A) Hydraulic pathway

B) Recovery pathways

C) Natural drainage into surface waters

Source: Crites, et al., 2000.

FIGURE 1 HYDRAULIC PATHWAYS FOR

RAPID INFILTRATION

United States

Environmental Protection

Agency

Wastewater Technology Fact Sheet

Rapid Infiltration Land Treatment

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

any adverse impact on the adjacent 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
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 MGD)

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
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 0.3-5.5 ha/103m3/d (3-

56 acres/MGD)

Hydraulic Loading Rate 6-90 m/yr (20-300 ft/yr)

[6-92 m3

/m2/yr (150-

2250 gal/ft2

/yr)]

BOD Loading 22-112 kg/ha/d (20 to

100 lb/acre/d)

Soil Depth at least 3-4.5 m (10-15

ft)

Soil Permeability at least 1.5 cm/hr (0.6

in/hr)

Wastewater Application Period 4 hrs to 2 wks

Drying Period 8 hrs to 4 wks

Soil Texture coarse sands, sandy

gravels

Individual Basin Size

(at least 2 basins in parallel)

0.4-4 ha (1-10 acres)

Height of Dikes 0.15 m (0.5 ft) above

maximum expected

water level

Application Method flooding or sprinkling

Pretreatment Required 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.

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 95 to 99 percent

TSS 95 to 99 percent

TN 25 to 90 percent

TP 0 to 90 percent

Fecal Coliform 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. 

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 MGD) wastewater flow and

use the following notation: C = costs in

million of dollars; Q = wastewater flow in

MGD.

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)0.888 C=0.054(Q)0.756

Case II Rapid Infiltration with 50 ft Deep Recovery Wells

C=0.597(Q)0.857 C=0.058(Q)0.756

Case III Rapid Infiltration with Underdrains

C=0.683(Q)0.886 C=0.075(Q)0.641

Source: Crites, et al., 2000
REFERENCES

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/mtbfact.htm

1. Crites, R. W. and G. Tchobanoglous, 1998.

Small and Decentralized Wastewater

Management Systems. McGraw Hill.

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.

ADDITIONAL INFORMATION

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

7th Floor, Mail Code 4204M

Washington, D.C. 20460