Analysis of Boron Removal for Reverse Osmosis, Ion Exchange, and Capacitive Deionization

Defluorination technology is crucial for ensuring the safety of accessible water. The application of capacitive deionization (CDI) technology faces challenges due to competitive adsorption of fluoride ions within complex natural fluoride-rich brackish water matrices, which often contain high levels of dissolved inorganic carbon (DIC) species (mainly HCO3– and CO32–). These DIC species are pH-dependent, playing a significant role in the selective removal of fluoride by the CDI process. Thus, there is a knowledge gap in understanding the effects of membranes in membrane capacitive deionization (MCDI) on fluoride removal. In this study, we examined the key operating parameters in CDI and MCDI, including applied constant voltages and different types of anion-exchange membranes (AEMs), on the desalination performance in F- and dissolved inorganic carbon water matrices. The application of AEMs significantly improve the salt adsorption capacity (SAC) for both F- and DIC species, and reduced energy consumption. However, it simultaneously results in a notable decrease in F- selectivity as membranes control mass transfer. Higher applied voltages enhance the SAC performance for F- and DIC species, but also induce more severe Faradaic reactions, leading to increased energy consumption and lower energy efficiency. Additionally, ion species and pH changes during CDI and MCDI processes are interrelated, indicating that stability tests of CDI electrodes in batch mode are not reliable when using the same testing solution repeatedly. The diverse valence states of ions in the solution impact pH variations under different voltages in the CDI/MCDI process. These findings provide valuable insights into the development of water purification and desalination technology, particularly for the application and further advancement of selective fluoride removal by the CDI process.

Automation of membrane capacitive deionization process using reinforcement learning

Although the energy efficiency of brackish water capacitive deionization (CDI) and reverse osmosis (RO) have been extensively compared, their relative costs remain poorly defined. We develop a parametric model to estimate the levelized cost of water (LCOW) of three CDI configurations (CDI, membrane CDI, and flow electrode CDI) and compare it with the LCOW of brackish water RO calculated using a process-based optimization model. We find significant deviations between cost-optimal and energy-optimal RO design and operation, highlighting the importance of LCOW in comparative evaluations of desalination technologies. Our results suggest that material (including electrode and ion exchange membrane) costs are the largest cost component for CDI processes. As such, the economic viability of CDI critically depends on the component lifespan, with lifespans longer than 1 year (105 cycles for 5 min cycle duration) required to reduce brackish water desalination costs relative to RO. Finally, sensitivity analyses indicate that CDI processes are unlikely to be cost-competitive against RO for feedwater concentrations greater than 2 g/L. Future research to enhance the economic feasibility of CDI processes should focus on developing more durable electrodes, increasing cost-normalized electrode capacitance, and developing low-cost ion exchange membranes and coatings.

Reverse osmosis (RO) brine from water reclamation facility is a potential untapped water source, provided a feasible and economical treatment process is available to recover this waste stream. Organic and inorganic compounds are two major groups of pollutants in the RO brine. In this study, an integrated treatment scheme consisting of a biological activated carbon (BAC) column and a low pressure Capacitive Deionization (CDI) process was investigated. BAC was used as a pretreatment to remove the organic compounds prior to the inorganic removal using the CDI process. Two empty bed contact times, namely 20 min and 40 min tested in the BAC columns provided similar TOC removal efficiency within the range of 15–21%. High ions removals of more than 85% from the RO brine were achieved in the CDI process when operated with water recovery up to 89%. This study has successfully demonstrated that the integrated BAC with CDI process has high potential to increase water recovery of a water reclamation plant while gaining the advantage of a reduced volume of RO brine for disposal. This system could further contribute to enhancement of sustainable water reclamation practice.

Optimizing energy efficiency in brackish water reverse osmosis (BWRO): A comprehensive study on prioritizing critical operating parameters for specific energy consumption minimization

Evaluation of operational performance in an integrated RO and RED hybrid desalination process using lava seawater

Integrated pretreatment with capacitive deionization for reverse osmosis reject recovery from water reclamation plant

Pretreatment for capacitive deionization: Feasibility tests using activated filter media and granule activated carbon filtration

Implication of zeta potential at different salinities on boron removal by RO membranes

Long-term intermittent operation of a full-scale BWRO desalination plant

Block copolymer coated carbon nanotube membrane anodes for enhanced and multipurpose hybrid capacitive deionization

Capacitive deionization (CDI) is a developing technology for water desalination applications with benefits over current methods such as reverse osmosis (RO) and distillation due to its operation at lower temperatures and pressure. CDI uses small applied potentials (<2.0 V) to selectively separate ions from feed water streams by reversibly adsorbing these ions in the electric double layer of conductive, high surface area, and porous carbon electrodes. This reversible absorption process can be used in a variety of configurations including flow-by, flow-through, and flowable electrode arrangements.1 Classic CDI systems have charge efficiency (equivalent charge of salt adsorbed per electronic charge input) values well below 100% due to parasitic discharge processes and Faradaic reactions.2,3 Therefore, ion-exchange membranes have been added into these systems to increase the effective adsorption capacity of the electrodes and limit the extent of parasitic processes. Overall, CDI systems are considered to be more efficient than RO membranes for many lower concentration salt streams (<5000 ppm) due to differences in its separation mechanism.4 Shown in Figure 1 is an energy cost comparison of CDI and RO technologies as a function of salt concentration. CDI is shown with energy recovery values of 0, 25, and 50%. As the concentration of NaCl decreases, there is a distinct region where the energy cost of this separation can best RO. The more recent addition of ion-exchange membranes in a membrane capacitive deionization (MCDI) process can further increase the charge efficiency to over 100% in certain instances and aid in further decreasing energy costs.5 The efficiency of both the CDI and MCDI process make them attractive options for many larger scale applications where energy cost becomes a dominant factor in choosing the water treatment system. In this talk, a comparison between the energy costs of RO and CDI/MCDI will be given. The use of recent surface chemistries to increase the charge efficiency will be shown to decrease the energy cost of CDI/MCDI processes further. Finally, applications will be reviewed where CDI and MCDI can be used as a standalone option as well as interface with existing membrane processes. Acknowledgements The authors are grateful to the U.S. - China Clean Energy Research Center, U.S. Department of Energy for project funding (No. DE-PI0000017).

The wastewater–seawater (WW-SW) integrated reverse osmosis (RO) process has gained much attention in and out of academia due to its energy saving capability, economic benefits, and sustainability. The other advantage of this process is to reduce boron concentration in the RO permeate that can exclude the post-treatment process. However, there are multiple design constraints regarding boron removal that restrict process design in the WW-SW integrated system. In this study, uncertainties in design factors of the WW-SW integrated system in consideration of boron removal have been explored. In comprehensive consideration of the blending ratio of between WW and SW, regulatory water quality standard, specific energy consumption (SEC), specific water cost, and RO recovery rate, a range of 15,000~20,000 mg/L feed turned out to be the most appropriate. Furthermore, boron rejection tests with SWRO (seawater reverse osmosis) and BWRO (brackish water reverse osmosis) membranes under actual WW-SW integration found a critical reduction in boron rejection at less than 20 bar of operating pressure. These findings emphasize the importance of caution in the use of BWRO membranes in the WW-SW integrated RO system.

Comparison of energy consumption in desalination by capacitive deionization and reverse osmosis

Performance comparison and energy consumption analysis of capacitive deionization and membrane capacitive deionization processes