Anaerobic digestion using ultrasound as pretreatment approach: Changes in waste activated sludge, anaerobic digestion performances and digestive microbial populations

To solve the problem of slow hydrolysis rate during anaerobic digestion, 20 kHz of ultrasound and energy density of 0.5 W/mL were applied as pretreatments for mesophilic anaerobic digestion of waste activated sludge. The sludge was digested at hydraulic retention time (HRT) of 20 d after sonication of 0-100 min. Results showed that ultrasonic pretreatment was beneficial to the dissolution of soluble chemical oxygen demand (SCOD) from the waste activated sludge, coupled with the rapid degradation in the dewatering performance of the sludge. In the succeeding anaerobic digestion, when sonication of the treated sludge was more than 80 min, the removal rate of total chemical oxygen demand (TCOD) and the sludge reduction rate also decreased significantly. Gas and methane productions were highest at the reactor fed with sludge sonicated at 80 min. At this condition, the content of CH4 was more than 53.8%, and the average yield of methane was about 36.2 mL/gVS. Through the analysis of microbial diversity, we observed that hydrolytic and acidification bacteria were abundant in the reactors. Methanocorpusculum and Methanosaeta were the alternating dominant methanogens in the anaerobic reactors, with adding sludge suffered different ultrasonic time. In the sludge pretreated with ultrasonication after 80 min, the relative abundance of Methanocorpusculum decreased rapidly, and the dominant substrate for anaerobic methanogenesis changed from hydrogen to acetic acid.