Sewage purification of multi-level composite constructed wetlands under low temperature in the winter in northern China：case of Heituwa wetland of the Guanting Reservoir

To address the challenge of overwinter operation for large artificial wetlands in northern China, in this study the Heituwa constructed wetlands of Guanting Reservoir was examined using a “stable pond + constructed wetland” combination process, to investigate water purification under low-temperature conditions from 2008-2020, incorporating continuous inflow, operating under ice cover, and maintaining wetland macrophytes． Water from the Yongding River was found to meet Class Ⅲ surface water standards after treatment by the Heituwa constructed wetlands． Removal rates for TN, \mathrmNH_4^+ -N, BOD₅, TP and orthophosphate were (30.84±25.74)%, (60.14±23.80)%, (36.84±32.16)%, (39.47±34.65)%, and (48.92±34.63)% respectively． The stabilization pond effectively degraded nitrogen and phosphorus nutrients, while the constructed wetland performed well removing \mathrmNH_4^+ -N, COD_Mn and BOD₅． Different types of constructed wetlands had their own advantages, the purification effect in terms of water quality in constructed wetlands was as follows: subsurface flow wetland zone Ⅰ > subsurface flow wetland zone Ⅳ > surface flow wetland． The combination process of “stable pond + constructed wetland” exhibited significantly high pollutant removal efficiency under low temperature in winter, providing a reference for the construction and operation management of wetlands in northern China．