Interpreting the synergistic effect in combined ultrasonication-ozonation sewage sludge pre-treatment

The sequential combination of ultrasonication and ozonation as sewage sludge treatment prior to anaerobic digestion was investigated. Synergistic volatile suspended solids (VSS) solubilization was observed when low energy ultrasonication (<= 12 kJ g(-1) TS) was followed by ozonation. 0.048 g O-3 g(-1) TS ozonation induced the maximum VSS solubilization of 41.3% when the sludge was pre-ultrasonicated at 9 kJ g(-1) TS; while, the same ozone dosage applied without prior ultrasonication only induced 21.1% VSS solubilization. High molecular weight (MW) components (MW > 500 kDa) were found to be the main solubilization products when sludge was only ozonated. However, solubilization products by ozone were mainly in the form of low MW components (MW < 27 kDa) when sludge was pre-ultrasonicated. The high MW products generated by ultrasound were effectively degraded in the subsequent ozonation. Anaerobic biodegradability increased by 34.7% when ultrasonication (9 kJ g(-1) TS) and ozonation (0.036 g O-3 g(-1) TS) were combined sequentially. The maximum methane production rate increased from 3.53 to 4.32, 4.21 and 4.54 mL CH4 d(-1) after ultrasonication, ozonation and ultrasonication-ozonation pre-treatments, respectively. (C) 2014 Elsevier Ltd. All rights reserved.