United States
Environmental Protection
Agency
Risk Reduction
Engineering Laboratory
Cincinnati, OH 45268
Research and Development
EPA/600/SR-94/129 September 1994
EPA Project Summary
Stormwater Pollution
Abatement Technologies
Richard Field, Michael P. Brown, and William V. Vilkelis
The report summarized here presents
information regarding best management
practices (BMPs) and pollution abate-
ment technologies that can provide
treatment of urban stormwater runoff.
The text includes a general approach
that considers small storm hydrology
and watershed practices that cover pub-
lic education, regulations, and source
control of pollutants. Also covered are
source treatments of pollutants, which
include vegetative BMPs and infiltra-
tion practices. Uses and modifications
of installed drainage systems, types of
end-of-pipe treatments including bio-
logical, chemical, and physical types
and storage and reuse of stormwater
are also covered.
This Project Summary was developed
by EPA's Risk Reduction Engineering
Laboratory, Cincinnati, OH, to announce
key findings of the research project
that is fully documented in a separate
report of the same title (see Project
Report ordering information at back).
Introduction
The full report covers the control and
treatment of stormwater in relation to the
removal or reduction of the stormwater
pollutant loads. Many of the pollution
abatement technologies discussed will help
attenuate stormwater flows. As they are
generally designed for small storm events,
however, they will not provide sufficient
capacity for the large events. Although
prevention of stormwater flooding is not
discussed, a drainage system design
should consider both pollutant and flood-
ing aspects of stormwater.
Strategically, the best way to control
and treat urban stormwater runoff is
through a combination of regulations,
BMPs, and treatment processes. The op-
timal combination will be site specific and
depend on site characteristics, specific
pollutants involved, and cost consider-
ations.
Regulations and BMPs are effective
tools in controlling urban stormwater run-
off because they tend to be preventive in
nature. Mandating effluent limits and cre-
ating zoning laws are regulatory examples.
BMPs may include upgrading current sys-
tems, developing proper management
techniques, using the existing drainage
systems for in-line or in-sewer storage, or
creating off-line storage facilities.
Designing devices that intercept or infil-
trate stormwater runoff back into the
groundwater system before it is introduced
into the stormwater or combined sewer
conveyance system can greatly save costs
in the design and construction of treat-
ment facilities. Examples of such devices
are swales, filter strips, porous pavement,
and stormwater wetlands.
General Approach and Strategy
Small Storm Hydrology
The selection of suitable abatement
technologies requires an understanding of
the size and distribution of storm events.
Generally, the smaller storm events are
the critical storms to consider, because
for many parts of the country, 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. The character-
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istics of small and large storms can be
very different in terms of the runoff gener-
ated, pollutant load, and receiving water
impacts. Frequent small storms have a
more persistent effect, whereas less fre-
quent large storms have a larger impact
but allow time for recovery between events.
For small storms, inaccurately estimating
the initial abstractions and the soil infiltra-
tion rates can significantly change the cal-
culated storm runoff pollutant load. Initial
abstractions include the rainfall depth re-
quired to satisfy surface wetting, surface
depression storage, interception by hang-
ing vegetation, and evaporation. Together
with soil infiltration rates, the initial ab-
stractions need to be accurately estimated
to calculate the storm runoff volume.
Strategy
Traditional wastewater treatment meth-
ods (i.e., secondary treatment processes)
tend to operate under conditions closer to
steady state and are usually unsuitable
for the fluctuating loads of stormwater run-
off. On the other hand, technologies used
to control and treat combined sewer over-
flows (CSOs) are more suitable for
stormwater runoff. Successful stormwater
management to control urban storm run-
off pollution requires an areawide approach
combining prevention, reduction, and treat-
ment practices/technologies. It is unlikely
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 pol-
lution prevention and control plan requires
a structured strategy that should include:
defining existing conditions; setting site-
specific goals; collecting and analyzing
data; refining site-specific goals; assess-
ing and ranking problems; screening and
selecting BMPs and treatment technolo-
gies; and, implementing, monitoring, and
reevaluating the plan.
