Chlorate and Perchlorate Minimization with On-Site Hypochlorite Generation


  • Aaron Duke - Hazen and Sawyer

As utilities evaluate the risk to operations personnel and the public posed by the use of chlorine gas for disinfection, many of them are considering conversion to sodium hypochlorite. Several options exist to implement disinfection via sodium hypochlorite including on-site hypochlorite generation systems where the utility owns and operates the system, bulk delivery of chemicals from outside suppliers, and “over-the-fence” systems where the chemical supplier owns and maintains the chemical storage and dosing facilities. While bulk delivery of sodium hypochlorite solutions from outside vendors is most prevalent, the use of on-site generation for sodium hypochlorite is gaining acceptance. The on-site systems are a solid alternative to bulk hypochlorite delivery, but they also present several challenges that must be understood and accounted for in good system designs.

The greatest challenge currently faced with hypochlorite solutions are the potential contaminants that may be introduced by their use. Both bulk delivered and on-site generated hypochlorite may contain regulated contaminants such as bromate and chlorite, in addition to unregulated contaminants such as chlorate and perchlorate, any of which may be formed during the manufacturing process or during storage. The formation of chlorate and perchlorate in stored hypochlorite solutions is exacerbated by elevated temperatures, concentration, and age of the hypochlorite solution, though careful management of the solution can minimize some of the impacts. While the concentration of chlorate in bulk hypochlorite solutions can be controlled by careful handling and storage, the production of chlorate in on-site generated hypochlorite appears to be beyond the ability of any utility to control and varies from generator to generator. The presentation will provide an overview of hypochlorite chemistry, contaminant occurrence in on-site generated hypochlorite, potential management strategies, a demonstration of the on-line degradation model, and will use the model results to demonstrate the impact of control strategies including dilution, pH control, temperature reduction, and use of low-bromide salts during hypochlorite production.

The presentation will also provide guidance to audience members on proper design of on-site generation systems to address lessons learned from past design projects. From a 21,000 pound per day facility in Florida, the largest installation in the United States, to systems that only utilize several hundred pounds per day, there are several key items that should be incorporated into good designs. These factors include controlling the water temperature entering the electrolytic cells, hydrogen gas management, and methods to control calcium build-up in system piping.

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