Dosing with pyrite significantly increases anammox performance: Its role in the electron transfer enhancement and the functions of the Fe-N-S cycle

•Firstly reported pyrite-induced nitrogen removal enhancement in anammox system.•Pyrite promoted the nitrogen removal rate by 52% under substrate shock loading.•Pyrite increased electron transfer to improve microbial activity.•Pyrite couples Fe-N-S cycle to remove undesired nitrate in the anammox process.