This strategy will provide the control
goal(s) to be achieved — the goal(s) that
are then used as the basis for selection of
suitable technologies or approaches. The
goal(s) should initially be broad and not
specific as 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 must also be sub-
jected to reassessment once feedback on
implementation is available.
The report is concerned with an over-
view of the abatement technologies avail-
able and reviews the technologies by
separating the drainage system into three
physical areas:
• watershed area (i.e., storm runoff
generation/collection area),
• installed and/or modified/natural
drainage system (i.e., conveyance
pipes, channels, storage, etc.), and
• end-of-pipe (i.e., point source).
Technologies applicable to each of these
areas can be divided into structural and
nonstructural. The nonstructural technolo-
gies cover approaches such as public edu-
cation, regulations, and local ordinances
and mainly apply to the upstream collec-
tion area. The structural approaches are
the main options for the drainage system
and end-of-pipe areas and tend to be the
more expensive items.
The optimal solution is likely to be an
integrated approach that employs several
practices and technologies. The manage-
ment of the watershed using BMPs to
prevent or control pollution at the source
is apt to offer the most cost effective solu-
tion and tends to be the basis of many
stormwater management plans. BMPs,
the preferred option, are, however, not
always feasible or sufficient to achieve
the control objectives by themselves. For
older and more heavily urbanized areas,
BMPs are likely to have limited applica-
tion, and some form of treatment before
discharge may be required.
Implementation of any stormwater man-
agement program needs to meet financial
and, probably, schedule restraints. There-
fore, an early review and improved use of
existing facilities offers several advantages.
These options, probably the quickest and
least costly to be implemented, must also
meet the objectives developed from the
earlier stormwater management planning
process. Examples might include the en-
forcement of existing regulations to con-
trol soil erosion during construction
activities and adaptation of existing
stormwater storage intended for flood con-
trol so that it also provides quality control
for small storm events. New installations
should consider design for both flood con-
trol and pollutant removals.
Watershed Area Technologies
and Practices
As already stated, BMPs are not suit-
able in every situation. It is important to
understand which BMPs are suitable for
the site conditions and can also achieve
the required goals. The realistic evalua-
tion for each practice includes: the techni-
cal feasibility, implementation costs, and
long-term maintenance requirements and
costs. It is also important to appreciate
that the reliability and performance of many
BMPs have not been well established,
with most BMPs still in the development
stage. This is not to say that BMPs can-
not be effective but rather that they do not
have a large enough bank of historical
data on which to base design to be confi-
dent 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 pre-
dicting performance, but showing prom-
ise, are sand filter beds, wetlands, and
infiltration basins.
The reported poor performance of some
of the BMPs is likely to be a function of:
the design, installation, maintenance, and/
or suitability of the area. Greater atten-
tion to these details is apt to significantly
reduce the failure rate of BMPs. Other
important design considerations include:
safety for maintenance access and opera-
tions, hazards to the general public through
safety or nuisance, acceptance by the pub-
lic, and assuming conservative perfor-
mances in the design until the historical
data can justify a higher reliable perfor-
mance.
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 pri-
mary objective because it probably is the
most cost effective. Mainly nonstructural
practices will be involved, and application
to new developments should be particu-
larly effective.
Source Control of Pollutants. Both
nonstructural and structural practices can
be used to prevent pollutants coming into
contact with the stormwater and hence
storm runoff. Management and structural
practices include: flow diversion (keeping
uncontaminated stormwater from contact-
ing contaminated surfaces or water by a
variety of structural means); exposure mini-
mization (minimizing stormwater contact
with pollutants by structure and manage-
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ment); mitigation (plans to recover released
or spilled pollutants in the advent of a
release); prevention (monitoring techniques
intended to prevent releases); control of
sediment and erosion; and infiltration.
Source Treatment, Flow Attenuation,
and Storm Runoff Infiltration. These are
mainly structural practices to provide up-
stream pollutant removal at the source,
controlled stormwater release to the down-
stream conveyance system, and ground
infiltration or reuse of the stormwater.
