FOR MORE THAN 2000 YEARS, a large variety of waste residuals (e.g., manure, sewage sludge, industrial residuals) in various forms have been land-applied as soil amendments to supplement and improve the soil (Moss et al., 2002). Once sewage nutrients were shown to beneficially affect crop growth, sewage farming was promoted as being profitable as well as a technology that helped alleviate the effects of gross waterborne pollution of surface waters. Pollutant and soil interactions were considered as purifying treatment processes, but with finite limitations that could be overloaded (with too much waste) and result in system failure (Jewell and Seabrook, 1979; Seabrook, 1975).
Many of the basic wastewater treatment processes in use today (e.g., chemical precipitation, activated carbon adsorption, trickling filters, biological contact beds, and intermittent filtration) were developed between 1840 and 1890 and land treatment was one of many available treatment alternatives (Fig. 1) . Along with greater acceptance and understanding of the germ theory, knowledge of disease-carrying agents provided the insight necessary to judge the public health hazards of effluents. Water supply treatment by filtration became widely adopted. The introduction of chlorination in 1910 eliminated major epidemics of typhoid and cholera. By the late 1890s, discharge of partially treated wastewater effluents was considered to be safe and cost effective. Many land-farming systems installed in the mid-1800s were used for 30 to 50 yr without size adjustment for growing populations, and resulted in unsightly overloaded conditions (Jewell and Seabrook, 1979).
Land treatment was considered to be the most effective alternative in the United States from 1980 to 1905, and was used by many communities with sewage treatment. Most of the 143 sewage treatment facilities in the United States and Canada as of 1899 were land treatment systems (Rafter, 1899
). In some cases (e.g., Calumet, MI; Woodland, Fresno, and Bakersfield, CA; Lubbock, TX; Vineland, NJ), municipal wastewater land treatment systems started in the late 1880s to early 1900s have been modified over time to accommodate changing conditions, and continue to operate successfully today as effective treatment systems (Crites et al., 2000
). However, from the beginning, many American engineers considered sewage farming, intermittent filtration, and other means of land application of wastes to be "disposal" systems (Jewell and Seabrook, 1979
). Various forms of land application have been used by industries to treat and dispose of industrial wastes, especially by food processors. Such projects often attempted to maximize the amount of waste applied per unit land area rather than to optimize waste use as a source of water for irrigation and/or nutrients. Similar practices were undertaken by some cities as a means of disposal of municipal effluents and sewage sludge. Conventional irrigation procedures and historically accepted practices for recycling animal manure back to the soil to fertilize food, fiber, and feed crops were frequently not followed. As a result, problems often developed such as elevated nitrates in the underlying shallow ground water, severe erosion and runoff from application sites into nearby water bodies, and/or poor cover crop performance. Odors and other undesirable site conditions developed from excess moisture, organic matter, and nutrient loadings. Similar problems have resulted from excessive manure applications to farmland in some areas, where the number and size of confined animal production facilities have dramatically increased. Reduced cropping acreage also limits the land base available for effectively recycling manure by land application.
Land treatment technologies have been used effectively for the treatment of many types of industrial wastewaters and residuals for many years. Residuals include a wide range of food processing wastes (e.g., brewery; canning and frozen foods, including vegetables, fruits, citrus, pineapple, coffee, and tea; dairy products, including milk and cheese; meat processing; winery and other wastes), as well as industrial residuals such as pulp and paper, tanning, pharmaceuticals, biological chemicals, and explosives (Crites et al., 2000; Reed and Crites, 1984; Overcash and Pal, 1979). In some cases (often based on trial and error experience), industrial land application projects that began as land "disposal" systems have evolved into land treatment projects. The latter limit residual application rates to avoid excessive effluent irrigation loading rates. Successful projects include Campbell Soup Co. projects in Paris, TX, and Napoleon, OH (Bendixen et al., 1969; Gilde et al., 1971; Law et al., 1970), Seabrook Farms vegetable canning and frozen foods processing operation in southern New Jersey (Pound and Crites, 1973), and the J.R. Simplot Co. potato processing operations in Idaho (Bruner et al., 1999; Crites et al., 2000).
Years of extensive research and demonstration efforts, as well as experience with both pilot- and field-scale projects, have provided the information needed to design and operate land application projects that can effectively treat and recycle wastewater effluents and organic residuals. Such systems use the soil as an integral part of the treatment system in a sustainable manner. Similarly, projects on agricultural lands, forests, and reclamation sites have focused on recycling treated sewage sludge ("biosolids"), industrial residuals, and manure. The residuals are used as organic soil conditioners and sources of macro- and micronutrients to enhance soil conditions and help establish sustainable vegetative cover and maximize crop yields (Jacobs et al., 1993). Today, application practices can be used in a sustainable manner to minimize negative effects on the environment and to restore disturbed areas with poor soil conditions (resulting from, for example, construction activities, surface mining, forest fires and clear cuts, and overgrazing) and highly contaminated sites (resulting from, for example, mining, smelting, and other industrial activity). Well-documented examples of long-term projects involving such land application practices exist in many parts of the country, for example, projects at Pennsylvania State University in University Park, PA (Kardos, 1974; Pennsylvania State University, 2001); Clayton and Dalton, GA (Reed and Bastian, 1991; Clayton County Water Authority, 2004; Dalton Utilities, 2004); Muskegon, MI (USEPA, 1976, 1980; Demirjian et al., 1980; Muskegon County, 2004); Lubbock, TX (Camann et al., 1985; Hinesly et al., 1978); Davis, CA (Smith and Schroeder, 1983; Kruzic and Schroeder, 1990); Madison, WI (USEPA, 1995a; Jacobs et al., 1993); Fort Collins, CO (Gallier et al., 1993); and Seattle, WA (USEPA, 1995a; Henry et al., 2000). These sustainable land application systems depend heavily on the soil as an integral part of the treatment and/or recycling system to effectively process and manage macro- and micronutrients, inorganic and organic contaminants, and pathogens.
The extensive operating experience with long-term pilot- and field-scale projects has often been complimented by extensive studies conducted by scientists in numerous disciplines, and an extensive body of literature exists. Efforts to compile and summarize the available information have led to identification of research needs in numerous formats. Conference proceedings (e.g., National Association of State Universities and Land-Grant Colleges, 1973; Loehr, 1977a, 1977b; Sopper and Kardos, 1973; Sopper and Kerr, 1979; Sopper et al., 1982; Page et al., 1983; Cole et al., 1986; Clapp et al., 1994; Henry et al., 2000; Rocky Mountain Water Environment Association, 2000) along with other sources (e.g., Overcash and Pal, 1979; Reed and Crites, 1984; Runge, 1986; Page et al., 1987; Sopper, 1993; National Research Council, 1996; Crites et al., 2000; Sharpley et al., 2003) have provided important information used in the development of USEPA technical guidance materials. Examples of these materials include the USEPA's process design manuals on land treatment of municipal wastewater, land application of sewage sludge, and guidelines for water reuse (USEPA, 1977, 1981, 1983, 1992, 1995a, 2004a); the USDA's Agricultural Waste Management Handbook and Comprehensive Nutrient Management Planning Technical Guidance (USDA Natural Resources Conservation Service, 1992, 2003); and the technical basis behind applicable regulations, such as 40 CFR Part 503 (USEPA, 1993) and the concentrated animal feeding operation (CAFO) rule (USEPA, 2003).