EPA/600/R-94/129
September 1994
Sformwater Pollution Abatement Technologies
by
Richard Field
Michael P. Brown
William V. Vilkelis
Storm and Combined Sewer Overflow Pollution Control Program
Risk Reduction Engineering Laboi'atory-- Cincinnati
Edison, New Jersey 08837
RISK REDUCTION ENGINEERING LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
CENTER FOR ENVIRONMENTAL RESEARCH INFORMATION
CINCINNATI, OHIO 45268
Printed on Recycled Paper
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Disclaimer
This report has been reviewed in accordance with the U.S. Environmental Protection
Agency's peer and administrative review policies and approved for publication.
Mention of trade names or commercial products does not constitute endorsement or
recommendation for use.
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Foreword
Today's rapidly developing and changing technologies and industrial products and
practices frequently carry with them the increased generation of materials that, if
improperly dealt with, can threaten both public health and the environment. The U.S.
Environmental Protection Agency is charged by Congress with protecting the Nation's
land air and water systems. 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. These laws direct the EPA to perform research to define our environmental
problems, measure the impacts, and search for solutions.
The Risk Reduction Engineering Laboratory is responsible for planning, implementing,
and managing research, development, and demonstration programs to provide an
authoritative, defensible engineering basis in support of the policies, programs, and
regulations of the EPA with respect to drinking water, wastewater, pesticides, toxic
substances, solid and hazardous wastes, and Superfund-related activities. This
publication is one of the products of that research and provides a vital communication
link between the researcher and the user community.
This publication covers the most current technologies and management practices
employed in remediating contaminated urban stormwater runoff. As society
progresses and expands, stormwater runoff becomes an increasingly more significant
source of pollutants entering our Nation's waterways. More impervious surfaces,
building in flood plains, and increased industrial/business/human activities all
contribute to diffuse source pollution. This significance is evident from the high
pollutant loadings from diffuse sources that still enter our waterways even though
tight controls have been initiated over previously unchecked point source discharges
from industry and wastewater treatment plants. Significant water quality problems
still exist and this publication presents the basic ideas, strategies and technologies
that can be used to comply with the Clean Water Act mandated urban stormwater
permit requirements and to satisfactorily combat the impact of diffuse source
pollutants on our waterways.
E. Timothy Oppelt, Director
Risk Reduction Engineering Laboratory
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Abstract
This publication presents information regarding best management practices (BMP's)
and pollution abatement technologies that can provide treatment of urban stormwater
runoff. Included in the text are a general approach which considers small storm
hydrology, and watershed practices which covers public education, regulations, and
source control of pollutants. Also covered are source treatment of pollutants which
include vegetative BMP's and infiltration practices. Uses and modifications of installed
drainage systems, types of end-of-pipe treatments including biological, chemical and
physical treatments and storage and reuse of stormwater are also covered.
Additionally, several tables list recommended publications should the reader wish to
explore any subject matter further.
IV
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Contents
Page
Disclaimer ii
Foreword . . iii
Abstract iv
Figures vii
Tables viii
Section 1 Introduction . . 1
Section 2 General Approach and Strategy . 3
Small Storm Hydrology . 3
Strategy 5
Section 3 Watershed Area Technologies and Practices . . 11
Regulations, Local Ordinances, and Public Education 13
Source Control of Pollutants 15
Section 4 Source Treatment, Flow Attenuation,
and Storm Runoff Infiltration 20
Vegetative BMPs . . 20
Detention Facilities 21
Infiltration Practices 22
Section 5 Installed Drainage System 26
Illicit or Inappropriate Cross-Connections 27
Catchbasin Cleaning 27
Critical Source Area Treatment Devices 28
Infiltration 32
In-Line Storage 32
Off-Line Storage 33
Maintenance 37
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Contents (continued)
Section 6
Section 7
Section 8
References
End-of Pipe Treatment 38
Biological Treatment 38
Use of Existing Treatment Facilities 38
Physical/Chemical Treatment .39
Storage and Treatment Optimization 56
Beneficial Reuse of Stormwater 58
59
VI
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List of Figures
Figures
Page
2-1 Runoff capture volume rates in Cincinnati, Ohio 4
3-1 Street cleaner productivity in Bellevue, Washington 17
3-2 Street sweeping: annual amount removed as a function
of the number of passes per year at San Jose, California
test site 18
4-1 Cross-section porous pavement and cellular porous pavement 25
5-1 Conceptual design of a sand filter system 29
5-2 Sand filter stormwater inlet 29
5-3 Multi-chambered enhanced treatment device 30
5-4 SAGES unit 31
5-5a Flow balance method-freshwater configuration 35
5-5b Flow balance method- seawater configuration 36
6-1 Isometric view of a swirl combined sewer
overflow regulator/separator 55
7-1 Estimated capital costs of storage and treatment for
200 MGD overflow 57
VII
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List of Tables
Table
Page
2-1 Planning approaches 6
2-2 Urban runoff and CSO BMP references 8
5-1 Pollutants retained in catchbasin 28
6-1 Description of screening devices used in CSO treatment 41
6-2 Design parameters for static screens 42
6-3 Design parameters for microstrainers,
drum screens, and disc screens 42
6-4 Design parameters for rotary screens 43
6-5 CSO-DMHRF average SS removals 45
6-6 CSO-DMHRF average BOD5 removals 45
6-7 Removal of heavy metals by DMHRF 46
6-8 Design parameters for DMHRF 46
6-9 Screening and DAF design parameters 47
6-10 Preliminary design parameters for
high gradient magnetic separators 48
6-11 Characteristics of principal
storm flow disinfectant agents 52
VIII
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Section 1
Introduction
This report covers the control and treatment of stoirmwater in relation to the removal
or reduction of the stormwater pollutant loads. The control of storrnwater to prevent
flooding is not the emphasis of this report. Many of the pollution abatement
technologies discussed will help attenuate stormwater flows. However, as they are
generally designed for small storm events they will not provide sufficient capacity for
the large events. Although prevention of stormwater flooding is not discussed in this
report a drainage system design should consider both pollutant and flooding aspects
of stormwater.
Strategically, the best way to control and treat urban stormwater runoff is through a
combination of regulations, best management practices (BMPs), and treatment
processes. The optimal combination will be site specific, depending on site
characteristics, specific pollutants involved, and cost considerations.
Regulations and BMPs will be effective tools in controlling urban stormwater runoff
because they tend to be preventative in nature. Mandating effluent limits and creating
zoning laws are examples of this. BMPs may also include such practices as upgrading
current systems (treatment devices, sewer systems, stormwater conveyance systems,
etc.) or developing proper management techniques (solid waste management, street
sweeping, etc.).
Source treatment and flow attenuation may come into play by designing such devices
that will intercept or infiltrate stormwater runoff back into the groundwater system
prior to introduction into the stormwater or combined sewer overflow (CSO)
conveyance system. This can have a huge savings on design and construction of
treatment facilities. Examples of such devices are swales, filter strips, porous
pavement, and stormwater wetlands.
BMPs may also include using the existing drainage systems fcr storage (in-line or in-
sewer storage) or creating off-line storage facilities,, A low cost example of an off-line
storage method is the Flow Balance Method (FBM). This method stores overflow from
storm or combined sewer outfalls until such time as the wastewater treatment plant
can handle the volume of overflow.
End-of-Pipe treatment may be necessary for controlling stormwater pollution. This
can be expensive if not used in conjunction with the other methods mentioned above.
There are times when this cannot be avoided and the costs may therefore be high.
The problem with End-of-Pipe treatment is that most of these systems are based on
continuous flow. Stormwater is intermittent in nature and can pose a problem for
1
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traditional treatment methods, e.g., maintaining biomass in a biological treatment
system, or proper amount of chemical addition to variable flows. Some methods will
function better than others and it is up to the system designer to determine the best
treatment system.
The reader may wish to delve into these subjects in greater detail than is presented
in this text. There are tables presented that list recommended literature as well as the
references listed in the end of the publication.
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Section 2
General Approach and Strategy
Small Storm Hydrology
The selection of suitable abatement technologies requires an understanding of the size
and distribution of storm events. These contribute to the total volume of storm runoff
and with knowledge of the pollutant concentrations provide the total pollutant load.
Generally the smaller storm events are the critical storms to consider, because for
many parts of the USA, 85% of all the rains are less than 0.6 in. (15 mm) in depth
and can generate about 70% of the total annual storm runoff (Pitt 1987). The
characteristics of small and large storm events can be very different in terms of the
storm runoff generated, pollutant load, and receiving water impacts. However, the
frequent small storms will have a more persistent impact, and less frequent large
storms will have a larger impact but allow time for recovery between events. For
small storm events, any inaccuracy in the estimation of the initial abstractions and the
soil infiltration rates can significantly change the calculated storm runoff pollutant
load. The initial abstractions include the rainfall depth required to satisfy surface
wetting, surface depression storage, interception by hanging vegetation, and
evaporation. Together with soil infiltration rates, the initial abstractions need to be
accurately estimated to calculate the storm runoff volume. Initial abstractions for
relatively impervious urban surfaces have been found to account for the first 0.2 - 0.4
in. (5-10 mm) of a storm event (Pitt 1987). Others (Pecher 1969, Viesman et al.
1977) have reported initial abstractions of between 0.02 - 0.14 in. (0.5 - 3.5 mm) for
pavement areas depending on whether the areas are flat or sloping steeply.. Figure 2-
1 illustrates the runoff capture volume rates in Cincinnati, Ohio. Note that 95% of the
runoff will be captured for the first 0.5 watershed in. (12.7 mm)(as stated above,
85% of all storms are less than 0.6 in. (15 mm)). This indicates that small
precipitation events need to be considered when designing stormwater quality
treatment facilities. Increases in design detention volume above these values will not
significantly affect the percent capture (Urbonas and Stahre 1993).
Traditional stormwater flood control is concerned with the peak storm runoff flow
rates from relatively infrequent large storm events and their conveyance to prevent
flooding. This is a different set of criteria from that needed for storm runoff pollution
control. Therefore the use of data, storm runoff coefficients, models, etc. intended
or developed to meet stormwater flood control requirements should be used with
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Figure 2-1. Runoff capture volumes in Cincinnati, Ohio (Urbonas and Stahre, 1993).
caution. This is illustrated by initial abstractions which can be a major portion of a
small storm but will be a relatively insignificant portion of a large storm. In other
words just because a model for an area has been verified as providing accurate
information for large storm events does not mean it will predict small events with the
same level of confidence.
A model developed and presently being updated for the calculation of urban
stormwater runoff pollutant loads from small storms is: Source Loading And
Management Model: An Urban Nonpoint Source Water Quality Model (SLAMM), (Pitt
1988). This model concentrates on the parameters discussed above to better
estimate the urban storm runoff pollutant loads before and after application of best
management practices (BMPs). However this is mainly applicable to small areas and
does not give a continuous time analysis. There are, however, a number of other
models such as the US EPA's Storm Water Management Model (SWMM) which will
allow a continuous time analysis for large drainage areas. Continuous time analysis
will provide an optimum design for storage and treatment facilities based on long term
historical weather patterns.
It should not be taken from the above that the large infrequent storm events do not
cause polluted urban storm runoff or significant impacts on receiving waters but that
their infrequency makes them a less significant factor than the smaller frequent
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storms. Communities must design control systems that meet applicable regulations
and these control systems may include large systems.
There are several other factors which will effect the stormwater runoff pollutants and
their concentrations, as discussed elsewhere, and these will also need to be taken into
consideration when estimates are made of the urban storm runoff pollutant load.
Strategy
The intermittent, widespread, and variable nature of urban stormwater runoff will
require a flexible and creative approach to achieve the optimal control and treatment
solution. This approach is likely to be responding to regulations and may be include
Best Management Practices (BMPs) and treatment processes. Traditional wastewater
treatment methods, particularly secondary treatment processes that tend to operate
under conditions closer to steady state, will not necessarily be suitable for the
fluctuating loads of stormwater runoff. On the other hand, technologies used to
control and treat combined sewer overflows (CSOs) are more likely to be applicable
for the stormwater runoff and advantage should be taken of any experience or
facilities of CSO origin that have application for separate stormwater runoff.
Successful stormwater management to control urban storm runoff pollution will
require an areawide approach combining prevention, reduction, and treatment
practices/technologies. It is highly unlikely that one method will provide the best
solution to control the widespread diffuse nature of stormwater runoff and achieve the
water quality required.
