According to experts, sewage treatment is divided into three levels, and the water quality effect achieved after different levels of treatment is different, and a reasonable treatment plan needs to be designed according to the needs of users.
Sewage primary treatment: sewage primary treatment, also known as sewage physical treatment, is a process that removes suspended solids in sewage, adjusts pH value and reduces the degree of decomposition of sewage through simple sedimentation, filtration or appropriate aeration. The treatment can be composed of screening, gravity sedimentation and flotation methods in tandem to remove most of the particulate matter in the wastewater with a particle size of more than 100 microns.
Sieve filtration can remove larger substances; Gravity precipitation can remove inorganic particles and cohesive organic particles with a relative density greater than 1; Flotation can remove particulate matter (oil, etc.) with a relative density of less than 1. Wastewater generally does not meet the discharge standard after primary treatment.
Secondary treatment of sewage: After primary treatment, the sewage is further purified by the treatment of aeration tank and sedimentation tank with activated sludge. Biological methods and flocculation methods are commonly used. The biological method is to use microorganisms to treat sewage, mainly to remove organic matter in the sewage after primary treatment; The flocculation method is to destroy the stability of the colloidal by adding flocculant, so that the colloidal particles will coagulate, produce flocculates and adsorption, mainly to remove inorganic suspended solids and colloidal particles or low-concentration organic matter in the sewage after primary treatment.
After secondary treatment, the sewage can generally meet the requirements of agricultural irrigation and wastewater discharge standards. However, under certain conditions, it may still cause pollution of natural water bodies.
Tertiary treatment of sewage: tertiary treatment of sewage, also known as advanced treatment: after secondary treatment, the sewage is further removed from other polluting components in the sewage (such as; Nitrogen, phosphorus, fine suspended solids, trace organic compounds and inorganic salts, etc.).
The main methods include biological denitrification method, agglutination and precipitation method, sand filtration method, diatomaceous earth filtration method, activated carbon filtration method, evaporation method, freezing method, reverse osmosis method, ion exchange method and electrodialysis method.
