Anaerobic Digestion of Waste Activated Sludge

The purpose of this laboratory study was to evaluate, and model the effect of solids retention time in the activated sludge process (θ_c^AS) on the performance of anaerobic digesters fed the resulting waste sludges. Activated sludge systems were operated over a range of θ_c^AS values, with the waste sludges used as feed sources for batch and semi-continuous anaerobic digestion studies. Previous models for aerobic biodegradability of activated sludges were found to accurately predict ultimate anaerobic digestibility. However, conventional kinetic models of digestion over-estimated actual performance with this substrate. An activated-sludge anaerobic digestion model is proposed which includes a first-order rate mechanism for preliminary conversion of viable activated sludge biomass into available anaerobic substrate. Decreasing θ_c^AS results in increased sludge solids for digestion. However, the solids are of sufficiently greater digestibility that both the quantity of digested solids for ultimate disposal, as well as the cost of digestion, are virtually independent of θ_c^AS.