septic tank or putrefaction tank is a kind of structure to treat sewage, especially the sewage with feces. it can settle the suspended solids heavier than water in the sewage and digest the sludge at the same time. when the sludge is digested at the bottom of the tank, gas can be generated to make part of the sludge float to the surface of the tank, thus preventing oxygen from dissolving in the pool water. therefore, sludge digestion in a septic tank belongs to anaerobic decomposition.
the time of sewage flowing through the septic tank is generally 12-24h, and the flow velocity is below 5-8mm / s. the sludge accumulates at the bottom of the tank for 3-6 months or more. during the residence period, the organic matter decreases continuously and the decomposition products can reach full stability. however, hydrogen sulfide and other gases will be produced in the process of sludge decomposition, and the rising gas can bring the sludge at the bottom of the tank to float up, which will affect the quality of effluent. generally, an underflow baffle is set at the outlet. the effluent is acidic and the content of dissolved oxygen is low. therefore, septic tank is often used to treat sewage
① before disinfection of hospital sewage, septic tank was used for pretreatment;
② the sewage discharged from buildings can be treated with septic tank before it enters the street sewer;
③ in areas with low groundwater level, the sewage discharged from single houses or small houses can be treated by septic tanks and then discharged to the underground irrigation system for further biological oxidation treatment. the work of pumping septic tank is essential and should be cleaned regularly.