This is the most common and oldest biotreatment process used to treat municipal and industrial wastewater. Typically wastewater after primary treatment is suspended impurities removal is treated in an activated sludge process based biological treatment system comprising aeration tank followed by secondary clarifier. The aeration tank is a completety mixed or a plug flow (in some cases) bioreator where specific concentration of biomass (measured as mixed liquor suspended solid (MLSS) or mixed liquor volatie suspended solid (MLVSS) is maintained along with sufficient 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 ta transfer required oxygen to the biomass and also ensure completely mixed reator. Roots type air blower is used to supply air to the diffused pipework. In several alder installations, mechanical surface aerators have been used to meet the aeration requirement. The aerated mixed liquor from the aeration tank verfltws 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. Excess biomass (producd during the biodegration process) is wasted to the sluge handling and dewatering facility.
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