The Erh-Ren River is one of the most polluted rivers in Taiwan. Although its flow rate is relatively low, the rate is still beyond the capacity of any traditional water treatment facility. A pilot-scale constructed wetland (CW) is the attempt used to purify the highly polluted river water and to collect data for the construction and operation of a full-scale system in the future. This article reports the results from this initial stage of our research work. During the study, the most efficient nutrient removal occurred between April and October. The monthly average removal rates of chemical oxygen demand (COD) ranged from 13-51% of ammonia-N (AN) from 78-100%, and of orthophosphate (OP) from 52-85%. After November, input COD levels increased, and the monthly average removal rates of ammonia-N dropped to 16% and of orthophosphate to 13%. The dramatic changes in removal efficiency suggest that the macrophytes in the CW had a direct influence on the water treatment and that the change of seasons and the quality of the river water inhibited the growth of the macrophytes.