Application of low-strength ultrasonication to the continuous anaerobic digestion processes: UASBr and dry digester

•The effects of low strength ultrasound on the AD performance were investigated.•In UASBr system, up to 38% higher CH4 production was achieved.•Higher enhancement was observed from ambient condition than mesophilic condition.•In dry digestion system, a 40% increase in the production of CH4 was achieved.•Finally, various mechanisms of enhanced CH4 production were discussed in detail.

In this study, the positive effects of low-strength ultrasonication (LS-ultrasonication) on the anaerobic digestion (AD) performance were investigated by continuously operating an upflow anaerobic sludge blanket reactor (UASBr) and a dry digester. In the ultrasonicated UASBr system (1 s per min, 0.05 W/mL), ultrasonication enhanced the CH4 production by 38% and 19% in an ambient and a mesophilic condition, respectively. In addition, a different sludge yield and a changed electron flow were observed after ultrasonication. In the ultrasonicated dry digestion system (2 s per 30 s, 0.0025 W/mL), a 40% increase in the production of CH4 was observed after lowering the total solid content of the reactor from 12% to 10%, implying that a high solid content diminished the ultrasonic stimulation effect. Moreover, the ultrasonication strength itself appeared to be a more crucial factor than the ultrasonication density during the application of LS-ultrasonication in the AD system.