## 5 Conclusions  
The direction of the quantitative changes in Cu, Zn, and Ni in the separated fractions and bioavailable contents was varied and depended on the experimental factors and the chemical nature of the metals. The used research methodology significantly presented the changes occurring during the successive stages of composting, and the indicators introduced had an important impact on the assessment of the environmental impact of the tested composts. Regardless of the composition of the mixtures, the amounts of Cu, Zn, and Ni generally increased for the majority of fractions during composting. The most substantial increment in copper content was evident in fraction I of C1 during the thermophilic phase (191.76%), as well as in C3 during the zinc bioavailability phase (75.13%) and in C3 within nickel fraction III (93.87%). Regardless of this, it should be emphasized that, throughout the composting process, the shifting levels of metal contents exhibited diverse trends, including both positive and negative values. Specifically, these variations were observed in C1 for Cu Fr I, Zn Fr IV, and bioavailable amounts, and Ni Fr I and II; C2 for Ni Fr IV and bioavailable amounts; and C3 for Znbioava, Ni Fr I, Fr II, Fr III, and bioavailable amounts. The obtained results confirm the initial assumptions about the significant influence of changes occurring during composting on the amount of microelements because the largest changes in copper, nickel, and zinc contents were observed during the thermophilic phase, except for zinc in C1 and nickel in C2. The smallest changes in the content of all discussed micronutrients were found in C1 and C3 during the cooling phase. For C2, the least changes occurred during the mature phase. According to the assumptions, the role of bulking agents in composting with SS was smaller in relation to the quantitative changes in the metals. Regardless of the bulking agent used, there was a reduction in the mobility of the quantity, and consequently, the availability of microelements, which was particularly visible in the case of Ni. Despite significant increases in the bioavailable amounts of Cu and Zn in all the tested mixtures, these composts did not have a potential negative impact on the environment, which was confirmed by the values of metal pollution indices. Valorizing composts as a source of available microelements, the most beneficial properties in this respect were observed in C3 and the least in C2. Regardless of this, it should be remembered and taken into account that composting SS with various bulking agents is justified and purposeful; however, the obtained composts are a slowly releasing source of Cu, Zn, and Ni, which indicates the need for supplementation with other fertilization.  
Conceptualization, M.J.; methodology, M.J.; software, M.G.; validation, M.J. and M.G.; formal analysis, M.G.; investigation, M.J.; resources, M.J.; data curation, M.G.; writing--original draft preparation, M.J. and M.G.; writing--review and editing, M.J. and M.G.; visualization, M.G.; supervision, M.J. and M.G.; project administration, M.J. and M.G. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
All data are included in this article.  
The authors declare no conflicts of interest.