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

Highlights: • CaO₂/MW pretreatment synergistically enhanced WAS solubilization and CH₄ production. • MW irradiation facilitated more ^·OH generation from CaO₂. • The optimal pretreatment condition for methane production was determined. • The growths of both hydrogenotrophic and acetate-utilizing methanogens were promoted. • The dewaterability of WAS was improved considerably by CaO₂/MW treatment. - Abstract: 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.