v>EPA
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 StormTreat™ 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.
Slotted PVC pipe
Infiltration
discharge pipe
Inflow from
catch basin
Outlet contol valve
Skimmers
- Slotted PVC pipe
Inverted elbow
Source: StormTreat™ Systems, 1998.
FIGURE 1 STORMTREAT™SYSTEM
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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 solar-
powered 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 tidally-
influenced 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.
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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:
• Area of impervious drainage surfaces.
• Design storm to be treated.
• 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
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TABLE 1 STORMTREAT™ SAMPLING
RESULTS FOR KINGSTON, MA
Pollutant
Percent
Removed
Fecal Coliform Bacteria
Total Suspended Solids
Chemical Oxygen Demand
Total Dissolved Nitrogen
Phosphorous
Total Petroleum Hydrocarbons
Lead
Chromium
Zinc
97
99
82
77
90
90
77
98
90
Source: StormTreat™ Systems, Inc., 1998.
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. StormTreat™ 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 StormTreat™
System - A New Technology for Treating
Storm Water.
3. Horsley, S. W., 1998. Personal
communication with Parsons Engineering
Science, Inc.
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4. Horsley & Witten, Inc., 1998. Fact Sheet
- Modeling of Water Flow Through the
StormTreat™ System.
5. Oregon Department of Environmental
Quality, 1998. Storm Water Management
Guidelines. Internet site at
[http://waterquality.deq.state.or.us/wq/gro
undwa/swmgtguide.htm], accessed
February, 1998.
6. StormTreat Systems, Inc., 1998.
StormTreat™ Systems Newsletter.
Barnstable, Massachusetts (relocated to
Hyannis, MA).
7. StormTreat Systems, Inc., Undated.
Technical Data for StormTreat™ System.
Barnstable, Massachusetts (relocated to
Hyannis, MA).
8. U.S. EPA, 1993. Subsurface Flow
Constructed Wetlands for Wastewater
Treatment: A Technology Assessment.
EPA832-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
StormTreat™ 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.
New Hampshire Department of Environmental
Services
Steve Landry
6 Hazen Drive
Concord, NH 03302
For more information contact:
Municipal Technology Branch
U.S. EPA
Mail Code 4204
401 M St., S.W.
MTB
Excelence fri compliance through optimal technical soLtfbns
MUNICIPAL TECHNOLOGY BRANCH
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