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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