Pollutant Removal Performance and Operational Responses of a Hybrid Constructed Wetland for Slow-Flowing Waters in the Pinglu Canal, China

Slow-flowing water bodies formed after the cutoff and straightening of large canals have low flow velocity and weak water exchange, allowing nitrogen and phosphorus to accumulate and induce eutrophication. To enhance canal ecological restoration, a hybrid constructed wetland (HCW) system was established in the ecological conservation area of the Pinglu Canal. The system consisted of a grass swale, surface-flow constructed wetland (CW), subsurface-flow CW, and filter, and its performance under different hydraulic conditions and hydraulic retention time (HRT) was evaluated based on the removal of chemical oxygen demand (COD), NH3-N, total nitrogen (TN), and total phosphorus (TP). In the operating-condition comparison, removal rates of all four pollutants were highest during the planted normal-water-level stage. Pollutant removal responses to HRT varied among pollutants and operating conditions and were not consistently monotonic. Considering the observed multi-pollutant removal performance and hydraulic throughput, 48 h was considered a practically suitable HRT under planted normal-water-level conditions within the tested range. Unit contributions varied among pollutants and operating conditions. The subsurface-flow CW mainly contributed to COD removal, the surface-flow CW contributed most to TP removal, and the four units presented functional complementarity. Under the recommended practical conditions (normal-water-level, vegetation, HRT 48 h), removal rates of COD, NH3-N, TN, and TP reached 33%, 91%, 68%, and 58%, respectively. These findings provide preliminary evidence for further evaluation of the tested HCW at the Pinglu Canal study site.