United States September
Environmental Protection 1983
<>EPA A Practical
Technology
Rapid
Infiltration
A Viable Land
Treatment
Alternative
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Rapid Infiltration - A Viabl
Objective: Simplicity
As we move through the 1980s, the need to protect
our streams, lakes, and ground waters continues to
warrant high priority. This task is made more
difficult by the ever-increasing costs of energy,
chemicals, and equipment necessary for
wastewater treatment systems. Today, municipal
officials and consultants agonize over the many
decisions required to achieve the objective of
satisfactory water quality at a cost which can be
borne by the community without creating serious
financial hardships.
If we are to achieve the objective of cost-effective
waste treatment, new alternatives are needed to
compete with the traditionally accepted methods. In
some cases, this means uncommon and innovative
techniques. In others, it simply means a reshaping
and refining of old ideas into improved processes/
for instance, the land treatment process known as
rapid infiltration. Used with established knowledge
and suitable site conditions, rapid infiltration can
offer a very efficient and cost-effective wastewater
treatment alternative.
Process Description
Rapid infiltration is a very site-specific wastewater
treatment process which can be used effectively
only at certain sites. These sites must contain well
drained soils which have moderate to moderately
high permeability rates. Wastewater is first given an
appropriate level of preapplication treatment and
then applied to the treatment site at relatively high
hydraulic loading rates.
A variety of application techniques can be used
such as sprinkler irrigation or, as shown on Figure
1, flooding of an infiltration basin. As a basin is
flooded, the applied wastewater percolates down
through the bottom of the basin, through the soil
profile, and eventually into the ground water.
Depending on site conditions the renovated water
may remain in the ground water aquifer or
eventually enter surface waters. As in any land
treatment system, the wastewater is renovated by a
combination of physical, chemical, and biological
processes prior to entering the aquifer.
After an individual basin is filled to a shallow depth,
it is allowed to drain and dry. This restores aerobic
conditions in the soil prior to the next application. In
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e Land Treatment Alternative
Figure 1 Rapid Infiltration Schematics
order to accomplish the alternate wetting and drying
cycles without interrupting continuous treatment,
more than one basin is required, and several are
often used in a single system.
Compared to other land treatment processes (e.g.
slow rate irrigation and overland flow), vegetation
plays a minor role, and in many cases, is not used
at all. Rapid infiltration systems have also shown an
ability to operate effectively in both wet and cold
weather, thereby minimizing or eliminating the need
for cold season storage.
Certain sites contain soils which are well suited for
the use of rapid infiltration, but also contain
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limitations which will restrict the effective use of the
process. Examples of these are sites with a high
ground water table and sites underlain with low
permeability soils or shallow fractured bedrock.
However, even here, if suitable soils are of
sufficient depth, it is often possible to utilize
subsurface wells or drainage systems to help
overcome many of these limitations (see Figure 1c).
Design Considerations
Rapid infiltration, following primary clarification, can
be used in lieu of a conventional secondary
treatment process, or it can be used as a polishing
process following conventional secondary
treatment. Many of the process design parameters
are very site specific. Therefore, it is not possible to
provide a set of design criteria which will be
applicable for all situations. Table 1 lists key design
factors and ranges which are applicable to most
rapid infiltration systems. Full details are contained
in the EPA Process Design Manual on Land
Treatment of Municipal Wastewater.
Table 1 Rapid Infiltration Design Factors
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Performance
Rapid infiltration systems are capable of providing
high levels of treatment. BOD and suspended
solids are removed almost completely by filtration
and biological degradation at or near the soil
surface. Fecal coliform removal efficiencies are
proportional to the depth of the soil through which
the wastewater must travel. Removal efficiencies
are excellent with adequate distance. Phosphorus
and trace element removals are dependent on the
composition of the soil as well as the distance of
travel. Travel of 10 feet to a few hundred feet
achieves excellent removals in most circumstances.
