United States
                      Environmental Protection
                      Agency
 Office of Water
 Washington, D.C.
EPA 832-F-00-040
September 2000
                      Decentralized  Systems
                      Technology  Fact Sheet
                      Septic System Tank
DESCRIPTION

A septic tank is an underground engineered tank
consisting of single or multiple units, together with
one or more connecting piping systems installed in
appropriate soils to receive wastewater flow from
one or more  residences or public buildings.
Wastewater is pretreated in the septic tank before
being discharged  to a final treatment system.
Annually or semi-annually, liquids  and solids
retained in the tank are pumped into a tank vehicle
which transports sewage to a final treatment site.

A septic tank is a traditional wastewater treatment
technology using a tank as the primary treatment
and holding device.  A system to handle multiple
residences may be designed as a collection  of
individual holding  tanks  with a  community
treatment and  disposal system  or a community
collection and treatment system.  The decision on
which type to  use is  based on available land,
existing systems, and maintenance issues. Figure 1
illustrates  a  septic tank  with  a leaching  field
downstream.

The primary device in treatment is the tank, an
enclosed watertight container that collects and
provides primary treatment of wastewater by
holding wastewater in the  tank and allowing
settleable  solids to  settle to the bottom while
floatable solids (oil and greases) rise to the top.
The tank should retain the wastewater for at least 24
hours.

Some solids are removed from the wastewater,
some are digested, and some are stored in the tank.
Up to 50 percent of the solids retained in the tank
                       Septic
                      System Tank
Source: U.S. EPA, 1991.

    FIGURE 1 SEPTIC SYSTEM TANK


decompose, while the remainder accumulate  as
sludge at the tank bottom and must be removed
periodically by pumping the tank.

There  are three main types of tanks for on-site
sewage holding and pretreatment:

      Concrete tanks.

      Fiberglass tanks.

      Polyethylene/plastic tanks.

All tanks must be watertight. Water entering the
system  can  saturate the soil  absorption field,
resulting in a failed system.

From the tank, the wastewater enters a sewer or is
passed directly  to a treatment system. The most
common outlet is a pipe fitting connected to the
septic tank. An effluent filter can be placed in the
outlet for additional filtering of the wastewater.

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Removing more solids from the wastewater helps to
prevent clogging the absorption field and causing
premature failure.

APPLICABILITY

A holding tank is used to pre-treat sewage and
make subsequent treatment systems more effective
by allowing a constant flow to enter the treatment
system. The effluent from the tank is consistent,
easy to convey, and easily treated by either aerobic
(with free  oxygen)  or  anaerobic (without  free
oxygen) processes.

ADVANTAGES AND DISADVANTAGES

Advantages

Subsurface infiltration systems are ideally suited for
decentralized treatment of wastewater because they
are buried. The tanks are relatively inexpensive and
can be installed in multiple tank installations.

Disadvantages

The sludge may pose an odor problem if the sewage
remains  untreated   for  an   extended  period.
Provisions for alarms and pumping are necessary if
the downstream treatment units  go off-line due to
power loss or equipment failure.

DESIGN CRITERIA

A holding tank must be the proper  size, have a
watertight design, and stable  structure for  proper
performance.

Tank size: The size of a tank for a single residence
depends upon the number of bedrooms, the number
of inhabitants,  the  home's square footage,  and
whether or not water-saving fixtures are used. For
example,  a  three-bedroom  house  with four
occupants and  no water-saving fixtures  would
require a 1,000-gallon septic tank. The tank should
be designed to hold at least one week of waste flow
(U.S. EPA,  1992.)  Holding tank  systems for
multiple units should include the above parameters
for each residence.  Commercial inputs should be
evaluated on a case by case basis and may need pre-
treatment to remove oil, grease, or solids.
Tank design: A key factor in the holding tank's
design is the relationship  between  the  liquid
surface area, the quantity of sewage it can store, and
the rate of wastewater discharged.  Each of these
factors will  impact the tank efficiency and the
amount of sludge it retains.

The greater the  liquid's  surface area, the more
sewage the  tank  can accommodate.  As solids
collect in the tank, the water depth decreases, which
reduces the  time sewage flow is  retained in the
tank.  Less solids will settle in the tank, resulting in
increased solids in the tank effluent that may have
a negative impact on the final treatment process.

Placing risers on the tank openings makes it easier
to access the tank for inspection and maintenance.
If a septic tank is buried more than 12 inches below
the soil  surface,  a riser  must be used  on the
openings to bring the lid to within 6 inches of the
soil surface.  Generally, the riser can be extended to
the ground surface and protected with a lid.

