What are the conventional treatment processes of urban sewage?

1. Primary processing

At the entrance of the sewage treatment facility, a grille must be installed, mainly to intercept larger floating or suspended matter by physical treatment, so as to reduce the load of the subsequent treatment structures and enable them to operate normally.

The grit chamber is generally located behind the grille, but can also be located in front of the primary sedimentation tank, in order to remove inorganic particles with a larger specific gravity. The primary sedimentation tank has a good removal effect on inorganic substances, and is generally located in front of the biological treatment structure.

After primary treatment, the BOD of sewage can generally be removed by about 30%, which cannot meet the discharge standard and can only be used as pretreatment for secondary treatment.

2. Secondary processing

It mainly removes organic pollutants in colloidal and soluble state in sewage, usually by biological treatment. The biological treatment structure is the most important part of the treatment process. It uses the metabolism of microorganisms to convert the organic pollutants in the soluble and colloidal state in the sewage into harmless substances, so as to meet the discharge requirements.

Generally, the removal rate can reach more than 90%, the organic pollutants can meet the emission standard, and the five-day biochemical oxygen demand (BOD5) after treatment can be reduced to 20-30 mgL.

The main function of the secondary sedimentation tank is to remove the biological shedding or dead organisms in the form of sludge produced in the biological treatment process.

3. Tertiary treatment

After primary and secondary treatment, it is used to further treat refractory organic matter, soluble inorganic matter such as phosphorus and nitrogen that can lead to eutrophication of water bodies.

The main treatment methods are biological denitrification and phosphorus removal, coagulation sedimentation, sand filtration, activated carbon adsorption, ion exchange and electrodialysis. Through tertiary treatment, BOD5 can be further reduced to below 5mgL.