The paper is intended to investigate the effect of denitrification and COD removal in the post-denitrification biological filter process, adding an external carbon source. Tow carbon sources are dosed, methanol and sewage. The experiment performs in a tow-classes biological filter, oxic zone and anoxic zone, indicate that denitrification rate can be instantaneously increased through the addition of either carbon source. The perfect inputs flow of methanol and sewage are 20 mg/L and 0.2Q (Q is the effluent flow). These results indicate that methanol is better than sewage to denitrification as carbon source and the effluent concentration of COD was low.