United States Environmental Protection Agency Wastewater Technology Fact Sheet Rapid Infiltration Land Treatment DESCRIPTION any adverse impact on the adjacent groundwater. It is also possible to use special management Rapid Infiltration (RI), which is also known as soil approaches that maximize the nitrification and aquifer treatment, is one of the three major land denitrification reactions, or to recycle the portion of treatment techniques that uses the soil ecosystem to the percolate with the highest nitrate concentration. treat wastewater. However, the RI process can treat a much larger volume of wastewater on a much smaller land area than other land treatment APPLICABILITY concepts. In RI systems, wastewater is applied to shallow basins constructed in deep and permeable RI is a simple and low cost wastewater treatment deposits of highly porous soils. Wastewater concept that has been used for more than 100 years. application can be by flooding, or occasionally by It is applicable for either primary or secondary sprinklers. Treatment, including filtration, effluent, and it has been used for treating municipal adsorption, ion exchange, precipitation, and and some industrial wastewaters. Industries which microbial action, occurs as the wastewater moves have successfully used RI to treat their wastewater 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 A) Hydraulic pathway 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. B) Recovery pathways 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 C) Natural drainage into surface waters bodies, particularly if hydrogeological studies show that the percolate will flow to the surface water Source: Crites, et al., 2000. system and will not impact the general groundwater quality. When using underdrains or wells, an FIGURE 1 HYDRAULIC PATHWAYS FOR alternative is to design for a discharge rate that only RAPID INFILTRATION slightly exceeds the percolation rate. This prevents include breweries, distilleries, food processing to Lake George or any of its tributaries. Therefore, plants, paper mills, and wool scouring plants. in order to dispose of its wastewater, the town discharges to natural basins consisting of more than RI can be used in a variety of different climates and 30 m (100 ft) of glacial sand deposits. The at varied site locations. Unlike other land treatment wastewater then percolates into the soil. After and aquiculture concepts, RI systems do not have percolation, the sand is raked and/or rototilled to any special seasonal constraints, and they have aerate the soil, and the beds can be reused. been successfully operated throughout the winter months in the northern United States and southern Currently, the Lake George WWTP discharges 1.3 Canada. RI is also very flexible in terms of site MGD during the summer, and between 0.5-0.6 location. Unless groundwater recharge and MGD in the winter. Treatment consists of recovery is intended, the most desirable sites are equalization, clarification, and trickling filters. located immediately adjacent to surface waters to After secondary settlement, wastewater is minimize any impact on the general groundwater discharged to one of 26 RI basins. Each basin is quality. An underdrained system can be located filled to just below the spillway, and the water is wherever suitable soil and groundwater conditions then allowed to infiltrate into the soil. During peak exist. flow periods in the summer, approximately one basin is filled per day. The basins take There are more than 350 RI systems operating in approximately 5 days to drain, and then each basin the United States. However, the potential difficulty is raked and is ready for reuse. in identifying appropriate sites for the construction of RI systems and more stringent standards that Because of the concerns that using these basins must be met before the effluent can be applied to RI could load high concentrations of nitrogen and basins have led to a decrease in the use of RI as a phosphorous into the groundwater, the town’s treatment process for primary wastewater. Instead, NPDES permit requires groundwater monitoring many of the systems currently in use in the U.S. are for increased nutrient concentrations. Nitrogen can used to polish secondary effluent. Other systems be a particular problem during the winter months serve primarily as a wastewater disposal method, or when nitrogen-fixing bacteria are less active. as a method to replenish groundwater supplies. For example, the Landis Sewerage Authority in New ADVANTAGES AND DISADVANTAGES Jersey operates an 3,100 m3/day (8.2 MGD) advanced wastewater treatment facility (AWTF). Advantages After being processed in the AWTF, all of the water is discharged back to the groundwater • Gravity distribution methods consume no through a RI basin, recharging the aquifer. RI energy. basins have also recently been installed to dispose of treated effluent from an industrial area consisting • No chemicals are required. of a hospital and a retirement home in Chester County, Pennsylvania. There are several basins • RI is a simple and economical treatment. covering a total of 1.2 ha (3 acres) in the system, and wastewater is applied by spraying it into each • The process is not constrained by seasonal basin on a rotating schedule. Once the basins have changes. reached their design effluent capacity, they are allowed to dry. The effluent then infiltrates • Effluent is of excellent quality. through the soil and into the groundwater, further improving its quality and recharging the aquifer • The process is very reliable with sufficient (Satterthwaite and Associates, 2003). resting periods.

