Establishing Direct Interspecies Electron Transfer during Laboratory-Scale Anaerobic Digestion of Waste Activated Sludge via Biological Ethanol-Type Fermentation Pretreatment

Direct interspecies electron transfer (DIET) has been considered as an effective working mode to proceed with syntrophic metabolism, which is not yet established in the anaerobic digesters treating waste activated sludge (WAS), due to the lack of DIET-based syntrophs capable of producing the biological electrical connection and slow hydrolysis/ acidification limiting the available substrates for DIET-based syntrophic partners. A strategy of initially pretreating WAS at pH 4.0-4.5 with ethanol type fermentation enrichments (referred to as Biological Ethanol-type Fermentation Pretreatment, BEFP) for stimulating the methanogenic communities to perform DIET was proposed in this study. Under the SRT of 24 days, methane production and sludge reduction in the digesters treating WAS with BEFP were about 29.8% and 12.3% higher than that without BEFP, respectively. The digested sludge with BEFP presented a high conductance (0.9722 +/- 0.0085 uS/cm/g/L-VSS), even more conductive than the aggregates in the digester initially fed with ethanol previously reported. Together with the special enrichment of Geobacter species and high abundance of Methanothrix species, it was suggested that the DIET-based methanogenic communities were established. Further investigations to compare the pretreatment at pH 10 and BEFP revealed that, although the pretreatment at pH 10 promoted the better hydrolysis and increased the content of acetate, the slow methanogenic metabolism via the aceticlastic pathway could not yet provide the sufficient energy to support the growth of Methanothrix species and further metabolize acetate. However, the analysis of the stable carbon isotope demonstrated that the DIET-based syntrophic metabolism established with BEFP could potentially participate in acetate oxidation that supported the better growth of Methanothrix species.