Zero valent iron catalyzing sulfite improves the quality and quantity of short-chain fatty acids from anaerobic fermentation of sewage sludge

•Fe0 catalyzing sulfite pretreatment promoted SCFAs production from sludge.•Acetate proportion was promoted by 275 % at 500 mg S/L and 1.25 ZVI/sulfite ratio.•Enhanced disintegration of sludge and loosen structure of protein were observed.•ZVI/sulfite pretreatment promoted hydrolysis, acidogenesis and acetogenesis.•Functional genes related to acetate formation were enriched by ZVI/sulfite pretreatment.