Nitrogen removal efficiency in sustainable eco-ditches with floating ryegrass mats: The effect of loading (hydraulic and nitrogen) and water level on N removal

Drainage ditches are used for surface water management as well as for the interception of pollutants from farmlands. In this study, ditches with floating ryegrass mat (FRM), Vallisneria natans (VN) and the combination of the two (FRMVN) (including an unamended control ditch) were trialed to evaluate the N interception rates of simulated, low-concentrated run offs at low temperature. The nitrogen removal efficiency of FRMs under low and high-water level (25 cm and 50 cm, respectively) was also investigated during a warmer period. The results showed that the NH4+ removal efficiency was fairly stable for the ditches with FRMs and the interception rates were 30.5% ∼ 46.0% for the low and high influent N concentration (5 mg L−1 and 15 mg L−1) respectively. These NH4+ interception rates were higher than those from the control, especially in the phases of high influent N. The NO3− and TN removal efficiency was kept low for all treatments especially during the cold period likely due to lack of readily available carbon source. Water level had no significant influence to the FRM-based TN, NH4+, and NO3− removal - interception rates though were higher compared to those from the Control ditches during the low N period. In terms of nitrogen budget, the N reduction from FRMs treatments was 8.4% ∼ 16.9% higher than those of the controls due to plant uptake, adsorption, and other pathways. Further improvement in denitrification can be achieved by supplementing cellulosic carbon.