v>EPA
United States
Environmental Protection
Agency
Office of Water
Washington, D.C.
EPA 932-F-99-075
September 1999
Decentralized Systems
Technology Fact Sheet
Septic Tank - Soil Absorption Systems
DESCRIPTION
An estimated 30 percent of all U.S. households use
on-site treatment methods (Hoover et a/., 1994).
Septic tank/soil absorption has been the most
popular on-site method (U.S. EPA, 1980a.) The
septic tank is an underground, watertight vessel
installed to receive wastewater from the home. It is
designed to allow the solids to settle out and
separate from the liquid, to allow for limited
digestion of organic matter, and to store the solids
while the clarified liquid is passed on for further
treatment and disposal. Though septic tank effluent
can be treated in a variety of ways, this Fact Sheet
describes the distribution of effluent wastewater into
a subsurface soil absorption area or drainfield.
APPLICABILITY
Septic tank/soil absorption systems are an option to
consider wherever a centralized treatment system is
not available. Since subsurface soil treatment and
disposal relies upon gradual seepage of wastewater
into the surrounding soils, these systems can only be
considered where favorable soil characteristics and
geology exist for treatment and subsequent disposal
of the treated wastewater into the environment.
For effective wastewater treatment, prospective
soils should be relatively permeable and should
remain unsaturated to several feet below the system
depth. Moreover, the soil absorption system should
be set well above water tables and bedrock.
Further, it cannot be easily located in steeply sloped
areas (U.S. EPA, 1980a.) For regions with high
water tables or shallow bedrock, other systems
using more advanced technology may be better
options for wastewater treatment. (See Wastewater
Technology Fact Sheet: Mound Systems.) In cases
where impermeable soils exist, fill systems and
sand-lined trench systems—in which fill material is
brought in to replace unsuitable soils—may be a
feasible alternative.
To avoid contamination of drinking water sources
and other problems, soil absorption systems must be
situated at prescribed distances from wells, surface
waters and springs, escarpments, property
boundaries and building foundations (U.S. EPA,
1980a). These regulations may restrict the
feasibility of septic system installation, depending on
property size, shape, and proximity to the features
noted.
Conventional septic systems are designed to operate
indefinitely if properly maintained. However,
because most household systems are not well-
maintained, the functioning life of septic systems is
typically 20 years or less. In contemporary practice,
it is commonly required that a second area of
suitable soil be reserved at the site as a "repair
area" in the event that the initial system fails to
operate properly or to allow for the possibility of a
future home addition project (Hoover, 1999.)
Since the soil absorption area must remain
unsaturated for proper system functioning, it may
not be feasible to install septic systems in regions
prone to frequent heavy rains and flooding, or in
topographical depressions where surface waters
accumulate.
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ADVANTAGES AND DISADVANTAGES
Advantages
• Simplicity, reliability and low cost.
• Low maintenance requirements.
• Nutrients in waste are returned to soil.
• A properly designed, well-maintained system
can last for more than twenty years.
Disadvantages
• Siting limitations for septic systems include
natural soil type and permeability, bedrock
and groundwater elevations, and site
topography.
• Regulations pertaining to set-backs from
water supply, lot lines, and drainage lines
must be taken into account.
• Restrictions on the character of influent
wastewater must be included in project
planning.
• Improperly functioning systems can introduce
nitrogen, phosphorus, organic matter, and
bacterial and viral pathogens into the
surrounding area and groundwater.
DESIGN CRITERIA
A septic system usually includes three components:
the septic tank, a drainfield and the soil beneath the
drainfield. The tank must be a watertight container
constructed of a sound, durable material resistant to
corrosion or decay (concrete, fiber reinforced
plastic, fiberglass, or polyethylene). The septic tank
is connected to a piping system that distributes
wastewater effluent into subsurface soil for
absorption and subsequent treatment.
Wastewater generated from a household is collected
and transported through the house drains to the
buried septic tank, where most of the solids settle
while grease and scum float to the surface. Inlet
baffles or effluent screens help to force wastewater
down into the tank, preventing short-circuiting
across the top. Outlet baffles keep the scum layer
from moving into the soil absorption system.
