5054
xvEPA
United States        June
Environmental Protection    1984
Agency
                905R84122
Land
Treatment:

Rapid
Infiltration
         Plan,
         Design,  and
         Construct
         for
         Success

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The Cautions
It is not possible in a limited space to deal in detail
with all of the potential problems listed in Table 1.
Many might not occur at all if the design team had
previous practical experience using and interpreting
the procedures described in the Process Design
Manual for Land Treatment of Municipal Wastewater
(EPA 625/1-81-013). It is relatively easy in a
hindsight analysis to conclude that a problem could
have been avoided if common sense had been
applied. For example, it seems obvious that field
tests should always be at the actual  location and
elevation of the proposed system. However, in the
real world it might be that a number of proper and
expensive tests were run to characterize a general
area.  Then, during development of the final design,
constraints arose so that a  slightly different location
or depth was selected for the basins. It would then
be a great temptation to assume that the original
investigation still applied since the available time and
funds remaining  are limited. In some cases, yielding
to such temptations may result in premature system
failure.

Other, Specific  Cautions
Basin construction in filled areas should be avoided
if possible. If construction in fill is absolutely
necessary, soils  should be  coarse-textured with a
fine fraction (passing #200 sieve) of 5% or less.
Pilot scale  infiltration tests in a constructed fill are
necessary, along with rigorous analysis of test
results and on-site control of construction.

Clayey sands with fines exceeding 10% (by weight)
have  not been successful for infiltration surfaces in
filled areas.

Typical  earthwork construction strives for the
maximum practical density  and structural stability of
the soil. In contrast, construction of the infiltration
surface in a Rl basin requires a different attitude and
approach, since  maintenance  and/or development of
the maximum possible hydraulic capacity is the goal.

When a clay fraction is  present,  placement of fill or
final construction activities in a cut are typically
limited to periods wh6n'fh@ soirrtiols'tire.rs'dn the
"dry"  side of optimum. •ThiS'pi-Qcautioq%ta^en to
avoid-reduction of soil pegpeabili|y,,by compaction.

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Land Treatment:  Rapid  Infil
The Concept
Rapid infiltration (Rl) land treatment can be the
simplest and least costly wastewater treatment
system available for a community, where site
conditions are suitable. Often all that is required is a
means for applying partially treated wastewater,
typically less than full secondary, to a set of basins
excavated in sandy  soils.

There are two basic types of Rl systems. In the
majority of the systems, the applied wastewater
seeps downward through the earth, joins the ground
water and eventually emerges in adjacent surface
waters. Most of the treatment occurs at shallow
depth during and after intermittent wastewater
applications. A long  travel path and travel time in the
soil allow for further  treatment so the liquid that
emerges in  the surface water is of better quality than
could be obtained with  most advanced wastewater
treatment techniques.

In the second type, where more direct control is
desired or where recovery of the treated water for
unrestricted agricultural irrigation is economical,
underdrains or recovery wells are sometimes used,
as shown in Figure  1.
     Wells
Figure 1.  Recovery of Treated Water

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ion  -  Plan,  Design,  and  Construe
   In some large-scale systems in arid climates, such
   as in Israel and Arizona, the treated wastewater is
   recovered through wells and used to irrigate a
   variety of crops. However, treated wastewater is not
   recovered and used in the majority of systems.
   Rather, the Rl concept is used because of its
   simplicity and low treatment costs.

   The major reason for the low cost is the use  of high
   wastewater hydraulic  loading rates relative to other
   land treatment systems. A rapid infiltration system
   on a typical sandy soil might easily accept 100
   acre-feet (748 gal/ft2)  per year, while irrigation of
   beans or corn on the  same soil might only need 3
   acre-feet of water per year. Therefore a much
   smaller land area is needed for rapid infiltration. A
   simpler distribution  system also contributes to cost
   savings.

   Present design criteria for rapid infiltration are
   conservative to ensure long-term successful
   operation. The 100 acre-feet per year,  for example,
   translates to a daily average of about 2
   gal/fp/day,  which is no greater than the application
   rate on household leachfield systems in similar soils.
   Many of the existing Rl systems in the U.S. have
   been successfully operating for 40 years or more.
   In 1981 there were about 320 rapid infiltration
   systems in the United States either operating or
   under construction.

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for Success
   The Concerns
   Most of the operating systems in the U.S. are
   successfully meeting all Rl performance standards.
   However, a very small number of systems that have
   been recently constructed do not satisfy all design
   expectations. These system designs did not
   adequately account for the volume of wastewater
   that can be applied and infiltrated within the time
   allowed. Therefore, sites with limitations that are
   difficult to discern may require additional construction
   or process modifications before the system is
   capable of performing as designed. Such  problems
   can be avoided in future designs. An analysis
   indicates that the problems can be grouped, as
   shown in Table 1, into four major categories related
   to: soil conditions, ground water conditions, design
   assumptions, and construction control.

