Treatment of groundwater by reverse osmosis and monovalent electrodialysis for irrigation of greenhouse crops

As water scarcity increasingly constrains agricultural production in arid regions with limited freshwater resources, there is a growing interest in considering brackish water sources for irrigation. However, elevated salinity of brackish water sources often limits direct use of this water for irrigation of high-value, salt-sensitive crops. Brackish water can be upgraded via reverse osmosis (RO) which effectively reduces salinity but also eliminates divalent ions required for plant growth. While divalent ions can be replenished in the RO permeate by blending with brackish groundwater this approach introduces undesirable levels of sodium and chloride ions. To overcome this limitation, this study employed monovalent selective electrodialysis (MSED) to reduce chloride and sodium concentrations while preserving divalent ions to prepare irrigation water from brackish water. To reduce MSED cost, a two-pronged approach was adopted combining blending of MSED diluate with RO permeate and optimization of MSED operation. The blending strategy reduced the required extent of monovalent ion removal in MSED (typically ∼40–60% of chloride). MSED unit size was further reduced by operating at 90% of the sodium-defined limiting current. Irrigation experiments with lettuce and strawberry demonstrated superior performance of RO:MSED water blends relative to RO-treated water and RO/ brackish water blends. RO:MSED water blending achieved higher crop yields (∼54% increase in strawberry fruit yield, 36% increase in lettuce yield) with minimal cost penalty relative to RO/brackish water blending. The study results demonstrate that integrated RO:MSED is a viable approach for brackish water upgrading, with flexibility to tailor irrigation water composition to crop-specific requirements.