## 4 Discussion  
Understanding changes in metal bioavailability during the composting of SS with various bulking agents largely depends on the changes in composting organic matter that occur during the process. Composating can be divided into four basic phases: mesophilic, thermophilic, cooling, and maturation [40]. They differ in temperature and pH, which have a direct impact on the microorganisms dominating in particular stages. In the mesophilic phase, at a temperature close to the ambient temperature, mesophilic bacteria predominate and use easily degradable organic compounds, such as simple sugars, proteins, starch, and fats. Thanks to thermophilic bacteria, the second phase of the process is the most active stage of the composting, during which most readily available organic compounds are decomposed into carbon dioxide and converted into humus. The material composted in the cooling phase consists of, among others, substances that are difficult to decompose: cellulose, chitin, lignin, and hemicellulose. Due to their structure and low susceptibility to degradation, the listed compounds can only be broken down by extracellular enzymes secreted by fungi and actinomycetes [41]. Therefore, the population of these organisms begins to dominate during the maturation phase. This stage of the process is the longest, and during this stage, the stabilization of the composted mixture occurs, expressed, among  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline  & & & \multicolumn{8}{c}{**Parameters**} \\ \hline  & & & \multicolumn{2}{c}{**Cu**} & \multicolumn{2}{c}{**Zn**} & \multicolumn{2}{c}{**Ni**} \\ \hline  & **C** & **Term** & **PGI** & **HMEI** & **PGI** & **HMEI** & **PGI** & **HMEI** & **HMPLI** \\ \hline Fr. IV & 1 & 1 & \(-\)3.994 & 0.094 & \(-\)3.639 & 0.120 & \(-\)4.935 & 0.049 & 0.082 \\ \hline  
other things, by an increased share of humus and a lack of phytotoxic substances [42]. As can be seen from this general description of the individual composoting phases, a number of microbiological and biochemical transformations occur during the process, which are reflected in the transformation of organic matter, pH, and temperature. These factors also have a direct and indirect impact on the mobility of metals and, consequently, their potential bioavailability expressed by their individual distribution in specific fractions and combinations. In general, the largest changes in the amounts of microelements were in the thermophilic phase, and the smallest changes were in the cooling phase. Changes in the organic matter of composots are one of the main drivers of the variable distribution of metals in their fractions. Therefore, special attention, especially in waste composoting, should be paid to reducing the mobility and availability of metals to plants, even when their total content increases as a result of reducing the composted mass. During the composoting process, a decrease in the amount of organic matter can cause either dilution or an increase in the concentration of nutrients, depending on the combinations in which metals occur [43]. The heterogeneity and dynamics of the transformations of individual metals were demonstrated in this research. The amounts of Cu and Zn in the separated fractions generally increased during the composoting process of the individual mixtures. Additionally, the bioavailable amounts of Cu and Zn were greater in mature composots than at the beginning of the composoting process, which the chain relative increment data show. The pattern of quantitative changes in Ni was slightly different because the content of Ni in Fr. I decreased while that in the remaining connections increased, which was particularly visible in the organic and residual bonds. The amounts of bioavailable Ni, apart from those in C1, were at a constant level. The bioavailable amounts of Ni in C1 decreased significantly during composoting. An experiment conducted with composed sewage sludge by [PERSON] [27] confirmed a significant reduced mobility of this metal during the composoting process.