Collected solids undergo some decay by anaerobic
digestion in the tank bottom. The capacity of a
septic tank typically ranges from 3,785 to 7570
liters (1,000 to 2,000 gallons).
Clarified septic tank effluent exits the septic tank
and enters the soil absorption system (also called a
"leachfield" or "drainfield") where a biological
"clogging mat" or "biomat" forms, contributing to
even distribution of the waste into the drainfield
(U.S. EPA, 1980a; Hoover et. al., 1996.) State
regulations usually require between two and four
feet (or sometimes less) of unsaturated soil beneath
the drainfield to renovate wastewater before it
reaches a "limiting layer"—the point at which
conditions for waste renovation become unsuitable.
The limiting layer may be bedrock, an impervious
soil layer or the seasonal high water table.
Absorption beds and trenches are the most common
design options for soil absorption systems.
Trenches are shallow, level excavations, usually
from 0.305 to 1.524 meters (one to five feet) deep
and 0.305 to 0.914 meters (one to three feet) wide
(see U.S. EPA, 1980a.) The bottom is filled with at
least 15.24 centimeters (six inches) of washed gravel
or crushed rock over which a single line of 10.16
centimeters (four-inch) perforated pipe is placed.
Additional rock is placed over and around the pipe.
A synthetic building fabric is laid on top of the
gravel to prevent backfill from migrating into the
gravel trench. Beds are constructed analogously to
trenches, but are more than three feet wide and may
contain multiple lines of distribution piping. While
beds are sometimes preferred for space savings in
more permeable soils, trench designs provide more
surface area for soil absorption (U.S. EPA,1980a;
Hoover, 1999.)
The size of a soil absorption system is based on the
size of the house and the soil characteristics.
Traditionally, soil is evaluated using a "percolation
rate", a measure of the water migration rate through
the candidate soil. Acceptable limits of percolation
for drainfield suitability range between 23 seconds
and 24 minutes per centimeter (1 and 60 minutes
per inch) (U.S. EPA, 1980a.) Percolation rates of
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1.18 and 24 minutes per centimeter (3 and 60
minutes per inch) would correspond to absorption
areas of about 70 and 340 square meters
respectively per bedroom of the house to be
serviced (Harlan and Dickey, 1999.) Though the
number of bedrooms has typically been used as a
rule-of-thumb measure for tank sizing, it should be
noted that this is only an approximation; by itself, it
is an unreliable way to gauge anticipated waste
volume (U.S. EPA, 1980a.)
While some states continue to use the percolation
rate as a criterion for site suitability, many use a
more comprehensive measure, the long-term
acceptance rate (LTAR), as part of a thorough site
evaluation (Hoover, 1999). The LTAR accounts
for the texture, structure, color, and consistency of
all soil layers beneath the drainfield, as well as the
local topography, to make a determination of the
wastewater loads the area is able to accept on a
long-term basis once the biomass has formed.
The character of wastewater flowing into the soil
absorption area is a critical variable for proper
functioning of septic systems. Soil absorption
systems work most effectively when the influent
wastewater does not contain significant levels of
settleable solids, greases and fats (U.S. EPA,
1980a), which can accelerate clogging of the
infiltrative soil. Accordingly, the use of household
garbage disposals and pouring of grease down
domestic drains can reduce the effectiveness of
septic tank/soil absorption systems (Gannon et a/.,
1998). To avoid infiltrative soil clogging, septic
tanks are fitted with outlet baffles to prevent
floating grease, scum, and entrained particles from
moving into the soil absorption system. Also, the
use of two-compartment tanks is recommended
over single-compartment designs. Even so, tanks
must be properly sized to avoid hydraulic overload
and the passing of unwanted materials into the soil
absorption system.
Digestion of wastes is a temperature dependent
process, and colder temperatures may hinder
effective breakdown of wastes in septic tanks
(Seifert, 1999.) Therefore, in cold climates tanks
may need to be buried more deeply, and/or
insulated.
