In energy production systems, the allowable concentration of hardness in feed water is 1-10 µg/l to ensure smooth operation of medium and high pressure (3.9-13.4 MPa) steam drum boilers. Calculations show that during reverse osmosis (RO) processing of Caspian Sea water, the hardness of sweetened water (permeate) is higher than required: depending on the prcentage of permeate (50-80%) and the selection of the membrane (99-99.8%), it varies between 0.23 mg/l and 2.1 mg/l. Therefore, softening of the permeate is required. Usually, the Na-cationization method is used for this purpose, and cationite regeneration is carried out with a 3-8% solution of external NaCl salt.
The article shows that in the softening of the permeate, the waste concentrate which is rich in NaCl and Na2SO4 salts of the RO process can be used instead of external NaCl salt for the regeneration of the cationite. Their concentration reaches 3-4%. In this case, the ion-exchange process takes place in equilibrium conditions. Approximately 40% of KU-2 cationite which has total ion-exchange capacity of 1700 mq/l is used in the permeate softening process. The residual hardness concentration of the softened permeate is 0.5-33 μg/l, and when the selectivity of the membrane is taken as 99.4-99.8%, the hardness concentration drops to the required level of 0.5-10 μg/l.
The authors believe that the proposed method can be effective for obtaining softened permeates from ocean and other saline waters.
Research were conducted by analytical method and some simplifying conditions. Therefore, it is planned to verify the obtained results by experimental method in the future.