Modular desalination concept with low-pressure reverse osmosis and capacitive deionization: Performance study of a pilot plant in Vietnam in comparison to seawater reverse osmosis

Brackish groundwater desalination by constant current membrane capacitive deionization (MCDI): Results of a long-term field trial in Central Australia

A long-term field trial of membrane capacitive deionization (MCDI) was conducted in a remote community in the Northern Territory of Australia, with the aim of producing safe palatable drinking water from groundwater that contains high concentrations of salt and hardness ions and other contaminants. This trial lasted for 1.5 years, which, to our knowledge, is one of the longest reported studies of pilot-scale MCDI field trials. The 8-module MCDI pilot unit reduced salt concentration to below the Australian Drinking Water Guideline value of 600 mg/L total dissolved solids (TDS) concentration with a relatively high water recovery of 71.6 ± 8.7 %. During continuous constant current operation and electrode discharging at near zero volts, a rapid performance deterioration occurred that was primarily attributed to insufficient desorption of multivalent ions from the porous carbon electrodes. Performance could be temporarily recovered using chemical cleaning and modified operating procedures however these approaches could not fundamentally resolve the issue of insufficient electrode performance regeneration. Constant current discharging of the electrodes to a negative cell cut-off voltage was hence employed to enhance the stability and overall performance of the MCDI unit during the continuous operation. An increase in selectivity of monovalent ions over divalent ions was also attained by implementing negative voltage discharging. The energy consumption of an MCDI system with a capacity of 1000 m3/day was projected to be 0.40∼0.53 kWh/m3, which is comparable to the energy consumption of electrodialysis reversal (EDR) and brackish water reverse osmosis (BWRO) systems of the same capacity. The relatively low maintenance requirements of the MCDI system rendered it the most cost-efficient water treatment technology for deployment in remote locations. The LCOW of an MCDI system with a capacity of 1000 m3/day was projected to be AU$1.059/m3 and AU$1.146/m3 under two operational modes, respectively. Further investigation of particular water-energy trade-offs amongst MCDI performance metrics is required to facilitate broader application of this promising water treatment technology.

Energetic Comparison of Flow-Electrode Capacitive Deionization and Membrane Technology: Assessment on Applicability in Desalination Fields

Flow-electrode capacitive deionization (FCDI) is a promising technology for sustainable water treatment. However, studies on the process have thus far been limited to lab-scale conditions and select fields of application. Such limitation is induced by several shortcomings, one of which is the absence of a comprehensive process model that accurately predicts the operational performance and the energy consumption of FCDI. In this study, a simulation model is newly proposed with initial validation based on experimental data and is then utilized to elucidate the performance and the specific energy consumption (SEC) of FCDI under multiple source water conditions ranging from near-groundwater to high salinity brine. Further, simulated pilot-scale FCDI system was compared with actual brackish water reverse osmosis (BWRO) and seawater reverse osmosis (SWRO) plant data with regard to SEC to determine the feasibility of FCDI as an alternative to the conventional membrane processes. Analysis showed that FCDI is competent for operation against brackish water solutions under all possible operational conditions with respect to the BWRO. Moreover, its distinction can be extended to the SWRO for seawater conditions through optimization of its total effective membrane area via scale-up. Accordingly, future directions for the advancement of FCDI was suggested to ultimately prompt the commercialization of the FCDI process.

