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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