Investigations on phosphorus retention in subsurface flow constructed wetlands

62 SSF sand-based constructed wetlands in Germany, Austria and Switzerland have been evaluated for their phosphorus performance. In addition intensive investigations on the removal of phosphorus and its accumulation within the filter bed were conducted at five wetlands. For the performance of horizontal flow (HF) reed beds a regression equation could be derived. This was not possible for vertical flow (VF) reed beds. The best removal efficiency was observed from HF systems with hydraulic loading rates of 10 mm d(-1) or less. 50 percent of all investigated HF wetlands had an average P output concentration of less than 2.1 mg l(-1) in comparison to 3.3 mg l(-1) of VF wetlands. Saturation, seasonal and redox effects could be derived from time series analysis of the performance rate and further investigations of a well monitored VF reed bed. The retention of phosphorus was highest during the warm season. A small positive effect on phosphorus removal was observed with intermittent low redox levels. Soil analyses showed the highest accumulation of P within the influent zone of either HF or VF wetlands.