Three-dimensional titanium mesh-based flow electrode capacitive deionization for salt separation and enrichment in high salinity water

Flow electrode capacitive deionization (FCDI) is a highly promising desalination technique known for its exceptional electrosorption capacity, making it suitable for efficient salt separation in high salinity water. However, the unsatisfactory charge transfer process between the flow electrode and current collector severely curtails the salt separation and enrichment performance of the FCDI device. To address this issue, three-dimensional titanium mesh (3D-TM) was proposed as a novel current collector for FCDI device, which significantly amplifies the charge transfer area and exhibits excellent salt separation performance. The 3D-TM current collector promotes the electron transfer, charge percolation, and ion migration processes through the electroconvection generated by the turbulence effect on the flow electrode. In the specific case of the 20-mesh 3D-TM, which is composed of 12 stacking layers of titanium mesh, the remarkable average salt removal rate and charge efficiency were achieved 5.06 μmol cm-2 min-1 and 92.9 % under an appropriate applied voltage of 2.0 V, respectively. Dramatically, the desalination performance maintained above 76.4 % over 100 desalination cycles at 2.0 V, demonstrating the exceptional cyclic stability of the 3D-TM FCDI cell. In the seawater desalination, the 3D-TM FCDI cell exhibited an impressive salt removal efficiency of 97.5 % (from 34.2 g L-1 to 0.84 g L-1) for 1 L East China seawater at 2.0 V for 24 h. For lithium-ion enrichment, the FCDI continuous desalting system achieved an astonishing concentration of 17.3 g L-1 for Li+ ions enrichment from an initial concentration of 1.30 g L-1, obtaining the average salt treating rate of 23.6 g m-2h-1 and charge efficiency of 80.0 %. Moreover, the lithium-sodium ions and lithium-magnesium ions enrichments were both conducted, yielding an enriched concentration of 10.4 g L-1 and 7.30 g L-1 for Li+ ions, respectively. These findings highlight the enormous potential of FCDI technology in industrial engineering applications, further establishing it as a highly viable solution.

Comparison of Pilot-Scale Capacitive Deionization (MCDI) and Low-Pressure Reverse Osmosis (LPRO) for PV-Powered Brackish Water Desalination in Morocco for Irrigation of Argan Trees

The use of saline water resources in agriculture is becoming a common practice in semi-arid and arid regions such as the Mediterranean. In the SmaCuMed project, the desalination of brackish groundwater (TDS = 2.8 g/L) for the irrigation of Argan trees in Essaouira, Morocco, to 2 g/L and 1 g/L (33% and 66% salt removal, respectively) using low-pressure reverse osmosis (LPRO) (p < 6 bar) and membrane capacitive deionization (MCDI) was tested at pilot scale. MCDI showed 40-70% lower specific energy consumption (SEC) and 10-20% higher water recovery; however, the throughput of LPRO (2.9 m³/h) was up to 1.5 times higher than that of MCDI. In addition, both technologies were successfully powered by PV solar energy with total water costs ranging from EUR 0.82 to EUR 1.34 per m³. In addition, the water quality in terms of sodium adsorption ratio was slightly higher with LPRO resulting in higher concentrations of Ca²⁺ and Mg²⁺, due to blending with feed water. In order to evaluate both technologies, additional criteria such as investment and specific water costs, operability and brine disposal have to be considered.

Optimization and Evaluation for the Capacitive Deionization Process of Wastewater Reuse in Combined Cycle Power Plants

Combined cycle power plants (CCPPs) use large amounts of water withdrawn from nearby rivers and generate wastewater containing ions and pollutants. Despite the need for wastewater reclamation, few technologies can successfully convert the wastewater into make-up water for CCPPs. Therefore, this study aimed to apply capacitive deionization (CDI) for wastewater reclamation in CCPPs. Using a bench-scale experimental unit, which included ion exchange membranes and carbon electrodes, response surface methodology (RSM) was used to optimize the operating conditions of the CDI process to increase the total dissolved solids (TDS) removal and product water ratio. The optimal conditions were found to be a voltage of 1.5 V, a flow rate of 15 mL/min, and an adsorption/desorption ratio of 1:0.8. The changes in CDI performance with time were also studied, and the foulants on the membranes, spacers, and electrodes were examined to understand the fouling mechanism. The TDS removal decreased from 93.65% to 55.70% after 10 days of operation due to the deposition of scale and organic matter. After chemical cleaning, the TDS removal rate recovered to 93.02%, which is close to the initial condition.