Chlorine Residuals
The presence of free chlorine residual in drinking water indicates that: 1) a sufficient amount of chlorine was added to the water to inactivate most of the bacteria and viruses that cause diarrheal disease; and, 2) the water is protected from recontamination during transport to the home, and during storage of water in the household. Because the presence of free residual chlorine in drinking water indicates the likely absence of disease-causing organisms, it is used as one measure of the potability of drinking water.
Content Table
- Chlorine Addition
- Chlorine Addition Flow Chart
- Testing Free Chlorine in Drinking Water
- Methods to Test Free Chlorine in the Field in Developing Countries
- Resources
- Related Articles
Chlorine Addition
When chlorine is added to water as a disinfectant, a series of reactions occurs. These reactions are graphically depicted later in this article. The first of these reactions occurs when organic materials and metals present in the water react with the chlorine and transform it into compounds that are unavailable for disinfection. The amount of chlorine used in these reactions is termed the chlorine demand of the water. Any remaining chlorine concentration after the chlorine demand is met is termed total chlorine. Total chlorine is further subdivided into: 1) the amount of chlorine that then reacts with nitrates present in the water and is transformed into compounds that are much less effective disinfectants than free chlorine (termed combined chlorine); and, 2) the free chlorine, which is the chlorine available to inactivate disease-causing organisms, and is thus a measure used to determine the potability of water.
For example, when chlorine is added to completely pure water the chlorine demand will be zero, and there will be no nitrates present, so no combined chlorine will be formed. Thus, the free chlorine concentration will be equal to the concentration of chlorine added. When chlorine is added to natural waters, especially water from surface sources such as rivers, organic material will exert a chlorine demand, and combined chlorine will be formed by reaction with nitrates. Thus, the free chlorine concentration will be less than the concentration of chlorine initially
added.
Chlorine Addition Flow Chart
Testing Free Chlorine in Drinking Water
Testing free chlorine is recommended in the following circumstances:
• To conduct dosage testing in project areas
• To monitor and evaluate projects by testing stored drinking water in households
The goal of dosage testing is to determine how much sodium hypochlorite solution to add to water that will be used for drinking to maintain free chlorine residual in the water for the average time of storage of water in the household (typically 24 hours). This goal differs from the goal of infrastructure-based (piped) water treatment systems, whose aim is effective disinfection at the endpoints (i.e., water taps) of the system. The WHO recommends “a residual concentration of free chlorine of greater than or equal to 0.5 mg/litre after at least 30 minutes contact time at pH less than 8.0.” This definition is only appropriate for users who obtain water directly from a flowing tap. A free chlorine level of 0.5 mg/L can maintain the quality of water through a distribution network, but is not optimal to maintain the quality of the water when it is stored in the home in a bucket or jerry can for 24 hours.
Recommendations:
1. At 1 hour after the addition of sodium hypochlorite solution to water there should be no more than 2.0 mg/L of free chlorine residual present (this ensures the water does not have an unpleasant taste or odor).
2. At 24 hours after the addition of sodium hypochlorite to water in containers that are used by families for water storage there should be a minimum of 0.2 mg/L of free chlorine residual present (this ensures microbiologically clean water).
This methodology is approved by the WHO, and is graphically depicted below. The maximum allowable WHO value for free chlorine residual in drinking water is 5 mg/L. The minimum recommended WHO value for free chlorine residual in treated drinking water is 0.2 mg/L. CDC recommends not exceeding 2.0 mg/L due to taste concerns, and chlorine residual decays over time in stored water.
Methods to Test Free Chlorine in the Field in Developing Countries
There are four main methods to test free and total chlorine residual in drinking water in the field in developing countries:
1) Pool test kits;
2) Color-change test tubes;
3) Color-wheel test kits;and,
4) Digital colorimeters.
All four methods depend on a color change to identify the presence of chlorine, and a measurement of the intensity of that color to determine how much chlorine is present. This article does not consider commercial test strips for chlorine residual testing, because of their relatively higher cost (0.15-1.00 USD) per test. When only a small number of tests are needed it is possible that commercial test strips could be a viable, cost-effective option.
Pool test kits
The first option for chlorine residual testing uses the liquid chemical OTO (othotolidine) that, when added to water with total chlorine in it, causes a color change to yellow. To complete the test, users simply fill a tube with water, add 1-5 drops of the solution, and look for the color change to yellow. These kits are sold in many stores as a way to test the concentration of total chlorine in swimming pool water. This method does not measure free chlorine.
Benefits of the pool test kits are:
• Low cost
• Very easy to use
• Easily purchasable
Drawbacks of the pool test kits are:
• Degradation of the OTO solution can cause
inaccurate readings over time
• Generally not reliable quantitative results
• Lack of calibration and standardization
• Measurement of total chlorine, not free chlorine
Cost and Ordering Information
Pool test kits can be obtained from many home supply and pool stores at a cost of approximately $5-20 each.
