Abstract: A biofilter reactor filled with lava as a carrier is used under the condition of room temperature(8-25 ℃) and low ρ(NH₄⁺-N)(60-90 mg/L).The NO₂^--N is accumulated through the method of aeration control in the reactor,and its accumulation rate can reach more than 80% after startup.Results show that DO control is the main approach for realizing nitrosification whereas the restraining of free ammonia can be the assistant method for choosing ammonia oxidization bacteria (AOB),and HRT adjusting is the key way to control the result of ρ(NH₄⁺-N)/ρ(NO₂^--N).According to the experiment result,the changes of ρ(NH₄⁺-N),ρ(NO₂^--N),and ρ(₃^--N) all have the dynamics characteristics of zero-grade reaction.Furthermore,the conversion rate of NH₄⁺-N is 4.32 mg/(L·h),and the accumulation rates of NO₂^--N and NO₃^--N are 3.05 and 0.40 mg/(L·h),respectively.Accordingly,the HRT for partial nitrosification should be controlled from 9 to 14 h.