Development of a hybrid biofilm reactor for nitrate removal from surface water with macrophyte residues as carbon substrate

Nitrate has become recognized as one of the most common contaminants in surface water. In this study, a hybrid biofilm bioreactor was developed to remove nitrate from surface water. Submerged macrophyte residues Ceratophyllum oryzetorum Kom (C. oryzetorum) harvested from a constructed wetland were utilized as an organic carbon source. The average nitrate removal efficiency and removal rate of the bioreactor were 81.6% and 50.8 g N m(-3)d(-1) within 270 day operation, respectively. The biodegradation kinetics constant of the C. oryzetorum residue was 0.039 d(-1). To eliminate the adverse effect of the bioreactor effluent on surface water quality, aeration was applied after 90 day operation with 15 L min(-1) air at a depth of 0-30 cm inside the reactor, which led to a reduction of 61.6% dissolved organic carbon liberated into the receiving water without decreasing the removal of nitrate. In addition, the change in chemical oxygen demand and its components can affect the abundance of denitrifying genes, nirK and nirS, in the bioreactor. The results suggest that macrophyte residues can act as an effective solid carbon substrate for the denitrifying bioreactor. The proposed denitrifying submerged macrophyte residues bioreactor is an effective technology for nitrate removal from surface water as well as macrophyte litter utilization.