Establishing an urban storm runoff pollution prevention and control plan requires a
structured strategy which will include the following steps:
- Define Existing Conditions
- Set Site-Specific Goals
- Collect and Analyze Data
- Refine Site-Specific Goals
- Assess and Rank Problems
- Screen BMPs and Treatment Technologies
- Select BMPs and Treatment Technologies
- Implement Plan
- Monitor and Re-evaluate
It is very likely that advantage can be taken of previous studies for either stormwater
or CSO to get a head start. The above strategy is described in Handbook: Urban
Runoff Pollution Prevention and Control Planning (U.S. Environmental Protection
Agency 1993a). Additional references that describe planning approaches for urban
storm runoff pollution prevention arid control are contained in Table 2-1.
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The above strategy will provide the control goal to be achieved which is then used as
the basis for selection of suitable technologies or approaches. The goal(s) should
initially be broad and not specific because the process of reviewing the technologies
or approaches available will in itself generate information to focus and refine the
goal(s) to meet cost, level of control, public opinion, feasibility, and other restraints.
A flexible approach, which, through an iterative process of review and adjustment is
focused to a specific action plan, is the only real method by which the complexity of
urban stormwater can be managed. The specific action plan will also need to be
subject to reassessment once feedback on its implementation is available.
The above is only a very brief indication of the extensive work which will be required
before the actual abatement technologies are implemented and more detail is given
in the above reference (U.S. Environmental Protection Agency 1993a). The remainder
of this report is concerned with an overview of the abatement technologies available.
The report reviews the technologies by separating the drainage system into three
physical areas of:
- watershed area (i.e., storm runoff generation/collection area)
- installed and/or modified/natural drainage system (i.e., conveyance pipes,
channels, storage, etc.)
- end-of-pipe (i.e., point source)
Technologies applicable to each of these areas are discussed and can be divided into
structural and non-structural. The non-structural will cover approaches such as public
education, regulations, and local ordinances which will have their main application to
the upstream collection area. The structural approaches will be the main options for
the drainage system and end-of-pipe areas and tend to be the more expensive items.
The technologies and approaches for stormwater management referred to as BMPs
generally cover the non-structural or low-structural stormwater runoff controls. The
point at which a stormwater management technology changes from a BMP to a unit
treatment process (i.e./'high-structural" control) is often unclear, therefore in this
report BMPs refer to the upstream watershed area prevention and/or control measures
only.
As stated previously4he optimal solution is likely to be an integrated approach using
several practices and technologies. The management of the watershed using BMPs
to prevent or control pollution at the source is likely to offer the most cost effective
solution and tend to be the basis of many stormwater management plans. However,
although BMPs will be the preferred option they will not always be feasible or by
themselves sufficient to achieve the control objectives. For older and more heavily
urbanized areas BMPs are likely to have a limited application and some form of
treatment prior to discharge may be required. There are a number of publications
cited in Table 2-2 which cover the present state-of-the-art on stormwater
management using BMPs but do not generally review the end-of-pipe treatments that
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could be applied to stormwater as a final line of control. This report will therefore
include the treatment options available for stormwater pollution control and which
appear to be ignored in many stormwater management manuals.
It should however be emphasized that it will be more cost effective to prevent
potential urban storm runoff pollution problems and protect existing resources than
to construct pollution controls once a problem exists. Unfortunately for many areas
the problems exist and retrospective prevention is not a feasible solution.
The implementation of_any stormwater management program will need to meet
financial and probably schedule restraints, therefore an early review and improved
utilization of existing facilities can offer several advantages. These options are likely
to be the quickest and least costly to be implemented but it is also important that they
should meet the objectives developed from the earlier stormwater management
planning process. Examples might include the enforcement of existing regulations to
control soil erosion during construction activities and adaption of existing stormwater
storage intended for flood control to also provide quality control for small storm
events. New installations should consider design for both flood control and pollutant
removals.
The public does not generally perceive stormwater to be an environmental pollution
problem. Furthermore they do not appreciate the direct connection between some of
their actions and the pollution consequences (e.g.,disposal of engine oil and household
toxic liquids down a storm drain or throwing litter on the street which is transported
by the storm runoff into the receiving water). Gaining public support to cooperate in
the implementation and to pay for a stormwater management plan will be a major
challenge. A strategy of concentrating efforts and resources on high priority areas
where results are likely to be achieved, and seen to be achieved, will help generate
public support.
Table 2-2. Urban Runoff and CSO BMP References
Document Title
Controlling Urban Runoff: A
Practical Manual for Planning
and Designing Urban BMPs,
1987
Author
or Editor
Schueler
BMPs Included
Detention
Infiltration
Vegetative
Filtration
Quality Inlets
Information
Available
General Description
Effectiveness
Design
Use Limitations
Maintenance
Cost Examples
8
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Table 2-2 (continued). Urban Runoff and CSO BMP References
Document Title
Protecting Water Quality in
Urban Areas, 1 989
Guide to NFS Control, 1987
Water Resource Protection
Technology: A Handbook of
Measures to Protect Water
Resources in Land Development,
1981
Urban Targeting and Urban BMP
Selection, 1990
Combined Sewer Overflow
Pollution Abatement, 1989
Author
or Editor
MFC A
US EPA
Urban Land
Institute
Woodward-
Clyde
WPCF
BMPs Included
Housekeeping
Detention
Infiltration
Vegetative
Quality Inlets
Housekeeping
Detention
Infiltration
Housekeeping
Detention
Infiltration
Vegetative
Quality Inlets
Housekeeping
Detention
Infiltration
Vegetative
Housekeeping
Collection System
Storage
Treatment
Information
Available
General Description
Effectiveness
Use Limitations
Maintenance
Cost
Examples
General Description
Effectiveness
Cost
General Description
Effectiveness
Design
Use Limitations
Maintenance
Costs
General Description
Effectiveness
Design
Use Limitations
General Description
Design
Effectiveness
Maintenance
Cost
Urban Stormwater Management
and Technology: An
Assessment, 1974
Decision Maker's Storm Water
Handbook: A Primer, 1992
Urban Storm Water Management
and Technology: Update and
User Guide, 1977
US EPA
Philips-
US EPA
Region V
US EPA
Housekeeping
Collection System
Storage
Treatment
Housekeeping
Detention
Infiltration
Vegetative
Filtration .
Quality Inlets
Source Control
Collection System
Storage
Treatment
General Description
Design
Maintenance
Use Limitations
General Description
Effectiveness
Design
Use Limitations
Maintenance
Examples
General Description
Design
Maintenance
Use Limitations
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Table 2-2 (continued). Urban Runoff and CSO BMP References
Document Title
Author
or Editor
BMPs Included
Information
Available
Control and Treatment of
Combined Sewer Overflows,
1990
Moffa
Source Control
Collection System
Storage
Treatment
General Description
Design
Maintenance
Use Limitations
Coastal Nonpoint Source Control
Program: Management Measures
Guidance, 1993
The Florida Development
Manual: A Guide to Sound Land
and Water Management, 1 992
x
Storm Water Management
Manual for the Puget Sound
Basin, 1991
Stormwater Management, 1 992
Stormwater: Best Management
Practices and Detention for
Water Quality, Drainage, and
CSO Management, 1 993
Integrated Stormwater
Management, 1993
US EPA Housekeeping
Infiltration
Vegetative
Filtration
Quality Inlets
"
Livingston, Housekeeping
et al. Infiltration
Vegetative
Quality Inlets
WA DOE Housekeeping
Infiltration
Vegetative
Quality Inlets
Wanielista Water Quality
and Yousef , Infiltration
Detention
Urbonas and Storage
Stahre Source Control
Detention
Treatment
Water Quality
Field, Detention
O'Shea and .Management
Chin Vegetative
Infiltration
Flood Control
Reclamation
Collection Systems
General Description
Effectiveness
Design
Use Limitations
Maintenance
Cost
Examples
General Description
Effectiveness
Design
Use Limitations
Maintenance
Cost
Examples
General Description
Effectiveness
Design
Use Limitations
Maintenance
Cost
Examples
General Description
Effectiveness
Examples
Cost
General Description
Effectiveness
Design
Use Limitations
General Description
Effectiveness
Design
Use Limitations
10
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Section 3
Watershed Area Technologies and Practices
There are many BMPs, but all BMPs are not suitable in every situation. It is important
to understand which BMPs are suitable for the site conditions and can also achieve
the required goals. This will assist in the realistic evaluation for each practice of: the
technical feasibility, implementation costs, and long-term maintenance requirements
and costs. It is also important to appreciate that the reliability and performance of
many BMPs has not been well established, with most BMPs still in the development
stage. This is not to say that BMPs cannot be effective, but that they do not have
a large bank of historical data on which to base design to be confident that the
performance criteria will be met under the local conditions. The most promising and
best understood BMPs are detention and extended detention basins and ponds. Less
reliable in terms of predicting performance, but showing promise, are sand filter beds,
wetlands, and infiltration basins (Roesner ef a/. 1989).
A study of 11 types of water quality and quantity BMPs in use in Prince George's
County, Maryland (Metropolitan Washington Council of Governments 1992a) was
conducted to examine their performance and longevity. The report concluded that
several of the BMPs had either failed or were not satisfying the designed performance.
Generally wet ponds, artificial marshes, sand filters, and infiltration trenches achieved
moderate to high levels of removal for both paniculate and soluble pollutants. Only
wet ponds and artificial marshes demonstrated an ability to function for a relatively
long time without routine maintenance. BMPs which were found to perform poorly
were infiltration basins, porous pavement, grass filters, swales, smaller "pocket"
wetlands, extended detention dry ponds, and oil/grit separators. Infiltration BMPs had
high failure rates which could often be attributed to poor initial site selection and/or
lack of proper maintenance.
The above report contains many more details and recommendations on the use of
BMPs. It is important to note that the reported poor performance of some of the
BMPs is likely to be a function of one or more of: the design, installation,
maintenance, or suitability of the area. Greater attention to these details is likely to
significantly reduce the failure rate of BMPs. Other important design considerations
include: safety for maintenance access and operations, hazards to the general public
through safety (e.g., drowning) or nuisance (e.g., mosquito breeding area), acceptance
by the public (e.g., enhance area aesthetics), and to assume conservative
performances in the design until the historical data can justify a higher reliable
performance.
11
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For any BMP involving soil infiltration of the storm runoff it is important to consider
the possible effects this could have on the groundwater. These could range from a
relatively minor local raising of the water table resulting in reduced infiltration rates
to more serious pollution of the groundwater, particularly if this is also used as a
water source. Stormwater runoff is likely to have very low levels of pollution when
compared to chemical and gasoline leaks/discharges and the soil will have some
natural capacity to hold pollutants. However the long-term build up of pollutants in
the soil and/or groundwater from storm runoff infiltration is not well known.
Therefore infiltration of urban storm runoff, especially from industrial and commercial
areas which are likely to have higher levels of pollution should be treated with caution.
Infiltration of storm runoff can offer significant advantages of controlling storm runoff
at the source, reduced risk of down stream flooding, recharge of groundwater, and
groundwater supply to streams (i.e., low-flow augmentation or maintaining stream
flow during dry-weather periods). All of these and probably other advantages can be
offered at a relatively low cost by infiltration and therefore the advantages will need
to be judged against any pollution risks from urban runoff.
The majority of treatment processes which can be readily applied to urban storm
runoff are only effective for removal of the settleable solids. Removal of dissolved or
colloidal pollutants will be minimal and therefore pollution prevention or control at the
source offers an effective way to control the dissolved pollutants. Fortunately
though, many pollutants in the form of heavy metals and organic chemicals show
significant association with the suspended solids (SS) (Pitt and Field 1990, Pitt et. a/.
1991, 1993, 1994). Therefore removal of the solids will also remove the associated
pollutants.
The previously mentioned goals for a stormwater management plan can be achieved
in the watershed area via three basic avenues:
Regulations, Local Ordinances, and Public Education. This should be the
primary objective because it is likely to be the most cost effective.
Mainly non-structural practices will be involved and application to new
developments should be particularly effective.
Source Control of Pollutants. This will be closely related to the above.
Both non-structural and structural practices can be used to prevent
pollutants coming into contact with the stormwater and hence storm
runoff. Management and structural practices will include: flow diversion
practices which keep uncontaminated stormwater from contacting
contaminated surfaces or keep contaminated stormwater from
contacting uncontaminated stormwater by a variety of structural means;
exposure minimization practices which minimize the possibility of
12
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stormwater contacting pollutants by structural (diking, curbs, etc.) and
management (coverings, loading and unloading practices) practices;
mitigative practices which include plans to recover released or spilled
pollutants in the advent of a release; preventative practices which
include a variety of monitoring techniques intended to prevent releases;
controlling sediment and erosion by vegetative and structural means; and
infiltration practices which provide for infiltration of stormwater into the
groundwater (structural and vegetative means) thereby reducing the total
runoff.
