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In southern Sweden, wetlands are constructed to remove nitrogen (N) in agricultural catchments. The possible effects of such wetlands on riverine phosphorus (P) were also estimated using input-output data from three well-monitored wetlands. This was done to formulate a simple model for removal of P that is dependent on inflow characteristics. Next, the N- and P-reducing effects of wetlands were modeled on a catchment scale (1900 km2) using the HBV-NP model and various assumptions about the wetland area and location. All three wetlands functioned as sinks for total P (tot-P) and for total suspended solids (TSS) with a removal of 10% to 31% and 28% to 50%, respectively. Mean P-removal rates of 17-49 kg ha(-1) yr(-1) were well simulated with the model. Catchment scale simulations indicated that wetlands were more efficient (in percentage of load) as traps for P than for N and that this may motivate the construction of wetlands for P removal far upstream from the catchment outlet.

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.