Synchronous vivianite and hydrogen recovery from waste activated sludge fermentation liquid via electro-fermentation mediated by iron anode

•Fe anode achieved 101–743% higher of hydrogen productivity than other iron sources.•Fe anode led complete removal of phosphate within 2 d (vs. 0–39.4% in other groups).•More crystalline vivianite particles were observed in EF system assisted by Fe anode.•Homo-acetogens (78.0%), EAB (8.0%) and DIRB (2.3%) were enriched in Fe anode group.•Correlation and syntrophy between functional microbes and metabolites were explored.