## 3 Results and Discussion
As is evident, during the natural deposition of the sludge, a significant portion of phosphorus was lost due to biochemical processes occurring, including under anaerobic conditions. Under these conditions, certain microorganisms facilitate the release of phosphorus from organic compounds through enzymatic hydrolysis. The released phosphorus can then bind with iron, aluminum, or calcium to form insoluble compounds that settle out of the sludge, effectively reducing the bioavailable phosphorus content. This is corroborated by studies related to the drying of sludge in natural environments [43], where prolonged exposure to variable conditions enhanced such losses. Composing sludge under aerobic conditions allows for the retention of a sufficient amount of phosphorus, which is one of the key elements in fertilizers. The aerobic environment promotes the activity of microorganisms that stabilize organic matter without significantly mobilizing phosphorus. These microbes decompose organic compounds while assimilating phosphorus into their biomass, thus retaining it within the compost matrix. Additionally, the formation of humic substances during  
Figure 8: Boxplots for phosphates in the samples. phosphorus, making it less prone to leaching. In evaluating the viability of this technology in each specific case, it is essential to assess the phosphorus content in the initial feedstock, as previously mentioned. It is recommended to implement wastewater treatment technologies that maximize the recovery of valuable elements for reuse. Currently, integrated technological solutions of this nature are being developed [33]. Figure 9 presents boxplots of petroleum product concentrations in the sludge. Overall, the collected samples exhibited a relatively high degree of contamination with petroleum products, which was related to the characteristics of the treated wastewater, with a significant fraction of these products being heavy. However, during composting, a substantial degradation of these substances was observed (part of which was adsorbed by the zeolite and remained in the compost). During the composting of wastewater sludge, the reduction in petroleum product concentration is driven by several interrelated processes. The primary mechanism is biodegradation of organic pollutants by the microbial community within the compost mass. Composing creates favorable conditions for the growth of various microorganisms, including bacteria and fungi, capable of utilizing hydrocarbons from petroleum products as carbon and energy sources. Aerobic conditions, maintained through regular mixing and aeration, stimulate microbial metabolic processes, accelerating the breakdown of complex organic compounds. The increase in temperature within the compost (thermophilic phase of composting) further accelerates biochemical reactions. At temperatures ranging from 45 \({}^{\circ}\)C to 65 \({}^{\circ}\)C, thermophilic microorganisms become active and are particularly effective in degrading the heavy fractions of petroleum hydrocarbons. In addition to biological degradation, the sorption and immobilization of petroleum products on solid particles of organic material occur. The addition of amendments, such as zeolite, enhances the sorption capacity of the compost, aiding in binding and retaining hydrocarbons, thereby reducing their mobility and bioavailability. This observation aligns with findings reported in recent studies [44]. Thus, composting may be applicable, even in cases of sludge contamination with petroleum products within known limits. Figure 9: Boxplots of petroleum products values in the samples. Figure 10 illustrates the overall trend in the distribution of metal concentrations in the samples. In the sludge after natural deposition, the quantity of metals was reduced; however, this may have reflected the characteristics of the wastewater at the time the sludge was generated (five years ago). During the composting of the sewage sludge, leachate was produced, which can carry away soluble substances, including metals.