This study was intended to investigate the effect of hydraulic retention time and the recycle ratio on the performance of the biofilter process for treating secondary sedimentation basin effluent. The biofilter process consist of an upflow anoxic reactor and two downflow oxic reactors. And operating conditions of biofiter process was that the hydraulic retention time decreased from 3.0 hr to 0.7 hr. BOD and TN removal rates achieved 87.7 % and 59.0 % at HRT 3.0 hr. According to decrease HRT 0.7 hr, BOD and TN removal rates were decreased as 84.9 % and 50.6 %. The performance of the biofilm process was also evaluated at different recycle ratios such as 50 %, 100 % and 150 %. At a recycle ratio of 150 %, the maximum removal rate of BOD and TN were 89.9 % and 73.4 %, respectively and concentrations of BOD and TN were less than 3.5 mg/L in effluent of biofilm process. And studying the removal characteristics of hydraulic retention time and distance because the biofilter process is plug flow reator. The organics removal and denitrification efficiency was highest at inlet(0.8 m ~ 1.5 m) of the anoxic reactor. In oxic reator, residual organics were removed at inlet(2.2 m ~ 2.9 m) and nitrification efficiency was active at middle section(1.5 m ~ 2.2 m). The results demonstrated that the biofilm process could treat low strength domestic wastewater effluent effectively at a short HRT. Furthermore, the recycle ratio had significant effect on the performance of the biofilm process.