Yields of biomass and extracellular polymers in four anaerobic sludges

Extracellular polymers (ECP) are responsible for the flocculation of activated sludge as well as the granulation of anaerobic sludge. In this study, the biomass and ECP yields of four anaerobic sludge were examined. The sludges were enriched by using acetate, propionate, butyrate and glucose, respectively, as the sole substrate. Throughout the enrichment process, which lasted up to 73 cycles, the sludge samples were analyzed for their volatile suspended solids (VSS) as well as ECP's protein (ECP(p)) and carbohydrate (ECP(c)) contents. Under steady-state condition, the net yields of biomass, ECP(p) and ECP(c) were measured for the conversion of each individual substrate to methane and carbon dioxide. Furthermore, the corresponding yields for the acetogenesis of propionate and butyrate, and the acidogenesis of glucose, were also estimated, based on the stoichiometry of the degradation reactions. Results show that acidogenesis of glucose produced more ECP(p) ECP(c) than acetogenesis and methanogenesis. This explains the reported observations that carbohydrate-degrading sludge produced better granules than the acid-degrading sludge.