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of wastewater. Because of the high degree of variability, most municipal treatment systems are not designed to treat a vast array of industrial wastes. Consequently, these wastes can interfere with the operation of treatment plants, contaminate receiving wa-terbodies, threaten worker health and safety, and increase the cost and risks of sludge treatment and disposal. Using proven pollution control technologies and practices that promote the reuse and recycling of material, however, industrial facilities can provide "pretreatment" by removing pollutants from their wastewater before discharging to the municipal wastewater system. In addition to the categorical standards for pretreatment established as part of the industrial effluent guideline process, local pretreatment limits are enforced by various municipal facilities to protect treatment processes, worker health and safety, and equipment. USEPA's National Pretreatment Program, a cooperative effort of federal, state, and local officials, is fostering this practice nationwide.

Combined Sewer Overflows In many older cities of the United States, urban sewer systems were originally designed to convey both raw sewage and stormwater runoff collected during rainstorms. These combined sewer overflow systems were also explicitly designed to discharge (overflow) the mixture of raw sewage and stormwater into the river if a heavy rainstorm exceeded the hydraulic capacity of the combined sewer system. As a vestige of public works practices from approximately 1850 to 1900, about 880 cities, mostly in the central and northeastern states, have combined sewer systems that continue to function in this manner (USEPA, 1997). Table 2-15 presents characteristic discharge concentrations of conventional and non-conventional pollutants in combined sewer overflows (CSOs).

In addition to raw sewage, a CSO system can discharge pretreated industrial waste and street debris washed off during a storm. Although pollutant loading from CSO systems is intermittent, occurring only under heavy rainstorm conditions, the high loading rates of sewage from CSO outlets frequently result in the closure of recreational beaches and shellfish beds to protect public health. Discharges from CSOs also are associated with depressed oxygen levels in poorly flushed waterbodies, accumulation of organics in sediments, and generally noxious conditions and odors.

National assessments show that the relative significance of annual loading of BOD5 from CSO systems is about the same as the effluent loading from secondary wastewater treatment facilities in the same urban area. In contrast to BOD5, annual loading of suspended solids and lead is about 15 times greater from CSO systems than from secondary wastewater treatment facilities. Annual loading rates of total nitrogen and phosphorus from CSOs, however, are only about one-fourth (total N) and one-seventh (total P) of the annual loads contributed by secondary facilities (Novotny and Olem, 1994).

Urban and Rural Nonpoint Sources Organic and inorganic materials, both naturally occurring and related to human activities, are transported to waterbodies within a drainage basin by surface runoff over the land as nonpoint, or diffuse, sources of pollutants. The magnitude and the timing of nonpoint pollutant loads are

TABLE 2-15 Effluent Characteristics of Urban Runoff and CSOs

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