Municipal wastewater treatment plants currently discharge more than 110 million cubic meters (30 billion gallons) of sewage per day into U.S. water bodies (van der Leeden et al., 1990). Manufacturing industries discharge an additional 91 million cubic meters (24 billion gallons) per day (van der Leeden et al., 1990). More difficult to manage than these point sources of pollution, however, are diffuse sources of pollution such as agricultural and urban runoff. For example, state water resource managers identified agricultural runoff as a cause of impairment of 49 percent of the damaged lakes, rivers, and streams they assessed; they identified urban runoff as a factor in the decline of 24 percent of damaged lakes and 10 percent of damaged rivers and streams (Environmental Protection Agency, 1994).
Three of the major impacts of excessive waste loadings to surface waters are loss of dissolved oxygen, cultural eutrophication, and buildup of toxic compounds.
Excess discharges of organic wastes cause the depletion of the receiving water's oxygen supply. Low oxygen levels threaten the survival of desirable sport fish species such as trout, salmon, and bass, which may be replaced by populations of catfish and carp. If the oxygen level drops low enough, even catfish and carp cannot survive. In the worst cases, when the oxygen concentration reaches zero for extended periods, no higher organisms can survive, and the only life remaining in the water body consists of anaerobic bacteria that produce gases such as CH4 and hydrogen sulfide.
Much of the earliest aquatic science research focused on understanding the effects of sewage and other organic waste discharges on dissolved oxygen concentrations. As a result of these early efforts, the effects of adding excess organic matter to a water body are well understood. For example, in 1884, Dupré recognized that oxygen depletion in a stored bottle of water occurred because of the activity of microscopic organisms, which he called "microphytes" (Phelps, 1944). By the early decades of this century, scientists had developed standardized procedures to determine the amount of oxygen that organisms will use when degrading a given waste; this amount is known as "biochemical oxygen demand" (BOD) (Streeter and Phelps, 1925). H. W. Streeter, a Public Health Service researcher, and E. B. Phelps, a professor of stream sanitation at Columbia University, developed an equation that predicts how much the oxygen concentration of a river will decline at given points downstream of a waste discharge (Streeter and Phelps, 1925). This equation is historically important as the first mathematical model used to predict water quality. It predicts oxygen levels based on rates of two processes: microbial oxygen