&EPA
United States
Environmental Protection
Agency
Decentralized Systems Technology Fact
Sheet
Septic Tank Polishing
DESCRIPTION
Polishing systems are used to improve the quality of
septic tank system effluent. Effluent polishing may be
necessary due to site constraints, regulations, or other
limiting factors. One of the most common technologies
used to polish septic tank effluent is the sand filter.
Because sand filters can be designed in various
configurations, they are highly flexible and can be
adapted to many different types of sites, making them
ideal for use in different community settings. The three
types of sand filters typically used for septic tank
polishing include buried, intermittent, and recirculating
sand filters.
Treatment of effluent by sand filter systems involves
physical, chemical, and biological processes.
Suspended solids are removed principally by
mechanical straining and sedimentation. Action by
bacteria that colonize sand grains further enhances the
removal of suspended solids. The removal of
biological oxygen demand (BOD) and the conversion
of ammonia to nitrate (nitrification) is performed under
aerobic conditions by microorganisms present in the
sand bed. The conversion of nitrate to nitrogen gas
(denitrification) is routinely performed by anaerobic
bacteria that exist in the anaerobic zones near the
bottom of the filter and in anaerobic tanks, resulting in
a significant (up to 45 percent) loss of nitrogen.
Specific constituents are removed by sorption, both
chemical and physical. Intermittent application and
venting of the underdrains helps to maintain aerobic
conditions in the filter, which helps achieve a high
performance level.
DESIGN CRITERIA
Buried sand filters are typically installed with
underdrains in 30 cm (1 ft) of coarse gravel, covered
with 60-90 cm of sand. Liquid enters through a
perforated pipe in another foot of gravel, and covered
with at least 15 cm (6 in) of topsoil. Intermittent sand
filters are divided into two or more units that are
alternately loaded and rested. Wastewater is applied
over a bed of sand 60 to 90 cm (2 to 3 ft) deep. The
sand should have an effective size of 0.2 to 0.6 mm,
with a uniformity coefficient less than 4.0. The filtrate
is collected by underdrains contained in a bottom layer
of gravel. The sand remains aerobic and serves as a
biological filter, removing suspended solids (SS) and
dissolved organics. Because of smaller sand size and
higher loading rates, these units must be accessible for
periodic servicing. The recirculating filter system
consists of a septic tank and a recirculation tank that
contains a timer-controlled sump pump for dosing onto
a sand filter. The filter bed contains 90 cm (3 ft) of
coarse sand and 30 cm (1 ft) or less of gravel
surrounding the underdrain system. In this case, the
sand should have an effective size of 0.6 to 1.5 mm
with less than a 2.5 uniformity coefficient. A
recirculation ratio of 4:1 (recycled filter effluent to
forward flow) is recommended. If tank effluent
requires disinfection, common methods used in on-site
systems include tablet chlorination, iodine crystals, or
ultraviolet irradiation. Designers must be careful when
specifying sand - minimum dust content is essential.
Although sand is the most common media, alternative
polishing media exist, including foam and geotextile
fabric, which produce high quality effluents. These
media are pre-fabricated, allowing performance to be
unaffected by the grading of the sand. However,
stringent fecal coliform effluent requirements may
require sand filter polishing in addition to textile filtering.
Buried sand filters are generally constructed in two
sections that are dosed separately from a tank with
alternating siphons. Above ground sand filters
(intermittent or recirculating) can be installed in areas
where subsurface construction is impossible. Dosing
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tanks with pumps or siphons feed these filters. The
filters may be open or covered, but must be accessible
for cleaning. Covering and insulation are
recommended for intermittent and recirculating filters to
minimize freezing in cold weather and potential health
risks and nuisances in warm weather.
Typical recommended loading rates from sand filter
systems are 30 to 60 L/m2 d (0.75 to 1.5 gal/ft2 d)
for buried sand filters; 200 L/m2 d (5 gal/ft2 d) for
intermittent sand filters; and 120 L/m2 d (3 gal/ft2 d)
for recirculating sand filters (based on forward flow
alone).
ADVANTAGES AND DISADVANTAGES
Advantages
Sand filters are relatively inexpensive, have low energy
requirements, and are highly flexible. They can be used
on sites with shallow soil cover, high groundwater, and
unsuitable permeability. Sand filters do not require
highly skilled operators because the process is stable
and no chemicals are required during operations.
Filters generally produce high quality effluents.
Disadvantages
Land availability may limit the application of polishing
systems. Furthermore, the amount of head required by
the filters typically exceeds 90 cm (3 ft). As a
consequence, pumping may be required if elevation
differentials are inadequate. Odors from anaerobic
portions of open, single pass filters used to treat septic
tank effluent may be a problem if not installed correctly,
and ongoing maintenance is necessary for the media,
pumps, and controls. Power is required for pumping
and some disinfection units. State or federal discharge
permits are required, accompanied by periodic
sampling and monitoring.
PERFORMANCE
Table 1 provides details of typical improvements in
effluent quality with intermittent sand filtration of lagoon
effluent.
