vvEPA
United States
Environmental Protection
Agency
Office of Water
Washington, D.C.
EPA 832-F-00-044
September 2000
Decentralized Systems
Technology Fact Sheet
Septic Tank Leaching Chamber
DESCRIPTION
A leaching chamber is a wastewater treatment system
consisting of trenches or beds, together with one or
more distribution pipes or open-bottomed plastic
chambers, installed in appropriate soils. These
chambers receive wastewater flow from a septic tank
or other treatment device and transmit it into soil for
final treatment and disposal.
A typical septic tank system consists of a septic tank
and a below-ground absorption field (also called a
drainfield, leaching field, or nitrification field). Leaching
chambers are drainfields used to dispose of previously
treated effluent. The drainfield system typically consists
of leaching chambers installed in trenches and
connected to the septic tank via pipe. Effluent flows
out of the septic tank and is distributed into the soil
through the leaching chamber system. The soil below
the drainfield provides final treatment and disposal of
the septic tank effluent. After the effluent has passed
into the soil, most of it percolates downward and
outward, eventually entering the shallow groundwater.
A small portion of the effluent is used by plants through
their roots or evaporates from the soil. Figure 1 shows
a typical leaching chamber.
Leaching chambers have two key functions: to dispose
of effluent from the septic tanks and to distribute this
effluent in a manner allowing adequate natural
wastewater treatment in the soil before the effluent
reaches the underlying groundwater aquifer. Although
the septic tank removes some pollutants from
wastewater, further treatment is required after the
effluent leaves the tank. Nitrogen compounds,
suspended solids, organic and inorganic materials,
Source: Infiltrator Systems Inc., 2000.
FIGURE 1 LEACHING CHAMBER
and bacteria and viruses must be reduced before the
effluent is considered purified. These pollutants are
reduced or completely removed from the wastewater
by the soil into which the wastewater drains from the
leaching chambers.
Depending on the drainfield size requirements, one or
more chambers are typically connected to form an
underground drainfield network. The leaching
chambers are usually made of sturdy plastic and do not
require gravel fill. The sides of each chamber have
several openings to allow wastewater to seep into the
surrounding soil.
A typical leaching chamber consists of several high-
density polyethylene arch-shaped, injection-molded
chamber segments. A typical chamber has an average
inside width of 51 to 102 centimeters (20 to 40 inches)
and an overall length of 1.8 to 2.4 meters (6 to 8 feet).
The chamber segments are usually one-foot high, with
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wide slotted sidewalls, which are usually 20 degrees
toward the chamber center or away from the trench
sidewall. Each chamber segment is designed to
mechanically interlock with the downstream chamber
segment, forming a complete drainfield trench that
consists of an inlet plate with a splash plate below the
inlet on the trench bottom, and a solid-end plate at the
distal end of the chamber drainfield line.
Common Modifications
Typical leaching chambers are gravelless systems, with
drainfield chambers with no bottoms and plastic
chamber sidewalls, available in a variety of shapes and
sizes. Some gravelless drainfield systems use large
diameter corrugated plastic tubing covered with
permeable nylon filter fabric not surrounded by gravel
or rock. The area of fabric in contact with the soil
provides the surface for the septic tank effluent to
infiltrate the soil. The pipe is a minimum of 25.4 to
30.5 centimeters (10 to 12 inches) in diameter covered
with spun bonded nylon filter fabric to distribute water
around the pipe. The pipe is placed in a 30.5 to 61
centimeter (12 to 24 inches) wide trench. These
systems can be installed in areas with steep slopes with
small equipment and in hand dug trenches where
conventional gravel systems would not be possible.
Use of these systems decreases overall drainfield costs
and may reduce the number of trees that must be
removed from the drainfield lot. However, fabric-
wrapped pipe cannot overcome unsuitable site
conditions and should not be installed where gravel
systems will not function properly or in fine sandy
organic rich, coastal plain soils with shallow
groundwater.
APPLICABILITY
Leaching chambers are widely used as drainfield
systems for septic tank effluent discharge. Many
leaching chambers have been installed in 50 states,
Canada, and overseas over the last 15 years.
Currently, a high percentage of new construction uses
lightweight plastic leaching chambers for new septic
tank systems in states such as Colorado.
