Recent progress in persulfate to improve waste activated sludge treatment: Principles, challenges and perspectives

•Persulfate usually needs to be activated for efficient WAS treatment.•Fermentation/digestion is improved as persulfate boosts bioprocesses and community.•Persulfate promotes sludge dewatering by decomposing EPS.•The oxides generated by persulfate help to control the environmental risk of WAS.•The knowledge gaps and prospects based on persulfate are identified and discussed.