United States June
Environmental Protection 1986
Agency
&EPA Large Soil
Absorption
Systems
Design
Suggestions
for Success
Prepared by Environmental Resources Management, Inc.
For additional information contact:
EPA-OMPC(WH-595)
401 M Street, SW
Washington, DC 20460
(202)382-7368/7369
EPA Region 1
John F. Kennedy Federal Building
Boston, MA 02203
EPA Region 2
26 Federal Plaza
New York, NY 10278
EPA Region 3
841 Chestnut Street
Philadelphia, PA 19107
EPA Region 4
345 Courtland Street, NE
Atlanta, GA 30365
EPA Region 5
230 South Dearborn Street
Chicago, IL 60604
EPA-WERL (489)
26 West St. Clair Street
Cincinnati, OH 45268
(513)684-7641
EPA Region 6
1201 Elm Street
Dallas, TX 75270
EPA Region 7
726 Minnesota Avenue
Kansas City, KS 66101
EPA Region 8
999 18th Street
Denver, CO 80202
EPA Region 9
215 Fremont Street
San Francisco, CA 94105
EPA Region 10
1200 6th Avenue
Seattle, WA 98101
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ess
Operating Tips
• One resting absorption module should be rotated into
service annually in late spring, when soil
temperatures are increasing.
• At least once per month, the absorption units should
be inspected for operational status and continuous
ponding. Whenever continuous ponding is noted, the
absorption module should be rested.
• A regular inspection and preventive maintenance
schedule for all mechanical equipment should be
established and implemented.
• If septic tanks are the form of pretreatment, the
septage must be removed periodically and the tanks
inspected for leaks.
Monitoring
• The composition of the applied wastewater should be
characterized during the first quarter of operation and
then annually thereafter. The parameters of concern
may include COD, TSS, TDS, TKN, NO3-N, and pH.
• The ground water quality should be monitored at
least semi-annually. Monitoring parameters may
include COD, TDS, TKN, NO3-N, pH, and fecal
coliform.
Design Techniques References
Several references are available which outline the
design options for LSAS in more detail. Four of these
references are:
1. High Rate Soil Absorption Systems, Task Force
Final Report, Minnesota Pollution Control Agency,
Roseville, MN.
2. Cogger, C. G., Carlile, B., Osborne, D. J., and
Holland, E., Design and Installation of Low-Pressure
Pipe Waste Treatment Systems, University of North
Carolina (UNC) Sea Grant College Publication
UNC-SG-82-03, Raleigh, NC, (1982).
3. On-Site Wastewater Treatment, Proceedings of the
4th National Symposium on Individual and Small
Community Sewage Systems. American Society of
Agricultural Engineers. Publication No. 07-85. St.
Joseph, Ml, (1985).
4. Technology Process Design Manual for Land
Treatment of Municipal Wastewater - Supplement
on Rapid Infiltration and Overland Flow, U.S.
Environmental Protection Agency, Center for
Environmental Research Information, EPA
Publication 625/1-81-013a, Cincinnati. OH,
(October, 1984).
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Introduction
Millions of homes in the United States use septic tanks
and leach fields to treat and dispose of their
wastewater. In these systems, the individual
homeowner is responsible for the cost of installation
and maintenance. By combining the wastewater from
several homes, the cost-effectiveness of these systems
can be improved. The result is a large soil absorption
system (LSAS). Hospitals, apartment and office
complexes, recreational areas, and small communities
are examples of situations where LSAS may be used.
The EPA Design Manual for Onsite Wastewater
Disposal Systems (EPA-625/1-80-012) provides design
guidance for small (single home) soil absorption
systems. Experience has shown that this guidance is
not necessarily appropriate for LSAS, which treat
substantially greater volumes of wastewater. A brief
overview of some key design and constuction
considerations for LSAS are presented herein.
The Concept
An LSAS consists of two components: a pretreatment
system and the soil absorption system. Septic tanks
are currently the predominant form of pretreatment
used. In a community LSAS, each house may
discharge into an individual septic tank which in turn
discharges into a sewer system leading to the soil
absorption or leach field. On the other hand, the use of
Effluent
From ,,„,(./,.
