Assessment of toxicological effects of raw and bioremediated olive mill waste in the earthworm Eisenia fetida: a biomarker approach for sustainable agriculture

The olive oil extraction process produces large amounts of olive mill waste (OMW) that can be re-used in agriculture by land spreading. OMW has a low pH, high salinity and high levels of polyphenols and is potentially dangerous for the environment. It is therefore mandatory to develop procedures able to produce OMW with lower environmental impact. The first aim of the present study was to evaluate the toxicological effects of two different typologies of OMW: raw Two-Phase Olive Mill Waste (TPOMW) and raw Olive Mill Waste Waters (OMWW), on a soil bioindicator, the earthworm Eisenia fetida, using neurotoxicity (AChE), oxidative stress (LPO, CAT) and genotoxicity (comet assay) biomarkers. The second aim was to test the efficacy of a composting/bioaugmentation process by investigating the toxicological effects of the bioremediated OMW. E. fetida was exposed for 72 h to increasing concentrations (12.5, 25, 50%) of raw and bioremediated TPOMW and OMWW in an artificial soil (50% potting soil + 50% quartz sand). Raw OMW caused earthworm mortality at high doses and measurable biochemical and cellular effects at lower doses. The main cause of these effects is probably the high level of polyphenols present in the OMW. Neurotoxic effects, induction of oxidative stress and genotoxic effects were highlighted. The most evident toxicological effects were produced by the TPMW. The evaluation of the bioremediation process efficacy revealed that a high decrease in acidity and polyphenol content corresponds to a decrease of toxicological effects. This study contributed to assess the environmental sustainability of the bioremediated OMWs, to be used as fertilizers in agriculture.