Septic systems can act as sources of nitrogen,
phosphorus, organic matter, and bacterial and viral
pathogens, which can have potentially serious
environmental and health impacts (Gannon et a/.,
1994.) Failure of systems to adequately treat
wastewater may be related to inadequate siting,
inappropriate installation, or neglectful operation.
Hydraulic overloading has been identified as a maj or
cause of system failure (Jarrett et a/., 1985). Since
septic wastewater contains various nitrogen
compounds (e.g., ammonia, ammonium compounds,
and organic forms of nitrogen) (Brown, 1998),
installation of septic systems in areas that are
densely developed can, in combination with other
factors, result in the introduction of nitrogen
contaminants into groundwater. Groundwater
impacts can occur even when soil conditions are
favorable because the unsaturated aerobic treatment
zone located beneath the drainfield—a zone
required for pathogen removal—promotes
conversion of wastewater-borne nitrogen to nitrates
(Hoover, 1999.) If nitrate contamination of
groundwater is a concern in the region, control
methods or denitrifying technologies may be
required for safe operation of a septic system.
Symptoms of a failing septic system can include
strong odors, ponding of improperly treated
wastewater or backup of wastewater into the home
(Hoover, 1999.) Less obvious symptoms arise
when systems are operating less-than-optimally,
including a measurable decline in water quality,
leading over the long term to local environmental
degradation (Brown, 1998).
Solvents, poisons, and other household chemicals
should not be allowed to flow into a septic system;
these substances may kill beneficial bacteria in the
tank and drainfield, and lead to system failure
(Montgomery, 1990.) Though some organic
solvents have been marketed as septic system
cleaners and substitutes for sludge pumping, there is
little evidence that such cleaners perform any of
their advertised functions. It is known that they can
exterminate useful microbes, resulting in increased
discharge of pollutants (Gannon et a/., 1999;
Montgomery, 1999.) In addition, the chemicals in
these products can contaminate receiving waters
(U.S. EPA, 1993). Additive restrictions are most
effective when used as part of a Best Management
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Practice system involving other source reduction
practices such as phosphate bans and use of low-
volume plumbing fixtures.
Design of subsurface disposal beds and trenches
varies greatly due to specific site conditions. In
sloping areas, a serial distribution system configures
the trenches so that each is used to its capacity
before effluent overflows into the succeeding trench.
A dosing or pressurized distribution system may be
installed to ensure complete distribution of the
effluent to each trench(U.S. EPA, 1980a.)
Alternating valves permit switching between beds or
trenches to allow drying out or resting of the system
(U.S. EPA, 1980a; Gannon etal, 1999). A dosing
system, such as a low-pressure pipe system, is
useful in areas of both high groundwater and
permeable soils, where shallow gravel ditches
installed from 22.86 to 30.48 centimeters (9 to 12
inches) below grade are employed. Another option
is the use of drip irrigation (Hoover, 1999.)
For systems that are properly sited, sized,
constructed, and maintained, septic tank/soil
absorption has proven to be an efficient and cost
effective method of onsite wastewater treatment and
disposal. Operating without mechanical equipment,
properly maintained soil absorption systems have a
service life in excess of 20 years. Several important
steps must be taken during construction to ensure
system reliability:
• Keep heavy equipment off the soil absorption
system area both before and after
construction. Soil compaction can result in
premature failure of the system.
• Divert rainwater from building roofs and
paved areas away from the soil absorption
system. This surface water can increase the
amount of water the soil has to absorb and
lead to premature failure.
• Ensure that the alternating device and the
trench bottoms are level to provide even
distribution of the septic tank effluent. If
settling and frost action cause shifting, part of
the soil absorption system may be overloaded.
• Avoid installing the septic tank and soil
absorption system when the soil is wet.
Construction in wet soil can cause puddling,
smearing, and increased soil compaction,
which greatly reduces soil permeability and
the life of a system.
• Install water-saving devices to reduce the
amount of wastewater entering the soil
absorption system.
• Have the septic tank pumped at least every
three to five years, and inspected regularly.
PERFORMANCE
When correctly installed and maintained, septic
tank/soil absorption systems are an effective way to
treat and dispose of domestic wastewaters.
