According to the degree of treatment, wastewater treatment (mainly urban domestic sewage and some industrial wastewater) can generally be divided into three levels.
the task of primary treatment is to remove suspended solid pollutants from wastewater. Therefore, physical treatment method is often used. Generally, after primary treatment, the removal rate of suspended solids is 70% ~ 80%, while the removal rate of biochemical oxygen demand (BOD) is only about 25% ~ 40%, and the purification degree of wastewater is not high.
the task of secondary treatment is to greatly remove organic pollutants in wastewater. Taking BOD as an example, after secondary treatment, BOD in wastewater can be
removed by 80% ~ 90%. For example, the BOD content in water after urban sewage treatment can be less than 30 mg / L. Most of the treatment units of aerobic biological treatment can meet this requirement.
the task of tertiary treatment is to further remove the pollutants that cannot be removed by secondary treatment, including organic matter, phosphorus, nitrogen and soluble inorganic matter that cannot be degraded by microorganisms.
tertiary processing is synonymous with * * processing, but the two are not completely consistent. Tertiary treatment refers to one or more additional treatment units added after secondary treatment in order to remove certain pollutants from wastewater, such as phosphorus and nitrogen** Treatment is often a treatment unit or system added after secondary treatment for the purpose of wastewater recovery and reuse. The tertiary treatment costs a lot and the management is complex, but it can make full use of water resources. A few countries have built some tertiary sewage treatment plants.