Biological wastewater treatmentThis is the most common and oldest biotreatment process used to treat municipal and industrial wastewater. Typically wastewater after primary treatment i.e. suspended impurities removal is treated in an activated sludge process based biological treatment system comprising aeration tank followed by sencondary clarifier. The aeration tank is a completely mixed or a plug flow ( in some cases ) bioreactor where specific concentration of biomass ( measured as mixed liuor suspended solids (MLSS) or mixed luquor volatile suspended solids (MLVSS)) is maintained along with sufficient dissolved oxygen (DO) concentration ( typically 2 mg/l) to affect biodegradation of soluble organic impurities measured as biochemical oxygen demand (BOD5) or chemical oxygen demand (COD).The aeration tank is provided with fine bubble diffused aeration pipework at the bottom to transfer required oxygen to the biomass and also ensure completely mixed reactor. Root type air blower is used to supply air to the diffuser pipework. In several older installations, mechanical surface aerators have been used to meet the aeration requirement. The aerated mixed liquor from the aeration tank verflows by gravity to the secondary clarifier unit to separate out the biomass and allow clarified, treated water to the downstream filtration system for finer removal of suspended solids. The separated biomass is returned to the aeration tank by means of return activated sludge (RAS) pump. Exess biomass ( produced during the biodegradation process) is wasted to the sludge handling and dewatering facility.
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