Potential sewage sludge utilization to fertilize forage surfaces in Galicia

Sewage sludge is an extensively used fertilizer in Galicia, where more than 70% of the sewage sludge produced is used in agriculture. Nevertheless, before using sewage sludge as a source of nutrients for soil, special attention has to be paid to its content of heavy metals, which could be quite high. For this reason, there are European and Spanish regulations that take account of the levels of heavy metals in soils, in sludge and the maximum amount of heavy metals that can be applied for a period of ten years. Based on previous evaluation at the Spanish scale, Zn is the regulated heavy metal with the highest levels in the sludge. In the present study, a survey of around 2600 soils was performed in order to evaluate the levels of heavy metals in soils and the implications of these levels on the potential use of sewage sludge. Sewage sludge could be used in more than 94% of the analysed soils. Ni was the most limiting heavy metal for sludge application, mostly associated with soils that have serpentine rock as the parent material.