United States
                    Environmental Protection
                    Office of Water
                    Washington, D.C.
EPA 832-F-99-029
September 1999
Storm  Water
Technology Fact Sheet
Water Quality  Inlets

Water quality inlets (WQIs), also commonly called
oil/grit separators or oil/water separators, consist of
a series of chambers that promote sedimentation of
coarse materials and separation of free  oil (as
opposed to emulsified or dissolved oil) from storm
water.  Most WQIs also  contain screens to help
retain larger or floating debris, and many of the
newer designs also include a coalescing unit that
                   helps to promote oil/water separation.   WQIs
                   typically capture only the first portion of runoff for
                   treatment and are generally used for pretreatment
                   before  discharging to other best management
                   practices (BMPs).

                   A typical WQI, as shown in Figure 1, consists of a
                   sedimentation chamber, an oil separation chamber,
                   and a discharge chamber. The basic WQI design is
                   often modified to improve performance. Possible
       Stormwater Inlet Pipe
                 Access Manhole
                      ccess Manhole
                        Trash Rack
                   Trapping Chamber
 Source: Berg, 1991.

modifications include: an  additional orifice and
chamber that replace the inverted pipe elbow; the
extension of the second chamber wall up to the top
of the structure; or the addition of a diffusion device
at the inlet.  The diffusion device is intended to
dissipate the velocity head and turbulence and
distribute the flow more evenly over the entire
cross-sectional area of the sedimentation chamber
(API, 1990).

The addition of a coalescing unit to the WQI can
dramatically increase its effectiveness in oil/water
separation while also  greatly reducing the size of
the required unit.  Coalescing units are made from
oil-attracting materials, such as polypropylene or
other materials.   These units attract  small  oil
droplets, which begin to concentrate until they are
large  enough to float to the surface  and separate
from the storm water. Without these units, the oil
and grease particles must concentrate and separate
naturally. This requires a much larger surface area;
and therefore, units that do not use the coalescing
process  must be  larger than units utilizing  a
coalescing unit.

WQIs can be purchased as pre-manufactured units
(primarily oil/water separator tanks) or constructed
on site.  Suppliers of pre-manufactured units (e.g.,
Highland Tank and Manufacturing, Jay R. Smith
Manufacturing, etc.) can also provide modifications
of the typical design for special conditions.


WQIs are  widely  used in the U.S. and  can be
adapted to all regions of the  country.  They are
often used  where  land requirements  and  cost
prohibit the use of larger BMP devices, such as
ponds or wetlands. WQIs are also used to treat
runoff prior to discharge to other BMPs.

Because of their ability to  remove hydrocarbons,
WQIs are typically located at sites with automotive-
related contamination or at other sites that generate
high hydrocarbon concentrations (MWCOG, 1993).
For example, WQIs may be ideal for small, highly
impervious  areas,  such as gas  stations, loading
areas, or parking areas (Schueler, 1992).  Many
WQIs, particularly those installed at industrial sites,
serve the dual purpose  of treating  storm water
runoff from contaminated areas, and  serving as
collection  and  treatment units  for  washdown
processes or petroleum spills.

Higher  residual  hydrocarbon  concentrations in
trapped sediments cause maintenance and residual
disposal costs associated with WQIs to be higher
than those of other BMPs.  Therefore, planners
should carefully evaluate maintenance and residual
disposal issues for the site before selecting a WQI.
Possible alternatives  to the WQI include  sand
filters, oil absorbent materials, and other innovative
BMPs (e.g., Stormceptor System).


WQIs can  effectively  trap trash, debris,  oil and
grease, and other floatables that would otherwise be
discharged to surface waters (Schueler, 1992). In
addition, a properly designed and maintained WQI
can  serve  as  an effective  BMP  for reducing
hydrocarbon  contamination  in receiving water
sediments.  While WQIs are effective in removing
heavy sediments and floating oil and grease, they
have  demonstrated  limited  ability to  separate
dissolved or emulsified oil from runoff. WQIs are
also not very effective at removing pollutants such
as nutrients or metals, except where  the metals
removal is directly related to sediment removal.

Several major constraints can limit the effectiveness
of WQIs. The first is the size of the drainage area.
WQIs are  generally recommended for drainage
areas of 0.4 hectares (1 acre) or less (Berg,  1991,
NVPDC, 1992). Construction costs often become
prohibitive for larger drainage areas.  However,
because WQIs are primarily designed for specific
industrial  sites  that  have  the  potential  for
petroleum-contaminated process washdown, spills,
and storm water runoff, sizing considerations are
not usually a problem.

