the aqueous phase is governed by Henry's Law, the
amount of turbulence of the wastewater and the pH of the solution.
Hydrogen Sulfide (Gaseous)
Once hydrogen sulfide leaves the dissolved phase and enters the gas phase it can cause odor
and corrosion. Hydrogen sulfide gas is a colorless but extremely odorous gas that can be
detected by the human sense of smell in very low concentrations. In high concentrations, it is also
very hazardous to humans. In concentrations as low as 10 ppm it can cause nausea, headache
and conjunctivitis of the eyes. Above 100 ppm it can cause serious breathing problems and loss
of the sense of smell along with burning of the eyes and respiratory tract. Above 300 ppm death
can occur within a few minutes. For these reasons, the Occupational Safety and Health
Administration (OSHA) has established an 8-hour, time-weighted, personal exposure limit of 10
ppm (U.S. EPA, 1985).
Due to the continuous production of sulfide in wastewater, hydrogen sulfide gas rarely, if ever, re-
enters the liquid phase. Sulfide continuously produced by the slime layer replaces that which is
lost to the atmosphere as hydrogen sulfide gas in the collection system. In addition, once the
hydrogen sulfide gas is released it usually disperses throughout the sewer environment and never
reaches a high enough concentration to be forced back into solution.
The four sulfide chemical species are related according to the following equilibrium:
pKa = 6.9 pKa = 14
H2s(g) ** H2S(aq) ** HS~ ** S~
hydrogen sulfide hydrogen sulfide bisulfide sulfide
gas (dissolved) ion ion
As indicated by the equilibrium equations, once hydrogen sulfide is released into the gas phase,
bisulfide ion is immediately transformed into more aqueous hydrogen sulfide to replace that which
is lost. Concurrently, sulfide ion is transformed into bisulfide to replace that lost to aqueous
hydrogen sulfide. Through this type of continuously shifting equilibrium it would be possible to
completely remove all sulfide from wastewater as hydrogen sulfide gas through stripping. This is
generally not recommended or advantageous due to odor releases and the accelerated corrosion
which can take place.
The quantitative relationship between the four sulfide species is controlled by the pH of the
wastewater. The sulfide ion (S=) does not exist below a pH of about 12 and as indicated by the
pKa, is in a 50/50 proportional relationship with the bisulfide ion (HS-) at a pH of 14. Since the
normal pH of wastewater is far lower, the sulfide ion is rarely experienced. The pKa of much
greater importance is the one controlling the proportional relationship between the bisulfide ion
and H2S(aq). Most domestic wastewater has a pH near 6.9. This means that at the pH of normal
-------
wastewater, half of all sulfide present exists as the bisulfide ion and the other half exists as
aqueous hydrogen sulfide (a dissolved gas). Since the concentration of dissolved gases in
solution are primarily controlled by the specific Henry's Law coefficient for that gas, they can be
released from solution to exist as the free gas form. Once subjected to turbulence or aeration,
wastewater can release the dissolved gas as free hydrogen sulfide gas, and more bisulfide ion is
transformed into the dissolved gas form to replace that lost to the atmosphere.
Settleable Solids
Periods of low flow in the collection system correlate to lower average wastewater velocities. Low
flow velocities allow material, usually grit and large organics, to settle in the collection system
piping. This increases the mass and surface area of material in the collection system upon which
sulfate reducing bacteria (slime layer) can grow, and can lead to an increased conversion of
sulfate to sulfide.
Collection systems with sedimentation problems can experience sulfide concentration spikes
during the historically high flow, cool temperature months. This phenomenon occurs when
significant sand or grit accumulations exist and the particles are covered by an anaerobic slime
layer that contains sulfate-reducing bacteria. Only the bacteria on the surface of the grit pile
receive a continuous supply of sulfate because they are exposed to the wastewater. The buried
sulfate-reducing bacteria are not exposed to a continuous supply of sulfate. This forces them to
exist in a semi-dormant, anaerobic state with very low cell activity (but they are not dead). When a
high flow event occurs, with sufficient velocity and shear force to re-suspend the sediment, this
enormous surface area of sulfate reducing bacteria is suddenly exposed to ample sulfate and they
rapidly convert it to dissolved sulfide. This causes a relatively short duration, high sulfide event
with resulting hydrogen sulfide gas release, odor and corrosion.
The grit particles and their attached sulfate-reducing bacteria that were semi-dormant are
suspended and exposed to a tremendous quantity of sulfate and quickly begin producing sulfide.
The interaction between a large quantity of bacteria and an almost unlimited food source will
create dissolved sulfide spikes that are subsequently released in areas of high turbulence. This
trend is common and well documented in many cities with similar grit deposition problems such as
Boston, Los Angeles, St. Louis, and Houston.
Temperature
In addition to the factors described above, summer conditions result in an increase of wastewater
temperatures. Greater wastewater temperatures increase the metabolic activity of the sulfate
reducing organisms, causing faster conversion of sulfate to sulfide and increased dissolved
sulfide concentrations. It has been estimated that each incremental 7 degree C (12.5 degree F)
increase in wastewater temperature doubles the production of sulfide.
Sulfuric Acid Production
Thiobacillus aerobic bacteria, which commonly colonize pipe crowns, walls and other surfaces
above the water-line in wastewater pipes and structures, has the ability to consume hydrogen
sulfide gas and oxidize it to sulfuric acid. This process can only take place where there is an
adequate supply of hydrogen sulfide gas (>2.0 ppm), high relative humidity and atmospheric
oxygen. These conditions exist in the majority of wastewater collection systems for some portion
of the year. A pH of 0.5 (which is approximately equivalent to a 7 percent sulfuric acid solution)
has been measured on surfaces exposed to severe hydrogen sulfide environments (>50 ppm in
air).
The simplified and balanced equation for the biological metabolic process which converts
hydrogen sulfide to sulfuric acid is presented below:
Thiobacillus Bacteria
-------
H2S(g) + 2O2 ~^ H2SO4
hydrogen sulfide atmospheric Sulfuric
gas oxygen acid
Turbulence Reduction
Turbulence is a critical parameter to consider in preventing hydrogen sulfide gas release from
wastewater. The effects of sulfide odor and corrosion are increased by orders of magnitude at
points of turbulence. Henry's law governs the concentration of gas over a liquid containing the
dissolved form of the gas. Henry's law states in effect:
The concentration of a gas over a liquid containing the dissolved form of the gas is
controlled by the partial pressure of that gas and the mole fraction of the dissolved gas in
solution.
Since this law governs the relationship between the dissolved form and gaseous form of sulfide
over a given surface area, any action which serves to increase the surface area of the liquid also
increases the driving force from the liquid to the gas phase.
The most common form of increased surface area is turbulence. In turbulent areas, small droplets
are temporarily formed. When this happens, the forces governing Henry's law (partial pressure)
quickly try to reach equilibrium between the liquid and atmospheric phases of the gas. The result
is often a dramatic release of sulfide from the dissolved to the gaseous form. Structures causing
turbulence should be identified and measures should be taken to protect and/or control the
subsequent hydrogen sulfide gas releases. This same release mechanism is exhibited whenever
wastewater containing dissolved sulfide is aerated.
Concrete Corrosion
The effect of sulfuric acid on concrete surfaces exposed to the sewer environment can be
devastating. Sections of collection interceptors and entire pump stations have been known to
collapse due to loss of structural stability from corrosion. The process of concrete corrosion,
however, is a step-wise process which can sometimes give misleading impressions. The following
briefly describes the general process of concrete corrosion in the presence of a sewer
atmosphere.
Freshly placed concrete has a pH of approximately 11 or 12, depending upon the mix design.
This high pH is the result of the formation of calcium hydroxide [Ca(OH)2] as a by-product of the
hydration of cement. Calcium hydroxide is a very caustic crystalline compound which can occupy
as much as 25 percent of the volume of concrete. A surface pH of 11 or 12 will not allow the
growth of any bacteria; however, the pH of the concrete is slowly lowered over time by the effect
of carbon dioxide (CO2) and hydrogen sulfide gas (H2S). These gases are both known as "acid"
gases because they form relatively weak acid solutions when dissolved in water. CO2 produces
carbonic acid and H2S produces thiosulfuric and polythionic acid. These gases dissolve into the
water on the moist surfaces above the sewage flow and react with the calcium hydroxide to
reduce the pH of the surface. Eventually the surface pH is reduced to a level that can support the
growth of bacteria (pH 9 to 9.5).
The time it takes to reduce the pH is a function of the concentration of carbon dioxide and
hydrogen sulfide in the sewer atmosphere. It can sometimes take years to lower the pH of
concrete from 12 to 9, however, in some severe situations it can be accomplished in a few
months.
Once the pH of the concrete is reduced to around pH 9, biological colonization can occur. Over 60
different species of bacteria are known to regularly colonize wastewater pipelines and structures
above the water line. Most species of bacteria in the genus Thiobacillus have the unique ability to
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convert hydrogen sulfide gas to sulfuric acid in the presence of oxygen. Because each species of
bacteria can only survive under a specific set of environmental conditions, the particular species
inhabiting the colonies changes with time. Since the production of sulfuric acid from hydrogen
sulfide is an aerobic biological process, it can only occur on surfaces exposed to atmospheric
oxygen.
As a simplified example, one species of Thiobacillus only grows well on surfaces with a pH
between 9 and 6.5. However, when the sulfuric acid waste product they excrete decreases the pH
of the surface below 6.5, they die off and another species takes up residence which can withstand
lower pH ranges. The succeeding species grows well on surfaces with a pH between 6.5 and 4.
When the acid produced by these species drops the pH below 4, a new species takes over. The
process of successive colonization continues until species, which can survive in extremely low pH
conditions, take over. One such specie is Thiobacillus thiooxidans, which is sometimes known by
its common name, Thiobacillus concretivorous, which is Latin for "eats concrete". This organism
has been known to grow well in the laboratory while exposed to a 7 percent solution of sulfuric
acid. This is equivalent to a pH of approximately 0.5.
Sulfuric acid attacks the matrix of the concrete, which is commonly composed of calcium silicate
hydrate gel (CSHG), calcium carbonate from aggregates (when present), and un-reacted calcium
hydroxide. Although the reaction products are complex and result in the formation of many
different compounds, the process can be generally illustrated by the following reactions:
H2SO4 + CaSi ^ CaS04 + Si + 2 H+
H2SO4 + CaCO3 ^ CaS04 + H2CO3
H2SO4 + Ca(OH)2 ^ CaS04 +2 H2O
The primary product of concrete decomposition by sulfuric acid is calcium sulfate (CaS04), more
commonly known by it's mineral name, gypsum. From experience with this material in its more
common form of drywall board, we know that it does not provide much structural support,
especially when wet. It is usually present in sewers and structures as a pasty white mass on
concrete surfaces above the water line. In areas where diurnal or other high flows intermittently
scour the walls above the water line, concrete loss can occur rapidly. The surface coating of
gypsum paste can protect underlying sound concrete by providing a buffer zone through which
freshly produced sulfuric acid must penetrate. Because Thiobacillus bacteria are aerobic, they
require free atmospheric oxygen to survive. Therefore, they can only live on the thin outer
covering of any surface. This means that acid produced on the surface must migrate through any
existing gypsum paste to reach sound concrete. When the gypsum is washed off fresh surfaces
are exposed to acid attack and this accelerates the corrosion.
The color of corroded concrete surfaces can be various shades of yellow caused by the direct
oxidation of hydrogen sulfide to elemental sulfur. This only occurs where a continuous high
concentration supply of atmospheric oxygen or other oxidants are available. The upper portions of
manholes and junction boxes exposed to high hydrogen sulfide concentrations are often yellow
because of the higher oxygen content there. This same phenomena can be observed around the
outlets of odor scrubbers using hypochlorite solutions to treat high concentrations of hydrogen
sulfide gas.
Another damaging effect of sulfuric acid corrosion of concrete is the formation of a mineral called
"ettringite". The chemical name for ettringite is calcium sulfbaluminate hydrate. It is produced by a
reaction between calcium sulfate and alumina, which is found in virtually all cements. It forms at
the boundary line between the soft calcium sulfate layer and the sound, uncorroded concrete
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surface. Ettringite is damaging because it is an expansive compound which occupies more space
that it's constituents. When ettringite forms, it lifts the corroded concrete away from the sound
concrete and causes a faster corrosion by continually exposing new surfaces to acid attack.
Although the rate of concrete loss is dependent upon a number of factors including ettringite
formation, it is not uncommon to see concrete loss of I inch per year in heavy sulfide
environments.
Metal Corrosion
Concrete is not the only material that can be affected by the corrosive action of hydrogen sulfide
gas. Most metals, including stainless steel, can also be attacked and destroyed by exposure to
the strong mineral acid, sulfuric acid. Metals can be corroded by two means, acid decomposition
by exposure to sulfuric acid produced by Thiobacillus bacteria, and direct molecular attack. Most
free metals are bi-valent cations, meaning that they carry two positive charges and react primarily
by gaining two electrons in their outer shell (M++). The sulfide component of hydrogen sulfide gas
supplies these two electrons resulting in a metal sulfide and two free hydrogen ions.
M++ + H2S ^ MS + 2H+
The metal has a much stronger affinity for the sulfide than hydrogen causing the release of two
free hydrogen ions. In this manner, the metal is converted from a strong metal-metal bonding
arrangement into a much weaker metal sulfide product. At the same time metals are exposed to
the acidic effect of the free hydrogen ions. This condition results in missing rungs of former
manhole steps, the corroded and weakened manhole covers and rings, and brass and copper
fittings turned dark bluish-black, the color of nickel and copper sulfide.
Trends in Sulfide Production
Hydrogen sulfide has always existed in wastewater. Recent trends of water conservation,
industrial pretreatment, and design deficiencies can significantly increase sulfide generation. This
increase in sulfide generation leads to additional odor and corrosion problems.
Water Conservation Practices
Utilities and water purveyors have recognized the benefit of promoting water conservation
practices. Prompted by water shortages around the country, water conservation practices were
shown to not only preserve a precious resource but also save utilities money by delaying planned
expenditures for plant upgrades, system capacity increases, and storage facilities. Although cost-
efficient and practical for the water industry, water conservation has caused an increase in
dissolved sulfide concentrations in wastewater systems.
Reduced wastewater flows from water conservation practices can cause reduced velocities and
longer residence time in the collection and transmission systems. These conditions allow more
time for the reduction of sulfate, creating higher dissolved sulfide concentrations and they also
increase the general septicity of the wastewater. Since less water is entering the wastewater
collection system while the organic load remains the same, the strength of the wastewater in
terms of BOD5 also increases. This increases the biological activity of the slime layer and causes
the faster consumption of dissolved oxygen and the creation of anaerobic conditions. Anaerobic
conditions also cause the production of organic acids, which drop the pH of the wastewater. Since
a small shift downward in the pH of the wastewater can cause a dramatic increase in hydrogen
sulfide gas release, the anaerobic drop in pH can also increase the release of odor-causing
hydrogen sulfide gas.
Although water conservation has increased the dissolved sulfide concentration of wastewater, little
evidence suggests a general increase in total sulfide mass. However, the Henry's Law coefficient
governing the release of hydrogen sulfide gas from wastewater is dependent only upon sulfide
concentration in the liquid phase, not mass. Therefore, any increase in the dissolved sulfide
concentration of wastewater will increase the release of hydrogen sulfide gas.
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Industrial Pretreatment
Perhaps the greatest contributor to increased dissolved sulfide concentrations in municipal
wastewater is industrial pretreatment (Public Law 92-500). The Clean Water Act, mandated that
the nation's water supplies be protected through a variety of mechanisms. One such mechanism
is to require cleaner wastewater discharges from our Publicly Owned Treatment Works (POTW).
One concern with treated wastewater discharges was the bio-accumulation of heavy metals in the
environment. High concentrations of certain metals, such as lead, copper, mercury, chromium,
and zinc were shown to have toxic effects on animals and plants. Some of these metals can pass
through a wastewater treatment plants and enter the receiving stream or water body where they
could be bio-accumulated to dangerous concentrations. As a result, the U.S. Environmental
Protection Agency (EPA) required municipalities to implement industrial pretreatment programs,
under which heavy metals and other contaminants from industrial sources were identified and
treated. Removing heavy metals removed a potential sulfide removal mechanism. Dissolved
forms of sulfide (bisulfide ion and sulfide ion) have a strong affinity for metals. One of the most
common methods used to remove sulfide from wastewater is to add metal salts, usually ferrous
iron compounds. Metal salts combine with sulfide and precipitate an insoluble metal sulfide.
Before 1980, a common source of heavy metals in wastewater was industrial discharges from
steel mills, electro-plating operations, photo-finishing, and electronics manufacturing. As of 1995,
all such operations pre-treat their wastewater to remove metals prior to discharge to a POTW.
The targeted heavy metals also exhibit a toxic effect on bacteria in the slime layer that produce
sulfide. Toxicity or inhibition of the sulfate-reducing bacteria in the slime layer by the targeted
heavy metals naturally reduced their health and activity, which reduced sulfide production.
Removing the source of the toxicity allowed the slime layer bacteria to flourish and produce even
more sulfide. In 1989, the United States Congress ordered a study to assess the impact of the
industrial pretreatment program on sulfide generation. The study indicated that both of the above
mechanisms were responsible for a general increase in sulfide in domestic wastewater.
Over a period of several years, influent wastewater at the Hyperion Wastewater Treatment Plant
in Los Angeles, California, was analyzed for several constituents including sulfide and total metals
concentrations. As the data in Table 4-1 dramatically indicate, influent sulfide to the plant
increased at the same time that total metals concentrations decreased. This trend has continued
since 1980 and is considered a direct effect of industrial pre-treatment. Therefore, industrial
pretreatment has resulted in an increase in sulfide in wastewater.
Design Deficiencies
Wastewater and collection system facility designers often under state the increasing trend in
sulfide odor and corrosion related problems and neglect to incorporate controls into their designs.
Often the relatively complex relationships between wastewater characteristics, slime layer
formation, sulfide production bio-kinetics, sulfide chemistry, corrosion biology, gas release, and
ventilation dynamics are not fully understood are not fully understood.
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Tabe 4-1. Dissolved Sulfide and Total Metals Concentration Trends at Hyperion WWTP.Los
Angeles, CA (Joyce, 1998)
Year
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
Total Metals
Concentration (mg/l)
19.0
17.0
16.0
14.0
13.0
12.5
11.0
10.0
8.0
7.0
6.0
5.0
3.5
3.0
Dissolved Sulfide
Concentration (mg/l)
1.0
2.0
3.0
4.0
4.5
5.0
7.0
8.0
10.0
12.0
12.5
13.0
13.5
15.0
The following examples should be noted as potential odor and corrosion catastrophes and should
be avoided:
force mains routed on a downhill gradient can experience premature crown failure
drop manholes with 4.5 to 6 meter (15 to 20 foot) hydraulic freefalls causing extreme
turbulence and hydrogen sulfide release
unlined, unprotected pipe conveying wastewater for distances up to 48 kilometers (30
miles)
manifolded force main systems for distances up to 24 kilometers (15 miles), detention
times of two or three days, and 360-degree slime layers producing dissolved sulfide with
no chance of reaeration
Flushing Relationship to Hydrogen Sulfide Generation
Description of the Process
Wastewater, stormwater or riverwater can be collected and stored in chambers or upstream
sewers for release as a flush wave. As the flush wave travels down the sewer, the shear forces
applied to the upper layer of the sediments exceeds the bonding strength of the solids, and
eroded particles are moved downstream with the flow. In the process of traveling down the sewer,
the upper layer of solids are removed and moved downstream with the flush water.
