Ultrasonication as anaerobic digestion pretreatment to improve sewage sludge methane production: Performance and microbial characterization.

A large amount of sludge is inevitably produced during sewage treatment. Ultrasonication (US) as anaerobic digestion (AD) pretreatment was implemented on different sludges and its effects on batch and semi-continuous AD performance were investigated. US was effective in sludge SCOD increase, size decrease, and CH production in the subsequent AD, and these effects were enhanced with an elevated specific energy input. As indicated by semi-continuous AD experiments, the mean daily CH production of US-pretreated AO-, AO-MBR-, and AO-AO-sludge were 176.9, 119.8, and 141.7 NmL/g-VS, which were 35.1%, 32.1% and 78.2% higher than methane production of their respective raw sludge. The US of AO-sludge achieved preferable US effects and CH production due to its high organic content and weak sludge structure stability. In response to US-pretreated sludge, a more diverse microbial community was observed in AD. The US-AD system showed negative net energy; however, it exhibited other positive effects, e.g., lower required sludge retention time and less residual total solids for disposal. US is a feasible option prior to AD to improve anaerobic bioconversion and CH yield although further studies are necessary to advance it in practice.