Low-temperature anaerobic digestion enhanced by bioelectrochemical systems equipped with graphene/PPy- and MnO2 nanoparticles/PPy-modified electrodes.

Bioelectrochemical systems (BESs) with graphene (Gr)/polypyrrole (PPy)- and MnO2 nanoparticles (NPs)/PPy-modified electrodes were developed to enhance low-temperature anaerobic digestion (LTAD) of low-strength wastewater. At 20 °C, the chemical oxygen demand removal efficiencies and CH4 yield of the BESs with Gr/PPy (R2) and MnO2 NPs/PPy (R3)-modified electrodes were 12.7% and 25.6%, and 43.9% and 66.3%, respectively, higher than those of the control (R1, without modification). Although the performance of all reactors decreased as temperature dropping to 12 °C, the CH4 yield rates of R2 and R3 were still 22.8% and 39.0% higher than that of R1. Further analysis indicated that the modified electrodes might stimulate the metabolic activity of the anaerobic digester sludge. Scanning electron microscopy observation showed that the modified electrodes had higher specific surface area, favoring the attachment and formation of dense biofilms on the surface of electrodes. 16S rRNA gene-sequencing results demonstrated that H2-consuming methanogens dominated in the BESs and the influence of Gr/PPy and MnO2 NPs/PPy differed on the microbial community structure of biofilms. These findings justify the wider use of Gr and MnO2 NPs in electrode modification to assist LTAD using BESs.