Slotted PVC pipe

Infiltration

discharge pipe

Inflow from

catch basin

Outlet contol valve

Slotted PVC pipe

Grit-filter bag

Inverted elbow

Skimmers

Constructed

wetland

Source: StormTreatTM Systems, 1998.

FIGURE 1 STORMTREATTMSYSTEM

United States

Environmental Protection

Agency

Office of Water

Washington, D.C.

EPA 832-F-99-044

September 1999

Storm Water 

Technology Fact Sheet

Modular Treatment Systems

DESCRIPTION

This fact sheet describes modular systems for

treating storm water. One of the primary modular

storm water treatment systems currently on the

market is the StormTreatTM System, or STS. The

STS, which was developed in 1994, is a storm water

treatment technology consisting of a series of

sedimentation chambers and constructed wetlands.

These wetlands are contained within a modular, 2.9-

meter (9.5 feet) diameter recycled-polyethylene

tank. The STS can be applied in many different

scenarios, ranging from residential areas to most

industrial parks, but should not be used in extremely

polluted areas, such as directly in wastewater

streams. Figure 1 is a diagram of the STS. The

STS works as follows: influent is piped into the

unit’s sedimentation chambers, where pollutants are

removed through sedimentation and filtration.

Storm water is then conveyed from the

sedimentation chambers to a surrounding

constructed wetland. Unlike most constructed

wetlands systems, STS conveys the storm water

directly into the subsurface of the wetland and

through the root zone. Pollutants are then removed

through filtration, adsorption, and biochemical

reactions. These processes occur at higher rates

within the root zone, making STS more efficient in

pollutant removal. Storm water is retained in the

wetlands for five to ten days prior to discharge.
The STS is suitable for use throughout the U.S.;

however, the system may require modification to

function in different environments. For example, as

an option in dry climates such as in the

southwestern U.S., STS has designed a solarpowered water pump to redirect water that is stored

in the bottom of the system to the wetland plants.

In addition, in arid regions such as these that do not

have enough groundwater to support the wetland

vegetation, the unit may be altered to release flow at

a slower rate, thereby increasing the amount of

water retained in the bottom of the unit; or it may

be designed with soils that retain water more

efficiently. Alternatively, the unit could have a

backup water supply to provide for extended dry

periods.

The STS design can be modified for areas with high

groundwater levels or tidal influence. In areas with

high groundwater, the discharge pipework can be

modified so that runoff is discharged downgradient

to an area with a lower water table. In tidallyinfluenced areas, a check valve can be installed to

prevent flow from re-entering the unit at its

discharge point. This will also allow discharge to be

released only during mid- to low-tide conditions.

Over 100 STS units have been installed nationwide,

including installations in California, Washington,

Oregon, Oklahoma, North Carolina, South Carolina,

Maryland, New York, Connecticut, New

Hampshire, Maine, Rhode Island, and

Massachusetts. An STS has been operating in

Kingston, Massachusetts, since November 1994.

This unit was installed to prevent bacterial

contamination from storm runoff from harming

shellfish beds in the Jones River. Additional systems

have been recently installed in various parts of

Massachusetts, as well as in Maine. In Hingham,

MA, six STSs were installed in an industrial park

bordering a wetland that is a tributary to a drinking

water supply. These STSs have been successful in

preventing contamination of the water supply. In

Ipswich, MA, and Barnstable, MA, several STS

tanks were installed to treat road and parking lot

drainage to prevent discharges to sensitive receiving

waters. Finally, in Manchester, ME, five STSs were

installed to help reduce the levels of phosphorus in

storm water effluent after new regulations tightened

runoff standards for phosphorous.

APPLICABILITY

The STS has applications in a wide range of

settings. The system's size and modular

configuration make it adaptable to a wide range of

site constraints and watershed sizes. Designers of

the system indicate that the system can be used to

treat runoff from highways, parking lots, airports,

marinas, and commercial, industrial, and residential

areas. The STS is an appropriate storm water

treatment technology for both coastal and inland

areas but is not designed to be used directly in

wastewater streams.

ADVANTAGES AND DISADVANTAGES

Regulators and environmental groups in

Massachusetts are utilizing storm water

management practices, including the STS, to

improve water quality in the shellfish beds located

downstream from potentially contaminated runoff.

