DETERMINING THE OPTIMAL ZLD METHOD FOR Ca AND SiO2 REMOVAL FROM RO SYSTEMS EFFLUENT

In RO (Reverse Osmosis), water on the feed side of the membrane becomes increasingly concentrated and supersaturated, with respect to sparingly soluble salts such as calcium carbonate (CaCO3), calcium sulfate (CaSO4), barium sulfate (BaSO4), and silica (SiO2). Therefore, recovery in RO systems is limited by the precipitation potential of sparingly soluble salts. Therefore, reuse of RO effluent could be limited by the precipitation potentials of these salts. Thus we were looking for procedures to prevent precipitation of soluble salts on the membrane surface in secondary RO. The objective of this research was to investigate methods to achieve maximum efficiency for precipitation of soluble salts in RO effluent by ZLD process as intermediate treatment step. This research was conducted on samples collected and tested from RO system effluent in Tange Alhad, Hajiabad, Zarin Dasht, Darab city, Fars province, Iran. In bench-scale for Ca removal, chemical precipitation with sodium hydroxide and fluidized bed crystallization and for silica removal adsorption with alum and sodium aluminate was used. The optimum ZLD processes for Ca and silica removal was fluidized bed crystallization with 100 mg/L of sodium aluminate and 100 mg/L NaOH. In this ZLD method, silica and calcium concentrations reduced by 90 and 55 percent, respectively.