What is the process and principle of activated sludge process?

The basic principle of activated sludge process is to continuously inject air into domestic sewage to maintain sufficient dissolved oxygen in the water. After a period of time, floc-activated sludge is formed in the sewage, which is composed of a large number of propagated microorganisms and is easy to precipitate and separate, making the sewage clear. Activated sludge method is to take activated sludge suspended in water as the main body, fully contact with sewage under environmental conditions conducive to microbial growth, and purify sewage. Its main structures are aeration tank and secondary sedimentation tank. Sewage to be treated and returned sludge enter the aeration tank together to form a suspended mixed solution, and compressed air is injected along the aeration tank for aeration, so that the sewage and activated sludge are fully mixed and sufficient dissolved oxygen is provided to the mixed solution. At this time, the organic matter in the sewage is decomposed by aerobic microorganisms in the activated sludge, and then the mixed liquid enters the secondary sedimentation tank, and the activated sludge and water are clarified and separated, and part of the activated sludge returns to the aeration tank to continue the purification process, and the clarified water is discharged. Due to the continuous growth of activated sludge in the treatment process, some surplus sludge is discharged from the system to maintain the stability of the system.

Water inflow → aeration tank (air) → secondary sedimentation tank (removing excess sludge before sludge returns to aeration tank) → water outlet.

Mechanism of activated sludge purification process: adsorption stage: After sewage comes into contact with activated sludge, organic matter (BOD) in water decreases rapidly in a short time, which is mainly caused by adsorption. Flocculent activated sludge has a large surface area and a polysaccharide mucus layer on the surface, which is beneficial to adsorption.

Oxidation stage: under aerobic conditions, part of the adsorbed organic matter is oxidized and decomposed by microorganisms to obtain energy, and part of it synthesizes new cells, which is much slower than the adsorption stage.

Flocculation formation and coagulation sedimentation stage: the bacteria synthesized in the oxidation stage form flocs, and the water quality is purified by gravity sedimentation.