United States
Environmental Protection
Rapid Infiltration (RI), which is also known as soil
aquifer treatment, is one of the three major land
treatment techniques that uses the soil ecosystem to
treat wastewater. However, the RI process can treat
a much larger volume of wastewater on a much
smaller land  area  than  other  land treatment
concepts. In RI systems, wastewater is applied to
shallow basins constructed in deep and permeable
deposits  of highly  porous  soils.   Wastewater
application can be by flooding, or occasionally by
sprinklers.  Treatment,   including   filtration,
adsorption,  ion  exchange,   precipitation,  and
microbial action, occurs as the wastewater moves
through the soil matrix.   Phosphorus and most
metals are retained in the soil while toxic organics
are degraded or adsorbed.

As wastewater percolates through the soil, it can be
collected, or it can flow to native surface water or
groundwater aquifers.  Where the groundwater
table is relatively  shallow,  the use of underdrains
allows control of groundwater  mounding and
recovery of the renovated water.  In areas with
deeper groundwater, wells are used to recover the
renovated water. This recovered water can be for
irrigating crops or for industrial  uses.  This is
known as "beneficial  reuse."  Water that is not
recovered can recharge groundwater aquifers. The
typical hydraulic pathways for water treated by RI
are shown in Figure 1.

Common Modifications

Concerns regarding  increased  nitrogen levels  in
aquifers near RI systems have prompted several
modifications to the general system design. RI sites
may be located next to rivers or other surface water
bodies, particularly if hydrogeological studies show
that the percolate will flow to the surface water
system and will not impact the general groundwater
quality.  When using underdrains or wells, an
alternative is to design for a discharge rate that only
slightly exceeds the percolation rate. This prevents
Wastewater Technology Fact Sheet
Rapid Infiltration Land Treatment
                        any adverse impact on the adj acent groundwater. It
                        is  also  possible  to use  special  management
                        approaches that maximize the nitrification and
                        denitrification reactions, or to recycle the portion of
                        the percolate with the highest nitrate concentration.

                        RI is a simple and low cost wastewater treatment
                        concept that has been used for more than 100 years.
                        It is  applicable  for either primary or secondary
                        effluent, and it has been used for treating municipal
                        and some industrial wastewaters.  Industries which
                        have successfully used RI to treat their wastewater
                                        A) Hydraulic pathway
                                     Flooding basins
                                                      Recovered water
                                       Ground water
                                        B) Recovery pathways
                                                         Flooding basin
                                    C) Natural drainage into surface waters
                        Source: Crites, et al., 2000.

                         FIGURE 1 HYDRAULIC PATHWAYS FOR
                                   RAPID INFILTRATION

include breweries,  distilleries, food  processing
plants, paper mills, and wool scouring plants.

RI can be used in a variety of different climates and
at varied site locations. Unlike other land treatment
and aquiculture concepts, RI systems do not have
any special  seasonal constraints,  and they have
been successfully operated throughout the winter
months in the northern United States and southern
Canada. RI  is also very flexible in  terms of site
location.    Unless  groundwater  recharge  and
recovery is intended, the most desirable sites are
located immediately adjacent to surface waters  to
minimize any impact on the general groundwater
quality. An underdrained system can be located
wherever suitable soil and groundwater conditions

There are more than 350 RI systems operating  in
the United States. However, the potential difficulty
in identifying appropriate sites for the construction
of RI systems and more stringent standards that
must be met before the effluent can be applied to RI
basins have led to a decrease in the use of RI as a
treatment process for primary wastewater.  Instead,
many of the systems currently in use in the U. S. are
used to polish secondary effluent.  Other systems
serve primarily as a wastewater disposal method,  or
as a method to replenish groundwater supplies. For
example, the Landis Sewerage Authority in New
Jersey operates an  3,100  m3/day  (8.2  MOD)
advanced wastewater treatment facility (AWTF).
After  being  processed  in the AWTF, all of the
water  is discharged back  to  the  groundwater
through a RI basin, recharging the aquifer.  RI
basins have also recently been installed to dispose
of treated effluent from an industrial area consisting
of a hospital and a retirement home  in Chester
County, Pennsylvania.   There are several basins
covering a total of 1.2 ha (3  acres) in the system,
and wastewater is applied by spraying it into each
basin on a rotating schedule.  Once the basins have
reached their design effluent capacity,  they are
allowed to  dry.    The effluent  then  infiltrates
through the soil and into the groundwater, further
improving its quality and recharging the aquifer
(Satterthwaite and Associates, 2003).

