Cataloging olive oil mill wastewater sludge based on toxicological profiles and functional microbial diversity

Olive oil mill wastewater (OMW) sludge, a byproduct of olive oil production stored in evaporation ponds, poses significant environmental and agronomic challenges due to its toxicity, heterogeneity, and high content of recalcitrant organic compounds. While most studies focus on the composition of OMW, research on its functional potential and practical applications for agriculture and sustainability remains limited. This study aimed to characterize the physicochemical, toxicological, and microbiological properties of OMW sludge from different geographical locations to identify patterns that could help standardize effective valorization strategies for agricultural use. Results showed considerable variability in pH (5.73–8.12), organic matter content (8.06–94.98 %), ecotoxicity (1.08–59.65 TU = Toxicity Units), and phytotoxicity (0.17–124 % GI = Germination Index) across samples. Some sludges, such as TED 19 and 25, exhibited phytostimulant properties (GI > 120 %), highlighting their potential to be used in sustainable agriculture. Functional biodiversity analyses revealed metabolic activities linked to the degradation of complex polymeric compounds and carboxylic acids, consistent with the recalcitrant nature of the sludges. Specifically, the samples that showed the highest values of microbial functional intensity were TED15, TED19 and TED25. A taxonomic analysis identified microbial families such as Moraxellaceae, Longimicrobiaceae, Ruminococcaceae, Sphingomonadaceae, Microbacteriaceae, and Lactobacillaceae, which may play a role in reducing toxicity and enhancing agronomic potential. Principal component analysis (PCA) classified the samples based on their characteristics, offering valuable insights into their functional potential. These findings support the development of tailored bioremediation strategies to mitigate the environmental risks of OMW sludge and harness its microbiological potential for sustainable agriculture.