Combining metal-microbe and microbe-microbe dual direct electron transfer on Fe(0)-cathode of bio-electrochemical system to enhance anaerobic digestion of cellulose wastewater

Abstract

Considering that cathode of microbial electrochemical system (MES) is a good electrons source for methane production via direct/indirect electron transfer to electroactive microorganisms, and that Fe(0) is also a confirmed electron donor for some electroactive microorganisms through metal-microbe direct electron transfer (DET), Fe(0)-cathode was equipped into an MES digester to enhance cathodic methane production. The results of this study indicated that the potential DET participator, Clostridium possibly obtained electrons directly from Fe(0)-cathode via metal-microbe electrons transfer, then transferred electrons directly to the definite DET participators, Methanosarcina/Methanothrix via microbe-microbe electrons transfer for CH4 production. In addition, Methanobacterium is another specially enriched methanogen on Fe(0)-cathode, which might obtain electrons directly from Fe(0)-cathode to produce CH4via metal/electrode-microbe DET. The increment of conductivity of cathodic sludge in Fe(0)-cathode MES digester (R1) further confirmed the enrichment of electroactive microorganisms participating in DET process. As a consequence, a higher CH4 production (1205–1508 mL/d) and chemical oxygen demand (COD) removal (79.0%-93.8%) were achieved in R1 compared with graphite-cathode MES digester (R2, 720–1090 mL/d and 63.6%-85.6%) and the conventional anaerobic digester (R3, 384–428 mL/d and 35.2%-41.0%). In addition, energy efficiency calculated indicated that the output energy of CH4 production was 8.16 folds of electricity input in Fe(0)-cathode MES digester.