Ferric chloride aiding nitrite pretreatment for the enhancement of the quantity and quality of short-chain fatty acids production in waste activated sludge

•The FC–N pretreatment increased the yield of SCFAs in WAS anaerobic fermentation.•The FC–N integrated pretreatment enhanced the conversion rate of propionic acid.•The FC–N pretreatment greatly improved the biodegradability of the sludge.•The FC–N pretreatment vastly increased the proportion of acid-producing bacteria.•FC could be applied as a substitute for HCl to promote free nitrite formation.