For biofilters treating waste gases containing volatile organic compounds (VOCs),biomass accumulation is a common problem which will induce bed clogging and significant decrease in VOCs removal efficiency during long-term operation.In this study,ozone injection was developed as a biomass control strategy,and its effects on the biofilter performance and the microbial community structure were investigated in long-term operation.Two biofilters,identified as BF1 and BF2,were operated continuously for 160 days treating gaseous toluene under the same conditions,except that 200 mg/m3 ozone was continuously injected into BF1 during days 45-160.During the operation period,ozone injection did not change the toluene removal efficiency,while the pressure drop of BF1 with ozone injection was significantly lowered compared with BF2.The wet biomass accumulation rate of BF1 was 11 g/m3/hr,which was only 46％ of that in BF2.According to the carbon balance result,ozone injection also increased the toluene mineralization rate from 83％ to 91％,which could be an important reason for the low biomass accumulation.The PMA-qPCR result indicated that ozone injection increased the microbial viability of the biofilm.The high-throughput sequencing result also revealed that the dominant phyla and genera were not changed significantly by ozone injection,but some ozonetolerant genera such as P hodanobacter,Dokdonella and Rhodococcus were enhanced by ozone exposure.All the results verified that ozone injection is capable of sustaining the long-term performance of biofilters by lowering the biomass accumulation,increasing the microbial viability and changing the microbial community structure.