The STS also protects groundwater by removing

pollutants prior to infiltration. The STS has shown

high total petroleum hydrocarbons (TPH), Total

Phosphorus (TP), metals, and suspended solids

removal rates, which improves water quality. An

additional benefit of the STS is the system's spill

containment feature, which can capture an upstream

release and therefore lessen the spill’s impact on the

environment. However, as previously discussed,

the STS is relatively new and remains to be

thoroughly tested in different geographical

locations. There may be possible limitations in

different areas, although soil types and high water

tables surrounding the modular unit will not limit the

system's effectiveness.

DESIGN CRITERIA

The STS is a modular, 2.9-meter (9.5-foot)

diameter recycled-polyethylene tank containing a

series of sedimentation chambers and constructed

wetlands. The sedimentation chambers are in the

inner ring of the tank, which has a diameter of

nearly 1.7-meters (5.5 feet). The 2.9-meter

diameter outer ring, which surrounds the

sedimentation chambers, contains the wetland. The

tank walls and bulkheads, which separate the

sedimentation chambers, are 1.2-meters (4 feet)

high.
STS tanks are designed to withstand the weight of

the saturated soils surrounding the tanks. Influent

is conveyed from a catch basin (and other

preliminary detention structures) through poly-vinyl

chloride (PVC) piping to the first of six internal

sedimentation chambers. A synthetic woven sack

placed at the end of the 10 centimeter (4 inch)

diameter inlet pipe traps large particles and debris.

Skimmers floating on the water surface within each

chamber convey flow to the following chamber

through an opening 15 centimeters (6 inches) below

the surface. This prevents sediment and floatables

from being transported to the subsequent chamber.

Sediments that collect in the bottom of the chamber

remain there until the unit is cleaned. The bulkhead

separating the last two sedimentation chambers is

fitted with an inverted elbow, which traps oil and

grease. The settling efficiency increases by

transferring water from the top of each chamber to

the subsequent chamber.

Flow is conveyed from the final sedimentation

chamber through four, slotted PVC outlet pipes,

each 10 cm (4 inches) in diameter, into the wetland

portion of the STS. Partially treated storm water

flows beneath the soil through the wetland. The

wetland has an approximate storage capacity of

2,880 liters (760 gallons). The entire system has a

static holding volume of 5,270 liters (1,390 gallons).

However, the system is sized based upon this

volume plus associated detention structures.

Vegetation within the wetland will vary depending

on the local conditions (climatological). Bulrush

and burreeds (which have maximum root depths of

0.8 and 0.6 meters (2.6 and 2 feet), respectively

[U.S. EPA, 1993]) have been used in

Massachusetts. Mature vegetation in the outer ring

should have roots that extend into the permanent 15

cm (6 inches) of water in the bottom of the tank.

Insufficient root depth may result in a lack of water

supply to the plants during the periods between

storm events.

Effluent from the wetland is discharged through a 5

centimeter (2 inch) diameter pipe that is controlled

by a valve. Flow rates and holding times can be

varied by manipulating the outlet control valve. At

the Kingston facility, the control valve is adjusted to

provide the recommended discharge rate of 0.1

liters per second (0.2 gallons per minute) and a five

day holding time in the wetland. The valve has an

added benefit that in the event of an upstream toxic

spill, it can be closed, trapping the pollutants in the

STS.

Tanks are available in one size, but several tanks can

be installed at a site to capture the projected volume

of runoff. The determination of the number of tanks

needed for a site is based on three factors:

C Area of impervious drainage surfaces.

C Design storm to be treated.

C Detention storage prior to the STS tanks.

Generally 1-2 units are required for each acre of

impervious surface. The system is sized based upon

the design storm which is determined by state

regulations (i.e., Maine requires treatment of first

half inch of storm and Washington requires

treatment of a six month storm). This first flush

storage volume is stored in preliminary storage

structures such as underground tanks and large

diameter pipes (which can be place under parking

areas).

PERFORMANCE

Runoff from the STS installed in Kingston, MA,

was analyzed to assess pollutant removal efficiency.

Thirty-three samples were collected over eight

independent storm events during both winter and

summer conditions. Sampling results are shown in

Table 1. The results indicate removal rates of 97

percent for fecal coliform bacteria, 99 percent for

total suspended solids, and 90 percent for total

petroleum hydrocarbons. Nutrient removal rates

were 82 percent chemical oxygen demand, 77

percent total dissolved nitrogen, and 90 percent

phosphorus. Metal removal rates were 77 percent

for lead, 98 percent for chromium, and 90 percent

for zinc.

In addition to the study in Kingston, MA, several

other studies are currently being conducted in

Connecticut, California, and Massachusetts. This
data has not been fully developed and is not yet

available.

OPERATION AND MAINTENANCE

Anticipated maintenance of the STS is minimal.

