Abstract
The wetland systems with emergent plants such as reeds and cattails are classified into free water surface systems and subsurface flow systems. In order to estimate the natural purification potential of the natural wetlands with free water surface systems, we investigated the natural wetland for four years which had been recieving the gray water from the residential area which consits of 45 households for twelve years. The removal rate of organic matter in the wetland ranged from 80% (COD) to 95% (BOD) throughout four years. However, the removal rate of nitrogen was lower than that of organic matter. NH4-N release from the bottom sediment and repression of nitrification in the wetland system seem to be responsible for the low removal rate of nitrogen in winter.
