Sludge obtained from water purification plants can be reused, as fertiliser for soils, for example or to reduce their acidity. The main aim of this University of the Basque Country (UPV/EHU) PhD research thesis was to study the effects of the application of EDAR (Estación Depuradora de Aguas Residuales - Waste Water Purification Plant) waste sludge on the chemical properties of the soil and on the water filtering through it — with special attention being paid to the behaviour of heavy metals. Moreover, the effects on forest plantations — concretely those of Pinus radiata —, have been studied. The author of the thesis is Dr. Goio Egiarte Castañeira, who presented it with the title, . Dr Egiarte is an agricultural engineer currently working as a secondary school teacher. He undertook his PhD under the direction of Ms Estilita Ruiz Romera from the Department of Chemical and Environmental Engineering at the Higher School of Engineering of the UPV/EHU and Ms Marta Camps Arbestain from the Department of Agricultural Systems and Natural Resources at Neiker-Tecnalia. When carrying out the research for the thesis, Dr. Egiarte had the help of the Dipartimento de Science Ambientali e delle Produzioni Vegetali at the Università Politecnica delle Marche in Ancona (Italy), of the Department of Pedology and Agricultural Chemistry at Biology Faculty of the University of Santiago de Compostela (Galicia) and the Bilbao Bizkaia Water Consortium. The study approach for the thesis was particularly innovative, not only for the possible use of fertilising sludge on forest soils, but also because it studied the effect of heavy metals and nitrogenated forms. To this end, experiments were carried out over a four-year period. With the results obtained from these experiments, Dr. Egiarte has come to the conclusion that adding high doses of EDAR sludge is totally inadvisable, even when the sludge complies with current norms as regards levels of heavy metals. When the compounds of the sludge begin to oxidise, they produce acidity and, in order to reduce this acidity, an alkaline system is necessary. But the alkanility of the system created on uniting the sludge with the soil is not capable of neutralising the acidity generated. On the other hand, the application of much lower doses of waste sludge — three times less —considerably reduces acidification of the soil and heavy metal content. Moreover, the production of forest biomass is very similar to that obtained by applying higher doses. Nevertheless, Dr. Egiarte emphasises that the use of other types of sludge from waste water purification plants — with a greater capacity of neutralisation of acids and a greater stability of organic elements — on acid soils, such as the one studied, is much more recommendable.