## 2 Materials and Methods  
Wastewater sludge from municipal treatment facilities located in various regions of the Russian Federation was used as the raw material for composting. This study focused on mixed sludge from both settlers. The sludge was not subjected to preliminary stabilization under anaerobic or aerobic conditions and was dewatered to 60 \(\pm\) 5% moisture content using mechanical methods. A filter press with a capacity of 85 cubic meters of dewatered sludge per day was utilized for the dewatering of the wastewater sludge. The sludge for composting was mixed with a filler to ensure proper air circulation within the sludge. Straw cuttings were used as the filler, as this material is readily available and provides a sufficiently low bulk density. Additionally, mineral zeolite was added to some of the sludge samples prior to composting to prevent nitrogen loss during the process. Zeolite effectively adsorbs ammonium ions, retaining a significant portion of nitrogen that would otherwise volatilize during the heating of the mixture [34]. In this study, calcined zeolite with an average particle diameter of 2 mm was used (as shown in Figure 1). Zeolite brand--SK002 (LLC \"SKAT3\", Salavat, Russia). The mass fraction of the filler comprised 5% of the total mixture, while the mass fraction of the zeolite was 2%. In parallel, the characteristics of the sludge collected from a disposal site (where the sludge had been deposited for approximately five years) were examined, as well as the characteristics of the sludge subjected to freeze-thaw-drying cycles (accelerated simulation of ten years of disposal under natural conditions). Figure 2 presents the fresh dewatered sludge at the point of collection, the sludge at the natural disposal site, and the sludge that underwent freeze-thaw-drying cycles. Figure 1: Zeolite used for sludge mixture before composting. After collection and prior to composing, the sludge samples were stored at a temperature of 5 \({}^{\circ}\)C without access to oxygen. Before the analyses, the sludge was conditioned at room temperature under normal conditions. The following parameters were determined in the sludge: mass fraction of ammonium nitrogen (N-NH\({}_{4}\)), mass fraction of nitrate nitrogen (N-NO\({}_{3}\)), anionic surfactants (A-S), pH of the aqueous extract, total iron (mobile form) (Fe), ash content (ASH), mass fraction of manganese (Mn), mass fraction of copper (Cu), mass fraction of petroleum products (P-P), mass fraction of nickel (Ni), exchangeable (mobile) aluminum (Al), mass fraction of mobile phosphorus compounds (calculated as P\({}_{2}\)O\({}_{5}\)) (P), mass fraction of chromium (Cr), and mass fraction of zinc (Zn). For the analysis of the moisture content (W) and dry matter content (DM), the sludge samples were dried in a Binder FD023-230V drying oven (Binder GmbH, Tuttlingen, Germany) at a temperature of 105 \({}^{\circ}\)C for 20 h in ceramic crucibles, after which the weight difference between the raw and dried samples was measured. To determine the ash content, the dried samples were incinerated for 5 h at a temperature of 600 \({}^{\circ}\)C in a SNOL 8.2/1100 LSM01 muffle furnace (SNOL, Narkanai, Lithuania). Subsequently, the parameters were determined according to standard Formulas (1)-(3):  
\[W(\%)=\frac{\left(W_{f}-W_{d}\right)}{\left(W_{f}-W_{c}\right)}\times 100 \tag{1}\]  
\[DM(\%)=100-W(\%) \tag{2}\]  
\[ASH(\%)=1-\frac{\left(W_{ash}-W_{c}\right)}{\left(W_{d}-W_{c}\right)}\times 100 \tag{3}\]  
where W (\(\%\)) represents the moisture content in the organic sample, W\({}_{\text{f}}\) is the weight of the fresh sample, W\({}_{\text{d}}\) is the weight of the dried sample, W\({}_{\text{c}}\) is the weight of the empty crucible, and DM (\(\%\)) denotes the dry matter content in the sample. ASH (\(\%\)) indicates the ash content, while W\({}_{\text{ash}}\) is the weight of the ash remaining after combustion.