United States
                     Environmental Protection
                     Office of Water
                     Washington, D.C.
EPA 832-F-99-044
September 1999
Storm  Water
Technology  Fact Sheet
Modular Treatment  Systems

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
     discharge pipe
       Inflow from
       catch basin
          Outlet contol valve
                              - Slotted PVC pipe
                                                Inverted elbow
   Source: StormTreat™ Systems, 1998.
                         FIGURE 1  STORMTREAT™SYSTEM

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

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.

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.


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.


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)

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

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


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

 Fecal Coliform Bacteria

 Total Suspended Solids

 Chemical Oxygen Demand

 Total Dissolved Nitrogen


 Total Petroleum Hydrocarbons











Source: StormTreat™ Systems, Inc., 1998.
data has  not been fully developed and is not yet


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.


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).


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.

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
       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.


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
New  Hampshire Department  of Environmental
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.
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