From wood to waste and waste to wood

In this thesis the flow of elements from the forest to the pulp mill and the possibility to recycle nutrients in solid pulp-mill residues back to the forest have been studied. To get improved estimations of element removal at whole tree harvesting (WTH), the changing element concentrations with changing diameter of Picea abies stems were investigated. The results showed that element concentration for Ba, Cd and Pb in wood and Ba, Ca, Cd, Co, Mn, Sr and Zn for bark were significantly positively correlated with diameter whereas Cu, Fe, K, Mg and P in wood and Cr, Cu, Fe, K, Na, Ni and P in bark were negatively correlated. In order to test the recycling potential of different solid waste products a mass-balance study was made for three pulp mills in Sweden. Bark-ash had the best composition of plant nutrients but high concentrations of some heavy metals. GLD and lime mud contain less nutrients, except Ca and Mg. Mixing of bark-ash and GLD can be a way to improve nutrient composition and achive a high degree of recycling. Pelleting and heat treatment of ash and GLD was evaluated as a way to get stable pellets with long-lasting effect in the field. Pelleting decreased the reactivity of the pellets, resulting in less effect on pH and low leaching rates of elements. The heating of ash pellets gave a decreased surface area and decreased reactivity in the soil. Mixing bark-ash with GLD resulted in an increased leaching of DOC and DON. Heating of GLD pellets increased pH significantly in the leachate due to formation and subsequent dissolution of MgO. The pre-treatments did not decrease the rapid leaching of K and Na from the pellets. There was no tendency for increased N or C mineralisation in the mor layer for any pellet type. Thus, when pure ash pellets are used, the risk of high N losses from mor layers in podsolised spruce stands after ash application is small, even under disturbed conditions. However, more caution should be taken with mixtures containing GLD, which show a greater interaction with the mor. Heating of GLD pellets should be avoided