Sustainable desalination of olive mill wastewater via electrodialysis

Electrodialysis (ED) is a membrane-based separation process that exploits the selective migration of ions under the influence of an electrical driving force. It removes ionized species from liquid solutions using alternating cation- and anion-exchange membranes, arranged between two electrodes. When a direct current is applied, cations move toward the cathode and pass through cation exchange membranes (CEM), while anions migrate toward the anode through anion exchange membranes (AEM). As a result, alternating compartments become either depleted of salts (diluate) or enriched (concentrate). ED is an environmentally friendly technology that competes with reverse osmosis for desalination of low-salinity streams. It shows advantages in fouling resistance and high sustainability due to its low chemical consumption. This study investigates the feasibility of using ED for demineralization of pretreated olive mill wastewater (OMW), with the goal of reducing salinity while preserving valuable phenolic compounds. Experiments were performed using a PCCell ED64 electrodialysis stack configured with five cell pairs and powered by a constant voltage (Fig. 1). The OMW was previously acidified, microfiltered, and nanofiltered to reduce organic load and suspended solids. A series of membrane combinations were tested in batch mode to evaluate their ionic removal efficiency and their impact on the retention of bioactive molecules. The tested membrane pairs included standard heterogeneous membranes (SK and SA), homogeneous monopolar pairs (MVK/MVA), and three specialty membranes (100D, 200D, 400D) coupled with SK. Among these, the standard SK/SA membranes and the 100D/SK configuration demonstrated the best trade-off between conductivity reduction and phenolic retention. Specifically, these two configurations minimized the migration of low molecular weight polyphenols (e.g., tyrosol and hydroxytyrosol) into the concentrate while achieving significant demineralization. In contrast, membranes such as 200D and 400D, characterized by looser structure and higher ion selectivity, led to increased phenolic losses. Although the present study focused on membrane screening under constant voltage conditions, the transition toward a semi-continuous process is planned in future work. This would aim to further enhance process control, energy efficiency, and scalability. This research is funded by Agritech Project (PNRR), National Center for Technology in Agriculture.