Major interactive components of the constructed wetlands such as aquatic vegetation, substratum, water, microorganisms and animals play significant roles in the treatment of wastewaters. For nitrogen (N) removal, a sequential nitrification/denitrification reaction and plant uptake are two major treatment mechanisms, while NH3 volatilization is considered to be insignificant under the liquid pH lower than 8. In this research, both laboratory- and pilot-scale constructed wetlands were operated in the free water surface (FWS) mode. In order to investigate the role of plant uptake of N, narrow-leave cattails (Typha augustifolia) were planted at the initial density of 35 rhizomes/m2. Based on N mass balance, approximately 50% of the total nitrogen (TN) loaded into the constructed wetlands, operating at the hydraulic retention time (HRT) of 2–10 days, was accounted for by the amount of N assimilation into the plant tissues, resulting in the total kjeldahl nitrogen (TKN) and chemical oxygen demand (COD) removal efficiencies of 40–70% and 71–83%, respectively. To further investigate the role of plant uptake, the cattail plants harvested at the intervals of 2, 4, 8, and 12 weeks of operation resulted in the TN removal efficiencies of 73, 78, 86 and 80%, respectively. The constructed wetland unit having the plant harvesting interval of 8 weeks yielded the N plant uptake of 7.1–7.5 kg/(ha.day) amounting to 66–71% of the TN input. Some biogeochemical parameters such as oxidation-reduction potential (Eh) and dissolved oxygen (DO) in the constructed wetland beds suggested the occurrence of anoxic and reduced conditions which were favorable for the N removal processes such as plant uptake, ammonification, and nitrification/denitrification.