Pretreatment of brackish water reverse osmosis (BWRO) concentrate to enhance water recovery in inland desalination plants by direct contact membrane distillation (DCMD)

Abstract

Inland brackish water desalination plants typically rely on reverse osmosis (RO) for the production of drinking water. However, the usual water recovery in these plants (i.e., 50–75%) results in fairly large quantities of the reject (concentrate) stream that needs to be disposed at considerable cost and with significant adverse environmental impacts. Membrane distillation (MD) offers a unique opportunity to improve the performance of these brackish water reverse osmosis (BWRO) plants by recovering additional clean permeate and reducing the volume of concentrate for disposal. In this study, barite precipitation was utilized to remove about 4000 mg/L of sulfate from actual BWRO concentrate to prevent interference with MD operation. The effect of barium concentration and mixing condition was evaluated for its impact on the removal of sulfate as the key ingredient of inorganic scales that would form in BWRO concentrate with increased water recovery. The pre-treated BWRO concentrate was used as feed in direct contact membrane distillation (DCMD) system operated at 60 °C to achieve at least 85% water recovery. The impact of concentration polarization on the performance of MD explored in a system operated at 80 °C revealed a potential for amorphous silica fouling of the membrane.