Performance evaluation and microbial community analysis of mesophilic and thermophilic sludge fermentation processes coupled with thermal hydrolysis

This research aims to provide new insights into the performance and the microbial dynamics of the sludge fermentation process coupled with thermal hydrolysis. Two semi-continuous fermenters were operated under the mesophilic and thermophilic conditions at the retention time of three days. Thermal hydrolysis (TH) was applied either before (pre-hydrolysis) or after (post-hydrolysis) the fermentation process at a temperature, pressure, and stationary time of 170 °C, 115 psi, and 30 min, respectively. The process incorporating pre-hydrolysis achieved 10–16% higher solubilization and volatile fatty acids (VFAs) production than that of post-hydrolysis. Acetate and propionate prevailed in both process schemes. However, significantly higher iso-butyrate and lower butyrate were recovered through the fermentation of pre-hydrolyzed sludge. Pre-hydrolysis stimulated the growth of more kinetically efficient fermentative bacteria such as Bacteroidetes and Firmicutes due to the abundance of soluble organic matters; however, it reduced the overall microbial diversity. The findings of this research suggest that coupling TH and acidogenic fermentation can provide an opportunity to enhance the production of soluble organics (e.g., VFAs) from wastewater sludge, which can be utilized for various applications including as an external carbon source for the on-site biological nutrient removal.