## 3 Results and Discussion
This effect has not been previously reported in studies on the composting of sludge in the presence of zeolite [34; 41] and warrants further investigation. The graph in Figure 6 illustrates that composting significantly reduced the pH value. During composting, the microbial decomposition of organic matter produces organic acids, such as acetic and butyrate acids, which lower the pH. In the initial stages, the rapid degradation of easily degradable substrates leads to a surge in acid production. As composing progresses, these acids are further broken down, and the pH may gradually rise again. In most cases, the compost decreases within a weakly alkaline range (pH = 8-9) [33], as seen with the sediment from natural deposition and the sediment subjected to freeze-thaw-drying cycles. In these treatments, the slower degradation rates result in less acid production, and the accumulation of ammonia from protein degradation can increase the pH. Ammonia acts as a buffering agent, maintaining a higher pH level. This phenomenon can generally be attributed to the increased formation of volatile fatty acids during the incomplete acidification of the sludge in anaerobic zones, as previously described [25]. In anaerobic conditions, acidogenic bacteria produce volatile fatty acids as intermediates, but without sufficient methanogenic activity to consume these acids, they accumulate and lower the pH. However, in the presence of adequate aeration during composting, these acids are oxidized by aerobic microorganisms, reducing their impact on the pH. To enhance the applicability of the compost, it is recommended to maintain a pH close to neutral, which necessitates monitoring the adequate intensity of aeration in the mixture. Proper aeration ensures that oxygen penetrates the compost pile, promoting aerobic microbial activity and preventing anaerobic pockets where acid accumulation can occur. Maintaining optimal moisture levels also facilitates microbial metabolism and pH stability. By controlling these parameters, the composting process can produce a final product with a neutral pH that is suitable for a wider range of agricultural applications. Figure 7 presents the concentrations of ammonium nitrogen in the analyzed samples. As observed, composting effectively preserved the high nitrogen concentrations (this also applied to nitrate nitrogen). Under aerobic composting conditions, microorganisms decompose organic matter, releasing organic nitrogen, which is then converted into its ammonium form. The controlled composting environment minimizes nitrogen losses  
through ammonia volatilization or nitrate leaching, thus retaining nitrogen within the compost and enhancing its value as a fertilizer. Figure 6: Boxplots for pH in the samples. Figure 7: Boxplots for ammonium in the samples. Additionally, zeolite demonstrated its expected effectiveness (+10-20% compared with composting without zeolite), as noted in previous studies [41; 42]. Zeolites are natural aluminosilicates with high ion exchange and adsorption capacities. When added to the compost mixture, zeolite adsorbs ammonium ions, retaining them within the compost structure and preventing their loss through leaching or volatilization as ammonia. This creates a reservoir of available nitrogen for plants and improves the overall efficiency of the fertilizer. It can be concluded that zeolite enhanced the quality of the composted product and may be considered a desirable additive in the composting process. Its inclusion not only increased the nitrogen content but also contributed to compost stabilization by improving its physical properties, such as porosity and water retention capacity. This, in turn, could positively impact the soil structure when the compost was applied, which enhanced the soil fertility and its capacity to support plant growth. Figure 8 illustrates the distribution of phosphorus values expressed as P\({}_{2}\)O\({}_{5}\).