Risk-based environmental assessment of the use of ten products from bottom ash treatment plants based on leaching and ecotoxicological tests.

Bottom ash (BA) treatment for metal recovery is widely implemented in European waste-to-energy plants. The resulting mineral fractions are increasingly recycled, mainly in construction applications. However, in many countries including Italy, assessments for verifying the environmental compatibility of the products are mainly performed through leaching compliance tests and comparing the results to generic limit values, which do not reflect actual use conditions. This study applies an integrated, risk-based assessment approach for ten mineral fraction samples produced by four full-scale Italian BA treatment plants. Standardized pH-dependence, up-flow percolation and batch leaching tests were performed on each type of sample, as well as aquatic and terrestrial ecotoxicological assays to evaluate the environmental compatibility of the materials for different use scenarios. Results showed that the groundwater application (free use) scenario was consistently critical, with exceedances for several contaminants (e.g. Al, Cr, Cu, Mo, Pb and Sb) and ecotoxicological effects observed for all samples. Conversely, for large-scale unbound applications, only limited exceedances were identified, generally within a factor of 1-3 relative to the risk-based thresholds, and mainly associated with chromium release at low liquid-to-solid ratios. Ecotoxicological results confirmed this trend, with only minor deviations (up to 1.5 times the acceptable dilution factor). No exceedances of human health risk-based limits or ecotoxicological effects were observed for applications such as road sub-base use. Overall, a good agreement was found between the leaching-based and ecotoxicological assessments, indicating that both approaches capture consistent risk patterns. The results demonstrate that, under realistic application conditions and without direct groundwater contact, the investigated materials can be safely used, supporting the adoption of scenario-based evaluation frameworks instead of generic compliance criteria.