United States
                      Environmental Protection
                      Off ice of Water
                      Washington, D.C.
EPA 832-F-99-007
September 1999
Storm Water
Technology  Fact  Sheet
Sand  Filters

Sand filters have proven effective in removing
several  common pollutants from storm water
runoff.  Sand filters generally control storm water
quality, providing very limited flow rate control.

A typical sand filter system consists of two or three
chambers or basins. The first is the sedimentation
chamber, which removes floatables  and heavy
sediments. The second is the filtration chamber,
which removes additional pollutants by filtering the
runoff through a sand bed.   The  third is the
discharge chamber. The treated filtrate normally is
then  discharged through  an underdrain system
                     either to a storm drainage system or directly to
                     surface waters. Sand filters take up little space and
                     can be used on highly developed sites and sites with
                     steep slopes.  They can be added to retrofit existing
                     sites. Sand filters are able to achieve high removal
                     efficiencies  for  sediment, biochemical oxygen
                     demand (BOD), and fecal coliform bacteria. Total
                     metal removal, however, is moderate, and nutrient
                     removal is often low.

                     There are three main sand filter designs currently in
                     common use: the Austin sand filter (Figure 1); the
                     Washington,  D.C., sand filter (Figure 2); and the
              To Stormwater
             Detention Basin
                   Energy Dissipators
                                               Filtration Basin
Sedimentation [ p^
op Inlet


*v — * 	
55:J.J .I.;.
if U L

                                                              -Filtered Outflow
       Channel Sloped to
       Facilitate Sediment
       Transport into
       Sedimentation Basin
                                             Weir to Achieve
                                            Uniform Discharge

                                                  Sand Bed
      Perforated Riser
      with Trash Rack
                            ELEVATION A - A
                          •—— zsana t

                          i •"'  %
                                          Underdrain Piping System
 Source: Schueler, 1992.

            30" Manhole
           Frame & Cover
 Ladder   30" Manhole
         Frame & Cover
                       24" Manhole
                      Frame & Cover
                                                                                  6" PVC
                                                                                  Drain with
                                                                                  PVC Gate
Washed iy
6" PVC Clean Out
Pipe with Cap
                                                                           ••-' -J  - SI   Outflow
  Source: Troung, 1989.

                                   Grated Cover
                     Solid Cover
                                              Grate (Fabric Wrapped
                                              Over Entire Grate Opening)
Source: Shaver, 1991.
Delaware sand filter (Figure 3).   The  primary
differences among these designs are location (i.e.,
above or below ground), the drainage area served,
their filter surface areas, their land requirements,
and the quantity of runoff they treat.
                    Modifications that may improve sand filter design
                    and  performance   are  being  tested.     One
                    modification is the addition of a peat layer in the
                    filtration chamber. The addition of peat to the sand

filter may increase microbial  growth within the
sand filter and improve metals and nutrient removal


Sand filters are intended primarily for water quality
enhancement. In general, sand filters are preferred
over infiltration  practices,  such  as  infiltration
trenches, when contamination of groundwater with
conventional pollutants - BOD, suspended solids,
and fecal coliform - is of concern. This usually
occurs in areas where underlying soils alone cannot
treat runoff adequately - or ground water tables are
high. In most cases, sand filters can be constructed
with impermeable basin or chamber bottoms, which
help to collect, treat, and release runoff to a storm
drainage system or directly to surface water with no
contact  between  contaminated  runoff  and

The selection of a sand filter design depends largely
on the drainage area's characteristics. For example,
the Washington, D.C., and Delaware sand filter
systems are well suited for highly impervious areas
where  land available  for structural controls is
limited, since both are installed underground. They
are often used  to treat runoff from parking lots,
driveways, loading docks, service stations, garages,
airport runways/taxiways, and  storage yards. The
Austin sand filtration system  is more  suited for
large drainage areas that have both impervious and
pervious surfaces.  This system is located at grade
and is often used at transportation facilities, in large
parking areas, and in commercial developments.

