t for Success

The Concerns

Most of the operating systems in the U.S. are
successfully meeting all RI 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.


Table 1. Rapid Infiltration - Potential Problems


ion - Plan, Design, and Construc

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 RI 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/ft”) 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

galft?/day, which is no greater than the application
tate on household leachfield systems in similar soils.
Many of the existing RI 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.


— DATE DUE _—

pen

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 feast 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.

US. Environmental Protection Agency

Region V, Library ;
230 South Dearborn Street ~~

Chicago, HWiinoia BO608~ wall


United States June
Environmental Protection 1984

5054 Agency

905R84122 —

SEPA Land
Treatment:

Rapid
Infiltration

Plan,
Design, and
Construct
for

Success



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 Steet, SW PO Box 1198
Washington, DC 20460 ‘Ada,OK 74820,
(202)382-7370 7369 (405) 322-8800

EPA Region 1 EPA Region 6

John F Kennedy Federal Butding 1201 Eim Street
Boston, MA 02203 Dallas, TX 75270

EPA Region 2 EPA Region 7

26 Federal Plaza 824 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 a? ‘San Francisco, CA 94105
EPA Region 5 a=. EPARegion 10
230 South Dearborr Street 1200 6th Avenue

‘Chicago, IL 60604 Seattie. WA 98101


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 alll 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 Ri 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 constructian activities in a cut are typically
* jimited to periods whén ‘thé Soil molstdre ison the

“dry” side of optimum. ‘This precaytion 4s .taken to
avoid reduction of soil permeability by compaction.
: anes :



Land Treatment: Rapid Infil

The Concept

Rapid infiltration (RI) 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 Ri 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.

Figure 1. Recovery of Treated Water