OPERATION AND MAINTENANCE
Sand filters require relatively little operational control
and maintenance. Primary servicing tasks include filter
surface maintenance, dosing equipment, and monitoring
of influent and effluent. With continued use, sand filter
surfaces will become clogged with organic biomass and
solids, and when operating infiltration rates fall below
the hydraulic loading rate, permanent ponding of the
filter surface will occur, indicating that the filter should
be taken off-line for rest or sand removal and
TABLE 1 TREATMENT PERFORMANCE OF ON-SITE SEPTIC TANK AND SAND
FILTER
Parameter
Raw Waste
Septic Tank Effluent Intermittent Sand Filter
Effluent
BOD, mg/L
SS, mg/L
Total nitrogen, mg/L
Ammonia-nitrogen, mg/L
Nitrate-nitrogen, mg/L
Total phosphorus, mg/L
Fecal coliforms (#/ 100
210
237
35
7
<
10
106
-530
-600
-80
-40
= 1
-27
-1010
140
50
25
20
<
10
103
-200
-90
-60
-60
: 1
-30
-106
< 10
< 10
-
< 0.5
25
-
102-104
mL)
Vi ruses (#/100 mL)
Unknown
105-107
Source: Adapted from Tchobanoglous and Burton, 1991.
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replacement. Inaccessible buried filters are designed to
operate without maintenance for their design life.
Filters exposed to sunlight may develop algae mats,
which can be controlled by shading the surface.
Disinfection is required prior to discharge in community
systems, but disinfectant quantity requirements are low
due to the high quality of the effluent from the sand
filter.
Weeding should be performed at the surface of above-
ground filters to prevent unwanted vegetative growth.
In cold climates, the filter should be insulated and the
distribution lines must be drained to prevent standing
water and to prevent freezing.
Although it is a common maintenance practice, surface
tilling is not recommended for slow sand filtering
systems. This process moves clogged zones to the
bottom of the tilled zone which may exacerbate surface
ponding problems.
COSTS
Filter costs depend on many factors including soil type,
cost of land, site topography, groundwater level, and
cost of filter media. These site and system specific
factors should be examined and incorporated when
preparing a polishing filter cost estimate.
Construction Costs
Under typical, favorable soil conditions, the cost to
install a polishing filter system is greater than the costs
of a conventional gravel pipe drainfield. Nonetheless,
while drainage pipe costs are lower, the drainfield
footprint may be up to two times larger than that of a
conventional gravel drainfield. Typical costs for a
single pass sand recirculating filter system range
between $7,000 and $15,000, including the septic tank
and soil adsorption field. System design by an
engineer, if required, will be an additional cost. If the
existing site is inadequate for a new drainfield or if the
existing field is no longer serviceable, removal and
disposal costs should be considered.
Operation and Maintenance Costs
Operation and maintenance costs for sand filtration filter
systems are minimal. Key costs associated with proper
functioning of drainfield systems include septic tank
cleaning, which ranges between $400 to $1,500 per
cleaning.
REFERENCES
Other EPA Fact Sheets can be found at the following
web address:
http://www.epa.gov/owm/mtb/mtbfact.htm
1. Amberg, Larry W., 1988. Rock-plant Filter-
An Alternative for Septic Tank Effluent
Treatment. Department of Health and
Hospitals, Office of Public Health, Bossier
City, Louisiana.
2. Barrett, Michael E. and J. F. Malina, Jr., 1991.
Technical Summary of Appropriate
Technologies for Small Community
Wastewater Treatment Systems. The
University of Texas at Austin.
3. Barrett, Michael E. and J. F. Malina, Jr., 1991.
Wastewater Treatment Systems for Small
Communities: A Guide for Local
Government Officials. The University of
Texas at Austin.
4. Jowett, E. Craig, 1997. Field Performance
of the Waterloo Biofilter with Different
Wastewaters. National Small Flows
Clearinghouse.
5. Tchobanoglous, G., and F. L. Burton, 1991.
Wastewater Engineering: Treatment,
Disposal and Reuse. McGraw-Hill Inc., New
York, New York.
6. Treanor, William O., 1995. Treatment
Capability of Three Filters for Septic Tank
Effluent. Tennessee Technological University.
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7. U.S. EPA, 1980. Design Manual: Onsite
Wastewater Treatment and Disposal
Systems. EPA 625/1-80/012.
8. U.S. EPA, 1989. Alternative Sewers
Operation and Maintenance Special
Evaluation Project. Office of Water.
9. U.S. EPA, 1991. Design Manual:
Alternative Wastewater Collection Systems.
EPA 625/1-91/024.
10. U.S. EPA, 1992. Summary Report Small
Community Water and Wastewater
Treatment. EPA 625/R-92/010.
ADDITIONAL INFORMATION
Ronald W. Crites
Brown and Caldwell
P.O. Box 8045
Walnut Creek, CA 94596
Sherwood Reed
Environmental Engineering Consultants
50 Butternut Road
Norwich, VT 05055
The mention of trade names or commercial products
does not constitute endorsement or recommendation
for use by the U. S. Environmental Protection Agency
(EPA).
Office of Water
EPA 832-F-02-021
September 2002
For more information contact:
Municipal Technology Branch
U.S. EPA
1200 Pennsylvania Avenue, NW
Mail Code 4204M
Washington, D.C. 20460
* 2002*
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CIJFAN WATER
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