The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. Citation Ahdab, Yvana D. et al. "Brackish water desalination for greenhouses: Improving groundwater quality for irrigation using monovalent selective electrodialysis reversal." Forthcoming in

The most common desalination technology for treating brackish irrigation water is reverse osmosis (RO). RO yields product waters low in monovalent ions that are harmful to crops (Na+ and Cl–) and in divalent ions that encourage crop growth (Ca2+, Mg2+, and SO4 2–). Fertilizer or divalent-rich brackish water must be mixed with the desalinated water to reintroduce these nutrients. Monovalent selective electrodialysis (MSED) provides an alternative to RO that selectively extracts monovalent ions while retaining divalent ions. This paper investigates the monovalent selectivity and potential of the new cost-effective Fujifilm MSED membranes to treat brackish source water in greenhouses, with a comparison to the widely used Neosepta MSED membranes. Thirteen groundwater compositions serve as feedwater to an MSED experimental setup to characterize membrane selectivity, ion transport, limiting current, and membrane resistance. The Fujifilm membranes demonstrate notable selectivity for all compositions. On average, they remove six sodium ions, compared to Neosepta’s four, for every calcium ion and 13 sodium ions, compared to Neosepta’s seven, for every magnesium ion, while their bench-scale cost is 68% lower than that of the Neosepta membranes. The Fujifilm selectivity values are used to calculate annual fertilizer savings of MSED relative to RO, which average $4995/ha for 6000 brackish groundwaters across the United States.

This paper uses chemical and physical data from a large 2017 U.S. Geological Survey groundwater dataset with wells in the U.S. and three smaller international groundwater datasets with wells primarily in Australia and Spain to carry out a comprehensive investigation of brackish groundwater composition in relation to minimum desalination energy costs. First, we compute the site-specific least work required for groundwater desalination. Least work of separation represents a baseline for specific energy consumption of desalination systems. We develop simplified equations based on the U.S. data for least work as a function of water recovery ratio and a proxy variable for composition, either total dissolved solids, specific conductance, molality or ionic strength. We show that the U.S. correlations for total dissolved solids and molality may be applied to the international datasets. We find that total molality can be used to calculate the least work of dilute solutions with very high accuracy. Then, we examine the effects of groundwater solute composition on minimum energy requirements, showing that separation requirements increase from calcium to sodium for cations and from sulfate to bicarbonate to chloride for anions, for any given TDS concentration. We study the geographic distribution of least work, total dissolved solids, and major ions concentration across the U.S. We determine areas with both low least work and high water stress in order to highlight regions holding potential for desalination to decrease the disparity between high water demand and low water supply. Finally, we discuss the implications of the USGS results on water resource planning, by comparing least work to the specific energy consumption of brackish water reverse osmosis plants and showing the scaling propensity of major electrolytes and silica in the U.S. groundwater samples.

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Monovalent selective electrodialysis (MSED) is a variant of conventional electrodialysis (ED) that employs selective ion exchange membranes to preferentially remove monovalent ions relative to divalent ions. This process can be beneficial when the divalent rich stream has potential applications. In agriculture, for example, a stream rich in calcium and magnesium is deemed beneficial for crops and can decrease the use of fertilizers that would otherwise need to be re-introduced to the source water prior to irrigation. MSED has been used for salt production, brine concentration, and irrigation. An experimentally validated computational model to predict its performance, however, is not available in the literature. The present work uses concepts from conventional ED modelling to build a high-resolution predictive model for the performance of MSED. The model was validated with over 32 experiments at different operating conditions and observed to fit the data to within 6% and 8% for two different types of membranes. All voltage predictions were within 10% of experiments conducted. The model was then used to predict permselectivity across different salinities and compositions. These values were extended to investigate the economic benefits of using MSED to save fertilizers for greenhouses across the U.S. Results showed an average of $4,991 saved per hectare when employing MSED technology. These values aligned with predictions from two previous techno-economic studies conducted investigating MSED for agriculture.