According to the degree of treatment, wastewater treatment (mainly urban domestic sewage and some industrial wastewater) can generally be divided into three levels:

Primary treatment:

The task is to remove suspended solid contaminants from wastewater. For this reason, more physical treatment methods are used. Generally, after primary treatment, the removal rate of suspended solids is 70% to 80%, while the removal rate of biochemical oxygen demand (BOD) is only about 25% to 40%, and the purification degree of wastewater is not high.

Secondary treatment:

The task is to substantially remove organic pollutants in the wastewater, taking BOD as an example, generally through secondary treatment, the BOD in the wastewater can be removed by 80% to 90%, such as the BOD content in the water after urban sewage treatment can be less than 30 mg/l. Most of the various processing units of aerobic biological treatment can meet this requirement.

Tertiary treatment:

The task is to further remove contaminants that secondary treatment fails to remove, including organic matter, phosphorus, nitrogen and soluble inorganic matter that microorganisms fail to degrade.

Tertiary processing is synonymous with advanced processing, but the two are not exactly the same. Tertiary treatment is the addition of one or more treatment units after secondary treatment in order to remove a specific pollutant from the wastewater, such as phosphorus, nitrogen, etc. Advanced treatment is often a treatment unit or system added after secondary treatment for the purpose of recycling and reuse of wastewater. Tertiary treatment is more expensive and complex to manage, but it can make full use of water resources. A few countries have established some tertiary sewage treatment plants.