   The simplicity of the concept tends to mislead
   individuals to assume that all that is needed is a
   hole in the ground with simple piping, and that
   nature will take care of the rest. In fact, from a
   construction engineering point of view,  the simplicity
   of the concept hides the variable and complex
   interrelationships of soil, water, and geohydrology.
   Understanding these relationships at sites with
   atypical limitations is essential for a successful
   design. However, the special skills and expertise
   that are required may not be present in the
   background of the wastewater system designer, so
   outside assistance may be needed. The criteria for
   rapid infiltration designs are conservative,  but the
   hydraulic loading rates are usually an order of
   magnitude greater than those  used for irrigation type
   systems. As a result, the margin  of safety is reduced
   somewhat and the system is less forgiving of errors
   and omissions in its design and construction.

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                 DATE DUE
Stabilization of the soil "fines" in the surface layer of
basins constructed in cut sections may be needed
when the fines exceed about 10% (by weight).
Vegetation (water tolerant grasses) is the only
effective stabilization method known to date.

A surface layer of gravel or coarse sand to filter
algae or excess suspended solids is not
recommended. This may cause a continuously wet
interface between the gravel and in-situ soil that may
result in system failure.

Site investigations which do not identify seasonally
high ground water levels may lead to biological
clogging and slow water movement.

A sufficient number of drill  holes and test wells must
be installed to define the ground water position and
flow direction.

A reliable estimate of horizontal hydraulic
conductivity at  the site is essential and special tests
for this purpose may be needed.

Long narrow basins, with the long axis parallel to the
ground water contours,  will have the least problem
with ground water mounding. Clusters of  basins will
require detailed mounding  calculations  and careful
operational scheduling.

At northern sites, where significant long-term ice
formation is possible, any vegetation in the basin
should be cut close to the ground or burned in late
fall to prevent ice adherence at the infiltration
surface. Construction of a ridge and furrow
configuration on the basin surface and  promoting
development of a floating ice sheet will allow
continuous winter operation.
      U.S.  Environment^ Protection
      Raglan V, Library
      230  South  Dearborn Street
      Chicago, Illlnoll   Wto&s'

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The Conclusions
Rapid infiltration is a reliable and cost effective
technique for wastewater treatment. The use of
routine planning,  design  and construction procedures
insures successful systems for favorable site
conditions. The design becomes increasingly
complex as key site conditions become less
favorable. These  factors include (1) increasing
percentage of fines  (passing #200 sieve) above 5%,
(2) a soil profile with fine-textured lenses, (3) a
seasonably high water table, and  (4) an undulating
topography requiring major cut and fill.
For additional information contact:


EPA-OWPO(WH-547)                  EPA-RSKERL
401 M Street, SW                    P 0 Box 1198
Washington, DC 20460                 Ada.OK 74820
(202)382-73707369                   (405)322-8800

EPA Region 1                      EPA Region 6
John F Kennedy Federal Building          1201 Elm Street
Boston, MA 02203                    Dallas. TX 75270

EPA Region 2                      EPA Region 7
26 Federal Plaza                     324 East 11th Street
New York, NY 10278                  Kansas City. MO 64106

EPA Region 3    '                 EPA Region 8
6th & Walnut Streets                  1860 Lincoln Street
Philadelphia, PA 19106                 Denver, CO 80203

EPA Region 4                      EPA Region 9
345 Courtland Street, NE                215 Fremont Street
Atlanta, GA 30308               "     San Francisco, CA 94105

EPA Region 5              4,.     EPA Region 10
230 South Dearborn Street      "       1200 6th Avenue
Chicago, IL 60604                    Seattle, WA 98101

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  Soils

  Layers of zone® ef tine'.
  permeable soils not revealed
  during site investigation may
  impede water rrtovernejit.

  Design may be based oft
  inappropriate data from field
 different tocafloo 0r itt
 different depth than the final
                  in
  infiltration basin may contain
 significant day tr si(l These
  "tries" may ^fipte tturtng
  flbbdingj resale mi»
  surface and impede future
  watsr movement,',.,'.


  Design Assumptions

  Less than design capacity for
  water movement because
  backfill operations have
  reduced soil permeability
  Construction).

  Actual wastewater
  characteristics (algae,
  suspended solids) different
  than assumed.

  Design based  on improper
  use of criteria.

  Design ignores potential for
 Jnwzlng during $»|r*er  .  /
  operations in cold
Ground Water
wJtfr§«bgurfae& water
W^fequpte edacity to rnwe
water away from the site,
                               SUbsuifaee
                               basin may influence the
bash*.
Construction  Control
Failure to remove all of the
                       or
specified zones of
Construction activity in the
feasin infiltration area when
soil moisture content is too
Rainfall sorting of fines into
layers of tow permeability
Table 1.  Rapid Infiltration - Potential Problems

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