Constructed wetlands in Flanders: a performance analysis

During the last decade, the number of constructed wetlands in Flanders (Belgium) increased exponentially. Extensive data collection resulted in a database of 107 constructed wetlands that was used to evaluate certain trends and treatment performances. Design sizes vary between 1 and 2000 population equivalents (PE), with the majority of reed beds having a size smaller than 500 PE. Most reed beds are used as single treatment units, although they are sometimes also combined with other reed beds or even conventional systems. The main purpose is to treat domestic and dairy wastewater. Average removal efficiencies were lowest with free-water-surface (FWS) reed beds (chemical oxygen demand (COD), 61%; suspended solids (SS), 75%; total nitrogen (TN), 31% and total phosphorus (TP), 26%). The best overall performance was obtained with vertical flow (VF) wetlands (COD, 94%; SS, 98%; TN, 52%; TP, 70%), except for total nitrogen removal where combined reed bed systems even did better (COD, 91%; SS, 94%; TN, 65%; TP, 52%). Despite this considerable achievement in removal, the effluent nutrient concentrations of many systems remain too high and entail a tangible danger of eutrophication. (C) 2004 Elsevier B.V. All rights reserved.