Nevertheless, even under the best of circumstances
septic systems allow a "planned release" of
contaminants into the groundwater (Tolman et al.,
1989) and must be designed and operated to
minimize the impact of this release. While
hydraulic overloading been identified as a major
cause of septic system failure (Jarrett et a/., 1985),
contamination due to system failure can be caused
by a variety of factors. In one study, widespread
septic failures in Illinois were primarily attributed to
unsuitability of soils, age of system, lack of
maintenance, and improper design and installation of
systems (Smith and Ince, 1989.) Likewise, a study
of septic systems in the Borough of Hopatcong,
New Jersey, found poor soil conditions and shallow
bedrock to be significant contributors to system
failure (HSAC, 1997.) By one estimate, only 32
percent of the total United States land area has soils
suitable for waste treatment by traditional septic
tank/soil absorption systems (U.S. EPA, 1980a.)
Frequency of use also affects system performance.
Drainfields installed on seasonally used properties
have been found to develop an incomplete biological
clogging mat, leading to uneven distribution and
absorption of wastewater (Postma et al., 1992.)
A critical factor in optimal system performance is
the depth of unsaturated soil beneath the soil
absorption field. A septic system performance study
conducted on a coastal barrier island (characterized
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by variably high water tables and sandy
soils—conditions unfavorable for septic system
operation) found that a 60-cm soil layer provided
adequate microbial treatment, even at the highest
loading rate studied (Cogger et a/., 1988.) By
contrast, the same study found that another system
of the same design having a 30-cm soil layer beneath
the leachfield suffered from rising water tables and
ineffective treatment. For the loading rates studied,
the depth of unsaturated soil beneath the system was
determined to be a more decisive factor in system
performance than hydraulic loading.
Despite the limitations discussed above, septic
systems tend to be preferred over other on-site
treatment methods for long-term domestic use. A
1980 study found septic tank/soil absorption
systems to offer the lowest cost and the highest
level of performance among six on-site treatment
techniques tested (U.S. EPA, 1980b). In addition to
septic tank/soil absorption, the other five techniques
included incinerating toilets, recycling toilets,
extended aeration units followed by open sand
filters, septic tanks followed by open sand filters,
and septic tanks followed by horizontal sand filters).
OPERATION AND MAINTENANCE
To keep the system healthy, care must be taken to
avoid putting high-solids or grease containing
materials down drains or toilets, including paper
towels, cigarettes, cat litter, feminine hygiene
products, and residual cooking fat (HSAC 1997).
In the past, pump-out of accumulated solids from
septic tanks every three to five years has been
recommended, however solids loading has been
shown to be extremely variable and for modern
tanks, pump-out may not need to occur as often
(U.S. EPA, 1994). Pump-out every four years
should be planned, but actual practice should be
determined by inspection.
Inspections should be conducted at least biannually
to confirm that baffles are operating correctly, that
no leaks are occurring, and to check the levels of
sludge and scum in the tank (U.S. EPA, 1994). The
tank should be pumped out if the sludge layer
thickness exceeds 25 percent of the working liquid
capacity of the tank (Hoover, 1999), or if the
bottom of the scum layer is within 7.62 centimeters
(three inches) of the bottom of the outlet baffle
(U.S. EPA, 1994). More frequent inspections are
required for systems using more advanced on-site
technologies (Hoover et a/., 1995.)
Though many enzyme additives are marketed as
septic system digestion aides, the effectiveness and
usefulness of many of these products is
questionable. (Seifert, 1999.) If waste products are
not being properly digested before they are
discharged, the most likely cause is hydraulic
overloading. In cold climates, lower average tank
temperatures can also inhibit digestion.
Similarly, many chemical additives are available for
system cleaning and rehabilitation. However, many
of these products are not effective (see Bicki and
Bettler, 1988, on use of peroxide for rehabilitation
of septic systems) and some may even harm the
system (Gannon et a/., 1998.) The use of chemical
additives should be avoided.
