A study was conducted to see how well a constructed wetland system could clean polluted water in cold winter conditions in northern China. The researchers found that a wetland system with insulation could work effectively in winter. The system removed around 65% of BOD5, 50% of nitrogen, and 35% of phosphorus from the water. However, the removal rates decreased when the water flow rate was too high. This shows that constructed wetlands can help clean polluted water even in winter.