Adding Fe0 powder to enhance the anaerobic conversion of propionate to acetate

Propionate is an unfavorable substrate for the anaerobic digestion because it is thermodynamically difficult to be decomposed into acetate. An attempt to enhance the decomposition of propionate by adding Fe0 powder (10 g) into an acidogenic reactor (A1) with propionate as the sole carbon source was made in this study. The results showed that the propionate conversion rate (67–89%) in A1 were higher than that in a reference reactor (43–77%) without dosing of Fe0 (A2). The enhanced conversion of propionate caused both chemical oxygen demand removal (COD) (57–79%) and acetate production (178–328 mg/L) in A1 to increase significantly. Although Fe0 contributed the H2 production chemically, the H2 content of A1 was less than that of A2. The reason was ascribed to the enhanced utilization of H2 for the homoacetogenesis. It was calculated that the Gibbs free energy in the decomposition of propionate was decreased by about 8.0–10.2% with the dosing of Fe0. Also, the activities of enzymes related to the acetogenesis were enhanced by 2–34-folds. Fluorescence in situ hybridization (FISH) and denaturing gradient gel electrophoresis (DGGE) analysis indicated that Fe0 increased the abundance of microbial communities, especially propionate-utilizing bacteria and homoacetogenic bacteria.

► Adding Fe0 to enhance propionate decomposition in an acidogenic reactor.
► Accelerating homoacetogenesis to reduce the H2 content with the presence of Fe0.
► Fe0 could decrease the Gibbs free energy and improve the enzymes activities.