Long-term performance of a denitrifying bioreactor for the treatment of nitrate-laden agricultural drainage water in northeastern Germany

Denitrifying bioreactors are a promising end-of-pipe technology to reduce nitrate losses in the receiving surface waters in agricultural tile-drained landscapes. However, this technology has been barely tested over longer time periods and under Northern German weather conditions. A denitrifying bioreactor was installed at a 2.9 ha tile-drained field alongside a brook in 2017. The bioreactor is 20 m long, 3.75 m wide and 2.2 m deep. We recorded weather data (precipitation and air temperature), water temperature inside the woodchip filter and flow rates at 15-min time intervals. Additionally, water samples were taken from the influent and effluent several times a week and analyzed for nitrate, dissolved reactive phosphorus (DRP), total phosphorus (TP) and total organic carbon concentration (TOC). After seven monitoring seasons (November to April), the main results can be summarized as follows. Nitrate removal efficiency and nitrate removal rate were on average 59 % and 5.5 g N m⁻³ d⁻¹, respectively, indicating that microbial denitrification takes place in winter at water temperatures <8 °C. After initial flushing, the bioreactor acted also as a sink for phosphorus (ca. 61 % TP load reduction). However, as a negative side effect, the bioreactor released organic carbon and the TOC load increased by ca. 59 % at the outlet as compared to the inflow. The results indicate further that the hydraulic retention time is crucial for an optimal management of denitrifying bioreactors. Increasing the hydraulic retention time leads to greater nitrate removal efficiencies and vice versa. In contrast, water temperature fluctuations throughout the drainage season did not influence the performance of the bioreactor. Overall, denitrifying bioreactors have the potential to significantly reduce nitrate pollution in hot-spot areas of agricultural used tile-drained landscapes.