Analysis of potentially toxic metal constraints to apply sewage sludge in Portuguese agricultural soils.

The application of sewage sludge (SS) in the soil can be a valuable way to increase its content of organic matter. However, the concentration of potentially toxic metal (PTM) in both SS and soil can hinder this route of management. Thus, the main objective of this work was to evaluate the compliance with the restrictions related to PTM contained in SS from wastewater treatment plants (WWTP) for agricultural land application. The regulatory constraints associated with SS and soil in respect to PTM (Cd, Cr, Cu, Ni, Pb, and Zn) were analyzed. These metals showed a deleterious effect on germination of Lepidium sativum seeds, and their phytotoxicity may be ranked as Cd > Cu > Cr(VI) > Cr(III) ~ Zn ~ Ni > Pb. Portuguese SS samples from different WWTP, from a national sludge management operator, and from the literature were considered. The results revealed that the content of these metals, in general, complies with the regulatory threshold values. The content of PTM in the soil is not restrictive to receive SS in at least 90% of the national territory. The assessment of ecological risk based on the geoaccumulation index (I), pollution index (PI), and potential ecological risk index (PERI) showed low risk for all metals. The exception was I of Cd, Cu, and Zn, which presented moderate to high level of pollution. According to the state of the art, no significant negative impacts have been detected on human health and the environment due to SS applications in the soil. Thus, in a country with low carbon content in the land and whenever compliance with regulations is achieved, the main route for SS management may be agricultural soil.