Investigations on Nitrogen Removal in a Two-Stage Subsurface Vertical Flow Constructed Wetland

A two-stage constructed wetland (CW) system consisting of two vertical flow beds with intermittent loading operated in series has been investigated. The first stage uses a grain size of 1–4 mm for the main layer and has a drainage layer that is operated under saturated conditions; the second stage uses a grain size of 0.06–4 mm and a drainage layer with free drainage. The two-stage system was operated with an organic load of 80 g COD m^–2 day^–1 for the first stage (1 m² per person equivalent), i.e. 40 g COD m^–2 day^–1 for the whole system (2 m² per person equivalent). The system has been investigated using indoor pilot-scale CWs. The results of the indoor experiments are currently verified outdoor. Organic matter and ammonia nitrogen (NH₄-N) removal has been very high and has been shown to be the same as for a single-stage CW system (grain size for main layer 0.06–4 mm, free drainage, organic load 20 g COD m^–2 day^–1, i.e. 4 m² per person). A total nitrogen elimination rate of 58% could be reached for the two-stage indoor system compared to almost no nitrogen elimination for a single-stage system. The first results of the outdoor experiments are similar compared to the indoor experiments regarding organic matter, nitrogen, and microbial parameters. The nitrogen elimination reached in the first 3 months of operation was 63%. By using the two-stage CW system it is possible to obtain a higher effluent quality compared to a standard single-stage CW system with only half of the specific surface area requirement. Nitrogen removal efficiencies of about 60% and nitrogen elimination rates of more than 1,400 g N m^–2 year^–1 can be reached without recirculation.