Disinfection Selection

Drinking water contaminants and upcoming regulations

U.S. drinking water systems are always on the alert for contaminants found in chemicals used for treatment or that develop during the treatment process. Researchers and regulators have identified many of these contaminants, which are regulated under NSF/ANSI Standard 60. Changes to Standard 60 constantly are under consideration. New changes that can impact the disinfection process are anticipated to go into effect in 2013. Publication of the changes is expected at the end of 2011.  

Standard 60 covers the chemicals used in water treatment, including those that act as coagulants, flocculants, softeners, pH controllers, corrosion controllers, scale controllers, disinfectants and otherwise. The concerns here are the disinfection chemicals—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 also can be produced by using an electrolytic cell for onsite generation with a brine feed.

Although there may be other changes to Standard 60, those effecting bleach are of great importance to the water industry. The changes relate to sodium hypochlorite and the contaminants in hypochlorite. These contaminants are chlorate, perchlorate and bromate.

Bromate may be present in the bleach or produced as a result of the disinfection treatment process in the presence of bleach. It is a potent human carcinogen.  Perchlorate affects the ability of the thyroid gland to take up iodine, which can affect its functions in the human body. Chlorate can affect the health of certain population groups (e.g., senior citizens and children). It is included in the contaminant candidate list and likely will be included in the unregulated contaminant mandatory rule.

Each compound can be traced to sodium hypochlorite, which is known to lose strength over time. Decomposition is caused by higher temperatures and higher concentrations. If dilution water is not softened, scaling will occur. Furthermore, pipe joints often leak and feed frequently is impacted by air binding in the pumps, among other problems.

Bromate: Tracing the Source

Bromate comes from two separate sources in drinking water. The first is in the hypochlorite manufacturing process. Sodium hypochlorite generally is produced for bulk use by the reaction of chlorine gas with sodium hydroxide, or it 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 onsite generation, which also produces hypochlorite with bromated levels.

The second source is the raw water supply. Bromide ions can be present in the raw water supply, surface water or groundwater. When water containing 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, treatment with hypochlorite does not require testing of 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

Disinfectant selection can have an impact on the presence of bromate in finished water. Both ozone and hypochlorite disinfection require additional awareness.

Control of the bromate content added by hypochlorite requires a sodium hypochlorite supplier to meet a more stringent hypochlorite quality guideline. The use of hypochlorite requires the water treatment facility to obtain a source of hypochlorite that meets maximum hypochlorite contaminant requirements. Plants using ozone must analyze the raw water and its bromide content. In addition, a monthly analysis for bromate in the finished water is required. Water treatment facilities must be aware of these needs.

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 disinfect using chlorine gas, as there is no reaction with chlorine gas that will produce bromates.

Perchlorate: Tracing the Source

Perchlorate is a product of sodium hypochlorite decomposition. The longer hypochlorite is kept by the utility before use, the more likely it is to experience a significant increase in perchlorate. The increasing use of perchlorate in rocket propellants and the improper disposal of wastes from the manufacture of these propellants have caused concerns regarding 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 help reduce perchlorate development. Conversion to chlorine gas from hypochlorite also can help because perchlorate is not present in chlorine.

Chlorate: Tracing the Source

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 chlorate levels low. Frequent storage tank cleaning can prevent a heel of old sodium hypochlorite from being kept in the tank. Temperature reduction and/or dilution on receipt from the supplier also reduce the formation of chlorate. Again, conversion of hypochlorite to chlorine gas would eliminate the concern of chlorate.

Regulatory Considerations

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 to a level of about 39 ppm by January 2013. Currently, 69 ppm of bromate is the maximum contaminant level allowed in sodium hypochlorite. Because bromate is produced in the ozonation process, the use of ozone in the treatment process may 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 U.S. Environmental Protection Agency 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 6 ppb; in Massachusetts, the maximum allowable is 2 ppb. New Jersey has a proposed 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 issued for chlorate. Canada currently has a maximum allowable contaminant level of 1 ppm for drinking water.

Gerald F. Connell is a consultant serving the water and wastewater industry. Connell can be reached at gfconnell@verizon.net.

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