# Risk Assessment of Soil Contamination with Heavy Metals from Municipal Sewage Sludge
## 3 Research Results and Discussion  
Table 8 shows the results of the speciation analysis of heavy metals in sewage sludge. The cadmium, nickel, and copper content of the sewage sludge from all three wastewater treatment plants did not show any values exceeding the acceptable limits given in the literature [8; 9; 10; 11; 12; 13; 14]. For chromium, this value was exceeded only for S2, as the value was 2760.3 mg\(\cdot\)kg\({}^{-1}\) d.m., and was several times higher than the limit values. A high lead content was found in S1, equalling 427.10 mg\(\cdot\)kg\({}^{-1}\) d.m. All tested sewage sludge, on the other hand, was characterised by a high zinc content, exceeding the limit values indicated in the regulation of the Minister of the Environment on municipal sewage sludge [9]. Zinc belongs to the migrating elements, and has a relatively low degree of toxicity to humans and animals. However, high doses of zinc cause damage to many biochemical processes, and it is deposited in the kidneys, the liver, and genital glands [45]. When assessing the value of the \(I_{geo}\) of heavy metals in the soil, it can be concluded that the dominant heavy metals causing a high risk of contamination are cadmium, zinc, and chromium (Figure 2). Zinc, the content of which exceeded the limit values outlined by legal acts, reached a very high level of potential risk in all three treatment plants. It should be taken into account that the \(I_{geo}\) compares the heavy metal content of the sewage sludge to that of potentially used soils. The difference between the two values was very high due to the low values of heavy metals in soils. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Heavy Metal** & **Fraction I** & **Fraction II** & **Fraction III** & **Fraction IV** & \(\sum\)**F1\(\div\) F4** \\ \hline \multicolumn{6}{c}{Sewage sludge—**S1**} \\ Cu & 0 \(\pm\) 0.1 & 0 \(\pm\) 0.1 & 14.6 \(\pm\) 0.9 & 6.5 \(\pm\) 0.4 & 21.1 \\ Cr & 13 \(\pm\) 0.9 & 4.2 \(\pm\) 0.2 & 29.4 \(\pm\) 1.6 & 59.1 \(\pm\) 2.3 & 105.7 \\ Cd & 1.2 \(\pm\) 0.1 & 1.5 \(\pm\) 0.1 & 3.7 \(\pm\) 0.1 & 3 \(\pm\) 0.1 & 9.4 \\ Ni & 2 \(\pm\) 0.2 & 0.9 \(\pm\) 0.1 & 6.1 \(\pm\) 0.5 & 7.7 \(\pm\) 0.6 & 16.7 \\ Pb & 2.5 \(\pm\) 0.2 & 0 \(\pm\) 0.2 & 16.2 \(\pm\) 0.3 & 408.4 \(\pm\) 9.1 & 427.1 \\ Zn & 509.9 \(\pm\) 9.0 & 447.3 \(\pm\) 9.2 & 1119 \(\pm\) 15.0 & 693.2 \(\pm\) 8.4 & 2770.0 \\ \multicolumn{6}{c}{Sewage sludge—**S2**} \\ Cu & 0 \(\pm\) 0.1 & 0 \(\pm\) 0.1 & 9.5 \(\pm\) 2.7 & 12.3 \(\pm\) 1.9 & 21.8 \\ Cr & 5 \(\pm\) 0.4 & 2.3 \(\pm\) 0.1 & 1284 \(\pm\) 12 & 1469 \(\pm\) 47 & 2760.3 \\ Cd & 0.3 \(\pm\) 0.1 & 0.7 \(\pm\) 0.2 & 1.1 \(\pm\) 0.3 & 10 \(\pm\) 1.7 & 12.1 \\ Ni & 4.5 \(\pm\) 0.1 & 1 \(\pm\) 0.1 & 14.1 \(\pm\) 0.3 & 8.9 \(\pm\) 0.1 & 28.5 \\ Pb & 0.7 \(\pm\) 0.2 & 1 \(\pm\) 0.2 & 0 \(\pm\) 0.1 & 29.6 \(\pm\) 7.8 & 31.3 \\ Zn & 152.9 \(\pm\) 0.7 & 144.5 \(\pm\) 0.2 & 537.5 \(\pm\) 23.6 & 4516 \(\pm\) 91.0 & 5351.0 \\ \multicolumn{6}{c}{Sewage sludge—**S3**} \\ Cu & 0.9 \(\pm\) 0.2 & 1.2 \(\pm\) 0.2 & 124 \(\pm\) 0.8 & 69.9 \(\pm\) 0.6 & 196.0 \\ Cr & 2.1 \(\pm\) 0.2 & 0.9 \(\pm\) 0.1 & 78.1 \(\pm\) 0.8 & 61.5 \(\pm\) 0.5 & 142.6 \\ Cd & 0.1 \(\pm\) 0.1 & 0.3 \(\pm\) 0.1 & 0.9 \(\pm\) 0.2 & 1.1 \(\pm\) 0.3 & 2.4 \\ Ni & 7.9 \(\pm\) 0.3 & 3.2 \(\pm\) 0.2 & 9.8 \(\pm\) 0.5 & 23 \(\pm\) 0.9 & 43.9 \\ Pb & 7.8 \(\pm\) 0.8 & 1 \(\pm\) 0.2 & 4.5 \(\pm\) 0.5 & 43.5 \(\pm\) 4.5 & 56.8 \\ Zn & 79 \(\pm\) 1.0 & 275 \(\pm\) 3.0 & 1491 \(\pm\) 21.0 & 932 \(\pm\) 82.0 & 2777.0 \\ \hline \hline \end{tabular}  
* \(\pm\) standard deviations.  
\end{table} 
Table 8: Chemical speciation of heavy metal * in sewage sludge, mg\(\cdot\)kg\({}^{-1}\). The values of the PERI indicate that the main element in the heavy metal group causing a very high PERI value in all three sewage sludges is cadmium (Figure 3). Chromium of S2 and copper of S3 also showed very high potential environmental risk values. Zinc from S1 showed a moderate level of risk, and high for the other two.