Can Bleach Degradation in Sodium Hypochlorite Storage Tanks Be Controlled?
By Mike Wilson
Is it possible to control the rate of sodium hypochlorite degradation in storage tanks? This is an important question for operators of cooling towers and other industrial process water users that depend on sodium hypochlorite (NaOCl) for microbial control. This rate may be impacted by a number of factors and this article explores ways users may take to minimize NaOCl decay in order to achieve effective water treatment.
NaOCl solution is a common biocide used to treat water in cooling towers and other industrial processes as well as drinking water and swimming pools. Sodium hypochlorite solutions ranging from 3% to 8.25% NaOCl by weight are sold as common household bleach. Recently, Clorox increased its standard bleach from 5.5% to 8.25% NaOCl by weight. Stronger solutions are sold for industrial use. As sodium hypochlorite decomposes, its dosage rate must be increased to maintain the same level of chlorination. Therefore, hypochlorite procurement cost can be minimized by preventing its decomposition, which is affected by seven factors:
- Initial strength of solution: Higher strength solutions decompose faster.
- Storage temperature: Higher temperatures cause rapid decomposition.
- Storage duration: Sodium hypochlorite continues to decompose over time.
- pH of solution: pH in the range of 11.9 to 13 results in lowest rate of decomposition.
- Ionic strength of solution: Higher ionic strength aggravates decomposition.
- Light: UV light causes rapid decomposition.
- Contaminants such as organic matter and certain heavy metal cations cause decomposition.
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