The role of sludge retention time in the hydrolysis and acidification of lipids, carbohydrates and proteins during digestion of primary sludge in CSTR systems

The effect of the sludge retention time (SRT) between 3 and 15 days, on hydrolysis, acidification and methanogenesis of domestic sewage was researched by simulating a sludge bed segment of an upflow anaerobic sludge bed (UASB) system as a completely stirred tank reactor (CSTR). The CSTR systems were fed with primary sludge (settled solids of domestic sewage) as the influent at 25°C. The study revealed that an SRT≤8 days resulted in acidogenic conditions with negligible biogas production, whereas an SRT>8 days resulted in methanogenic conditions. The hydrolysis of lipids and carbohydrates increased with increasing SRT, whereas protein hydrolysis only occurred under methanogenic conditions. Approximately 20 and 60% of the particulate biopolymers are hydrolysed under acidogenic and methanogenic conditions, respectively. Hydrolysis was found to be the rate-limiting step for the conversion of carbohydrates. Under acidogenic conditions, acidification was the rate-limiting step for conversion of lipids, while both hydrolysis and acidification were limiting for the conversion of proteins. Under methanogenic conditions, hydrolysis was the rate-limiting step in the whole digestion process. None of the main components of primary sludge followed first order kinetics with respect to hydrolysis. The dewaterability of the sludges from the CSTRs operated under acidogenic conditions deteriorated, whereas the dewaterability of the methanogenic sludge improved in comparison with the dewaterability of the raw primary sludge.