Effects of low-strength ultrasonication on dark fermentative hydrogen production: Start-up performance and microbial community analysis

Our previous studies have demonstrated the positive impacts of low-strength (LS)-ultrasonication on biological methane production via anaerobic digestion. In this study, the effects of LS-ultrasonication and effluent recirculation on dark fermentative hydrogen production using upflow anaerobic sludge blanket reactor were investigated. Significant increase in hydrogen (H₂) yield (up to 0.89 mol H₂/mol hexose) was observed under LS-ultrasonication conditions. This enhanced H₂ yield seems to be associated with the decrease in dissolved H₂ concentration via a degassing effect, as evident from increased H₂ content (up to 20%). Effluent recirculation alone led to positive effects. However, applying both LS-ultrasonication and effluent recirculation decreased H₂ yield compared to the trial with only LS-ultrasonication (0.89–0.72 mol H₂/mol hexose), possibly due to the cumulative shear stress and sludge bed expansion above the ultrasonication zone. Microbial community analysis revealed that the presence/absence of LS-ultrasonication and/or effluent recirculation significantly affected the microbial communities. Ethanoligenes cloacae, Aeromonas hydrophila and Clostridium pasteurianum were identified as principal H₂ producers under the LS-ultrasonic conditions, while C. butyricum, the most commonly reported H₂ producer, was more abundant in the controls. The energy balance of the LS-ultrasonication system was evaluated and future research directions were suggested.