Upstream pollutant removal provides treat-
ment of stormwater runoff at the specific,
highly polluting locations where it enters
the stormwater conveyance system. Ar-
eas of this type include but are not limited
to vehicular parking areas, vehicular ser-
vice stations, bus depots, industrial load-
ing areas, etc.
Source Treatment, Flow
Attenuation, and Storm Runoff
Infiltration
Vegetative BMPs
These practices have been the subject
of many publications in the last 20 yr.
Existing urbanized areas are unlikely to
have the land space available for installa-
tion of many of these practices and, in
these situations, their application will be
restricted.
Swales are generally grassed
stormwater conveyance channels that re-
move pollutants by filtration through the
grass and infiltration through the soil. A
slow velocity of flow, <1.5 ft/s (<46 cm/s),
a nearly flat longitudinal slope, <5%, and
a vertical stand of dense vegetation higher
than the water surface, >6 in. (15 cm)
total height, are important for effective op-
eration.
Filter strips are vegetated strips of land
that act as "buffers" by accepting storm
runoff as overland sheet flow from up-
stream developments before discharge to
the storm drainage system. Filter strips
provide potential treatment mechanisms
similar to that of swales.
Stormwater wetlands can be natural,
modified natural, or constructed wetlands
that remove pollutants by sedimentation,
plant uptake, microbial decomposition,
sorption, filtration, and exchange capac-
ity. Note that natural wetlands are cov-
ered by regulations that limit discharges
to the wetland and limit modifications to
enhance the wetland performance.
Detention Facilities
One of the most common structural con-
trols for urban storm runoff and pollution
loading is the construction of local ponds
(including wetlands) to collect storm run-
off, hold it long enough to improve its
quality, and release it to receiving waters
in a controlled manner. The basic re-
moval mechanism is through settling of
the suspended solids (SS) with any asso-
ciated pollutants, but controlled release
will also attenuate the stormwater flows,
which can benefit receiving streams that
suffer from erosion and disturbance of
aquatic habitat during peak flow condi-
tions.
Extended detention dry ponds tempo-
rarily detain a portion of stormwater runoff
for up to 48 h (24 h is more common)
using an outlet control. They provide:
moderate but variable removal of particu-
late pollutants; negligible soluble pollutant
removal; and quick accumulation of de-
bris and sediment. Performance can be
enhanced by using a forebay to allow
sedimentation and easier removal from
one area.
Wet ponds have greater capacity than
the permanent volume of the pond; this
permits storage of the stormwater runoff
and controlled release of the mixed influ-
ent and permanent pond water. They can
provide moderate to high removal of par-
ticulate pollutants and reliable removal
rates with pool sizes ranging from 0.5 to
1.0 in. (12.7 to 25.4 mm) of storm runoff
per impervious acre. Wet ponds offer
better removals and less maintenance than
do dry ponds. But they need to be well
designed to ensure beneficial use and not
cause aesthetic, safety, or mosquito breed-
ing problems. A forebay here also im-
proves performance and maintenance.
Infiltration Practices
These practices have a high potential
to control stormwater runoff by disposing
of it at a local site. Infiltration in its sim-
plest form involves maximizing the pervi-
ous area of available ground 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 ex-
ists. Regulations that encourage the in-
corporation of a high proportion of pervious
areas, particularly for new developments,
can be effective; however, soil and water
table conditions have to be suitable, a
conservative design has to be used, and
maintenance has to be undertaken to mini-
mize the possibility of system failure. The
possible effects the storm runoff could
have on the groundwater must also be
considered. These could range from a
relatively minor local raising of the water
table that results in reduced infiltration
rates to more serious pollution of the
groundwater, particularly if this is also used
as a water source. In many cases
stormwater runoff will have low levels of
pollution; however, the long-term effects
of pollutant buildup in the soil and/or
groundwater from storm runoff infiltration
is not well known. Therefore, infiltration of
urban storm runoff, especially from indus-
trial and commercial areas that have higher
levels of pollution, should be treated with
caution.
