Bio-electrochemically assisting low-temperature anaerobic digestion of low-organic strength wastewater

•BES significantly enhanced low-temperature AD of low-organic strength wastewater.•With 0.4 V voltage applied, both COD removal and CH4 yield were promoted at 20–8 °C.•More EPS was produced in BES, suggesting a faster metabolism activity of microbes.•Both H2-producing bacteria and H2-utilizing methanogens were enriched in BES.•New crucial H2-mediated synergy raised in BES at low temperatures.