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Background: During power plant operations, significant amounts of (PCBs)-contaminated soil are generated. PCBs enter the environment in various forms, causing toxicity in humans and other living organisms. Various technologies are used to remove PCBs from contaminated soils and sediments. Objectives: Bioremediation is an environmentally friendly method to improve areas infected with PCBs. In this study, to decontaminate PCB-contaminated soil, the effect of biological sludge in contaminated soil on the removal of PCBs was investigated in a vermicomposting process at different mix ratios. Methods: Mixtures of PCB-contaminated soil and biological sludge were prepared in sample containers at different mix ratios, and then, earthworms were added. Over 80 days, besides the reduction of PCBs, pH, volatile solid content, and fixed solid content changes were also examined. Results: The highest reduction in PCBs (47.4%) was reported in pilot D at a contaminated soil-to-biological sludge volume ratio of 1:4. Moreover, in the same pilot, the pH level decreased from 7.9 to 7.2, indicating a further decline compared to other pilots, attributed to the further reduction of volatile solids. The highest reduction in volatile solids occurred in pilot D, decreasing from 74.8% at the beginning of the experiment to 42.9% at the end. The largest amount of mineralization also occurred in the same pilot. Conclusions: In this study, it was found that Eisenia fetida worms, along with microorganisms in the biological sludge, could function properly at all volume ratios of contaminated soil to biological sludge and could purify contaminants in a vermicomposting process.

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