The operation of constructed wetland (CW) in winter is prone to fail partially due to the vegetation senescence and subsequent nutrient release. Water celery (Oenanthe javanica (Blume) DC), an amphibian vegetable that can grow well in winter and has edible seedlings and leaves, was therefore selected to be planted in a lab-scale free-water-surface (FWS) system in order to purify municipal sewage in winter at a mean water temperature of 7.3-8.3 degrees C. Under the quasi-steady-state conditions at 6- and 2-d hydraulic retention times, pollutant removal through the system was 40-61% for chemical oxygen demand, 17-36% for total phosphorus, 16-30% for total nitrogen, 6-32% for NH3-N, and 82-84% for nitrogen oxides (including NO2-Nand NO3-N). An optimal aspect ratio, i.e., the ratio of the length to the width of the reactor, of 3 was recommended on the basis of the variation of water quality indicators along the waterway of the FWS system. It was demonstrated that water celery played a minor role in both pollutant removal and oxygen release. (C) 2009 Elsevier B.V. All rights reserved.