Recent advancements in nanobubble water technology and its application in energy recovery from organic solid wastes towards a greater environmental friendliness of anaerobic digestion system

Anaerobic digestion (AD) is an effective biological process to recover energy from organic solid wastes. There is a growing interest in utilization of renewable or sustainable bioenergy sources as future alternatives to fossil fuels. However, low hydrolysis rate, high heterogeneity of plant cell-wall structure of lignocellulose, process instability at high organic loading rate, and low metabolic activity of microbes limit the practical application of AD of organic solid wastes. A novel nanobubble water (NBW) based-AD system has been proposed to achieve enhanced methane production from typical organic solid wastes. This review summarizes the recent NBW studies, particularly on its utilization in AD processes and its stimulating effects on microorganisms and enzyme activities. The NBW generation approaches and its unique properties are firstly reviewed; then the roles of O 2 in the O 2 -containing gas NBW in AD are also outlined and discussed. A deep insight into the feasibility of applying NBW-based AD system in the promotion of energy production from organic solid wastes is provided. Based on the reviewed literature works, the challenges and prospects of adopting the NBW-based AD system as a promising and sustainable technology for energy recovery from organic solid wastes are highlighted. • Gas-water circulation can be used to generate nanobubbles for NBW-based AD system. • Bioactivities of anaerobic microbes and enzymes can be stimulated in NBW-AD process. • NBW supplementation mainly contributes to enhanced hydrolysis/acidification stages. • Conversion pathways of organic solid wastes in O 2 -NBW-based AD system are proposed. • NBW-based AD can achieve sustainable energy recovery from organic solid wastes.