United States
Environmental Protection
Agency
Decentralized Systems Technology Fact Sheet
Septic Tank Effluent Screens
DESCRIPTION
A septic tank is a traditional wastewater treatment
technology that uses an underground tank to hold
and treat wastewater. As wastewater flows into the
tank, heavier materials settle to the bottom and
form a sludge layer, while lighter greases and fats
float to the top, forming a scum layer. Clarified
effluent is piped from the center of the tank and
into a drainfield, where it percolates into the
surrounding soil.
An effluent screen (Figure 1) is a physical device
that is placed on the outlet pipe of the septic tank to
enhance solids removal from the septic tank
effluent. In addition, by preventing excess solids
from flowing out into the drainfields with the
clarified effluent, these screens help to prevent
blockages that can damage the drainfield. Finally,
in some cases, a thin layer of organic growth called
a "biomat" may build up on the screen. This
biomat is rich in anaerobic bacteria, which can help
to remove viruses and pathogens from the effluent.
FIGURE 1 EFFLUENT SCREEN
APPLICABILITY
The use of effluent screens in septic tanks is
becoming more common in the U.S. Installation of
effluent screens on septic tanks is mandatory in
more than 50 counties nationwide, as well as in the
states of Florida, Georgia, North Carolina, and
Connecticut.
ADVANTAGES AND DISADVANTAGES
The two primary benefits of using effluent screens
in septic systems are that screens improve the
quality of the effluent and extend the life of the
leach field. Additional advantages and
disadvantages of using effluent screens in septic
systems are listed below.
Advantages
• Helps prevent solids from clogging the
drainfield.
Keeps non-biodegradeable objects from
entering the drainfield.
Can be placed in existing or new septic
tanks.
Requires little routine maintenance because
there are no moving parts.
• Units are relatively inexpensive.
Disadvantages
Regular clean-out of the effluent screen is
required to maintain optimal total
suspended solids removal.
Requires surface access for servicing.
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DESIGN CRITERIA
The two primary design considerations for septic
tank effluent screens are the location of the screen
and the flow area of the screen relative to the size
of the tank.
Effluent screens can be placed directly in the septic
tank's outlet tee, or in a separate housing unit.
When the screen is placed in a housing unit (Figure
2), the housing unit can act as a second settling
chamber, increasing the clarity of the effluent
before it goes through the screen.
If the effluent screen is located in the outlet tee
within the septic tank, it should be placed in the
clear-water zone beneath the scum layer and above
the sludge layer (Figure 3). The bottom of the
screen should extend into the liquid a distance
equal to 40 percent of the liquid depth. This should
Ground
To leachfield
Filter
Outlet
Filter
Chamber
FIGURE 2 EFFLUENT SCREEN LOCATED
OUTSIDE THE SEPTIC TANK
Source: Barnstable County (Mass.) Department of Health
and Environment (use of Zabel filter), 2003.
ensure that neither scum nor sludge will be
transferred onto the screen, and will therefore
maximize the clarity of the effluent flowing out of
the tank.
Protected Septic Tank
SmarlFilter1"
Switch
Outlet:
Filtered
wastewater
leaves the Lank
FIGURE 3 FILTER PLACEMENT BETWEEN THE SCUM AND SLUDGE LAYERS
Source: Zabel, 2003.
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The other major design consideration is ensuring
that the flow area ( the combined area of the
perforations or openings in the screen through
which liquid passes) is sufficient for the flow rate
and the solids concentrations in the system.
Screens placed in systems with high flow rates
and/or high solids content will need higher flow
areas to avoid screen clogging. Some screens have
an alarm to alert the owner if the filter becomes
clogged. This can allow the owner to clean the
effluent screen before effluent backs up in the tank.
PERFORMANCE
As described above, effluent screens are designed
to remove solids. Most effluent screens have the
capability to retain solids that are greater than 3
mm (Vs in) in diameter. However, solids removal
performance for any given septic tank effluent
screen will depend on a number of factors, the most
important of which is daily flow. The higher the
flow, the more likely it is to overload the filter,
even at average solids loadings. Larger systems
may require multiple filters in a manifold
arrangement to treat the daily flow.
