Utilization of concentrate from reverse osmosis units in the production of electrolytic sodium hypochlorite

Summary

Owing to high efficiency and minimum chemical consumption reverse osmosis and nanofiltration have been widely used in process flow schemes of drinking water demineralization and softening and water preparation for industrial use (in steam boiler-houses, recycling cooling circuits, heat network make up etc.). However, the methods of membrane separation are accompanied by concentrate generation which is very difficult to process and utilize. The data on utilization of concentrates of membrane separation units with obtaining chloride-sodium raw material for the production of electrolytic low-concentrated sodium hypochlorite is presented. Since the waste stream of reverse osmosis units contains elevated concentrations not only of chloride ions but also of Са²⁺, Mg²⁺, НCO^3- and SO₄^2- ions it would be reasonable at the first stage to reduce the amount of process concentrate by repeated concentrating as per flowchart «nanofiltration-reverse osmosis». Further on the concentrate of nanofiltration containing mainly Са²⁺, Mg²⁺ and SO₄^2- divalent ions is subject to chemical treatment as per flowchart: at the first stage with barium compounds; at the second stage with sodium carbonate and hydroxide. This will allow separating practically insoluble BaSO₄ from the solution with its precipitation in a vortex reactor or first-stage lamellar separator; then CaCO₃ and Mg(OH)₂ low-soluble in alkaline environment are precipitated in the second stage reactor. BaSO₄, CaCO₃ and Mg(OH)₂ insoluble salts removed from the mass balance are dewatered in a filter-press and sold as commodity or raw products. Aqueous solution of sodium chloride is repeatedly concentrated by three-stage reverse osmosis to obtain 2–2.5% aqueous solution of table salt – high-grade raw material for the production of electrolytic sodium hypochlorite with 6–8 g/l chlorine equivalent concentration. Chlorine-containing product can be used for drinking and waste water disinfection, biocidal processing of cooling towers, heat exchanging units for preventing and removing biofouling, washing ultra-and microfiltration membranes, disinfecting water treatment facilities and equipment, pipelines and other components used in the production of drinking and process water.