Synergistic pretreatment of waste activated sludge using CaO2 in combination with microwave irradiation to enhance methane production during anaerobic digestion

To investigate the effects of combined calcium peroxide (CaO2) and microwave pretreatment on anaerobic digestion of waste activated sludge, lab-scale experiments were conducted to measure the solubilization, biodegradation, and dewaterability of the waste activated sludge. Additionally, the synergistic effects between CaO2 and microwave were studied, and the microbial activity and methanogenic archaea community structure were analyzed. Combined pretreatment considerably facilitated the solubilization and subsequent anaerobic digestion of the waste activated sludge. The optimal pretreatment condition was CaO2 (0.1 g/gVSS)/microwave (480 W, 2 min) for methane production during the subsequent anaerobic digestion process. Under this condition, 80.2% higher CH4 accumulation yield was achieved after 16 d of anaerobic digestion when compared with the control. The synergistic effects of CaO2/microwave pretreatment resulted from the different mechanisms of CaO2 and microwave treatments. Further, microwave irradiation increased OH generation from CaO2 and significantly alleviated the inhibitory effect of CaO2 on methanogens. The activities of hydrolytic enzymes and acid-forming enzymes in the waste activated sludge were improved after CaO2 (0.1 g/gVSS)/microwave (480 W, 2 min) pretreatment. Methanogenesis enzyme activity was also higher after CaO2 treatment (0.1 g/gVSS)/microwave (480 W, 2 min) following a lag period. Illumina MiSeq sequencing analysis indicated that acetate-utilizing methanogen (Methanosaeta sp.) and H2/CO2-utilizing methanogen (Methanospirillum sp.) abundance improved greatly following CaO2 (0.1 g/gVSS)/microwave (480 W, 2 min) pretreatment. The percentage of CH4 in biogas with CaO2/microwave pretreatment increased by 25.4% relative to the control. The dewaterability of the waste activated sludge also improved considerably after anaerobic digestion with combined CaO2/microwave treatment. Therefore, CaO2/microwave pretreatment can be used as an effective method to recover energy from waste activated sludge.