United States
                       Environmental Protection
                       Office of Water
                       Washington, D.C.
EPA 932-F-99-075
September 1999
Decentralized  Systems
Technology Fact Sheet
Septic Tank - Soil Absorption  Systems

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.


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

                      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



•     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.


•     Siting limitations for septic systems include
      natural soil type and permeability, bedrock
      and  groundwater  elevations,   and  site

•     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

•     Improperly functioning systems can introduce
      nitrogen, phosphorus, organic matter, and
      bacterial  and   viral  pathogens  into the
      surrounding area and groundwater.


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

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
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

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.


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

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).


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 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.

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.)


1.      Anchorage  HHS  (Health   and  Human
       Services).     Internet  site   at  http://
       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

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-
State University  Water  Quality  Group.
Internet site  at   http://h2osparc.wq.ncsu
.edu/estuary/rec/ septic.html, accessed June

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://
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

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

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/
      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,

21.    U.S.  EPA,  1994.    Guide  to Septage
       Treatment  and Disposal.  EPA  625/R-
       94/002, U.S. EPA, Washington, DC.


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-

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

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
                                                             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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