Integrated two-phase acidogenic-methanogenic treatment of municipal sludge with thermal hydrolysis

The purpose of this research was to investigate the impact of two process configurations integrating two-phase anaerobic digestion (AD) of municipal sludge with thermal hydrolysis (TH). The TH was positioned either before or after the acidogenic fermentation phase. The fermentation process was carried out under the semi-continuous flow regime with a retention time of three days. The TH was done at a temperature of 170 °C and for 30 min. Among all the tested scenarios, the TH of sludge followed by the acidogenic fermentation resulted in the highest COD solubilization ratio (39.5%) and volatile fatty acids production (6,420 ± 400 mg/L), which was 630% and 500% more than that of the raw sludge, respectively. The sequential TH/fermentation process achieved 40% higher ultimate methane yield (240 mL/g COD) than the non-pretreated (raw) sludge. Positioning TH after the fermentation process reduced the ultimate methane yield to 231 240 mL/g COD, although it was still 32% higher than that of the raw sludge. The analysis of methane production rate and biodegradation kinetics data suggested the formation of refractory intermediates during the thermal process of sludge, which reduced the overall performance rate during the first week of the AD process. It was also revealed that acidogenic fermentation of thermally-processed sludge could diminish the adverse effect of the recalcitrant compounds formed during the thermal hydrolysis on the subsequent AD process.