Enhancement of nitrate removal at the sediment–water interface by carbon addition plus vertical mixing

• The water–sediment interface has great potential for nitrate removal.
• Adding additional carbon plus mixing significantly increases the nitrate removal.
• Mixing benefits the diffusion across sediment–water interface.
• Wetland microcosm experiment confirmed the efficiency of such simple measure.

Wetlands and ponds are frequently used to remove nitrate from effluents or runoffs. However, the efficiency of this approach is limited. Based on the assumption that introducing vertical mixing to water column plus carbon addition would benefit the diffusion across the sediment–water interface, we conducted simulation experiments to identify a method for enhancing nitrate removal. The results suggested that the sediment–water interface has a great potential for nitrate removal, and the potential can be activated after several days of acclimation. Adding additional carbon plus mixing significantly increases the nitrate removal capacity, and the removal of total nitrogen (TN) and nitrate-nitrogen (NO3−-N) is well fitted to a first-order reaction model. Adding Hydrilla verticillata debris as a carbon source increased nitrate removal, whereas adding Eichhornia crassipe decreased it. Adding ethanol plus mixing greatly improved the removal performance, with the removal rate of NO3−-N and TN reaching 15.0–16.5 g m−2 d−1. The feasibility of this enhancement method was further confirmed with a wetland microcosm, and the NO3−-N removal rate maintained at 10.0–12.0 g m−2 d−1 at a hydraulic loading rate of 0.5 m d−1.