Microbial electrolysis cells (MEC) accelerated methane production from the enhanced hydrolysis and acidogenesis of raw waste activated sludge

The anaerobic digestion coupled with the microbial electrolysis cell (MEC-AD) is generally thought to accelerate methane production from the hydrolysate of biomass. This work studied the methanogenesis performance and response of functional microorganisms in MEC-AD feeding with raw waste activated sludge (rWAS) and heat pretreated waste activated sludge (hWAS), respectively. The results showed that the methane productivity of rWAS and hWAS were both substantially enhanced by applied voltage at 0.8 V, being 7.8 times and 2.1 times higher than that without voltage supply (the first open circuit, stage I). Surprisingly, when applied voltage was afterward decreased back to 0 V, the enhanced observation of hWAS almost gone, but the methane productivity of rWAS remained as high as 6.3 mL gVSS_in⁻¹d⁻¹, which were still 6.2 times higher than that in the first open circuit. Different from the enhancement of methanogenesis on hWAS, the applied voltage in rWAS not only benefited to enrich the electricigens and methanogens, but also specially to enrich the fermentative bacteria and syntrophic acetogenic bacteria in both electrode biofilms. While this advantageous microbial structure in rWAS did not disappear with the removal of voltage supply except electricigens. Therefore, the enhanced hydrolysis-fermentation and synergy of acetogenic bacteria and hydrogenotrophic methanogens might be the main reason for keeping the high efficiency of methanogenesis with rWAS as substrate in MEC-AD. The findings reported in this study is economically and environmentally attractive, which might support high and stable methane production in traditional AD by enriching functional microorganisms with less energy input in the future.