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

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.