Wastewater treatment (mainly urban sewage and some industrial wastewater) can generally be divided into three levels based on the degree of treatment required.
Primary treatment aims to remove suspended solids from wastewater. Physical treatment methods are commonly used for this purpose. Typically, 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%, resulting in a low level of wastewater purification.
Secondary treatment aims to significantly remove organic pollutants from wastewater. For example, after secondary treatment, 80%–90% of BOD in wastewater can be removed. For instance, the BOD content in treated urban sewage can be below 30 mg/L. Most aerobic biological treatment units can achieve this requirement.
Tertiary treatment aims to further remove pollutants that secondary treatment failed to remove, including organic matter, phosphorus, nitrogen, and soluble inorganic matter that microorganisms have not been able to degrade. Tertiary treatment is synonymous with advanced treatment, but the two are not entirely identical. Tertiary treatment is an additional treatment unit or systems added after secondary treatment to remove specific pollutants from wastewater, such as phosphorus and nitrogen. Advanced treatment, on the other hand, is often an additional unit or system added after secondary treatment for the purpose of wastewater recycling and reuse. Tertiary treatment is more expensive and more complex to manage, but it makes full use of water resources. A few countries have built some tertiary wastewater treatment plants.