The town of Lake George, New York, has been
using  a RI system for over 60 years. The use of RI
basins at Lake George  stems from  a  1942 New
York state law that forbids discharge of wastewater
to Lake George or any of its tributaries. Therefore,
in order  to  dispose of its wastewater, the town
discharges to natural basins consisting of more than
30  m  (100  ft) of  glacial  sand deposits.   The
wastewater then percolates into the soil.   After
percolation,  the sand is raked and/or rototilled to
aerate the soil, and the beds can be reused.

Currently, the Lake George WWTP discharges 1.3
MGD  during the summer, and between 0.5-0.6
MGD  in the winter.    Treatment  consists  of
equalization, clarification,  and trickling  filters.
After  secondary   settlement,  wastewater   is
discharged to one of 26 RI basins.  Each basin is
filled to just below the spillway, and the water is
then allowed to infiltrate into the soil. During peak
flow periods in the summer, approximately one
basin  is  filled   per  day.    The  basins take
approximately 5 days to drain, and then each basin
is raked and is ready for reuse.

Because  of the concerns that using these basins
could load high concentrations of nitrogen and
phosphorous into  the  groundwater, the town's
NPDES permit requires groundwater monitoring
for increased nutrient concentrations. Nitrogen can
be a particular problem during the winter months
when nitrogen-fixing bacteria are less active.



      Gravity distribution methods consume no

       No chemicals are required.

      RI is a simple and economical treatment.

      The process is not constrained by seasonal

       Effluent is of excellent quality.

      The process is very reliable with sufficient
       resting periods.

      RI provides  a means  for  groundwater
       recharge,  controlling groundwater levels,
       recovering renovated water for  reuse  or
       discharge  to a particular  surface  water

       body, and temporary storage of renovated
       water in the aquifer.

      The process is suitable for small  plants
       where operator expertise is limited.


      As typically operated, RI systems will not
       usually meet the stringent nitrogen levels
       required for discharge to drinking water

      Requires  long  term  commitment  of a
       significant land area  for treatment, with
       minimal secondary benefits such as are
       possible  with  other  natural  treatment
       systems (i.e., crop or forest production,
       habitat enhancement, etc.).

       Requires annual  removal of accumulated
       deposits of organic matter on the infiltration
       surfaces in the basins.

      May  require  occasional  removal  and
       disposal of  the top few inches of soil to
       expose  clean material.

      Clogging can occur  when  influent is
       received at high application rates from algal
       laden facultative  lagoons  and polishing


Most RI failures are due to improper or incomplete
site  evaluation.  Therefore,  the primary design
consideration for an RI system is site selection.
Soil  depth,  soil   permeability,  and  depth  to
groundwater are the most important factors in site
evaluation.   All of these factors  must  be very
carefully  evaluated  during   site  investigation,
regardless of system size, to ensure a  successful

Once a suitable site has been selected, hydraulic
loading  rates,  nitrogen  loading rates,  organic
loading rates,  land  area requirements, hydraulic
loading  cycle,  infiltration  system design,  and
groundwater mounding  must all be  taken into
account in designing the RI system. General design
parameters for  RI systems  are shown in Table 1.
As described above, the RI process is entirely
dependent  on  the  soil   and  hydrogeological
characteristics  at  a  particular  site,  and these
characteristics must be carefully considered before
choosing the site for  a RI system.  The  soil must
have  sufficient  hydraulic  capacity to allow the
wastewater to infiltrate,  then percolate and move
either to the groundwater or into underdrains. Any
fine textured top soil must be removed from the site
so as to utilize the underlying coarse  soils as the
basin bottom and percolation media. In addition,
the top  1.5-3 m (5-10 ft) of soil beneath the basin
must  be unsaturated  at  the  start of the flooding
cycle to allow the expected treatment  to  occur.
There must be suitable subsurface conditions (i.e.,
slope and/or hydraulic gradient) to  ensure that the
percolate can flow away from the site  at expected
rates. The use of RI basins on fill  material is not
recommended because of potential  damage to soil
structure   and   hydraulic   capacity   during

 Basin Infiltration Area

 Hydraulic Loading Rate

 BOD Loading

 Soil Depth

 Soil Permeability

 Wastewater Application Period

 Drying Period

 Soil Texture

 Individual Basin Size
 (at least 2 basins in parallel)

 Height of Dikes

 Application Method

 Pretreatment Required	
0.3-5.5 ha/103m3/d (3-
56 acres/MGD)

