High-recovery inland desalination: Concentrate treatment by electrodialysis and batch reverse osmosis
Since fresh water resources are finite and stressed, it is of the utmost importance to develop alternative water resources. Efforts to recover and treat impaired water are happening in arid areas around the world, where water resources are already scarce. Such efforts are particularly focused on high-recovery treatment systems, accomplished by reducing the amount of liquid discharge with the intent to eventually reach zero liquid discharge. The goal of this research is to improve the feasibility of high-recovery inland desalination systems. Two types of high-recovery treatment systems were selected for study: (1) combined reverse osmosis (RO) and electrodialysis (ED) systems, such as the Zero Discharge Desalination (ZDD) process; and (2) reverse osmosis systems with concentrate recycling, such as the Concentrate Enhanced Recovery Reverse Osmosis® (CERRO) process. The objectives of this research were to investigate two challenges associated with electrodialysis (ED) used in high-recovery desalination processes, and to evaluate a reverse osmosis (RO) system with concentrate recycling for desalination and contaminant removal of recreational water. Experimental evaluation was performed with laboratory-scale-ED systems, to investigate the voltage loss associated with the electrodes and rinse solution. Large scale ED processes can be precisely simulated in laboratory-scale ED systems, but power calculations will be inaccurate if the electrode and electrode solution voltage losses are not properly considered. A mathematical model was developed using electrochemistry theory to effectively predict the voltage loss associated with the electrodes and rinse solutions by considering the membrane specifications, electrode compartment geometries, solution conductivity, and electrode material. The model was compared against experimental data and results demonstrated the prediction of voltage drop within 5% error. A second project was conducted to investigate a membrane-modification process to minimize the shunt currents in high-recovery ED applications. In high-recovery desalination systems, the ED process can produce a concentrate stream with a very high electrical conductivity, and this concentrate solution flowing through the manifold can become a significant short-circuiting path for the electrical current. (The electrical current passes through the electrolyte and then shunts into the manifold distribution system of the ED cell.) This short-circuiting in the compartments causes serious overheating to the membranes and spacer and may cause permanent damage to the system. This research project evaluates the use of chemicals to intentionally neutralize the ion-exchange capacity of the membrane surrounding the manifold and thereby increase the manifold resistance to minimize the shunt current. Results identified neutralizing chemicals that were able to reduce the membrane in-plane conductivity by 90% and decrease the ion exchange capacity more than 80%. Neutralized membranes were analyzed with Fourier-transform infrared (FTIR) spectroscopy and atomic force microscopy (AFM), which revealed distinct changes in chemical moiety and surface morphology. Experimentation with neutralized membranes demonstrated stable long-term performance. A third investigation was performed to evaluate a batch reverse osmosis process with concentrate recycling (UTEP-patented process called CERRO ®) for desalination and contaminant removal from sun-exposed swimming pools. Cyanuric acid (CYA) is a stabilizer added to the pools to reduce chlorine photodegradation. However, as water evaporates from the pool, CYA and total dissolved solids (TDS) are concentrated. Subsequent chlorination is then primarily sequestered to the concentrated CYA, which may result in ineffective disinfection. Experimentation with the CERRO ® process was performed using nanofiltration (NF) and seawater reverse osmosis (SWRO) membranes, which demonstrated removal of more than 90% of CYA and TDS, achieving 70% and 85% recoveries using NF and SWRO, respectively of water that would otherwise be lost.
Civil engineering|Environmental engineering
Ortega-Corral, Noe, "High-recovery inland desalination: Concentrate treatment by electrodialysis and batch reverse osmosis" (2013). ETD Collection for University of Texas, El Paso. AAI3682476.