Sewage Treatment Plant

a set of engineering structures in the sewerage system of an inhabited location or industrial enterprise, designed to remove impurities from effluents. Treatment is designed to prepare effluents for reuse by industry or discharge into basins. As a rule, industrial effluents are initially treated in local facilities to lower the concentration of impurities, extract and utilize useful materials, and prepare effluents for any major treatment that may be necessary. Once treated locally or processed in major plants, effluents may be reused in industrial processes. In some cases, treated effluents are discharged into basins or (after partial treatment) into city sewage systems. Depending on the objectionable constituents in effluents and the required degree of treatment, plants may include facilities for mechanical, biological, and physicochemical treatment, as well as for posttreatment.

Mechanical treatment facilities remove up to 75 percent of insoluble impurities from effluents, including fine mineral impurities, petroleum products, grease, and other materials. Floating materials are trapped by grids or screens, removed from the water, reduced in size by hammer crushers, and either returned to the effluent stream or treated together with the sediment. Sand and other fine mineral impurities are retained as effluents pass through sand traps. Sand that has settled is transported by water-jet pumps to sand platforms or bunkers for later use as landfill. Insoluble suspended materials are retained mainly in settling basins and septic tanks. Oil traps, grease traps, and flotation cleaners remove petroleum products, grease, and other materials with density close to that of water.

Organic impurities in the form of colloids and dissolved materials are removed with 90 to 95 percent efficiency in biological treatment facilities.

Chemical treatment methods are based on the use of solutions of certain reagents that cause flakes to form in effluents, thus promoting the precipitation of suspended materials. Physico-chemical treatment facilities consist of units that prepare and add reagents (if used), devices for mixing effluents with the reagents, reaction chambers for primary flocculation, and settling basins, in which suspended materials and some colloids precipitate. In addition to treatment by reagents, physicochemical methods include electrochemical techniques, hyperfiltration, and oxidation. If necessary, effluents are posttreated after biological treatment; filtration and treatment with reagents are among the basic methods used to remove the remaining organic suspended materials, phosphorus, and nitrogen.

The last stage of treatment involves disinfecting effluents with chlorine to remove the bacterial impurities remaining after biological, chemical, or supplementary treatment. Chlorinators and contact beds (primary settling basins) are used for disinfection.

Treatment of effluents in settling basins results in the accumulation of sediment, which is difficult to dry, has an unpleasant odor, and is hazardous to health. Fermented, or rotted, sediment does not have these undesirable properties; various means are therefore used to treat the sediment and render it harmless. Facilities include septic tanks, double-deck settling tanks, digestion tanks, sludge basins, centrifuges, and filter presses.