BACKGROUND Phenols are compounds found in agricultural by-products, with high antioxidant activity and positive impact on human health. As a result, their isolation and purification is of interest for the production of cosmetics, nutritional and pharmaceutical supplements. A preliminary design for a plant with a treatment capacity of 1.1 m(3) h(-1) of liquid by-products or extracts is presented herein, based on experiments carried out with three materials rich in phenolic compounds. The proposed process for the separation of phenols implements physicochemical methods such as solvent extraction, filtration through membranes, adsorption/desorption on resins and vacuum evaporation. The materials tested were olive mill wastewater (OMW), grape marc and olive leaves. RESULTS The final products of the proposed process were rich in phenolic compounds, with the OMW final concentrate containing 378 g L-1 phenols in gallic acid equivalents, 84.8 g L-1 being hydroxytyrosol (61% of the initial amount of hydroxytyrosol was retrieved in the final concentrate in 0.05% of the initial OMW volume). The final concentrate of olive leaf extract contained 98 g L-1 phenols in gallic acid equivalents, and the final concentrate of grape marc phenols 190 g L-1 in gallic acid equivalents, containing 4.7 g L-1 catechin (25% of the initial amount of catechin was retrieved in the final concentrate in 0.04% of the initial extract volume). CONCLUSIONS The combination of the proposed techniques proved to be effective for the separation and purification of phenols contained in agro-industrial by-products. As a result, a new process was designed for their separation and purification. (c) 2019 Society of Chemical Industry