COSTS
Costs for installation and maintenance of septic
systems vary according to geographical region,
system size and type, and the specific soil and
geological characteristics of the selected site.
Installation of a new bed or trench septic system on
a site meeting the criteria for such systems varies
widely in cost. Figures range from as low as $1,500
to more than $8,000 (Montgomery, 1990;
Anchorage HHS, 1999; Ingersoll, 1994.) An
average installation cost of $4,000 is assumed for a
traditional septic tank/soil absorption system in a
geologically favorable area.
The cost of tank pump-out varies from as low as
$60 to(Ingersoll, 1994) to as much as $260 (HSAC,
1997.) For a pumping cost of $150, assuming
pump-out every four years, the total pump-out cost
over a 20-year period would be $750 (subject to
inflation). Biannual inspections cost between $50
and $250 (Scott County, 1999); for a $125 fee, the
cumulative inspection cost over 20 years would be
$1,250. Non-inflation adjusted inspection and
maintenance costs for a properly functioning septic
system average $ 100 per year for a hypothetical 20-
year system life.
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The total (non-inflation adjusted) cost including
purchase price averaged over a 20-year period
comes to $300 per year. It should be noted,
however, that if a system is properly maintained, its
life should exceed 20 years.
The value of proper maintenance is further
underscored by the costs associated with repairing
failing septic systems. These can range widely,
depending on the nature of the problem and on the
location of the site. A typical range would be
$1,200 to $2,500 for revitalization or repair of an
exhausted drainfield. Complete removal and
replacement of existing systems can cost five to ten
times more than this (see, for example, HS AC, 1997;
Ingersoll, 1994.)
REFERENCES
1. Anchorage HHS (Health and Human
Services). Internet site at http://
www.ci.anchorage.ak.uk.us/Services/
Departments/Health/que stions.html,
accessed July 1999.
2. Bicki, T.J.; and Bettler, R., 1988. Potential
Nitrate Contamination of Shallow Ground
Water Following Chemical Rehabilitation of
a Septic System. In Proceedings of the
FOCUS Conference on Eastern Regional
Ground Water Issues, Stamford, CT, pp.
169-177. Dublin, OH: National Well Water
Association.
3. Brown, R.B., 1998. Soils and Septic
Systems. Fact Sheet SL-118. University of
Florida Cooperative Extension Service.
Internet site athttp://edis.ifas.ful.edu/scripts/
htmlgen.exe?body&DOCUMENT_SS 114,
accessed July 1999.
4. Cogger, C.G.; Hajjar, L.M.; Moe, C.L.;
and Sobsey, M.D., 1988. Septic System
Performance on a Coastal Barrier Island.
Journal of Environmental Quality. 17: 401-
9.
10.
State University Water Quality Group.
Internet site at http://h2osparc.wq.ncsu
.edu/estuary/rec/ septic.html, accessed June
1999.
Harlan, P.W.; and Dickey, B.C., 1999.
Soils, Absorption Fields and Percolation
Tests for Home Sewage Treatment.
Cooperative Extension, Institute of
Agriculture and Natural Resources,
University of Nebraska, Lincoln. Internet
site at http://www.ianr.unl.edu/pubs/
wastemgt/g514.htm, accessed June, 1999.
Hoover, M.T. Professor of Soil Science and
Extension Specialist, North Carolina State
University, Raleigh, NC. Personal
communication with Donna Messner,
Parsons Engineering Science, Inc. 1999.
Hoover, M.T.; Disy, T.M.; Pfeiffer, M.A.;
Dudley, N.; Mayer, R.B.; and Buffington,
B., 1996. North Carolina Subsurface
System Operators Training School Manual.
Raleigh, NC: Soil Science Department,
College of Agriculture and Life Sciences,
North Carolina State University and North
Carolina Department of Environment,
Health and Natural Resources.
HSAC (Hopatcong Sewer Advisory
Committee), 1997. Benefits and
Consequences of the Choice Between:
Septic Systems or Sewers. HSAC
Publication #1. Internet site at http://
www.hopatcong.org/sewers/sewer.htm,
accessed July, 1999.
