By Gerald F. Connell
United States drinking water systems are always on the alert for contaminants that are in the chemicals used in the treatment process or that develop in the treatment process. Many of these contaminants have been identified by researchers and regulators and are regulated under the NSF/ANSI Standard 60. Changes to Standard 60 are always under consideration and new changes that can impact the disinfection process are anticipated to be effective in the year 2013. Publication of the changes is expected in the year 2011.
Standard 60 covers the chemicals used in the treatment of water. There are many chemicals used including those that act as coagulants, flocculants, softeners, pH controllers, corrosion controllers, scale controllers, disinfection and others. The concerns here are the disinfection chemicals, and sodium hypochlorite and chlorine, in particular.
Sodium hypochlorite for bulk use is produced by reacting chlorine (liquid or gas) with sodium hydroxide. Sodium hydroxide solution contains bromine as sodium bromide, a salt of bromine, dissolved in the sodium hydroxide solution. Chlorine also can contain bromine in the liquid chlorine as elemental bromine. Almost all of the bromine in the two chemicals becomes bromate in the resultant hypochlorite solution. Sodium hypochlorite can also be produced using an electrolytic cell, on-site generation (OSG), or using a brine feed.
Although there may be other changes to Standard 60, those effecting bleach are of great importance to the industry. The changes relate to sodium hypochlorite and the contaminants in hypochlorite. These contaminants are chlorate, perchlorate and bromate.
• Bromate may either be present in the bleach or produced as a result of the disinfection treatment process in the presence of bleach. Bromate is a potent human carcinogen.
• Perchlorate affects the ability of the thyroid gland to take up iodine. This would affect the functions of the thyroid gland and its performance in the body.
• Chlorate can affect the health of certain population groups such as senior citizens, children, etc. Chlorate is included in the contaminant candidate list and will probably be included in the unregulated contaminant mandatory rule.
It is important to look at each compound and their sources. Each can be traced to sodium hypochlorite. Hypochlorite is already known to have its strength decompose over time. Decomposition is caused by higher temperatures and higher concentrations. Scaling will occur if dilution water is not softened, also pipe joints often leak and feed is frequently impacted by air binding in the pumps among other problems.
Bromate comes from two separate sources in drinking water. The first is in the hypochlorite manufacturing process itself. Sodium hypochlorite is generally produced for bulk use by the reaction of chlorine gas with sodium hydroxide or can be produced on-site by the electrolysis of brine (salt) solutions. The compounds used in bulk production (chlorine gas and sodium hydroxide) can contain bromine (chlorine gas) or bromide (sodium hydroxide). The reaction of chlorine gas and sodium hydroxide produces sodium hypochlorite. The bromine in the chlorine gas and bromide in sodium hydroxide are converted to bromate at the pH level of the sodium hypochlorite solution produced by the reaction. The addition of this hypochlorite to water in the disinfection process adds the bromate present in the hypochlorite solution to the finished water. Brine is used to feed on-site generation, which also produces hypochlorite with bromated levels.
The second is the raw water supply. Bromide ions can be present in the raw water supply, surface or ground water. When water containing these bromide ions is exposed to disinfection using the ozonation process, the reaction of bromide with ozone will produce bromate ions. As a result, all water treatment plants using the ozonation process will be required to test for bromates in their finished water.
Whereas the development of bromate in the ozonation process requires testing of the finished water for bromate content, the treatment with hypochlorite does not require testing the finished water for bromates. The quantity of bromate present in the finished water from the hypochlorite disinfection process can be controlled by limiting the bromide concentration in the hypochlorite source or the hypochlorite manufacturing process.
Options in Bromate Influence
Choice of disinfectant, then, can have a significant impact upon the presence of bromate in the finished water. Both ozone and hypochlorite disinfection would require additional awareness. Testing of the finished water would be necessary if ozone is used. Control of the bromated content added by hypochlorite would require the supplier of the sodium hypochlorite to meet a more stringent guideline on the quality of the hypochlorite.
