The excess nitrate in groundwater could be removed by heterotrophic denitrifying bacteria, though often limited by scarce carbon source present. Denitrification in two columns filled with walnut shell and its mixture with silica sand was studied. Both media showed high nitrate removal (>90%) with trace amounts of nitrite left over. The nitrate removal rates of two columns, 0.039±0.003, 0.029±0.003 g N L-1d-1, respectively, were zero order (r2=0.9181, 0.9998 for each column) in kinetics, which demonstrated a low C/N ratio herein. Despite of the implication that denitrification was carbon-limited, the organic matter residue in both columns, especially in pure walnut shell, were still problematic. The mixture media was suggested as potential PRB media for its satisfactory nitrate removal and relatively low COD residue; though we believed these hardly degradable organic residues require more study.