Sediment can also cause problems for WQIs. There
are several reasons for this.  First,  high sediment
loads can interfere with the ability of the WQI to
effectively separate oil  and grease from the runoff.
Second,  during periods  of high flow, sediment
residuals may be resuspended and released from the
WQI to surface  waters.   A  1993 Metropolitan
Washington Council of Governments (MWCOG)

long-term study evaluating the performance  and
effectiveness of more than 100 WQIs found  that
pollutants in the WQI sediments were similar to
those pollutants found in downstream receiving
water sediments (the tidal Anacostia River).  This
information suggests that downstream  sediment
contamination is linked to contaminated runoff and
pass-through from WQIs (MWCOG, 1993). Third,
WQI residuals accumulate  quickly  and require
frequent removal. There is also some concern that
because the collected residuals contain hydrocarbon
by-products, the residuals may be considered too
toxic for conventional landfill disposal. The 1993
MWCOG study found that the residuals from WQIs
typically contain many priority pollutants, including
polyaromatic  hydrocarbons,   trace   metals,
phthalates, phenol, toluene, and possibly methylene
chloride (MWCOG,  1993).   Based on  these
considerations, WQIs should not be implemented at
sites that generate large amounts of sediment in the
runoff unless the runoff has been pretreated to
reduce the sediment loads to manageable levels.

WQIs  are  also   limited   by   maintenance
requirements. Maintenance of underground WQIs
can be easily neglected because the WQI is often
"out  of  sight  and  out  of  mind."    Regular
maintenance is  essential to ensuring  effective
pollutant removal.  As discussed above, lack of
maintenance will often result in resuspension of
settled pollutants.

Finally, WQIs generally provide limited hydraulic
and residuals storage. Due to the limited storage,
WQIs do not provide adequate storm water quantity


Prior to WQI design, the site should be evaluated to
determine  if another  BMP would  be  more
cost-effective  in  removing  the  pollutants  of
concern. WQIs should be used when no other BMP
is feasible. The WQI should be constructed near a
storm  drain network so that flow can be easily
diverted to the WQI for treatment (NVPDC, 1992).
Any construction activities within the drainage area
should be completed before installation of the WQI,
and the drainage area should be revegetated so that
the sediment loading to the WQI is minimized.
Upstream sediment control measures  should  be
implemented to decrease sediment loading.

WQIs are most effective for small drainage areas.
Drainage areas of 0.4 hectares (1 acre)  or less are
often recommended. WQIs are typically used in an
off-line configuration  (i.e., portions of runoff are
diverted to the WQI), but they can be used as
on-line  units (i.e., receive all runoff).   Generally,
off-line units are designed to handle the first 1.3
centimeters (0.5 inches) of runoff from the drainage
areas. Upstream isolation/diversion structures can
be used to divert the water to the off-line structure
(Schueler,  1992).  On-line units receive higher
flows that will likely  cause increased  turbulence
and  resuspension  of  settled material,  thereby
reducing WQI performance.

As discussed above, oil/water separation tank units
are often utilized in specific industrial areas, such as
airport  aprons, equipment washdown  areas,  or
vehicle  storage areas.  In these instances, runoff
from the area  of concern will usually be diverted
directly into the unit, while all other runoff is sent
to the storm drain downstream from the oil/water
separator.  Oil/water  separation tanks  are  often
fitted with diffusion baffles at the inlets to prevent
turbulent  flow  from  entering  the  unit and
resuspending settled pollutants.

WQIs are available as pre-manufactured units or
can be cast in place. Reinforced concrete should be
used to construct below-grade WQIs.   The WQIs
should be water tight to prevent possible ground
water contamination.

Chamber Design

Structural loadings  should be considered in the
WQI design (Berg, 1991), particularly with respect
to the sizing of the chambers.  When the combined
length of the first two chambers exceeds 4 meters
(12 feet), the chambers are typically designed with
the length of the first and second chamber being
two-thirds and one-third of the combined length of
the unit, respectively. Each of the chambers should
have a  separate manhole to provide  access for
cleaning and inspection.

