PhD Thesis Defence Presentations - Spyridon S. Kontos

Phenolic compounds are among, the most complex and difficult to remove compounds from the by-products of Olive Mill Wastewater (OMW). Due to significant properties, including stability and anti-oxidative activity, the recovery of poly-phenols from OMW is of paramount importance. A potential recovery of high added value products, in high purification levels, can lead to their exploitation in different scientific fields such as the improvement of human health, the food industry, cosmetics etc. For this reason, the present work is focused on the investigation of the possibility of recovering phenolic compounds at high levels of purity from synthetic mixtures, based on the combination of different physicochemical methods (membrane filtration and cooling crystallization), with an ultimate objective of applying the method to OMW.

The theoretical and experimental study of the recovery of phenolic compounds from aqueous solutions by cooling crystallization is initially examined. The theoretical study of the recovery of a phenolic compound from aqueous solutions for different crystallization time intervals is investigated. The comparison of the theoretical results with the experimental data shows a very small deviation rate (3.5%). The present work is also associated with the investigation of the efficiency of solution crystallization for the separation of a mixture of phenolic compounds from standard aqueous solutions in the presence and absence of sugars at high initial concentration levels. The experimental recovery results are very promising. In a subsequent step, the efficiency of the melt crystallization process of phenolic compounds is examined, with an ultimate target of applying the process as the final treatment step in order to obtain a phenolic crystal layer at high purification levels. The main objective is the development of a mathematical model for the prediction of the crystal growth of a phenolic compound, deposited on the cooled surface from its melt solution as a function of crystallization time. Subsequently, the theoretical results are compared with the experimental data showing very satisfactory predictions for the developed thickness of the phenolic compound.

As a next step, the recovery of high added value products from OMW may be achieved by the application of a novel methodology that consists of a combination of different physicochemical methods. The recovery of phenolic compounds is investigated from synthetic OMW, prepared according to literature data. Initially, the synthetic solution is used as a feed stream and is filtered through Nanofiltration and Reverse Osmosis membranes for separation of the compounds on the basis of the differences of their molar masses. In a subsequent step, the RO retentate, enriched in simple sugars and phenolics, is distilled under vacuum (or freeze-dried) for the removal of the residual solvent (water). As a final treatment step, phenolic compounds are recovered by solution crystallization and melt crystallization based on the differences of solubility and melting point of each compound respectively. Finally, the combined treatment process is applied to a three-phase oil mill waste.