Ingersoll, J.H., 1994. "Septic Tank Sense
(Country Property Dollars and Sense)."
Country Living 17': 148-9.
5. Gannon, R.W.; Bartenhagen, K.A.; and
Hargrove, L.L., 1999. Septic Systems: Best
Management Practices. North Carolina
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11. Jarrett, A.R.; Fritton, D.D.; and Sharpe,
W.E., 1985. Renovation of Failing
Absorption Fields by Water Conservation
and Resting. American Association of
Agricultural Engineers paper 85-2630.
12. Montgomery, T., 1990. On-Site
Wastewater Treatment Systems: A Brief
Description of Ecological, Economic and
Regulatory Factors. The New Alchemy
Institute, Technical Bulletin No. 6. Internet
site at http://www.fuzzlu.com/
greencenter/tb/ tb006.htm, accessed July
1999.
13. Postma, F.B.; Gold, A.J.; and Loomis,
G.W., 1992. Nutrient and Microbial
Movement from Seasonally-Used Septic
Sy stem s. Journal of Environmental Health
55: (2)5-10.
14. Scott County Geology, Minnesota. Internet
site at http://www.co.scott.mn.us/EH/eh/
ehgeology.htm, accessed July 1999.
15. Seifert, R. 1999. Septic System Fact
Sheets. Alaska Cooperative Extension,
Univeristy of Alaska, Fairbanks. Internet
site at http://zorba.uafadm.alaska.edu/
coop-ext/faculty/seifert/septicsys.html,
accessed June 1999.
19. U.S. EPA, 1980b. Evaluation of 19 On-Site
Waste Treatment Systems in Southeastern
Kentucky. EPA 600/2-80-101, U.S. EPA,
Washington, DC.
20. U.S. EPA, 1993. Guidance Specifying
Management Measures for Sources of
Nonpoint Pollution in Coastal Waters.
EPA 840-B92-002, U.S. EPA, Washington,
DC.
21. U.S. EPA, 1994. Guide to Septage
Treatment and Disposal. EPA 625/R-
94/002, U.S. EPA, Washington, DC.
ADDITIONAL INFORMATION
Contact your local county extension office and your
state department of health for information and
region-specific details. Additional information is
available from:
American Society of Civil Engineers
World Headquarters
1801 Alexander Bell Drive
Reston, VA 20191-4400
American Society of Home Inspectors
Contact: Rob Paterkiewicz
932 Lee St., Suite 101
DesPlaines, IL60016
16. Smith, T.; and Ince, M., 1989. Septic
System Density and Goundwater
Contamination in Illinois: A Survey of State
and Local Regulation. NTIS Report PB89-
178545.
17. Tolman, A.L.; Gerber, R.G.; and Hebson,
C.S., 1989. Nitrate Loading Methodologies
for Septic System Performance Prediction:
State of an Art. In Proceedings of the
FOCUS Conference on Eastern Regional
Ground Water Issues., pp 167-180. Dublin,
Ohio: National Water Well Association.
18. U.S. EPA, 1980a. Design Manual: Onsite
Wastewater Treatment and Disposal
Systems. EPA 625/1-80-012, U.S. EPA,
Washington, DC.
Dr. Michael T. Hoover
Professor of Soil Science/Extension Specialist
North Carolina Cooperative Extension Service
North Carolina State University
Soil Science Department
Raleigh, NC 27695-7619
Dr. R.B. Brown
Professor and Extension Specialist
Florida Cooperative Extension Service
Institute of Food and Agricultural Services
University of Florida
Gainesville, FL 32611-0510
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National Society of Consulting Soil Scientists
Mary Reed, Executive Secretary
Chuck Jackson, Executive Director
National Society of Consulting Soil Scientists, Inc.
325 Pennsylvania Ave., S.E., Suite 700
Washington, DC. 20003
The mention of trade names or commercial products
does not constitute endorsement or recommendation
for use by the U.S. Environmental Protection
Agency.
For more information contact:
Municipal Technology Branch
U.S. EPA
Mail Code 4204
401 M St., S.W.
Washington, D.C., 20460
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