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

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 CH₄ 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 CH₄ production of US-pretreated A²O-, A²O-MBR-, and AO-AO-sludge were 176.9, 119.8, and 141.7 NmL/g-VS_added, which were 35.1%, 32.1% and 78.2% higher than methane production of their respective raw sludge. The US of A²O-sludge achieved preferable US effects and CH₄ 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 CH₄ yield although further studies are necessary to advance it in practice.