Long-term behaviour of a two-stage CW system regarding nitrogen removal

In the first two years of operation a nitrogen removal efficiency of 53% and a high average elimination rate of 1,000 g N m⁻² yr⁻¹ could be observed for a two-stage vertical flow (VF) constructed wetland (CW) system. The two-stage system consists of two VF beds with intermittent loading operated in series, each stage having a surface area of 10 m². The first stage uses sand with a grain size of 2–3.2 mm for the 50 cm main layer and has a drainage layer that is impounded; the second stage sand with a grain size of 0.06–4 mm and a conventional drainage layer (with free drainage). The two-stage VF system was designed for and operated with an organic load of 40 g COD m⁻² d⁻¹ (i.e. 2 m² per person equivalent). Data from the following years of operation showed that from the third year nitrogen elimination increased and stabilized. The median values of the nitrogen elimination rate in the first five years of operation have been 3.51, 2.76, 4.20, 3.84 and 4.07 g N m⁻² d⁻¹, the median value of the last three years being 3.8 g N m⁻² d⁻¹ and 1,380 g N m⁻² yr⁻¹, respectively, and the nitrogen removal >60%. It can be assumed that the vegetation as well as the biofilm development in the two-stage VF CW system plays the major role for the enhancement of the nitrogen elimination rate.