Effluent screens can also enhance the
decomposition of solids within the tank. Effluent
passes through the effluent screen through vertical
inlet holes, while larger particles are retained in the
tank. As these particles settle in the tank, further
decomposition of organic materials occurs.
OPERATION AND MAINTENANCE
Because of their lack of moving parts, effluent
screens require minimal maintenance. Nonetheless,
lack of attention will lower their overall efficiency,
and regular maintenance is important to ensure
efficient screen operation.
The primary maintenance activity is cleaning the
screen to prevent plugging. When an effluent
screen plugs, liquid backs up and cannot exit the
tank. To avoid this problem, effluent screens must
be cleaned on a regular basis. The cleaning
frequency will be dependent on the size of the
screen, environmental conditions, and the type of
material entering the septic system. Smaller flow
areas and smaller effluent screen openings increase
the need for maintenance. Most manufacturers
recommend cleaning the screen every one to three
years, depending on site characteristics.
COSTS
Effluent screens cost from $70-$300 per unit.
Installation and servicing add additional costs.
REFERENCES
Other Related Fact Sheets
Septic System Tank
EPA 832-F-00-040
September 2000
Septic Tank Leaching Chamber
EPA 832-F-00-044
September 2000
Other EPA Fact Sheets can be found at the
following web address:
http://www.epa.gov/owm/mtb/mtbfact.htm
1. Architerra Enterprises, Inc. The Natural
Home Building Source, 2001. Website at
http://www.thenaturalhome.com/septicfilt
er.htm. Accessed 2001.
2. Arrow Septic and Sewer, 2001. Website at
http ://www. arrowseptic. com/effluent_filte
rs.htm. Accessed May 31, 2001.
3. Barnstable County (Mass.) Department of
Health and Environment. "Effluent and
Vent Filters" Website at
http://www.barnstablecountyhealth.org/
AlternativeWebpage/Filters/Filters.htm.
Accessed August 2003.
4. Byers, M..E., K.E Zoeller, and J.D.
Fletcher, 2001. "Septic Tank Effluent
Filters: Monitoring and Assessment." On-
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5.
6.
7.
9.
10.
11.
Site Wastewater Treatment; Proceedings of
the Ninth National Symposium on
Individual and Small Community Sewage
Systems.
Crites, R. and G. Tchobanoglous, 1998.
Small and Decentralized Wastewater
Management Systems, WCB. McGraw-
Hill, Inc., Boston, Massachusetts.
Hammer, M.J., 1975.
Wastewater Technology.
Sons, Inc., New York.
Water and
John Wiley &
Metcalf and Eddy, Inc., 1991. Wastewater
Engineering: Treatment, Disposal, and
Reuse. McGraw-Hill, Inc., Boston,
Massachusetts.
National Precast Concrete Organization,
2001. Website at http://www.precast.org.
Accessed September 21, 2001.
North Carolina State University Department
of Soil Science, 2001. Website at
http://ces.soil.ncsu.edu/soilscience/publica
tions/Soilfacts/AG-439-13/. Accessed May
31,2001.
U.S. EPA, 1980. Onsite Wastewater
Treatment and Disposal Systems.
Zabel Industries. Website at
http://www.zabelzone.com. Accessed
August 2003.
ADDITIONAL INFORMATION
Brown and Caldwell
Ronald W. Crites
P.O. Box 8045
Walnut Creek, California 94596
Flowlink Manufacturing Company
7225 Pacific Avenue, SE
Olympia, Washington 98503
Orenco Systems, Inc.
814 Airway Avenue
Sutherlin, Oregon 97479
Environmental Engineering Consultants
Sherwood Reed
50 Butternut Road
Norwich, Vermont 05055
Zabel Environmental Technology
10409 Watterson Trail
Jeffersontown, Kentucky 40229
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-03-023
September 2003
For more information contact:
Municipal Technology Branch
U.S. EPA
1200 Pennsylvania Avenue, NW
Mail Code 4204M
Washington, D.C. 20460
* 2002 *
THE YEAR OF
CLEAN WATER
iMTB
Excellence in compliance through optimal technical solutions
MUNICIPAL TECHNOLOGY BRANCH
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