In general, all three types of sand filters can be used
as alternatives for water quality inlets.  They are
more frequently used to treat runoff contaminated
with oil and grease from drainage areas with heavy
vehicle  usage.    In  regions  where  evaporation
exceeds rainfall and a wet pond would be unlikely
to maintain the required permanent pool, the Austin
sand filtration system can be used.


Sand filters can be highly effective storm water best
management practices (BMPs). All three types of
sand  filters  achieve  high   removal  rates  for
sediment, BOD, and fecal coliform bacteria.  The
filter media is periodically removed from the filter
unit,  thus  also permanently  removing  trapped
contaminants.  Waste media from the filters does
not appear to be toxic and is environmentally safe
for landfill disposal. If they are designed with an
impermeable basin liner,  sand  filters can also
reduce the potential for groundwater contamination.
Finally sand filters also generally require less land
than other BMPs, such as ponds or wetlands.

The size and characteristics of the drainage area, as
well as the pollutant loading, will  greatly influence
the effectiveness  of the sand filter system.   For
example, sand filters may be of limited value in
some  applications because of they are designed to
handle runoff from relatively small drainage areas
and they have low nutrient removal and  metal
removal capabilities. In these cases, other BMPs,
such as wet ponds, may be less costly and/or more
effective.    The  system  also requires  routine
maintenance to prevent sediment from clogging the
filter.  In some cases, filter media may need to be
replaced 3 to 5 years. Lastly, sand filters generally
do not control storm water flow, and consequently,
they do not prevent downstream  stream bank  and
channel erosion.

Climatic  conditions may also limit the filter's
performance. For example, it is not yet known how
well sand filters will operate in colder climates or in
freezing conditions.


Typically the Austin sand filter system is designed
to handle runoff from  drainage  areas up  to  20
hectares (50 acres).  The collected runoff is first
diverted to the sedimentation basin, where heavy
sediments and floatables are removed.  There are
two designs for the sedimentation basin: the  full
sedimentation  system, as shown in Figure 1; and a
partial sedimentation system, where only the initial
flow is diverted. Both systems  are located off-line
and are designed to collect and treat the first 1.3
centimeters  (0.5 inches) of runoff.  The partial
system has the capacity to hold only a portion (at
least 20 percent)  of the first flush volume in the
sedimentation  basin,  whereas the  full  system
captures  and  holds  the  entire  flow  volume.

Equations used to determine the sedimentation
basin surface areas (As) in square and meters acres
are shown in Table 1.

  Partial Sedimentation   Full Sedimentation


  Note:   Designed to collect and treat 0.5 inches of runoff.
         Ds (feet)=depth of the sedimentation basin.
         H (feet)=depth of rainfall, 0.042ft (0.5 in).
         AD(acres)=impervious and pervious areas that
         provide contributing drainage.

  Source: Galli, 1990.
Flow is conveyed from the sedimentation basin,
through a perforated riser, a gabion wall, or a berm,
to the filtration basin.  The filtration basin consists
of a 45-centimeter (18-inch) layer of sand particles
0.05 to  0.10 centimeters (0.02 to 0.04 inches) in
diameter that may be  underlain by a gravel layer.
Equations used to determine the surface areas (Af)
in acres are also shown in Table 1.  The filtrate is
discharged  from  the filtration basin  through
underdrain piping 10  to 15  centimeters  (4 to 6
inches)  in  diameter with 1-centimeter (0.4 inch)
perforations.   Filter fabric is placed around the
underdrain piping  to prevent  sand and  other
particulates from being discharged.

