Integrated treatment strategies for olive mill wastewater: bridging conventional processes with biochar-enhanced nature-based solutions

Olive mill wastewater (OMW) is one of the most challenging agro-industrial effluents, characterized by extremely high organic load (COD: 50–150 g/L), total phenol concentrations exceeding 10 g/L, high salinity (EC: 2–30 mS/cm), and a dark, phytotoxic composition that severely limits the performance of biological treatment systems. Developing effective, sustainable, and scalable treatment strategies has become a critical environmental and socio-economic priority for Mediterranean olive-producing regions. This review critically assesses existing OMW treatment technologies, evaluating their mechanisms, performance ranges, failure modes, and integration potential. Conventional physicochemical methods, such as sedimentation, filtration, coagulation-flocculation, membrane processes, and adsorption, are effective for particular pollutant fractions but individually insufficient for complete treatment (respectively achieving 20–30%, 30–40%, ~50% COD reduction with 60–70% phenolic removal, 60–95% COD rejection, and 70–90% phenolic removal). Chemical oxidation and advanced oxidation processes (AOPs) can degrade recalcitrant phenolic structures (75–90% removal of phenolics) but are constrained by energy demand and sludge management. Biological processes offer COD reductions of 60–90% but are frequently inhibited by OMW’s phenolic toxicity. Integrated natural treatment systems that combine sequential physicochemical pretreatment, oxidative degradation, and biological processing emerge as the most technically viable approach. Within this framework, this review gives particular and sustained attention to biochar-enhanced constructed wetlands (BCW) as a low-energy tertiary polishing stage. Critically, BCW systems are not suited for raw or minimally pre-treated OMW, as total phenol concentrations above 400–500 mg/L inhibit nitrifying bacteria and electrical conductivity above 6–10 mS/cm impairs macrophyte survival. Applied to pre-treated effluents, conventional CW systems have achieved 74–87% phenolic removal and 83–98% COD reduction, while biochar-specific validation on OMW matrices remains an identified gap; BCW systems are expected to provide additional benefits for nutrient removal and carbon sequestration based on evidence from non-OMW wastewater matrices. Biochar derived from olive mill by-products themselves offers a particularly attractive circular economy pathway. This work highlights the need to move beyond single-process optimization toward integrated, scalable frameworks that combine high removal efficiency with sustainability and circular-economy principles and identifies the key scientific and engineering gaps that remain to be resolved.

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