Hydrolysis and methane yield enhancement of anaerobic co-digestion of kitchen waste and sewage sludge by air/carbon dioxide nanobubble water

Anaerobic co-digestion of kitchen waste and sewage sludge has attracted considerable attention, but the accumulation of volatile fatty acid can affect biogas yield and hinder its application. In this study, the effects of air/ carbon dioxide nanobubble water (NBW) on hydrolysis and methane yield were investigated. Under mesophilic conditions, the addition of different NBW (Air, CO2, and Air + CO2) increased the total ammonia nitrogen in the hydrolysis stage by 6.6 %, 11.5 %, and 7.0 %, respectively. Meanwhile, the volatile fatty acid ratio total alkalinity of NBWAir+CO2 was lower than that of the other NBW digesters, improving system stability. In addition, NBW increased ultimate methane yield by 4.7 %, 25.3 %, and 21.0 %, respectively. The addition of NBW also affected the microbial diversity and relative abundance of microorganisms in the digesters. The alpha diversity index of NBWAir+CO2 is superior to other NBW digesters. Bacteria enriched with Firmicutes, Bacteroidetes, Proteobacteria, and Chloroflexi improved the hydrolysis, and methanogens were dominated by hydrotrophic types. In summary, the introduction of NBWAir+CO2 provides new insights to improve hydrolysis, system stability, and methane yield efficiency.