High-rate nitrogen removal in a two-stage subsurface vertical flow constructed wetland

A two-stage constructed wetland (CW) system consisting of two vertical flow (VF) beds with intermittent loading operated in series has been investigated. The first stage uses sand with a grain size of 2–3.2mm for the main layer and has a drainage layer that is impounded; the second stage uses sand with a grain size of 0.06–4mm and a conventional drainage layer. The two-stage system was operated with an organic load of 40 gCODm−2d−1 for the whole system, that is with a specific surface area requirement of 2 m2 per person equivalent. The system was operated over a period of 2 years including two full winter seasons. No operational problems occurred, and the Austrian effluent standards for organic matter and ammonium nitrogen could be met during the whole investigation period. Additionally, average nitrogen removal efficiencies of 53% and average nitrogen elimination rates of 2.7 g Nm−2d−1 and 986 g Nm−2yr−1, respectively, could be achieved without recirculation, thus allowing construction with energy-free loading systems. It can be concluded that a better effluent quality can be reached with such two-stage systems as compared to a conventional single-stage VF CW system with intermittent loading, even if the two-stage system is loaded with the double organic load and therefore has only half of the specific surface area requirement.