Enhanced ethanol-driven carboxylate chain elongation by MOF-808 from waste activated sludge: Process and mechanism.

Carboxylate chain elongation can create value-added bioproducts from waste activated sludge (WAS). The bioconversion of WAS during anaerobic fermentation is often constrained by inefficient hydrolysis. The addition of MOF-808 (200 mg MOF-808/g volatile solids (VS)) increased caproate production and selectivity by approximately 38.9 % and 28.9 %, respectively. MOF-808 significantly promoted the hydrolysis of WAS, accelerated the degradation of extracellular polymeric substances, and enhanced acetate accumulation. Absolute quantitative metagenomics conducted during the acidification and chain elongation phases demonstrated that MOF-808 markedly improved enzymatic hydrolysis. The absolute gene abundance of protease and α-glucosidase increased by 168.9 % and 191.2 %, respectively, compared to the control trial. Furthermore, the reverse β-oxidation (RBO) pathway, the primary route for chain elongation, exhibited a 19.2 %-76.1 % increase in gene abundance for enzymes involved in this pathway in the presence of MOF-808. Notably, the absolute gene abundance of electron-bifurcating enzyme complexes, including butyryl-CoA dehydrogenase-electron transferring flavoprotein complex (Bcd-EtfAB), proton-translocating NAD(P) transhydrogenase, ATPase (subunits A-I), and NAD oxidoreductase (RnfA-E), was significantly elevated in the MOF-808 trial. These findings provide valuable insights into enhancing the efficiency of chain elongation fermentation of WAS using MOF-like materials.