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In order to investigate the effect of temperature, hydraulic residence time (HRT), vegetation type, substrate material and wetland shape on the performance of free-water surface (FWS) constructed wetlands treating wastewater, 5 pilot-scale units were constructed and operated continuously from December 2004 until March 2007 in parallel experiments. Four of the units (A, B, C, D) were rectangular in plan view with dimensions 3.40m in length and 0.85m in width, and contained substrate material at a thickness of 0.45m. The fifth unit (E) had a trapezoidal plan view shape, with a width at the inlet of 1.15m and at the outlet of 0.55m, while the length and the thickness of the substrate were the same as in the other four. All units operated at a water depth of 0.10m. Units B–E contained clay substrate and unit A contained sand. The four units with clay were planted as follows: two with cattails (B and E), one with common …