Source Treatment, Flow Attenuation, and Storm Runoff Infiltration.
These are mainly structural practices to provide upstream pollutant
removal at the source, controlled stormwater release to the downstream
conveyance system, and ground infiltration or reuse of the stormwater.
Upstream pollutant removal provides treatment of stormwater runoff at
the specific highly polluting locations where it enters the stormwater
conveyance system. Areas of this type include but are not limited to
vehicular parking areas, vehicular service stations, bus depots, industrial
loading areas, etc.
The following provides brief details of BMPs, but a reader should appreciate many of
these BMPs can be combined and/or modified to best suit the conditions of the
watershed under consideration. More information on BMPs can be found in the
references listed in Table 2-2.
Regulations, Local Ordinances, and Public Education
The regulatory approach can address a wide variety of stormwater management
aspects, some of which are listed below. For any regulations to work there will need
to be an existing framework within which to place the regulations (e.g., local
ordinances, zoning, planning regulations, etc.) together with dedicated resources to
enforce them. Without the institutional systems to set them up and enforce them,
they will not be effective.
Regulations can be an important pollution prevention BMP with particular application
to new developments to ensure that the pollution is prevented or controlled at the
source and any implementation and maintenance costs are included in the
development costs. New York State has compiled a manual on BMPs for new
developments (New York State 1992).
13
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Some typical regulations include:
-Land use regulations
•zoning ordinances
•subdivision regulations
•site plan review procedures
• natural resource protection
-Comprehensive storm runoff control regulations
-Land acquisition
Further details on a regulatory approach are contained in Handbook: Urban Runoff
Pollution Prevention and Control Planning (U.S. Environmental Protection Agency
1993a) and Urban Stormwater Management and Technology: Update and Users'
Guide (U.S. Environmental Protection Agency 1977).
Public education can have a significant role to play because an aroused and
concerned public has the power to alter behavior at all levels. However if the
Stormwater management plans are not adequately communicated and public opinion
responded to, this power of the public can work against the implementation of a
Stormwater plan if viewed as an unnecessary extra cost and restriction on freedom.
Gaining the public support as with all education does not stop but is a continuous
process and applies to all sectors of the public. These sectors are listed below and
discussed in the following paragraphs:
- residential
- commercial
- industrial
- governmental
The residential sector is made up of everyone living in a drainage area and therefore
education should focus on large groups. Long range education goals can be tackled
through school programs and shorter range goals may be achieved through community
groups. Advantage should be taken of working with groups looking for community
improvement projects and opportunities arising from news media coverage and the
associated publicity.
The commercial sector is a fairly large and often diffuse group to communicate with.
Both the owners/managers and their staff will need to be included in any
communication together with new businesses opening; existing businesses moving,
expanding, and closing; and employee turnover. Methods of communication may
include news announcements in the local press, mailed news items, individual contact
by a public official and follow up repeated contacts to answer questions and cope
14
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with employee turnover. Public education can benefit from failures, such as violations
of regulations which result in a citation or fine, and reported in the local press. This
not only informs the public about regulations but also provides an incentive for the
regulations to be followed because they are seen to be enforced.
The industrial sector is a smaller group and can be educated by direct contact with
public officials, education of the consultants from whom industry seeks advice, and
by education of trade associations. Indirect education opportunities are provided by
speaking to meetings of professional organizations and by writing in professional
newsletters and journals. Industrial decision makers are a relatively small group,
which when informed or made aware of their obligations are likely to respond.
Public officials should also communicate with other public officials and governmental
institutions to ensure that they are aware of a stormwater management program and
its implications. Examples include: road, sanitation, and parks departments; and
workers at public institutions such as hospitals arid prisons.
A multi-level, multi-target public education program can help to avoid problems in
implementing a stormwater management program. Further information on
communicating a stormwater management program to the public can be found in
Designing an Effective Communication Program: A Blueprint for Success (U.S.
Environmental Protection Agency 1992a), and Urban Runoff Management
Information/Education Products (U.S. Environmental Protection Agency 1993b). The
latter reference is a catalog of material and publications which are available.
Source Control of Pollutants
Source controls are generally non-structural practices many of which can be termed
as "good housekeeping" practices. They can be very effective in that they are
pollution prevention options some of which are listed below:
- Cross-connection identification and removal
- Controlled construction activities
- Street sweeping
- Solid waste management
- Animal waste removal
- Toxic and hazardous waste management
- Reduced use of fertilizer, pesticide, and herbicide
- Reduced roadway sanding and salting
- Material and chemical substitution
Research of illicit or inappropriate cross-connections into separate stormwater
drainage systems has shown that these can add a significant pollutant loading
15
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(Montoya 1987, Pitt and McLean 1986, Schmidt eta/. 1986, and Washtenaw Co.
1988). This is also recognized in the National Pollution Discharge Elimination System
(NPDES) permits forstormwater discharges which require investigation of dry-weather
flows (DWF) at stormwater outfalls. This will involve inspecting outfalls for DWFs,
identification of illicit discharges from analysis of DWF samples, tracing the discharge
source, and corrective action. DWF can originate from many sources. The most
important sources may include sanitary wastewater (from sewerlines or septic tank
systems), industrial and commercial pollutant entries, and vehicle maintenance
activities. It should be recognized that not all DWF will be a pollutant source and they
may be caused by infiltrating potable water supplies and clean groundwater. A full
illicit connections investigation is likely to be time consuming and costly. A
methodology for identifying illicit discharges in the DWF and tracing the source using
distinct characteristics of potential sources is described in Investigation of
Inappropriate Pollutant Entries into Storm Drainage Systems: A User's Guide (U.S.
Environmental Protection Agency 1993c) and Investigation of Dry-Weather Pollutant
Entries into Storm Drainage Systems (Field et a/. 1994). The User's Guide
concentrates on procedures which are relatively simple and which do not require
sophisticated equipment or training. At a minimum the procedures should identify the
most severely contaminated outfalls to assist in prioritizing areas to be investigated
first and at best will identify the pollutant source. A stormwater management plan
that ignores investigation of DWF is very likely to find that goals set to improve
receiving water quality will not be achieved due to pollutants discharged in DWF.
Soil erosion from construction sites together with wash off from stock piled material
and ready mix concrete trucks can be a major source of pollutants (suspended solids)
for the relatively short construction duration. Requirements for phased removal of
vegetative cover and early re-establishment of ground cover combined with detention
of stormwater for sedimentation and filtering will help reduce the pollution from
construction site stormwater runoff. It is important to also consider the period
following construction when vegetative ground cover has still to be fully established
and occupants of new buildings may undertake landscaping. Further information can
be found in Reducing the Impacts of Stormwater Runoff from New Development (New
York State 1992) and Storm Water Pollution Prevention for Construction Activities
(U.S. Environmental Protection Agency 1992b).
Street sweeping studies (U.S. Environmental Protection Agency 1979d, 1985)
concluded: that typical reduction in storm runoff pollutant loadings can be between
5 and 10 percent for street sweeping carried out every two days (beyond two days
a week does not significantly reduce the solids loading any further, as illustrated in
Figure 3-1); street cleaners did not significantly remove the smallest particulates
(<100//m) that the rain washes off; street cleaners were able to remove large
fractions of large particulates (>200yt/m); the reduction in storm runoff pollutant load
is much less than the pollutant load removed by sweeping (since street surfaces only
16
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I
JQ
L.
3
UJ O
O \
K °
S9
S3
Ul
(K C/J
H- Q
W -;
700 _
600 _
500 -
400 _.
300 .
200 _
100 _
TOTAL SOLIDS
50
100 150 200 250
NUMBER OF PASSES PER YEAR
i
300
Figure 3-1. Street cleaner productivity Bellevue,, Washington (Data from U.S. EPA
1985).
contribute <. 0.5 the total pollutant load) which can lead to a false sense of
effectiveness; pavement type and condition have a pronounced affect on performance
(as illustrated in Figure 3-2); and street sweeping results are highly variable and the
results from one city cannot be applied to another city. The above comments
together with the fact that street storm runoff is only a part of the outfall discharge
would imply that street cleaning is not particularly effective on its own but should be
part of an overall program. Street cleaning is likely to be more effective for removal
of heavy metals from vehicle emissions which tend to associate with the particulates.
17
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Sweeping of parking areas, storage, and loading/transfer areas should be included in
a cleaning program. Concentrated cleaning during certain seasons is likely to be
effective, e.g., during early spring in the snowbelt, when leaves accumulate in the fall,
Q
LU
UJ ^
K$
cn f
§4
o£
(/) g,
10,000 _
4O.OOO -
30.000 _
20.000 _
10.000 _
OIL AND SCREENS SURFACED
smccra on ASPHALT sincere
M no*
/
ASPHMJ sneers m
GOOD CONDinON
1.000
NUMBER OF PASSES PER YEAR
Figure 3-2. Street Sweeping: Annual amount removed as a function of the number
of passes per year at San Jose, California test site. (U.S. EPA 1979d).
and prior to rainy seasons. Although the effectiveness of the above is not generally
proven, street cleaning does offer aesthetic improvements in the removal of large
items from the streets and receiving water. Fugitive emissions from street sweeping
will lead to increased air pollution and may need to be considered if an intensive street
sweeping program is part of a stormwater management plan.
Solid Waste Management involves the collection and proper disposal of solid waste
to maintain clean streets, residences and businesses. It can also be extended for
collection of items such as leaves during the fall. A study of stormwater runoff into
Minneapolis lakes found that phosphorus levels were reduced by 30 - 40% when
street gutters were kept free of leaves and lawn clippings (Minnesota Pollution Control
Agency 1989).
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Domesticated and wild animal wastes represent a source of bacteria and other
pollutants such as nitrogen that can be washed into the receiving waters. A study in
San Francisco, California (Colt 1977) estimated that the dogs, cats, and pigeons
produced 54,500, 9,000, and 2,200 Ib (247, 4, and 1 metric tonnes), respectively
of nitrogen a year for an area of 30,480 acres (12,343 ha). On an annual basis, bulk
precipitation, dog wastes, and fertilizer respectively, accounted for 49, 23, and 22%
of the total nitrogen runoff. Controls through regulation and public education, if
successful, could therefore have a major impact based on these figures.
Toxic and hazardous waste management should review methods to prevent the
dumping of household and automotive toxic and hazardous wastes into municipal
stormwater inlets, catchbasins, and other storm drainage system entry points. Public
education, special collection days for toxic materials, and posting of labels on
stormwater inlets to warn of the pollution problems of dumping wastes are possible
management options.
Fertilizers, pesticides, and herbicides washed off the ground during storms can
contribute to water pollution. Agriculture, recreation parks, and gardens can be
sources of these pollutants. Controlling the use of these chemicals on municipal lands
and educating gardeners and farmers to use the minimum amounts required and
appropriate application methods can help reduce nutrient and toxic pollutants washed
off by storm runoff.
Sand and salt are applied as deicing agents to roads in many areas of the United
States that experience freezing conditions and are then washed off by the melt water
and stormwater runoff. Effects of highway deicing appear most significant in causing
contamination and damage of groundwater, public water supplies, roadside wells,
farm supply ponds, and roadside soils, vegetation, and trees (U.S. Environmental
Protection Agency 1971), Deicers also contribute to deterioration of highway
structures and pavements, and to accelerated corrosion of vehicles. Studies (U.S.
Environmental Protection Agency 1971) indicated that major problems in the control
of deicing chemicals were the excessive application, misdirected spreading, poor
storage practices, inaccurate weather forecasting and the logistics of setting up the
deicing operation. To address these problems two manuals of practice on the
application and storage of deicing chemicals (U.S. Environmental Protection Agency
1974a, 1974c) were produced to give recommendations and improvements. They
provide comprehensive details on storage management, layout, handling, application
for various storm and temperature conditions, and use and calibration of equipment
to minimize the amount of chemicals used. Studies were conducted on alternative
deicing methods (U.S. Environmental Protection Agency 1972a, 1976a, 1978) but
these were more costly than the use of rock salt and therefore would be unlikely to
have general economic application.
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Section 4
Source Treatment, Flow Attenuation,
and Storm Runoff Infiltration
Vegetative BMPs
These developing practices have been the subject of many publications in the last 20
years, a few of which are listed in Table 2-2. Readers are directed to these or similar
publications for more detailed information.
Knowledge of the performance of these systems is limited but the above publications
do contain lessons learned from their implementation and in some cases failure.
Existing urbanized areas are unlikely to have the land space available for installation
of many of these practices and in these situations their application will be restricted.
