Sludge-based biosolids can be used for crop fertilization and for soil enrichment with organic matter. The prerequisites for their application are laid out in 86/278/EEC which sets limits for metals of toxicological concern in the biosolid and in the recipient soil. In this context, three kinds of sludge-based biosolids from a municipal wastewater treatment plant were submitted to the leaching tests EN 12457-2 and ΝΕΝ 7341. The leachates were examined for their physicochemical, chemical, and ecotoxicological characteristics four times within a year. Even in the more aggressive metal leaching method (ΝΕΝ 7341), results were much lower than the limits set in Council Directive 86/278/EEC. Correlations were noted between numerous parameters and principal component analysis grouped them in 3 components explaining 76% of total variation. These leaching tests were also performed on soil-biosolid samples from another investigative research in which two of the three kinds of biosolids had been incorporated in corn fields at doses 0, 20, and 40 tn/ha. EN 12457-2 leaching tests indicated that Ni and Cd leaching was affected by biosolid incorporation in soil, in relation to the type of the receiving soil (clay or sand). The leachates from the soil-biosolid mixtures were practically not toxic for the organisms tested (D. magna, V. fischeri, higher plants). In summary the biosolids were considered of low environmental risk when utilized in the field. A detailed risk assessment encompassing both chemical and ecotoxicological analysis is necessary for integrated evaluation of biosolids.
Keywords: Bioassays; Biosolids; Metals; Risk assessment; Sludge.