ES&T FEATURE
Land use of wastewater and sludge Here is a review of current technologies, treatment performance, and research needs for these municipal wastes Ronald W. Crites George S. Nolte and Associates 1700 L Street Sacramento, Calif. 95814 Serious research concerning the use of municipal wastewater and sludge on land started in the late 1960s. In July 1973, a workshop was held at the University of Illinois (/) to define the existing state of knowledge and determine research needs for land treatment of municipal waste. In the ensuing 10 years, much information on the technology has been developed, and many questions posed have been answered. Nevertheless, not only do some of the research needs remain unfulfilled, but several new issues have been raised. During the 1960s, the technology of land treatment was viewed mainly as a disposal technology with all necessary treatment carried out before land disposal. Similarly, land application of sludge was seen as a disposal approach only; little thought was given to the beneficial uses of sludge on land. Currently, there is a much wider understanding of the ability of soil systems to treat wastewater and acceptance of the view that sludge can be safely and beneficially applied to cropland, forest land, and disturbed lands. Land treatment of wastewater The three land treatment processes for wastewater are slow rate, rapid infiltration, and overland flow. During the early 1970s, only slow rate (irrigation) and rapid infiltration were in use for municipal wastewater. Over the next 10 years, overland flow land treatment emerged as an effec140A
Environ. Sci. Technol., Vol. 18, No. 5, 1984
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tive treatment process for screened raw wastewater as well as primary and secondary effluent. A comparison of the number of municipal land treatment systems existing in 1972 with those in 1981 is presented in Table 1. Slow rate. Slow rate land treatment is the controlled application of wastewater by sprinkler or surface means to a vegetated land surface. Systems are designed to avoid offsite runoff of the wastewater. A portion of the applied wastewater percolates to the subsurface, and the remainder is used by the vegetation. The largest slow rate system, the Metro system in Muskegon County, Mich., went into operation in 1974 to treat 1.2 m 3 / s [27.5 million gallons per day (mgd)] of domestic and industrial wastewater (4). Center pivot sprinklers apply aerated lagoon effluent onto 2240 ha (5540 acres) of land on which field corn is planted annually. The underdrain system (which is generally unique to slow rate systems), consisting of corrugated polyethylene tubing with nylon sock-type filter sheaths, was installed at a 150m (500-ft) spacing and at depths of 1.5-3.7 m (5-12 ft). Problems with drainage have limited the hydraulic capacity of the site (4). The system is to be expanded to a 2.1-m 3 /s (49mgd) capacity by the addition of a rapid infiltration system in the welldrained north area. Another development in the 1970s and early 1980s was the full-scale use of forested land for slow rate land treatment. This grew out of research carried out during the 1960s and 1970s at the Pennsylvania State University (5) and similar research undertaken in 1973 in Georgia (6"). On the basis of this research, a full-scale 0.86-m 3 /s (19.6-mgd) forested slow rate system was designed for Clayton County, Ga. The subsurface flow from this system, placed into operation in 1981, augments stream flow, which is used for water supply for the community (7). Slow rate land treatment normally requires 2.6-3.5 ha/m 3 -s (150-200 acres/mgd). To minimize the capital costs of new slow rate systems, several have been developed to irrigate private land. The cost of purchasing the land is thereby avoided. In 1979, a 525-ha ( 1300-acre) system was developed at Mitchell, S.D., where the farmers own the land and operate and maintain the center pivot machines. Other systems using private land include El Reno, Okla., Vandalia, Mo., Lubbock, Tex., and Petaluma, Calif.
