New Application of Ethanol-Type Fermentation: Stimulating Methanogenic Communities with Ethanol to Perform Direct Interspecies Electron Transfer

Direct interspecies electron transfer (DIET) has been considered as an effective mechanism to proceed syntrophic methanogenic metabolism. However, up to now, this working mode has been still not widely established in the Geobacter-rare methanogenic digesters. In this study, a strategy that could enrich Geobacter species and stimulate the methanogenic communities to continuously perform DIET was proposed in a two-phase anaerobic digestion (AD) system with the aim to enhance and stabilize the better AD. The results demonstrated that, under the conditions employed, the ethanol-abundant acidogenic products could be produced via ethanol-type fermentation when acidogenic-phase pH was kept at 4.0–4.5. Enrichments in the methanogenic phase continuously stimulated with the ethanol-abundant acidogenic products presented a higher conductivity, as well as more positive response to granular activated carbon (GAC) supplemented, compared with the enrichments without this stimulation, suggesting that DIET might be established. Microbial community analysis showed that Geobacter species were only detected in the methanogenic enrichments stimulated by the ethanol-abundant acidogenic products. Together with the significant increase of Methanosarcina species in these enrichments, the potential DIET between Geobacter and Methanosarcina species might be permanently established in the methanogenic digester to maintain the acidic balance as well as syntrophic metabolism stable.