Typically, the Washington, D.C., sand filter system
is designed to  handle runoff from completely
impervious drainage areas of 0.4 hectares (1 acre)
or less.  The system, as shown in Figure 2, consists
of three underground  chambers:  a sedimentation
chamber,  a filtration   chamber,  and a discharge
chamber.   The sand filter system is designed to
accept the first  1.3  centimeters (0.5 inches) of
runoff.   Coarse sediments  and floatables are
removed from the runoff within the sedimentation
chamber.    Runoff   is   discharged  from  the
sedimentation chamber through a submerged weir,
into  the filtration  chamber, which  consists of a
combination of sand and gravel layers totaling 1
meter (3 feet) in  depth with underdrain  piping
wrapped in filter fabric.   The underdrain system
collects the filtered water and discharges it to the
third chamber, where the water is collected and
discharged to a  storm  water channel  or sewer
system. An overflow weir is located between the
second and third chambers to bypass excess flow.
The  Washington,  D.C.,  sand  filter  is  often
constructed on-line, but can be constructed off-line.
When the system is off-line, the overflow between
the second and third chambers is not included.

The Delaware sand filter, shown  in Figure 3,  is
similar to the Washington, D.C., sand filter in that
both utilize underground concrete vaults. However,
the Delaware sand filter has only two chambers: a
sedimentation chamber and a filtration chamber.  A
2.5-centimeter (1 inch) design storm was selected
for sizing  the sedimentation basin because  it  is
representative of large storm events: in Delaware,
92  percent  of all storms  are  less  than 2.5
centimeters (1 inch) in depth.  Runoff enters the
sedimentation chamber through a grated cover and
then overflows into the filtration chamber, which
contains a sand layer 45 centimeters (18 inches) in
depth.  Gravel is not normally used in the filtration
chamber although the filter  can be modified  to
include it.  Typical systems are designed to handle
runoff from drainage areas of 2 hectares (5 acres) or
less. A maj or advantage of the Delaware sand filter
is its shallow structure depth of only 76 centimeters
(30  inches),  which  reduces  construction  and
maintenance costs.

Proper design and maintenance are also critical
factors in maintaining the operating life of any filter
system.   The life of the filter media  may be
increased by a number of methods, including:

•      Stabilizing  the   drainage  area  so  that
       sediment   loadings  in  the  runoff are

•      Providing adequate storm water detention
       times  to   enhance   sedimentation  and

•      Inspecting and maintaining the sand filter
       frequently   enough   to  ensure  proper


Sand  filters are  currently  in use in Delaware,
Maryland,  Florida,  Texas,   Virginia,   and
Washington,  D.C.    Studies on  the  systems'
pollutant removal efficiencies are currently being
performed in Washington, D.C., and Austin, TX.
Additional  evaluations  are needed  to  evaluate
alternative  sand filter designs  and media.  Sand
filters remove particulates in both the sedimentation
and the filtration chambers.  The City of Austin has
estimated  their   systems'   pollutant  removal
efficiencies based on preliminary findings of their
storm water monitoring program (Austin, 1988).
The estimates shown in Table 2 are average values
for various sand filters serving drainage areas of
several different sizes.  As  shown  in Table 2, no
removal  of nitrate  was  observed.   No  other
dissolved pollutants were monitored.  Additional
monitoring is currently being performed by the City
of Austin to supplement the preliminary estimates.


All filter system  designs must provide  adequate
access to the filter for inspection and maintenance.
The sand filters should be inspected after all storm
events to verify that they are working as intended.
Since the Washington, D.C., and Austin sand filter
systems can be deep, they  may be designated as
confined spaces  and require  compliance with
confined space entry safety procedures.

Typically, sand filters begin to experience clogging
problems within  3  to 5  years  (NVPDC, 1992).
Accumulated trash,  paper  and  debris should be
removed from the sand filters every 6 months or as
necessary to keep the filter clean. A record should
be kept of the dewatering times for all sand filters
to  determine  if  maintenance   is  necessary.
Corrective maintenance  of  the filtration chamber
includes removal and replacement of the top layers
of sand, gravel and/or filter fabric that has become
clogged.   The removed media may  usually be
disposed in a landfill. The City of Austin tests their
waste media before disposal.  Results thus far
indicate that the waste media is not toxic and can be
safely landfilled  (Schueler,  1992).   Sand  filter
systems may also require the periodic removal of
vegetative growth.
Percent Removal
  Fecal Coliform

