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Why is some activated sludge returned to the aeration tank?
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wastewater treatment
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In the activated sludge process, a portion of the sludge—typically about 30 percent—is recirculated from the secondary clarifier back into the aeration tank to maintain a high concentration of active microorganisms.1
This recirculation is essential for several reasons:
- Acclimation: The recycled microbes are already well acclimated to the sewage environment, allowing them to immediately and effectively metabolize the organic materials present in the incoming primary effluent.1
- Biological Efficiency: By "seeding" the incoming wastewater with these established biological solids, the system ensures that the microorganisms can rapidly absorb dissolved organics and reduce the biochemical oxygen demand (BOD) of the sewage.12
Without this continuous return of active biomass, the aeration tank would lack the necessary microbial population density required to treat the wastewater efficiently within the standard detention time.1