The role of willow plants in the treatment of iron-rich landfill leachate

Leachate arising from closed landfill sites, which often contain high concentrations of ammonia and iron, must be contained and treated in order to avoid pollution of ground and surface waters. At one such landfill site (Nant Mel) in Powys, Wales, leachate is treated on site, firstly in an aeration tower and subsequently by passing through a series of gravel beds planted with reeds or willow trees. Aeration is effective in removing a large proportion of the dissolved iron as a solid precipitate. More iron is removed by deposition within the gravel of the reed beds and ammonia is removed by oxidation to nitrate. This paper examines the effectiveness of three gravel-filled tanks planted with willow cuttings to provide tertiary treatment of leachate. After passing through the tanks there was little change in various water quality parameters except dissolved oxygen and iron. Willow tree growth showed a marked decline between the inlet and outlet ends of all three tanks. Possible reasons for this, and the implications for leachate treatment, are discussed. It is concluded that gravel beds with reed and willow plants offer an effective, low-cost method of in situ treatment of leachate, particularly in isolated rural areas such as mid Wales.