Enhanced methane production from waste activated sludge by microbial electrolysis cell assisted anaerobic digestion: Fate and effect of humic substances.

[Display omitted] • 102.1 mg/g-VS humic substances reduced the methane production rate by 33.8 % in AD. • 0.6 V MEC-AD maintained stable when 102.1 mg/g-VS humic substances were added. • Humic substances acted as carbon sources and electron shuttles in 0.6 V MEC-AD. • The newly formed humic substances have lower aromaticity and higher reducibility. • The abundance of hydrogenotrophic methanogens on the cathode reached 42.9 %. Humic substances as major components of waste activated sludge are refractory to degrade and have inhibition in traditional anaerobic digestion (AD). This study for the first time investigated the feasibility and mechanism of microbial electrolysis cell assisted anaerobic digestion (MEC-AD) to break the recalcitrance and inhibition of humic substances. The cumulative methane production of AD decreased from 134.7 to 117.6 mL/g-VS with the addition of humic acids and fulvic acids at 25.2–102.1 mg/g-VS. However, 0.6 V MEC-AD maintained stable methane production (155.5–158.2 mL/g-VS) under the effect of humic substances. 0.6 V MEC-AD formed electrical stimulation on microbial cells, provided anodic oxidation and cathodic reduction transformation pathways for humic substances (acting as carbon sources and electron shuttles), and aggregated functional microorganisms on electrodes, facilitating the degradation of humic substances and generation of methane. This study provides a theoretical basis for improving the energy recovery and system stability of sludge treatment. [ABSTRACT FROM AUTHOR]