Infiltration of storm runoff can offer sig-
nificant advantages of controlling storm
runoff at the source, reducing the risk of
downstream flooding, recharging ground-
water, and supplying groundwater to
streams (i.e., low-flow augmentation or
maintaining stream flow during dry-weather
periods). These advantages need to be
judged against any pollution risks from
urban runoff.
Infiltration trenches, infiltration basins,
and porous pavement are all applications
of infiltration practices. Performance of
these applications can be improved
through regular maintenance, protective
practices against clogging (e.g., protec-
tive screening from nearby construction),
grass filter strips to filter out particulates,
and sub-surface piping installed to direct
the stormwater away.
Installed Drainage System
Control practices that can be applied to
the drainage system are relatively limited,
especially for existing systems, and in-
volve the removal of illicit or inappropriate
cross-connections, catchbasin and inlet
cleaning, critical source area treatment
devices, infiltration, and in-line and off-line
storage.
New separate (or combined) systems
can take advantage of increasing the pipe
size and gradient to provide in-line stor-
age and self cleaning, respectively. Exist-
ing separate (or combined) drainage
systems can be modified for in-line stor-
age by adding flow control devices (weirs,
flow regulators, etc.).
Established urban areas with separate
stormwater drainage systems are most
likely to have an existing stormwater pol-
lution problem that needs to be rectified.
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 pol-
lutant load. Treatment of the critical source
areas can, therefore, offer the potential
for a greater benefit than end-of-pipe or
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drainage system control, to reduce down-
stream pollutant loads. Potential critical
sources include: vehicle service, garage,
or parking areas; storage and transfer
yards; and industrial materials handling
areas exposed to precipitation.
In-line Storage
In-line storage uses the unused volume
in the drainage system network of pipes
and channels to store storm runoff that
can also be provided by storage tanks,
basins, tunnels, or surface ponds con-
nected to the conveyance network. In-
line storage will probably not offer any
treatment in itself as the intent will be to
make the system self-cleaning to reduce
maintenance requirements. If storage is
combined with an end-of-pipe treatment,
however, the flow attenuation will help
equalize the load to the treatment process
and, hence, optimize the treatment plant
size and costs. Other cost effective solu-
tions might be found if existing treatment
facilities can be used, such as connection
to an existing wastewater system.
The degree to which the existing con-
veyance system can be used for storage
is a function of: pipe or channel sizes;
pipe or channel gradient (relatively flat
lines provide the most storage capacity
without susceptibility to flooding low ar-
eas); suitable locations for installation of
control devices; and the reliability of the
installed control. It is essential that accu-
rate details of the existing system be col-
lected from field surveys and as-built
drawings. This allows the assessment of
the storage capacity, number and loca-
tions of controls, and risk of upstream
flooding. In new drainage system design,
conveyance pipes and channels can be
up-sized and hydraulic controls can be
designed into the system for added sys-
tem storage and routing.
Controls to restrict flow can either be
fixed or adjustable. Fixed systems will
probably be cheaper and require less
maintenance. Some examples of fixed
regulators are: orifices, weirs (lateral and
longitudinal), steinscrews, hydrobrakes,
wirbeldrossels, swirls, and stilling-pond
weirs.
Adjustable systems can offer the ad-
vantage of being connected to a real-time
control (RTC) system, which can be ad-
justed to hold back or release stormwater,
to maximize storage capacity of the whole
drainage system. The sophistication of
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 adjustable regulators are: inflatable
dams, tilting plate regulators, reverse-
tainter gates, float-controlled gates, and
motor-operated or hydraulic gates.
Some of the above are relatively inex-
pensive, quick to install, and effective
means of increasing storage. Also some
will concentrate the heavier solids in the
stored storm runoff for a more concen-
trated later release.
