Upgrading groundwater for irrigation using monovalent selective electrodialysis

Desalinated water reduces salt load and water needs in irrigation but lack hardness and sulfate ions required for optimal plant growth. Mixing with groundwater adds hardness but raises chloride and sodium levels limiting the amount of groundwater that can be used in the blend. Monovalent selective electrodialysis (SED) allows reducing chloride and sodium while preserving most of the hardness ions. This study examined under what conditions monovalent selective membranes are effective in treating groundwater for irrigation. All monovalent selective anion exchange membranes tested showed excellent selectivity for sulfate over chloride. Monovalent selective cation exchange membranes worked less well at the lower conductivities typical of brackish water, with only CSO membrane showing significant monovalent permselectivity (Pdm < 1) at conductivities < 4.5 dS/m. Keeping current densities for treating groundwater below limiting current found for solutions of pure sodium chloride at sodium levels found in the groundwater was key to preserving monovalent selectivity. Poisoning of CSO membranes monovalent cation selectivity could be reversed by acid wash and electrically driven re-deposition of polyethyleneimine on the membrane. When compared to irrigation with mix of groundwater and RO water, mix of SED and RO water used in irrigation gave better results in growing bell peppers.