The denitrification process for treating low carbon/nitrogen (C/N) wastewater in horizontal subsurface flow constructed wetlands (CWs) is often restricted by the carbon source and both abundance and activity of denitrifying bacteria. In this study, the effects of rice husks or immobilized Pseudomonas fluorescens on N removal were investigated under various conditions in a synthetic wastewater solution. CW mesocosms were added with granules formed by co-immobilizing rice husks and P. fluorescens were also used to investigate organics and N removal performance. A removal efficiency of 96% was achieved for nitrate nitrogen at 30 degrees C and pH of 7.5 for synthetic wastewaters solutions at different C/N ratios. Much higher average removal rates of COD (79%), ammonia nitrogen (85%), and total nitrogen (78%) were achieved in CW mesocosms with granule addition. These results suggest granule addition to the CW mesocosms as an effective strategy for treating the low C/N wastewater.