The basic method of sewage treatment is to separate, remove, recycle or convert pollutants in sewage into harmless substances through various means and technologies, so as to purify sewage. Generally divided into water supply treatment and sewage treatment.

Self-purification means that the concentration of pollutants in polluted rivers is reduced or transformed through physical, chemical and biological effects, and the water body is restored to its original state, or reduced from exceeding the water quality standard at first to equal the water quality standard.