# Risk Assessment of Soil Contamination with Heavy Metals from Municipal Sewage Sludge
## 4 Conclusions  
The tested sewage sludge was characterised by a high zinc content in relation to the applicable legal regulations. However, analysis of the potential environmental risks has shown that despite exceeding the limit values for concentrations of heavy metals in sewage sludge, its use for natural purposes does not necessarily involve a high risk of environmental contamination. A key role is played here by a speculation analysis of heavy metals in sludge. The mobility of HMs is important in terms of the possibility of natural or agricultural use of sewage sludge. Consideration must be given to the extent to which heavy metals in sewage sludge are permanently bound, and whether there are external factors that can influence the change in the speculative fractions in which they are currently found.  
Figure 5: The environmental risk factor (ERF) of HMs in SS.  
Figure 6: The environmental risk determinant (ERD) indicator of HMs in SS.  
The \(I_{geo}\) showed a very high or high risk of organic contamination with zinc. On the other hand, the PERI for all three sludge samples showed a very high risk of cadmium contamination, while for zinc it was moderate and high. The RAC for all metals showed mostly a medium to low risk. For zinc, it was low in each case. The environmental risk factor analysis showed low levels of contamination for all three heavy metals, except for zinc for S1 and cadmium for S2.  
The authors' own indicator, the ERD, is also based on the issue of metal mobility. It determines the content of a given element from the group of heavy metals, depending on its content in four fractions. It takes into account only three fractions (FI, FII, and FIII), with the highest value of potential contamination assigned to metals contained in the FI fraction and the lowest in the FIII fraction. Zinc in all three sewage sludge samples showed a low risk of contamination. The highest indicator level for zinc was obtained for S1. However, the ERD indicator did not show the average risk when compared to the ERF indicator, due to the fact that the ERF indicator treats the FI and FII fractions equally.  
The heavy metal content of FII and FIII fractions, which are conditionally mobile, cannot be ignored. However, they cannot be treated in the same way as the metals in the most mobile FI fraction. The proposal to use the indicator suggested by the authors is a response to the differences in the mobility of heavy metals depending on the form in which they occur in sewage sludge.  
Conceptualization, [PERSON] and J.G.; methodology, [PERSON] and R.K.; formal analysis, J.L., J.G. and R.K.; investigation, [PERSON] and R.K.; writing--original draft preparation, J.L. and R.K.; writing--review and editing, J.L.; visualization, J.L. and R.K. All authors have read and agreed to the published version of the manuscript.  
This research and the APC were funded by the Polish Ministry of Science and Higher Education, The Programme of the Polish Ministry of Science and Higher Education--the Regional Initiative of Excellence on the basis of the contract number 025/RID/2018/19 of 28 December 2018; the amount of funding: 12 million PLN.  
Not applicable.  
The authors declare no conflict of interest.