The  State of Maryland design standards indicate
that the combined volume of the first and second
chambers should be determined based on 1.1 cubic
meters (40 cubic feet) per 0.04 hectares (0.10 acres)
draining to the WQI.    In Maryland,  this  is
equivalent to capturing the first 0.33 centimeters
(0.133 inches)  of runoff from the contributing
drainage area.

Permanent pools within the chambers help prevent
the possibility of sediment resuspension. The first
and second chambers should have permanent pools
with depths of 1.2 meters (4 feet). If possible, the
third chamber should also contain a permanent pool
(NVPDC, 1992).

The  first and  second chambers  are  generally
connected by an opening  covered by a trash rack,
a PVC pipe, or other suitable material pipe (Berg,
1991). If a pipe is used,  it should also be covered
by a trash rack or screen.  The  opening or pipe
between the first and second chambers should be
designed  to pass  the   design  storm  without
surcharging  the first chamber (Berg, 1991).  The
design storm will vary depending on geographical
location  and  is  generally  defined by  local

In the standard WQI, an inverted elbow is installed
between the second and third chamber.  The elbow
should extend a minimum of 1 meter (3 feet) into
the second chamber's permanent pool. Because oil
will naturally separate from, and float on top of, the
water, water will be forced through the submerged
elbow and into the third chamber while oil will be
retained in the second chamber (NVPDC,  1992).
The  depth of the elbow into the permanent pool
should  should be.  The size of the elbow or the
number of elbows can be adjusted to accommodate
the  design  flow   and   prevent  discharge  of
accumulated oil(Berg, 1991).

Pre-manufactured oil/water separation tanks do not
usually  follow  the  separated-chamber  design;
instead, these units often rely on baffle units to
separate the different removal process. Particulates
are thus retained near the inlet to the tank, while
oil/water separation takes place closer to the tank

WQIs are primarily utilized to remove sediments
from storm water runoff. Grit and sediments are
partially removed by gravity settling within the first
two chambers. A WQI with a detention time of 1
hour may expect to have 20 to 40 percent removal
of sediments.  Hydrocarbons associated with the
accumulated sediments are also often removed from
the runoff through this process.  The WQI achieves
slight,  if any, removal  of nutrients, metals and
organic  pollutants  other  than  free  petroleum
products  (Schueler, 1992).

The 1993 MWCOG study discussed above  found
that an average of less than 5 centimeters (2 inches)
of sediments  (mostly  coarse-grained  grit  and
organic  matter)  were  trapped  in  the WQIs.
Hydrocarbon and total organic  carbon (TOC)
concentrations of the sediments averaged 8,150 and
53,900 milligrams per kilogram, respectively.  The
mean hydrocarbon concentration in the WQI water
column was 10 milligrams per liter. The study also
indicated that  sediment accumulation  did not
increase  over time, suggesting  that the sediments
become re-suspended during storm  events.  The
authors  concluded  that  although  the   WQI
effectively  separates oil and grease from  water,
re-suspension of the settled matter appears to limit
removal efficiencies. Actual removal only occurs
when the residuals are  removed from the WQI
(Schueler 1992).

A 1990 report by API found that the efficiency  of
oil and water separation in  a WQI is inversely
proportional to the ratio of the discharge rate to the
unit's surface area. Due to the small capacity of the
WQI, the discharge rate is typically very high and
the detention time is very short. For example, the
MWCOG study  found that the average detention
time in a WQI is less than 0.5 hour. This can result
in  minimal  pollutant  settling  (API,   1990).
However, the addition of coalescing units in many
current WQI units may increase oil/water separation
efficiency.  Most coalescing units are designed  to
achieve a specific outlet concentration of oil and
grease (for example, 10-15 parts per million oil and


The   key  to  the  performance   of WQIs  is
maintenance.  When properly maintained, WQIs
should experience very few separation, clogging  ~-
structural problems.
Basic maintenance should consist of regularly
checking and cleaning out the sediment that has
accumulated  in  the  WQI.   A  lack  of regular
clean-outs can lead to the resuspension of collected
sediments;  therefore, WQIs  should be inspected
after every storm event to determine if maintenance
is required.  At a minimum, each WQI should be
cleaned at  the beginning  of  each  season (Berg,
1991).    The  required   maintenance  will  be
site-specific due to  variations  in  sediment and
hydrocarbon loading. Maintenance should include
clean out, disposal of the sediments, and removal of
trash  and debris.   The clean out  and disposal
techniques  should  be environmentally acceptable
and in accordance  with local regulations.   Since
WQI residuals contain hydrocarbon by-products,
they may require  disposal as hazardous  waste.
Many WQI owners  coordinate with waste haulers to
collect and dispose  of these residuals. Since WQIs
can be relatively deep, they may be designated as
confined spaces. Caution should be exercised to
comply with confined space entry safety regulations
if it is required.

