Characterization of the COD/VSS Ratio during Anaerobic Digestion of Waste Activated Sludge: Experimental and Modeling Studies

An experimental study that examined the elemental composition of waste activated sludge as it progressed through mesophilic and thermophilic anaerobic digestion was completed. It was found that the oxygen mass fraction of the VSS increased while the carbon content decreased as digestion progressed. Small differences were observed between the responses observed for mesophilic and thermophilic digestion with the former having a greater increase in oxygen content. With the exception of nitrogen, the average molecular composition of the VSS was found to be consistent with that previously reported for microbial cells. The COD/VSS ratio of the solids decreased as digestion progressed and varied from 1.6-1.2 g COD/g VSS. When employing fixed default values for the COD/VSS ratio, the BioWin model was able to adequately predict biogas production but the prediction of COD/VSS deviated from the observed values as digestion progressed.