High-Efficiency Ethanol Yield from Anaerobic Fermentation of Organic Wastes via Stimulating Growth of Ethanol-Producing Fe(III)-Reducing Bacteria with Magnetite

Anaerobic fermentation of organic wastes to ethanol in mixed cultures has been considered as a promising strategy for simultaneous reduction of organic wastes and recovery of bioenergy. However, a major factor limiting the efficiency of ethanol production is that the formation of ethanol in the cells of ethanol-producing bacteria is usually accompanied by NADH/NAD+ transformation, but without ATP generation, to support their growth to resist the stressed conditions (pH: 4.0–5.0). In the presence of extracellular electron acceptors, such as Fe(III), the electrons released from NADH to NAD+ can be consumed for Fe(III) reduction via crossing cytomembrane and leaving H+ translocate back to the cytoplasm to produce ATP. Therefore, the study presented here proposed a strategy for enhancing the ethanol production from anaerobic fermentation of organic wastes via stimulating ATP synthase of ethanol-producing Fe(III)-reducing bacteria with magnetite. Under the conditions employed, the ethanol yield with magnetite (4507.3 ± 212.6 to 6112.0 ± 364.1 mg/L) was 4.52–6.13-folds higher than that without magnetite (996.0 ± 45.2 mg/L). Scanning electron microscopy showed that the sludge was dense and even aggregated with magnetite but which was sparse without magnetite. Analysis of microbial communities revealed that the relative abundance of both ethanol-type fermentation bacteria (Ethanoligenens species) and ethanol-producing Fe(III)-reducing bacteria (Clostridium species) increased in the presence of magnetite. Further analysis by fluorescence in situ hybridization revealed that the number of cells involved in both Bacteria and Clostridium species with magnetite was evidently more than that without magnetite.