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
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 210 - 530 140 - 200 < 10

SS, mg/L 237 - 600 50 - 90 < 10

Total nitrogen, mg/L 35 - 80 25 - 60 --

Ammonia-nitrogen, mg/L 7 - 40 20 - 60 < 0.5

Nitrate-nitrogen, mg/L < 1 < 1 25

Total phosphorus, mg/L 10 - 27 10 - 30 --

Fecal coliforms (# / 100

mL)

106 - 1010 103 - 106 102 - 104

Viruses (# / 100 mL) Unknown 105 - 107

--

Source: Adapted from Tchobanoglous and Burton, 1991.
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 aboveground 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.