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secondary treatment

*Numbers represent millions of people tna = not available.

secondary treatment

*Numbers represent millions of people tna = not available.

FIG. 7.2.2 Population served by municipal wastewater treatment systems. (Reprinted from Council on Environmental Quality 1992, Environmental trends, Washington, D.C.).

of specific pollutants. Figure 7.2.6 shows various transfer routes and processes related to pollutant movement within the hydrosphere and biosphere. Examples of specific bio-geochemical cycles for one nutrient (nitrogen) and one metal (mercury) are shown in Figures 7.2.7 and 7.2.8, respectively. The information in these examples indicates that the biological environment is a dynamic system that can be stressed as a result of various wastewater discharges.

Environmental engineers can use chromium to illustrate changes within aqueous systems since it is a transition metal that exhibits various oxidation states and behavior patterns (Canter and Gloyna 1968). Trivalent chromium is generally present as a cation, Cr(OH)+, and is chemically reactive, tending to sorb on suspended materials and subsequently settle from the liquid phase. Hexavalent chromium is anionic (CrO^) and chemically unreactive,

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