Odor Producing Conditions
The following scenarios illustrate how flushing may produce odor conditions.
Remaining water in the pipe can contain high concentrations of dissolved sulfide. The
turbulence associated with the flush wave may release peak concentrations of
hydrogen sulfide gas at the downstream flushing terminus.
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Decomposing and hydrolyzing organic solids in the sewer materials can produce
locally high soluble BOD concentrations in the matrix of the solids. Due to the
presence of sulfate and sulfate reducing organisms in the matrix of the solids, it can
be expected that high dissolved sulfide concentrations can exist in the upper layer of
solids (approximately 2 to 6 centimeters). When this layer of solids is removed by a
flush event, the dissolved sulfide present inside the matrix of the solids will be
released into the flush water. Turbulence associated with the flush water will then
cause release of the dissolved sulfide as hydrogen sulfide gas. This may cause odor
and corrosion producing conditions.
When flushing deep deposits, once a fresh layer of solids is scoured away the new
surface of the solids will presumably be the solids previously buried deeper within the
solids mass. The biological population in this freshly exposed layer of solids will likely
be strict anaerobes. This class of bacteria also contains species that can reduce
sulfate. Prior to being exposed to the flush event, these solids received very little flux
of sulfate, since most sulfate was reduced to sulfide by the previous layer. Since the
now exposed solids have been buried for some time, the anaerobic decomposition of
organic matter can produce a rich mixture of organic acids and other short-chain
carbon compounds. These organic acids and other anaerobic products make the
soluble BOD of the mixture very high. Now that the freshly exposed layer is subjected
to ample sulfate in the flush water flow, the metabolism of the sulfate reducing
organisms is greatly accelerated and significant quantities of dissolved sulfide will
again be formed. This newly formed sulfide is therefore available for release as
hydrogen sulfide during the next flushing event.
As the flush wave moves down the sewer at significant velocities, friction between the
air and water creates a very large bowlus of air traveling down the pipe with very high
concentrations of hydrogen sulfide present. When this air arrives at the downstream
flushing terminus, a peak spike of hydrogen sulfide odor may need controlling.
Invert Erosion
It is important to consider the effect of flushing on the condition of the partially corroded pipe. Will
the high velocity water cause erosion of the partially corroded and pasty pipe walls? This may be a
concern when flushing older pipe systems or new systems recently impacted by corrosion. The
basis of this concern is a series of observations noted in 1997 in Phoenix, Arizona. The following
is an excerpt from a recent investigation in Phoenix, Arizona noting the erosion of stones and
aggregates on a downstream new interceptor system from upstream piping areas (OCTC, 1997).
Example of Invert Erosion: Phoenix, Arizona
During an investigation of the twin 1.8 meter (72 inch) diameter sewers entering the 91st Avenue
Wastewater Treatment Plant (WWTP) on June 18, 1997, the flow in the pipe was approximately
0.9 meters (3 feet) deep at the invert and moving at an estimated 1.5 m/s (5 fps). This was a
reduced flow condition from previous inspections in this sewer, and it allowed the inspector to
physically make contact with the invert of the sewer. As the inspector stood on the bottom of the
pipe, a groove, approximately 8 to 10 centimeters (3 to 4 inches) wide and 10 centimeters (3
inches) deep, was detected in the sewer invert. As the inspector probed the length and depth of
the groove with his boot to determine the dimensions, a rock approximately 4 centimeters (1.5
inches) in diameter struck the toe of his boot with considerable force. The rock was retrieved,
examined, and found to be a semi-round, river-gravel type silica rock with one fracture plane. The
rock was relatively clean and did not have a slime layer attached. The rock resembled what could
be used as concrete aggregate in upstream piping. The precise source of the rock could not be
determined. During the 15 minutes that the inspector was in the pipe more than two dozen rocks
were noted to travel down the pipe inside the groove in single-file fashion.
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The groove was probed further and found to extend as far as the inspector could reach both in an
upstream and downstream direction from the manhole. It is believed that the groove extends the
entire length of the sewer in this reach. It was concluded that the groove is caused by the impacts
caused by stones, rocks, and other hard debris traveling down the pipe with the flow. Because of
the high wastewater velocity and hydraulic shear forces developed in the fast-flowing Salt River
Outfall (SRO), it is suspected that all hard debris and rocks which enter the SRO (and pipes of
similar hydraulic characteristics) produce invert erosion until they reach the 91st Avenue WWTP.
It could not be determined with certainty that the reinforcing steel has been severed by the groove;
however, estimating from the depth of the groove and the normal reinforcing mesh coverage in
reinforced concrete pipe (RCP), the steel reinforcing in the bottom of the pipe has most likely
been compromised.
The source of the debris that is causing the invert corrosion cannot be definitely determined,
although the type and size of the rocks are very similar to what would normally be used as
concrete pipe aggregate. The pure silica, acid-proof, river-gravel stone aggregate historically used
as concrete aggregate in the Phoenix area comes from the Salt River basin. The stones are
crushed and screened to produce the proper gradation for concrete mix designs. The crushing
operation produces very hard, sharp, angular pieces ideal for concrete aggregate. It is this type of
aggregate that has been noticed during these inspections to be protruding from the surface of
corroded pipes. The aggregate protrudes from the concrete matrix because silicate stones are
inert to strong reducing acids such as the sulfuric acid produced by Thiobacillus bacteria from
hydrogen sulfide gas. The concrete matrix (predominantly calcium silicate, calcium carbonate,
and calcium hydroxide), however, is easily dissolved by sulfuric acid leaving the inert silicate
stones protruding from the corroded concrete surface.
The older, shallow surface sewers of all SRO member communities are most likely the source of
the rocks. The rocks move slowly or intermittently in these shallow sewers due to the lack of
sustained flushing velocities. When moving slowly or intermittently the rocks do not tumble with
great force and their impact locations are scattered across a large area of the bottom of the pipe.
When these rocks enter the fast-flowing SRO or other large diameter sewers with sustained high
velocities, the rocks are overwhelmed by the hydraulic forces and align themselves in single-file
fashion as they move quickly down the pipe. This causes the impacts from all these stones to be
concentrated in a very narrow band directly in the invert of the pipe. The combination of the mass
(weight) of the stones, their velocity, tumbling action caused by asymmetrical shapes and the
concentration of all impact sites directly at the invert of the pipe is causing the groove. If not
addressed or corrected the erosion of the invert will continue to the point of pipe breach. Once this
happens, the pipe bedding and surrounding soils will likely be liquefied and either moved or
transported downstream. This will leave portions of the bottom of the pipe unsupported and
promote pipe settlement, fracture, and eventual destruction of the pipe requiring replacement.
From a brief study of the physics of this erosion phenomenon, it can be determined that the
greatest damage is caused by those stones and rocks having sufficient mass to impart a
significant impact under the acceleration of gravity. As the stones tumble within the pipe they are
continuously striking the bottom of the pipe, rebounding, being launched briefly a few inches
above the invert, and then strike the invert again a short distance downstream. It can be
determined that the larger stones and rocks cause the greatest damage; however, it can certainly
be argued that the smaller stones also contribute to the phenomenon (less than #4 sieve) but to a
much less degree.
10
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Dissolved Sulfide Prediction Procedure
The dissolved sulfide estimation procedure used in the desktop analyses was developed by
Montgomery Watson to estimate the amount of dissolved sulfide which can be produced by
wastewater under a variety of gravity and force main situations and the corrosion which results
from the release of dissolved sulfide to hydrogen sulfide gas. This procedure is used in
conjunction with the solids deposition and erosion procedure noted earlier in this chapter. This
procedure uses the classical Pomeroy/Parkhurst dissolved sulfide generation equations, with the
exception that additional logic have been added to account for variables not anticipated in the
original equations. The procedure consists of two parts: one for dissolved sulfide generation
based on the Pomeroy equations, and a second part to estimate corrosion rates in wastewater
collection systems. This second part estimates the corrosion rate of concrete, steel, brick, mortar
and other materials subjected to hydrogen sulfide environments. This procedure is for use in
gravity sewers, wet wells, junction boxes, siphons and force main discharge manholes
downstream of sulfide producing sewers and force mains.
Sulfide Model Development
The dissolved sulfide generation rate is most affected by BOD and temperature. An increase in
temperature increases the metabolic rate of the bacteria and the rate of sulfide production. The
term "effective BOD" has been used as a convenient way to combine the temperature and BOD
effects. The equation for this relationship is as follows:
(EBOD) = (BOD) x (1.07) (T'2ฐ)
Where:
EBOD = effective BOD, mg/l
BOD = standard BOD5, mg/l
T = temperature, deg C
1.07 = empirical factor
The Pomeroy and Parkhurst equation noted below is used to predict dissolved sulfide buildup.
This equation accounts for the various factors affecting sulfide buildup in typical municipal
wastewater applications (Pomeroy, 1985). A common form of the equation is presented below
(where D/4 represents the hydraulic radius):
S2 = S,| + (M) (t) [EBOD (D/4 + 1.57)]
Where:
82 = predicted sulfide concentration at time t^. mg/l
S^ = sulfide concentration at time t^. mg/l
t = \2 -1-| = flฐw time in a given sewer reach with constant slope, diameter, and flow; hr
M = specific sulfide flux coefficient; m/hr
D = pipe diameter; ft
An M factor of 1 x 10 "^ m/hr is generally reasonable for force mains in which conditions are
favorable for sulfide buildup (ie: infrequent flow, low velocities, high temperatures, long retention
times, very low dissolved oxygen, and moderate to high BOD). The default value for M in the
sulfide generation procedure is 3 x 10 -4 m/hr and has been found to be a good approximate
value when all force mains and gravity sewers are considered. This value can be adjusted up or
down to account for the specifics of each system to be modeled.
11
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Dissolved Oxygen Effects
The Pomeroy/Parkhurst equations for prediction of sulfide buildup assume that little or no
dissolved oxygen is present. One of the requirements for use of the equations is that less than 0.5
mg/l dissolved oxygen be present for the equations to be accurate. This is due to the sulfide
oxidation effects of the aerobic zone on top of the slime layer. Typically, wastewater contains
more than 0.5 mg/l dissolved oxygen. If the effect of this oxygen is ignored, the model will over-
estimate sulfide production. Facultative bacteria are a type of common sewer bacteria that utilize
dissolved oxygen, when available, but can also respire in its absence. These bacteria will utilize
the available dissolved oxygen (D.O.) until it is depleted. Then, in the absence of dissolved
oxygen, the bacteria will begin utilizing first nitrate and then sulfate as their oxygen source.
Unprocessed municipal wastewater typically has a limited nitrate concentration. Once oxygen and
any nitrate is depleted, the bacteria will begin to utilize sulfate as an oxygen source and produce
sulfide. The time required for depletion of the DO should be accounted for in the determination of
sulfide buildup so that the most accurate results are obtained.
To account for dissolved oxygen concentrations higher than 0.5 mg/l, the Pomeroy/Parkhurst
equations have been modified to include logic which estimates the time for oxygen depletion. The
time required for depletion of the dissolved oxygen is calculated with the following equation:
tDO= DO
SOUR(1/103)(VSS)
Where:
t = time required for bacterial dissolved oxygen depletion, hr
DO
DO = initial dissolved oxygen concentration, mg/l
SOUR = specific oxygen uptake rate, mg O2/g VSS-hr
VSS = volatile suspended solids concentration, mg/l (represents bacterial concentration)
Subtracting the time required for oxygen depletion from the detention time yields the time
remaining for sulfide production. This assumes that sulfide accumulation does not occur during
periods when dissolved oxygen concentrations are greater than 0.5 mg/l. Substituting the time
remaining for bacterial sulfide production into the equation for prediction of sulfide buildup results
in an equation which accounts for the presence of DO. The equation is expressed as follows for a
force main or full flowing pipe:
S2 = S,| + (M) ts [EBOD (D/4 + 1.57)]
Where:
S2 = predicted sulfide concentration at time t^; mg/l
S^ = sulfide concentration at time t^; mg/l
M = specific sulfide flux coefficient; m/hr
ts =t- tDO = time remaining for sulfide accumulation; hr
D = pipe diameter; ft
If enough dissolved oxygen is present, many bacteria will not utilize sulfate as an oxygen source
and therefore not produce sulfide, and other bacteria in the aerobic zone on the slime layer will
oxidize sulfide produced in the anoxic zone and suppress sulfide release back into the
wastewater.
12
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Chapter 5
Overview of Sewer Cleaning
Flushing Systems and CSO Tank Cleaning Technology
The deposition of sewage solids during dry weather in combined sewers has long been recognized as a
major contributor to "first-flush" phenomena. Another manifestation of first-flush, in addition to the scouring
of materials already deposited in the lines, is the first flush of loose solid particles on the urban ground
surface that are transported into the sewerage system and not trapped by catch basins or inlets. These
particulates may settle out in the system and be scoured and resuspended during wet periods. Such
materials also create first flush loading from storm drainage systems. Deposition of heavy solids is also a
problem in separate sanitary systems.
One of the underlying reasons for considerable sewage solids deposition in combined sewers is the
hydraulic design. Combined sewers are sized to convey many times the anticipated peak dry weather
sewage flow. Combined sewer laterals can carry up to 1000 times the expected background sewage flow.
Ratios of peak to average dry weather flow usually range from 2 to 10 for interceptor sewers. The
oversized combined sewer pipes possess substantial sedimentation potential during dry weather periods.
Dry weather flow velocities are typically inadequate to maintain settleable solids in suspension which tend
to accumulate in the pipes. During rainstorms, the accumulated solids can re-suspend and overflow to
receiving waters.
Generally if sediments are left to accumulate in pipes, hydraulic restrictions can result in blockages in
flowline discontinuities. Otherwise, the bed level reaches an equilibrium level. A number of conventional
cleaning techniques are described below, followed by a discussion of various manual and automated
flushing methods.
Over the past 50 years nearly 15,000 CSO tanks have been constructed world-wide. In the US there are
approximately 300 facilities mostly off line at the end of collection systems. The balance are mostly in
Europe with nearly 14,000 constructed in Germany. Tank cleaning methods are reviewed.
Conventional Sewer Cleaning Techniques
Conventional sewer cleaning techniques include rodding, balling, flushing, poly pigs and bucket machines.
These methods are used to clear blockages once they have formed, but also serve as preventative
maintenance tools to minimize future problems. With the exception of flushing these methods are
generally used in a "reactive" mode to prevent or clear up hydraulic restrictions. As a control concept,
flushing of sewers is viewed as a means to reduce hydraulic restriction problems as well as a pollution
prevention approach.
Power Rodding
Power rodding includes an engine and drive unit, steel rods and a variety of cleaning and driving units.
The power equipment applies torque to the rod as it is pushed through the line, rotating the cleaning
device attached to the lead end. Power rodders can be used for routine preventative maintenance, cutting
roots and breaking up grease deposits. Power rodders are efficient in lines up to 0.30 meters (12 inches).
Balling
Balling is a hydraulic cleaning method in which the pressure of a water head creates high velocity water
flow around an inflated rubber cleaning ball. The ball has an outside spiral thread and swivel connection
that causes it to spin, resulting in a scrubbing action of the water along the pipe. Balls remove settled grit
and grease buildup inside the line. This technique is useful for sewers up to 0.60 meters (24 inches).
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Jetting
Jetting is a hydraulic cleaning method that removes grease buildup and debris by directing high velocities
of water against the pipe walls at various angles. The basic jetting machine equipment is usually mounted
on a truck or trailer. It consists of water supply tank of at least 3.8 cubic meters (1,000 gallons), a high
pressure water pump, an auxiliary engine, a powered drum reel holding at least 152 meters (500 feet) of
one inch hose on a reel having speed and direction controls and a variety of nozzles. Jetting is efficient for
routine cleaning of small diameter, low flow sewers.
Pigging
Poly pigs, kites, and bags are used in a similar manner as balls. The rigid rims of bags and kites cause the
scouring action. Water pressure moves these devices against the tension of restraining lines. The shape
of the devices creates a forward jet of water. The poly pig is used for large sanitary sewers and is not
restrained by a line, but moves through the pipe segment with water pressure buildup behind it.
Power Bucket
The power bucket machine is a mechanical cleaning device effective in partially removing large deposits
of silt, sand, gravel, and grit. These machines are used mainly to remove debris from a break or an
accumulation that cannot be cleared by hydraulic methods. In cases where the line is so completely
plugged that a cable cannot be threaded between manholes, the bucket machine cannot be used. The
bucket machine is usually trailer or truck mounted and consists mainly a cable storage drum coupled with
an engine with controllable drive train, up to 300 meters (1000 feet) of 1.3 centimeter (1/2-inch) steel cable
and various sized buckets and tools ranging up to in diameter. The cable drum and engine are mounted
on a framework that includes a 0.9 meter (36 inch) vertical A-frame high enough to permit lifting the
cleaning bucket above ground level. Typically two machines of same design are required. One machine at
the upstream manhole is used to thread the cable from manhole to manhole. The other machine is used
at the downstream manhole has a small swing boom or arm attached to the top of the A-frame for
emptying buckets. The bucket is cylindrical. The bottom of the bucket has two opposing hinged jaws.
When the bucket is plugged through the material obstructing the line, these jaws are open and dig into
and scrape off the material and fill the bucket. When the bucket is pulled in the reverse direction, the jaws
are forced closed by a slide action. Any material in the bucket is retained as the bucket is pulled out
through the manhole.
S/7f Traps
Silt traps (or grit sumps) have successfully been used to collect sewer sediments at convenient locations
within the system with the traps being periodically emptied as part of a planned maintenance program.
The design and operational performance of two experimental rectangular (plan) shaped silt traps in
French sewer systems was reported (Bertrand-Krajewsk, Madiec, and Moine, 1996). Information on
design procedures and methodology for silt traps is scarce.
Sewer Flushing
Flushing of sewers has been a concern dating back to the Romans. Ogden (1892) described early
historical efforts for cleaning sewers in Syracuse, New York at the turn of the century. The concept of
sewer flushing is to induce an unsteady wave by either rapidly adding external water or creating a
"dambreak" effect by quick opening a restraining gate. This aim is to re-suspend, scour and transport
deposited pollutants to the sewage treatment facility during dry weather and/or to displace solids deposited
in the upper reaches of large collection systems closer to the system outlet. The control idea is either to
reduce depositing pollutants that may be resuspended and overflow during wet events and/or to decrease
the time of concentration of the solids transport within the collection system. During wet weather events
these accumulated loads may then be more quickly displaced to the treatment headworks before
overflows occur or be more efficiently captured by wet weather first flush capture storage facilities.