On the other hand the use of hypochlorite would require the water treatment facility to obtain a source of hypochlorite that would meet maximum hypochlorite contaminant requirements. Water treatment facilities must be aware of these needs. Analysis of the raw water and its bromide content would be a requirement for plants using ozone. In addition, a monthly analysis for bromated in the finished water would also be required. It is clear that the use of either sodium hypochlorite would require additional testing and analytical costs on the part of the water treatment plant or the hypochlorite supplier.
What choice does the utility or water treatment facility have? One clear choice is to use chlorine gas as the disinfectant since there is no reaction with chlorine gas that will produce bromates.
Perchlorate is a product of sodium hypochlorite decomposition. The longer hypochlorite is kept by the utility before use, the more likely the significant increase in perchlorate. Also, the development of perchlorate's use in rocket propellants and the improper disposal of wastes from the manufacture of these propellants has been a cause for the appearance of perchlorate in raw water supplies.
Options In Perchlorate Influence
Rapid turnover of the hypochlorite and/or a reduction in inventory of the sodium hypochlorite at the treatment plant will aid in the reduction of perchlorate development. Conversion to chlorine gas from hypochlorite would also be of help since perchlorate is not present in chlorine.
Chlorate is formed when sodium hypochlorite decomposes in the bleach solution. Thermal decomposition of bleach is the primary source of chlorate.
Options In Chlorate Influence
Quick turnover of sodium hypochlorite is the best method known to keep the chlorate level low. This could require frequent cleaning of the storage tanks so that a heel of "old" sodium hypochlorite is not kept in the tank. Temperature reduction and/or dilution on receipt from the supplier would also reduce the formation of chlorate. Once again, conversion of hypochlorite to chlorine gas would eliminate the concern over chlorate.
The following are the current or proposed regulatory actions or regulations for each of the three contaminants or byproducts mentioned above:
1. Bromate -- The maximum level of bromine allowed in sodium hypochlorite is expected to be reduced by January, 2013 to around a level of 39 ppm. Currently, 69 ppm of bromate is the Maximum Contaminant Level allowed in sodium hypochlorite. Since bromated is produced in the ozonation process, ozone's use in the treatment process could be reconsidered.
2. Perchlorate -- Several states have established regulatory limits for perchlorate in drinking water. Standard 60 is expected to have a perchlorate limit established by January, 2013. The EPA has an advisory 15 ppb of chlorate per liter of water but there is no deferral regulation for perchlorate in drinking water at this time. California has a maximum level of 6ppb and in Massachusetts the maximum allowable is 2 ppb for perchlorate. New Jersey has a proposed MCL (Maximum Contaminant Level) of 5 ppb.
3. Chlorate -- Limits are being considered for addition to Standard 60 with a target date of January, 2013. No current Federal regulations have been issues for chlorate. Currently Canada has guidelines established for drinking water for a MAC of 1 ppm.
1. Occurrence Of Perchlorate In Sodium Hypochlorite, Journal of AWWA, November, 2008, pp 68-73.
2. What Every Operator Should Know About Sodium Hypochlorite, Water Engineering and Technology, March 2011, pp 66 and 67.
3. Bleach Chemistry & Pending Changes For Drinking Water Treatment Chemicals, Chlorine Institute Packagers & Bleachers Seminar, Feb, 2011, Orlando Fl.
4. Bromine As A Disinfectant, Lenntech, www.lenntech.com, November, 2010
5. Bromates In Sodium Hypochlorite, DX Terminals, LTD, www.dxgroup.com, October, 2010.
6. Bromine Compounds In Dow Caustic Soda Solution, 50%, Dow Chemical, www.dow.com, October 2010.
7. Bromate In Sodium Hypochlorite Potable Water Treatment, Chlorine Institute, www.cl2.com, November, 2004.
8. Perchlorate, Bromated and Chlorate in Hypochlorite Solutions: Guidelines for Utilities, Stanford, et al, Journal of AWWA, June, 2011, pp71-83.
About the Author: Gerald F. Connell is an independent consultant with over 35 years in the use and handling of chlorine in water treatment. He is the author of "The Chlorination/Chloramination Handbook" as well as "The Chlorination/Dechlorination Handbook."