Organic compounds in domestic wastewater have a highly differentiated composition, which depends on the source of wastewater. Detailed fractionation of organic pollutants is indispensible to determine their transformation during treatment processes. Mathematical modeling requires the division of organic compounds in wastewater into the fractions: dissolved, suspended, biodegradable, and unbiodegradable. It is possible to determine the content of fractions based on BOD₅, COD, and reaction rate coefficients.
In this work, fractions of organic pollutants in wastewater, originating from small sources and treated in small treatment plants (0.66-22.00 m³/d), were identified. For comparison, fractions of organic pollutants in a medium municipal wastewater treatment plant (WWTP) of 4,000 m³/d capacity have been determined.
It was found that the wastewater from small sources and households differ in composition from typical municipal sewage. They are characterized by higher contents of dissolved and suspended biodegradable fractions. A similar result was found for septic tank effluent. The high content of biodegradable compounds and relatively large reaction rate coefficients positively affect the efficiency of pollutant removal.