The town of Lake George, New York, has been • RI provides a means for groundwater using a RI system for over 60 years. The use of RI recharge, controlling groundwater levels, basins at Lake George stems from a 1942 New recovering renovated water for reuse or York state law that forbids discharge of wastewater discharge to a particular surface water body, and temporary storage of renovated As described above, the RI process is entirely water in the aquifer. dependent on the soil and hydrogeological characteristics at a particular site, and these • The process is suitable for small plants characteristics must be carefully considered before where operator expertise is limited. choosing the site for a RI system. The soil must have sufficient hydraulic capacity to allow the Disadvantages wastewater to infiltrate, then percolate and move either to the groundwater or into underdrains. Any • As typically operated, RI systems will not fine textured top soil must be removed from the site usually meet the stringent nitrogen levels so as to utilize the underlying coarse soils as the required for discharge to drinking water basin bottom and percolation media. In addition, aquifers. the top 1.5-3 m (5-10 ft) of soil beneath the basin must be unsaturated at the start of the flooding • Requires long term commitment of a cycle to allow the expected treatment to occur. significant land area for treatment, with There must be suitable subsurface conditions (i.e., minimal secondary benefits such as are slope and/or hydraulic gradient) to ensure that the possible with other natural treatment percolate can flow away from the site at expected systems (i.e., crop or forest production, rates. The use of RI basins on fill material is not habitat enhancement, etc.). recommended because of potential damage to soil structure and hydraulic capacity during • Requires annual removal of accumulated deposits of organic matter on the infiltration TABLE 1 DESIGN CRITERIA surfaces in the basins. Item Range

Basin Infiltration Area 0.3-5.5 ha/103m3/d (3- • May require occasional removal and 56 acres/MGD) disposal of the top few inches of soil to expose clean material. Hydraulic Loading Rate 6-90 m/yr (20-300 ft/yr) [6-92 m3/m2/yr (150- 2250 gal/ft2/yr)] • Clogging can occur when influent is BOD Loading 22-112 kg/ha/d (20 to received at high application rates from algal 100 lb/acre/d) laden facultative lagoons and polishing ponds. Soil Depth at least 3-4.5 m (10-15 ft) DESIGN CRITERIA Soil Permeability at least 1.5 cm/hr (0.6 in/hr) Most RI failures are due to improper or incomplete Wastewater Application Period 4 hrs to 2 wks site evaluation. Therefore, the primary design Drying Period 8 hrs to 4 wks consideration for an RI system is site selection. Soil depth, soil permeability, and depth to Soil Texture coarse sands, sandy gravels groundwater are the most important factors in site evaluation. All of these factors must be very Individual Basin Size 0.4-4 ha (1-10 acres) carefully evaluated during site investigation, (at least 2 basins in parallel) regardless of system size, to ensure a successful Height of Dikes 0.15 m (0.5 ft) above design. maximum expected water level Once a suitable site has been selected, hydraulic Application Method flooding or sprinkling loading rates, nitrogen loading rates, organic Pretreatment Required primary or secondary loading rates, land area requirements, hydraulic loading cycle, infiltration system design, and Source: Crites, et al., 2000. groundwater mounding must all be taken into account in designing the RI system. General design parameters for RI systems are shown in Table 1. construction. Exceptions may be possible for very expected performance of the system. Table 2 coarse textured soils, but only if the hydraulic shows expected removal percentages for typical capacity is tested in a full scale fill. Performance pollution parameters using RI. limitations relate to removal of nitrogen, as discussed previously. OPERATION AND MAINTENANCE

Some system designs include an underdrain, which RI has excellent reliability. With proper operation is used to collect renovated water. In order for and management, several systems in the percolating water to move down through the soil northeastern United States have operated and into an underdrain, the soil must be saturated. continuously for more than 50 years without Therefore, the use of an underdrain pipe network problems. for percolate recovery is not feasible unless the native groundwater is less than 3 m (10 ft) deep Operation beneath the bottoms of the basins. This should allow for soil saturation during the flooding cycle. Preapplication treatment can be used to reduce the concentration of excess solids in the wastewater Once the proper site is chosen, a preliminary prior to introduction of the wastewater into the RI estimate of the treatment area required for an RI basin. Use of secondary effluent will allow a system can be made with the following equation: higher hydraulic loading rate and therefore a smaller RI basin system. RI basins receiving A = (0.250)(Q)/(L)(P) influent at high application rates from algal laden facultative lagoons and polishing ponds often Where: A = RI treatment area in acres; Q = experience rapid clogging. wastewater flow, gal/d; L = annual hydraulic loading into the basin, ft/yr (typical range 6-90 m Proper operation of a RI system requires a periodic [20-300 ft]; higher values for coarse soils and cycle of flooding and drying of each basin at the secondary treated wastewater); P = number of site. First, wastewater is added to a dry bed in the weeks per year the system is operated. “flooding” stage. The length of the flooding stage is determined by the design infiltration rate and the If the RI system operates on a year-round basis, the treatment requirements. After the bed is flooded equation reduces to: for the appropriate period, it is allowed to dry. During the drying stage, wastewater infiltrates into A = (0.0048)(Q)/(L) the soil or is evapotranspired into the atmosphere. The drying period is essential to restore aerobic This is an estimate of the basin treatment area. The conditions in the soil profile and to allow for total site area would also include dikes and berms, desiccation and decomposition of the organic solid access roads, etc. matter retained on the soil surface. The drying period can range from several hours to several Design of an RI basin must include mechanical equipment. Typical equipment associated with RI TABLE 2 EFFLUENT QUALITY systems includes distribution piping or troughs, pumps, underdrain piping (if used), well piping and Parameter Percent Removal pumps (if used), and storage tanks or lined basins BOD 95 to 99 percent (if needed). Sprinklers or pumped groundwater 5 recovery will require appropriate energy sources. TSS 95 to 99 percent TN 25 to 90 percent PERFORMANCE TP 0 to 90 percent