Test-tube DPD color comparator
The Lamotte Company developed a rapid presence/absence test kit for free chlorine residual in response to the 2004 tsunami in Indonesia. The test tube color comparator uses DPD (N,N diethyl-p-phenylene diamine) tablets that causes a color change to pink in the presence of chlorine. To use the kit, users add 5 mL of water to the test tube, add one rapidly dissolving DPD-1R tablet, and compare the results to a color chart. This kit combines the simplicity of the pool test kits, with the benefit of testing for free chlorine residual instead of total chlorine residual. To use this kit to measure total chlorine, a DPD-4R tablet can be added instead of the DPD-1R tablet to the water sample, or a DPD-3R tablet can be added to the sample water after the DPD-1R tablet has been added and the free chlorine residual has been read. Although the test kit is not available as a package from Lamotte, the individual components (test tube, DPD tablets, and color chart) can be ordered individually. Test kit instructions and part numbers can be found at: http://www.lamotte.com/pages/common/pdf/instruct/3529.pdf.
Benefits of the test tube kit are:
• Low cost
• Very easy to use
• Measurement of free chlorine
Drawbacks of the test tube kit are:
• Generally not reliable quantitative results
• Lack of calibration and standardization
Cost and Ordering Information
The kit parts are available from the Lamotte Company (www.lamotte.com) for a cost of approximately $5 for both the test tube and color comparator chart, and approximately $70 for 1,000 DPD tablets.
Color wheel test kits
Color wheel test kits also use DPD tablets or powder that, when added to water with free or total chlorine present in it, cause a color change to pink. The color wheel test kits are more accurate than the pool test kits / test tube DPD color comparators and simpler and less expensive than digital meters. This is because users measure the intensity of the color change, as compared with a sample of water to which no DPD has been added, using a color wheel to visually match the color change to a numerical reading. The test kit can be used to measure either free chlorine or total chlorine, or both, generally within a range of 0-3.5 mg/L, equivalent to 0-3.5 ppm (parts per million). Please note that DPD tablets and powders are company specific, and that using one company’s test kit with another company’s DPD tablets is not recommended. Also note that although all DPD-1 tablets measure free chlorine, there is variation between companies as to whether the total chlorine tablet is added to the same sample water the free chlorine is tested in or whether it is added to fresh sample water.
Benefits of the color-wheel test kits are:
• Accurate quantitative readings if used correctly
• Low cost
Drawbacks of the color-wheel test kits are:
• Potential for user error
• Lack of calibration and standardization
Cost and Ordering Information:
Color wheel test kits can be ordered from:
Hach Company
Part Number:
CN-66 Approximately $44
Lamotte Company
Part Number: 3308
Approximately $55
Additional DPD tablets or powders can be ordered as well, at a cost of approximately $70 per 1,000 tablets or sachets.
Digital Colorimeters
Digital colorimeters are the most accurate way to measure free chlorine and/or total chlorine residual in the field in developing countries. To use the colorimeters:
1) a DPD-1 (free chlorine) or DPD-3 (total chlorine) tablet or powder is added to a vial of sample water that causes a color change to pink; and,
2) the vial is inserted into a meter that reads the intensity of the color change by emitting a wavelength of light and automatically determining and displaying the color intensity (the free or total chlorine residual) digitally.
The range of the meters is generally 0-4 mg/L, equivalent to 0-4 ppm (parts per million). Please note that DPD tablets and powders are
company specific, and that using one company’s test kit with another company’s DPD tablets is not recommended.
Benefits of the digital colorimeters are:
• Highly accurate readings
• Fast results
• EPA approved
Drawbacks of the digital colorimeters are:
• Expense
• Necessity of calibration with standards
• Necessity for skilled operator
Cost and Ordering Information
Digital colorimeters can be ordered from:
Lamotte Company www.lamotte.com
Part Number 3670-01 Approximately $350
Standards: Part Number: 4140 Approximately $70
Hach Company
Part Number 5870000 Approximately $352
Standards: Part Number: 2635300 Approximately $115
Additional DPD tablets or powders can be ordered as well, at a cost of approximately $70 per
1,000 tablets or sachets.
Resources
The Issues in this article have been adapted from the Chlorine Residual Testing Fact Sheet, CDC SWS Project, safewater@cdc.gov.
The CDC Safe Water System (SWS) program is an intervention proven to reduce diarrheal disease incidence in developing countries that consists of three elements: water treatment with dilute sodium hypochlorite at the point-of-us; storage of water in a safe container; and, education to improve hygiene and water and food handling practices. The sodium hypochlorite solution is generally packaged in a bottle with directions instructing users to add one full bottle cap of the solution to clear water (or 2 caps to turbid water) in a standard-sized storage container, agitate, and wait 30 minutes before drinking.
The SWS project recommends testing free chlorine in homes of SWS users to evaluate whether or not users are using the SWS (as measured by presence or absence of free chlorine residual) and whether they are using it correctly (as measured by free chlorine residuals in the 0.2-2.0 mg/L range). This approach is useful for program monitoring because the presence of free chlorine residuals in stored water obtained from an unchlorinated source is an objective, quantitative measure that people are using the SWS hypochlorite solution.
