United States      September
       Environmental Protection  1983
<>EPA A Practical


       A Viable Land

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

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

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

 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
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.

Table 3   Selected Operational Rapid
          Infiltration Facilities

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

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.
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
 Low construction costs.

 Low energy requirements and operating costs.

 Simplicity of operation.
 Performance equal to or better than comparable
  conventional treatment processes.
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

 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.

Figure 2   Flooding of al Rapid Infiltration Basin
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