xvEPA
United States
Environmental Protection
Agency
DESCRIPTION
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.
APPLICABILITY
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
Evaporation
A) Hydraulic pathway
Flooding basins
Recovered water
Ground water
Underdrains
B) Recovery pathways
Wells
Flooding basin
C) Natural drainage into surface waters
Source: Crites, et al., 2000.
FIGURE 1 HYDRAULIC PATHWAYS FOR
RAPID INFILTRATION
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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
exist.
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.
ADVANTAGES AND DISADVANTAGES
Advantages
• Gravity distribution methods consume no
energy.
No chemicals are required.
• RI is a simple and economical treatment.
• The process is not constrained by seasonal
changes.
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
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body, and temporary storage of renovated
water in the aquifer.
• The process is suitable for small plants
where operator expertise is limited.
Disadvantages
• As typically operated, RI systems will not
usually meet the stringent nitrogen levels
required for discharge to drinking water
aquifers.
• 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
ponds.
DESIGN CRITERIA
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
design.
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
TABLE 1 DESIGN CRITERIA
Item
Range
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)]
22-112kg/ha/d(20to
100lb/acre/d)
at least 3-4.5 m (10-15
ft)
at least 1.5 cm/hr(0.6
in/hr)
4 hrs to 2 wks
8 hrs to 4 wks
coarse sands, sandy
gravels
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.
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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.
PERFORMANCE
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.
OPERATION AND MAINTENANCE
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
problems.
Operation
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
TABLE 2 EFFLUENT QUALITY
Parameter
Percent Removal
BOD5
TSS
TN
TP
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.
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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.
Maintenance
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.
COSTS
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
perimeter.
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
MOD.
Costs of preliminary treatment, monitoring wells,
and transmission from preliminary treatment
facility to the RI site are not included.
TABLE 3 COST ESTIMATION
EQUATIONS
Construction ($)
Operation and
Maintenance ($)
Case I Rapid Infiltration - No Underdrains, No Recovery
Wells
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
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REFERENCES
ADDITIONAL INFORMATION
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.
McGraw-Hill.
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
Tth
Floor, Mail Code 4204M
Washington, D.C. 20460
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