This study explores the interaction of saponification products with microbial communities in aerobic and anaerobic sewage sludge from wastewater treatment plants (WWTPs). It focuses on the reutilisation of waste cooking oils into soap and evaluates the biodegradation of these products using microbial respiration activity and biological oxygen demand (BOD) as indicators. Results demonstrate that soaps degrade effectively under both aerobic and anaerobic conditions, with anaerobic degradation contributing to methane production-a valuable biofuel. Importantly, no toxic effects on sludge microorganisms were observed. The research highlights that these saponification products can be fully integrated into the wastewater treatment process without adverse effects on microbial dynamics. Moreover, the economic analysis reveals that biosurfactants derived from used oils can be produced at a cost of approximately 0.12-3.0 EUR/kg, significantly lower than the 1-20 EUR/kg typically spent on chemical coagulants or synthetic surfactants used in WWTPs. These findings support the feasibility of repurposing waste oils into environmentally friendly, cost-effective treatment additives, enhancing microbial performance and promoting circular economy practices in wastewater management.

Department of Agricultural Food and Environmental Engineering Faculty of AgriSciences Mendel University in Brno Brno Czech Republic

Department of Experimental Biology Faculty of Science Masaryk University Brno Czech Republic

Department of Plant Origin Food Sciences Faculty of Veterinary Hygiene and Ecology University of Veterinary Sciences Brno Brno Czech Republic

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