6-90 m/yr (20-300 ft/yr)
[6-92 m3/m2/yr( 150-
2250 gal/ft2/yr)]


at least 3-4.5 m (10-15
at least 1.5 cm/hr(0.6

4 hrs to 2 wks

8 hrs to 4 wks

coarse sands, sandy

0.4-4 ha (1-10 acres)
0.15 m (0.5ft) above
maximum expected
water level

flooding or sprinkling

primary or secondary
Source: Crites, et al., 2000.

construction. Exceptions may be possible for very
coarse textured  soils, but  only if the hydraulic
capacity is tested in a full scale fill.  Performance
limitations  relate  to removal of  nitrogen,  as
discussed previously.

Some system designs include an underdrain, which
is used to collect renovated water.  In order for
percolating water to move down through the soil
and into an underdrain, the soil must be saturated.
Therefore, the use of an underdrain pipe network
for  percolate recovery is not feasible unless the
native groundwater is less than 3 m (10 ft)  deep
beneath the bottoms of the basins.  This should
allow for soil saturation during the flooding cycle.

Once the proper site  is chosen, a  preliminary
estimate of the treatment area required for an RI
system can be made with the following equation:

       A = (0.250)(Q)/(L)(P)

Where: A = RI treatment area  in  acres;  Q =
wastewater  flow,  gal/d; L = annual  hydraulic
loading into the basin, ft/yr (typical range 6-90 m
[20-300 ft]; higher values for coarse soils and
secondary treated  wastewater); P = number  of
weeks per year the system is operated.

If the RI system operates on a year-round basis, the
equation reduces to:

       A = (0.0048)(Q)/(L)

This is an estimate of the basin treatment area. The
total site area would also include dikes and berms,
access roads, etc.

Design of an RI basin must include mechanical
equipment.  Typical equipment associated with RI
systems includes distribution piping or troughs,
pumps, underdrain piping (if used), well piping and
pumps (if used),  and storage tanks or lined basins
(if needed).   Sprinklers or pumped groundwater
recovery will require appropriate energy sources.


RI systems  produce effluent of excellent quality
with sufficient travel distance through soil. The use
of  primary  versus  secondary  level  influent
influences the hydraulic loading rate but not the
expected performance of the system.  Table  2
shows expected removal percentages for typical
pollution parameters using RI.


RI has excellent reliability.  With proper operation
and   management,   several  systems  in   the
northeastern  United  States   have   operated
continuously for more than 50 years  without


Preapplication treatment can be used to reduce the
concentration of excess solids in the wastewater
prior to introduction of the wastewater into the RI
basin.  Use of secondary  effluent  will allow  a
higher hydraulic loading  rate  and  therefore  a
smaller RI basin system.   RI  basins receiving
influent at high application rates from algal laden
facultative  lagoons  and polishing  ponds  often
experience rapid clogging.

Proper operation of a RI system requires a periodic
cycle of flooding and drying of each basin at the
site.  First, wastewater is added to a dry bed in the
"flooding" stage.  The length of the flooding stage
is determined by the design infiltration rate and the
treatment requirements. After the bed is flooded
for the appropriate period,  it is allowed to dry.
During the drying stage, wastewater infiltrates into
the soil or is evapotranspired into the atmosphere.
The drying period is  essential to restore aerobic
conditions in  the soil profile and  to allow for
desiccation and decomposition of the organic  solid
matter retained on the soil  surface.  The drying
period can range from several  hours  to several

Percent Removal




 Fecal Coliform
95 to 99 percent

95 to 99 percent

25 to 90 percent

0 to 90 percent

99.9 to 99.99+ percent
Source: Crites, et al., 2000.

weeks depending on the flooding period selected
and the type of wastewater applied. Typically, the
drying period is at least equal to the flooding period
and may be twice as long.  In cold climates, the
drying period may be extended and the flooding
period shortened  during  the  winter  months  to
compensate for the lower rate of treatment during
that season.


The same maintenance  requirements used at any
earthen basin are applicable to RI systems. Special
requirements for RI systems pertain to preserving
the design infiltration capacity of the basins. The
operator  should  perform  daily  inspections and
record drainage time for  the  basins so  that the
infiltration rate can be tracked. Restoration of the
infiltrative surface may be necessary  when the
infiltration rate decreases.  Accumulated organic
deposits  are typically removed at least annually,
and the infiltration surface is raked, disked or tilled
to restore infiltration capacity. On a more extended
interval, it may be necessary to remove the top few
inches of soil to expose clean material.   These
maintenance activities should only occur when the
basin bottom is  dry to avoid soil compaction.
Dikes and berms should also be monitored for signs
of decay  or erosion.


