BACKGROUND: This study used particle size distribution (PSD) to reveal how chemical conditioning inflicted changes on different chemical oxygen demand (COD) size fractions to improve the potential of two-phase membrane separation as an effective preliminary step for the sustainable management of olive processing wastewaters. RESULTS: Experiments were designed to explore: (i) the potential of membrane separation as a pretreatment step; and (ii) the way in which chemical conditioning improved the removal potential of membrane treatment. Particle size distribution analysis was conducted to observe changes inflicted on different COD size fractions by chemical conditioning. The chemically enhanced membrane process achieved 95% COD removal, reducing the effluent COD to 6000-8000mgL(-1) range and the total phenol down to 100mgL(-1). Chemical conditioning also removed a significant portion of soluble COD, so that the resulting PSD fingerprint in the effluent was properly reshaped to allow effective removal by membrane treatment. CONCLUSION: The investigated treatment schemes involving pretreatment of oil wastewater by chemical conditioning together with membrane filtration offered high COD reduction in the effluent and biodegradable permeate when properly diluted for co-treatment with a compatible wastewater stream. The use of chemical conditioning prior to membrane filtration improved final permeate quality and ultrafiltration membrane flux. (C) 2016 Society of Chemical Industry