Major olive mill aqueous effluents as industrial sources of hydroxytyrosol, 3,4-dihydroxyphenylglycol and tyrosol for circular valorisation

The selective recovery of valuable chemicals from waste streams is a central objective of Green Chemistry, particularly when waste prevention and the use of renewable feedstocks can be combined. In two-phase olive oil extraction systems, several aqueous effluents are generated that are commonly treated as waste despite their significant organic and phenolic content. In this work, four industrial effluents—olive washing water (OWW), hopper drip water (HDW), vertical centrifuge water (VCW) and oil decanting water (ODW)—were systematically characterised in six olive oil mills over two consecutive production seasons to evaluate their potential as sources of bioactive phenolic compounds. HDW exhibited the highest phenolic concentration, reaching up to 3.5 g/L, together with elevated sugar contents, identifying it as the most promising stream for the recovery of hydroxytyrosol (HT) and 3,4-dihydroxyphenylglycol (DHPG). VCW, although more diluted, was generated in the largest volumes and contained substantial amounts of both compounds, while ODW showed an unusual predominance of DHPG over HT, enabling selective recovery. An industrial chromatographic process was optimised to maximise phenolic retention and HT purity in the most concentrated streams. Based on national production data, the valorisation of these effluents could enable the recovery of up to 80 t/year of HT and 56 t/year of DHPG. Overall, this study highlights the potential of phenolic recovery to support waste valorisation strategies and facilitate subsequent wastewater management within a circular-economy framework.