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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