United States              June
                                                               Environmental Protection       1986
                                                 &EPA    Large  Soil

                                                               for  Success
Prepared by Environmental Resources Management, Inc.
For additional information contact:
401 M Street, SW
Washington, DC 20460

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

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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    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.
    • 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
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).

 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
   From          ,,„,(./,.
   Pretreatment^ • *.;"'•}"'•'•• >'•' i'
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|»|^||^,,^  \a

                                   Laterals (Number,
                                      & Hole Spacings
                                      are Variable)
     See Figure 2
     for Details -

                                    - Filter Fabric

                                                         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

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
• 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
• 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
    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.