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Recently, Gatke and Borcherding (1996) investigated the effectiveness of long distance flushing of a 4.5
meter (14.8 feet) diameter CSO tunnel 360 meters (1180 feet) in length using a physical (1:24 scale)
hydraulic model coupled with numerical simulation techniques. The work showed that a reservoir 15.5
meters (50.8 feet) high with about a release volume of 360 cubic meters (95,100 gallons) would be
adequate for cleansing sediments.
Manual flushing methods usually involve discharge from a fire hydrant or quick opening valve from tank
truck to introduce a heavy flow of water into the line at a manhole. Flushing removes floatables and some
sand and grit, but is not very effective for removing heavy solids. In recent years, automated flushing
equipment has emerged in France and Germany.
Hydrass ฎ
The Hydrass flushing system developed in France, and shown in Figure 5-1, is comprised of a balanced
hinged gate with the same shape as the cross section as the sewer. At low flows the self-weight of the
gate holds the gate in the vertical position and the sewer flow builds up behind the gate. The depth of flow
continues to build up behind the gate until the force created by the retained water becomes sufficient to tilt
the gate. As the gate pivots about the hinge to a near horizontal position, the sewer flow is released and
this creates a flush wave which travels downstream and subsequently cleans any deposited sediment
from the invert of the sewer. The gate then returns to the vertical position and the cyclic process is
repeated, thus maintaining the sewer free of sediment. Gates are positioned in series at intervals dictated
by the nature, magnitude and location of the sedimentation problem. Chebbo, Laplace, Bachoc, Sanchez
and LeGuennec (1995) reported the effective operation of the Hydrass system. This system has been
installed on a segment of the Marseilles Number 13 trunk. A 100 meter (328 feet) stretch required about
700 flushes to clean an initial deposit of about 100 millimeters (4 inches). Flushing frequency can be
reduced if the upstream head can be increased, i.e., the number of flushes with a 0.5 meter (1.6 feet)
head is 24 times that required for a 1.5 meter (4.9 feet) head.
Diagram showing how the HYDRASS gate operates
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Figure 5-1. Hydrassฎ
Hydroselfฎ
In recent years pollution caused by CSO has become a serious environmental concern. Over 13,000 CSO
tanks have been constructed with over 500 being in-line pipe storage tanks 1.8 to 2.1 meter (6 to 7 feet)
diameter with lengths 125 to 180 meters (400 to 600 feet). Discharge throttles control the outlet discharge
to about twice average dry weather flow plus infiltration. Many different methods for cleaning these pipes
were tried over the years. The most popular has been the HYDROSELFฎ system developed by Steinhardt
Wassertechnik, Taunusstein about 11 years ago.
The HYDROSELFฎ system is a simple method that uses a wash water storage area and hydraulically
operated flap gates to create a cleaning wave to scour inverts of sewers. This system consists of a
hydraulically operated flap gate, a flush water storage area created by the erection of a concrete wall
section, a float or pump to supply hydraulic pressure and valves controlled by either a float system or an
electronic control panel. The water level in the sewer is used to activate the release and/or closure of the
gate using a permanently sealed float controlled hydraulic system. The flushing system is designed to
operate automatically whenever the in-system water level reached a pre-determined level, thereby
releasing the gate and causing a "dambreak" flushing wave to occur. Activation by remote control is also
possible. This technology does not require an outside water supply, can be easily retrofitted in existing
installations with a minimal loss of storage space, and may operate without any external energy source.
The system consists of a hydraulically operated flap gate, a flush water storage area created by the
erection of a concrete wall section, a float or pump to supply hydraulic pressure and valves controlled by
either a float system or an electronic control panel. See Figure 5-2. The actual arrangement for a given
installation is site dependent. The flushing length, slope and width determine the flush water volume
needed for an effective single flush of the system.
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FLUSHING LENGTH = 770m
PLAN VIEW
ENLARGED SECTION A-A
(Q) MAX. WATER SURFACE
-FLUSHING GATE
SECTION B-B
FLUSHING STORAGE CONFIGURATION (Whitten, Germany)
Figure 5-2. Flushing Gate
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The HYDROSELFฎ system has been used to clean settled debris in sewers, interceptors, tunnels,
retention and detention tanks in Germany and Switzerland. This technology was first used in 1986 for
cleaning a tank in Bad Marienberg (a small town with a population less than 10,000 people, about 100
kilometers northeast of Frankfurt). In that same year the first two pipe storage projects using the flushing
gate technology were implemented. This system has been used extensively in Europe with 284
installations with over 600 units in operation. Approximately 37 percent of the projects are designed to
flush sewers, interceptors and tunnels ranging from 0.25 meters to 4.3 meters (0.8 to 14 feet) in diameter
and flushing lengths of up to 340 meters (1100 feet) for large diameter pipes and up to 1000 meters
(3300 feet) for small diameter pipes. The balance of flushing gate installations is for cleaning sediments
from CSO tanks. The largest project in Paris, France cleans an underground 120,000 cubic meter (31.6
million gallons) tank beneath a soccer field using 43 flushing gates.
For large diameter sewers greater than 2 meters (78 inches) the flushing system may be installed in the
sewer pipe itself. The required storage volume for the flush water is created by erecting two walls in the
sewer pipe to form a flush water storage area in between the two walls. For the area to remain free of
debris, a reasonable floor slope (5 to 20 percent) must be provided in the storage area. The requirements
for the storage area slope will determine, in most instances, the maximum flushing length possible for a
single flush gate. Should the actual flushing length be longer than this value, then additional flushing gates
must be installed to operate in series with the first one. In order to increase the maximum flushing length it
is also possible to build additional flush water storage area by creating a rectangular chamber in-line or
adjacent to the sewer line itself.
BIOGESTฎ Vacuum Flushing System
A variation of the HYDROSELFฎ is the BIOGESTฎ, which is a system comprised of a concrete storage
vault and a vacuum pump system to create a cleaning wave to scour the inverts of sewers. The system
consists of a flush water storage area, diaphragm valve, vacuum pump, level switches, and a control
panel for automatic operation of the system. The water level in the sewer is used to activate the vacuum
pump. The vacuum pump evacuates the air volume from the flush chamber and as the air is evacuated
the water is drawn in from the sewer and rises in the chamber. The vacuum pumps shuts off when a
predetermined level in the flushing vault is reached. A second level sensor detects the water level in the
sewer and activates the flush wave. The flush wave is initiated by opening the diaphragm valve above the
flush chamber and subsequently releasing the vacuum and vault contents.
Storage Tank Cleaning Alternatives
Introduction
There are many ways to clean debris and sediment in storage tanks. The most simple and primitive
cleaning methods include hand labor with shovels, brooms and high-pressure hoses for small tanks, or
small bulldozers and clamshells for larger tanks. The most modern and sophisticated technologies include
tipping flushers and flushing gates and are often self-actuating.
Originally tanks were cleaned utilizing automated cleaning options such as traveling bridges, fixed spray
headers and nozzles and submerged mixers. These types of automated cleaning options are primary
cleaning operations. Ineffective primary cleaning options often required manual cleaning such as water
cannons or high pressure hoses to be an integral part of the overall tank cleaning procedure. Manual
cleaning procedures such as water cannons or high pressure hoses are secondary cleaning options.
However, as technology and personnel confidence has evolved many tanks now incorporate only a
primary source of cleaning because it operates efficiently (i.e., tipping flushers and flush gates). From a
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functional perspective a primary method of cleaning is considered highly effective if little "mop-up"
cleaning is required. Often the "mop-up" incorporates visual tank inspection and periodic washdown of
debris in tank corners and other locations that were bypassed by the primary flushing operation. Some
flushing methods are nearly self sufficient and require little or no personnel interaction other than starting
the system (tipping flushers and flush gates), while others need operators to guide the cleaning operation
(water cannons and traveling bridge). In Germany, over 13,000 CSO tanks (mostly rectangular) have been
constructed, and two premier technologies have evolved: tipping flushers and flush gates.
It is important to note that the method of flushing also impacts the configuration of the tank's bottom.
Bottom sloping enhances the removal of settled solids during tank draining and cleaning operations. If
header nozzle systems are used for washdown the tank is typically configured with a center trough,
traversing the length of the tank, and sloping towards the effluent end and the tank bottom slopes from the
side walls to the trough at 3-10 percent. Where tipping flushers or "flush" gates are used the channel
bottom slopes at 2-3 percent from the flusher end toward a large, wide collecting trough at the opposite
end of the tank.
The floor design should consider the maximum admissible slopes to ensure high scouring velocities
during drainage and cleaning operations, while optimizing the depth, area and overall storage tank
volume. Tank bottom design should incorporate input from the flushing equipment supplier to assure
proper operation and sizing. The design of the end trough for tipping flusher and flush gate installation is
as critical as the tank design and must be sufficiently wide in its cross section to prevent "splash back"
from occurring. Peculiarities in terms of special side sloping are discussed with each method.
There are five practical methods that are feasible from an operational standpoint for cleaning the
accumulated sludge and debris in storage tanks. Two of the methods are similar and include wash down
nozzles attached to a moveable bridge and fixed headers and jetting nozzles. The three remaining
cleaning methods are mixers, tipping flushers and flushing gates.
Primary Flushing Systems
Traveling Bridge
There are three general types of traveling bridges that have been used to clean CSO storage tanks.
Discussions specific to each of the three are provided below.
Traveling Bridge - Scraper
The Ruhrverbund Sewage Authority in Essen, Germany maintains 93 WWTP's, several hundred CSO
tanks and a number of multi-purpose water resource reservoirs, water treatment plants and groundwater
recharge systems. In the last 50 years the Ruhrverbund has tried and discarded many types of cleaning
equipment and dozens of different types of channels on the tank floor to increase tractive shear on
draindown. One fairly common method developed 10-15 years ago (but not having any other installations
since) was a traveling bridge with a hard rubber blade or "squeegee" that moved sludge on the tank
bottom to a side channel. From the side channel sludge was pumped vertically to a channel trough at top
of the tank wall, then drained to a local sewer. Montgomery Watson personnel visited Germany in April
1994 to inspect various tank cleaning systems. Visual inspection of the traveling bridge scraper operation
at several facilities showed poor scraper performance (it was impossible to get the floor cleaned and
odors were a problem). However, the side channel sludge pickup system worked satisfactorily.
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Traveling Bridge - Suction Pickup
The Ruhrverbund maintains about 6 tanks with traveling bridges having pumped suction manifolds that
"suck" up sludge to a discharge channel at the top of the tanks and ultimately drain to WWTP's. Such
arrangements are commonly used in secondary clarifiers in Europe and the Ruhrverbund tried this idea on
CSO tank sludge. Visual inspection of the latest constructed facility indicated that small "wind rows" of
sludge remained, and additional, extensive secondary washdown was necessary to supplement the bridge
performance.
Traveling Bridge with Washdown Nozzles
There are only two known traveling bridges with washdown nozzles installed in the United States,
Worcester, Massachusetts and Spring Creek, New York. The Worcester tank will be discussed for the
purpose of this report.
Figure 5-3(a) depicts the traveling bridge arrangement installed at this facility. The storage tank has a
volume of 5700 cubic meters (1.5 million gallons) with two cells 57 meters by 15.3 meters by 5.8 meters
(187 feet by 50 feet and 19 feet) deep. The slope from the sidewalls to the center of the cell is 4 percent
and a sloped sump is located at the center of each cell. Each cell is equipped with a moveable bridge
which is operated by a two-speed chain drive at 1.5 and 2.5 meters (5 and 8 feet) per minute. The bridge
is equipped with a traveling pump (95 liters/second, 1500 gpm), a ductile iron pipe system with 52 spray
nozzles spaced 0.3 meters (1 foot) on center for the horizontal bottom and 3 nozzles on each of the
vertical pipe assemblies, and a water cannon. On the outside of each basin is a 1.35 meter (4.5 feet) wide
water trough that runs the entire length of the tank, and supplies water for the washdown system. Two
passes are generally sufficient to clean the tank. Visual inspection indicates that the side vertical nozzles
are not required to clean the tank walls because nothing adheres to them.
This system has been in operation for five years and has performed very efficiently although it requires
significant operation and maintenance and is costly. One advantage of this system is that the primary and
secondary modes of cleaning are located in a central location, the bridge. The disadvantages of this
system include:
Significant water consumption during the cleaning operation;
The traveling bridge mechanism requires frequent maintenance;
The initial installation requires extensive alignment of the bridge mechanism;
Many mechanical components;
The system requires a secondary mode of cleaning (i.e., water cannons);
High structural costs associated with the water supply reservoir for each basin; and
Since the traveling bridge is located above the high water elevation in the storage tank, it must
be used with an open tank concept.
Mechanical Mixers and Submerged Jets
The basic principle involved with this technique is different from washdown systems that aim to clean
sediments from empty tanks. Mechanical mixers and water jets operate on the premise of resuspending
settled debris before drawdown begins and maintaining all materials in suspension until the tank is
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completely drained. Re-suspension of solids requires an introduction of energy to create necessary
turbulence. This method is very popular for small tanks in central Germany. The actual placement of the
mixers requires considerable adjustment to optimize mixing and establish proper mixing currents and flow
directions
Estimates of required mixing energy could be assumed using criteria established for aerobic digesters
depending on inorganic solids concentration. As the water depth in a tank decreases during the drawdown
procedure, the mixing power must increase proportionally to properly maintain solids suspension.
Although capital cost data is not available for installations of this type, it is clear that this system requires a
considerable amount of energy in addition to equipment and operational costs similar to those for header
and nozzle systems. A serious disadvantage of this system is the requirement to manually clean the
residual inorganic solids on the tank floor following draindown, consequently requiring extensive
secondary cleaning operations. Since a secondary cleaning operation must be utilized in conjunction with
this technology, an open tank layout must be employed. Advantages of this system are that the captured
solids are more uniformly returned to WWTP via the pumpback operation and that little additional water is
used.
This technology is most appropriate for circular tanks because circulation can easily be accomplished with
few dead spots. Designers are constantly experimenting with fillets and baffles in rectangular tanks
because flow currents are extremely complicated to predict when using mixers. A submerged jet is
depicted in Figure 5-3(b).
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TRAVELING BRIDGE
HEADER
SUMP
HEADER PIPE
Figure 5-3 (a) Traveling Bridge (Adapted City of Worcester, 1998)
(b) Submerged Jet
10
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Fixed Spray Nozzles and Headers
Fixed spray nozzle and header systems are generally comprised of an extensive piping network with
multiple valves, booster pumps and controls and a washdown wet well. The spray headers and nozzles
are generally suspended inside the tanks. This alternative requires secondary cleaning operations,
typically with water cannons, because of the inefficiency of the spray nozzles to clean the entire tank. A
dedicated secondary cleaning system must be used in conjunction with this technology, and thereby
necessitates an open tank concept be incorporated. The tank bottom required for this system slopes
steeply (approximately 10 percent) from the sidewalls to a center trough, and the center trough slopes at 2
- 3 percent toward an effluent trough. Disadvantages associated with this alternative include:
Significant water consumption during the cleaning operation;
The system requires a secondary mode of cleaning (i.e., water cannons);
Floatables get caught on the header system; and
Excessive sediment and debris can accumulate in areas where the nozzles do not reach.
Experience has shown that fixed spray headers and nozzles have been somewhat effective at some
installations but do have some limitations. The Toronto Easterly Beaches Phase 1 tank (3800 cubic
meters, 1 million gallons) could not be washed all at once because the washdown system demand, both
pressure and flowrate, depleted the city supply system. Ultimately the tank was washed in quadrants to
relieve the strain on the city system. The Saginaw, Michigan Weiss Street Facility 36,100 cubic meters
(9.5 million gallons) tank had 1830 meters (6000 feet) of 0.41 meter (16 inch) diameter pipe with nozzles
spaced 1.2 meters (4 feet) on-center and 24 water cannons to perform the cleaning operations. Since this
system was installed four other tanks have been built by the city and all have incorporated tipping flushers
as the primary cleaning technology. The cost and disadvantages associated with this alternative do not
make it a feasible option for this installation. A typical spray header and nozzle arrangement is depicted in
Figure 5-4.
11
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TANK OVERFLOW
ACCESS OPENING
INFLOW PIPE
FLOWPIPI
STORM INTERCEPTOR
SEWER
CONTROLLED
OUTFLOW
A - TANK CROSSECTION
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Figure 5-4. Fixed Spray Header and Nozzle Arrangement
Tipping Flushers
Tipping flushers (TF) systems have been used in North America for three years (about 15 tanks with
flushers in the US, with most located in Michigan area), and have been operational in Germany and
Switzerland for over 9 years. Tipping flushers are extremely effective for subsequent cleansing of debris
from the floors of all types of urban runoff tanks. These devices were initially developed in Switzerland.
The system generally include filling pipes and valves, a pumping system and wet well (where restricted by
the site conditions), and the tipping flusher vessels. The TF is a cylindrical stainless steel vessel that is
ideally suspended above the maximum water level on the back wall of the storage tank. The units can be
filled with river water; ground water or potable water, but require a filling system consisting of 5 to 7.6
centimeters (2 to 3 inches) headers with appropriate controls. Just prior to overtopping the vessel with
water, the center of gravity shifts and causes the unit to rotate and discharge its contents down the back
wall of the tank. A curved fillet at the intersection of the wall and tank floor redirects the flushwater (with
minimum energy loss) horizontally across the floor of the tank. The fillet size depends on the size of the
flusher. The flushing force removes the sedimented debris from the tank floor and transports it to a
collection sump located at the opposite end of the tank.
12
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The experience with US TF systems to date indicates that dedicated secondary cleaning operations, using
concurrently operated water cannons or high pressure hoses, are not needed. If the first flush of the basin
does not remove all of the sediment, the basin can be re-flushed or "mopped-up" by fire hoses. In
Germany and in Switzerland, tank sidewalls are generally hand trowelled to a very smooth finish to
prevent buildup from occurring, and consequently don't require frequent washdown. "Mop-up" cleaning of
the influent and washdown channels has been done utilizing small tipping flushers in large German tanks
and in Saginaw, Michigan. See Figure 5-5 for an example of a tipping flusher installation.
TYPICAL TIPPING
FLUSHER INSTALLATION
Figure 5-5. Tipping Flusher (adapted from LIFT, 1998)
13
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Flushing Gates
The flushing gate was originally developed in Germany (1985) as a method for flushing sediments in pipe segments
(in-line storage or troublesome flat trunk and interceptor sewers), and has evolved for use in CSO tanks. As
described earlier in this chapter, flushing gates have been used as the means to flush and cleanse deposits and debris
from CSO tanks in about 350 German installations. In concept this scheme is depicted in Figure 5-6 and 5-7 from a
recent design in Cincinnati, Ohio.
