Valorization of surface-water RO brines via Assisted-Reverse Electrodialysis for minerals recovery: performance analysis and scale-up perspectives

Reverse Osmosis (RO) plays a key role in seawater and brackish water desalination to fulfill the growing demand for fresh water. In recent years, RO has also been more and more adopted for the treatment and potabilization of surface waters, leading to two main problems: (i) the depletion in minerals of the product water, making it aggressive and unsuitable for drinking purposes and (ii) the production of a concentrated brine requiring proper disposal. Permeate remineralization post-treatments include pH adjustment and addition of minerals, such as bicarbonates, calcium and magnesium, which are essential for human health and required to meet drinking water guidelines. However, such solutions are exposed to critical issues related to supply, cost as well as extraction and transportation of chemicals. In this work, we investigate, via modelling tools, the use of Assisted-Reverse Electrodialysis (A-RED) to remineralize a surface water RO permeate stream by recovering minerals from its corresponding brine itself. This concept (patent pending [1]) was explored at the bench scale [2] and the subsequent experimental data used to validate the multi-scale model used in this study. The validated model was implemented to perform a process parametric analysis aiming at the design and optimization of a pilot-scale plant. Sensitivity analysis was performed considering the use of different stacks in series and hybrid configurations, including feed & bleed loops for one or both compartments and permeate by-pass. Moreover, the use of simplified techno-economic analysis tools allowed to identify the most promising configurations, which reduce the main cost items relevant to the industrial scale-up of the technology. More than 150 simulations were performed to compare different scenarios on the basis of the main performance parameters characterizing electro-membrane processes such as energy consumption, apparent flux and remineralization capacity. Results have shown that energy consumption can be reduced to less than 0.1 kWh/kg of salt transported to the permeate, in the case of permeate flow rates up to ~2 m3/h, while apparent fluxes can rise to ~170 l/(h*m2) for larger bypass flow rates, resulting in lower capital and overall costs.