MUNICIPAL WASTEWATER

Characteristics

The quantity and quality of wastewater delivered from varies among communities, depending on the number of commercial and industrial establishments in the area, the per capita in-house water use (which may vary from 400 1/day or more in industrialized countries to 40 1/day or less in developing or water-short countries), and the condition of the sewer system. Raw municipal wastewater may include contributions from domestic and industrial sources, infiltration and inflow from the collection system, and, in the case of combined sewer systems, urban stormwater runoff. The typical composition of untreated municipal wastewater appears in Table 2.1.

The occurrence and concentration of pathogenic microorganisms in raw wastewater depend on a number of factors, and it is not possible to predict with any degree of assurance what the general characteristics of a particular wastewater will be with respect to infectious agents. Important variables include the sources contributing to the wastewater, the original purpose of the water use, the general health of the contributing population, the existence of "disease carriers" in the population, and the ability of infectious agents to survive outside their hosts under a variety of environmental conditions. Table 2.2 lists infectious agents potentially present in untreated municipal wastewater. Table 2.3 illustrates the variety and order of magnitude of the concentration of microorganisms in untreated municipal wastewater.

Viruses are not normally excreted for prolonged periods by healthy individuals, and the occurrence of viruses in municipal wastewater fluctuates widely. Viral concentrations are generally highest during the summer and early autumn months. Viruses shed from an infected individual commonly range from 1,000 to 100,000 infective or plaque forming units (pfu's) per gram (g) of feces, but may be as high as 1,000,000 pfu/g of feces (Feachem et al., 1983). Viruses as a group are generally more resistant to environmental stresses than many of the bacteria, although some viruses persist for only a short time in municipal wastewater. Vital levels in the United States have been reported to be as high as 700 pfu/100 ml, but are typically less than 100 pfu/100 ml (American Society of Civil Engineers, 1970; Melnick et al., 1978).

Dissolved inorganic solids (total dissolved solids or salts, TDS) are not altered substantially in most wastewater treatment processes. In some cases, they may increase as a result of evaporation in lagoons or storage reservoirs. Therefore, unless wastewater treatment processes specifically intended to remove mineral constituents are employed, the composition of dissolved minerals in treated wastewater used for ground water recharge can be expected to be similar to the composition in the raw wastewater. The concentration of dissolved minerals in untreated wastewater is determined by the concentration in the domestic water



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