24"x24" SLUICE-
GATE
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-TRAINING WALL TYPE "A"
-TRAINING WALL TYPE "B"
-TRAINING WALL TYPE "A"
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-DEWATERING PUMP
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Figure 5-6. Plan View, Clough Creek CSO Treatment Facility, Cincinnati, Ohio
FIN. GRADE
GRADE.
^4" FLUSH WATER
ADJUSTMENT PIPE
FLUSHING GATE (TYP.)
Figure 5-7. Section View, Clough Creek CSO Treatment Facility, Cincinnati, Ohio
The system is comprised of two basic elements, a gate and a closed circuit hydraulic actuation system
utilizing a float control mechanism. A low-level wall is constructed across the short axis of the influent end
of the tank approximately 1.5 to 2 meters (5 to 6.5 feet) high. The wall is located on the influent end of the
tank to guarantee filling the space behind the wall prior to filling the rest of the tank. The instantaneous
opening of a stainless steel gate that is mounted on the face of the wall activates the system. The release
of the gate creates a "dam break" scenario, which generates a high velocity flush wave (generally trying to
14
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maintain a velocity in excess of 1.8 meters/second (6 feet/second). Normally the width of the flushing gate
is approximately 0.7 the effective flushing lane width. The volume retained behind the wall required for
proper cleaning is a function of flushing length and floor slope. The "nominal" design volume can be
adjusted by changing the height of a level standpipe on the backside of wall. The hydraulic system can
also be connected to central control system (on or off-site) with auto or manual override.
These systems have tank floors that slope from the flush gate location to the collection trough at 1 - 3
percent. The flush gates require training walls on the tank bottom that are about 0.4 to 0.5 meters (15
inches to 18 inches) high, and run the full length of the tank to control the flow direction of the wave. All
walls parallel to the path of flushing flow should be perpendicular to the tank bottom, with no fillets, to
ensure the lower wall edges are cleaned.
In function, this technology is similar in concept to tipping flushers. One main difference between the two
technologies is that the tipping flushers are suspended above the tank floor and flush down sidewall,
thereby taking advantage of the energy conversion from potential to kinetic. In practice, this means that
the flushing gate needs about 20 percent more flushing volume than tipping flushers for comparable tank
floor slope and tank lengths. However, since the flush volume consists of stored CSO, there is no
additional cost associated with this volume. The experience with flush gate systems to date indicates that
dedicated concurrent secondary cleaning operations, using water cannons or high pressure hoses, are not
needed. If the flush of the basin using tank contents does not remove all of the sediment, the basin can
either be re-flushed (requiring and external water source for filling), or "mopped-up" using fire hoses. To
date flushing basins with tank contents has not required mop-up in German tanks. The largest length
flushed with flushing gates is 90 meters (295 feet) while flushing lengths of 70 meters (230 feet) are fairly
common.
Secondary Flushing Systems
Water Cannons
Water cannons are typically used to washdown corners, areas around piping, and other hard to reach
places. They are typically used in conjunction with spray header systems (both fixed and traveling bridge)
and mechanical mixers and submerged jets. These systems require extensive piping and valve networks,
booster pumps, a supply wet well and are strictly operated manually. The use of water cannons requires
an open storage tank configuration.
Water cannons typically have a maximum discharge rate of 25 liters/second (400 gpm) each at a working
pressure of 2.8 to 5 cm/m2 (40 to 70 psi) and have a useful working spray radius of 20 to 30 meters (70 to
100 feet). Cannons can rotate 360 degrees horizontally and have about 100 to 120 degrees range of
motion in the vertical direction. Water cannons should be provided with shut-off/isolation valves and 2.5
centimeter (1 inch) nozzles. Spray down and cleanup times required per cannon vary depending on the
facility, type of solids loading, and time of solids exposure (open tanks). However, cleanup times of 5 to 15
minutes per water cannon are common.
High Pressure Hoses
Most CSO facilities have washdown high pressure hose systems on-site for miscellaneous cleanup
operations. This system can often be utilized for secondary cleaning operations by providing hose gates
where required. This system requires a piping and valve network, booster pumps and a supply wet well.
Hose gate connections are provided throughout the facility to accommodate cleaning operations. If this
technology is used as a true secondary flushing system it will require an open tank layout. Hose
15
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connections are typically 3 centimeters (1-1/4 inch) and utilize similar water usage and pressure
requirements to a water cannon system. Hoses allow the flexibility to move the discharge point around
because the hose is not fixed like water cannons.
Novel Approach For Cleaning Circular Tanks
The Lincoln Park facility in Decatur, Illinois consists of two mechanical screening facilities, a 7500 cubic
meter (2 million gallons) open circular "first flush" storage tank, a 11 meter (36 feet) diameter vortex flow
dividing chamber (two asymmetric flow inputs of 11,000 l/s and 7200 l/s (176,000 gpm and 114,000 gpm)
are divided into four 4550 l/s (72,000 gpm)) waste streams, four 13.6 meter (45 feet) diameter vortex
solids separators, and a treated effluent to the Sagamon River. Figure 5-8 presents on overall schematic
for the facility.
Inlet =12,000 l/s
Influent flow meter
2) Influent sampler
Effluent flow meter
4) Effluent sampler
5) Underflow Sampler
Dortex Separator
istribution Struc
I
Effluent to
Sangamon River
Inlet =3150 l/s
Lincoln Park Facility Monitoring Locations
Figure 5-8. Lincoln Park Schematic
Diverted wet-weather flows (WWF) (two inputs) are first passed through mechanically cleaned,
automatically controlled, catenary screens. A manually cleaned bar screen is provided for emergency
bypass. Downstream of each screen chamber are two liquid-level actuated, motorized sluice gates
16
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directing flow into the "first flush" retention tank. The tank diameter is 36 meters (118 feet) with a side
water depth of 8.5 meters (28 feet). The tank is equipped with mixers and aerators and the tank floor
slopes to a circular gutter draining to a pumping station. When the tank level rises to a pre-set level,
control gates direct any additional inflows to the vortex flow divider with outputs into the four vortex solids
separators. Foul underflows from the bottom of the four separators are pumped into the "first flush" tank.
The pumping station also dewaters the tank and the separators after an event.
Cleanout of the circular "first flush" tank includes several novel design and operational concepts. First, the
two main gravity feeds to the tank are tangentially fed during an event. Secondary flow currents are
established moving most of the solids to the center zone of the tank, a common design attribute on
hundreds of circular tanks in Germany. After an event the cleaning operation involves two steps. The
contents of the tank are kept in motion while being slowly pumped to the interceptor. This circulation
feature is accomplished by a feed from the center of the tank with a separate recirculation loop and a
tangential return. When the tank is fully drained, no sediment is typically observed from the perimeter
tank walls inward for about 4.5 to 6 meters (15 to 20 feet). From there the sediments grade from about 1
cm in depth to about 15 centimeters (6 inches) in the center region of the tank. Two high-pressure water
monitors on opposite sides of the tank are then used to cleanse the remaining sediments to the center
well in about six minutes.
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Chapter 6
Case Evaluations
This chapter presents facility evaluation data of combined sewer in-line and CSO storage tank flushing
systems. The objectives of these evaluations included the following:
Collect dimensional and operational data of combined sewer in-line and CSO storage tank
facilities that utilize flushing gates or tipping flushers for cleaning;
Evaluate the effectiveness of the system design in terms of sediment removal;
Compare capital and operation and maintenance costs of flushing gate and tipping flusher
facilities with other cleaning methods.
Table 6-1 presents a guide outlining the major features of the 18 case studies. Contents of the table
include location; flushing function, i.e. flushing of storage pipe, conveyance pipe or tank; tank geometry
(rectangular or circular); flushing method, i.e. flushing gate, tipping flusher or other; flushing volumes for
pipe configurations, either by generated off-line or in-line compartments; flushing volumes for tanks are
noted as in line. Information for other miscellaneous tank reviews are provided following the 18 case
studies.
Table 6-1. Overview of Case Studies
Case
Number
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Location
Marht Wiesentheid, Germany
Gemeinde Schauenburg, GER
Stadt Kirchhain, GER
Stadt Heidenheim, GER
Markt Grossostheim, GER
Osterbruch-Opperhausen,
GER
Gemeinde Hettstadt, GER
Filterstadt-Bernhausen, GER
Stadt-Essen, GER
Markt- Wiesentheid, GER
Stuttgart- Wangen, GER
Heidenheim-Kleiner-Buhl,
GER
Cheboygan, Michigan
Sarnia, Ontario, Canada
Port Colborne, Ontario,
Canada
Wheeler Avenue, Kentucky
14tn Street Pumping Station,
Ml
Saginaw Township, Ml
Flushing Function
Pipe
Storage
X
X
X
X
Pipe
Convey.
X
X
X
Tank
Rect.
X
X
X
X
X
X
X
X
X
X
Circular
X
Flushing Method
Flush Gate
In-line
X
X
X
X
X
X
X
X
X
X
Off-line
X
X
X
X
Tipping
Flusher
X
X
X
X
A standard evaluation form was prepared and distributed to operators at several facilities in Germany and
in North America. In-line versus off-line refers to the relative location of the flushing volume. Due to
-------
space limitations, flush volumes are often generated in-line with main convergence function accomplished
by an underflow conduit or channel under the flush volume chamber. Vaults with large flushing volumes
are commonly provided by off-line configurations. Average slope refers to the slope of the conduit or
section being flushed. Slope of flush volume refers to the floor slope of the flush vault. Flush gate
activation is accomplished either by passive float operation termed "hydraulic" or by an active electrical
signal from an external location termed "electrical". Water source refers to the source of the water for
flushing, i.e. "local waste" or "external supply". Performance assessment is defined as follows: "Excellent"
- all sediments in channel or bay cleaned with flush; "Good" - substantial removal of sediments in
channel, i.e. 90% of flush lane or pipe is cleaned; "Fair" - partial removal of sediments, i.e. 50-70% of
flush lane or channel cleaned with flush. The following case studies, consisting of tables and narratives
summarize the findings for each site.
Case Studies: Combined Sewer Flushing Facilities using Flushing Gates
The following are summaries of the pipe flushing facilities that were evaluated in Germany.
Case No. 1- Marht Wiesentheid
Details of the Marht Wiesentheid flushing facility are noted in Table 6-2. The in-line CSO storage conduit
is a 1.8 m (6 ft diameter) circular pipe, 46.8 m (153 ft) in length. The storage pipe is throttled at its outlet
using a flow regulator to maintain outflow equal to twice average dry weather flow (plus infiltration). An
inline flushing vault holding 14 cubic meters (3700 gal.) is used to cleanse the storage pipe. The flushing
gate is activated by hydraulic float control. Operators note excellent performance in cleansing deposits
during flushing.
Table 6-2. Marht Wiesentheid, Germany
Question
In-Line or Off-line
Year Constructed
Flow type
Pipe size and shape
Length
Average Slope
Pipe Material
Flush Gate
Dimensions
(I w)/M ate rial
Flush Vault Volume
Flush Vault
Dimensions
(Iwh)
Slope of flush vault
Water Source
Method of Gate
Activation
Frequency of
Inspection/Crew Size
Performance
Assessment
Response
In-Line
1992
Combined
1800 mm circular
46.8m
1%
Concrete
2.8 mx1.31 m/
Stainless steel
14 cubic meters
2.5 mx4.84 mx
1.15m
20%
Local waste
Hydraulic
After each activation/1
person
Excellent
Case No. 2 - Gemeinde Schauenburg
The Gemeinde Schauenburg storage facility (Germany) is very similar to the facility at Marht Wiesentheid
(Case 1). Details are noted in Table 6-3. This facility is a CSO storage pipe 2 m (78 inch) circular in
-------
diameter 64 m (210 ft) in length. Flushing volume of 5.5 m (1450 gal.) is used and the flushing gate
operates by hydraulic float activation. The gate activates when the downstream flow controller permits the
storage to drain. The operators note "fair" performance for this facility.
Table 6-3. Gemeinde Schauenburg, Germany
Question
In-Line or Off-line
Year Constructed
Flow type
Pipe size and shape
Length
Average Slope
Pipe material
Flush Gate
Dimensions (I w) /
Material
Flusher Volume
Flusher Dimensions
(Iwh)
Slope of flush vault
Water Source
Method of gate
Activation
Frequency of
Inspection/Crew Size
Performance
Assessment
Response
In-Line
1988
Combined
2000 mm circular
64 m
1 %
Asbestos Cement
1.2 mxO.4 m
Stainless steel
5.5 cubic meters
2 mx3 mxO.91 m
20%
Local waste
Hydraulic
After each activation/2
person
Fair
Case No. 3 - Stadt Kirchhain
The Stadt Kirchhain facility, listed in Table 6-4, has an off-line flushing gate that is used to flush the
deposited sediments in the 1600 mm pipe to a downstream regulator which empties into a downstream
300 mm sewer. Figure 6-1 depicts the plan view of one of the two off-line flushing vaults used in the Stadt
Kirchhain facility (Germany). Figure 6-2 depicts the plan view of the downstream flow control chamber
throttling the twin storage pipes.
-------
Table 6-4. Stadt Kirchhain, Germany
Question
In-Line or Off-line
Year Constructed
Flow type
Pipe size and shape
Length
Average Slope
Pipe material
Flush Gate
Dimensions
(1 w)/M ate rial
Flush Vault Volume
Flush Vault
Dimensions
(Iwh)
Slope of flush vault
Water Source
Method of gate
Activation
Frequency of
Inspection/Crew Size
Performance
Assessment
Response
Off-line
1991
Combined
1600 mm circular
115m
0.4%
Concrete
1.2 mxO.4 m
Stainless steel
4 cubic meters
2 m x2.5 m x 1 m
20%
Local Waste
Electrical
Not Available
Fair
o
in
cxi
1
* .
* * *
*ป
*
t
*
* **
* *
*
All Dimensions in Meters
* .'.*: * *ซ.
*- _/ :'?
^-^uLUm
ADJUSTABLE ^
VAUI T 1 FVFI
STANf) PIPF O_
,-.
^^-rTTTTTTT
*. **.*''.?
.'*'.** * . .
*
V
^
*
4
*
.FLUSH
/GATE
ซ * / *
//Y///
I ^S
/
J 1
f
0
0
t
1
1
>
^
X\\\V
* '' / * .
H
.
. *
*
.
cso
STORAGE
PIPE
0=1,600 mm
^
\
\
\
\
^ 250 mm
MAI P r
2.00
1.00
Figure 6-1. Stadt Kirchhain - Off-Line Flushing Vault Plan View
-------
INFLUENT
CSO
STORAGE
FLUSH VAULTS
BOTH SIDES
STORAGE
^THROTTLED
OUTFLOW
Figure 6-2. Stadt Kirchhain - Plan View Downstream Control Chamber
Case No. 4- Stadt Heidenheim
Details of the Stadt Heidenheim facility are noted in Table 6-5. The Stadt Heidenheim facility has an in-
line flushing chamber that is located within the regulator structure on the dry weather conduit. Extreme
wet weather flows overtop the weir in the center of the structure into a 1200 mm bypass conduit. Plan and
section views of the overall regulator, flushing vault and bypass chamber are depicted in Figures 6-3 and
Figure 6-4.
-------
Table 6-5. Stadt Heidenheim, Germany
Question
In-Line or Off-line
Year Constructed
Flow type
Pipe size and shape
Length
Average Slope
Pipe Material
Flush Gate
Dimensions
(1 w)/M ate rial
Flush Vault Volume
Flush Vault
Dimensions
(Iwh)
Slope of flush vault
Water Source
Method of Gate
Activation
Frequency of
Inspection/Crew Size
Performance
Assessment
Response
In-Line
1993
Combined
2200 mm circular
240m
Not Available
Concrete
1.2 mxO.4 m
Stainless steel
10 cubic meters
3 m x5 m xO.7 m
10%
Local Waste
Hydraulic
After each activation/2
person
Good
-------
OVERFLOW
WEIR
BYPASS
Figure 6-3. Stadt Heidenheim - Plan View
-------
EXTREME FLOW
BYPASS WEIR
INFLUENT
UNDERFLOW
Figure 6-4. Stadt Heidenheim - Section View
FLUSH GATE
Case No. 5 - Markt Grossostheim
Details of the Marktt Grossostheim facility are presented in Table 6-6. The flushing vault and storage
facility involves an off-line storage compartment created by a spill weir from the dry weather channel.
Extreme overflows can then bypass the side spill storage compartment. These bypasses are controlled
by a mechanical operated bending weir. The downstream flow throttle is a mechanical knife valve
controlled by direct measurements of a magnetic meter.
Table 6-6. Markt Grossostheim, Germany
Question
In-Line or Off-Line
Year Constructed
Flow type
Pipe size and shape
Length
Average Slope
Pipe material
Flush Gate Dimensions(l w)
Material
Flush Vault Volume
Flush Vault Dimensions
(Iwh)
Slope of flush vault
Water Source
Method of Gate Activation
Frequency of
Inspection/Crew Size
Performance Assessment
Response
Off-Line
1993
Combined
2200 mm circular
190m
9.4%
Not Available
1.5 m xO.4 m
Stainless steel
15 mj
6.5 m x 2.2 m x
1.03m
15%
Local Waste
Hydraulic
After each
activation/2person
Excellent/Good
-------
Case No. 6 - Osterbruch-Opperhausen
Details of the Osterbruch-Opperhausen facility are noted in Table 6-7. Sectional views of this facility are
presented in Figure 6-5. The flushwater chamber is filled by an upstream 100 mm pumped pipe. The
flushing gate is utilized to clean the downstream 250 mm combined sewer conduit. The flush vault is
placed on the head of the combined sewer and 1000 m (3250 ft) of downstream conduit to a regulator
location where flushed solids are discharged into receiving sewer.
Table 6-7. Osterbruch-Opperhausen, Germany
Question
In-Line or Off-line
Year Constructed
Flow type
Pipe size and shape
Length
Average Slope
Pipe material
Flush Gate Dimensions
(I w)/M ate rial
Flusher Volume
Flusher Dimensions
(Iwh)
Slope of flush vault
Water Source
Method of Gate Activation
Frequency of
Inspection/Crew Size
Performance Assessment
Response
Off-line
1989
Sanitary
250 mm circular
1000m
0.1%
Vetrified Clay Pipe
0.5 m xO.4 m
Stainless steel
2mJ
2.4 m x 1.6 m x
0.75m
20%
River Water
Hydraulic
After each
activation/2person
Excellent/Good
-------
SECTION VIEW
PUMPED
FLUSH WATER
FEED
All Dimensions in Meters
i_
PLAN VIEW
FLUSH GATE'
Figure 6-5. Osterbruch-Opperhausen - Sectional Views
Case No. 7 - Geimeide Hettstadt
Details of the Geimeide Hettstadt facility are noted in Table 6-8. The storage element is an off-line CSO
pipe conduit 1600 mm (66 inch) in diameter, 224 m (735 ft) in length. The off-line flushing chamber holds
10 m3 (2700 gal.) and is filled during overflow events. Operators have noted good to fair performance.
