| Abstrakt: |
Subsurface flow wetlands with a woodchip‐based biofilter are known to act as effective measures in mitigating site‐specific nutrient losses in agricultural drainage. To optimize operation, the combined effect of water temperature, hydraulic residence time (HRT), and filter design is fundamental. This 2‐yr study evaluated a combination of three flow designs and two woodchips mixtures at a test facility receiving agricultural drainage discharge from a 78‐ha catchment. The biofilter test facility consisted of six independent parallel‐constructed wetlands (CW1–CW6), with two types of filter mixtures and three different hydraulic designs (horizontal, vertical upward, and vertical downward flow). Horizontal CWs performed best with an annual removal of total N (TN) amounting to 53 to 54% of the load, corresponding to a daily removal of 2.16 to 2.32 g N m−3 d−1. Removal by the vertical CWs amounted to 38 to 60% of the TN load or 1.19 to 2.31 g N m−3 d−1. Despite low temperatures and HRT down to 3 h, all CWs functioned well in winter with 20 to 30% N removal. In contrast, during summer when the hydraulic load was low and HRT was up to >96 h, N removal was ∼100%, and the reduction sequence proceeded with sulfate reduction and formation of H2S. Statistical analysis and model development revealed that the percentage removal of both TN and nitrate N was mostly dependent on water temperature and HRT, explaining 81.7 to 91.1 and 83.5 to 89.5% of the variation, respectively. Core Ideas: The hydraulic design influences seasonal and annual N removal efficiency.Water temperature and hydraulic residence time control N removal efficiency.Operational models can be used for dimensioning biofilters for N removal efficiency. [ABSTRACT FROM AUTHOR] |
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