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The composting of sewage sludge (SS) with structure-forming additives is a popular and inexpensive method for the management of biodegradable waste. During this process, a number of transformations of organic matter and nutrients occur. This study presents the rates and directions of quantitative changes in Cu, Zn, and Ni during the composting of various mixtures. The following substances were used for preparing compost mixtures: SS, sawdust, straw, and bark. These substances were mixed together in appropriate proportions as follows: C1: 45% SS + 50% sawdust + 5% straw; C2: 45% SS + 50% bark+ 5% straw; and C3: 45% SS + 35% bark + 15% sawdust + 5% straw. Single (DTPA solution) and sequential extraction methods (BCR protocol) were used for microelement mobility assessment. Compost valorization was performed in each individual composting phase. The chain relative increment analysis was used for such assessment. Additionally, the potential metal pollution in the composts was evaluated by applying the following indices: the pollutant accumulation index (PAI), heavy metal enrichment index (HMEI), and heavy metal pollution load index (HMPLI). During composting, generally, the amounts of Ni, Cu, and Zn increased in the various extracted combinations, which was independent of their susceptibility to decomposition. Despite of this, the tested composts should be considered as a source of slowly available microelements for plants. The most intense quantitative changes in metals occurred in the thermophilic phase, and the weakest changes occurred in the cooling phase. At the same time, the calculated indices indicated a lack of contamination of the composts with the analyzed metals, emphasizing their environmental safety and lack of negative impact. The bulking agents used for composting with SS did not significantly influence the intensity of the quantitative changes in the tested metals.