Before the 1972 CWA, state officials made waterbody-dependent decisions about the required level of municipal wastewater treatment needed to attain compliance with ambient water quality criteria or standards. After the 1972 CWA, the USEPA implemented a technology-based policy to regulate pollutant loading from municipal and industrial point sources. Under the 1972 CWA, municipalities were required to achieve at least a minimum level of secondary treatment to remove approximately 85 percent of the oxygen-demanding material from wastewater. In cases where the minimum level of secondary treatment was not sufficient to meet water quality criteria or standards, ambient criteria were used to determine a water quality-based level of wastewater treatment greater than secondary treatment. From a policy and planning perspective, the key question for water quality management decision makers is: What level of municipal wastewater treatment is needed to ensure compliance with water quality criteria or standards under critical conditions?
For the Delaware, Potomac, James, and Upper Mississippi case studies, validated water quality models have been used to provide quantitative answers to evaluate the changes in water quality conditions achieved as a result of either actual or hypothetical upgrades to municipal wastewater treatment facilities. Effluent loading rates for the primary and secondary loading scenarios were based on existing population served and effluent flow data with typical effluent concentrations characteristic of primary and secondary treatment facilities; existing loading rates were used to define the better-than-secondary (actual) scenario. Receiving water streamflow was based on the existing "dry" summer streamflow measurements used to validate the models. The water quality models were used to simulate the impact of the primary, secondary, and actual better than secondary loading scenarios on the spatial distributions of DO, BOD5, nitrogen, phosphorus, and algal biomass.
Figure 4-4 summarizes the key results for the model simulations for dissolved oxygen simulated at the worst-case critical oxygen sag location along the length of each river. As shown in these results, the primary effluent scenario results in ex-
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