Previous research has shown that nitrogen from municipal wastewater could be eliminated by a biofilter system. In this study a system of combined pre-denitrification/nitrification biofilters was set up. It is to investigate the effect of the hydraulic loading and recycled ratio on nitrogen removal. The characteristics of bacterial activity at different heights is discussed. The experiment shows that longer hydraulic loading would result in better total nitrogen removal. Total nitrogen removal might be not dependent on denitrification but nitrification. Hydraulic loading that affects nitrification might be due to the diffusion of NH3–N from the bulk solution to the inner biofilm. The recycling NO3–N could be completely eliminated in the anoxic biofilter. The operation with longer retention time (HRT of 12 hours) would result in inner denitrification in the aerobic biofilter. Biological activity could be determined by the distribution of bacteria. The specific rates of pollutant decomposition depend on biological activity and effective biological VSS. The effect of the recycled ratio on the nitrogen removal is significant. Total nitrogen removal rate and nitrogen type of effluent would be determined by recycle ratio. The operation at low recycled ratio would result in worse total nitrogen removal, but the NH3–N of effluent would be lower. The operation in higher recycle ratio would be opposite to low recycle ratio.

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