Effects of Sodium Hypochlorite in Water Treatment on Chlorate, Bromate and Perchlorate
In order to be able to determine the effects of sodium hypochlorite in water treatment on the contaminants chlorate, bromate, and perchlorate, a literature review has been undertaken with the intent of being able to determine several key pieces of information regarding these four compounds. The formation of chlorate, bromate, and perchlorate in an on-site setting as a result of sodium hypochlorite will be undertaken, discussing how these three components occur, the typical observations associated with their formation, and addressing the chemistry and mechanics of their formation. The formation of chlorate, bromate, and perchlorate by sources other than sodium hypochlorite will be reviewed, looking at the alternative causes of these three contaminants, and the health effects of sodium hypochlorite and these three contaminants will be addressed, including how to treat the byproducts and the disinfection processes. Information regarding the treatment of these three contaminants will be reviewed, and a discussion of potential future treatments will be included. Ways to measure chlorate, bromate, and perchlorate will be reviewed, including information regarding their measurement both in an on-site facility and within a laboratory based setting, including the typical measurements for each of these three contaminants. Through the gathering of this information, the research question for this thesis will be able to be adequately addressed, providing not only answers to key research questions, but working to serve as a building block for future research into this important topic.
Sodium hypochlorite is formed in the manufacturing industry and on-site in the water treatment plants themselves. On-site generation of sodium hypochlorite is done through the use of electrolyzer systems, wherein crystallized salt is dissolved and used for electrolysis, providing a resulting one to three days of stored sodium hypochlorite (Casson & Bess, 2006). The water supply flows through the water softener, reducing water hardness, and a portion of this softened water is added in the salt dissolver, making a concentrated salt brine solution, which is them pumped through the electrolyzer, creating sodium hypochlorite (Casson & Bess, 2006). This is done through the following equation: NaCl + H2O +2e = NaOCl +H2; 3.5 lbs + 15gal. +2.5KwH = 0.8% NaOCl (Casson & Bess, 2006). These electrolyzers will need cleaning every one month to every six months, depending on usage.
Manufactured bleach is created through a several step process as well; first, caustic soda, chlorine, and water must be gathered. The caustic soda is shipped as a concentrate solution and must be diluted by half with water in order to form a 25% solution, causing a chemical reaction; the solution must be allowed to cool before proceeding. The chlorine and the caustic soda react together to form sodium hypochlorite, taking place in a continuous reactor, an almost instantaneous process; the bleach is then filtered to remove the impurities that may cause discoloration or speed up decomposition, after which time it is bottled up and shipped out for distribution; both range manufactured sodium hypochlorite and on-site manufactured sodium hypochlorite range in concentrations of 11-15%, with most being between 11-13%. Sodium hypochlorite should be, ideally, no more than four days old at the time of delivery to the treatment plant, and should be stored at temperatures no greater than 68o during that time period; however the age of the sodium hypochlorite and the subsequent amount of time that it may be stored prior to delivery can vary depending on the contracts that the water treatment plant makes with the manufacturer of the bleach itself. Sodium hypochlorite, when stored, has an average shelf life of six months, assuming that ideal storage conditions were maintained. After such a time, the sodium hypochlorite starts to degrade, becoming 20% less effective each subsequent year of storage, even when in its original bottle.
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