Swales
These are generally grassed stormwater conveyance channels which remove
pollutants by filtration through the grass and infiltration through the soil. A slow
velocity of flow, <1.5 ft/s «46 cm/s), nearly flat longitudinal slope, <5%, ft/s and
vertical stand of dense vegetation higher than the water surface, >6 in. (15 cm) total
height, are important for effective operation (Metropolitan Washington Council of
Governments 1992b). Swales can be enhanced by the addition of check dams and
wide depressions to increase storm runoff storage and promote greater settling of
pollutants. A further enhancement would be in the development of a wetland channel
(Urbonas and Stahre 1993), but good design would be necessary to minimize the
disadvantages of difficult maintenance access, mosquito breeding and aesthetics to
maximize the benefits of greater treatment potential.
Filter Strips
These are vegetated strips of land which act as "buffers" by accepting storm runoff
as overland sheet flow from upstream developments and providing similar treatment
potential mechanisms to that of swales, prior to discharge of the storm runoff to the
storm drainage system. Low velocity flows, installation of a level spreader and/or land
grading to ensure sheet flow over the filter strip, and dense vegetative cover will
enhance the filter strip performance (Metropolitan Washington Council of
Governments 1992b, Yu et al. 1993).
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Stormwater Wetlands
These can be natural, modified natural, or constructed wetlands and remove pollutants
through sedimentation, plant uptake, microbial decomposition, sorption, filtration, and
exchange capacity. It is important to note that natural wetlands will be covered by
regulations that will limit what can be discharged to the wetland and any
modifications to enhance the wetland performance. Constructed Stormwater
wetlands can be designed for more effective pollutant removal with elements such as:
a forebay for solids capture; meandering flow for extended detention of low flows;
benching of bottom for different water depths and associated plants; and pondscaping
with multiple species of wetland trees, shrubs and plants (Metropolitan Washington
Council of Governments 1992b).
Constructed wetland systems are increasingly being used and developed for
wastewater treatment and this area could be a source of information (e.g., Water
Environment Federation 1990, U.S. Environmental Protection Agency 1988} in
addition to information in Stormwater BMP publications.
Detention Facilities
One of the most common structural controls for urban storm runoff and pollution
loading is the construction of local ponds (including wetlands) 10 collect storm runoff,
hold it long enough to improve its quality, and release it to receiving waters in a
controlled manner. The basic removal mechanism for detention ponds is through
settling of the solids with any associated pollutants, but controlled release will also
attenuate the Stormwater flows which can be a benefit to receiving water streams
that suffer from erosion and disturbance of aquatic habitat during peak flow
conditions.
It should be realized that a detention facility designed to provide pollution control for
a particular size of storm is not likely to provide the same level of treatment for
smaller or larger storms. Therefore as an example a detention facility designed to
capture and release over a certain period a 10 year storm event may need to have the
discharge control orifice designed for a 2 year storm in addition to the 10 year storm
to provide discharge control and hence treatment over a spread of storm events
(Urbonas and Stahre 1993). Detention ponds are in effect small dams and the safety
aspects associated with failure and overtopping should also be considered in the
design.
In a heavily urbanized landscape there is likely to be limited opportunities to use the
types of detention facilities mentioned below, but use can be made of flat roof
storage, temporary flooding of recreational areas such as parks and paved precinct
areas, and automobile parking areas. Use of these facilities will obviously cause user
21
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inconvenience and possible hazard which will need to be assessed along with the
frequency and duration of flooding. Also the users and people responsible for
maintenance should be aware of the designed function of these detention facilities so
that they do not take measures to prevent their flooding.
Extended Detention Dry Ponds
These temporarily detain a portion of stormwater runoff for up to 48 h (a 24 h limit
is more common) using an outlet control. They provide: moderate but variable
removal of particulate pollutants; negligible soluble pollutant removal; and quick
accumulation of debris and sediment (Metropolitan Washington Council of
Governments 1992b). The performance can be enhanced by use of a forebay to allow
sedimentation and easier removal from one area. Many dry ponds which were
originally intended for flood control can be modified or retrofitted to serve as wet
ponds thereby providing the additional benefit of removing pollutants as well.
Wet Ponds
These have a permanent pool of water for treating incoming stormwater runoff. Wet
ponds have capacity greater than the permanent pond volume which permits storage
of the influence stormwater runoff and controlled release of the mixed influent and
permanent pond water. They can provide moderate to high removal of particulate
pollutants and reliable removal rates with pool sizes ranging form 0.5 - 1.0 in. (12.7 -
25.4 mm) of storm runoff per impervious acre (Metropolitan Washington Council of
Governments 1992b). Wet ponds offer better removals and less maintenance than
dry ponds but need to be well designed to ensure they are a benefit to an area and do
not cause aesthetic, safety, or mosquito breeding problems. The performance and
maintenance requirements can be helped by installing a forebay to trap sediments and
allow easier removal, and through use of a fringe wetland on a shallow water bench
around the pond perimeter.
There are several variations and combinations that can be used for the above
detention systems to enhance the stormwater treatment and/or better suit local
conditions. Further details on the design, performance, maintenance and any special
requirements/problems, are available (Metropolitan Washington Council of
Governments 1992b, 1987, Wanielista and Yousef 1992, and Urbonas and Stahre
1993).
infiltration Practices
Infiltration practices have a high potential of controlling stormwater runoff by disposal
at a local site level. However, the soil and water table conditions have to be suitable,
22
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a sufficiently conservative design has to be used, and adequate maintenance has to
be undertaken to minimize the possibility of system failure. The importance of using
only suitable sites together with adequate design and maintenance cannot be over
stressed for this BMP. Another important aspect is the potential for groundwater
pollution. Dissolved pollutants which show little association with solids would be the
immediate concern but other pollutants could be more of a problem in the long term.
Sandy soils generally have high infiltration rates and a potential to filter the
stormwater well. However they are unlikely to provide good removals through
sorption or ion exchange. Soil with a high organic content is likely to offer better
capacity to absorb pollutants but at a slower infiltration rate.
Infiltration in its simplest form involves maximizing the pervious area of ground
available to allow infiltration of stormwater and minimize the storm runoff. This can
be enhanced by directing storm runoff from impervious paved and roof areas to
pervious areas, assuming sufficient infiltration capacity exists. Regulations which
encourage the incorporation of a high proportion of pervious areas, particularly for
new developments, can be effective.
Infiltration Trenches
These are shallow, excavated trenches that have been backfilled with stone to create
an underground reservoir. Stormwater runoff which is diverted into the trench
gradually exfiltrates from the trench into the surrounding soil and in many cases
eventually to the water table. There is no real performance data on infiltration trench
removals but they are believed to have a good capacity to remove particulate
pollutants and a moderate capability to remove soluble pollutants. Variations on this
system include the use of perforated pipes to allow exfiltration and conveyance or
storage of stormwater in excess of the filtration rate. Clogging of infiltration trenches
is the most common cause of their failure. It is important to protect them from
sediment loads during and after construction until the surrounding runoff area has
eveloped ground cover to minimize erosion and sediment transport. The system can
be enhanced and clogging reduced by providing pretreatment in the form of grass filter
strips to filter particulates out of the storm runoff before reaching the infiltration
trench (Metropolitan Washington Council of Governments 1992b).
Infiltration Basins
These are similar to dry ponds (unlined), except that infiltration basins have an
emergency spillway only and no standard outlet structure. The incoming stormwater
runoff is stored until it gradually exfiltrates through the soil of the basin floor. The
comments made about infiltration trenches will also apply to infiltration basins.
Additionally, unlined detention ponds will allow some degree of infiltration.
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Porous Pavement
This is a permeable, specially designed, concrete or asphalt mix which provides an
alternative to conventional pavement, allowing stormwater to percolate through the
porous pavement into a deep gravel storage base area that also acts as a subsurface
foundation (Figure 4-1). The stored storm runoff then gradually exfiltrates into the
surrounding soil. In areas where soil has a slow infiltration rate sub-surface piping
may be installed to direct the stormwater away. Field studies have shown that porous
pavement systems can remove significant levels of both soluble and particulate
pollutants (Metropolitan Washington Council of Governments 1992b, U.S.
Environmental Protection Agency 1981b, 1980). The previous caution about
infiltration BMPs affecting the groundwater also applies here. This system tends to
be used in areas such as parking areas with gentle slopes and relatively light traffic.
Sites which lack areas to form detention ponds or provide sufficient pervious areas
can find this an attractive alternative. Sediment loads will clog the surface and should
be avoided, this will be particularly important during construction. Also regular
maintenance of cleaning the surface should be done. On some installations the gravel
bed/storage layer has been extended beyond the plan limits of the pavement and
returned up at the edge of the pavement. This can enable, with suitable design, any
excess storm runoff to be collected by the perimeter gravel. Construction costs of a
porous pavement parking lot will be approximately equal to that of a conventional
pavement parking lot requiring stormwater inlets and subsurface piping (Field et. a/,
1993).
Another type of porous pavement is constructed using modular interlocking blocks
with open cells which are placed over a deep stone storage base similar to the above
porous pavements. This is illustrated in Figure 4-1.
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Porous concrete or asphalt
Granular graded filter
Uniform graded rock base
Geotextile filter
Native soil subbase
Coarse sand or sandy
loam turf in all spaces
Interlocking concrete
blocks with vertical
holes
Fine gravel filter layer
Granular graded filter
Uniform graded rock base
Geotextile filter
Native soil subbase
Figure 4-1. Cross-section porous pavement and cellular porous pavement (Urbonas
and Stahre 1993).
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Section 5
Installed Drainage System
The goal of upstream BMPs will be to provide sufficient stormwater control to ensure
that further downstream treatment is not needed. However, particularly for urban
areas it is highly unlikely that this goal will be totally achieved and further drainage
system and end-of-pipe controls will need to be considered.
Control practices which can be applied to the drainage system are relatively limited,
especially for existing systems, and involve the items listed below:
- Removal of illicit or inappropriate cross-connections
- Catchbasin cleaning
- Critical source area treatment devices
- Infiltration
- In-line storage
- Off-line storage
Many of the control options are similar to those used for CSO control and in the case
of new developments there is the option to install either separate or combined sewer
systems. A combined sewer system with treatment is likely to provide the most
effective solution in an urban/commercial environment where BMPs are unlikely to
provide sufficient reduction of urban storm runoff pollution. Whether a combined
sewer would be the most effective choice would depend on many factors including
the required degree treatment of separate stormwater discharges. In less urbanized
areas with strongly enforced BMPs and public support there is a much greater
possibility of the downstream stormwater in a separate system needing no further
treatment.
For new combined or separate systems, advantage can be taken of increasing the pipe
size and gradient to provide in-line storage and self cleaning, respectively. This will
incur an additional cost which should be relatively small, but the feasibility will be
subject to site conditions and available hydraulic head. Existing separate (and
combined) drainage systems can be modified for in-line storage by the addition of flow
control devices (weirs, flow regulators, etc.).
Established urban areas with separate stormwater drainage systems are most likely
to have an existing stormwater pollution problem which needs to be rectified. The
following covers some of the options available.
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illicit or Inappropriate Cross-Connections
This control was discussed in Section 3 under source control but also appears here
because of its close relation to the drainage system. Identification and removal of
illicit or inappropriate connections may provide a partial or complete solution but will
be time consuming and costly with no guarantee of success. Depending on the likely
magnitude of the cross-connection problem it is worth considering the alternative of
accepting the pollution problem and providing treatment. If this decision is made early
in the investigation there is the potential to maximize the use of resources on the
treatment option.
Catchbasin Cleaning
A catchbasin has a sump below the outlet invert to capture settled solids, usually has
a baffle or inserted pipe to capture floatables, and is distinct from an inlet which has
no sump. Pollution control performance is variable with the trapped liquid generally
having a high dissolved pollutant content, which is purged from catchbasins during
a storm event contributing to intensification of the stormwater runoff pollutant
loading. Countering this negative impact io the removal of pollutants associated with
the settled solids and floatables (e.g., heavy metals and organics) retained in, and
subsequently cleaned from the basin (U.S. Environmental Protection Agency 1977b).
A regular cleaning schedule is important to maintain the catchbasin performance with
a frequency such that sediment build up is limited to 40 - 50% of the sump capacity
(U.S. Environmental Protection Agency 1977b) or at least twice a year depending
upon conditions.
