The use of horizontal subsurface flow constructed wetlands (CW SSF) for the treatment of municipal wastewater in small communities is becoming a commendable alternative to conventional wastewater management in a society led to the sustainable living. From 2001 to 2003, the Environmental Engineering Division of the Hydraulics Department of the UPC Barcelona Tech had carried out an experiment on CW SSFs in the small town of Les Franqueses, Spain. The CW SSFs were connected in parallel to a wastewater source so that each CW SSF received the same discharge along the observation time. The CW SSFs were designed with different aspect ratios (ratio “length : width” of each CW SSF): A(1:1), B(1.5:1), C(2:1) and D(2.5:1). In type A CW SSFs, water depth was 0.5 m; in type B CW SSFs it was 0.5 m in 2001 and 2002 and changed to 0.27 m in 2003; in type C CW SSFs, it was 0.5 m and for CW SSFs D it was 0.27 m. For each of the CW SSF groups described, there were 2 subgroups with different substrate medium sizes (D60) and porosities ( ): subgroup 1 had a substrate medium size of 10 mm and a porosity of 39% while subgroup 2 had a substrate medium size of 3.5 mm and a porosity of 40%. Additionally, a variety of hydraulic load rates (HLR) were applied in order to test HLR effects on CW SSF water depuration: 20, 27, 36 and 45 mm/day. Concentrations of affluent and effluent flow were taken in 824 measurements. This data was analyzed by J. Garc´ia et al. (2005) using conventional statistics. Compositional data analysis and linear models are used in the present analysis, which is meant to provide complementary information to the analyses carried out by García. The results determine that the most efficient CW SSFs have the following features: Aspect ratio equal to 2.5:1, porosity equal to 40 % (substrate medium size equal to 3.5 mm), depth equal to 0.27 m and HLR equal to 20-27 mm/day. Temperatures around 5-15 C (mild weather) are also recommended

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