Operating an activated sludge system requires wasting to maintain a healthy biomass (MLSS). While conventional activated sludge and contact stabilization units have small aeration basins with high cell yield per unit of water treated, they normally have sizable aerobic digesters to continue organic decomposition and take the biomass further down the growth curve where cells begin to lyse (decline or endogenous phase growth). Many industrial wastewater systems and smaller municipal systems now use extended aeration where part or nearly all of the aerobic digester function is moved into a larger main aeration basin. This means lower F/M or loading rates per unit of MLSS.
With extended aeration, cell yield is lower and cells actually lyse and enter endogenous respiration in the aeration basin. Often in an attempt to lower solids disposal costs, operators reduce wasting below design rates. The idea is the bacteria will continue to thrive by lysing more cells, leaving a higher non-volatile percentage in the biomass. While this can save money in the very short run, here is why not wasting ends up costing money:
Erik Rumbaugh has been involved in biological waste treatment for over 20 years. He has worked with industrial and municipal wastewater facilities to ensure optimal performance of their treatment systems. He is a founder of Aster Bio (www.asterbio.com) specializing in biological waste treatment.
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