A study (U.S. Environmental Protection Agency 1983a) conducted in West Roxbury,
Boston, Massachusetts took three catchbasins, cleaned them and monitored four
runoff events at each catchbasin. The average pollutant removals per storm are
shown in Table 5-1. The same study also looked at the effectiveness of screening the
stormwater runoff through U.S. standard no. 8 brass mesh installed in the three
catchbasins. The results indicated screens offered a slight gain in overall pollutant
removal efficiency for catchbasins. The screens were effective for the removal of
coarse material that could cause aesthetic problems in the receiving water but the
potential for clogging and decomposition of trapped material reduced their value
unless weekly cleaning was carried out. The present increased emphasis on
stormwater management has resulted in a review of the role that screening at inlets
and catchbasins can play. The City of Austin, Texas has developed its own form of
inlet filter (Captur™) which is a relatively coarse screen for removal of larger
stormwater debris, and others (Emcon North West) have developed screens utilizing
filter material (5-100 //m) for removal of SS. The Storm and Combined Sewer
Pollution Control Research Program of the U.£. EPA through the University of
27
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Alabama at Birmingham is presently evaluating a number of inlet or catchbasin
screening/filtering devices (U.S. Environmental Protection Agency 1992c).
Table 5-1. Pollutants Retained in Catchbasin
Constituent
% Retained
SS'
Volatile SS
COD"
BODC
60-97
48-97
10-56
54-88
'Suspended solids
bChemical oxygen demand
c5-day biochemical oxygen demand
(EPA-600/2-83/043)
Critical Source Area Treatment Devices
Research into the source of stormwater pollutants has shown that certain critical
source areas can contribute a significant portion of the total urban storm runoff
pollutant load (Pitt et. a/. 1991, 1993, 1994). Treatment of the critical source areas
can therefore offer the potential for a greater benefit to reduce downstream pollutant
loads. Potential critical source areas include: vehicle service, garage, or parking areas;
storage and transfer yards; and industrial materials handling areas exposed to
precipitation.
Sand Filters
These use a bed of sand through which the storm runoff is filtered prior to discharge
to the drainage system or ground infiltration. Sand filters can offer high removal rates
for sediment and trace metals, and moderate removals for nutrients, BOD, and fecal
coliform (Metropolitan Washington Council of Governments 1992a). The arrangement
of the sand filter bed can vary from an open pit with perforated pipes under the sand
bed as shown in Figure 5-1, to a more sophisticated trench stormwater inlet as shown
in Figure 5-2 which includes a sediment chamber, weir and sand filter chamber.
Washington DC has installed a few sand filters in chambers in the line of the drainage
pipes for treatment of urban storm runoff. The storm runoff passes along the
drainage pipe, enters the chamber, passes through the sand filter bed, and returns to
the drainage pipe. An overflow bypass is incorporated in the chamber to handle flows
in excess of the filter bed capacity.
28
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Geotextile fabric
Cleanout
pipe
n
n i
-8-in perforated pipe
Geomembrane
Figure 5-1. Conceptual design of a sand filter system MWCG 1992a.
Weir flow
(one weir for
each inlet grate)
Sedimentation chamber
(heavy sediments,
organics, debris)
Covor lids
Inlet grates
10 in.. ,.
2£icm)vX Trapped solids
Filtration chamber
Screen covered
with filter fabric
Figure 5-2. Sand filter stormwater inlet (Urbonas and Stahre 1993).
Maintenance of sand filter beds involves removal of debris from the surface,
replacement of the top layer of sand, and raking of the surface. The frequency of this
maintenance will be controlled by the rate of accumulation of filtered material.
Oil-Grit Separators
These are usually three stage underground chambers designed to retain storm runoff,
remove heavy particulate by settling, and remove hydrocarbons by trapping floating
29
-------
material or adsorption onto settled solids. They have limited pollutant removal
capability and only appear to trap coarse-grained solids and some hydrocarbons.
Removal of silt and clay, nutrients, trace metals, and organic matter is expected to be
slight. Without regular clean out maintenance (e.g., every 3 months), resuspension
is likely to limit any long term removal.
Enhanced Treatment Device
Research is presently being conducted to develop a treatment device for runoff
generated by small but critical toxicant source areas. This will consist of first, a
relatively small chamber filled with plastic, hollow slotted media to promote cascading
and aeration of the inflow and volatilization of volatile compounds, together with a
sump to collect any heavier solids that settle out. The first chamber will then feed the
runoff into a second, sedimentation chamber incorporating tube or plate settling for
enhancing sedimentation with floating sorption pillows to remove floating oil and
grease. This chamber may also be fitted with aeration facilities depending on the
results of the demonstration. The final chamber will contain a sand filter bed which
will may also be enhanced with either a homogeneously mixed layer of sand and peat,
or a granular activated carbon layer to improve removals (U.S. Environmental
Protection Agency 1992c). This treatment device is shown in Figure 5-3.
Catchbasin Main Settling Chamber
Packed Column ~ sorbent pillows
aerators ~ f'ne bubble aerators
— tube settlers
Q,
0°00
O O O
° o°<> o o —
-------
The above research study will also install and monitor a filtering and pre-infiltration
device (SAGES), shown in Figure 5-4. The intention of this device is to provide a high
level of filtration treatment to the storm runoff prior to local infiltration into the
ground.
i— GROUND SURFACE
U\\X
V\ V /\ V l\ \/A V A \/\ \S\ TTI
4_
\\\
SURFAC
WATER FROM L fc. '
\\
E IN FLOOD
"DIRTY"
. fc PYi^TiNr;
UPSTREAM
"DIRTY
EXISTING SYSTEM
OUTFLOW
"DIRTY-
CABLE SUPPORT
DRILL THROUGH FLOOR
OF CATCH BASIN
TOP OF
KNOWN
AQUIFER
SUPPORT GRATE
REMOVABLE FILTERS
GRAVEL FILTER
(OVERSIZE REMOVAL)
SAND FILTER
(SILT REMOVAL)
CARBON FILTER
(SOLUTES REMOVAL)
NEW
INFILTRATION
ZONE FOR PURIFIED
DISCHARGE
TO AQUIFER
Figure 5-4. SAGES Unit (931026 Ontario Limited, J. Van Egmond).
31
-------
infiltration
New installations offer the possibility of using porous conveyance pipes to promote
infiltration, but this can only be recommended where the soil and water table
conditions are suitable and stormwater pollutants will not cause a problem.
In-line Storage
This is the use of the unused volume in the drainage system network of pipes and
channels to store storm runoff. In-line storage capacity can also be provided by
storage tanks, basins, tunnels, or surface ponds which are connected in-line to the
conveyance network. To gain maximum benefit from in-line storage it should be
combined with some form of treatment, otherwise only flow attenuation will be
achieved. The in-line storage is unlikely to offer any treatment in itself through
settling as the intent will be to make the system self cleaning to reduce maintenance
requirements. However, if the storage is combined with an end of pipe treatment the
flow attenuation will help equalize the load to the treatment process, hence optimizing
the size of the treatment plant and costs.
The concept of combining storage and treatment to minimize the storage and
treatment capacity required and hence optimize the cost to control polluted
stormwater is an important relationship. Further cost effective solutions might be
found if existing treatment facilities can be used, such as connection to an existing
wastewater system. This is discussed in more detail later in the report as the storage
does not necessarily need to be provided by in-line storage.
Even without treatment, flow attenuation will help equalize the pollutant loading to
be assimilated by the receiving water and reduce the peak flows and consequent
erosion in the receiving stream. This can have a major benefit to reduced disturbance
of the aquatic ecosystem.
The degree to which the existing conveyance system can be used for storage will be
a function of: the pipe or channel sizes which will provide the storage volume; the
pipe or channel gradient (relatively flat lines are likely to provide the most storage
capacity without susceptibility to flooding of low areas); suitable locations for
installation of control devices such as weirs; and the reliability of the installed control.
It will be essential that accurate details of the existing system be collected from field
surveys and as-built drawings. This will allow the storage capacity, numbers and
locations of controls, and risk of upstream flooding to be assessed. This will also be
invaluable in new drainage system design where conveyance pipes and channels can
be up-sized and hydraulic controls can be designed into the system for added system
storage and routing.
32
-------
Controls used to restrict flow causing a back up and storage in the system fall into
two categories, either fixed or adjustable. Fixed systems are likely to be cheaper and
require less maintenance but do not offer the flexibility and potential to maximize the
storage potential. Adjustable systems can offer the advantage of being connected to
a real-time control (RTC) system which, via a system of rainfall measurements and
forecasts, monitoring of stormwater levels in critical sections of the drainage system,
and input of this data into a computer system, can be adjusted to hold back or release
stormwater to maximize storage capacity of the whole drainage system. RTC
systems have been installed and are being further developed to control complex
sewerage systems in the CSO field. The sophistication offered by an RTC system is
unlikely to offer a cost effective solution for a separate storm drainage system unless
there is a large in-line storage capacity and the stored runoff is to be treated.
Typical examples of fixed and adjustable flow regulators are:
Fixed Regulators
Orifices
Weirs (lateral and longitudinal)
Steinscrew
Hydrobrake
Wirbeldrossel
Swirl
Stilling-pond weir
Adjustable Regulators
Inflatable dams
Tilting plate regulators
Reverse-tainter gates
Float-controlled gates
Motor-operated
or Hydraulic gates
Some of the above are relatively inexpensive, quick to install, and an effective means
of increasing storage. Several publications (Urbonas and Stahre 1993, U.S.
Environmental Protection Agency 1977a, 1970a, 1970b) on CSO control can provide
more information on the above regulators. However, as stated earlier, without
treatment the advantage of storage is only in flow attenuation. It should also be
noted that some of the above regulators will concentrate the heavier solids in the
stored storm runoff for a more concentrated later release.
Off-line Storage
This refers to storage that is not in-line to the drainage conveyance system. Storage
is achieved by diverting flow from the drainage system when a certain flowrate is
exceeded. The diverted water is stored until sufficient capacity is available
downstream. The off-line storage can be provided by any arrangement of basins,
tanks, tunnels, etc. and, if gravity filling and emptying is not possible, it will involve
pumping the water into or out of storage.
Off-line storage as with in-line storage can be designed to be relatively self cleaning
or have facilities to re-suspend the settleable solids. Examples can be found in books
33
-------
on stormwater (Metcalf & Eddy 1981, Field 1990, Urbonas and Stahre 1993). Off-
line storage can also be used to provide treatment by sedimentation with the sludge
either collected or diverted to a wastewater treatment plant.
Many of the regulators listed under in-line storage can be used to divert the flow once
the predetermined flowrate has been exceeded. In addition to the above listed
regulators, vortex and helical bend regulators/concentrators can be used. As their
name suggests they will concentrate the heavier solids into the underflow which will
continue to be conveyed along the drainage pipes. Therefore end-of-pipe treatment
is required if this concentrated pollutant load is to be prevented from reaching the
receiving water. The regulator/concentrator can offer advantages for end-of-pipe
treatment when the flow needs to be regulated to prevent the treatment capacity
being exceeded. End-of-pipe treatment can be satellite or central treatment. This is
discussed further in the end-of-pipe treatment section.
Flow Balance Method (FBM)
The system provides a means of storing discharged urban storm runoff in the
receiving water. This allows either pump back for treatment, when capacity is
available, or treatment of the runoff by sedimentation until the next storm runoff
event displaces the stored volume. The method was first developed in Sweden
(Soderlund 1988) as a means of protecting lakes against pollution from stormwater
runoff and has since been demonstrated for control of CSO in a marine receiving
water in Jamaica Bay, New York City, NY (Field et a/. 1990, Forndran et al. 1991).
Storage in the receiving water is achieved by forming a tank using flexible plastic
curtains suspended from pontoons. The curtains are anchored to the receiving water
bottom by concrete weights and the base of the tank is formed by the receiving water
bed. The relatively low cost of the materials and construction gives this system cost
advantages over conventional concrete and steel tank systems (estimated to be one-
fifth to one-tenth the cost), requires only a minimal amount of land space for controls
and access, and has flexibility to expand the volume if required at a later date.
The Swedish freshwater lake installations use a connected system of bays with
openings between adjacent/sequential tanks to facilitate movement of the stormwater
and lake water between tanks. Lake water can enter and leave these FBMs via the
last tank in the series which has an opening to the lake. Plug flow set up by the
discharging stormwater displaces the lake water from the first to the second bay and
on down the line until the discharge finishes or each bay is filled with stormwater (i.e.,
stormwater has to pass through all of the bays to gain access to the lake). A reverse
flow sequence occurs during pumpback of the stormwater to the wastewater
treatment plant (WWTP). Figure 5-5a shows the FBM freshwater system.
34
-------
—u
ilischargc
feed pump("Yj
floating tank
Kt/"""'
1
L T
'I
1 '
^
*V plastic cloth
- pressure line
Irealnivnl plant
-
--
11
"^oiK'ninj*
i
/
- pontoons
^
7T
HI
~ f
II V*
a
drainage
sludge drying beds
outlet pipe
r- Stormwoter
To I Rump
•=-\ Concrete
iV\ Weights
Figure 5-5a. Flow Balance Method (FBM) - freshwater configuration.