  Biochemical Oxygen
  Demand (BOD)

  Total Suspended Solids

  Total Organic Carbon

  Total Nitrogen (TN)

  Total Kjeldahl Nitrogen

  Nitrate as Nitrogen

  Total Phosphorus (TP)

  Iron (Fe)


  Zinc (Zn)	








  Source: Galli, 1990

The construction cost for an Austin sand filtration
system is approximately $18,500 (1997 dollars) for
a 0.4 hectare- (1 acre-) drainage area.  The cost per
hectare decreases with increasing drainage area.
The cost for precast Washington, D.C. sand filters,
with drainage areas  of less than 0.4 hectares (1
acre), ranges  between $6,600 and $11,000 (1997
dollars).  This is considerably less than the cost for
the same size cast-in-place system.  Costs for the
Delaware sand filter are similar to that of the D.C.
system, with the exception of the lower excavation
costs due to the Delaware filters' shallowness.

Annual costs for maintaining sand filter systems
average about 5 percent of the initial construction
cost (Schueler, 1992). Media is replaced as needed.
Currently the sand is being replaced in the D.C.
filter systems about  every  2  years.   The cost to
replace the gravel layer, filter fabric and top portion
of the sand for D.C.  sand filters is approximately

$1,700  (1997   dollars).    Improvements  in
Washington, D.C.'s maintenance procedures may
extend the life of the filter media and reduce the
overall maintenance costs.


1.      City  of Austin, Texas,   1988.   Design
       Guidelines for  Water  Quality  Control
       Basins. Environmental Criteria Manual.

2.      City of Austin, Texas,  1990a.  Removal
       Efficiencies  of  Storm  Water  Control
       Structures.     Environmental  Resource
       Division, Environmental and Conservation
       Services Department.

3.      City  of Austin,  Texas,  June 20,1990.
       Memo from Leslie Tull,  Water  Quality
       Management Section.  Updated December
       14, 1998.

4.      Galli,  I, 1990.  Peat Sand Filters:  A
       Proposed  Storm  Water  Management
       Practice  for   Urbanized  Areas.
       Metropolitan  Washington  Council  of

5.      Naval  Facilities   Engineering   Service
       Center, 1997.  "Sand Filter for Treating
       Storm  Water  Runoff."  Joint   Service
       Pollution  Prevention   Opportunity
       Handbook, Version 1.1.   Internet site at
       1297.html], accessed June, 1999.

6.      Northern  Virginia  Planning  District
       Commission  (NVPDC), 1992.  Northern
       Virginia BMP Handbook.

1.      Northern  Virginia  Planning  District
       Commission  (NVPDC), 1996.  Northern
       Virginia BMP Handbook Addendum," S and
       Filtration Systems."

8.      Schueler,  T.R.,   1992.     A  Current
       Assessment of Urban Best Management
       Practices.     Metropolitan  Washington
       Council of Governments.
9.      Shaver, E., 1991.  Sand Filter Design for
       Water  Quality  Treatment.    Delaware
       Department  of  Natural  Resources  and
       Environmental   Control.     Updated
       December, 1998.

10.    Troung, H., 1989.  The Sand Filter Water
       Quality  Structure.    The   District  of

11.    City of Washington, D. C., 1992. Personal
       Communication with Parsons Engineering
       Science, Inc.


City of Austin, Texas
Leslie Tull
Water Quality Management Section
206 E.  9th Street
Austin, TX 78767

Baltimore County, Maryland
Al Wirth
Department  of Environmental  Protection  and
Resource Management,  Stormwater Management
401 Bosley Avenue
Towson, MD 21204

Center for Watershed Protection
Tom Schueler
8391 Main St.
Ellicott City, MD21043

State of Delaware
Earl Shaver
Delaware Department of Natural  Resources and
Environmental Control
59 King's Highway, P.O. Box 1401
Dover, DE 19903

Northern Virginia Planning District Commission
David Bulova
7535 Little River Turnpike, Suite 100
Annandale, VA 22003

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, DC, 20460
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