The educational publication, written by scientists from Denmark, summarizes modern biochemical ideas about wastewater treatment processes (removal of organic pollutants, nitrogen and phosphorus compounds, etc.). Theoretical bases of processes and possible engineering and technological solutions are considered in detail. Significant attention is paid to mathematical modeling as a tool in the design of belt treatment facilities. For undergraduate and graduate students of engineering, construction, chemical, biotechnological and environmentally oriented universities, as well as specialists in the design and operation of treatment facilities.