E AP Project Summary

United States

Environmental Protection

Agency

Stormwater Pollution

Abatement Technologies

Richard Field, Michael P. Brown, and William V. Vilkelis

Risk Reduction

Engineering Laboratory

Cincinnati, OH 45268

Research and Development

The report summarized here presents

information regarding best management

practices (BMPs) and pollution abatement 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 public education, regulations, and source

control of pollutants. Also covered are

source treatments of pollutants, which

include vegetative BMPs and infiltration practices. Uses and modifications

of installed drainage systems, types of

end-of-pipe treatments including biological, 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 flooding 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 optimal combination will be site specific and

depend on site characteristics, specific

pollutants involved, and cost considerations.

Regulations and BMPs are effective

tools in controlling urban stormwater runoff because they tend to be preventive in

nature. Mandating effluent limits and creating zoning laws are regulatory examples.

BMPs may include upgrading current systems, 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 infiltrate 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 treatment 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 characterEPA/600/SR-94/129 September 1994
2

istics of small and large storms can be

very different in terms of the runoff generated, pollutant load, and receiving water

impacts. Frequent small storms have a

more persistent effect, whereas less frequent large storms have a larger impact

but allow time for recovery between events.

For small storms, inaccurately estimating

the initial abstractions and the soil infiltration rates can significantly change the calculated storm runoff pollutant load. 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.

Strategy

Traditional wastewater treatment methods (i.e., secondary treatment processes)

tend to operate under conditions closer to

steady state and are usually unsuitable

for the fluctuating loads of stormwater runoff. On the other hand, technologies used

to control and treat combined sewer overflows (CSOs) are more suitable for

stormwater runoff. Successful stormwater

management to control urban storm runoff pollution requires an areawide approach

combining prevention, reduction, and treatment 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 pollution prevention and control plan requires

a structured strategy that should include:

defining existing conditions; setting sitespecific goals; collecting and analyzing

data; refining site-specific goals; assessing and ranking problems; screening and

selecting BMPs and treatment technologies; 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 subjected to reassessment once feedback on

implementation is available.

The report is concerned with an overview of the abatement technologies available 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 technologies cover approaches such as public education, regulations, and local ordinances

and mainly apply to the upstream collection 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 management of the watershed using BMPs to

prevent or control pollution at the source

is apt to offer the most cost effective solution 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 application, and some form of treatment before

discharge may be required.

Implementation of any stormwater management program needs to meet financial

and, probably, schedule restraints. Therefore, 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 enforcement of existing regulations to control soil erosion during construction

activities and adaptation of existing

stormwater storage intended for flood control so that it also provides quality control

for small storm events. New installations

should consider design for both flood control and pollutant removals.

Watershed Area Technologies

and Practices

As already stated, 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. The realistic evaluation for each practice includes: 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 have not been well established,

with most BMPs still in the development

stage. This is not to say that BMPs cannot be effective but rather that they do not

have a large enough 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.

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 attention to these details is apt 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 or nuisance, acceptance by the public, and assuming conservative performances in the design until the historical

data can justify a higher reliable performance.

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 probably is the

most cost effective. Mainly nonstructural

practices will be involved, and application

to new developments should be particularly 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 contacting contaminated surfaces or water by a

variety of structural means); exposure minimization (minimizing stormwater contact

with pollutants by structure and manage-
3

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 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.

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 installation of many of these practices and, in

these situations, their application will be

restricted.

Swales are generally grassed

stormwater conveyance channels that remove 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 operation.

Filter strips are vegetated strips of land

that act as “buffers” by accepting storm

runoff as overland sheet flow from upstream 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 capacity. Note that natural wetlands are covered by regulations that limit discharges

to the wetland and limit modifications to

enhance the wetland performance.

Detention Facilities

One of the most common structural controls for urban storm runoff and pollution

loading is the construction of local ponds

(including wetlands) to 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 is through settling of

the suspended solids (SS) with any associated 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 conditions.

Extended detention dry ponds temporarily 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 particulate pollutants; negligible soluble pollutant

removal; and quick accumulation of debris 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 influent and permanent pond water. They can

provide moderate to high removal of particulate 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 breeding problems. A forebay here also improves 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 simplest form involves maximizing the pervious 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 exists. Regulations that encourage the incorporation 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 minimize 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 industrial and commercial areas that 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, reducing the risk of

downstream flooding, recharging groundwater, 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., protective 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 involve 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 storage and self cleaning, respectively. Existing separate (or combined) drainage

systems can be modified for in-line storage 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 pollution 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 pollutant load. Treatment of the critical source

areas can, therefore, offer the potential

for a greater benefit than end-of-pipe or
4

drainage system control, to reduce downstream 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 connected to the conveyance network. Inline 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 solutions might be found if existing treatment

facilities can be used, such as connection

to an existing wastewater system.

The degree to which the existing conveyance 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 areas); suitable locations for installation of

control devices; and the reliability of the

installed control. It is essential that accurate details of the existing system be collected from field surveys and as-built

drawings. This allows the assessment of

the storage capacity, number and locations 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 system 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 advantage of being connected to a real-time

control (RTC) system, which can be adjusted 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, reversetainter gates, float-controlled gates, and

motor-operated or hydraulic gates.

