Improved methane production from corn straw using anaerobically digested sludge pre-augmented by nanobubble water

[Display omitted] •NBW enhanced ETS activity and methane yield during pre-augmentation process.•NBW pre-augmented AD sludge enhanced hydrogen production in BMP tests.•O2-NBW pre-augmented AD sludge achieved the highest VFAs consumption in BMP tests.•22% higher CH4 yield was obtained from corn straw by O2-NBW pre-augmented sludge.•No increase in reactor volume was realized by using O2-NBW pre-augmented AD sludge. Nanobubble water (NBW) has been proven to efficiently improve methane production from organic solid wastes. However, the increase in reactor volume due to addition of NBW hinders its practical applications. In this study, anaerobically digested sludge was first pre-augmented by N2-NBW and O2-NBW using corn straw as sole substrate for methane production with electron transfer activity being monitored. 20%, 33% and 38% of cellulose and 29%, 35% and 35% of hemicellulose were reduced respectively from the control, N2-NBW and O2-NBW pre-augmented sludge reactors. N2-NBW and O2-NBW pre-augmented sludge reactors achieved methane yields of 127 and 142 mL/g-VS, about 10% and 22% higher than that from the control. Results show that use of NBW pre-augmented anaerobically digested sludge as inoculum can remarkably enhance methane yield from corn straw, providing a novel concept for NBW-based anaerobic digestion system with no increase of reactor volume and construction cost in practice.