Vegetationsfilter för rening av lakvatten - kväveaspekter
Area technology | Water in nature and society
Short rotation energy forests with willows (Salix spp) have come into increased use in Sweden for treatment of different types of waste water, sludge and ash. The vegetation does not only have a cleaning effect on the water, it is also appaerent that the water, sludge or ash often function as fertiliser for the fast growing crop. Leachate water from municipal landfills in the methanogenic phase can be cleaned with short rotation forests through a so called vegetation filter. In a small scale project in 1999 at Rävstatippen in the community of Sigtuna north of Stockholm a landfill in the methanogenic phase was irrigated with leachate water. Two areas, a cultivated area with Salix viminalis (0.1 ha) and a stand of hybrid aspen, Populus triploides (0.1 ha), were irrigated from June to September. The purpose of the project was to determine the capacity of the vegetation filter to accumulate nitrogen and minimise the amount of leachate water formed. Then a valuable wetland next to the landfill could be protected from eutrophication. The movement of water in volume over time was determined both by instrumental mesurement (Flygel) and by the land area and the specific runoff. In April in the area down to the wetland the movement of water over time was 18 l/s and 15 l/s respectively. In June-August 1.4 l/s and 1.2 l/s respectively and for the year 5 l/s and 4 l/s respectively. The loading of nitrogen on the wetland was in June-August 30 kg per month and for the year 110 kg per month. The amount of leachate water irrigated during the period was 529 mm, the real evapotranspiration amounted to 406 mm (576 mm) and the precipitation was only 160 mm (240 mm). The growth of the salix plants during the period of irrigation was very high. The maximum growth speed was 30 cm per week and the highest plants reached 325-375 cm at the end of the period. The stem growth at breast height for the irrigated trees in mean was 4.9 mm in diameter and 3 times higher than the non irrigated trees. The difference in growth between the two groups of was proved significant by statistical analysis. The main explanations for the difference in stem growth is the amount of water reachable for the plants and the high content of nutrients (mostly nitrogen) in the water. Big trees grew better than small ones, which could be anticipated, and the growth was favoured in parcels with organic material mixed with moraine. Nitrogen content in stem wood dry matter after the irrigation period in the treated trees was 0.41 % of DM and in the untreated trees 0.34 % of DM. The total amount of nitrogen uptake was 69.1 kg/ha and 18.6 kg/ha respectively. The increase of nitrogen content in the topsoil due to nitrification in both treated and untreated plots suggest that recycling of inorganic nitrogen was good. Although not measured, it could be assumed that a big part, up to 60 kg/halyear, of the nitrogen has probably left the soil as nitrogenous gas due to denitrification. Nitrogen not taken up by plants or denitrificated has been recycled back into the leachate water. On a long-term basis, the salix plantation needs to increase 7 times to reduce the leachate water from the landfill at Rävsta. The result of this project indicate good possibilities of establishment and potentials for growth of energy forests on Rävstatippen. By an optimised composition of the topsoil, efficient weed treatment and optimised irrigation the vegetation filter is weIl developed. That results in good possibilities for nitrogen trapping from the leachate water and to reduce the amount of the leachate water.