Table 6-8. Gemeinde Hettstadt, Germany
Question
In-Line or Off-line
Year Constructed
Flow type
Pipe size and shape
Length
Average Slope
Pipe material
Flush Gate Dimensions
(I w)/M ate rial
Flusher Volume
Flusher Dimensions
(Iwh)
Slope of flush vault
Water Source
Method of Gate Activation
Frequency of
Inspection/Crew Size
Performance Assessment
Response
Off-line
1992
Combined
1600 mm circular
224m
0.5%
Concrete
1.5 m xO.4 m
Stainless steel
10 cubic meters
5 m x 2.7 m x
0.75m
15%
Local Waste
Hydraulic
After each
activation/2person
Good/Fair
Hydraulic Analysis Flushing Gate Performance for Sewers
10
-------
The Stormwater Management Model (SWMM) with Extended Transport Block (EXTRAN) was used to
investigate the efficiency of the German sewer pipes that use flushing technology. Simulation output
takes the form of water surface elevations and discharge at selected system locations. EXTRAN was
developed by the U.S. EPA and is described in total in the User's Manual, EPA/600/3-88/001 b.
The basic conveyance element input data required in EXTRAN are specifications for shape, size, length,
roughness, connecting junctions and ground (rim) and invert elevations. These data for pipe and tank
flushing were obtained from the evaluations of the German facilities. Pipe and tank lengths were
discretized into two or three equal sections. These discretizied sections varied from 15 to 30 meters (50 to
100 feet). Pipe sections were assumed to be circular (equivalent diameters calculated) and tank sections
(flushing bays) were assumed to be rectangular. An additional 30 meter (100 feet) section was added to
the downstream tank conduit to simulate a grit pit. The following parameters were kept constant in both
pipe and tank simulations:
Computation time increment = 1 second
Manning roughness coefficient = 0.015
Gate opening time in 10 seconds
Flow hydrographs at the flushing gate are assumed to increase linearly from zero to a constant flow
rate in 5 seconds and also to decrease linearly from the constant rate to zero in 5 seconds.
Upstream of the conduit/tank was assumed to be the input and downstream was assumed to be a
free overflow.
Table 6-9 summarize the hydraulic data (length, slope, size and flush volume) and results (velocity, depth,
and flush volume/length of flush) from the respective flushing gates determined from the evaluations of
the German facilities. The listed results are at the downstream end of the pipe or channel flushed. At the
far right hand side of Table 6-9 is listed the operator observation. Qualitative operator observations have
excellent agreement with the quantitative modeled velocity. For example the terminal velocity of Stadt
Kirchhain is 0.60 m/s, which is the lowest velocity, and the operator observe only "Fair" flushing results.
Table 6-9. Summary of Pipe Flushing Results
Location
Marht Wiesentheid
Stadt Heidenheim
Stadt Kirchhain
Markt Grossostheim
Length
(m)
47
241
115
191
Slope
1 .0%
1 .0%
0.4%
0.94%
Size
(m)
1.8
2.2
1.6
2.2
Velocity
(m/s)
3.1
1.0
0.60
1.2
Depth
(m)
0.40
0.09
0.07
0.11
Flush Vol.
(m3)
14
10
4
15
Operator
Observation
Excellent
Good
Fair
Excellent / Good
Case Studies: CSO Storage Tank Flushing Facilities using Flush Gates
The following are summaries of the CSO tank flushing facilities that were evaluated in Germany and North
America.
Case No. 8 - Filterstadt-Bernhausen
Details of the Filterstadt-Bernhausen CSO tank facility using, flush gates to cleanse the tank after
activations are noted in Table 6-10. The tank volume is 1000 m and consists of two unequal sized bays
(5.65 m and 4.65 m). Flushing is accomplished by two vaults with two flusher systems per bay. No
flushing training walls are provided. The end channel transports the flushed deposits to a central throttled
11
-------
outlet. The flush gates are activated by hydraulic float control when the tank drains after an event through
a 300 mm (12 inch) throttle.
Table 6-10. Filterstadt-Bernhausen, Germany
Question
Year Constructed
Type of Flow
Covered or Open Tank
Number Of Bays
Number of Flushers/Bay
Training Walls per Flusher
Flush Channel Dimensions
(Iwh)
End Trough Dimensions
Iwd
Flush Channel Slope
Flush Gate Dimensions(l w)
Material
Flush Vault Volume
Flush Vault Dimensions
(Iwh)
Slope of flush vault
Water Source
Method of Gate Activation
Method of Removing
Flushed Sediments
Frequency of
Inspection/Crew Size
HVAC System/Type
Odor Control System/Type
Electrical System Explosion
Proof
Performance Assessment
Response
1990
Combined
Covered
2
2
No
36 m x 5 m x 3.5
m
8 m x 12 m x 1.2
m
2.5%
1.5/1.75 m x 0.4
m
Stainless steel
2@10mJ/
2@ 8.5 m3
2.5 m x 5.65 m x
1.0
2.5 m x 4.65 m x
1.0m
20%
Combined
Hydraulic
Underflow Throttle
(400 mm) to
WWTP
After each
activation/Not
Available
Yes/Grating
No
No
Good
Case No. 9 - Stadt-Essen
Details of the Stadt-Essen storage facility are noted in Table 6-11, has each flush vault equipped with two
gates, and training walls are not installed within individual bays. The Stadt Essen layout is depicted in
Figures 6-6 and 6-7. It features a unique inlet scheme where the underflow is concentrated in a vortex
chamber that discharges back to the dry weather sewer. Once the underflow capacity is exceeded, the
influent channel to the flushing gates fills which in turn fills the flush vaults and tank. The flushed
sediments are collected in the mud sump for discharge back to the dry weather sewer.
Table 6-11. Stadt-Essen, Germany
12
-------
Question
Year Constructed
Type of Flow
Covered or Open Tank
Number Of Bays
Number of Flushers/Bay
Training Walls per Flusher
Flush Channel Dimensions
(Iwh)
End Trough Dimensions
Iwd
Flush Channel Slope
Flush Gate Dimensions
(1 w)/M ate rial
Flusher Volume
Flusher Dimensions
(Iwh)
Slope of flush vault
Water Source
Method of Gate Activation
Method of Removing
Flushed Sediments
Frequency of
Inspection/Crew Size
HVAC System/Type
Odor Control System/Type
Electrical System
Expolosion Proof
Performance Assessment
Response
1993
Combined
Covered
3
2
Yes
55 m x 3 m x 3.4
m
5 m x 20 m x 0.5
m
0.5%
1.5/1.75 m x 0.4
m
Stainless steel
12 mj
4 m x 1.4 m x
2.85 m (2)
3.20 m (4)
20%
Combined
Electric
Underflow Sluice
Gate (200 mm) to
WWTP
Not Available
Yes/ Ventilation
Pipes - 300 mm
Intake and 450
mm Exhaust
No
No
Good
13
-------
FLUSH VAULTS
. v . '..''./. . . **ป!
_ _ _ Jr
INFLUENT
All Dimensions are in Meters
Figure 6-6. Stadt-Essen - Plan View, Tank Influent
14
-------
All Dimensions are in Meters
t
t
c
ZN
t
_i_i_
.a.
rsi
_n_
_n_
3.20
i
J-L
_n_
Figure 6-7. Stadt-Essen - Plan View, Tank Effluent
Case No. 10- Markt-Wiesentheid
A section view of this facility is depicted in Figure 6-8. This facility has a unique flush vault filling
arrangement. Flow enters the facility at the effluent end of the tank via a dry weather conduit integral to the
tank that is regulated downstream. As flow in the dry weather conduit increases during wet weather, flow
surcharges a 300 millimeter conduit that is connected at the springline of the dry weather conduit on an
adverse slope which fills the flush vaults.
15
-------
Table 6-12. Markt-Wiesentheid, Germany
Question
Year Constructed
Type of Flow
Covered or Open Tank
Number Of Bays
Number of Flushers/Bay
Training Walls/Flusher
Flush Channel Dimensions
(Iwh)
End Trough Dimensions
I wd
Flush Channel Slope
Flush Gate Dimensions
(I w)/M ate rial
Flusher Volume
Flusher Dimensions
(Iwh)
Slope of flush vault
Water Source
Method of Gate Activation
Method of Removing
Flushed Sediments
Frequency of
Inspection/Crew Size
HVAC System/Type
Odor Control System/Type
Electrical System
Expolosion Proof
Performance Assessment
Response
1992
Combined
Open
5
1
Yes
41 mx4.84 m x3
m
2 m x 25 m x 0.5
m
1 .5%
2.8 mxO.4 m
Stainless steel
12.7mJ
2.5 m x 4.8 m x
1.3m
20%
Combined
Electric
Underflow Flap
Gate
Not Available
No
No
No
Excellent/Good
rt
r
::
1 [
y
rcr
? .
1 1
cc
^
^U-^
i ii i i
.
1 ^
i i i i i i
1
= == T~ -
'.--.-: ::-:V-----v.-;...-:-.W
r
3-
_ซ
1
I/ 1
,-vJ
1
' ;. -..
ฐ'1\\\
^60U-
All Dimensions are in Meters
17.0
- 20 -
-H1.50
20
4k ^
46.80
Figure 6-8. Markt-Wiesentheid - CSO Storage Tank, Section View
16
-------
Case No. 11- Stuttgart-Wangen
Details of the Stuttgart-Wangen CSO storage facility are provided in Table 6-13. The tank consists of a
single rectangular bay 67m (220 ft) in length with rectangular cross section 3.6 m x 1.8 m (12 ft x 6 ft).
The flush gate is activated by external electric signal after the tank section has been noted by level sensor
to drain.
Table 6-13. Stuttgart-Wangen, Germany
Question
Year Constructed
Type of Flow
Covered or Open Tank
Number Of Bays
Flush Channel Dimensions
(Iwh)
End Trough Dimensions
I wd
Flush Channel Slope
Flush Gate Dimensions
(I w)/M ate rial
Flusher Volume
Flusher Dimensions
(Iwh)
Slope of flush vault
Water Source
Method of Gate Activation
Method of Removing
Flushed Sediments
Frequency of
Inspection/Crew Size
HVAC System/Type
Odor Control System/Type
Electrical System
Expolosion Proof
Performance Assessment
Response
1993
Combined
Covered
1
67 m x 3.6 m x
1.8m
6.5 m x 3.6 m x
0.9 m
0.5%
2.8 mxO.4 m
Stainless steel
18 mj
5 m x 3.6 m x
1.15m
14.5%
Combined
Electric
Pumps
Not Available
Yes/Ventilated
Manhole Lids
No
No
Excellent/Good
Case No. 12 - Heidenheim-Kleiner-Buhl
Details of the Heidenheim-Kleiner-Buhl CSO storage facility are provided in Table 6-14. The flush gates
are activated in sequence by external electric signal after the tank contents have been noted to drain.
Level in the receiving sewer is also noted prior to activating the flush gates
17
-------
Table 6-14. Heidenheim-Kleiner-Buhl, Germany
Question
Year Constructed
Type of Flow
Covered or Open Tank
Number Of Bays
Number Flushers/Bay
Training Walls/Flusher
Flush Channel Dimensions
(Iwh)
End Trough Dimensions
I wd
Flush Channel Slope
Flush Gate Dimensions
(I w)/M ate rial
Flusher Volume
Flusher Dimensions
(Iwh)
Slope of flush vault
Water Source
Method of Gate Activation
Method of Removing
Flushed Sediments
Frequency of
Inspection/Crew Size
HVAC System/Type
Odor Control System/Type
Electrical System Explosion
Proof
Performance Assessment
Response
1992
Combined
Covered
6
1
Yes
30 m x 4.85 m
3.6 m
5 m x 17.5 m
1.5m
x
x
1 .0%
1.85 mxO.4 m
Stainless steel
28.5 mj
5 m x 2.5 m
1.45m
x
15%
Combined
Electric
Underflow Sluice
Gate to WWTP
Not Available
Yes/Ventilation
Shafts
No
Yes
Excellent/Good
Case No. 13 - Cheboyan
Details of the circular CSO storage facility utilizing the flush gate technology in Cheboyan, Michigan are
noted in Table 6-15. This facility features a circular tank construction with a diameter of 30.5 m (100 ft)
Seven flushing bays direct flush waves across the length of the tank to a sump channel on one side of the
tank. This flushing configuration is the first ever for circular tanks. In Germany, there are 3 circular tanks
using flush gate technology but the flushing arrangement features flushing vaults located at the center of
the tank with flushing lanes extending in a radial direction to the end trough at the tank perimeter. Figure
6-9 depicts plan and section views of the CSO circular storage tank in Cheboyan, Michigan. Photos of
this facility are presented in Figure 6-10.
18
-------
Table 6-15. Cheboygan, Michigan
Question
Year Constructed
Type of Flow
Covered or Open Tank
Number Of Bays
Number Flushers/Bay
Training Walls/Flusher
Flush Channel Dimensions
(Iwh)
End Trough Dimensions
I wd
Flush Gate Dimensions
(I w)/M ate rial
Flush Channel Slope
Flusher Volume
Flusher Dimensions (I w h)
Slope of flush vault
Water Source
Method of Gate Activation
Method of Removing
Flushed Sediments
Frequency of
Inspection/Crew Size
HVAC System/Type
Odor Control System/Type
Electrical System Explosion
Proof
Performance Assessment
Response
1997
Combined
Open
7
1
Yes
See Figure 6-9
See Figure 6-9
2.8 mxO.4 m
Stainless steel
2.0%
13-16mJ
See Figure 6-9
20%
WWTP Effluent
Hydraulic
Underflow Pipe to
WWTP
Not Available
No
No
No
Excellent/Good
19
-------
Storage Volume
Adjustment Pipe
Flushwater
Storage Area
Flushwater
Supply Pipe
Training Wall
s Flushing Gate (typ.)
Plan View
Section "A"
Figure 6-9. Cheboygan - Circular CSO Storage Tank, Plan and Section Views
20
-------
Figure 6-10. Cheboygan - Photograhs of Circular CSO Storage Tank
21
-------
Case No. 14 - Sarnia
Details of the first North American CSO storage facility located in Sarnia, Ontario, Canada utilizing the
flush gate technology are given in Table 6-16. This facility is an underground covered tank with a storage
volume of 8400 m (2.2 million gallons). Twenty flushers cleanse the tank after each event. Plan and
sectional views of this facility are provided in Figure 6-11 and Figure 6-12. Photos of the hydraulic gate
opening mechanism are shown in Figure 6-13. The operation of this facility has been noted as excellent.
Table 6-16. Sarnia, Ontario Canada
Question
Year Constructed
Type of Flow
Covered or Open Tank
Number Of Bays
Number of Flushers/Bay
Training Walls/Flusher
Flush Channel Dimensions
(Iwh)
End Trough Dimensions
Iwd
Flush Gate Dimensions
(I w)/M ate rial
Flush Channel Slope
Flusher Volume
Flusher Dimensions
(Iwh)
Slope of flush vault
Water Source
Method of Gate Activation
Method of Removing
Flushed Sediments
Frequency of
Inspection/Crew Size
HVAC System/Type
Odor Control System/Type
Electrical System Explosion
Proof
Performance Assessment
Response
1996
Combined
Closed
10
2
Yes
38.5 m x 3.9 m
0.5 m
2 m x 79.7 m
0.7m
x
x
2.8 mxO.4 m
Stainless steel
2.0%
10 mj
2.5 m x 3.5 m
1.6m
x
20%
Combined
Hydraulic
Sluice Gate
WWTP
to
Not Available
Not Available
Not Available
Not Available
Excellent
22
-------
Influent Sewer
1650 mm Diameter
Location of float
ele. 175.75
r
15
LLJ
OL
O
I
w
w
z
O
w
z
LU
Figure 6-11. Sarnia - CSO Storage Tank, Plan View
23
-------
1550 1450
700
T
Secondary Concrete
Flushwater Storage
Area, VOL = 2750 l/m
308-
1750
Flushwater Storage
Adjustment Pipe
600
405
T
-2500-
200
- Flushing Gate
Gate Model HF-28
43000
-Training Wall
BENCHING
J
500
I500
2000-
ALL DIMENSIONS SHOWN ARE IN mm
Figure 6-12. Sarnia - CSO Storage Tank, Section View
24
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Figure 6-13. Sarnia - Photographs of Hydraulic Opening Mechanism
Hydraulic Analysis of Flushing Gates for Rectangular Tanks
Summary results of SWMM EXTRAN simulations of flush gate performance for five rectangular tank for
five rectangular tanks are presented in Table 6-17. Flushing volumes computed from the construction
drawings are used as inputs into rectangualer open channels (flushing lane). Velocities shown are
computed at the end section of the flush lane, just prior to discharge into the end channel. There is good
agreement between velocity and operator observations.
Table 6-17. Summary of Tank Flushing Results
Length Slope Width Height Velocity Depth Flush
(m) (m) (m) (m/s) (m) Vol.
(m3)
Observation
Filderstadt-Berhausen
Stadt Essen
Markt Wiesentheid
Stuttgart Wangen
Heidenheim Kleiner
Buhl
36
55
41
67
30
2.5%
0.5%
1 .5%
0.5%
1 .0%
5
3
4.84
3.6
4.85
3.5
3.4
3.0
1.8
3.6
1.33
0.92
1.30
0.83
1.61
0.05
0.07
0.06
0.06
0.06
8.5
12
12.7
18
28.5
Good
Good
Excellent / Good
Excellent / Good
Excellent / Good
Evaluation of Flushing Gates for Tanks
Flushing gates use a flushing water storage compartment separated from the actual CSO tank to produce
a flushing transient wave for cleaning the tank floor and to wash the settled matter into a mud sump of
adequate volume and suitable benching at the opposite tank side.
25
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The following design considerations are important for maximizing the effectiveness of flushing rectangular
tanks using flush gate technology:
The tank inlet should be designed such that flushing water reservoirs are filled before the
storm tank compartments. This ensures that the necessary supply of flushing water to clean
tanks from short storms of small volume. Filling the tank at the effluent side of the tank
requires additional conduits to fill the flushing water reservoirs located at the influent tank
side. Careful hydraulic considerations are necessary to assure adequate filling of all desired
storage units.
Depending on the tank filling concept, the inlet overflows should route the inflow sequentially
into the different tank compartments. Overflow weirs of different heights can control inflow to
individual compartments.
The tank floor should be horizontal across the direction of flushing, and sloped from 0.5 to
2%.
The flusher opening must be a minimum of 150 mm above the tank floor.
Larger tanks should be compartmentalized into bays with a width of 3 to 5 m. Training walls
should be sized to prevent the flush wave from spilling into adjacent bays.
The tank side walls should be perpendicular to tank floor with no fillets to ensure cleaning of
side walls.
The tank side walls should be hand troweled to a smooth finish. No sidewall spray systems
are necessary provided a smooth finish is achieved.