With suitable soil and hydrogeologic conditions, RI
systems can produce a percolate that is essentially
equal  in  quality to  that  produced  by  more
conventional  advanced  wastewater  treatment
processes,  at a fraction of the  cost.    General
equations  for  estimating  preliminary  costs for
construction and O&M of RI systems are shown in
Table 3.  The following assumptions were made in
developing the equations:

       Costs are based  on May 2001  data (ENR
       Index 6318).

       Basin  construction  costs  include  field
       preparation, no seasonal storage, assumed
       hydraulic loading of 60 m/yr (200 ft/yr) [61
       m3/m2/yr (1496 gal/ft2/yr),] gravel service
       roads,   and  stock  fence  around site
       O&M  includes  the  annual  tillage  of
       infiltration surfaces, and the repair of dikes,
       fences, and roads every 10 years.

      Construction for underdrained case also
       includes drain pipes at 2.5 m (8 ft) depth on
       120  m  (400 ft)   spacing,  with  drains
       connecting to an interception ditch at the
       edge of the site.

      Construction of the  recovery  well  case
       includes gravel packed well, vertical turbine
       pumps, simple shelter over well, and a 15 m
       (50 ft) vertical pumping head.

       Special O&M for underdrains includes jet
       cleaning  of pipes every five years,  and
       annual cleaning of interceptor ditch.

      Equations in Table 3  are valid for up to
       3785 m3/d (10 MOD) wastewater flow and
       use the following notation:  C = costs in
       million of dollars; Q = wastewater  flow in

Costs of preliminary treatment,  monitoring wells,
and  transmission  from  preliminary  treatment
facility to the RI site are not included.

   Construction ($)
 Operation and
Maintenance ($)
  Case I Rapid Infiltration - No Underdrains, No Recovery

 C=0.580(Q)0888         C=0.054(Q)0756

  Case II Rapid Infiltration with 50 ft Deep Recovery Wells

    C=0.597(Q)0857           C=0.058(Q)0756

      Case III Rapid Infiltration with Underdrains

    C=0.683(Q)0886           C=0.075(Q)0641

 Source: Crites, et al., 2000

Other Related Fact Sheets

Slow Rate Land Treatment
EPA 832-F-02-12
September 2002

Other EPA Fact  Sheets  can be  found at the
following web address:
http://www.epa. gov/owm/mtb/mtb fact, htm

1.     Crites, R. W. and G. Tchobanoglous, 1998.
      Small  and  Decentralized   Wastewater
      Management Systems. McGrawHill.

2.     Crites, R.  W.,  S.  C.  Reed, and R. K.
      Bastian, 2000. Land Treatment Systems for
      Municipal  and   Industrial  Wastes.

3.     Satterthwaite   Associates,   Inc.,   2003.
      Internet    site     at
      http://www.wbsatterthwaite.com/ accessed
      August, 2003.

4.     U.S. EPA, 1980. Innovative and Alternative
      Technology Assessment Manual. U.S. EPA
      MERL, Cincinnati, Ohio.

5.     U.S. EPA, 1981.  Process Design Manual:
      Land Treatment of Municipal Wastewater.
      U.S. EPA CERI, Cincinnati, Ohio.

6.     U.S. EPA, 1984.  Process Design Manual:
      Land Treatment of Municipal Wastewater,
      Supplement on  Rapid  Infiltration  and
      Overland  Flow.  U.S.  EPA  CERI,
      Cincinnati, Ohio.
The Town of Lake George, NY
Reggie Burlingame
P.O. 791,26 Old Post Road
Lake George, NY 12845

Brown and Caldwell
Ronald W. Crites
P.O. Box 8045
Walnut Creek, CA 94596

Environmental Engineering Consultants
Sherwood Reed
RR1 Box 572
Norwich, VT 05055

The  mention  of trade names or  commercial
products  does  not  constitute endorsement  or
recommendation for use by the U.S. Environmental
Protection Agency.

              Office of Water
             EPA 832-F-03-025
                 June 2003
      For more information contact:

      Municipal Technology Branch
      U.S. EPA
      ICC Building
      1200 Pennsylvania Ave., NW
        Floor, Mail Code 4204M
      Washington, D.C. 20460
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