(8). The city of Petaluma saved more than $1 million on the capital costs of installing a slow rate system on 222 ha (550 acres) by not purchasing the land. Rapid infiltration. Rapid infiltration is the application of wastewater to permeable soils, such as sands or sandy loams, for treatment in shallow spreading basins. Wastewater undergoes treatment by chemical, physical, and biological processes as it travels through the soil. Vegetation is not normally a part of rapid infiltration, because loading rates are too high for nutrient uptake by plants to be effective. There are, however, instances in which vegetation plays an integral role in stabilizing surface soils and maintaining high infiltration rates. The treated water generally flows through the subsurface until it joins a surface stream. The quality of this water is typically very good and generally comparable to tertiary effluent (9). A major advance in the use of rapid infiltration in the 1970s arose from the realization that with application of primary effluent, the quality of the percolate was usually comparable with, and often better than percolate from systems using secondary effluent. For many years, the use of primary effluent at Ft. Devens, Mass. (70), and Hollister, Calif. ( / / ) , and unt r e a t e d w a s t e w a t e r at C a l u m e t , Mich. {12), has resulted in high-quality percolates. Primary effluent enhances the denitrification reaction in the soil and allows greater nitrogen removal (3). Another development was the use of underground aquifers for wastewater storage, followed by recovery and reuse of the water. Research at Phoenix, Ariz., has shown that intermittent infiltration, followed by recovery with wells, can produce highquality water suitable for unrestricted irrigation (13). A similar system employing ponds, rapid infiltration, and recovery wells is in use in the Dan region of Israel (14). This underground storage can allow cost-effective reuse of the treated water for crop irrigation. Overland flow. Overland flow is a fixed-film biological treatment process by which wastewater is applied at the upper portions of grass-covered slopes and allowed to flow down the slope to runoff collection ditches. The wastewater is treated by physical, chemical, and biological means as it flows in a thin sheet down the slope, which is between 30 and 45 m (1001 50 ft) long. Slowly permeable soils
are used, and slopes have a grade of 2 8%. As indicated in Table 1, no municipal overland flow system was in use in the U.S. in 1972. EPA conducted the first research on overland flow treatment of municipal wastewater at Ada, Okla. This stimulated research and pilot projects at several other locations during the mid-1970s (15). A list of research and demonstration systems is presented in Table 2. Most of the research at these facilities has been completed. With this field research plus laboratory-scale research and development concerning overland flow for t r e a t m e n t of c e r t a i n i n d u s t r i a l wastes, there was a substantial evolution of the technology. By the late 1970s, research at Hanover, N.H., and Davis, Calif., led to rational design equations based on slope length and application rate (16, 17). These equations, based upon fixed-film biological kinetics, predicted removal of organic matter over shorter distances,
TABLE 1
Municipal land treatment systems 1972 and 1981 Type of system
Slow rate Rapid infiltration Overland flow Total
1972
1981
315 256 0 571
839 323 18 1180
Sources: References 2 and 3
TABLE 2
U.S. overland flow municipal wastewater treatment systems Pilot-scale or research systems 1972-81
Full-scale systems 1981-83
Alum Creek Lake, Ohio Davis, Calif. Beltsville, Md. Easley, S.C. Carbondale, III. Hanover, N.H. Cleveland, Miss. Harriman, N.Y Corsicana, Tex. Pauls Valley, Davis, Calif. Okla. Falkner, Miss. Utica. Miss. Fillmore, III. Gretna, Va. Heavener, Okla. Lamar, Ark. Minden-Gardnerville, Nev. Mt. Olive, N.J. Newman, Calif. Norwalk, Iowa Vinton, La. Ada, Okla.
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at higher application rates than are normally thought possible, and indi cated the degree of conservatism in t h e c u r r e n t empirical design a p proach. The largest overland flow system treating municipal wastewater is the one at Davis, Calif., designed for 20 000 m 3 / d (5 mgd) (18). Oxidation pond effluent is applied at the top of the slope through gated aluminum pipe. The gated pipe allows surface distribution, which saves energy com pared with over sprinkler application. Problems related to the physical de sign of the Davis system have been detailed recently. Land treatment performance In the early 1970s, very little was known about the removal of waste water constituents by land treatment. As a result of research, pilot systems, and investigations into the perfor mance of long-term operations, there
is now a better understanding of the functioning of land treatment pro cesses. BOD and suspended solids. Land treatment processes are very efficient in removing biodegradable organics and suspended solids. Typical remov als and concentrations at selected sites are presented in Table 3. Remov al mechanisms include filtration, ad sorption, and biological oxidation. The removals termed "typical" in Ta ble 3 for slow rate and rapid infiltra tion are based on applying primary effluent. Existing systems at Dickin son, N.D., Muskegon, Mich., Lake George, N.Y., and Phoenix, Ariz., use secondary effluent. At Hanover, N.H., the slow rate systems have re ceived primary effluent and have achieved results comparable to sys tems in which secondary effluent has been applied. For overland flow, screened raw wastewater is typically applied (see Table 3).
Nitrogen. Both ammonia and total nitrogen removal occur in all land treatment processes. In slow rate, the principal mechanism is uptake by crops; denitrification is generally of secondary importance. In rapid infil tration and overland flow, denitrifica tion is the principal nitrogen removal mechanism. Removals of ammonia and total nitrogen are presented in Table 3. At Hanover, N.H., the slow rate test cell was loaded with primary effluent on a year-round basis (20). Higher nitrogen removal could be ex pected for systems that store waste water during the nongrowing season. Nitrogen removals for the four rap id infiltration systems illustrate the range typically found with existing systems. The removals where primary effluent is applied (Ft. Devens, Mass., and Hollister, Calif.) are higher than the 50% normally expected. At Hol lister, the 93% total nitrogen removal is attributable, in part, to the avail-
TABLE 3
Removals of BOD, suspended solids, nitrogen and phosphorus by land treatment
Process/location
BOO, mg/L Treated Applied water
Suspended solids, mg/L Applied
Treated water
Total N, mg/L Applied
Treated water
Ammonia-N, mg/L Applied
Total phosphorus, mg/L
Treated water
Applied
Treated water
Percent phosphorus removal
Slow rate Typical (primary eff.)
150
