The aim of this study is to investigate the denitrification efficiency enhancement and to evaluate the denitrification potential at the condition of lower temperature. Three identical biofilter were operated in parallel at batch tests. Results showed that the denitrification potential can be substantially increased by addition of ethanol and hydrolysate of sludge as external carbon sources compared with raw sewage. The mean denitrification rates of ethanol and hydrolysate reached up to 0.058 and 0.08 mgN/(g MLVSS•h), respectively, while that of raw sewage was only 0.041 mgN/(g MLVSS•h). The denitrification potential increased respectively from 0.76 to 1.54 and 5.3 mg NO3-N/L owing to ethanol and hydrolysate addition. By comparison, hydrolysate of sludge was found to be the better carbon source for denitrification considering strengthening denitrification performance and price. This study provides evidence for external carbon sources choice of sewage plant and surplus sludge reutilization.