Leaching chambers are an alternative to the
conventional septic tank drainfield, which consists of
several trenches with gravel beds and perforated
plastic pipes. Leaching chambers allow more of the
soil profile to be used since the septic tank effluent is
distributed to the ground below and the soil
surrounding the chamber. Therefore, leaching
chambers are more effective than traditional gravel
drainfields, especially when the drainfield must be
located on a steep slope. Leaching chambers are
suitable for lots with tight sizing constraints or where
water tables or bedrock limit the depth of the
drainfield. Some states offer up to 50 percent sizing
reduction allowance when using leaching chambers
instead of conventional septic tank gravel drainfields.
Because they can be installed without heavy
equipment, leaching chamber systems are easy to install
and repair. These high-capacity open-bottom
drainfield systems can provide greater storage and
more time for proper infiltration than conventional
gravel systems and, therefore, are also suitable for
stormwater management.
Current Status
Septic tank system drainfields are usually classified as
two types: gravel or gravelless systems. In gravel
drainfield systems, the pipelines distributing septic tank
wastewater are placed over a layer of gravel. Four
inches of additional rock are then typically placed
around the pipe and two inches above the pipe.
Gravelless systems provide the same functions as
gravel drainfields while overcoming the potentially
damaging impacts of gravel such as compaction of
moist soil during installation and reduction of infiltration
by obstructing the soil. The leaching chambers create
a larger contact area for effluent to infiltrate into the
soil, providing efficient treatment.
Typically, leaching chambers consist of series of large,
two to four foot wide modular plastic arch segments
that snap together. These arch segments replace the
perforated drainpipes used in gravel drainfields. The
wide chambers are manifolded with conventional
plastic pipe such as high-density polyethylene (HOPE).
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ADVANTAGES AND DISADVANTAGES
Limitations
Leaching chamber application is limited under certain
conditions. The main limitations for installation and
normal operation are small lot sizes, inappropriate soils,
and shallow water tables. Leaching chamber systems
can be used only in areas with soils that have
percolation rates of 0.2 to 2.4 minutes per millimeter (5
to 60 minutes per inch). Neglect of septic tank and
leaching chamber maintenance can lead to drainfield
failure and soil and groundwater contamination.
Reliability
Leaching chambers are reliable, do not have moving
parts, and need little maintenance to function properly.
They are usually made of plastic materials, with a useful
life of 20 years or more in contrast to the average
useful life of a drainfield of 15 years, with a maximum
of 20 to 25 years.
Some systems can be combined with other drainfield
systems such as mounds and pressure distribution
systems. Some can also be used for stormwater
applications. Leaching chambers do not require more
maintenance than conventional drainfield systems.
Advantages
Key advantages of leaching chamber systems
compared to gravel drainfields include:
• Easier and faster to install.
• Soil in the trenches is not as likely to be
compacted.
• Less expensive in areas where gravel must be
transported over a long distance, such as parts
of eastern North Carolina, the Rocky
Mountains, eastern Oregon, and Connecticut.
• Leaching chambers allow for lower intrusion of
soil and silt into the drainfield and thereby
extend the useful life of the drainfields.
Some leaching chambers have greater storage
volumes than gravel trenches or beds.
• Inspection of the chambers is easier.
• Eliminates the need for gravel.
• Leaching chambers require a smaller footprint.
Some states allow up to a 50 percent
reduction in drainfield size compared to
conventional gravel drainfield systems.
The lightweight chamber segments available on the
market stack together compactly for efficient transport.
Some chambers interlock with ribs without fasteners,
cutting installation time by more than 50 percent over
conventional gravel/pipe systems. Such systems can
be relocated if the site owner decides to build on the
drainfield site. Leaching chamber systems can be
installed below paved areas and areas of high traffic.
Disadvantages
A key disadvantage of leaching chambers compared to
gravel drainfields is that they can be expensive if a low-
cost source of gravel is readily available. Also, tests to
assess the effectiveness of these drainfield systems have
yielded mixed results. Direct effluent infiltration is
advantageous in some soils yet detrimental in others.
While open chambers can break up tight, clay soils and
open up more airspace for biological treatment, they
are less effective than gravel drainfields in preventing
groundwater pollution. Because the open bottom
allows septic tank effluent to infiltrate the soil unfiltered,
high percolation rates (sandy soils) and groundwater
levels must be carefully considered before installing
such systems.
