Surface waters treated sewage discharges

Impact of discharges of treated sewage on surface water quality... 

The standards applied to lead in industrial effluent depend upon whether the effluent is discharged directly to a surface water or indirectly via a sewage treatment works (Table 6.2). The use of municipal treatment plants to treat industrial effluent is favoured in Britain and elsewhere. Treatment of the mixed and diluted wastes that result can prove highly effective (Section 6.4.2). Furthermore, it is administratively easier to maintain control of polluting discharges if they are routed via one control plant. 

Human acfivifies can release chemical substances at several points in the water cycle. Leachate from an unlined landfill or sewage treat in a septic tank may infiltrate groundwater. Treated wastewater commonly discharges to surface water. And air pollutants enter the water cycle when they dissolve in precipitation and return to earth. 

Sources of pollutants are commonly divided into two categories point sources and nonpoint sources. Point sources of pollution refer to discrete, localized, and often readily measurable discharges of chemicals. Examples of point sources are industrial outfall pipes, treated sewage outfalls from wastewater treatment plants, and untreated storm water discharge pipes. A spill of chemicals, due fo an accident on or near a surface water body, can also be regarded as a point source because its initial areal extent is limited. Point sources are often modeled by "plume" or "cloud" models that explicitly consider where and when a chemical release occurs, and then mathematically model the concentration of the chemical as a function of time and location. 

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