Biological treatment is principally used to process organic contaminants in wastewaters that may be subject to primary treatment prior to this step. The BOD5 level of primary sewage effluent is usually around 100 ppm. Although municipal treatment works are typically designed to process sewage, they may also process industrial wastewaters containing organic wastes such as food processing and paper products waste. Sewage plus industrial wastewater is referred to as combined wastewater. The level of contamination in industrial wastewaters discharged to municipal treatment works would be controlled through local government (or authority) ordinances based on pre-treatment standards established by federal (EPA), and state (DEQ) agencies. The effluent from biological treatment should contain less than 30 ppm of BOD5 and less than 30 ppm of suspended solids while maintaining a ph between 6.5 and 8.5. Nearly complete oxidation of the influent ammonia content (nitrification) can also be accomplished by biological treatment. A well nitrified effluent would contain less than 1 ppm of ammonia.
Biological treatment involves the use of a reactor, to establish a concentrated population of microorganisms (Biomass), followed by clarification operations designed to retain the biomass within the treatment process. Clarification utilizes natural flocculation and gravity to separate the suspended solids (TSS) from the liquid flow slowly passing through settling basins over a period of several hours. The reactor biomass reduces the strength of the clarified effluent by greatly concentrating the natural process of biological water purification within the reactor. A portion of the separated biomass is returned to the reactor to maintain a design operating level. A variety of biological reactors may be utilized by a treatment works to achieve permit requirements. The selection of a particular type of reactor is typically based on economic considerations. A key factor in this determination involves the means of supplying the level of dissolved oxygen necessary to provide aerobic conditions for optimum biological activity. Compressed air may be diffused into the wastewater contained in a reactor basin, or it may be otherwise mechanically induced through mixing and agitation to expose the wastewater to turbulent contact with the ambient air. The energy and maintenance requirements to provide aeration are major cost items associated with biological treatment. Additional, or post, aeration may be required to meet permit requirements for effluent dissolved oxygen levels of 5 ppm or more. Combinations of reactors or compartmentalized reactors may be used as selector systems to provide a competitive growth advantage to a desired group of microorganisms.