Total nitrogen removal will vary according to the
operating scheme used. Very high levels of
nitrification are typical in a rapid infiltration system.
Rapid infiltration systems can be managed to
achieve microbial denitrification to change nitrate to
nitrogen gas and prevent the nitrate from entering
the ground water. Managing to achieve
denitrification requires special expertise and
considerations in the design and operation of a
project at the present time.
Table 2 shows the type of treatment performance
which can be expected from a rapid infiltration
system.
Table 2 Expected Quality of the Percolate
from a Rapid Infiltration System
Current Use
The rapid infiltration process is currently being used
for the treatment of both municipal and industrial
wastewaters. Table 3 shows the location and size
of selected rapid infiltration systems that are
presently treating municipal wastewaters.
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Table 3 Selected Operational Rapid
Infiltration Facilities
Costs
Both the capital and operating and maintenance
costs for a rapid infiltration system can be very
competitive with the costs of other treatment
technologies. The actual costs will depend on
various factors such as land costs, transport
distance, soil permeability, and degree of
preapplication treatment. However, four important
factors help keep the costs of these systems low.
These factors, common to most rapid infiltration
systems, are: (1) high hydraulic loading rates that
keep the land area requirements to a minimum; (2)
minimal need for seasonal storage; (3) no need for
a high pressure application system; and (4) very
low operation and maintenance costs. This makes
both construction costs and energy requirements
very low.
One example of the potential savings offered by the
rapid infiltration process is the Bozeman, Montana,
project, a 5.57 mgd system which became
operational in 1982. The stream discharge
requirements for this facility included an ammonia
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limitation. The Facilities Plan recommended the use
of a rapid infiltration system, preceded by activated
sludge, at an estimated cost of $9.0 million. The
next lowest treatment option which could meet the
ammonia limitations was a secondary treatment
system followed by a fixed growth process at a
projected cost of $11.1 million, 23% higher than the
rapid infiltration process option. The actual
construction cost of the combination activated
sludge/rapid infiltration system was only $7.8
million, due chiefly to the elimination of the
ammonia limitation during winter.
Advantages
Used under the proper conditions, rapid infiltration
systems offer the following advantages:
• Lower land requirements compared to other land
treatment processes.
• Can be operated in cold and wet weather without
storage.
• Low construction costs.
• Low energy requirements and operating costs.
• Simplicity of operation.
• Performance equal to or better than comparable
conventional treatment processes.
Limitations
The following factors must be adequately
considered in order to use rapid infiltration as an
effective wastewater treatment process:
• The soil conditions must be favorable and
construction must not destroy the needed
permeability.
• The hydrogeologic conditions must be favorable
(i.e. adequate depth of soils and depth to ground
water, favorable subsurface drainage).
• Careful site testing and conservative safety
factors are needed to select successful design
hydraulic load rates.
• Close operational control is needed when
nitrogen removal must be achieved to meet
system requirements.
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Figure 2 Flooding of al Rapid Infiltration Basin
For additional information contact:
EPA-OWPO(WH-547)
401 M Street, SW
Washington, DC 20460
(202)382-7370/7369
EPA Region 1
John F. Kennedy Federal Building
Boston, MA 02203
EPA Region 2
26 Federal Plaza
New York, NY 10278
EPA Region 3
6th & Walnut Streets
Philadelphia, PA 19106
EPA Region 4
345 Courtland Street, NE
Atlanta, GA 30308
EPA Region 5
230 South Dearborn Street
Chicago, IL 60604
EPA-MERL|
26 West St. Clair Street
Cincinnati, OH 45268
(513)684-7614
EPA Region 6
1201 Elm Street
Dallas, TX 75270
EPA Region 7
324 East 11th Street
Kansas City, MO 64106
EPA Region 8
1860 Lincoln Street
Denver, CO 80203
EPA Region 9
215 Fremont Street
San Francisco, CA 94105
EPA Region 10
1200 6th Avenue
Seattle, WA 98101
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