RI systems produce effluent of excellent quality Fecal Coliform 99.9 to 99.99+ percent with sufficient travel distance through soil. The use of primary versus secondary level influent Source: Crites, et al., 2000. influences the hydraulic loading rate but not the weeks depending on the flooding period selected • O&M includes the annual tillage of and the type of wastewater applied. Typically, the infiltration surfaces, and the repair of dikes, drying period is at least equal to the flooding period fences, and roads every 10 years. and may be twice as long. In cold climates, the drying period may be extended and the flooding • Construction for underdrained case also period shortened during the winter months to includes drain pipes at 2.5 m (8 ft) depth on compensate for the lower rate of treatment during 120 m (400 ft) spacing, with drains that season. connecting to an interception ditch at the edge of the site. Maintenance • Construction of the recovery well case The same maintenance requirements used at any includes gravel packed well, vertical turbine earthen basin are applicable to RI systems. Special pumps, simple shelter over well, and a 15 m requirements for RI systems pertain to preserving (50 ft) vertical pumping head. the design infiltration capacity of the basins. The operator should perform daily inspections and • Special O&M for underdrains includes jet record drainage time for the basins so that the cleaning of pipes every five years, and infiltration rate can be tracked. Restoration of the annual cleaning of interceptor ditch. infiltrative surface may be necessary when the infiltration rate decreases. Accumulated organic • Equations in Table 3 are valid for up to deposits are typically removed at least annually, 3785 m3/d (10 MGD) wastewater flow and and the infiltration surface is raked, disked or tilled use the following notation: C = costs in to restore infiltration capacity. On a more extended million of dollars; Q = wastewater flow in interval, it may be necessary to remove the top few MGD. inches of soil to expose clean material. These maintenance activities should only occur when the Costs of preliminary treatment, monitoring wells, basin bottom is dry to avoid soil compaction. and transmission from preliminary treatment Dikes and berms should also be monitored for signs facility to the RI site are not included. of decay or erosion. TABLE 3 COST ESTIMATION COSTS EQUATIONS

With suitable soil and hydrogeologic conditions, RI Construction ($) Operation and systems can produce a percolate that is essentially Maintenance ($) equal in quality to that produced by more Case I Rapid Infiltration - No Underdrains, No Recovery conventional advanced wastewater treatment Wells processes, at a fraction of the cost. General C=0.580(Q)0.888 C=0.054(Q)0.756 equations for estimating preliminary costs for Case II Rapid Infiltration with 50 ft Deep Recovery Wells construction and O&M of RI systems are shown in Table 3. The following assumptions were made in C=0.597(Q)0.857 C=0.058(Q)0.756 developing the equations: Case III Rapid Infiltration with Underdrains

0.886 0.641 • Costs are based on May 2001 data (ENR C=0.683(Q) C=0.075(Q) Index 6318). Source: Crites, et al., 2000

• 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. REFERENCES ADDITIONAL INFORMATION

Other Related Fact Sheets The Town of Lake George, NY Reggie Burlingame Slow Rate Land Treatment P.O. 791, 26 Old Post Road EPA 832-F-02-12 Lake George, NY 12845 September 2002 Brown and Caldwell Other EPA Fact Sheets can be found at the Ronald W. Crites following web address: P.O. Box 8045 http://www.epa.gov/owm/mtb/mtbfact.htm Walnut Creek, CA 94596

1. Crites, R. W. and G. Tchobanoglous, 1998. Environmental Engineering Consultants Small and Decentralized Wastewater Sherwood Reed Management Systems. McGraw Hill. RR1 Box 572 Norwich, VT 05055 2. Crites, R. W., S. C. Reed, and R. K. Bastian, 2000. Land Treatment Systems for The mention of trade names or commercial Municipal and Industrial Wastes. products does not constitute endorsement or McGraw-Hill. recommendation for use by the U.S. Environmental Protection Agency. 3. Satterthwaite Associates, Inc., 2003. Internet site at Office of Water http://www.wbsatterthwaite.com/ accessed EPA 832-F-03-025 August, 2003. June 2003

4. U.S. EPA, 1980. Innovative and Alternative Technology Assessment Manual. U.S. EPA MERL, Cincinnati, Ohio. For more information contact: 5. U.S. EPA, 1981. Process Design Manual: Land Treatment of Municipal Wastewater. Municipal Technology Branch U.S. EPA CERI, Cincinnati, Ohio. U.S. EPA ICC Building 6. U.S. EPA, 1984. Process Design Manual: 1200 Pennsylvania Ave., NW th Land Treatment of Municipal Wastewater, 7 Floor, Mail Code 4204M Supplement on Rapid Infiltration and Washington, D.C. 20460 Overland Flow. U.S. EPA CERI, Cincinnati, Ohio.