Phosphorus and nitrogen are the important eutrophication nutrients. They are removed in the anoxic/oxic reactor through simultaneous precipitation and biological nitrogen removal. The effect of alum a commonly used simultaneous precipitant on biological nitrification and denitrification are investigated in the present study. Simultaneous removal of phosphorus was carried out using the coagulant alum $Al_2(SO_4)_3cdot14H_2O$ at 2.2 mol ratio. Before the start of simultaneous precipitation the nitrification rate of the A/O reactor was found to be 0.05 g N-$NH_4{^+}/g$ VSS/d. It starts to decrease with increase in coagulant dosage. The nitrification rate for alum dosage 97.13 mg/L was 0.38 g N-$NH_4{^+}/g$ VSS/d. There was no accumulation of nitrate in anoxic tank. The nitrogen removal efficiency of the reactor was affected and it fell from 88 to 78%. There was a slight decrease in effluent COD from 16~20 mg/L to 8~12 mg/L after the introduction of simultaneous precipitation into the reactor. The usage of alum as a simultaneous precipitant in the anoxic/oxic reactor was limited due to its inhibition on nitrification. Alum did not have any affect over denitrification process.