The usability of the IR, RAC and MRI indices of heavy metal distribution to assess the environmental quality of sewage sludge composts

•Heavy metal redistribution during sewage sludge composting in a two-stage system was studied.•Metal stability and environmental risk were evaluated by three indices: IR, RAC and MRI.•Bonding metal intensity, based on IR and RAC was highest for Ni, medium for Cu and Pb, and lowest for Zn.•To estimate the potential ecological risk of compost, the MRI index should be used.

To assess the environmental quality of compost, it is insufficient to use only total metal concentration. Therefore in this study, the stability of metals in compost and the environmental risk they pose were assessed by three indices that have been proposed for soils or sediments: the IR, the RAC and the MRI. In mature composts, the highest bonding intensity was for Ni (0.79 < IR < 0.93), then for Cu (0.56 < IR < 0.65) and Pb (0.55 < IR < 0.73), and the lowest for Zn (0.19 < IR < 0.25). Although, both the IR and the RAC are useful indices for evaluating the mobility of metals, they do not take into account their toxicity. Therefore, the overall classification of compost should also include the MRI, at which metal toxicity from the most available fractions is considered. Based on the MRI ranged between 10.0 and 11.6, all composts evaluated posed a low risk.