Sulfite serving as a pretreatment method for alkaline fermentation to enhance short-chain fatty acid production from waste activated sludge

•Sulfite pretreatment enhanced SCFAs production from WAS alkaline fermentation.•H2SO3 and HSO3− were the major contributors to the enhanced SCFAs production.•SO32− facilitated the conversion of propionate and butyrate acid to acetate.•Sulfite + pH 9.5 caused positive synergies on fermentation process.