Valorization of Treated Olive Mill Wastewater and Olive Pits in Hydroponic Systems for Lettuce Production

Significant volumes of wastewater and solid by-products are produced by olive oil industries worldwide, posing serious environmental challenges. This study presents an innovative circular economy and environmental sustainability approach that simultaneously valorizes liquid (olive mill wastewater, OMW) and solid by-products (crushed olive pits) rom olive oil production through hydroponic lettuce cultivation. The OMW was pretreated and supplemented with nutrients (OMW-N) to create a hydroponic solution for lettuce (Lactuca sativa) cultivation using crushed olive pits as growing substrate. A hydroponic system fed with a nutritive solution was used as a control. Lettuces grown in the OMW-N system achieved a 100% survival rate with no signs of phytotoxicity, although they exhibited a significant reduction in fresh mass (approx. 66%) and size, compared to the control. The sensory analysis revealed no significant differences in consumer acceptance, except for slightly lower color intensity, with 40% of participants explicitly indicating a purchase preference for the OMW-N lettuce, validating its commercial feasibility. Results demonstrated that OMW-N system functioned as a tertiary treatment, achieving additional removal of nutrients. Overall, integrating treated OMW and olive pits into hydroponics is a feasible strategy to convert agro-industrial waste into value-added food products, reducing the environmental footprint of the olive sector.