Nanobubbles drive advanced anaerobic treatment of swine wastewater for efficient methane recovery: Performance gains and multi-pathway enhancement

Swine wastewater contains recoverable energy, but anaerobic digestion is often limited by complex organics and slow hydrolysis. To overcome this limitation, this study introduced nanobubble technology using three gas media (air, O₂, and O₃) and systematically studied their effects on methane recovery during the anaerobic digestion of swine wastewater. Batch experiments showed that O₃ nanobubbles achieved the strongest enhancement, increasing cumulative methane production by 87.5% compared with the control. This improvement may result from the strong oxidative capacity of O₃ nanobubbles to degrade recalcitrant organics, as indicated by the second methane production peak observed only in the O₃ nanobubbles. In contrast, O₂ nanobubbles provided the weakest improvement, potentially because excess dissolved oxygen stimulated facultative aerobic respiration, converting substrates to CO₂ and lowering availability for methanogenesis. Further analysis revealed that all nanobubble treatments accelerated volatile fatty acid turnover and enriched key hydrolytic and acidogenic microbes, particularly under O₃ nanobubbles. The enrichment of Methanothrix and downregulation of the energy-intensive PilA gene suggest promoted electron transfer. Negatively charged nanobubbles may act as abiotic mediators that facilitate direct interspecies electron transfer. Metabolic analysis indicated enhanced hydrogenotrophic, methylotrophic, and acetoclastic methanogenesis, implying strengthened synergy among pathways. Overall, O₃ nanobubbles show promise for resource recovery from organic waste.