The "mud sump" volume depends on the flushing volume, and hence of the width of the
individual flushing bay as well as the possibility to flush the entire tank with multiple cells in
sequential mode. The mud sump generally has a central emptying pit with a sluice gate or
pump outlet. The bottom of the sump slopes gently towards this outlet sump, with a step
between the tank bottom and the beginning of the slope. This step is important in order to
contain the reflected wave in the mud sump.
An adjustable standpipe in the flushing water reservoir allows for "fine-tuning" of the flushing
volume in case the reflected wave throws water and debris back onto the tank floor.
A 5% lateral slope of the mud sump is frequently used. In case of large tanks and one outlet
this will result in fairly deep outlets.
The flush volume depends on the length, width and slope of the flushing bay, and is roughly
1m3/m width and per 10 m length at a slope of 0.5%. A slope of 1 % reduces this requirement
by 15%, and a 2% slope reduces the requirement by 25%. A maximum flushing length of
105m for normal tanks is suggested.
The flushing gate mechanism is released hydraulically, either by means of an autonomous
float mechanism, or by means of a more sophisticated control system that utilizes other
criteria such as interceptor and WWTP capacity, for flushing and emptying (Parente et al.,
1995).
26
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Case Studies: CSO Storage Tanks Utilizing Tipping Flusher Technology
The following tables summarize the evaluations of the tipping flusher rectangular tank facilities.
Case No. 15 - Port Colborne
Details of the Port Colborne, Ontario CSO storage facility utilizing tipping flushers are given in Table 6-18.
Plan and profile views of this facility are presented in Figure 6-11. Photos of this facility are shown in
Figure 6-12.
Table 6-18. Port Colborne, Ontario, Canada
Question
Year Constructed
Type of Flow
Covered or Open Tank
Number Of Bays
Flushers/Bay
Training Wall/Flusher
Flush Channel Dimensions
(Iwh)
Height Off Floor
Front or Rear Tip
Fillet Radius
End Trough Dimensions
I wd
Flush Channel Slope
Flush Volume
Water Source
Method of Removing
Flushed Sediments
Frequency of
Inspection/Crew Size
HVAC System/Type
Odor Control System/Type
Electrical System Explosion
Proof
Performance Assessment
Response
1996
Combined
Open
2
2
Yes
57 m x 7.2 m x
0.6 m
5.3 m
Rear
1.4m
3.4 m x 13.8 m x
0.65 m
1 .5%
8.2 mj
Fresh
Underflow Sluice
Gate
Not Available
No
No
No
Excellent
27
-------
"A"
800
'
6782
7200 1
+/- 12 mm
I L
"inn
7200
6782
+/- 1 2 mm 1
i.L
500
ty
\
s
*
ELE. 174.500
TIPPING FLU
_1
|
600
ELE. 174.500
800 -J L c of Unit
SHE
800
3410
i
/
/
ELE. 172.500 '
R ELE. 172.200 '
200
1
f
SLUICE GATE
762x762 Opening
INV. ELE. 171.850
ELE. 172.500 >
1400
SUMP
\
LXJ
PLAN VIEW
ALL DIMENSIONS SHOWN ARE IN mm
7200 ._ 7200
+/- 12 mm
L- 6782 J
600
800
n
+/- 12 mm
I- 6782 -I
800
-ELE. 181.000
-C_ELE. 179.782
- ELE. 177.500
-ELE. 174.500
SECTION "A"-"A"
Figure 6-14 (alb). Port Colborne - CSO Storage Tank, Plan and Section Views
"B"
28
-------
Figure 6-15. Port Colborne - CSO Storage Tank, Plan View
Case No. 16- Wheeler Avenue
Details of the Wheeler Avenue, Louisville, Kentucky CSO storage facility are provided in Table 6-19. Plan
view of the Wheeler Avenue facility is presented in Figure 6-16. Photos of the facility are provided in
Figure 6-17.
29
-------
Table 6-19. Wheeler Avenue, Louisville, Kentucky
Question
Year Constructed
Type of Flow
Covered or Open Tank
Number Of Bays
Number Flusher/Bay
Training Wall/Flusher
Flush Channel Dimensions
(Iwh)
Height Off Floor
Front or Rear Tip
Fillet Radius
End Trough Dimensions
Iwd
Flush Channel Slope
Flush Volume
Water Source
Method of Removing
Flushed Sediments
Frequency of
Inspection/Crew Size
HVAC System/Type
Odor Control System/Type
Electrical System Explosion
Proof
Performance Assessment
Response
1997
Combined
Open
1
4
Yes
38 mx4 m x 0.41
m
2.9m
Rear
1.2m
2.4 m x 19 m x
0.6 m
1 .6%
6mJ
Fresh
Underflow Sluice
Gate
Not Available
No
No
No
Excellent
30
-------
- PAVED AREA
7
DRIVE SLOPE
All Dimensions are in Meters
SLOPE 1.6%
SLOPE 1.6%
4m
Unit
4m
Unit
4m
Unit
4m
Unit
TIPPING FLUSHER
Figure 6-16. Wheeler Avenue - CSO Storage Tank, Plan View
31
-------
Figure 6-17a. Wheeler Avenue - Photographs of Overall CSO Storage Tank
32
-------
Figure 6-17b. Wheeler Avenue - Photographs of Filling Tipping Flushers and Overall CSO Storage
Tank
Case No. 17- 14th Street Pumping Station
The 14th Street Pumping Station in Saginaw Michigan functions as a regulation facility to pass dry weather
flow to the East Side Tunnel leading to the WWTP, and as a pump station with 3 pumps, each with 2800
Ips (100 cfs) capacity, to dewater drainage from four low-level trunk sewers discharging from the 14th
Street district 80 hectares (200 acres). Land is mostly heavy industrial car manufacturing together with
scattered residences. Pumped overflows occur about 12 times per year. The fifth main sewer to the
station drains the 16th Street district with an 340 hectares (850 acre) catchment. The 16th Street district is
long and narrow in shape, flat, and discharges into a 14-foot arch pipe with little slope. Solids deposition is
considerable. Excess wet weather flows discharge over backwater gates into box culverts (collecting the
pumped discharge from the station), and then proceeds through a 762 m (2500 ft) open ditch to the
Saginaw River. Overflows occur 30-40 times per year. A wet weather connection from the 16th Street
sewer into the station's wet well is used only in emergency. The total hydraulic flow capacity tributary to the
14th Street Pumping Station from both districts is about 13,000 Ips (460 cfs).
33
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The layout of the 14th Street Pumping Station treatment complex during wet weather operation is shown
in Figures 6-18. It includes the following features:
22,000 m3 (6.0 million gallons) of various storage elements consisting of in-line, first flush
off-line, detention sedimentation treatment storage, underflow storage from vortex separators,
and ditch storage overflow with pump back.
Outflows from all storage facilities are equipped with vortex throttles to permit continuous
gravity drainage to WWTP.
All concrete tanks use tipping bucket flushers for ease in cleanup.
Vortex solids separators precede conventional sedimentation tanks to ease solids cleanup.
Initially, first flush related flow from both the 16th Street and 14th Street districts are captured by in-system
storage (2850 m3, 0.75 million gallons) and then by tanks B1 and B2 (8550 m3, 1.5 million gallons), Both
tanks have vortex throttles to permit continuous drainage to the WWTP. This is the first in the United
States of this type of CSO storage.
When Tanks B1 and B2 are filled, pumped discharge occurs with initial processing by three 11 m (36 ft)
diameter reinforced concrete vortex solids separators each with a maximum capacity of 2830 l/s (100 cfs).
Pumped underflow from the separators passes to Tank B3 (throttled outflow to WWTP). Treated overflow
from the vortex separators discharges in a channel over the top of existing box culverts and into a new
7600 m3 (2 million gallons) detention and storage facility (Tank B4) with chlorination. Outflow from the
separators can also bypass Tank B4 and discharge directly into box culverts draining to the Saginaw
River.
Tank B4 will provide 45 minutes of detention with "one pump on" (common event) and will provide further
treatment of "clear water" overflow from the vortex solids separator. The overflow from Tank B4
discharges into the box culverts draining to the ditch. New backwater gate structures were installed near
the River and the ditch rehabbed (stone lined and reverse graded) to create 9500 cubic meters (2.5 million
gallons) of inexpensive storage. Retained ditch storage is drained after an event to a new drop shaft
discharging to the East Side Tunnel.
Long term simulations indicate that the "continuous drain" operation for Tanks B1 and B2 permit capture
with bleedback to WWTP without disinfection of half the annual runoff volume. Although the 1 year 1 -hour
storm is the most severe design condition, the more likely events of concern are spring multiple day
intermittent rainfall events. The vortex tank throttles permits optimized overall utilization of Tank B1 and B2
and the complex of vortex separators with Tank B4 treatment.
The sum of all tank discharges (vortex throttles) plus the two dry weather underflow vortex throttles had to
equal existing wet weather discharge from this catchment. This new requirement was imposed by
regulating agencies since the City was planning to accept dry weather flows from nearby communities
deactivating small WWTPs.
Additional estimated cost savings associated with the sedimentation tank tipping bucket flushers in
contrast to conventional high pressure spray systems are significant (approximately 33 percent) while the
requirement for flushing water was decreased by 25 percent (Pisano, 1985).
A total of 23 tipping flushers are used to clean tanks B1 (4 tipping flushers), B2 (2 tipping flushers), B3 (6
tipping flushers), and B4 (10 tipping flushers). The performance of this cleansing system has been noted
by the maintenance staff to be very satisfactory. Only one flush per lane is required. Figure 6-20 depicts
a photo of tanks B1 through B3. Figure 6-22 depicts a photo of tank B4 after overflow activation. Since
the facility is very close to the WWTP, draindown of all tanks is typically accomplished in 6-12 hours.
Photos depicting a cleaning sequence for Tank B4 are depicted in Figures 6-23 (a to d).
34
-------
jth
Figure 6-18. 14 Street Pumping Station - Tanks B1, B2 and B3
jth
Figure 6-19.14 Street Pumping Station - Tank B4
-------
jth
Figure 6-20a. 14 Street Pumping Station - Flushing and Sequence for Tank B4
jth
Figure 6-20b. 14 Street Pumping Station - Flushing and Sequence for Tank B4
36
-------
jth
Figure 6-20c. 14 Street Pumping Station - Flushing and Sequence for Tank B4
Case No. 18- Saginaw Township Center Road Storage Tank
The first operational tipping flusher sedimentation tank installation within the United States, the Center
Road Storage Tank, is located in Saginaw Township, Michigan. The tank was constructed in 1989 and
handles overflows from a catchment of 514 hectares (1270 acres) with a design peak flow of 5943 Ips
(210cfs).
The tank has no odor control provisions, and contains multiple bays with a total volume of 19000 m3 (5
million gallons). In the first third of the tank which is utilized as first-flush storage, three tipping flushers are
used to clean heavy sediments and sand. Water cannons and fire hoses are used to clean the remainder
of the tank.
The tipping flushers cleanse a length of approximately 55 m (180 ft). Generally the performance of the
tipping flushers has been satisfactory. However, two flushes are necessary after major storms as
sediments contain some inorganic material. Flush waves have been observed to break up into rivulets
near the end of the flushing lane.
Evaluation of Tipping Flushers
The following design considerations are important for maximizing the effectiveness of flushing rectangular
tanks using tipper flusher technology:
The size of a tipping flusher (TF) required to clean a given tank depends on the flushing distance, the
height the unit is suspended above the tank invert, and the slope of tank floor in the flushing direction.
The TF's vary in size from 0.02 - 2 m3/ m (12 to 160 gal/foot) of flusher length, with a maximum
length/span of approximately 11 m (35 feet).
The maximum effective flushing length is between 160-175 ft because the wave cannot be sustained
and tends to break up into rivulets beyond this distance. Experience with flushing in the 55 to 60
37
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meters (180 to 200 ft) range is mixed, and it has been observed that often 2 flushes are necessary to
achieve adequate cleaning.
Tank floors should slope from the flusher location to the collection trough at 1 - 3 % (2 % is desirable
because quality control is difficult for flatter slopes).
Tipping flushers require flushing lanes that are about equal to the width of the flusher to control the
flow direction of the flush wave. This is accomplished by training walls that are about 0.4 to 0.6 m (15
to 18 inches) high, and run the full length of the tank terminating at the collection trough.
All walls parallel to the path of flushing flow should be perpendicular to the tank bottom, with no fillets,
to ensure the lower wall edges are cleaned.
There is an upper effective height limit for placement of the tipping flusher where additional height
does not enhance the flusher's performance further, (approximately 5.5 m, or 18 ft).
The wall beneath the flushers must be continuous (i.e., contain no openings, penetrations or
obstructions), to assure wave continuity.
Side walls should be hand troweled to ensure smooth surface. No sidewall spray systems are
necessary.
Tanks with shallow sidewater depth of sidewater depth of long length are a concern using tipping
flusher technology as the gate will tip near floor level.
Other Flushing Case Studies
Other researchers (Parente et. al., 1995) have collected survey data for several European CSO
rectangular tank facilities using flush gates. The following are brief summaries of the findings.
City of Essen, Germany
The City of Essen has two CSO tanks utilizing flushing gates. One facility, in operation since 1991, has a
volume of 1500 m3 (395,000 gallons). The tank is divided into 3 bays with each bay approximately 5 m (16
ft) wide and 34 m (112 ft) long. The cleaning effectiveness was reported as very good without a need for
any manual cleaning.
The second facility, in operation since 1993, has a volume of 2700 m3 (710,000 gallons). The tank is
divided into 6 bays with each bay approximately 2.5 m (8 ft) wide and 46 meters (151 ft) long. The
cleaning effectiveness was reported as very good with a need to clean walls once a year due to the
heavily polluted corrosive wastewater handled by the tank.
City of Konstanz, Germany
The City of Konstanz has a CSO tank utilizing flushing gates. The facility, in operation since 1992, hs a
volume of 2000 m3. The tank is divided into 3 bays with each bay approximately 7 m (23 ft) wide and 18
m (59 ft) long. The cleaning effectiveness was reported as generally good. Konstanz is located on a
lakeshore with minimal slopes and heavy accumulation of sediment inside the sewer system during dry
weather. During rain events, excess water is pumped to storm overflow tanks, with heavy first-flush
concentration. The filling of the tank is from flushing water reservoir side, with heavy matter spilling into
the flushing vaults. These materials clutters the flusher gate and mechanism, which occasionally fails to
close properly because of debris and clogging. His preference was to use pre-clarified water for the
flushing reservoirs to reduce potential for clogging the gate. The tank is drained by gravity through a
regulator back to the pump station. The tank control system is programmable, allowing for adjustments in
accordance with operation experience.
City of Augsburg, Germany
The City of Augsburg has a covered CSO tank utilizing flushing gates. The facility, in operation since
1994, has a volume of 4900 m3 (1.3 million gallons). The tank is divided into 9 bays with each bay
approximately 5 m (16 ft) wide and 50 m (164 ft) long. The cleaning effectiveness was reported as very
good without a need for any manual cleaning. The flushing reservoirs are filled first and no problems with
solids accumulating/clogging the gate have been reported. Some sediments deposit on wall crowns and
weir sills and require occasional hosing down were reported. The tank floor was clean, with no deposition
38
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in the corners. In order to prevent clogging of the sludge pumps when emptying the mud sump, the owner
installed mixing equipment to keep heavy sludge concentrations and debris in suspension during
operation. The City has found stainless steel conduits to be more dependable then copper for the
hydraulic flushing gate actuators.
City of Elizabeth, New Jersey - Mechanical Flap Gate Flusher
Flushing of large diameter combined sewers was investigated during the City of Elizabeth Combined
Sewer Pollution Abatement Program completed in 1986. The program concluded that daily flushing of
sewers particulary prone to solids deposition in seven subsections of the City by constructing 12 flushing
modules would reduce the first flush overflow pollution by approximately 28%.
A total of 12 flushing modules were installed in sewers ranging in size from 0.45 to 1.4 m (18 to 54 inches)
with slopes as low as 0.02% and were designed to produce sewer flushing in pipe runs up to 305 m
(1000ft) long. Figure 6-22 presents plan and section views for a typical Elizabeth flushing system.
The flushing modules are underground structures with above ground control cabinets. Each module has a
dry and a wet chamber. The wet chamber is constructed around the existing sewer and houses a
hydraulically operated flap gate mounted on a shaft across the sewer supported on two bearings. The flap
gate is sized to match the dimension of the sewer in which it is installed. The flap gate is normally open
but closes at a controlled time (usually at night, during periods of low flow). The gate is designed to
automatically open when the stored sewage volume rises to a preset level needed to create a flushing
wave adequate to resuspend and transport the deposited sewage solids to the interceptor at a flow rate
which will not cause overflow at the downstream regulators. The wet chamber also contains level
switches and ultrasonic level sensors to provide signals for opening and closing of the gate. The dry
chamber is adjacent to the wet chamber and houses the mechanical and electrical equipment such as the
flap gate actuator, the hydraulic power unit as well as auxiliary equipment. A number of facilities are still in
operation. No performance evaluations of these facilities have been conducted. This facility is unique in
the United States and another such facility has yet to be installed.
Case Studies: Cost Effectiveness Studies of CSO Tank Cleaning Methods
The following two case studies present cost effectiveness evaluations that were prepared for CSO storage
tanks in North America. The Eastern Beaches study compares cost effectiveness of tipping flushers with
other methods. The Sarnia study compares cost effectiveness of flushing gates and tipping flushers with
other methods.
Case Study: Eastern Beaches, Toronto, Ontario, Canada
Environment Canada in conjunction with the International Joint Commission has designated the City of
Toronto Waterfront as one of the 42 Remedial Action Plan (RAP) areas within the Great Lakes drainage
basin. The City of Toronto has been carrying out an extensive program of pollution abatement over the
past 30 years to meet the RAP objectives. The City has prepared a pollution abatement program for CSO
and stormwater runoff and has developed a Sewer System Master Plan for the combined and storm
sewers. As a component of the Master Plan, the Eastern Beaches, located within the City of Toronto, were
determined to have first priority in pollution abatement to ensure that the beaches remain open for body
contact water recreational purposes. Environmental Assessment studies analyzed various pollution
abatement strategies and determined that detention tanks would be the most feasible alternative for
controlling pollution originating from CSO and stormwater runoff entering the nearshore waters along the
Eastern Beaches (Parente et. al., 1994).
The first of two tanks, located at Kennilworth Avenue, was put into service in July 1990 to intercept 1 CSO
and 5 storm sewer outfalls. High volume spray nozzle cleaning system was used to cleanse sediments
from the tank after activations. This tank has been in operation for the past 4 years and has been
monitored with respect to pollution abatement efficiency, beach closure impacts and
operation/maintenance impacts. Monitored data from this tank indicated that the seasonal pollutant
removal from the beach area has averaged approximately 2,800 kilograms (6160 pounds) of sediments
and 130 kilograms (286 pounds) of BOD. The resultant beach closures has been reduced from 35% prior
to tank installation down to 4% of the swimming season.
