Supplementation of O2-containing gas nanobubble water to enhance methane production from anaerobic digestion of cellulose

Nanobubble water (NBW) contains 106–108 gas bubbles per milliliter with diameter <1 µm. These fine bubbles possess high mass transfer efficiency and hydrophobic property beneficial for agricultural and biomedical application. Nowadays, much attention has been paid to the secure and sustainable management of lignocellulosic wastes. Cellulose is regarded as one kind of slowly biodegradable organic components owing to its complex crystalline structure. This study investigated the effect of O2-containing gas NBW on anaerobic digestion (AD) of cellulose for methane production. Results show that the cumulative methane yields from the reactors with Air-NBW (193 NmL/g-VSreduced), N2-NBW (196 NmL/g-VSreduced) and O2-NBW (233 NmL/g-VSreduced) addition were increased by 8–30% in comparison to the control (with the same amount of deionized water addition) (179 NmL/g-VSreduced). Under NBW addition, the reductions in cellulose content and cellulose crystallinity were respectively enhanced by 8–14% and 9–21% during AD, in which cellulase activity was elevated by 10–38%. The O2-NBW reactor was found to have the highest electron transport system activity, increasing by 1.7 times compared to the control, most probably due to the collapse of O2-nanobubbles and release of O2 resulting in micro-oxygen environment under the test condition. Besides, microbial community analysis suggests that the direct interspecies electron transfer could be quickly established with the addition of NBW. Results from this study also shed light on the mechanisms involved in the bioconversion of cellulose to methane via AD supplemented with NBW.