35
-------
r
Outfoll
Stormwoter
Pump
^"
r
Pontoons
V
STORMWATER
SEAWATER
'— End Curtain
f'— Window Opening
For Flow In/Out
^^^^
Figure 5-5b. Flow Balance Method (FBM) - seawater configuration.
Sweden has invested in three of these installations which have all been in operation
for a number of years. The systems have withstood wave action up to 3 ft (0.9 m)
as well as severe icing conditions. If a wall is punctured, patching is easily
accomplished and general maintenance has been found to be inexpensive. The FBM
has been successfully demonstrated in these lakes resulting in improved water quality
in the lakes (Soderlund 1988, Pitt and Dunkers 1993). The marine FBM
demonstration (see Figure 5-5b) utilizes a different operating principle of density
difference for displacement instead of plug flow. One tank is used and the seawater
is displaced vertically by the lower density CSO influent floating on the higher density
seawater, and hence forming a stratified layer of CSO above the lower seawater layer.
The demonstration project is in two phases. The first phase concentrated on proving
the feasibility of the system concept to: displace seawater; form a stable CSO layer;
pump the CSO back to the WWTP; and for the system structure to withstand a
marine environment including tidal exchange, freezing, and coastal storms. The
second phase (presently in progress) expands the system capacity from 0.41 Mgal
(1550 m3) to 2 Mgal (7570 m3), and concentrates on monitoring the system
performance (U.S. Environmental Protection Agency 1990).
36
-------
During the two year demonstration project the system withstood the marine
environment (the FBM was located in a relatively sheltered seawater creek) with no
structural damage or material degradation observed. The system was exposed to tidal
ranges up to 7 ft (2 m), winds gusting to 40 mph (64 km/h), and icing conditions.
The system was shown to retain CSO in a stratified layer which remained relatively
stable and could be pumped back to the WWTP. The FBM proved effective in
trapping floatable material and a means of floatable material removal is part of the
next phase. Pumpback of the settled solids from the FBM bed has been incorporated
into both phases.
It is important to note that, although an FBM can offer a cost effective and quick to
construct storage facility, it requires a suitable location and does have limits on its
performance. There will be a certain amount of imixing with the receiving water. Not
all of the stored volume will be pumped back, and any settleable solids will settle out
of the stored storm runoff (regular pumpback of the accumulated sediment would help
over come this problem). The low cost and quick construction potential of the FBM
could favor the use of this system as a temporary measure in cases of a severe
problem which needs attention. The FBM does use the existing natural receiving
water and therefore will require all the necessary permits involved in these situations.
Maintenance
In order for the drainage system and the controls to work efficiently they should all
be regularly maintained. This will generally consist of removing sediments from
control devices, flushing drainage lines, and general inspections to identify any
problems. Regular maintenance will also minimize any build up of material which
could be flushed out by a surge from a large storm event, and thereby minimize the
shock loading caused by intermittent storm events.
37
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Section 6
End-of-Pipe Treatment
Biological Treatment
Biological treatment provides a means of removing organic pollutants from the storm
runoff either aerobically or anaerobically. For this treatment to be effective the
systems must be operated continuously to maintain an active biomass or be able to
borrow the biomass from a system which does operate continuously. Biological
processes are relatively sensitive and can be affected by the variable flow conditions
and the relatively high concentration of nonbiodegradable solids in storm runoff.
These factors tend to make high-rate physical treatment processes more suitable for
stormwater applications with their ability to handle high and variable flow rates and
solids concentrations.
Partial exceptions to the above are biological systems which include attached growth,
e.g. the trickling filter (with honeycomb plastic medium) and the rotating biological
contactor (RBC) which are less susceptible to overloading shock loads compared to
other biological systems, e.g. activated sludge processes. RBCs have achieved high
removals at flows 8-10 times their base flow for CSO treatment (U.S. Environmental
Protection Agency 1974d). RBCs though like all biological processes need a food
source to keep the microbes alive during extended dry periods and therefore have their
limitations. The remainder of this section will therefore concentrate on the
physical/chemical treatment processes which tend to be more suitable for treatment
of stormwater.
Use of Existing Treatment Facilities
As stated earlier any use of existing facilities is likely to provide cost effective
treatment, providing an economic means of connecting the stormwater drainage
system to the facility is possible.
Use of spare capacity at wastewater treatment plants is one option, particularly if
storage can be provided to equalize the storm runoff load. Even if the biological
system has very little capacity the primary treatment systems can often function well
at somewhat higher overflow rates which if combined with disinfection of the
discharged storm runoff will offer significant treatment. Stormwater also tends to
have a higher percentage of heavier solids than sanitary sewage which will benefit
removals at higher overflow rates.
38
-------
An alternative could be to construct additional primary treatment at a wastewater
treatment plant (WWTP) to run in series with existing facilities during dry-weather
flow (DWF) for improved treatment of DWF and run in parallel during wet-weather
flow for some control over the total flow.
Use of any storage facilities, either at an end-of-pipe or an upstream location, could
provide treatment by sedimentation or storage to be released when treatment capacity
is available.
Physical/Chemical Treatment
These processes generally offer: good resistance to shock loads, ability to consistently
producea low suspended solids (SS) effluent, and adaptability to automatic operation.
Those described below are, with the exception of high gradient magnetic separation
and powdered activated carbon, only suitable for removal of SS and associated
pollutants. The extent of removals will depend on the SS characteristics and the level
of treatment applied. The physical/chemical systems to be discussed are:
- Screening
- Filtration
- Dissolved air flotatipn
- High gradient magnetic separation
- Powdered activated carbon-alum coagulation
- Disinfection
- Swirl concentrators/regulators
Screening
Screens can be divided into four categories with the size of the SS removed directly
related to the screen aperture size:
Screen Type
Bar screen
Coarse screen
Fine screen
Microscreen
Opening Size
>1 in. O25.4 mm)
3/16-1 in. (4.8-25.4 mm)
1/250-3/16 in. (0.1-4.8 mm)
<1/250 in. «0.1 mm)
Bar and coarse screens have been used extensively in WWTP at the headworks to
remove large objects. Depending on the level of treatment required for the storm
runoff the smaller aperture sized coarse screens may be sufficient, however a higher
level of treatment can be achieved using the bar and coarse screens in conjunction
with the fine or microscreens. Design of screens can be similar to that for WWTP and
CSO, but with consideration for stormwater characteristics of intermittent operation
39
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and possible very high initial loads which may not reflect WWTP operation character-
istics. A self-cleaning system should be included for static screens to save manual
cleaning during storm events together with automatic start up and shut down.
Catenary screens fall into the coarse screen category. They are rugged and reliable
and commonly used for CSO facilities. Therefore they are likely to be a good screen
for use with storm runoff.
Table 6-1 lists screening devices that fall into the fine screen and microscreen
category, and were developed and used for SS removal from CSO. With no
information on screening of separate stormwater, the information on screening CSO
is a good starting point and the information given below is from CSO studies. Design
parameters for static screens, microstrainers, drum screens, disc screens, and rotary
screens are presented in Tables 6-2, 6-3, and 6-4. The removal efficiency of
screening devices is adjustable by changing the aperture (size of opening) of the
screen placed on the unit, making these devices very versatile. In other words the
efficiencies of a screen treating a waste with a typical distribution of particle sizes will
increase as the screen aperture decreases.
Solids removal efficiencies are affected by two mechanisms; straining by the screen,
and filtering of smaller particles by the mat deposited by the initial straining.
Suspended matter removal will increase with increasing thickness of filter mat
because of the filtering action of the mat itself, which is especially true for
microstrainers. This will also increase the headless across the screen. A study in
Philadelphia (Field 1972) showed (on a 23 //m aperture microscreen (Microstrainer))
that with a large variation in the influent SS, the effluent SS stayed relatively constant
(e.g., if a 1000 mg/L influent SS gave a 10 mg/L effluent SS, then a 20 mg/L influent
SS would still give a 10 mg/L effluent SS). Accordingly, treatment efficiencies vary
with influent concentration.
Microscreens and fine screens remove 25 - 90% of the SS, and 10 - 70% of the
BOD5, depending on the screen aperture used and the wastewater being treated. The
above Philadelphia study showed that improved removals and increased flux densities
(hydraulic loadings) are possible using polyelectrolyte addition. This is also likely to
be the case with storm runoff but laboratory coagulation studies would be needed to
find the best polyelectrolyte and dosage for the particular storm runoff characteristics.
The optimum dosage will change with changes in the storm runoff characteristics
requiring some form of automated monitoring (e.g., SS monitoring) for adjustment of
dosage or setting of an average effective dosage.
More detailed descriptions of the various screening devices are available in the
literature (U.S. Environmental Protection Agency 1977a, Metcalf & Eddy 1981, Field
1990, Water Environment Federation 1992).
40
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Table 6-1. Description of Screening Devices Used in CSO Treatment
Type of
Screen
General
Description
Process
Application
Comments
Drum Screen
Microstrainars*
Rotostrainer
Disc Strainer
Rotary Screen
Static Screen
Horizontally mounted cylinder with screen
fabric aperture in the range of 100-843
Operates at 2-7 r/min.
Horizontally mounted cylinder with screen
fabric aperture 23-100 (jm. Operates at 2-
7 r/min.
Horizontally mounted cylinder made of
parallel bars perpendicular to axis of drum.
Slot spacing in the range of 250-2500 //m.
Operates at 1-10 r/min.
Series of horizontally mounted woven wire
discs mounted on a center shaft. Screen
aperture in the range of 45-500 //m.
Operates at 5-15 r/min.
Vertically aligned drum with screen fabric
aperture in the range of 74-167 //m.
Operates at 30-65 r/min.
Stationary inclined screening surface with
slot spacing in the range of 250-1600 pm.
Pretreatment
Main
Treatment
Pretreatment
Pretreatment,
main
treatment, or
posttreatment
of
concentrated
effluents.
Main
treatment
Pretreatment
Solids are trapped on
inside of drum and are
backwashed to a
collection trough.
Solids are trapped on
inside of drum and are
backwashed to a
collection trough.
Solids are retained on
surface of drum and
are removed by a
scraper blade.
Unit achieves a 12-
15% solids cake.
Splits flow into two
distinct streams: unit
effluent and
concentrate flow, in
the proportion of
approximately 85:15.
No moving parts.
Used for removal of
large suspended and
settleable solids.
*A vertically mounted microstrainer is available, which operates totally submerged at approximately
65 r/min. Aperture range is 10-70 //m (microns). Solids are removed from the screen by a sonic
cleaning device.
(EPA-600/8-77/014
41
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Table 6-2. Design Parameters for Static Screens
Hydraulic loading, gal/min/ft of width
Incline of screens, degrees from vertical
Slot space, fjm
Automatic controls
100-180
35*
250-1600
None
'Bauer Hydrasieves™ have 3-stage slopes on each screen: 25°, 35°, 45°.
gal/min/ft X 0.207 = l/m/s
(EPA-600/8-77/014)
Table 6-3. Design Parameters for Microstrainers. Drum Screens, and Disc Screens
Parameter
Screen aperture, yum
Screen material
Drum speed, r/min
Speed range
Recommended speed
Submergence of drum, %
Flux density, gal/ftz/min
of submergence screen
Headless, in.
Backwash
Volume, % of inflow
Pressure, Ib/in.2
Microstrainers
23-100
stainless steel
or plastic
2-7
5
60-80
10-45
10-24
0.5-3
30-50
Drum Screen
100-420
stainless steel
or plastic
2-7
5
60-70
20-50
6-24
0.5-3
30-50
Disc Screen
45-500
wire cloth
5-15
50
20-25
18-24
•>
'Unit's waste product is a solids cake of 12-15% solids content
gal/min/ft2 X 2.445 = m3/h/m2
in. X 2.54 = cm
ft X 0.305 « cm
Ib/in.2 X 0.0703 = kg/cm2
(EPA-600/8-77/014)
42
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Table 6-4. Design Parameters for Rotary Screens
Screen aperture,
Range
Recommended aperture
Screen material
Peripheral speed of screen, ft/s
Drum speed, r/min
Range
Recommended speed
Flux density, gal/fWmin
Hydraulic efficiency, % of inflow
Backwash
Volume, % of inflow
Pressure, Ib/in.2
74-167
105
stainless steel or plastic
14-16
30-65
55
70-150
75-90
0.02-2.5
50
ft/s X 0.305 = m/s
gal/ft2/min X 2.445 = m3/m2/h
Ib/in2 X 0.0703 = kg/cm2
(EPA-600/8-77/014)
Filtration
Dual-Media High-Rate Filtration (DMHRF) (>8 gal/ft2/min (20 m3/m2/h)) removes
small participates that remain after screening and floe remaining after
polyelectrolyte and/or coagulant addition. As implied this provides a high level of
treatment that can be applied after screening together with automated operation
and limited space requirements. To be most effective, filtration through media that
are graded from coarse to fine in the direction of flow is desirable. A single filter
material with constant specific gravity cannot conform to this principle since
backwashirig of the bed automatically grades the bed from coarse to fine in the
direction of washing; however, the concept can be approached by using a two
layer bed. A typical case is the use of coarse anthracite particles on top of less
coarse sand. Since anthracite is less dense than sand, it can be coarse and still
remain on top of the bed after the backwash operation. Typically a unit is
comprised of 5 ft of No. 3 anthracite (effective size 0.16 in. (4.0 mm)) placed over
3 ft of No. 612 sand (effective size 0.08 in. (2.0 mm)). This arrangement was
shown superior to both coarser and finer media tested separately (U.S.