Off-line Storage
This refers to storage that is achieved
by diverting flow from the drainage con-
veyance system when a certain flowrate
is exceeded. The diverted water is stored
until sufficient capacity is available down-
stream. Storage can be provided by any
arrangement of basins, tanks, tunnels, etc.
If gravity filling and emptying are not pos-
sible, pumping the water into or out of
storage is involved.
Off-line storage can be designed to be
relatively self-cleaning or to have facilities
to resuspend the settleable solids. It can
also be used to provide treatment by sedi-
mentation with the sludge either collected
or diverted to a wastewater treatment plant
(WWTP).
Flow balance method (FBM) provides a
means of storing discharged urban storm
runoff in the receiving water. This is done
by forming a tank with the use of flexible
plastic curtains suspended from pontoons.
The curtains are anchored to the receiv-
ing 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 conven-
tional concrete and steel tank systems.
The FBM requires a suitable location
and has limits on performance: a certain
amount of mixing exists with the receiving
water, not all the stored volume will be
pumped back, and settleable solids re-
quire regular pumpback of the accumu-
lated sediment. The quick construction
potential of the FBM could favor the use
of this system as a temporary measure in
cases of a severe problem that needs
attention. Since the FBM uses the exist-
ing receiving water, permits will probably
be required.
Maintenance
Regular maintenance should be con-
ducted for the drainage system and the
controls to work efficiently. This generally
consists of removing sediments from con-
trol devices, flushing drainage lines, and
conducting inspections to identify any prob-
lems. Maintenance minimizes buildup of
materials that can be flushed out by a
surge from a large storm event and,
thereby, minimizes the shock loading
caused by intermittent storm events.
End-of-Pipe Treatment
Use of Existing Treatment
Facilities
Use of existing facilities is apt to pro-
vide cost effective treatment as long as
an economic means of connecting the
stormwater drainage system to the facility
is possible.
Spare capacity at the WWTP is one
option, particularly if storage can be pro-
vided to equalize the storm runoff load.
Even if the biological system has very
little capacity, the primary treatment sys-
tems can often function well at somewhat
higher overflow rates that, 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 does
sanitary sewage, which will benefit remov-
als at higher overflow rates.
An alternative could be to construct ad-
ditional primary treatment at a WWTP to
run in series with existing facilities during
dry-weather flow (DWF) for improved treat-
ment of DWF and to run in parallel during
wet-weather flow for some control over
the total flow. Also, use of any storage
facilities, either at an end-of-pipe or at an
upstream location, can provide treatment
by sedimentation or storage to be released
when treatment capacity is available.
Physical/Chemical Treatment
Physical/chemical treatment processes
generally offer: good resistance to shock
loads, ability to consistently produce a low
SS effluent, and adaptability to automatic
operation. Those described below are only
suitable for removal of SS and associated
pollutants. Other treatment methods (de-
scribed more fully in the report), which
may apply to a wider variety of stormwater
pollutants, are high gradient magnetic
separation and powdered activated car-
bon-alum coagulation. The extent of re-
movals will depend on the SS
characteristics and the level of treatment
applied.
Screening can be divided into four cat-
egories with the size of the SS removed
directly related to the screen aperture size
(Table 1).
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Table 1. Screening Categories
Screen Type Opening Size
Bar screen
Coarse screen
Fine screen
Microscreen
>1 in. (>25.4 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 con-
junction with the fine or microscreens.
Design of screens can be similar to that
for WWTP and CSO. Consideration, how-
ever, must be given to stormwater char-
acteristics of intermittent operation and to
possible very high initial loads, which may
not reflect WWTP operation characteris-
tics. 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. Cat-
enary screens (a coarse screen) are rug-
ged and reliable and commonly used for
CSO facilities. Therefore, they are likely
to be a good screen for use with storm
runoff.
