Primary treatment
The main removal of solid pollutants in the sewage in suspension, most of the physical treatment method can only complete the requirements of primary treatment. After primary treatment of sewage, BOD can generally be removed about 30%, can not meet the discharge standards. Primary treatment belongs to the pretreatment of secondary treatment.
Secondary treatment
The main removal of colloidal and dissolved organic pollutants in the sewage (BOD, COD substances), the removal rate of up to 90% or more, so that organic pollutants to meet the discharge standards.
Tertiary treatment
Further treatment of difficult to degrade organic matter, nitrogen and phosphorus can lead to eutrophication of the water body soluble inorganic substances. The main methods include biological nitrogen and phosphorus removal, coagulation and precipitation, sand filtration, activated carbon adsorption, ion exchange and electrodialysis.
From the source of pollution discharged from the sewage (wastewater) water, due to the total amount of pollutants or high concentration, can not meet the requirements of the discharge standards or do not adapt to the environmental capacity requirements, thereby reducing the quality of the water environment and the functional objectives of the site must be artificially strengthened treatment