Pretreatment^ • *.;"'•}"'•'•• >'•' i'
1&P V"r':'' "^; - A:';.'.'V/ <•. ^Latei
»,v,^::^.;v-^^^^\ f
|»|^||^,,^ \a
^^/^^S'^M^0SS^
Laterals (Number,
Placement,
& Hole Spacings
are Variable)
See Figure 2
for Details -
-Backfill
^
- Filter Fabric
"Aggregate
~Soil
one large septic tank may be more economical than
the use of single septic tanks. In the soil absorption
component of an LSAS, the effluent from the septic
tank or other pretreatment system is discharged to a
subsurface absorption field or leach field. The
absorption field consists of trenches into which
perforated pipe is placed in a porous medium, typically
clean gravel. A filter fabric is placed over the gravel
and the trenches are then backfilled. A typical trench
system layout is presented in Figure 1. A cross-section
of a typical trench is presented in Figure 2.
Treatment Mechanisms
Since a septic tank produces only primary quality
effluent, the soil absorption system must provide the
remaining treatment. In a municipal wastewater soil
absorption system, nitrogen, pathogens, metals,
phosphorus, and organics are the primary
contaminants of concern. Septic tank effluents are
typically high in ammonia. In a properly functioning
LSAS, nitrification will convert almost all of the
ammonia to nitrates. Since denitrification is minor, most
of the nitrates may pass readily through the soil. LSAS
systems must thereifore be designed to ensure that
adequate dilution of the nitrates is obtained in the
ground water. Selective control of the dosing and
resting cycles in uniformly dosed systems can improve
.Water Table or
Creviced Bedrock
Figure 1. Typical Trench System Layout
Figure 2. Typical Trench System Cross-Section
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Large Soil Absorption Systems - Design Suggestions for Sued
nitrogen removal. Pathogens are removed primarily by
filtration and adsorption. Unsaturated soil conditions
enhance pathogen removal, and at least 3 to 4 feet of
unsaturated soil must be maintained beneath the
bottom of the LSAS during operation. Metals and
phosphorus are removed primarily by soil adsorption
and precipitation, and treatment performance will
depend upon the soil characteristics.
Detailed Site Evaluation: A Necessity
* The site evaluation should be performed by a
professional soil scientist / hydrogeologist
experienced in the siting and design of large
wastewater absorption facilities.
• Detailed inspections of the soil morphological
characteristics to a depth of at least 6 feet below the
infiltrative surface should be conducted. Special
attention should be given to both the vertical
permeability characteristics and the horizontal flow
potential of the site. Detailed1 procedures for this type
of evaluation are given in EPA's 1981 Process
Design Manual for Land Treatment of Wastewater.
• The depth to restrictive layers or ground water affects
treatment performance. While supportive data are
lacking, a minimum vertical separation of
approximately 3 feet (depending on soil type) from
the top of the ground water mound is recommended.
Seasonal changes in the ground water depth should
be taken into consideration.
• Sites with convex contours are preferred. Sites with
concave slopes or which receive drainage from the
surrounding area should be avoided.
Design Suggestions
• The design flow should be based upon accurate
population projections.
• Seasonal or diurnal changes in flow rate should be
considered in the design of the dosing system.
• Alternating operating and resting cycles are a
necessity for good performance. Dosing frequencies
of 2 to 4 cycles per day have been found to be most
satisfactory. The selection of dosing frequencies are
usually related to soil types, with more permeable
soils being able to accomodate higher dosing
frequencies.
• Trench orientation should be parallel to ground
contours, and the trenches should be as shallow as
possible to maximize aeration and to ensure that
treatment occurs in the most chemically and
biologically active soil zone.
• Recommended design loadings, based upon trench
bottom area, for different soil textures, are presented
in Table 1. Design infiltration rates should be selected
cautiously based upon the soil morphology and
hydraulic capacity.
• Pretreatment through multiple chambers in series
generally provides the highest degree of
pretreatment.
• At least four separate absorption modules should be
provided for alternating service, with two resting at
any one time.
» Pressure distribution is required to uniformly dose
each module.
• LSAS systems are most suitable for flows under
30,000 GPD.
• The capability for flow measurement and for influent
and ground water sampling should be included in the
design of an LSAS.
Soil Texture
Application Rate
(gal/ft2/day)*
Gravel, Very Coarse Sand
Coarse to Medium Sand
Fine Sand, Loamy Sand
Sand Loam, Porous Loam
Loarn, Silt Loam
Clay Loam and Clay
Not Recommended
0.79 - 0.98
0.61 - 0.74
0.52 - 0.61
0.25 - 0.52
Not Recommended
* To convert gal/ft2/day to cm/day,
multiply by 4.07.
Table 1. Recommended Application Rates for LSAS.
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