Enhancing semi-continuous anaerobic digestion efficiency with nanobubble water: thermophysical properties optimization and microbial regulation

The utilization of nanobubbles to enhance biomass anaerobic digestion (AD) for bioenergy production has emerged as a prominent research focus. However, the mechanisms governing their effects on semi-continuous AD characteristics and heat-mass transfer processes remain inadequately elucidated. This research employed corn stover as feedstock for semi-continuous AD at 35 ± 1 °C, investigating the impacts of nanobubble water (NBW) prepared with H2, CH4, and CO2 on digestion performance, thermophysical properties (thermal conductivity, specific heat capacity, density, viscosity, surface tension), and microbial dynamics. The results demonstrated that NBW-supplemented systems achieved up to 15.67 ± 1.96 % higher daily methane production compared to the control. Compared to the control group, NBW-supplemented systems demonstrated improvements of 7.22–29.50 % and 5.06–18.85 % in total solids and volatile solids removal efficiencies, respectively. Concurrently, soluble chemical oxygen demand and reducing sugar concentrations decreased by 7.36–13.07 % and 3.34–10.61 % in NBW groups. NBW enhanced mass transfer efficiency, thereby inducing selective microbial enrichment and functional augmentation that improved substrate utilization. Furthermore, NBW optimized digestate thermal stability and flow properties, consequently enhancing heat-mass transfer efficiency during AD. This work elucidates nanobubble-mediated enhancement mechanisms in semi-continuous AD through integrated thermophysical and microbiological perspectives, thus providing a theoretical foundation for advanced biogas production technologies.