Valuable Materials from Olive Mill Wastewater

Abstract

AbstractThe olive oil industry represents one of the most economically essential agri-food sectors. The worldwide olive oil production goes beyond 2.5 million tons each year, with more than 95% of this production being generated in the Mediterranean countries. This industry produces unexploited agronomic residues of more than 20 million m3 of olive mill wastewater (OMW). OMW is typically acidic (pH ~ 4–5) and highly toxic to plants and microbes because of its high phenolic compounds (up to 30 g L−1) content. Hence, OMW is claimed to be one of the most polluting effluents. Several physical, biological, and physicochemical strategies have been proposed to treat OMW. Besides being costly, these techniques cause biodegradation or destruction of the phenolic fraction of OMW, thus resulting in a loss of valuable, functionally active, and exploitable compounds. These polyphenols are essential as natural antioxidants, antibacterial, antiviral, and antifungal compounds, and their potential as active components for increasing the tolerance of salt-stressed plants. Soil salinity is one of the global problems that affect approximately 20% of irrigated land, which can cause a fundamental reduction in crop yields. The severe result is translated into causing risks to food security for the escalating world population. Most crops species are not salt-tolerant in soil. The toxic effects, the concentration of salt, and the stress duration are the main factors determining the adverse plant responses to salinity stress. Plants respond to stress by modulating gene expression, which eventually restores cellular homeostasis, detoxifies toxins, and recovery of growth. Consequently, these diverse environmental stresses often activate similar cell signaling pathways and cellular responses, such as the creation of stress proteins, up-regulation of antioxidants, and accumulation of compatible solutes. In this regard, Ascorbic acid (ASA), Jasmonic, and the salicylic acid act as a primary substrate in the cyclic pathway for enzymatic detoxification of hydrogen peroxide. Additionally, they have emerged as an otent antioxidant and play a key role in plant stress signaling and growth, physiological and biochemical processes. In recent studies (Asli et al. in Sci Total Environ 630:728–737, 2022) have demonstrated that phenolic compounds from OMW (such as hydroxytyrosol) showed a significant increase in the tolerance of the salt-stressed plant. Therefore, OMW may be regarded as an inexpensive source of organic compounds to be recovered because of their ability to be transformed into valuable products in agricultural applications.KeywordsOlive mill wastewaterPhenolsAntioxidantsNanoparticlesHydroxytyrosolSalt stressSalt tolerancePlant water balance