Anaerobic Volatile Fatty Acid Production Performance and Microbial Community Characteristics from Solid Fraction of Alkali-Thermal Treated Waste-Activated Sludge: Focusing on the Effects of Different pH Conditions.

The waste-activated sludge (WAS) is rich in organic matter and various nutrients. Alkali-thermal hydrolysis of WAS can be employed to produce a liquid fertilizer with high plant-promoting nutrient content. However, the solid fraction (abbreviated as SF) generated from this process requires further treatment. Although there have been studies on the recovery of plant nutrients from WAS via alkali-thermal hydrolysis, researches on the safe treatment of the SF are limited. This study aims to explore the potential and the microbiological mechanisms on anaerobic volatile fatty acid (VFA) production from the SF under different pH conditions (i.e., 6, 7, 8, 9, and 10). The results showed that the VFA yield was highest at pH 6, reaching 4095.84 mg COD/L (i.e., 0.16 g-COD/g-volatile solids), followed by pH 10, 8, 7, and 9, with acetate being the main component (> 56%). Microbial community analysis revealed that members in phyla Firmicutes and Bacteroidota constituted the main acid-producing microbial community during the anaerobic fermentation of SF. Furthermore, different pH conditions influenced the yield and composition of VFAs by altering the structure and functions of microbial community. This research provides a new direction for the fully resourceful utilization of sludge by producing both liquid fertilizer and VFAs from WAS.