Some of the above are relatively inexpensive, quick to install, and effective

means of increasing storage. Also some

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 achieved

by diverting flow from the drainage conveyance system when a certain flowrate

is exceeded. The diverted water is stored

until sufficient capacity is available downstream. Storage can be provided by any

arrangement of basins, tanks, tunnels, etc.

If gravity filling and emptying are not possible, 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 sedimentation 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 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.

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 require regular pumpback of the accumulated 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 existing receiving water, permits will probably

be required.

Maintenance

Regular maintenance should be conducted for the drainage system and the

controls to work efficiently. This generally

consists of removing sediments from control devices, flushing drainage lines, and

conducting inspections to identify any problems. 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 provide 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 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 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 removals at higher overflow rates.

An alternative could be to construct additional primary treatment at a WWTP to

run in series with existing facilities during

dry-weather flow (DWF) for improved treatment 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 (described more fully in the report), which

may apply to a wider variety of stormwater

pollutants, are high gradient magnetic

separation and powdered activated carbon-alum coagulation. The extent of removals will depend on the SS

characteristics and the level of treatment

applied.

Screening can be divided into four categories with the size of the SS removed

directly related to the screen aperture size

(Table 1).
5

Table 1. Screening Categories

Screen Type Opening Size

Bar screen >1 in. (>25.4 mm)

Coarse screen 3/16-1 in. (4.8-25.4 mm)

Fine screen 1/250-3/16 in. (0.1-4.8 mm)

Microscreen <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. Consideration, however, must be given to stormwater characteristics of intermittent operation and to

possible very high initial loads, which may

not reflect WWTP operation characteristics. 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 (a coarse screen) are rugged 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 because of the filtering action of the mat

itself. This also increases the headloss

across the screen. One study showed

(on a 23 μm 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 concentration.

Generally, microscreens and fine

screens remove 25% to 90% of the SS,

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 m3/m2/h]) removes small

particulates that remain after screening

and floc 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 drinking water, but a number of pilot studies

have also been done with the use of CSO,

which should provide more relevant information. The studies on CSO used various diameter filter columns, with anthracite

and sand media with and without various

dosages of coagulants and/or polyelectrolytes. 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 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 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 the entire stormwater

flow, a portion of the stormwater flow (split

flow pressurization), or recycled DAF effluent.

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 consisting of screening (using a 297μm 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. 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 treatment system.

Disinfection of storm runoff requires a

different approach from conventional disinfection because the flows have characteristics of intermittency, higher rates, high

SS content, wide temperature variation,

and variable bacterial quality. Residual

disinfecting capability may not be permitted, as chlorine residuals and compounds

discharged to natural waters may be harmful to human and aquatic life. Coliform

counts are increased by surface runoff in

quantities unrelated to pathogenic organism concentration. Total or fecal coliform

levels may not be the most useful indication of disinfection requirements and efficiencies. Discharge points requiring

disinfection are often at outlying points on

the drainage system and require unmanned, automated installations. In addition, a number of nonenteric pathogens

found in stormwater runoff have been

linked to respiratory illnesses and skin infections.

Table 2 shows disinfectants that might

be used for storm flow disinfection. 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 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 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 to 5 min), adequate mixing is a critical parameter; it

provides complete dispersion of the disinfectant and forces disinfectant contact with

the maximum number of microorganisms.

Mixing can be done by mechanical flash

mixers at the point where disinfectant is

* Mention of trade names or commercial products does

not constitute endorsement or recommendation for

use.
6

Table 2. Characteristics of Principal Storm Flow Disinfection Agents

Characteristics Chlorine Hypochlorite Chlorine Dioxide Ozone

Stability Stable 6-mo half-life Unstable Unstable

Reacts with ammonia Yes Yes No No

to form chloramines

Destroys phenols At high concentrations At high concentrations Yes Yes

Produces a residual Yes Yes Short-lived No

Affected by pH More effective More effective Slightly No

at pH < 7.5 at pH < 7.5

Hazards Toxic Slight Toxic; Explosive Toxic

added 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.

Swirl regulators/concentrators are compact flow-throttling and solids-separation

devices that also collect floatable material. Swirls are compact units that function 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 effective at removing solids with characteristics

similar to grit (≥0.008 in. [0.2 mm] effective diameter, 2.65 specific gravity). It is

important to appreciate this aspect of swirl

devices and to not expect significant removals of fine and low specific gravity

solids.

The three most common configurations

are the EPA swirl concentrator, the

FluidsepTM vortex separator, and the Storm

KingTM 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 swirling, 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 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. It is reasonable to expect that reuse of treated wastewater 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 increasingly more viable option.
7

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

BULK RATE

POSTAGE & FEES PAID

EPA

PERMIT No. G-35

United States

Environmental Protection Agency

Center for Environmental Research Information

Cincinnati, OH 45268

Official Business

Penalty for Private Use

$300

EPA/600/SR-94/129