Fine screens and microscreens have
been developed and used for SS removal
from CSO. The removal efficiency of
screening devices is dependent on the
aperture (size of opening) of the screen
placed on the unit, making these devices
very versatile. 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 increases with
the increasing thickness of filter mat be-
cause of the filtering action of the mat
itself. This also increases the headless
across the screen. One study showed
(on a 23 |im aperture microscreen
[Microstrainer*]) that with a large variation
in the influent SS, the effluent SS stayed
relatively constant (e.g., both 1000 mg/L
and 20 mg/L influent SS would give a 10
mg/L effluent SS). Accordingly, treatment
efficiencies vary with influent concentra-
tion.
Generally, microscreens and fine
screens remove 25% to 90% of the SS,
* Mention of trade names or commercial products does
not constitute endorsement or recommendation for
use.
and 10% to 70% of the BOD5, depending
on the screen aperture used and the
wastewater being treated.
Dual-media high-rate filtration (DMHRF)
(>8 gal/ft2/min [20 nf/nf/h]) removes small
particulates that remain after screening
and floe remaining after polyelectrolytes
and/or coagulants are added. As implied,
this provides a high level of treatment that
can be applied after screening together
with automated operation and limited
space requirements. Typically a unit is
composed of 5 ft of No. 3 anthracite coal
(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.
Information is available on the use and
design of DMHRF for treatment of drink-
ing water, but a number of pilot studies
have also been done with the use of CSO,
which should provide more relevant infor-
mation. The studies on CSO used vari-
ous diameter filter columns, with anthracite
and sand media with and without various
dosages of coagulants and/or polyelectro-
lytes. 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 re-
moval to 65%. The average filtration run
was 6 h at a hydraulic loading of 24 gal/
ft2/min (59 m3/m2/h). SS removal increased
as influent SS concentration increased and
decreased as hydraulic loading increased.
Dissolved air flotation (DAF) separates
solid particles or liquid droplets from a
liquid phase 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 par-
ticle 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 pressur-
ized flow carrying the dissolved air to the
flotation tank is either the entire stormwater
flow, a portion of the stormwater flow (split
flow pressurization), or recycled DAF ef-
fluent.
Higher overflow rates (1.3 to 10.0 gal/
ft2/min [3.2 to 25 m3/m2/h]) and shorter
detention times (0.2 to 1.0 h) can be used
for DAF when compared with conventional
settling (0.2 to 0.7 gal/ft2/min [0.5 to 1.7
m3/m2/h]; 1.0 to 3.0 h). Studies for CSO
have shown that a treatment system con-
sisting of screening (using a 297|im aper-
ture with a hydraulic loading rate of 50
gal/ft2/min [122.3 m3/m2/h]) followed by
DAF can offer an effective level of treat-
ment. The addition of chemical flocculent
in the form of ferric chloride and cationic
polyelectrolyte was also shown to improve
the removals. There are no data available
for treatment of separate storm runoff;
however, from the CSO data, it would
appear that, except for sedimentation,
screening DAF is the most expensive treat-
ment system.
Disinfection of storm runoff requires a
different approach from conventional dis-
infection because the flows have charac-
teristics of intermittency, higher rates, high
SS content, wide temperature variation,
and variable bacterial quality. Residual
disinfecting capability may not be permit-
ted, as chlorine residuals and compounds
discharged to natural waters may be harm-
ful to human and aquatic life. Coliform
counts are increased by surface runoff in
quantities unrelated to pathogenic organ-
ism concentration. Total or fecal coliform
levels may not be the most useful indica-
tion of disinfection requirements and effi-
ciencies. Discharge points requiring
disinfection are often at outlying points on
the drainage system and require un-
manned, automated installations. In addi-
tion, a number of nonenteric pathogens
found in stormwater runoff have been
linked to respiratory illnesses and skin in-
fections.
Table 2 shows disinfectants that might
be used for storm flow disinfection. Con-
ventional municipal sewage disinfection
generally involves the use of chlorine gas
or sodium hypochlorite as the disinfec-
tant. To be effective for disinfection pur-
poses, a contact time of not less than 15
min at peak flowrate and a chlorine re-
sidual of 0.2 to 2.0 mg/L are commonly
recommended.
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 fa-
vorably to use of high-rate disinfection.
