Enhancement of medium-chain fatty acids production from sewage sludge fermentation by zero-valent iron

The effect of zero-valent iron (ZVI) dosage on medium-chain fatty acids (MCFAs) production from sewage sludge fermentation was explored. ZVI within a dosage of 2-20 g/L favored MCFAs production. Adding 20 g/L ZVI (ZVI20) increased the MCFAs and long-chain alcohols (LCAs) production to 4079.0 mg/L and 93.1 mg/L, the electron transfer efficiency of MCFAs and MCFAs selectivity were also increased by over 40% and 25% than the control. This may be due to the enriched MCFAs-producing genera, like Romboutsia and Paraclostridium. 2 g/L ZVI favorably strengthened the RBO pathway and facilitated intracellular electron generation. Moreover, ZVI facilitated the extracellular electron transfer, and cytochrome C was most enriched by ZVI20. The low MCFAs production in the ZVI50 group might be due to the inhibition of acetyl-CoA and ATP synthesis. These results provided a deep insight into the effects of ZVI dosage on MCFAs production and the specific mechanisms.