Evaluation of hidden H2-consuming pathways using metabolic flux-based analysis for a fermentative side-stream dynamic membrane bioreactor using untreated seed sludge

This study investigates the performance and hidden hydrogen consuming metabolic pathways of a fermentative side stream dynamic membrane (DM) bioreactor using flux balance analysis (FBA). The bioreactor was inoculated with untreated methanogenic seed sludge. It was found that fouling rate aggravated with increasing COD concentration (10–30 g/L) and was positively correlated to it rather than to the applied solid flux on the DM module. Due to increased fouling rate the hydraulic retention time (HRT) could not be reduced less than 0.82 ± 0.02 d. An increase in the organic loading rate (OLR) led to an increase in H₂ yield from 0.01 to 0.76 mol H₂/mol of sucrose. FBA revealed that homoacetogenesis was the main H₂-consuming pathway at lower OLRs (corresponding to 10 and 15 g COD/L), while for the OLR corresponding to 30 g COD/L, homoacetogens were suppressed. More importantly, caproic acid production pathway was identified for the first time as another H₂-consuming pathway at high OLR which was not significant at lower OLRs during fermentative dynamic membrane bioreactor operations.