Abstract: The influence and recovery of pipeline biofilm purification efficiency in closed aqueduct by pre-ozonation were studied by simulations with a BAR(biofilms annular reactor) and with addition of ozone.The results show that the amount of heterotrophic bacteria quickly reduces after ozonation and the inactivation ratio of ammonia oxidizing bacteria reaches 100%.Oxygen,produced by ozone decomposition,is conductive to biofilm update,so the biofilm is recovered quickly.Ammonia nitrogen degradation evidently reduces after ozone addition and,the oxidization ability of ozone leads to NH₄⁺-N increasing.Removal efficiency drops from 81.85%to 38.49%,17.60%and-5.50%respectively,as concentration of ozone is at 0.7,1.5 and 3.2 mg/L.Moreover,significant nitrite nitrogen accumulation appears,and recovery of ammonia nitrogen removal is faster than that of nitrite nitrogen,which is recovered after 5 and 8 days respectively.Total organic carbon increases over inflow because oxidation caused by ozone,and water purification efficiency is recovered after 6 days.