43
-------
Environmental Protection Agency 1972b). Another alternative would be an upflow
filter, but these units have limitations in that they cannot accept high hydraulic
loadings (filtration rates).
The principal parameters to be evaluated in selecting a DMHRF system are the
media size, media depth, and filtration rate. Since much of the removal of solids
from the water takes place within the filter media, their structure and composition
is of major importance. Too fine a medium may produce a high-quality effluent,
but also may cause excessive headlosses and extremely short filter runs. On the
other hand, media that are too coarse may fail to produce the desired clarity of the
effluent. Therefore, the selection of media for DMHRF should be made by pilot
testing using various materials in different proportions and at different flow rates.
Depth of media is limited by headless and backwash considerations. The deeper
the bed, the greater the headloss and the harder to clean. However, there should
be sufficient bed depth to retain the removed solids without break-through during
the filter run period at the design hydraulic loading.
Information is available on the use and design of DMHRF for treatment of drinking
water, but a number of pilot studies have also been done using CSO which should
provide more relevant information. The studies (U.S. Environmental Protection
Agency 1972b, 1979a, 1979b) used 6, 12, and 30 in. (15, 30, and 76 cm)
diameter filter columns, with anthracite and sand media with and without various
dosages of coagulants and/or polyelectrolytes. A preliminary (420 //m) screening
process was used upstream of the DMHRF to extend the treatment run time
before backwashing. It was found that SS removal increased as influent SS
concentration increased, and decreased as hydraulic loading increased.
Removal efficiency for the filter unit was about 65% for SS, 40% for BOD5 and
60% for chemical oxygen demand (COD). The addition of polyelectrolyte
increased the SS removal to 94%, the BOD5 removal to 65%, and the COD
removal to 65%. The length of filtration run averaged 6 h at a hydraulic loading of
24 gal/ft2/min (59 m3/m2/h).
Tables 6-5, 6-6, and 6-7 show removals of SS, BOD5, and heavy metals for a
study in New York, New York (U.S. Environmental Protection Agency 1979a).
Design parameters for DMHRF are presented in Table 6-8 (U.S. Environmental
Protection Agency 1977a).
44
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Table 6-5. CSO-DMHRF Average SS Removals (New York, NY)
No Chemicals
Poly Only
Poly & Alum
Plant
Influent
(mg/l)
175
209
152
Filter
Influent
(mg/l)
150
183
142
Filter
Effluent
(mg/l)
67
68
47
Filter
Removals
55
63
67
System
Removals
62
67
69
(EPA-600/2-79/015)
Table 6-6. CSO-DMHRF Average BOD6 Removals (New York, NY)
No Chemicals
Poly Only
Poly & Alum
Plant
Influent
(mg/l)
164
143
92
Filter
Influent
(mg/l)
131
129
85
Filter
Efifluent
(mg/l)
96
84
53
Filter
Removals
(%)
27
35
38
System
Removals
(%) s
41
41
43
(EPA-600/2-79/015)
Dissolved Air Flotation (DAF)
This is a unit operation used to separate solid particles or liquid droplets from a
liquid phase. Separation is brought about by introducing fine air bubbles into the
liquid phase. As the bubbles attach to the solid particles, the buoyant force of the
combined particle and air bubbles is great enough to cause the particle to rise.
Once the particles have floated to the surface, they are removed by skimming.
The most common process for forming the air bubbles is to dissolve air into the
waste stream under pressure and then release the pressure to allow the air to
come out of solution. The pressurized flow carrying the dissolved air to the
flotation tank is either (1) the entire stormwater flow, (2) a portion of the
stormwater flow (split flow pressur-ization), or (3) recycled DAF effluent.
Higher overflow rates (1.3 - 10.0 gal/ft2/min (3.2 - 25 m3/m2/h)) and shorter
detention times (0.2 -1.0 h) can be used for DAF when compared to conventional
settling (0.2 -0.7 gal/ft2/min (0.5 - 1.7 m3/m2/h]; 1.0 - 3.0 h). Studies for CSO
have shown that a treatment system consisting of screening (using a 297//m
45
-------
Table 6-7. Removal of Heavy Metals by DMHRF (New York, NY)
Cadmium Chromium Copper Mercury Nickel Lead Zinc
Average
removal, %' 56 50 39 0 13 65 48
'concentration basis
(EPA-600/2-79/015)
Table 6-8. Design Parameters for DMHRF
Filter Media Depth (ft)
No. 3 anthracite
No. 612 sand
4-5
2-3
Headloss (ft)
Backwash
5-30
Effective Size (mm)
Anthracite
Sand
Flux density (gal/ft2/min)
Range
Design
4
2
Volume (% of flow) 4-10
Air
Rate (standard (ft3/min/ft2) 10
Time (min) 10
Water
Rate gal/ft2/min) 60
Time (min) 15-20
8-40
24
ft X 0.305 - m
gal/ft2/min X 2.445 - m3/m2/h
standard ft3/min/ft2 X 0.305 = m3/m2/min
(EPA-600/8-77/014)
aperture with a hydraulic loading rate of 50 gal/ft2/min (122.3 m3/m2/h)) followed
by DAF can offer an effective level of treatment (U.S. Environmental Protection
Agency 1977c, 1979c).
The basis of the system being that the screening removes the particles that are too
heavy for the air bubbles to carry, and the DAF system removes the floating,
neutral buoyancy and remaining negative buoyancy particles. The addition of
chemical flocculent in the form of ferric chloride and cationic polyelectrolyte was
shown in the above two references to improve the removals. Table 6-9 shows the
screening-DAF system design parameters (U.S. Environmental Protection Agency
1977a).
46
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Table 6-9. Screening and DAF Design Parameters
Overflow Rate (gal/ft2/min)
Low rate
High rate
Horizontal Velocity (ft/min)
Detention Time (min)
Flotation cell range
Floatation cell average
Saturation tank
Mixing chamber
Pressurized Flow (% of total flow)
Split-flow pressurization
Effluent recycle pressurization
Air to Pressurized Flow Ratio {standard ft3/min/100 gal)
Air to Solids Ratio
Pressure in Saturation Tank (Ib/in.2)
Float
Volume (% of total volume)
Solids concentration (% dry weight basis)
1.3-4.0
4.0-10.0
1.3-3.8
10-60
25
1-3
1
20-30
25-45
1.0
0.05-0.35
40-70
0.75-1.4
1-2
gal/ft2/min X 2.445 - m3/m2/h
ft/min X 0.00508 = m/s
standard ft3/min/100 gal X 0.00747 = m3/min/100 I
Ib/in.2 X 0.0703 = kg/m2
(EPA-600/8-77/014)
As with the other treatment processes discussed there is not any data available for
treatment of separate storm runoff, however from the CSO data it would appear
that except for sedimentation, screening-DAF is likely lo be the most expensive
treatment system.
High-Gradient Magnetic Separation (HGMS)
This is a relatively new treatment technology for treatment of storm runoff or CSO
but has been used successfully for a number of years in the treatment of water for
or from industrial processes. A high degree of treatment is possible with this
process, which will probably be greater than required to meet permitting
requirements alone.
47
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In its simplest form, the high gradient magnetic separator consists of a canister
packed with a fibrous ferromagnetic material that is magnetized by a strong
external magnetic field (coils surround the canister). The water to be treated is
passed through the canister and the fibrous ferromagnetic matrix causes only a
small hydraulic resistance because it occupies less than 5% of the canister volume.
Upstream of the canister the water is prepared by binding finely divided magnetic
seed particles, such as magnetic iron oxide (magnetite), to the nonmagnetic
contaminants. Binding the magnetic seed is accomplished in two general ways:
adsorption of the contaminant to the magnetic seed; and chemical coagulation
(alum).
The magnetic particles are trapped on the edges of the magnetized fibers in the
canister as the water passes through. When the matrix has become loaded with
magnetic particles, they are easily washed off by turning off the magnetic field and
backflushing. Particles ranging in size from soluble through settleable (>0.001//m)
may be removed with this process and design parameters for HGMS are presented
in Table 6-10.
Table 6-10. Preliminary Design Parameters for High Gradient Magnetic Separators
Magnetic Field Strength, kG* 0.5-1.5
Maximum Flux Density, gal/ftz/min 100
Maximum Detention Tfme, min 3
Matrix Loading, g solids/g of matrix fiber 0.1-0.5
Magnetite Addition, mg/l 100-500
Magnetite to SS Ratio 0.4-3.0
Alum Addition, mg/l
Range
Average
Polyelectrolyte Addition, mg/l
90-120
100
0.5-1.0
'Kilogauss.
gal/ft2/min X 2.445 = m3/m2/h
(EPA-600/8-77/014)
48
-------
HGMS can offer rapid filtration for many pollutants with greater efficiency than for
sedimentation because the magnetic forces on the fine particles may be many
times greater than gravitational forces. Urban Stormwater Management and
Technology: Update and User's Guide (U.S. Environmental Protection Agency
1977a) provides details of bench and pilot scale studies which have been
conducted using HGMS to treat CSO (U.S. Environmental Protection Agency
1977a).
For HGMS and all other treatments that involve an additive to enhance the solids
removal, there is a need to accommodate the variation in storm runoff SS
concentrations. This will require automatic monitoring and adjustment of the
additive dosage for efficient operation.
Powdered Activated Carbon-Alum Coagulation
A treatment option which has the potential to remove dissolved organics is the use
of powdered activated carbon with alum added to aid in subsequent clarification.
This was demonstrated at a 100,000 gal/d (379 m3/d) pilot unit in Albany, New
York (Field 1990, U.S. Environmental Protection Agency 1973c) using municipal
sewage and CSO. A short flocculation period followed the addition of alum with
settling of solids by gravity and disinfection of the effluent or filtering (tri-media)
and disinfection prior to discharge.
Carbon regeneration in a fluidized bed furnace and alum recovery from the calcined
sludge were also demonstrated, as was reuse of the reclaimed chemicals. Average
carbon losses per regeneration cycle were 9.7%. Average removals were in
excess of 94% for COD, 94% for BOD5, and 99% for SS with no filtration.
Disinfection
Disinfection is generally practiced at WWTPs to control pathogenic
microorganisms. The development of disinfection techniques and measurement of
their effectiveness to kill pathogens has been mainly derived from the sanitary
wastewater field, where the concern has been to measure the presence of fecal
contamination and ability to kill any pathogens and viruses of human origin.
Because it is both difficult and expensive to isolate and measure specific pathogens
in water, methods were developed to monitor certain indicator organisms, i.e.,
microorganisms indicative of the presence of fecal contamination. Bacteria of the
total coliform (TC) group became the generally accepted indicator for fecal
pollution, but includes different genera which do not all originate from fecal wastes
(e.g., Citrobacter, Klebsiella, and Enterobacter).
An improvement over the TC test is the more selective fecal coliform (FC) test,
49
-------
which selects primarily for Klebsiella and Escherichia coli (E. coif) bacteria. £. coli
is the bacteria of interest because it is a consistent inhabitant of the intestinal tract
of humans and other warm-blooded animals. The FC test though, is still not
specific to enteric bacteria, and human-enteric bacteria in particular. In 1986 a US
EPA publication (U.S. Environmental Protection Agency 1986) recommended that
states "begin the transition process to the new (E. coli and enterococci)
indicators." However, many states still retain the TC and FC criteria and the most
widely used bacteriological criterion in the United States is the maximum of 200
FC/100 Ml.
