The Mineral Recovery Enhanced Desalination (MRED) process for improved brackish water desalination

A 4 L/min pilot study of a pretreatment process for brackish water desalination, known as the Mineral Recovery Enhanced Desalination process, was designed and tested. The objectives of the process are to (1) reduce the volume and mass of concentrate requiring disposal, (2) remove scale-forming constituents, (3) recover commodity minerals, and (4) increase feed water recovery by a subsequent desalination system. The process consists of (1) air stripping to remove dissolved inorganic carbon (DIC), (2) high pH precipitation and membrane filtration to remove high-purity magnesium hydroxide, (3) ion exchange (IX) to remove calcium, and (4) nanofiltration (NF) to remove sulfate. The pretreated water consists of a monovalent salt solution with low scaling potential that allows increased feed water recovery and reduced waste production. The IX regenerant containing calcium can be combined with the high sulfate concentrate from the NF process to precipitate gypsum. The system achieved more than 98% removal of calcium, magnesium, and sulfate and more than 90% removal of DIC. No fouling of NF and reverse osmosis membranes was observed. A steady-state model was developed and calibrated to calculate the chemical quality and material balances for water and major ions.