Environmental aspects of using waste waters and sludges in energy forest cultivation

Abstract

From a waste management point of view, energy crops offer the advantage of being non-food crops. Poplar and willow coppice have longer growing seasons and deeper, longer lasting root systems than annual crops, which enables them to have a better utilization of the pulses of mineralised nutrients from waste products. When using waste waters they have the further advantage of a high evapotranspiration. The removal of nutrients from the growth system by harvests is limited, however, and the applied amounts should be regulated accordingly, to avoid leaching of nutrients. The nutrient composition in waste products is often quite different from plant demand. To ensure optimum use of the nutrient resource, this has to be balanced, either by mixing with complementary waste products and/or by adding a mineral fertilizer. Heavy metal contents of waste products for land use are controlled by national regulations. Applications several times higher than what is now allowed has had but little documented effect on the environment due to the binding of metals to soil particles. Willow crops seem to take up relatively large amounts of cadmium and zinc. During combustion all metals except mercury can be concentrated in the ash. The nitrogen content of wood fuels converts mainly to atmospheric nitrogen, but with high temperatures where the combustion is most efficient, there is also some conversion to nitrogen oxides.