Deposition of calcium and magnesium from ro concentrate by means of seed crystallization and production of softened water for technical purposes

Abstract

Existence of reverse osmosis (RO) concentrate effluents often creates a problem of its handling and utilization. This limits industrial applications of this technique for drinking and industrial water supply. The article presents the results of development and field testing of new proposed technique that removes calcium carbonate and magnesium hydroxide from RO retentate streams using seeding techniques. This new concentrate utilization technique uses RO membrane unit operated in circulation mode. Seed crystals of calcium carbonate and magnesium hydroxide are added to the circulating concentrate flow. Calcium and magnesium deposit on the seed crystals due to their concentrations increase in membrane channel. The test unit is tailored with membrane modules that have “open channels” and eliminate trapping of particles by a spacer and increase of flow resistance. Seed crystals are suspended in circulating concentrate stream. The seed crystallization procedure provides up to 90% removal of hardness and alkalinity from RO concentrate and decrease concentrate total dissolved solids due to calcium and magnesium salts precipitation. RO retentate thus becomes a softened water that can be blended with product water to produce technical water used for cooling or heating, as well as for drinking purposes. The developed process does not require softening reagents that are used in stoichiometric amounts to deposit calcium carbonate and magnesium hydroxide during chemical softening. Sodium hydroxide is used only to produce seed crystals and to maintain pH of feed water at value of 8.8–9.0. Consumption of caustic by new seed crystallization process constitutes 2%–4% of the stoichiometric amount.