For discharges of separate storm runoff the above criterion is unlikely to give a true
indication of the potential risk of infection, as many of these indicator bacteria also
originate from soils, vegetation, and animal feces. Stormwater runoff can contain
high densities of the nonhuman indicator bacteria and epidemiological studies of
recreational waters receiving stormwater runoff have found little correlation
between indicator densities and swimming related illnesses (U.S. Environmental
Protection Agency 1983b, 1984a, Calderon et at. 1991). In addition a number of
non-enteric pathogens found in stormwater runoff have been linked to respiratory
illnesses and skin infections, a risk which is not assessed by the present fecal
indicators.
Although the present standards and indicators are unlikely to reflect the actual
human disease contraction potential, i.e., pathogenicity of a storm flow and its
receiving water, they are the only practical standards available. Also urban storm
runoff has a high potential to be contaminated by sanitary cross connections which
would make the standards more relevant. Therefore, until other more relevant
indicators are developed and proven the present standards should be used but with
the caution that they may over or under estimate the true risk. The paper entitled
The Detection and Disinfection of Pathogens in Storm-Generated Flows (O'Shea
and Field 1992) covers this subject in more detail.
Conventional municipal sewage disinfection generally involves the use of chlorine
gas or sodium hypochlorite as the disinfectant. To be effective for disinfection
purposes, a contact time of not less than 15 min at peak flowrate and a chlorine
residual of 0.2 -2.0 mg/L are commonly recommended. However, a different
approach is required for storm runoff, because the flows have characteristics of
intermittency, high flowrate, high SS content, wide temperature variation, and
variable bacterial quality. Further aspects of disinfection practices which require
consideration for storm runoff are:
1- A residual disinfecting capability may not be permitted, because
chlorine residuals and compounds discharged to natural waters may
be harmful to human and aquatic life (i.e., formation of carcinogens,
50
-------
e.g., tri-halomethanes).
2- The coliform count is increased by surface runoff in quantities
unrelated to pathogenic organism concentration. Total coliform or
fecal coliform levels may not be the most useful indication of
disinfection requirements and efficiencies.
3- Discharge points requiring disinfection are often at outlying points on
the drainage system and require unmanned, automated installations.
The disinfectant used to treat storm runoff should be adaptable to intermittent use,
be effective, and be safe and easy to dose the effluent with. Table 6-11 shows
disinfectants that might be used for storm flow disinfection. Chlorine and
hypochlorite will react with ammonia to form chloramines and with phenols to form
chorophenols. These are toxic to aquatic life and the latter also produce taste and
odor in the water. Chlorine dioxide (CI02) does not react with ammonia and
completely oxidizes phenols. Ozone has a more rapid disinfecting rate than
chlorine, is effective in oxidizing phenols, and has the further advantage of
supplying additional oxygen to the effluent. The increased disinfecting rate of
ozone requires shorter contact times and results in a lower capital cost for a
contactor, as compared to that for a chlorine contact tank. Ozone does not
produce chlorinated hydrocarbons or a long lasting residual as chlorine does, but it
is unstable and must be generated on-site just prior to application. Therefore,
capital investment in a generating plant is required along with the operation and
maintenance.
Another disinfection technique that promises short detention times and the
absence of toxic reaction products is the use of ultraviolet (UV) light irradiation.
The effectiveness of the early systems was limited for water with high
concentrations of solids which tended to attenuate the UV energy. Later systems
emit higher intensity radiation for more effective treatment. More recently
modulated UV light has been reported to reduce viable bacteria by approximately
100 fold compared to populations observed after similar exposure to UV light that
lacked modulation (Bank et al. 1990).
The characteristics of storm runoff (i.e., intermittent and often high flows) together
with the need to minimize capital costs for a treatment operation, lend themselves
favorably to use of high rate disinfection. This refers to achieving either a given
percent or a given bacterial count reduction through the use of: decreased
disinfectant contact time; increased mixing intensity; increased disinfectant
concentration; chemicals having higher oxidizing rates; or various combinations of
these. Where contact times are less than 10 min, (usually in the range 1 - 5 min),
adequate mixing is a critical parameter, providing complete dispersion of the
51
-------
disinfectant and forcing disinfectant contact with the maximum number of
microorganisms. The more physical collisions high-intensity mixing causes, the
lower the contact time requirements. Mixing can be accomplished by mechanical
flash mixers at the point of disinfectant addition and at intermittent points, or by
specially designed plug flow contact chambers containing closely spaced,
corrugated parallel baffles that create a meandering path for the wastewater (U.S.
Environmental Protection Agency 1973b).
Table 6-11. Characteristics of Principal Storm Flow Disinfection Agents
Characteristics
Stability
Reacts with ammonia to
form chloramines
Destroys phenols
Produces a residual
Affected by pH
Hazards
Chlorine
Stable
Yes
At high
concentrations
Yes
More effective
at pH < 7.5
Toxic
Hypochlorite
6-month
half-life
Yes
At high
concentrations
Yes
More effective
at pH < 7.5
Slight
Chlorine
Dioxide
Unstable
No
Yes
Short-lived*
Slightly
Toxic;
Explosive
Ozpne
Unstable
No
Yes
No
No
Toxic
'Chlorine dioxide dissociates rapidly.
(EPA-600/8-77/014)
High-rate disinfection was shown (for CSO) to be enhanced beyond the expected
additive effect by sequential addition of CI2 followed by CI02 at intervals of 15 - 30
s (U.S. Environmental Protection Agency 1975a, 1976b). A minimum effective
combination of 8 mg/L of CI2 followed by 2 mg/L of CIO2 was effective in reducing
TC, FC, fecal streptococci, and viruses to acceptable target levels and compared to
25 mg/L CI2 or 12 mg/L CI02. It was surmised that the presence of free CI2 in
solution with chlorite ions (CIO'2/ (the reduced state of CI02)) may cause the
oxidation of CIO'2 back to its original state. This process would prolong the
existence of CI02, the more potent disinfectant.
An equation and concept to enable the effect of high rate mixing to be taken into
52
-------
account in the disinfection process is provided (U.S. Environmental Protection
Agency 1973b). A velocity gradient (G), as defined in the equation below is used
as a measure of the mixing intensity, but is also a measure of the opportunities for
microorganism and disinfectant matter collisions per unit time per unit volume.
G= (17'30/'^viscosity(centipoise)) (y/( velocity If t/sj) ([channel slope[£t/ft\)
The product of velocity gradient and contact time (GT) is the number of
opportunities for collisions per unit volume during the contact time.
It is important to note that if high-rate mixing is to be relied upon to provide
effective disinfection the velocity gradient should not reduce if the flowrate
reduces i.e., if the mixing intensity depends on the velocity of flow and not
mechanical mixing then the level of disinfection will be reduced at low flowrates.
There will be some offset of this due to longer detention times at lower flowrates
but the intensity of mixing will be the more significant parameter. Use of a Sutro
weir for the influent and effluent will help maintain the peak rate velocity at all
flowrates.
Swirl Regulators/Concentrators
These are compact flow throttling, and solids separation devices which also collect
floatable material. The performance of the swirl device is very dependant on the
settling characteristics of the solids in the stormwater. The EPA swirl is most
effective at removing solids with characteristics similar to grit (3*0.008 in. (0.2
mm) effective diameter, 2.65 specific gravity). It is important to appreciate this
aspect of swirl devices and not expect significant removals of fine and low specific
gravity solids.
The three most common configurations are the EPA swirl concentrator, the
Fluidsep™ vortex separator, and the Storm King™ hydrodynamic separator.
Although each of the separators is configured differently, the operation of each
unit and the mechanism for solids separation are similar. The flow enters the unit
tangentially and follows the perimeter wall of the cylindrical shell, creating a
swirling, quiescent vortex flow pattern. The swirling action throttles the influent
flow, and causes solids to be concentrated at the bottom of the unit. The throttled
underflow containing the concentrated solids passes out through an outlet in the
bottom of the unit, while the clarified supernatant passes out through the top of
the unit. Various baffle arrangements are provided to capture floatables in the
supernatant which are then usually carried out in the underflow as the storm
subsides and the water level in the swirl unit falls. During low flow conditions all
53
-------
of the flow passes out via the bottom outlet and only when the flow increases
does the throttling effect and build up of water in the swirl occur.
The solids separation is helped by the flow patterns, with the influent being
deflected into a slower moving inner swirl pattern after one revolution around the
perimeter of the swirl unit. Gravity separation occurs as particles follow a "long
path" through the outer and inner swirl. Solids separation is also assisted by the
shear forces set up between the inner and outer swirls, along the perimeter walls,
and bottom. An EPA swirl regulator/concentrator is shown in Figure 6-1.
The swirl device can offer a compact unit which functions as both a regulator for
flow control and a solids concentrator, and when combined with treatment of the
relatively heavy settleable solids can provide an effective treatment system. There
are a number of references (U.S. Environmental Protection Agency 1973a, 1974b,
1977d, 1982, 1984b) which provide performance and design information for the
EPA swirl regulator/concentrator. A degritter version of the EPA swirl has also
been developed (U.S. Environmental Protection Agency 1977d, 1981 a) which has
no underflow and only removes the grit (detritus) portion.
54
-------
Inflow
Overflow
A Inlet ramp
B Flow deflector
C Scum ring
D Overflow weir and weir plate
E Spoilers
F Floatables trap
G Foul sewer outlet
H Floor gutters
I Downshaft
J Secondary overflow weir
K Secondary gutter
Figure 6-1. Isometric view of a swirl combined sewer overflow
regulator/separator (U.S. EPA 1982K
55
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Section 7
Storage and Treatment Optimization
As stated previously, storage alone will only offer flow attenuation, and treatment
alone will only treat a fraction of the stormwater flow, or have such a large capacity
to handle peak flows that the costs will be prohibitive. Therefore combining the
storage/treatment, finding the best balance, and if possible using existing facilities is
likely to provide the most cost effective solution for treatment of urban storm runoff.
No two situations are likely to be the same but a cost analysis to produce curves of
storage alone, treatment alone, and the combined cost will produce an optimized cost
curve as shown in Figure 7-1. Factors such as the number of storms which are likely
to exceed the capacity of the combined system need to be taken into account, but
this approach will provide useful information on which to base a decision (Field et al.
1994, U.S. Environmental Protection Agency 1972b).
Due to the variable nature of storm events there will always be some storm events
which generate runoff in excess of the storage/treatment capacity. The excess runoff
will be treated by gravity settling in the storage basin prior to being discharged to the
receiving water. Use of the swirl regulator/concentrator or degritter described above
can provide some treatment to the runoff which is either diverted to storage
(alleviating bottom solids accumulation problems) or the receiving water.
56
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8.0
7.0
NOTES
I. TOTAL SYSTEM COSTS INCLUDE
STORAGE AND TREATMENT COSTS.
2. TREATMENT COSTS INCLUDE COST
OF INFLUENT PUMP STATION AND
HIGH RATE FILTRATION PLANT.
3. STORAGE COST IS BASED ON
CONCRETE TANK COSTING $ 700
PER 1000 GALLONS OFSTORAGED
VOLUME.
TOTAL
SYSTEM
COSTS-v
STORAGE
FACILITIES
COSTS
50 100 ISO 200
TREATMENT PLANT CAPACITY -(MGD)
Figure 7-1. Estimated capital costs of storage and treatment for 200 MGD
overflow (U.S. EPA 1972b).
57
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Section 8
Beneficial Reuse of Stormwater
The reuse of municipal wastewater for industry, non-potable domestic usages, and
groundwater recharge has been practiced for many years. In 1971 an EPA nationwide
survey estimated that current reuse of treated municipal wastewater for industrial
water supply, irrigation, and groundwater recharge was 53.5 billion gal/yr, 77 billion
gal/yr, and 12 billion gal/yr (200 million m3/yr, 290 million m3/yr, and 45 million
m3/yr), respectively (Environmental Protection Agency 1975b). It is reasonable to
expect that reuse of treated wastewater and/or stormwater for industrial cooling, non-
potable domestic water supply, and park and golf course irrigation will increase in the
future.
Many of the treatments discussed above are likely to produce an effluent quality
which is of a higher standard than that required to meet a stormwater permit. Where
there are suitable circumstances, an opportunity exists to take advantage of this
higher effluent quality for reuse of the storm runoff. The intended reuse will govern
the level of treatment required but careful selection, design, and use of pilot studies
should result in the required combination of the above technologies to achieved
required effluent quality.
The additional cost to provide treatment above that required to satisfy a discharge
permit will need to be less than the cost of water from other sources for economic
viability. With increasing demands on potable water supplies the concept of reuse,
in particular where a non-potable water quality standard is required, will make this an
increasingly more viable option. The report "Reclamation of Urban Stormwater" from
Integrated Stormwater Management provides details and a hypothetical case study
(Field eta/. 1993).
58
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07 ^US. GOVERNMENT PRINTING OFFICE: MM- SSMOI/002W
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