Evaluation of Paper Mill Sludge Using Bioindicators: Response of Soil Microorganisms and Plants

The growing demand for sustainable waste management practices has prompted interest in the land application of paper sludge as an alternative to landfilling and incineration. This study evaluates the environmental potential of paper sludge derived from recycled hygienic paper production by investigating its effects on soil respiration, seed germination, and seedling development. A comprehensive set of respirometric tests using the OxiTop® system assessed microbial activity in soil amended with various concentrations of paper sludge (1–100%). Concurrently, bioassays using Lepidium sativum L. and Pisum sativum L. seeds examined the phytotoxicity and physiological response during germination. The results show that low to moderate sludge concentrations (1–20%) stimulated microbial activity and enhanced germination parameters, with a germination index (GI) up to 150% at 1%. However, higher concentrations (>40%) led to oxygen depletion, microbial stress, and decreased plant growth, indicating potential phytotoxicity and the need for application thresholds. For certain intermediate concentrations (e.g., 30–40%), a delay of approximately 21 days before sowing is recommended to allow microbial communities to stabilize and avoid initial stress conditions for plants. This study demonstrates that controlled application of paper sludge in soil systems can serve as a viable and sustainable disposal method, supporting circular economy principles and reducing the environmental burden of paper industry by-products.