Olive oil industry is an important sector within the agro-food industries in the Mediterranean countries but constitutes a major environmental problem regarding the disposal of its wastewaters. Olive mill wastewater (OMW) is a turbid, dark colored, foul-smelling and acidic effluent. It presents a low biodegradability due to its antibacterial activity, given by the phenolic content¹. Bioremediation through microalgae is an interesting option, since it is an environmentally friendly process, as wastewaters can be used as cheap nutrient sources for microalgal biomass production that could be a source of stored chemical bond energy, especially into lipids, carbohydrates and proteins². The main objective of this work was to evaluate the potential for bioremediation of two OMW with different origin - olives washing (OW-OMW) and olive oil extraction (OMW) - by microalgae. It were used three species of green microalgae, Chlorella vulgaris, Chlorella protothecoides and Scenedesmus obliquus and the cyanobacterium Arthrospira maxima. Due to the complexity of the effluents, it was necessary a dilution of media: 5% for OMW and 50% for OW-OMW. The best removals of chemical oxygen demand (COD) removal were 61.6% for OMW and 67.9% for OW-OMW in cultures of C. protothecoides and C. vulgaris, respectively. Significant removals of phosphorus (P-PO₄) were only verified with Arthrospira (67.0% for OMW and 36.0% for washing wastewater). Regarding nitrates (N-NO₃), very satisfactory removal rates were obtained with all microalgae (around 80 %). Due to difficult biological degradation, the removal of polyphenols did not exceed 40 % for both OMWs. Although microalgae can grow in these OMWs and show potential for its bioremediation, further studies will not be feasible if this effluent is not subjected to a primary treatment, since its toxicity causes cellular death after 4 days.

Acknowledgements

Authors acknowledge the OBTain project (NORTE-01-0145-FEDER-000084), co-financed by the European Regional Development Fund (ERDF) through NORTE 2020, and FCT for the financial support to CQVR (UIDB/00616/2020).

References

1. Amor, C., Marchão, L., Lucas, M. S. & Peres, J. A. Application of Advanced Oxidation Processes for the Treatment of Recalcitrant Agro-Industrial Wastewater: A Review. Water 11, 205 (2019).
2. Marchão, L. et al. Microalgae and immobilized TiO2/UV-A LEDs as a sustainable alternative for winery wastewater treatment. Water Res. 203, 117464 (2021).