AWWA ACE95084

Sodium hypochlorite (NaOCl-also referred to as hypo or bleach) can be used as an alternative to chlorine gas for introducing chlorine into water (chlorination) for the purpose of disinfection. Several major cities have converted or are in the process of converting to hypochlorination for potable water disinfection: San Francisco, Seattle, Philadelphia, Norfolk, and Elizabethtown. The primary advantage of sodium hypochlorite lies in the area of safety. Because of the nature of chlorine gas (gaseous--with associated inhalation and atmospheric plume transport risks), release poses a serious threat to operating personnel as well as nearby communities. Being a strong oxidizing agent, sodium hypochlorite can cause chemical burns on contact; however, since it is stored and applied as a liquid, safety risks associated with inhalation are minimal. Although safer for persons working at and living near a chlorination facility, hypochlorite does have disadvantages: cost, more frequent deliveries, corrosion problems, and chlorate and other impurities. While evaluating disinfection options for New York City's Hill View Reservoir, several key issues were identified that could influence the recommended mode of chlorine addition. Safety and associated zoning issues favor hypochlorite use. However, the extent and impact of chlorate contamination of hypochlorite stock solutions was of concern. As such, the specific objectives of this project and the focus of this paper are: Evaluate initial chlorate concentrations in sodium hypochlorite solution of current and previous New York hypochlorite suppliers; evaluate chlorate formation in sodium hypochlorite solutions as functions of time and temperature. This paper provides a brief discussion of the regulatory aspects of chlorate in drinking water supply and the chemistry of chlorate formation in sodium hypochlorite stock solutions and offers several design suggestions for a sodium hypochlorite facility.