Source: Schueler, 1992.

FIGURE 1 TYPICAL AUSTIN SAND FILTER DESIGN

United States

Environmental Protection

Agency

Office 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
Source: Shaver, 1991.

FIGURE 3 TYPICAL DELAWARE SAND FILTER DESIGN

Source: Troung, 1989.

FIGURE 2 TYPICAL WASHINGTON, D.C. 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
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.
TABLE 1 SURFACE AREA EQUATION

FOR AUSTIN SAND FILTER SYSTEM

Partial Sedimentation Full Sedimentation

As=(AD)(H)/(1/Ds-1/10) As=(AD)(H)/10

Af=(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.

Equations used to determine the sedimentation

basin surface areas (As) in square and meters acres

are shown in Table 1.

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 major 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.
TABLE 2 TYPICAL POLLUTANT

REMOVAL EFFICIENCY

Pollutant Percent Removal

Fecal Coliform 76

Biochemical Oxygen

Demand (BOD)

70

Total Suspended Solids

(TSS)

70

Total Organic Carbon

(TOC)

48

Total Nitrogen (TN) 21

Total Kjeldahl Nitrogen

(TKN)

46

Nitrate as Nitrogen

(NO3-N)

0

Total Phosphorus (TP) 33

Iron (Fe) 45

Lead (Pb) 45

Zinc (Zn) 45

Source: Galli, 1990

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.

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
$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, J., 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/p2library/10-

1297.html], accessed June, 1999.

6. Northern Virginia Planning District

Commission (NVPDC), 1992. Northern

Virginia BMP Handbook.

7. Northern Virginia Planning District

Commission (NVPDC), 1996. Northern

Virginia BMP Handbook Addendum, “Sand

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, MD 21043

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
For more information contact:

Municipal Technology Branch

U.S. EPA

Mail Code 4204

401 M St., S.W.

Washington, DC, 20460

recommendation for the use by the U.S.

Environmental Protection Agency.