Sewage sludge and ecological risk of heavy metals

In the European Union (EU), a common understanding of the potential harmful effect of sewage sludge (SS) on the environment is regulated by the Sewage Sludge Directive 86/278/EEC (SSD). The SS Directive (SSD), with its amendments and updates, regulates the use of SS in agricultural land and sets rules on the type of treatment and sites where SS is used, how farmers can use SS as a fertilizer and soil improver, on the regular recording and reporting of the sludge quantities produced and applied. In 2018, Decision (EU) 2018/853 amended the SSD regarding procedural rules in environmental reporting. In 2019, the SSD was amended by Regulation (EU) 2019/1010, which aligns and streamlines the reporting requirements in environmental legislation. The SSD aims to increase SS use in agriculture while preventing harm to the environment and human health by ensuring that heavy metals in soil and sludge do not exceed set limits.

Limit values (LVs) for concentrations of heavy metals in soil are listed in Impact Assessment of this directive, and they were transposed by EU member states using different criteria. Member states adopted either single limit values or based on soil factors such as pH and texture to define the maximum limit values for concentrations of heavy metals in soils. Our work presents the first quantitative analysis of the SSD at the European level by using the Land Use and Coverage Area Frame Survey (LUCAS) 2009 topsoil database. The reference values at the European level were arranged taking into account the upper value (EU_UL) and the lower value (EU_LL) for each heavy metal (arsenic, cadmium, copper, chromium, mercury, nickel, lead, and zinc) as well as taking into account the pH of the soil (cadmium, copper, mercury, nickel, lead, and zinc) as introduced in the SSD Annex IA. Single and integrated contamination rate indices were developed to identify those agricultural soils that exceeded the reference values for each heavy metal. In total, 10%, 36%, and 19% of the LUCAS 2009 topsoil samples exceeded the limit values. Additionally, 12% and 16% of agricultural soils exceeded the concentration of at least one single heavy metal when European LVs were fixed following the soil pH in Strategy II compared to those national ones in Strategy I. Generally, all member states apply similar or stricter limit values than those laid down in the SSD. Our work indicates that choosing LVs quantitatively affects further actions such as monitoring and remediation of contaminated soils. The actual soil parameters, such as heavy metal concentrations and soil pH values from the LUCAS 2009 topsoil database, could be used by SSD-involved policy stakeholders not only to lay down the LVs for concentrations of heavy metal in soils but also for monitoring the SSD compliance grade by using the LUCAS surveys over time (past and upcoming LUCAS datasets).

In terms of heavy metals, the list of annexes in the SSD reports the specific LVs (LVs) of heavy metals (HMs) in the soil (Annex IA), maximum concentrations of HMs in sludge (Annex IB), and maximum annual quantities of HMs that may be added to the soil (Annex IC). JRC developed a methodology which contributes to the analysis of the implementation grade of the EU Sewage Sludge Directive by using the actual concentration of heavy metals in agricultural soils of Europe from the Land Use/Cover Area Frame Statistical Survey (LUCAS) 2009 topsoil survey. The quantitative analysis of the compliance grade of the SS Directive in Europe has been performed in this work by using the concentrations of heavy metals (As, Cd, Cu, Cr, Hg, Ni, Pb, and Zn). The use of a single contamination rate index (CRI) and integrated Contamination Index (OCRI) not only identifies the agricultural soils exceeding the LVs and no sludge could be applied but also quantifies the grade of contamination to predict warning agricultural soils where further analysis and monitoring activities should be implemented before application of sludges.

 Accumulation of HMs in soils was modelled by using a representative SS composition, distributed over 10 successive years at 5 Mg ha−1 year−1 rate. Ecological risk impact was assessed by using both the single ecological risk index (Er) and the integrated potential ecological risk index (RI). Maximum quantities of SS applied on agricultural soils in EU + UK were estimated to be 45 Mg ha−1 at the country level. We found that 19% of agricultural land (around 28,471,900 ha) in the EU + UK shows a higher RI than moderate risk after long time application of the representative SS. We show that the combination of the HM concentrations from the LUCAS topsoil survey and assumptions on the SS composition and soil HM partitioning can be used to define the actual and potential soil pollution rate in EU + UK.

Data available : Risk of heavy metals from application of sewage sludge

References:

Yunta, F., Schillaci, C., Panagos, P., Van Eynde, E., Wojda, P. and Jones, A., 2024. Quantitative analysis of the compliance of EU Sewage Sludge Directive by using the heavy metal concentrations from LUCAS topsoil database. Environmental Science and Pollution Research, DOI: https://doi.org/10.1007/s11356-024-31835-y

Yunta, F., Schillaci, C., Panagos, P., Van Eynde, E., Wojda, P. and Jones, A., 2024. Ecological risk assessment of heavy metals from application of sewage sludge on agricultural soils in Europe. European Journal of Soil Science, 75(5), p.e13562.