Application of combined physicochemical techniques for the efficient treatment of olive mill wastewaters

Abstract

Olive Mill Wastewaters (OMW), produced from olive processing for the extraction of olive oil by units scattered in the Mediterranean countries, is a hardly degradable byproduct due to very high concentration of organic load of complex composition. Several treatment methods have been proposed for the efficient treatment of OMW, but because of the high operational cost, the application of these methods seems to be prohibitive, as olive mills are small family units and operate only 3–4 months per year. In the present work, the implementation of a combination of physicochemical treatment methods is proposed for the effective treatment of OMW. Initially, a membrane filtration process was used (ultrafiltration, nanofiltration, and reverse osmosis membranes) for the fractionation of OMW in concentrate and permeate streams. OMW was thus sufficiently treated and as a result the final permeate of the reverse osmosis was a pure water stream appropriate for irrigation purposes or for reuse in the OMW premises. The concentrated fraction from NF and RO membranes was further treated, to separate and isolate carbohydrates and the phenolic compounds, through the implementation of adsorption on specific resins. Alternatively, the enriched in phenolic content concentrated fractions could be investigated for the possibility of their selective recovery from the respective solution by cooling crystallization, applied directly to the concentrated fractions. During this process, it is possible to recover a number of components from their solutions according to the respective freezing points. Two Poly-Phenols (PP) contained in the OMW, namely trans-cinnamic acid (TCA) and ferulic acid (FA) were examined for the possibility of their selective extraction from their aqueous solutions by cooling crystallization, in order an operational model of the process to be developed. Initially, short cooling crystallization experiment cycles were done to test the effect of molecular diffusion and convection phenomena to the total PP recovery. Furthermore, crystallization of FA and TCA was followed during one cooling cycle as a function of time past the immersion of a cold surface in hot homogeneous solutions of the test compounds. It was estimated that in the case of mixtures, a total recovery of 66% FA and 50% TCA was achieved. The combination of membrane filtration and cooling crystallization may be quite promising for the development of more effective and integrated exploitation of OMW abiding to the zero waste targets.