Intensified nitrogen removal in immobilized nitrifier enhanced constructed wetlands with external carbon addition

•Immobilized nitrifier was used to improve NH4-N removal in CWs.•Optimal immobilized conditions for efficient nitrification was investigated.•External carbon addition was performed to promote denitrification in CWs.•Efficient nitrification and denitrification promoted TN removal in CWs.•Immobilized nitrifier with high influent COD/N ratio improved nitrogen removal. Nitrogen removal performance response of twelve constructed wetlands (CWs) to immobilized nitrifier pellets and different influent COD/N ratios (chemical oxygen demand: total nitrogen in influent) were investigated via 7-month experiments. Nitrifier was immobilized on a carrier pellet containing 10% polyvinyl alcohol (PVA), 2.0% sodium alginate (SA) and 2.0% calcium chloride (CaCl2). A batch experiment demonstrated that 73% COD and 85% ammonia nitrogen (NH4-N) were degraded using the pellets with immobilized nitrifier cells. In addition, different carbon source supplement strategies were applied to remove the nitrate (NO3-N) transformed from NH4-N. An increase in COD/N ratio led to increasing reduction in NO3-N. Efficient nitrification and denitrification promoted total nitrogen (TN) removal in immobilized nitrifier biofortified constructed wetlands (INB-CWs). The results suggested that immobilized nitrifier pellets combined with high influent COD/N ratios could effectively improve the nitrogen removal performance in CWs.