Experts introduce that sewage treatment is divided into three levels, and the water quality effect achieved after different levels of treatment is different. It is necessary to design a reasonable treatment plan according to the needs of users.
Primary treatment of sewage: Primary treatment of sewage, also known as physical treatment of sewage, is a process that involves simple sedimentation, filtration, or appropriate aeration to remove suspended solids, adjust pH value, and reduce the degree of sewage decay. The treatment can be composed of a series of methods such as screening, gravity precipitation, and flotation to remove most of the particulate matter with a particle size of over 100 microns in the wastewater.
Screening can remove larger substances; Gravity precipitation can remove inorganic particles and cohesive organic particles with a relative density greater than 1; Flotation can remove particles (such as oils) with a relative density less than 1. After primary treatment, wastewater generally still does not meet the discharge standards.
Secondary treatment of sewage: After primary treatment, sewage is further purified by aeration tanks and sedimentation tanks with activated sludge. Commonly used biological methods and flocculation methods. Biological method is the use of microorganisms to treat wastewater, mainly removing organic matter from the wastewater after primary treatment; The flocculation method destroys the stability of colloids by adding coagulants, causing colloidal particles to coagulate and produce flocs for adsorption. It mainly removes inorganic suspended solids and colloidal particles or low concentration organic matter in the wastewater after primary treatment.
After secondary treatment, the sewage can generally meet the requirements of agricultural irrigation and wastewater discharge standards. But under certain conditions, it may still cause pollution to natural water bodies.
Third level treatment of sewage: Third level treatment of sewage, also known as deep treatment, is the process of further removing other pollutants (such as nitrogen, phosphorus, fine suspended solids, trace organic matter, and inorganic salts) from sewage after secondary treatment.
The main methods include biological denitrification, coagulation precipitation, sand filtration, diatomaceous earth filtration, activated carbon filtration, evaporation, freezing, reverse osmosis, ion exchange, and electrodialysis.
