Effect of enhanced biomass retention by sequencing batch operation on biomethanation of sulfur-rich macroalgal biomass: Process performance and microbial ecology

This study investigated the anaerobic digestion of Ulva biomass as an approach to diversifying energy sources and managing seaweed waste in a cost-effective manner. Ulva species are often identified as the main culprit of serious macroalgal blooms around the world, and their sulfur-rich nature causes difficulties in handling the biomass. Two reactors in continuous (Rc) and sequencing batch (Rs) modes were operated with decreasing hydraulic retention time in a stepwise manner from 20 to 6 days. Rs allowed significantly higher methane productivity (0.19-0.22 L/g chemical oxygen demand [COD] fed) and biomass retention capacity than Rc (0-0.16 L/g COD fed) throughout the experiment. Interestingly, sulfide production was also higher in Rs than in Rc. These findings, together with microbial quantification results, suggested that Rs operation enhanced biomass retention and the activity of both methanogens and sulfate-reducing bacteria. The Rc microbial community was less diverse and more variable than the Rs community. Accordingly, the performance of Rc was more significantly affected by changes in hydraulic and thus organic loads. Aceticlastic Methanosaetaceae dominated the methanogen community in both reactors, with the abundance of methanogens being significantly higher in Rs than in Rc. This may explain the more efficient and stable methane production despite the greater sulfidogenic activity in Rs, particularly at high hydraulic loads. Together, the results suggest that sequencing batch operation is advantageous over conventional continuous flow operation for the biomethanation of Ulva biomass and potentially other sulfur-rich feedstocks.