Hydraulic Loading Rate

The design capacity of the holding tank is  related
to the hydraulic  loading  rate  of the treatment
system.   For a ground  absorption system, it is
determined  by  soil characteristics,  groundwater
mounding  potential,  and  applied  wastewater
quality. Prolonged wastewater loading will clog the
infiltrative surface, reduce the capacity of the soil to
accept the wastewater, and may back  up the
wastewater into the holding tank. However, if the
loading is controlled, biological  activity  at the
infiltrative   surface  will   maintain   waste
accumulations in  relative  equilibrium so  that
reasonable infiltration rates and pass through in the
holding tank can be sustained.

PERFORMANCE

To keep a holding tank system operating efficiently,
the tank  should be pumped  periodically.  As the
system is used, sludge accumulates in the bottom of
the tank.  As the sludge level increases, wastewater
spends less time in the tank, and solids are more
likely to  escape into the absorption area. Properly
sized tanks can accumulate sludge for at least three
years.

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The frequency of pumping depends on:

•      Tank capacity.

•      Amount of wastewater flowing into the tank
       related to size of household(s).

•      Amount of solids  in the wastewater. For
       example, there will  be  more  solids  if
       garbage disposals are used.

•      Performance of the final treatment system.

OPERATION AND MAINTENANCE

A well-designed  holding tank requires limited
operator  attention.  Management needs include
tracking   system  status,  testing   for   solids
accumulation, evaluating pump performance, and
monitoring   system   controls.     Monitoring
performance  of pretreatment units,  mechanical
components, and wastewater ponding levels above
the filtration surface is essential. If a performance
or level change is noted, the operator should inspect
the system to determine  if additional service is
required.  Routine servicing of a holding tank is
limited to annual or semiannual inspection and
cleaning, if necessary.

COSTS

The costs for tanks greatly vary for each site. Land
and earthworks are the most significant capital
costs.  Where a  select fill must be used to bed the
tank, the cost of transporting this material may be
significant.  The factors that affect costs include
location, access, subsurface site conditions, and the
type of tank installed.  A general cost range for
tanks is from $1.00 to $4.00 per gallon. (A 1,000
gallon  tank installed in the City of Austin cost
$2,000.)   Other costs include  installation  of
equipment to  transport the wastewater  to the
holding and/or treatment site.
REFERENCES

Other Related Fact Sheets

Septic Tank Leaching Chamber
EPA 832-F-00-044
September 2000

Septic Treatment/Disposal
EPA 832-F-99-068
September 1999

Septic Tank - Soil Absorption Systems
EPA 832-F-99-075
September 1999

Other EPA Fact Sheets  can  be found  at  the
following web address:
http://www.epa.gov/owmitnet/mtbfact.htm

1.     Barret, Michael E.  and J. F. Malina,  Jr.,
      Sep.  1,  1991.   Technical Summary  of
      Appropriate  Technologies  for  Small
      Community Wastewater Treatment Systems.
      The University of Texas at Austin.

2.     City  of Austin, "Septic  Tank".   Site
      accessed    May     2000.
      http://www.ci.austin.tx.us/wri/treatl.htm

3.     Corbitt,  Robert  A.,  1990.     Standard
      Handbook of Environmental Engineering
      McGraw-Hill, Inc., New York, New York.

4.     Crites, R.  and G. Tchobanoglous,  1998.
      Small and   Decentralized Wastewater
      Management Systems, WCB.   McGraw-
      Hill, Inc. Boston, Massachusetts.

5.     U.S.  Environmental  Protection Agency.
      1980.  Design Manual: Onsite Wastewater
      Treatment & Disposal Systems. EPA Office
      of Water.   EPA Office  of Research  &
      Development.   Cincinnati,  Ohio.  EPA
      625/1-80/012.

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6.     U.S. Environmental Protection  Agency.
      Sep. 1992.  Design Manual: Wastewater
      Treatment   and  Disposal  for  Small
      Communities. EPA Office of Water. EPA
      Office   of  Research  &  Development.
      Cincinnati, Ohio. EPA 625/R-92/005.

The  mention   of trade  names  or  commercial
products  does not  constitute  endorsement  or
recommendation for use by the U. S. Environmental
Protection Agency (EPA).
                                                        For more information contact:

                                                        Municipal Technology Branch
                                                        U.S. EPA
                                                        Mail Code 4204
                                                        1200 Pennsylvania Avenue, NW
                                                        Washington, D.C. 20460
                                                          MTB
                                                        Excellence in compliance through optimal technical solutions
                                                        MUNICIPAL TECHNOLOGY B R A iT

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