Writer: Will Keener
New Mexico State University researchers are chipping away at the secrets of a promising water desalination technology to use in rural communities.
Graduate and post-graduate students, from NMSU’s Chemical Engineering Department, are cooperating on the project through the Institute for Energy and the Environment (IEE) and WERC, a consortium for environmental education and technology development. Most of the work has been accomplished at the U.S. Bureau of Reclamation’s Brackish Groundwater National Desalination Research Facility (BGNDRF) in Alamogordo.
Researchers reported on the work in Socorro, N.M., at the Water Resources Research Institute’s annual Water Research Symposium. Several papers and posters addressed the effort to better understand the Electrodialysis Reversal (EDR) process for desalination of brackish waters.
“The technology (EDR) is not 100 percent commercially deployable at this time, but pre-deployable,” said Abbas Ghassemi IEE executive director. “If we can get units with seven to 30 gallons per minute to work with different water chemistries, the system can be used in community settings.”
EDR is based on the behavior of solutions with dissolved salts when subjected to direct current, explained Lakshmi Pradeepa Vennam, a graduate student on the project. EDR performance is evaluated by two measures: separation percentage (SP), the amount of salts removed from the feed stream to obtain potable water, and current efficiency (CE), the efficiency that ions are moved across exchange membranes for a given current. Vennam studied the influence of temperature, flow-rate, and voltage on these two measures. Experiments were conducted at rates of 7, 9, and 11 gallons per minute, temperatures of 15 and 30 degrees C, and voltages of 15, 25, and 35 volts.
In her first round of experiments, she reported that higher temperatures showed improvements to both SP and CE. These temperatures also prevent damage to the membranes and increase their lifetimes. On the downside, heating the water is expensive and creates more evaporative loss. Her flow-rate experiments show that lower rates reduced scaling and fouling. Higher flow-rates reduce SP, because of insufficient time for ion exchange. Voltage increases cause both SP and CE to go up to a point, and then drop. Current inefficiencies and higher power requirements present problems for the technology, she said.
Using concentrated solutions of sodium, magnesium, and calcium, Vennam also noticed differences between bivalent and monovalent ion separation at temperature. The separation of the doubly charged or divalent ions, probably increased because of a higher mobility of these ions in water, she said. After a second round of testing, she hopes to move to a pilot test of the technology.
Purnima Praturi, a graduate student working on the project, is using a mathematical model to predict energy consumption by EDR. Models developed previously were based on solutions with equal numbers of positive and negative ions, while Praturi’s work makes use of experimental data using actual brackish groundwater. Praturi developed modeling equations to predict current, based on different concentrations of source water and energy consumption. She validated these predictions with experiments at different current levels and different temperatures. One thing the models show, she said, is how important the spacers in the stack are to efficiency.
Graduate student Ramya V. Chintakindi is looking at optimal current levels for EDR. Although suitable voltages for removing salt can cover a wide range, costs and other factors dictate that power levels be optimized. Chintakindi’s approach is to determine what current achieves maximum salt separation.
Using a pilot setup at the Bureau of Reclamation’s Alamogordo facility, she varied concentration of salts, flow velocity, temperature of the feedwater and applied voltages to study salt removal. She described a three-phase reaction curve of current climbing linearly, leveling, and then increasing again, as power (voltage) levels increased. This phase-curve has a deflection point, called the Limiting Current Density (LCD), which is the focus of her studies. She is studying the effect of temperature, flow-velocity and concentration changes on the LCD to better understand relationships between all the variables. Understanding these relationships is essential for end-cost reduction when operating at large municipal or industrial scales, she said.
The experimental results showing relations between temperature, flow-rate, and concentrations of feedwater were calculated because there were no previous equations with these particular water conditions. Significant to these experiments is the fact that previous research was confined to lab-prepared samples with deionized water or seawater, while Chintakindi’s work examined multi-salt solutions of brackish water.
In a related project, graduate student Venkat Ravi Kiran Paruchuri is addressing the question of what to do with concentrate removed in the EDR process. Paruchuri’s poster examined the possibility that some species of algae might be compatible with concentrates recovered in the EDR process. If this is so, the concentrate could be used to create biofuel stock or for other applications. Total dissolved solids in brackish waters can range from 1,200 parts per million (ppm) to 6,000 ppm, Paruchuri said. The concentrate from EDR has dissolved solids ranging from 3,000 ppm to16,000 ppm. His initial screening compared the growth media of the algae species with the water chemistry of the concentrate to identify any growth inhibitors in the concentrate. His goal is to follow up the initial work with experiments.
“At the end of the day, we need a combination of answers to questions that will make this technology affordable and applicable for small users,” said Ghassemi, who also acts as faculty adviser for the students. “Communities with high calcium or sulfate will want to know the effects of temperature and pH on the system, or if pretreatment will help. These are the kinds of questions we’re examining.