v>EPA
United States
Environmental Protection
Agency
Off ice of Water
Washington, D.C.
EPA 832-F-99-007
September 1999
Storm Water
Technology Fact Sheet
Sand Filters
DESCRIPTION
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^
\
Channel
op Inlet
i
V
*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.
FIGURE 1 TYPICAL AUSTIN SAND FILTER DESIGN
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30" Manhole
Frame & Cover
Ladder 30" Manhole
Frame & Cover
24" Manhole
Frame & Cover
Inflow
Pipe
6" PVC
^Dewatering
Drain with
PVC Gate
valve.
Washed iy
Aggregate
6" PVC Clean Out
Pipe with Cap
Filter
Fabric
-' -J - SI Outflow
Pipe
Source: Troung, 1989.
FIGURE 2 TYPICAL WASHINGTON, D.C. SAND FILTER DESIGN
Grated Cover
Solid Cover
Flow
Grate (Fabric Wrapped
Over Entire Grate Opening)
Source: Shaver, 1991.
FIGURE 3 TYPICAL DELAWARE SAND FILTER DESIGN
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
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filter may increase microbial growth within the
sand filter and improve metals and nutrient removal
rates.
APPLICABILITY
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
groundwater.
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.
ADVANTAGES AND DISADVANTAGES
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.
DESIGN CRITERIA
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.
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Equations used to determine the sedimentation
basin surface areas (As) in square and meters acres
are shown in Table 1.
TABLE 1 SURFACE AREA EQUATION
FOR AUSTIN SAND FILTER SYSTEM
Partial Sedimentation Full Sedimentation
As=(AD)(H)/(1/Ds-1/10)
Af=(AD)(H)/10
As=(AD)(H)/10
Af=(AD)(H)/18
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
minimized.
Providing adequate storm water detention
times to enhance sedimentation and
filtration.
Inspecting and maintaining the sand filter
frequently enough to ensure proper
operation.
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PERFORMANCE
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.
OPERATION AND MAINTENANCE
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.
TABLE 2 TYPICAL POLLUTANT
REMOVAL EFFICIENCY
Pollutant
Percent Removal
Fecal Coliform
Biochemical Oxygen
Demand (BOD)
Total Suspended Solids
(TSS)
Total Organic Carbon
(TOC)
Total Nitrogen (TN)
Total Kjeldahl Nitrogen
(TKN)
Nitrate as Nitrogen
(NO3-N)
Total Phosphorus (TP)
Iron (Fe)
Lead(Pb)
Zinc (Zn)
76
70
70
48
21
46
33
45
45
45
Source: Galli, 1990
COSTS
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
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$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.
REFERENCES
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
Governments.
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
[http://enviro.nfesc.navy.mil/p21ibrary/10-
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
Columbia.
11. City of Washington, D. C., 1992. Personal
Communication with Parsons Engineering
Science, Inc.
ADDITIONAL INFORMATION
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
Section
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
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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
IMTB
Excellence in compliance through optimal technical solutions
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