39
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Based on the encouraging monitoring results from the first installation, the City has proceeded with the
installation of a second tank at Maclean Avenue to remove all other CSO within the immediate area. This
second tank, approximately 3.5 times larger than the first tank, has a storage volume of 8,000 m3 (2.1
million gallons) and is located approximately 1 kilometer (0.62 miles) east of the first tank.
Design criteria for the Maclean Avenue detention tank included the following:
Provide 8,000 m3 ( 2.1 million gallons) of storage (4,000 m3 for CSO and 4000 m3 for
stormwater runoff) to eliminate untreated CSO and stormwater discharge to beach area;
Operation and maintenance concerns to address the most efficient cleaning system, water
conservation considerations, minimal entry requirements, and control of possible odors.
Due to the site constraints, the detention tank dimensions were finalized having a length of
100 m (330 ft), a width of 15 m (49 ft) and average depth of 6 m (20 ft).
Operation and Maintenance
During the design phase of the Maclean Avenue tank, discussions were held with the operating staff
regarding their experience in operating the first tank for the past 4 years and also their experience in
operating a combined sewage balancing tank further up the system which was constructed in 1914. In
addition to this transfer of operating experience, contact was made with several other municipalities
designing or operating combined sewage tanks to obtain their experience. The main
comments/requirements from the various sources identified the following operating features related to
tank flushing and odor control:
Efficient method of cleaning is essential to minimize manned entry requirements and to
ensure deposition of sediments does not occur;
An odor control system is essential to prevent public complaints during tank operation.
During the process of detaining the flows, settlement of suspended solids will occur. To minimize and
facilitate removal of the settled solids, the detention tank will be cleaned after every usage. Three methods
for removal of the sediments were investigated: tipping flushers, flushing spray, and manual cleaning.
Flushing spray and manual cleaning systems are described below.
Flushing Spray
A spray flushing system utilizes spray nozzles oriented in such a manner that the spray from the nozzles
covers the entire floor area and the floor has sufficient grade to ensure effective sediment transport.
These nozzles generally require water at a relatively high pressure and large volumes to provide the
scouring of the sediments. The tank floor requires a longitudinal slope of 2% and lateral slope of 10%. The
floor is graded to divert the wash water to a sump where it is pumped to the sanitary sewer.
The spray nozzles are selected to provide sufficient wash water volume and pressure that the settled
sludge is re-suspended. From experience at the Kennilworth detention tank, selected nozzles with a
capacity of 3.85 Ips (50.8 gpm) at a pressure of 415 kPa (60 psi) are required, Because of the high flow
and pressure demands, cleaning of the first tank is carried out in sections.
Manual Cleaning
This method requires manned entry to the tank during the cleaning process and the use of hoses to flush
the tank floor. Manual cleaning ensures a high level of effectiveness because the operators can be flexible
with the usage of time and water based on the level of sediments to be removed. This method is the most
labor intensive and most hazardous due to the requirement of working in a confined space. The
equipment cost of this alternative is minimal, but the operational cost is higher due to the longer time the
maintenance crews have to remain on site. This does not include cost of mobile equipment such as
hoses, safety equipment, etc. The volume of flushing water required will vary due to personnel work
40
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practices, but is anticipated to be equal to or greater than that required by the flushing spray method. This
method is being applied for cleaning the High Park tank constructed in 1914.
The cost comparison of the three cleaning systems is shown in Table 6-20.
Table 6-20. Capital and Operation and Maintenance Cost Comparison
Type of Cleaning System Storage Volume Floor Area Unit Construction UnitO&M
(m3) (m2) Cost Cost
($/m2) ($/m2)
Tipping Flusher 8000 1360 141 0.075
(Maclean Avenue)
Flushing Spray 2250 660 290 0.30
(Kennilworth Avenue)
Manual Cleaning 2400 790 2 1.77
(High Park)
*Excludes water and electrical costs.
In evaluating these tank flushing alternatives and evaluating experiences with all three practices, it was
determined that the tipping flusher alternative was the most effective and economical. The flushing system
for each compartment consists of a TF at each end of the tank compartment with the tank floor sloped at
a grade of 2% to a central channel designed to intercept the flush wave and its re-suspended sediments.
To incorporate water conservation practices in this design and since the flush water supply is not required
to be at a high operating pressure, a small pump has been installed to pump lake water through the outfall
force main and into the TF. A valve system has been installed such that the force main may be operated
both as a discharge main and also as an intake main.
Odor Control
The odor control for the venting system of the second tank is similar to the first tank. The venting system
is designed to convey the displaced air during the tank filling process by means of an underground pipe
system to the surface control structure where the air is passed through an air filter system consisting of
activated carbon. The treated air is then expelled to the atmosphere approximately 6 m (20 ft) above
ground surface. A similar system has been in operation at the first tank with the filter media requiring
replacement after 3 years of operation.
The housing for the filter system is located approximately 130 m (426 ft) west of the tank. The structure
also houses the main control center for all the mechanical equipment and monitoring system.
Case Study: Sarnia Ontario
The pollutant loadings from combined sewer overflows (CSO's) and storm water outfall discharges to the
Sarnia waterfront and the St. Clair River have resulted in beach postings, reduced waterfront recreational
activities and degradation of aquatic habitat. The City of Sarnia carried out a Pollution Control Planning
(PCP) Study to develop a master plan in reducing the pollution loadings to the Sarnia waterfront. One of
the recommendations of the PCP study is to construct four CSO tanks that would intercept and retain
CSO during storm events. The first of the recommended CSO tanks is located at the Devine Street outfall,
which has a contributory area equivalent to 40 percent of the combined sewer area within the City of
Sarnia. The required size of the tank was determined to be 10,700 m3 (2.8 million gallons) such that the
tank would reduce CSO discharge events to the St. Clair River to between 3 to 5 events per year (Parente
et.al., 1995).
The purpose of this detention tank (see Case No. 14, Table 6-16 for evaluation data of this facility) is to
detain the overflow volume until the conveyance capacity and/or treatment capacity is available. During
the operation of the facility, settlement of suspended solids will occur along the bottom of the tank. It is
therefore necessary to remove the settled solids after each event to eliminate caking, the accumulation of
these solids, and to avoid the formation of gases and odors as a result of decaying organics.
41
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The design of the tank was to incorporate the most efficient and reliable cleaning method such that
operation and maintenance would not impose high demands during tank operations. The detention tank is
situated within an existing residential park with an available effective hydraulic depth (outlet elevation to
overflow elevation) of 4.3 m (14 ft). Based on the existing site restrictions and storage volume
requirements this tank requires a cleaning system to effectively clean a floor with a surface area of 3,440
m2 (37,000 ft2).
Cleaning Alternatives
The following four cleaning methods were identified for the Devine Street CSO tank:
Manual Cleaning
Flushing Spray
Tipping Flusher, and
Flushing Gate.
The estimated capital, operation and maintenance costs for the four flushing alternatives for the tank are
provided in Table 6-21.
Table 6-21. Alternatives Capital and Operation and Maintenance Cost Assessment
Alternative
Manual
Flushing Spray
Tipping Flusher
Flushing Gate
Capital Cost
$10,000
$680,000
$525,000
350,000
Capital Cost/rn^
$2.91
$197.67
$152.62
$101.74
O & M Cost/Event
$6600
$1548
$378
$250
O & M Cost/nf
$1.92
$0.45
$0.11
$0.07
Operation and maintenance costs include labor, cost of potable water for cleaning at $0.40/m3, and cost of
sewage treatment for external flushwater at $0.45/m3.
Due to water conservation and the lower capital cost, the flushing gate system using detained sewage was
selected to be the most cost effective alternative. This system requires less mechanical equipment such
as valves and supply lines that are necessary for an external water source. The operation and
maintenance costs tend to be marginally lower since no additional costs are incurred for the supply of
potable water for flushing purposes and the associated treatment costs for the flush water.
42
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Chapter 7
Desktop Analysis
This chapter describes three case studies aimed at assessing the cost-effectiveness of sewer
flushing technology from different performance perspectives These performance perspectives
are minimization of maintenance costs, reduction of sediments CSO first flush, and reduction of
sediments to lower hydrogen sulfide levels. The first case study utilizes information developed
for Fresh Pond Parkway Sewer Separation and Surface Enhancement Project in Cambridge, MA.
A cost analysis was performed in this case study to compare flushing technologies versus
conventional sewer cleaning methods. The second case study uses the desktop procedure
described in Chapter 3 to investigate the pollution control effectiveness for a typical Northeast
combined sewer catchment. The number of combined sewer overflows was determined using
long term flow measurements. A cost analysis was performed to investigate present worth costs
of satellite treatment versus flushing technology. The last case study investigated the cost
effectiveness of sewer flushing versus chemical addition for hydrogen sulfide control.
Case Study One: Fresh Pond Parkway Sewer Separation and Surface
Enhancement Project Storm and Sanitary Sewer Flushing
In Chapter 5, different methods of manual cleaning are presented which are costly and
maintenance intensive. In this case study, the cost effectiveness of sewer flushing, utilizing
flushing gates, versus periodic manual cleaning and sediment removal is investigated.
Over the last twenty years, the City of Cambridge has aggressively separated old combined and
over and under sewerage systems throughout the City to enhance drainage service and to
improve the water quality in the Alewife Brook and the Charles River. Presently, the City is in the
construction phase of separating the CAM 004 area (25 hectares, 250 acres,) catchment. This
area is north and west of Harvard Square and within dense heavily traveled urban regions.
Grit deposition within both sewerage and storm drainage systems is a major problem because of
general flatness of the area, presence of several shallow streams that the sewerage (storm and
sanitary) systems must cross under as siphons, and the hydraulic level of the receiving water
body that frequently backwaters the storm systems. To overcome this problem in the CAM 004
area, automated flushing systems using quick opening (hydraulic operated) flushing gates to
discharge collected stormwater will flush grit and debris to downstream collector grit pits.
Description of Piping Systems to be Flushed
The storm and sanitary sewer systems to be flushed are located within the CAM 004 catchment
area. These systems start on the Fresh Pond Parkway near the Cambridge water treatment plant,
continue east to the Concord Circle and then northeast to the Fresh Pond Circle. Both systems
then proceed down Wheeler Street under the MBTA/Conrail railroad tracks and terminate near
the Alewife Parking Garage. The piping systems consist of approximately 1400 m (4,666 ft) of
sanitary trunk sewers, ranging from 460 mm to 1.2 m (18 inch to 48 inches), and approximately
1620 m (5,400 ft) of existing storm drains with pipe sizes ranging from 600 mm (24 inches) to
1.2m by 1.8 m (4 ft by 6 ft). There is a major overflow into the Alewife Brook from the sanitary
sewer system just beyond the Alewife Parking Garage.
Two construction contracts have been prepared for the overall sewer separation and surface
enhancement project along the Fresh Pond Parkway between Huron Avenue and Fresh Pond
Circle. Flushing systems are included in one of the two construction contracts. Construction
startup is expected in September 1998. Figure 7-1 depicts the general locations of the flushing
vaults.
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Fresh Pond Pkwy.
1m x 1.1m CS
DRAIN VAULT 2
Figure 7-1. Fresh Pond Parkway - Locations of Flushing Vaults
Description of Flushing Vaults
Another alternative is to retain pipes with flat slopes, but provide periodic cleaning of these pipes
by automatic passive means to maintain hydraulic capacities. The use of flushing chambers at
specific locations, with grit collection chambers downstream were designed for the Fresh Pond
Project. The design utilized quick opening flushing gates (hydraulically driven) that release stored
water to create a "dambreak" flush wave to cleanse and move sediments downstream to a grit pit.
Several hundred such installations have been implemented in Western Europe since 1985.
Figure 7-2 shows a typical storm sewer flushing chamber with quick opening gate designed for
the City of Cambridge. During a rainfall event, stormwater from the incoming storm drain fills the
sump adjacent to the flush chamber. Submersible pumps then pump stormwater from the sump
into the flush chamber. Each flush chamber volume was sized based on the roughness, slope,
size and length of the pipe being flushed. The "flush wave" is designed to have a depth of
approximately 100 to 150 millimeters (4 to 6 inches) and a velocity range between 0.9 to 1.2
meters/second (3 to 4-feet/second) at the end of the pipe segment being flushed. Once filled, the
pumps shut off and a timer is initiated that automatically initiates the flushing sequence 24 hours
after the rain event.
Process water (back wash) from the new Cambridge water treatment plant will be pumped to the
new sanitary system and collected in sanitary sewer flushing vaults for periodic flushing of the
sanitary sewers.
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LEVEL PROBE -
(TYP. OF 2)
FLUSHING GATE
FLUSHWATER STORAGE AREA
\
fur)
45ฐ BEND, DOWN
4" PVC-
FLUSHWATER
MANHOLE ABOVE
(2 TYP.)
DRAIN VAULT NO. 1-PLAN
GRADE
WEIR OPENING
FLUSHING GATE
FILLET (BEYOND)
1m DRAIN
SECTION
Figure 7-2. Fresh Pond Parkway - Flushing Gate Chamber
This approach is intended to minimize the daily operation of the system and provide the flexibility
of cleaning the pipes on demand. It would be cost-effective due to reduced initial capital costs
and minimal long term operational and maintenance costs versus a typical pumping station that
requires daily maintenance and power.
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Life Cycle Cost Comparison: Automated Flushing versus Periodic Manual
Removal
This preliminary analysis presents life cycle costs for two alternative systems to clean the major
storm and sanitary systems described above over a thirty-year period. Catch basin cleaning and
cleaning of all incidental lateral lines tributary to both systems were not included. The cost of each
alternative system does not include estimates of materials to be removed and disposed.
Notwithstanding this limitation, all costs necessary to remove deposits to street level using either
scheme are included.
Details of the Automated Flushing Systems
The automated flushing scheme consists of the following three separate flushing and grit capture
systems:
Near the new water treatment plant on Fresh Pond Parkway to the Concord Circle
(sanitary and storm) including costs of collector manholes for the storm lines. Daily
flushing of the sanitary system and periodic flushing of the storm system (assumed
every two weeks; flush vaults are filled using captured stormwater);
Concord (Sozio) Circle to the Fresh Pond Circle (sanitary and storm) including the
cost of collector manholes for the storm system. Daily flushing of sanitary system and
periodic flushing of storm system (assumed every two weeks; flush vaults are filled
with pumped captured stormwater);
Fresh Pond Circle to the south side of the B&M railroad crossing at the site of a new
grit and sand collector area. The storm drain and both sanitary trunk sewers will be
flushed. Twice weekly flushing of sanitary systems and periodic flushing of storm
system (assumed every two weeks); flush vaults are filled with pumped captured
stormwater);
The capital costs of the flushing systems include the flushing vaults, the grit capture chambers
(storm only), small above ground vaults to house the hydraulic power pack units to trigger the
flushing systems, and chambers as appropriate to pump storm water into the flushing chambers.
External flush water (stormwater runoff) will be used to flush the sanitary systems from the Fresh
Pond Circle to the Alewife rotary garage. The additional cost of sewage treatment of added
flushwaters was included for the two sanitary sewer chambers at Fresh Pond Circle.
Approximately 757,060 liters (200,000 gallons) are needed for flushing on an annual basis. No
such costs are included for the storm system, as collected storm water will be used to flush the
storm lines. Incidental costs of pumping storm water to flushing vaults are included. It is assumed
that on a quarterly basis all vaults will be cleaned of collected materials. Trucking and disposal
costs are not included. Police detail costs are also not included. Pertinent summary details of the
flushing systems are given in Table 7-1.
Table 7-1. Flushing System Summary
Location
Drain Vault #1
Drain Vault #2
Drain Vault #3
Drain Vault #4
Drain Vault #5
Sanitary Vault #1
Pipe Service
Drain
Drain
Drain
Drain
Drain
Sanitary
Pipe Diameter
(meters)
0.91, 1.06, 1.37
1.06
1.37
1.22, 1.22 by 1.83
1.83
0.4
Flushing
Segment
Length
(meters)
390
235
240
350
443
215
Flushing
Volume (liters)
12,083
9,725
10,917
12,655
39,640
5,845
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Sanitary Vault #2 Sanitary 0.6 350 6,363
Sanitary Vault #3 Sanitary 0.4 426 9,202
Sanitary Vault #4 Sanitary 1.22 426 24,059
Table 7-2 summarizes the life cycle costs for sanitary and storm sewer flushing vaults for the
locations noted above. Present worth costs per gallon flushing volume averaged about $44 per
liter ($165 per gallon).
Details of the Manual Cleaning System
It is assumed that the sanitary systems will be cleaned on a three year cycle and the storm lines
cleaned on a five-year cycle. Existing sediment levels (about one-third of pipe depths) can
reoccur in a five year period (estimated).
Unit cleaning costs were obtained from contractor bids for the cleaning construction package of
the storm and sanitary sewers within the project area as follows:
914 mm (36 inch) Storm Drain -$75.00/meter ($25.00/foot)
1067 mm (42 inch) Storm Drain -$102.00/meter ($34.00/foot)
1219 mm (48inch) Storm Drain -$129.00/meter ($43.00/foot)
1372 mm (54 inch) Storm Drain -$163.50/meter ($54.50/foot)
1829 mm (72inch) Storm Drain- $267.00/meter ($89.00/foot)
1.22 m x 1.83 m (4'x 6') Storm Drain -$232.50/meter ($77.50/foot)
457 mm (18inch) Sanitary -$19.50/meter ($6.50/foot)
610 mm (24") Sanitary -$21.00/meter ($7.00/foot)
1219 mm (48") Sanitary -$60.00/meter ($20.00/foot)
No trucking and disposal costs are assumed. On a life cycle basis, the automated flushing
scheme is about $400,000 cheaper. The reader must also be aware that the avoidance of
potential real and societal costs of flooding caused by surcharged and clogged drains and sewers
is not reflected in this cost estimate. In addition, the nuisance level costs associated with traffic
disruption on Fresh Pond Parkway (4 lanes with 50,000 vehicles per day) are also not reflected.
An independent estimate of traffic disruption places a present value of $3 million. Last, the
analysis does not take into account the fact that even after separation is completed, overflows
can occur at the end of the sanitary system which is 2 kilometer (1.2 miles) downstream. Periodic
flushing of the sanitary sewer trunk lines will minimize the amount of scoured and suspended
solids discharged from this overflow into Alewife Brook during major wet periods.
Table No. 7- 2. Cost Effectiveness Analysis Flushing versus Manual Cleaning
Manual Pipe Cleaning Present Worth Flushing Chamber Sites Present Worth Cost
Cost ($M) ($M)
Sanitary Sewer Cleaning 0.9 Fresh Pond Circle Site 1.1
Storm Drain Cleaning 3.4 Concord Circle Site 1.5
Water Treatment Plant Site 1.3
Total 4.3 Total 3.9
Notes:
1. Pipe cleaning costs assume inflation rate of 3.12% per year.
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2. Stormwater pipes are cleaned every 5 years, and sanitary pipes are cleaned every 3 years.
