The use of constructed wetlands for the treatment of secondary effluents, to allow for safe river discharge, was the subject of a research project carried out at the Alexander River basin in Israel. Foul pilot-scale units, each about 100 m(2), were built: two were designed for subsurface flow with gravel as the medium and two units were designed for free water surface with local soil as the medium. Secondary treated effluent from the Netarzya sewage treatment plant was used as the influent. To support the field study and broaden the understanding of tire processes involved, two smaller concrete units, 10 m(2) each, were built at the Technion site. Results from the first 6 months of the constructed wetlands operation at the Alexander River site and the Technion site showed that the removal of BOD and SS was very efficient under all the conditions studied: retention time of 4-15 days (hydraulic loading of 20-80 m(3)/0.1 hectare/day) and BOD loading of 1-5.5 kg BOD/0.1 hectare/day: BOD and SS concentrations in treated liquids were always lower than 20 mg/l and most of the time lower than 10 mg/l. However; the removal efficiency of the nitrogen and phosphorous compounds varied within a very wide range; from 95% to as low as zero removal. Copyright (C) 1996 Elsevier Science Ltd.