Free water surface wetlands for wastewater treatment in Sweden: nitrogen and phosphorus removal

In South Sweden, free water surface wetlands have been built to treat wastewater from municipal wastewater treatment plants. Commonly, nitrogen removal has been the prime aim, though a significant removal of tot-P and BOD₇ has been observed. In this study, performance data for 3–8 years from four large (20–28 ha) FWS wetlands have been evaluated. Two of them receive effluent from WWTP with only mechanical and chemical treatment. At the other two, the wastewater has also been treated biologically resulting in lower concentrations of BOD₇ and NH₄⁺-N. The wetlands performed satisfactorily and removed 0.7–1.5 ton N ha⁻¹ yr⁻¹ as an average for the time period investigated, with loads between 1.7 and 6.3 ton N ha⁻¹ yr⁻¹. Treatment capacity depended on the pre-treatment of the water, as reflected in the k₂₀-values for N removal (first order area based model). In the wetlands with no biological pre-treatment, the k₂₀-values were 0.61 and 1.1 m month⁻¹, whereas for the other two they were 1.7 and 2.5 m month⁻¹. P removal varied between 10 and 41 kg ha⁻¹ yr⁻¹, and was related to differences in loads, P speciation and to the internal cycling of P in the wetlands.