The system should be observed at least once a year

to be sure that it is operating effectively. At that

time, the burlap sack that covers the influent line

should be replaced. If the installed system uses

filters, these should be removed, cleaned, and

reinstalled. Sediment should be removed from the

system once every three to five years, more often if

the system has higher than normal sediment loads.

The sediment level may be measured with a probe

or even a yard stick. It is recommended that the

sediment be removed when 0.3 meters (1 foot) of

sediment has accumulated. After six months of

operation the unit installed in Kingston, MA was

found to have 5 centimeters (2 inches) of

accumulated sediment. The sediment can be

pumped from the tank by septic haulers or by

maintenance personnel responsible for sediment

removal from catch basins. It is not anticipated that

the sediment will be toxic, and it may be safely

landfilled. However, sediment toxicity will depend

on the activities in the contributing drainage area

and testing of the sediment may be required to

determine if it is considered hazardous. Because the

STS system is relatively new, there is no definitive

data on the lifetime of the plants and gravel in the

system. However, it is estimated that these will

need to be replaced every 10 to 20 years.

COSTS

The STS is a prefabricated unit that is easily

installed in most locations. The cost for one unit is

$4,900, and the installation cost is usually between

$500 and $1,000 (which is provided by the

manufacture). Additional materials required include

gravel, PVC piping, and wetland plants, at a total

cost of about $350 to $400 per tank. Capital and

installation costs per tank decrease as the number of

units on a site increases. Installation will cost less if

construction on that site is new (not retrofitted)

because drainage lines will be more easily accessible.

Installation will cost more if there are extra

construction costs (for example, retrofit design) or

if there are complications. StormTreatTM Systems

recommends one STS unit per one acre of

impervious surface.

The estimated maintenance cost for removal of

sediment from one tank ranges from $80 to $120.

This cost is incurred every three to five years, when

sediment is removed. Costs have not been

determined for an annual site inspection or for

removing any debris from the wetland area.

However, these costs should be minimal (i.e., one

day of labor for one person per year).

REFERENCES

1. Horsley, S. W. and W. Platz, 1995.

Progress Report: Water Quality

Monitoring at Elm Street Facility.

Barnstable, Massachusetts (relocated to

Hyannis, MA).

2. Horsley, S. W., 1995. The StormTreatTM

System - A New Technology for Treating

Storm Water.

3. Horsley, S. W., 1998. Personal

communication with Parsons Engineering

Science, Inc.

TABLE 1 STORMTREATTM SAMPLING

RESULTS FOR KINGSTON, MA

Pollutant Percent

Removed

Fecal Coliform Bacteria 97

Total Suspended Solids 99

Chemical Oxygen Demand 82

Total Dissolved Nitrogen 77

Phosphorous 90

Total Petroleum Hydrocarbons 90

Lead 77

Chromium 98

Zinc 90

Source: StormTreatTM Systems, Inc., 1998.
For more information contact:

Municipal Technology Branch

U.S. EPA

Mail Code 4204

401 M St., S.W.

4. Horsley & Witten, Inc., 1998. Fact Sheet

- Modeling of Water Flow Through the

StormTreatTM System.

5. Oregon Department of Environmental

Quality, 1998. Storm Water Management

Guidelines. Internet site a t

[http://waterquality.deq.state.or.us/wq/gro

undwa/swmgtguide.htm], accessed

February, 1998. 

6. StormTreat Systems, Inc., 1998.

StormTreatTM Systems Newsletter.

Barnstable, Massachusetts (relocated to

Hyannis, MA). 

7. StormTreat Systems, Inc., Undated.

Technical Data for StormTreatTM System.

Barnstable, Massachusetts (relocated to

Hyannis, MA).

8. U.S. EPA, 1993. Subsurface Flow

Constructed Wetlands for Wastewater

Treatment: A Technology Assessment.

EPA 832-R-93-001. 

ADDITIONAL INFORMATION

Ecocycle

George Lord

P.O. Box 228

Manchester, ME 04351

Land Use Consultants, Inc.

Pat Clark

966 Riverside Street

Portland, ME 04103

New Hampshire Department of Environmental

Services

Steve Landry

6 Hazen Drive

Concord, NH 03302

StormTreatTM Systems, Inc.

Scott Horsley

90 Route 6A, Sextant Hill

Sandwich, MA 02563

University of Washington

Chris May

Applied Physics Laboratory

UW Box 355640

Seattle, WA 98105

The mention of trade names or commercial products

does not constitute endorsement or recommendation

for the use by the U.S. Environmental Protection

Agency.