Effect of stone filters in a pond–wetland system treating raw wastewater from a university campus

Dissolved biochemical demand (BOD), total organic carbon (TOC) and NH4-N were analysed in seven sampling points in a pond–wetland system treating raw wastewater from 2001 to 2005 to determine the efficiency of its constructive elements. The wetland consists of horizontal flow stone filters, free water wetlands and a final subsurface flow wetland. Elimination (g d-1) and elimination efficiency (g d-1 m-2) were calculated for each constructive element. The highest BOD elimination was achieved by the pond (1714 g d-1), particularly when the highest BOD loads were applied. However, the highest BOD elimination efficiency was given by the first stone filter (45 g m-2 d-1). Although NH4-N elimination was highest in the pond and the horizontal subsurface flow constructed wetland with 432 and 104 g d-1, respectively, the stone filters achieved much higher elimination efficiencies than the other elements, about 15.8 g m-2 d-1 each. These results can be explained in terms of the combination of high bacterial activity in the stone filters with higher oxygenation in the pond and surface flow wetlands, and suggest that NH4-N elimination can be improved by combining short horizontal surface and subsurface flow wetlands instead of using only a longer subsurface flow constructed wetland.