Biological Sewage Treatment

a method of purifying domestic and industrial sewage, consisting of biochemical decomposition (mineralization) by microorganisms of organic substances (impurities of organic origin) dissolved and emulsified in sewage. Microorganisms (bacteria) use these substances as sources of food and energy for their life processes. In the process of the microorganisms’ respiration, organic substances are oxidized and energy necessary for their life functions is released. Part of this energy is used for the processes of synthesizing cellular substances, that is, for increasing the mass of bacteria, the quantity of active sludge, and the thickness of the biologic film in the purifying structures. Bacteria that participate in the mineralization of organic compounds in sewage may be divided into two groups, according to their relationship to oxygen: aerobes (which use oxygen dissolved in water for respiration) and anaerobes (which develop in the absence of free oxygen). Besides dissolved organic substances, sewage contains suspended substances, tars, and oils, which must be removed before biologic treatment. For this purpose, gratings, sand traps, and settling tanks are used. Aerobic biological purification is accomplished under conditions close to natural ones, in irrigation fields and filtration fields and in biologic ponds; and, when the life processes of the microorganisms become intensified, in an artificially created environment, in aerotanks, aerofilters, and biofilters. In the anaerobic method of purification, methane tanks are used. Choice of the type of structure is determined by the character and quantity of the sewage, local conditions, requirements as to quality of the purified sewage, and so on. As a result of complete purification, biochemically oxidized organic substances are virtually absent from the water. The water loses its ability to putrefy and becomes transparent, and bacterial contamination is significantly decreased. Biologic treatment is only used for sewage containing a sufficient quantity of biogenic elements (nitrogen, phosphorus, potassium) necessary to the life processes of microorganisms. In a number of cases, biogenic elements are added to sewage before biologic treatment, in the form of salt solutions.