Nickel (ii) (Ni2+) is considered as one of the necessary trace elements in the process of Anammox culture, but it may have toxic effects at high concentration. This study explored the long-term influence of Ni2+ on the denitrification efficiency of Anammox bioreactors. The results showed that when the concentration of Ni2+ was 0.5 mg/L, the bioreactor had the highest denitrification efficiency, while the removal efficiency of ammonia nitrogen and nitrite nitrogen gradually decreased at concentrations...