## 1 Introduction  
The generation of a large amount of waste is a characteristic of human civilization development. One of the most troublesome wastes systematically generated by humans is sewage sludge (SS). According to the Local Data Bank [1], in the last 12 years (2010-2022), the amount of SS increased by 10% in Poland. This may not be a dramatic pattern, but the proper management of this waste is needed, especially since the landfill of SS is prohibited in Poland, and its incineration is not the most popular method. The above data [1] indicate that the use of SS for agricultural and reclamation purposes is a popular method of utilization, in addition to its use for composting and the production of organic fertilizer, i.e., compost. The basis for this direction of SS management is its chemical properties related to its significant and attractive fertilization value [2; 3; 4; 5; 6; 7]. On the other hand, the possibility of the presence of pollutants that have a toxic effect on the environment, such as heavy metals, PAHs, PCBs, and AOX, should be considered [8; 9]. In particular, the last negative aspect resulted in the introduction of standards tightening the conditions for the use of SS in agriculture in some European countries (Germany, Norway, and Sweden), up to including a complete ban (e.g., in Switzerland) [10]. However, in many countries around the world (the USA, China, and South Korea), SS has been successfully used for agricultural purposes because the chemical composition of SS is rich in organic matter and nutrients; thus, it contributes significantly to the improvement in soil fertility [11]. However, when SS is applied repeatedly, there is a possibility of heavy metal accumulation in the soil and its subsequent incorporation into the food chain [12; 13; 14]. Considering the dual nature of SS, an alternative is composting. It should be emphasized that the composting process is an environmentally friendly technique that is relatively inexpensive and attractive for agriculture. At the household scale, composting has been used since immemorial times and is likely the oldest form of recycling [15]. As a biological process, composting involves simultaneous biochemical and microbiological processes (mineralization and humification), which lead to the transformation of organic matter and physicochemical changes in the composted materials. Additionally, the composting of SS solves many environmental problems. The most frequently mentioned are the avoidance of waste disposal, elimination of bothersome odors, reduction in the mass of waste, degradation of toxic organic compounds, reduction in the bioavailability of heavy metals or transformation of organic matter, and maintenance of nutrient circulation [2]. Moreover, during the composting process of SS, the quality of organic matter improves, and the sludge is dewatered and sanitized [15]. The most important product of the entire process of composting SS is the production of organic fertilizer, compost, which is a valuable source of both macro- and micronutrients and organic matter [11; 16; 17; 18]. However, the use of compost from organic waste is not only related to the assessment of its effectiveness as a fertilizer but also to concerns about whether it can be safely used in agriculture due to the potential content of heavy metals. The origin of the raw materials used for compost production is the most important factor when determining the level of heavy metals in the mature product. In general, composts made from plant waste contain small amounts of metals, and the highest content is found in composts with a high proportion of sludge sewage [19; 20]. This raises concerns about their possible excessive accumulation in the soil when using composts prepared from sewage sludge. When assessing the value of compost, we consider, among other factors, the total amount of metals that cannot exceed the permissible standards for use as fertilizer.