This refers to achieving either a given
percent or a given bacterial count reduc-
tion through the use of: decreased disin-
fectant contact time, increased mixing
intensity, increased disinfectant concen-
tration, chemicals having higher oxidizing
rates, or various combinations of these.
Where contact times are less than 10 min
(usually in the range 1 to 5 min), ad-
equate mixing is a critical parameter; it
provides complete dispersion of the disin-
fectant and forces disinfectant contact with
the maximum number of microorganisms.
Mixing can be done by mechanical flash
mixers at the point where disinfectant is
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Table 2. Characteristics of Principal Storm Flow Disinfection Agents
Characteristics
Chlorine
Hypochlorite
Chlorine Dioxide
Ozone
Stability
Reacts with ammonia
to form chloramines
Destroys phenols
Produces a residual
Affected by pH
Hazards
Stable
Yes
At high concentrations
Yes
More effective
atpH<7.5
Toxic
6-mo half-life
Yes
At high concentrations
Yes
More effective
atpH<7.5
Slight
Unstable
No
Yes
Short-lived
Slightly
Toxic; Explosive
Unstable
No
Yes
No
No
Toxic
added and at intermittent points, or by
specially designed plug flow contact cham-
bers containing closely spaced, corrugated
parallel baffles that create a meandering
path for the wastewater.
Swirl regulators/concentrators are com-
pact flow-throttling and solids-separation
devices that also collect floatable mate-
rial. Swirls are compact units that func-
tion as both a regulator for flow control
and as a solids concentrator and, when
combined with treatment of the relatively
heavy settleable solids, can provide an
effective treatment system. Performance
of swirls is very dependent on the settling
characteristics of the solids in the
stormwater. The EPA swirl is most effec-
tive at removing solids with characteristics
similar to grit (>0.008 in. [0.2 mm] effec-
tive diameter, 2.65 specific gravity). It is
important to appreciate this aspect of swirl
devices and to not expect significant re-
movals 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 separator is configured differently,
operation and the mechanism for solids
separation are similar. Flow enters the
unit tangentially and follows the perimeter
wall of the cylindrical shell, creating a swirl-
ing, quiescent vortex flow pattern. The
swirling action throttles the influent flow
and causes solids to be concentrated at
the bottom of the unit. A degritter version
of the EPA swirl has also been developed
that has no underflow and only removes
the grit (detritus) portion.
Beneficial Reuse of Stormwater
The reuse of municipal wastewater for
industry, nonpotable domestic usages, and
groundwater recharge has been practiced
for many years. In 1971, an EPA nation-
wide survey estimated that current reuse
of treated municipal wastewater for indus-
trial water supply, irrigation, and ground-
water 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. It is reason-
able to expect that reuse of treated waste-
water and/or stormwater will increase in
the future.
Many of the treatments discussed are
apt to produce an effluent quality of a
higher standard than that required to meet
a stormwater permit. The intended reuse
will govern the level of treatment required,
but careful selection, design, and use of
pilot studies should result in the required
effluent quality.
Increasing demands on potable water
supplies, in particular where a nonpotable
water quality standard is required, will
make the concept of reuse an increas-
ingly more viable option.
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Richard Field (also the EPA Project Officer, see below), Michael P. Brown,
and William V. Vilkelis are with the Risk Reduction Engineering Laboratory,
Edison, NJ 08837-3679.
The complete report, entitled "Stormwater Pollution Abatement Technologies,"
(Order No. PB95-100053AS; Cost: $19.50, subject to change) will be
available from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
Copies will also be available free of charge until supply is exhausted from:
ORD Publications
U.S. EPA-CERI, G-72
Cincinnati, OH 45268
Telephone: 513-569-7562
FAX: 513-569-7566
The EPA Project Officer can be contacted at:
Risk Reduction Engineering Laboratory
U. S. Environmental Protection Agency
Edison, NJ 08837-3679
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
Official Business
Penalty for Private Use
$300
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
EPA/600/SR-94/129
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