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DESIGN CRITERIA
The size of a leaching chamber system is based on the
wastewater flow and soil properties. For a three
bedroom home, the area needed for a leaching
chamber system could range from 18.6 sq. meters
(200 square feet) for a coarse-textured soil up to
185.8 sq. meters (2,000 square feet) for a fine-
textured soil. When the total drainfield area is
estimated, setbacks from the house and property lines
must be provided. These are usually state-regulated
and vary from state to state. Table 1 recommended
TABLE 1 SETBACK DISTANCES
FROM LEACHING CHAMBER
DISPOSAL AREAS
Item Minimum Distance, ft
Private Water Supply Well
Public Water Supply Well
Leak or Impoundment
Stream or Open Ditch
Property Lines
Water Line Under
Pressure
Sewer Interceptor Drain
100
300
50
25
10
10
25
Source: Schultheis, 1999.
setback distances.
The key design parameter for leaching chambers is the
maximum long-term acceptance rate (LTAR), which
depends on the type of drainfield soils. Table 2
presents recommended LTARs for leaching chamber
sizing.
The design LTAR should be based on the most
hydraulically limiting naturally occurring soil horizon
within three feet of the ground surface or to a depth of
one foot below trench bottom, whichever is deeper.
To determine the total trench bottom area required, the
design daily wastewater flow should be divided by the
applicable LTAR. The minimum linear footage of the
leaching chamber system should be determined by
dividing the total trench bottom area by 1.2 meters (4
feet), when used in a conventional drainfield trench.
No reduction area is allowed for leaching chamber
systems installed in bed or fill systems. In addition to
the area needed for the leach field, space should be
reserved for possible expansion (for example, a 50
percent expansion area is required in New York State;
a 100 percent repair area is required in North
Carolina).
Leaching chamber systems in septic tank drainfields are
typically installed in three foot wide trenches, separated
by at least nine feet, edge to edge. Soil backfill is
placed along the chamber sidewall area to a minimum
compacted (walked-in) height of eight inches above the
trench bottom. Additional backfill is placed to a
minimum compacted height of 30.5 centimeters
(12 inches) above the chamber. The leaching chamber
trench bottom is usually at least 61 centimeters
(24 inches) below finished grade, and the inlet invert is
approximately 20.3 centimeters (8 inches) above the
trench bottom, and at least 43.2 centimeters
(17 inches) below the finished grade. Most health
codes limit the length of individual trenches to
18.3 meters (60 feet). A leaching chamber system
should have at least two trenches. Figure 2 shows a
schematic of a leaching chamber trench.
TABLE 2 LEACHING CHAMBER LONG-TERM ACCEPTANCE RATE
Soil Type
Long-Term Acceptance Rate (gpd/ft./yr)
Natural Soil Saprolite
Sands
Coarse Loams
Fine Loams
Clays
0.8-1.0
0.6-0.8
0.3-0.6
0.1-0.4
0.4-0.6
0.1-0.4
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Source: Infiltrator Systems Inc., 2000.
Individual chamber trenches should be leveled in all
directions and follow the contour of the ground surface
elevation without any dams or other water stops.
Leaching systems installed on sloping sites may use
distribution devices or step-downs when necessary to
channel the level of the leaching chamber segments
from upper to lower elevations. The manufacturer's
installation instructions should be followed and systems
should be installed by an authorized contractor.
PERFORMANCE
The performance of leaching chamber systems is
determined by the characteristics of the soil, available
slope, space, soil depth over the groundwater table,
and other site-specific factors. The overall performance
of leaching chambers is highly dependent on the
performance of the connected septic tanks.
OPERATION AND MAINTENANCE
Septic tank/leaching chamber systems can operate
independently and require little day-to-day
maintenance. Proper maintenance of the septic tank
includes inspection to determine the rate of sludge and
scum accumulation in the tank every three to five years.
Under normal conditions, the septic tank should be
pumped every five to eight years.
Materials that do not readily decompose (grease and
cooking oil, coffee grounds, disposable diapers,
tampons, cigarette butts, condoms, plastics, etc.)
should not be flushed into septic tanks because they
may clog the tank inlet and/or outlet and cause the
leaching chambers to fail. Harmful chemicals, such as
pesticides, herbicides, gasoline, oil, paint and paint
thinners should not be discharged to sanitary drains
because they may harm soil microorganisms in the
drainfield which provide natural wastewater treatment.
