Hydrazine addition enhances the nitrogen removal capacity in an anaerobic ammonium oxidation system through accelerating ammonium and nitrite degradation and reducing nitrate production

•Hydrazine addition improves specific anammox activity by 42%.•Hydrazine degradation strengthens anammox, increasing the ammonia removal rate.•Hydrazine addition accelerates nitrite degradation, improving nitrite conversion rate.•Hydrazine addition reduces nitrate production, avoiding nitrate accumulation.•A model in which hydrazine addition enhances the nitrogen removal capacity in an anammox system was established.