A study looked at how well a wetland in Lianhua Lake cleans up sewage. They found that the wetland is good at removing pollutants like COD, TN, and TP. Vertical subsurface flow wetlands are better at cleaning up COD than horizontal ones. Subsurface flow wetlands are better at removing COD than surface flow wetlands. Down-flow wetlands are not as good at removing TN. Vertical subsurface flow wetlands are better at removing TP than horizontal ones. Surface flow wetlands are best at removing TP. Wetlands work best at cleaning up COD and TP in August and September, and TN in July, August, and late September. In late October, the wetland's ability to remove TP decreases.