Excessive use of chlorine-based cleaners can harm the
normal operation of leaching chambers because they
may cause soil dispersion and sealing, reducing soil
treatment capabilities.
COSTS
Leaching chamber costs depend on many factors,
including:
1. Soil type.
2. Cost of land.
3. Site topography.
4. Groundwater level.
These site and system specific factors must be
examined and incorporated when preparing a leaching
chamber cost estimate.
Construction Costs
Even with favorable soil conditions, a leaching chamber
system is more expensive then a conventional gravel-
pipe drainfield. The cost of a standardized, 2.13 meter
(seven foot) leaching chamber segment ranges from
$50 to $150. While drainage pipe is less expensive
per foot, a larger drainfield footprint is needed for
conventional gravel drainfields. The cost for a single-
family septic tank leaching chamber drainfield typically
ranges between $2,000 and $5,000 in 1993 dollars.
If the site is inadequate for a new drainfield and the
field must be removed and replaced with a new one,
the cost of a new leaching chamber system may exceed
$10,000.
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Operation and Maintenance Costs
Operation and maintenance costs for these systems are
minimal. Key costs associated with the proper
functioning of the drainfield systems include septic tank
cleaning, which typically ranges between $500 to
$1,500 per cleaning.
REFERENCES
Other Related Fact Sheets
Septic System Tank
EPA 832-F-00-040
September 2000
Septage 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. Anderson, J.L., R.E. Machmeier, and MJ.
Hansel. Long-term Acceptance Rates of
Soils for Wastewater. Proceedings of the
Third National Symposium on Individual and
Small Community Sewage Treatment Systems.
1981.
2. Crites R. and Tchobanoglous G. 1998. Small
and Decentralized Wastewater
Management Systems. McGrill-Hill, New
York.
3. Dix S. P. and V. Nelson. The Onsite
Revolution: New Technology, Better
Solutions. Water Engineering and
Management Journal, October 1998.
Infiltrator Systems, Inc.
httD://www.infiltratorsvstems.com/.
2000.
5. Keys, J. R., E. J. Tyler, and J. C. Converse.
1998. Predicting Life for Wastewater
Absorption Systems. Proceedings of the
Eighth National Symposium on Individual and
Small Community Sewage Treatment Systems.
6. May, R. March/April, 1991. Chamber
Leachfield Systems. Journal of Environmental
Health,
7. North Carolina Department of Environment,
Health, and Natural Resources, Division of
Environmental Health. 1995. On-site
Wastewater Section. Innovative Wastewater
System Approval Guidelines.
8. North Carolina Rural Communities Assistance
Project, Inc. February 1994. Considering
the Alternatives: A Guide to Onsite
Wastewater Systems in North Carolina.
9. Ricklefs, S. Evaluating Innovative Systems:
A Field Study of Leaching Chamber Design.
In Proceedings of the 1992 Texas On-site
Wastewater Treatment and Research
Conference.
10. Schultheis, Robert A. and Gwen Hubbie. A
Homeowner's Guide: Septic
Tank/A bsorption Field Systems. University
of Missouri, WQ0401, 1999.
11. Tobias, S. 1990. Onsite and Alternative
Wastewater Treatment Systems,
Sacramento, California Rural Community
Assistance Corporation.
12. U.S. EPA. Design Manual: Onsite
Wastewater Treatment and Disposal
Systems, EPA/625/1-80-012. Cincinnati,
Ohio. Center for Environmental Research
Information, 1988.
13. U.S. EPA. 1992. Septic Systems and
Groundwater Protection: A Program
Manager's Guide and Reference Book.
Washington, D.C.: Office of Water.
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ADDITIONAL INFORMATION
Alternative Septic System Test Center
Tony Millham, Project Manager
2 Spring Street
Marion, MA 0273 8
Bio Systems Ag Engineering
David Gustafson
1390 Eckles Avenue
StPaul,MN55108
Extension Service
David A. Bryant, Director
Montana State University
Bozeman, MT 59717
Lockwood, Dietershagen Associates
Ken Lockwood
1 Earl M. Court
Clifton Park, NY 12065
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
IMTB
MUNICIPAL TECHNOLOGY
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