Removal of calcium carbonate from reverse osmosis concentrate by seed crystallization

A process for the removal of calcium carbonate from reverse osmosis (RO) units by seeded crystallization has been described. Seed crystals are suspended in the circulating concentrate stream. Owing to the precipitation of calcium carbonate on the seed, the hardness and total dissolved solids of the concentrate are reduced and higher recovery rates are achieved, thereby facilitating concentrate disposal and reuse. The process has been implemented using open-channel membrane modules resistant to the impact of scaling and fouling on membrane transport. Seed crystals are produced by introducing a sodium hydroxide solution into the concentrate. Partial removal of grown seed crystals is accomplished by settling after membrane backwashing. The rate of calcium carbonate crystallization, depending on the feed water composition, the amount of crystals introduced, and the permeate recovery rate, can be determined using experimental plots. A flow diagram of an RO unit operating in the continuous mode is proposed. Reverse-osmosis process design procedures to determine the amount of circulating seed crystals, the caustic soda consumption, and the size of calcium carbonate sedimentation tanks have been described. Calculations have shown that in order to run the calcium carbonate crystallization process in a continuous-flow RO unit, the circulating concentrate stream is to be periodically dosed with a sodium hydroxide solution in an amount of no more than 5% of its stoichiometric amount required to precipitate the same amount of calcium carbonate in feed water.