Efficacy of thermal hydrolysis for boosting specific methane yield depending on temperature-normalized solids retention time in an activated sludge process

The impact of varying operating conditions of a full-scale activated sludge process (ASP) on the efficacy of thermal hydrolysis in terms of increasing the specific methane yield (SMY) of waste activated sludge (WAS) during anaerobic digestion was evaluated. For this purpose, batch and semi-continuous anaerobic digestion tests (long-term study) were carried out with untreated and thermal hydrolyzed WAS on a laboratory scale. For the long-term study, thermal hydrolysis was set up at 160 degrees C for 30 min. A temperature-normalized solids retention time (SRTASP,T) was used to account for varying operating conditions in the ASP. In the semi-continuous experiments, the SMY of WAS decreased by 20% due to endogenous respiration as SRTASP,T increased from 26 d to 60 d. At the same time, thermal hydrolysis increased the SMY of WAS by 31% to 53%. Since the SMY of WAS mainly depends on active organic biomass, non-biodegradable or slowly degradable components are made more bioavailable for anaerobic digestion by thermal hydrolysis.