Assessment of metals in dry-toilet collected matters from suburban areas of Ulaanbaatar, Mongolia, using biosolids quality guidelines and potential ecological risk index

Abstract

Dry-toilet collected matter (DCM) from traditional dry-toilet pits are a potential health and ecological risk in suburban areas. In this study, the characteristics of metals in DCMs from suburban areas of Ulaanbaatar were surveyed. The results indicate that DCMs contain a high percentage of organic matter and nutrients, while heavy metals are at low levels, which shows good agricultural potential. The concentration ranges of Cr, Cu, Hg, Ni, Pb, and Zn were 11±5, 46±9, 0.08±0.05, 9±3, 17±9, and 338±86 mg·kg−1, respectively. The concentration of Cd was below 0.5 mg·kg−1, and a high positive relation was shown between chromium and nickel concentrations. The heavy metals in DCMs were safe for land application but Zn in DCMs was close to the effects range median (ERM), which is toxic in some cases, such as amphipod bioassays. Because it is mandatory to treat DCMs to reduce pathogens, in the case of heavy metal enrichment and agricultural reuse, composting or pyrolysis are better choices than incineration. Compared with global soil background values, the heavy metals in DCMs showed a low level of ecological risk, but a medium level when compared with Mongolian soil background values. The ecological risk of six heavy metals was in the descending order Hg>Cu>Zn>Pb> Ni>Cr and the contribution rate of Hg exceeded 60%.