Oil/water separator tank units can be  fitted with
sensing units that will indicate when the units need
to be cleaned. Because most of oil/water separator
tank units  are designed  for specific  industrial
applications, their maintenance schedule should be
closely tied to the industrial process schedule.
However, these units  should also be inspected after
rain events.


The construction costs for WQIs will vary greatly
depending  on  their  size  and depth.    The
construction costs (in 1993 dollars) for cast-in-place
WQIs range  from  $5,000 to $16,000, with  the
average WQI costing around $8,500 (Schueler,
1992). For the basic design  and construction of
WQIs, the pre-manufactured units are generally less
expensive than those that are cast in place (Berg,

Maintenance costs will also vary greatly depending
on the size of the drainage area, the amount of the
residuals collected, and the clean out and disposal
methods available (Schueler, 1992).  The cost of
residuals removal, analysis, and disposal can be a
major maintenance expense, particularly if the
residuals are toxic and are not suitable for disposal
in a conventional landfill.


1.    American Petroleum  Institute  (API), 1990.
      Monographs  on Refinery Environmental
      Control -Management of Water Discharges
      (Design  and  Operation  of Oil-Water
      Separators). Publication 421, First Edition.

2.    BaysaverŪ,   Inc.,   1998.    BaysaverŪ
      Separation  System Technical  and Design

3.    Berg, V.H,  1991.   Water  Quality Inlets
      (Oil/Grit  Separators).     Maryland
      Department of the Environment,  Sediment
      and Storm Water Administration.

4.    Boelke, Art, E.L. Shannon & Associates,
       1999.   Personal  communication with
      Parsons Engineering Science, Inc.

5.    Fibresep Limited, Not dated.  Informative
      literature on  the  Stormceptor  System.
      Oakville, Ontario, Canada.

6.    Highland   Tank  and   Manufacturing
      Company, 1999.  Personal communication
      with Parsons Engineering Science, Inc.

7.    Metropolitan   Washington  Council  of
      Governments  (MWCOG), 1993.    The
      Quality of  Trapped  Sediments  and Pool
       Water  Within  Oil   Grit  Separators  in
      Suburban Maryland.  Interim Report.

8.     Northern   Virginia   Planning   District
      Commission (NVPDC) and Engineers and
      Surveyors  Institute,  1992.    Northern
      Virginia BMP Handbook.

9.     Schueler,   T.R.,   1992.     A   Current
      Assessment of  Urban Best Management
      Practices.   Metropolitan  Washington
      Council of Governments.

10.    Xerxes  Corporation,  1999.    Personal
      communication with Parsons Engineering
      Science, Inc.


Perry Hall High School
Baltimore County, Maryland
Bay saverŪ, Inc.
Mark Hausner
1010 Deer Hollow Drive
Mount Airy, MD 21771

Cronin   Department  Store  Site,  Waltham,
Paul Finger
Beals and Thomas, Inc.
200 Friberg Parkway
Westborough, MA 01581

E.L. Shannon & Associates
Art Boelke
627 Minuet Lane, Suite E
Charlotte, NC 28217

Environmental  Technology Evaluation Center
Will Kirksey
Civil Engineering Research Center
101515th Street, NW
Washington, D.C.  20005

Shell Superstop
Bowman, South Carolina
Highland Tank and Manufacturing Company
Tom Schoendorf
99 West Elizabethtown Road
Manheim, PA  17545
Volk Field ANG Base
Bill Buth
Mead and Hunt
6501 Watts Road
Madison, WI  53719

Laredo Bus Facility
Metro Area Rapid Transit Authority, Atlanta
Xerxes Corporation
Mark Trau
7901 Xerxes Avenue, South
Minneapolis, MN 55431

The  mention of trade  names  or commercial
products  does not  constitute  endorsement  or
recommendation  for  the  use   by  the   U.S.
Environmental Protection Agency.
          For more information contact:

          Municipal Technology Branch
          U.S. EPA
          Mail Code 4204
          401 M St., S.W.
          Washington, D.C., 20460
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