Intermittent energization catalyzes direct interspecies electron transfer in electro-anaerobic digestion of sewage sludge

Abstract

Microbial electrolysis cell-assisted anaerobic digestion (MEC-AD) is a promising technology for enhancing biomethane recovery over conventional AD, while external energy input is required for its operation. A few previous reports suggested that periodic power supply in MEC-AD systems could improve their performance with synthetic substrates, while significant gaps remain in understanding the underlying fundamental features of such operation schemes. Here, the impacts of intermittently applied potential on a sewage-sludge fed MEC-AD system were investigated to shed new light on microbial syntropy, and functional methanogenesis pathways under continuous (24 h/d) vs. intermittent (12–18 h/d) applied potential modes. In general, sludge hydrolysis and methane production increased with a more extended switch-off period of applied potential. The results showed that periodic applied potential could improve direct interspecies electron transfer (DIET) based syntrophy between anodic electroactive bacteria (Geobacter sp.) and known electrotrophic methanogen (Methanobacterium). Consistently, extracellular electron transfer (EET)-related genes (omcC, omcZ, omcB, omcE, omcS, and pilA) were upregulated due to switching between on and off modes of applied potential. It was found that the intermittent application of applied potential was also effective in alleviating microbial competition between methanogens and acetogens in the biocathode. Such microbial competition was evident due to excessive H2 production under continuous applied potential mode. These results indicate the potential of operationally flexible MEC-AD systems for more economic sustainability.