The objectives of biological wastewater treatment are to coagulate and remove nonsettlable colloidal solids and to stabilize organic matter. For example, in municipal waste-water treatment, the objective is usually to reduce organic content and, if necessary, to remove nutrients such as nitrogen and phosphorus. In some cases, trace concentrations of toxic organic compounds also require removal.
In industrial wastewater treatment, reduction or removal of organic and inorganic compound concentrations is essential. Microorganisms (see Figure 7.22.1) play a ma jor role in decomposing waste organic matter, removing carbonaceous BOD, coagulating nonsettlable colloidal solids, and stabilizing organic matter. These microorganisms convert colloidal and dissolved carbonaceous organic matter into various gases and cell tissue. The cell tissue, having a specific gravity greater than water, can then be removed from treated water through gravity settling. Thus, wastewater treatment facilities use these microorganisms in biological wastewater treatment processes to dispose of wastes in a nontoxic and sanitary manner.
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