3. Flushing costs are based on inflation rate of 3.12% per year and discount rate of 7.1% per
year.
4. Term = 30 years
5. Flushing costs for the sanitary systems include payment to the MWRA for all external applied
flushing water. The annual flush waters for the sanitary systems = 757,060 liters (200,000
gallons). Current cost factor of $5.68/3,785 liters (1000 gallons) used.
6. Maintenance labor cost = $60/hour.
7. Sanitary systems to Fresh Pond Circle assumed to be flushed.
8. Storm systems will be flushed approximately every two weeks depending on rainfall.
9. Capital costs for flushing sites include excavation and backfill, hauling, pavement, gravel,
dewatering, hazardous soil disposal, piping, traffic maintenance, equipment, structures and
mobilization.
10. Operation and maintenance costs for flushing sites include hydraulic oil, routine inspection
and servicing, power, and removal of collected sediments. Trucking and disposal costs are
not included.
Case Study Two: Cost Effectiveness of Sewer Flushing versus
Conventional Treatment
Over the last two decades, numerous investigators have noted that routine flushing of flat sewers
on a continual (i.e., one to three day interval) basis could decrease the amount of solids available
for scour, resuspension and transport to overflows. Until recently, flushing equipment has not
been available to accomplish this idea in practice.
This case study investigates the cost effectiveness of utilizing the flush gate technology to flush
on a routine basis sewer deposits within a large flat sewer to minimize "first flush" at a
downstream overflow. The alternative conventional approach would be to use a satellite
treatment facility such as a retention or detention treatment tank or vortex separator technology.
The basic idea is to ascertain whether flushing on a routine basis can be a cost-effective adjunct
to other treatment schemes or even viewed as a stand-alone control.
The case study is developed from actual data in a Northeast community. The first step in the
investigation is to compute solids loadings in the overflow over the course of a year. Life cycle
costs to handle these loadings using satellite CSO treatment are next computed. Next, flush gate
technology is used to flush on a routine basis the same flat stretch of sewer, thereby reducing the
amount of available solids that would be scoured and carried out the downstream overflow during
high flow events. Life cycle costs are computed for this alternative "preventative" scheme and
compared with the costs involved with satellite treatment.
Description of Area
The sewer catchment covers an area of 1600 hectares (4000 acres). The land use is mixed with
a portion of heavy industrial and food processing establishments. The sewers are old, separated
and carry heavy inorganic and organic settleables solids loadings. Inorganic loadings generally
inflow from cracks in sewer and manholes. Organic loadings derive from food processing wastes,
average about 15 mg/l, and are generally large rapidly settling particles.
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Flow monitoring data collected at the end of the catchment prior to entry into a 1.8 m (72 inch)
line were used as input to the desktop procedure. A summary of the input flow data is presented
below Statistics of the average daily velocity, average, maximum hourly velocity, average daily
and average peak hourly shear stress levels within the 1.8 meter (72 inch) sewer are presented in
Table 7-3.
Overflow at the downstream regulator occurs when hourly flow levels exceed 2632 Ips (60 MGD).
The desktop procedure notes events occurred when the average daily flow rate exceeded 2632
Ips (60 MGD) or when the peak hourly flow rate was less then 2632 Ips (60 MGD). A summary of
the events including number of activation's, peak overflow rate and total volume of overflow are
listed below in Table 7-4. There are 21 events with a total annual overflow volume of 676,400
cubic meters (178 million gallons). Overflow peaks range up to 877 Ips (20 MGD). The organic
settleable solids input to the outfall line from the catchment is 879 metric tons (970 tons). During
the 21 overflow events, about 230 metric tons (256 tons) of organic solids discharge to the
receiving water (average concentration equals 290 mg/L.)
Table 7-3. Average Daily and Average Maximum Hourly Velocity and Shear Stress
Average Daily Velocity:
Average Maximum Hourly Velocity:
Average Daily Shear Stress:
Average Maximum Hourly Shear Stress:
0.70 m/s (2.3 fps)
0.85 m/s (2.8 fps)
0.84 N/m2
1 .2 N/m2
Table 7-4. Pertinent Overflow Characteristics
Number of Overflows:
Total Volume Overflow:
Range Overflow Discharges:
Total Hours Activation:
Total Organic Settleable Solids:
Total Organic Settleable Solids:
Average Concentration:
Average Daily Input:
21
676,400 m3(1 78 MG)
43 to 868 Ips (1-20 MGD)
228 hour
232,000 kg (512, 200 Ib) Loadings in Overflows
881 ,000 kg (1 ,938,000 Ib) Input to System
290 mg/l Organic Settleable Solids overflow
15 mg/l Organic Settleable Solids
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FLUSHING ALTERNATOR
38 m3 FLUSHING VAULT
FLUSHING EVERY 3RD DAY
HIGH RATE TREATMENT
ALTERNATIVE & SYSTEM
DIAMETER=1.8m PIPE
LENGTH= 330m
SLOPE= 0.0008
VORTEX SEPARATOR,
UNDERFLOW TANK,
PUMPTOWWTP
REGULATOR
WWTP
OVERFLOW TO RECEIVING WATER
CONCEPTUAL SCHEMATIC
FOR HIGH RATE TREATMENT
VERSUS FLUSHING ALTERNATIVE
Figure 7-3. Conceptual Schematic for High Rate Treatment.
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A conventional satellite treatment at the overflow facility consists of a vortex separator and
underflow storage facility. A conceptual schematic of alternative controls are depicted in Figure
7-3. A satellite treatment system to handle the 21 overflows is assumed. The facility consists of
a 9 meter (30 foot) diameter vortex separator, a 760 cubic meters (0.2 million gallon) underflow
tank (sized to retain 5% maximum overflow), headworks including course screening, dewatering
pumps (both vortex separator and underflow tank need pumping to WWTP after events),
electrical and instrumentation controls, and general site civil and yard piping. Capital and
operational costs were estimated for the facility using a procedure previously developed and used
for CSO facility planning in Cincinnati and Toronto.
Present value costs for this configuration are shown below:
Capital
O&M
Present Worth Cost
$2,620,000
$49, 000 /yr
$3,150,000(1 = 9%, 30 years)
Estimated effectiveness of this satellite facility is depicted in below. It is assumed that the vortex
separator will remove 75% of the incoming organic settleable solids. The maximum influent
applied hydraulic loading is 8.5 Ips/m2 (12 gpm/ft2) during the 21 overflow periods. Average
applied loading on the separator is 6.6 Ips/m2 (9.3 gpm/ft2). Vortex separators are typically
designed to remove heavy settleable solids with hydraulic loadings typically in the 14 to 21 Ips/m2
(20 to 30 gpm/ft2) range. High performance is expected in this case study.
In an alternative scheme, an off-line flushing module just upstream of the 330 m (1000 ft) reach of
1.8 m (72 inch) pipe having with a flush volume of 38 cubic meters (10,000 gallons) is
programmed to flush during night hours every three days except when flows are above average
in the trunk sewer (wet overflow periods). It is assumed that the flush wave on each day of
flushing will move 80 percent of the existing sediment build up (noted in the desktop procedure)
to the regulator and pass to the downstream WWTP.
This flushing scheme is primitive but efficient. The summary results using the desktop procedure
including the flushing scheme are shown below in Table 7-5. This simple scheme reduces the
annual organic settleable solids in the overflow by 92.8%. These solids would have to otherwise
be handled during wet periods by the satellite treatment scheme. As stated above, the fixed
interval-flushing scheme could be modified to include more intelligent options such as more
frequent flushing during extended dry periods when severe deposition is occurring. Such
schemes could easily be programmed.
Table 7-5. Comparison Satellite Treatment Versus Automatic Flushing
Satellite Treatment Alternative
Number Overflows:
Organic Settleable Solids Overflow:
Organic Settleable Solids Removable
by Satellite Treatment :
Residual Organic Settleable Solids:
21
232,700 kg/yr (512,200 Ib./yr)
1 74,600 kg/yr (384,1 50 Ib/yr)
(Assume 75 % Capture)
58,200 kg/yr (128, 050 Ib/yr) out to
receiving water
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Automatic Flushing Alternative
Residual Organic Settleable Solids
overflow:
Organic Settleable Solids to WWTP:
Percent Reduction of Organic
Settleable Solids loadings Attributable
to flushing:
1 6,490 kg/yr (36,280 Ib/yr)
216,300 kg/yr (475,920 Ib/yr) instead
of overflow (i.e., attributable to
flushing)
92.8%
As noted in the Case 1 Study examining flushing vaults in the Fresh Pond Parkway project in
Cambridge, the average present worth per gallon of flushing vaults equals $44 per liter ($165 per
gallon) The largest flush scheme proposed in the Fresh Pond Project was an 42 m3 (11,000
gallons) vault flushing 427 m (1400 ft) of 1.8 m (72 inch) storm drain.
Using this estimate the present worth cost for the 42 m3 (11,000 gallon) flushing scheme
assumed in this case study is $1,760,000. The summary cost effectiveness comparison of the
two technologies are noted below in Table 7-6.
Table 7-6 - Present Worth Cost Comparison - Flushing Gate vs. Satellite Treatment
Satellite Treatment
Flush Gate Technology
Effectiveness
(% Capture)
75%
92.8%
Present Worth
$3,150,00
$1,760,000
It is evident that the flushing scheme utilizing the flush technology appears to be more cost
effective than conventional high rate satellite treatment.
Case Study Three: Cost Effectiveness of Sewer Flushing versus Chemical
Addition for Hydrogen Sulfide Control
General
Hydrogen sulfide will be produced as previously discussed in Chapter 3 in sewers with an
anaerobic environment and an active slime layer. Sediment accumulations can further increase
the generation of dissolved hydrogen sulfide. Hydrogen sulfide can be treated in a dissolved state
or as a gas once it is stripped from wastewater. The analysis in this section will focus on treating
dissolved sulfide because dissolved sulfide treatment will ultimately reduce the quantity of
hydrogen sulfide that is stripped from solution. This section presents a cost analysis of chemically
treating dissolved sulfide within a long, flat depositing sewer for a city in the southern United
States with high BOD, high volatile solids, and elevated wastewater temperature. Flush gate
technology will be used to minimize sediments, thereby reducing hydrogen sulfide contributions
generated from the deposited solids.
The sewer solids deposition and erosion procedure described in Chapter 4 is used to compute
daily solids deposition levels. Once the annual history of deposition and erosion of solids has
been computed using this procedure, dissolved hydrogen sulfide levels are next computed. First,
10
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dissolved hydrogen sulfide levels are computed for the slime layer covering the perimeter of the
pipe. Pomeroy's method is used and is described in Chapter 5. Next, dissolved hydrogen levels
are computed attributable to accumulated sediments. In the initial deposition and erosion
calculations, surface area for each sediment size remaining on a given day is tallied. Total
sediment surface area is computed on a daily basis. In the dissolved sulfide calculations, a
portion of the surface area of sediments is used within the Pomeroy formulation to calculate the
incremental gain of dissolved hydrogen sulfide attributable to the sediment layer. The total
dissolved hydrogen sulfide levels at the end of the pipe segment is the sum of the initial levels
entering the pipe segment, the increment attributable to the slime layer, and if present, the
additional increment associated with the sediments.
Chemical Treatment of Dissolved Sulfide
Numerous chemicals can be used to treat dissolved sulfide through oxidation, precipitation, or
preventing sulfide formation. These chemicals include pure oxygen injection, hydrogen peroxide,
chlorine, potassium permanganate, nitrate solutions, and iron salts. Chemicals must provide
sulfide treatment from the point of application to the WWTP for this case study comparison.
Following are descriptions of each of the potential chemicals and their potential application for
this case study.
Pure Oxygen
Pure oxygen or air can be added to sewers to oxidize dissolved sulfide. However, more than one
injection point is required for prolonged treatment of dissolved sulfide because of the tremendous
oxygen uptake demand of the wastewater and slime layer. This option is not considered for the
case study comparison because more than one injection point is required to treat dissolved
sulfide along the length of sewer.
Hydrogen Peroxide
Hydrogen peroxide can oxidize hydrogen sulfide very quickly but it is a non-specific oxidant and
will oxidize other compounds beside sulfide. However, peroxide is only effective up to 45 minutes
after application before it decomposes to water and oxygen. Hydrogen peroxide is dangerous to
handle in high concentrations and hazardous to humans. It would require specific chemical
storage and handling equipment procedures, as do most of the oxidizing odor control chemicals.
Hydrogen peroxide is not recommended for this case study because of its quick decomposition
after application.
Chlorine
Chlorine could also oxidize the same compounds as peroxide but the longer half-life and toxic
nature of chlorine would prohibit dosing large slugs of the chemical required to be effective.
Chlorine would also react with organics to produce a variety of chlorinated organic compounds
(chloroform, formaldehyde and other mono and polysubstituted chlorinated organic compounds of
indeterminable chemical composition). These compounds could then, depending upon the
physical and chemical properties, be released at downstream processes. Chlorine is also
hazardous to handle and requires strict adherence to health and safety procedures and is not
considered the most desirable alternative for this case study.
Potassium Permanganate
Potassium permanganate could be used but the production of manganese dioxide from its use
and the batch nature of the chemical mixing process would make it a difficult system to operate.
Potassium permanganate is also a hazardous chemical to handle and has explosion precautions
for contact with dry powder. Permanganate application would require particulate control during
mixing and personnel respirators must be worn during mixing. This is also the most expensive
chemical (on a per pound of sulfide removal basis) to use for sulfide removal and is not
considered appropriate for this
11
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Nitrate Solution
Nitrate, if provided in sufficiently high concentrations directly onto the solids in the siphon, would
have the effect of suppressing or stopping sulfide production in the upper layers of the sediments.
Bacteria will use free oxygen first, then reduce nitrate before reducing sulfate. Therefore, if nitrate
is present the bacteria will not reduce sulfate to sulfide. This would reduce, although not
eliminate, the evolution and release of hydrogen sulfide at the treatment plant. Nitrate can not act
quickly enough to be of much use over the entire length of the siphon and an overdose could
violate discharge requirements or cause bulking in the secondary clarifiers at the WWTP
However, the massive logistical effort and cost of applying that much nitrate uniformly to the
siphon is impractical.
Iron Salts
Iron salts will react with dissolved sulfide to form metal-sulfide precipitates. The metal-sulfide floe
typically does not settle in the collection system because of its characteristics and is often
removed in the secondary treatment stage of a WWTP. A metal salt residual can be maintained in
the collection system and effective sulfide control can be accomplished up to 40 kilometers from
an application point with the proper environment. Because of these reasons the case study cost
analysis was performed using iron salts to treat the anticipated dissolved sulfide loading.
Flushing Gates for Sulfide Reduction
As discussed in Chapter 4, flushing gates are used to periodically clean pipes that can
accumulate sediment. However, sediment in collection systems can also contribute and actually
increase hydrogen sulfide production. Therefore, if a flushing gate is installed on a segment of a
collection system to keep the pipes relatively free of debris then the corresponding sulfide
generation should decrease.
Case Study Description
The case study in this section represents a city located in the southern part of the United States.
Cities in this part of the country typically have wastewater characteristics with high BOD
concentrations and elevated temperatures that can lead to substantial hydrogen sulfide
concentrations (when compared with locations in cooler climates). The specific mean and
maximum wastewater characteristics for this case are contained in Table 7-7.
Table 7-7. Wastewater Characteristics
Parameter Average Daily Average Daily Average Daily
Maximum Minimum
Discharge (Ips) 3468 6571 2707
BOD5(mg/l) 570 928 197
VSS(mg/l) 376 760 39
TSS(mg/l) 464 913 272
Temperature (deg. C) 29 32 16
The desktop procedure described in Case Study 2 was used to analyze a 1.8 m diameter sewer.
This procedure calculated sediment and erosion behavior, calculated dissolved sulfide
concentrations using the Pomeroy method described in the Chapter 4, and calculated dissolved
sulfide attributed to the accumulated sediments. The sewer is 915 meters long and has a slope of
0.0007. The dissolved sulfide concentration entering the pipe was assumed to be 0.25 mg/l. It is
assumed that the allowable dissolved sulfide concentration entering the WWTP is 0.75 mg/l.
Three 50 m3 gallon flush gates must be installed to clean the 915 m long conduit (flush gates are
spaced equally along the pipe length to clean 305 meters each). The flush gates are programmed
to flush every third day during low flow conditions.
Based on the above information the desk-top procedure revealed that 383,750 kilograms of
sulfide was produced in this segment of the sewer per year. Approximately 208,200 kilograms of
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sulfide was produced in the sediment layer of the siphon and the remainder was attributed to the
slime layer on the pipe walls.
Cost Analysis
Two conditions were analyzed for this case study; 1) treating the yearly hydrogen sulfide mass
with chemicals and 2) using a combination of flushing gates and chemicals to treat the total
sulfide mass. Present worth calculations will be estimated for both scenarios.
Iron salts are assumed to treat the dissolved sulfide concentration for this sewer segment.
Specifically, ferrous chloride will be used for the analysis. Ferrous chloride solution has
approximately 0.12 kilograms of iron per liter of solution and a capital cost of $0.16/liter. Field
experience indicates that approximately 0.82 kilograms of iron is required to treat 0.45 kilograms
of sulfide. Therefore, the general cost of treatment per kilogram of sulfide is approximately $2.42.
The capital cost for the chemical addition equipment is approximately $50,000 (installed) and the
yearly operation and maintenance cost is $10,000. The flushing gate systems are based on a
flushing vault volume of 50 m3 and a present value cost of $35,200/m3. Costs assume an inflation
rate of 3.12% per year, discount rate of 7.1%, and a 30-year term.
Condition 1 - Sulfide Treatment with Ferrous Chloride
The estimated present worth cost for treatment of 383,750 kilograms of dissolved sulfide per year
(assuming that sediment contributes to the total sulfide mass) is $15.5 million.
Condition 2 - Sulfide Treatment with Ferrous Chloride and Flushing Gates
This condition assumes that the total sulfide mass contribution from the sediment ( 208,200 kg)
bed is eliminated by using the flushing gates to minimize sediment accumulation in the pipe. The
estimated present worth cost for treatment of 175,900 kilograms of dissolved sulfide per year
using ferrous chloride to treat the dissolved sulfide and plus the present worth cost of the flushing
gate system to minimize the sulfide formation is $12.5 million.
Summary of chemical treatment costs and flushing gate costs for both conditions are shown in
Table 5-10.
Table 5-10. Chemical Treatment Costs
Chemical Cost ($) Flushing Cost ($) Total Cost ($)
Condition 1 15,500,00 N/A 15,500,00
Condition 2 7,200,000 5,200,000 12,500,000
It appears that the overall system of employing flushing in combination with chemical treatment of
residual unacceptable dissolved hydrogen sulfide levels is cheaper than only chemical treatment.
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