Effects of annual harvesting on plants growth and nutrients removal in surface-flow constructed wetlands in northwestern China

•Large-scale FWS CWs were built to treat polluted river water in northern China.•Annual harvesting of Phragmites australis increased shoot biomass and density.•Annual harvesting improved overall nutrients removal.

The importance of harvest management for the growth and development of plants and nutrients removal in constructed wetlands (CWs) is highly controversial. This study evaluated the effects of annual harvesting on the growth and productivity of local Phragmites australis in northwestern China. Growth characteristics such as shoot density, biomass and height were studied using two pilot-scale surface-flow CWs over a two-year operation period. Plants were kept unharvested in one CW for comparative studies with the second CW, which was harvested at the end of the growing season. Each CW of 400 m2 was operated with a hydraulic loading of 34 m3/d for the treatment of water from an urban river polluted with municipal and industrial wastewater. The harvested CW recorded a higher shoot density (175 shoots/m2), biomass (1.4 kg/m2) and peak height (3.4 m) than the unharvested one (130 shoots/m2, 1.2 kg/m2 and 3.2 m). The overall nutrients removals were also slightly higher for the harvested CW (46.0% TN and 38.1% TP) than the unharvested CW (40.6% TN and 29.1% TP). Plants harvesting in the first year improved nutrients removal by plant uptake (41.9 g N/m2 and 3.7 g P/m2 versus 37.3 g N/m2 and 3.2 g P/m2) as well as in the substrate layer (216.9 g N/m2 and 8.0 g P/m2 versus 191.0 g N/m2 and 5.7 g P/m2) during the second year. Nonetheless, the increase in nutrients removal by harvesting was minimal.