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Safe Water System (SWS) Publications - Disinfectant

  1. Is sodium hypochlorite (SH) the best disinfectant to use?
  2. What about using HTH or chlorine tablets instead?
  3. How do you ensure adequate shelf life of SH?
  4. Will adding NaOH change the effectiveness of the SH?
  5. Does the SH inactivate giardia and cryptosporidium?
  6. What are disinfection by-products, and are they an issue in the SWS?
  7. If you add SH to water already treated in a municipal water treatment plant, is there a risk of a chlorine overdose?
  8. What happens if a child accidentally drinks the SH?
  9. Which hypochlorite generator do you recommend?
  10. Are there advantages to having a private company manufacture the SH?
  11. What characteristics are required for the SH?
  12. How much does it cost to manufacture the SH?
  13. How do we determine the appropriate SH dose?
  14. What characteristics do you recommend for the SH bottle?
  15. How can we save money on manufacturing the SH bottle?
  16. What do we do if the source water is turbid?
  17. Why does free chlorine in treated water decline over time?
  18. How can you distribute the SH throughout the country?

1. Is sodium hypochlorite (SH) the best disinfectant to use?

Our investigations have shown sodium hypochlorite to be effective and have broad applications. We have investigated a number of other disinfectants (calcium hypochlorite, ozone, UV, solar disinfection) and treatment processes (filters, slow sand filtration) and feel that sodium hypochlorite offers the best mix of low cost, ease of use, safety, and effectiveness in areas where there is enough water to drink and water is not excessively turbid. We feel that these characteristics are the reasons why most water treatment systems in the US and Europe have been using chlorine for disinfecting drinking water for nearly 100 years. We do, however, recognize that the other disinfection methods noted above also effectively disinfect water and are useful in a number of settings.

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2. What about using HTH or chlorine tablets instead?

Chlorine tablets and/or HTH (also named calcium hypochlorite) are widely available in some areas. We have found that a number of potential users of the SWS know that if they add the tablets to water, it will disinfect the water. However, we have also found that people have very different ideas of the appropriate dosing and the tablets vary in strength considerably. In Haiti, a small saran wrap bag of approximately 100 HTH pellets is widely available and inexpensive. However, the pellets are of varying size, the quality of the pellets is unknown, and depending on impurities in the manufacturing process they can degrade quickly. In other countries we have seen very high strength tablets sold, which, if added to water for disinfection would impart a strong, unpalatable taste to the water. Thus, it is extremely important to investigate the quality and strength of the HTH or chlorine tablets, and the appropriate dosing strategy before attempting to use them for drinking water treatment. For these reasons, we have found that the hypochlorite solution is a better option.

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3. How do you ensure adequate shelf life of SH?

Sodium hypochlorite is highly reactive and volatile. At normal pH (6-8) sodium hypochlorite can degrade substantially within 2-3 weeks. This shelf life is not adequate for use in the SWS, for the hypochlorite must remain at high enough concentration to inactivate disease-causing organisms. By raising the pH of the hypochlorite solution, you stabilize the solution. The pH can be raised by the addition of sodium hydroxide, which is widely available. In order to determine the amount of sodium hydroxide to add to your sodium hypochlorite solution, you will need to complete trial-and-error testing. Add a known volume of sodium hydroxide to a known volume of sodium hypochlorite, and then measure the pH with a meter or kit. Because source water quality is different in each location, there is not one standard volume of sodium hydroxide to add to ensure pH is above 11. You will have to start with a known volume (perhaps 1 tablespoon in 1 gallon, or 5 ml in 1 liter) and complete iterative trial-and-error testing. The exact pH is not important in this context - you simply need to ensure that the pH is above 11.

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4. Will adding NaOH change the effectiveness of the SH?

No, because when the sodium hypochlorite solution is added to water, the water decreases the pH and the sodium hypochlorite more active. The chemistry behind this is: the pH scale is from 0 to 14. Acids have a pH below 7, bases are above 7, and 7 is neutral. Most natural water is around pH 6-7. When sodium hypochlorite is in water, it breaks up into two compounds, with the concentration of each compound dependent on pH. One of these compounds is significantly more reactive, volatile, and more effective at inactivating bacteria than the other. At high pH (above 11) the majority of the sodium hypochlorite is in the form of the less-reactive compound. Thus, when you add sodium hydroxide to the sodium hypochlorite, you are converting it into the less-reactive form. However, water is around pH 6-7. When you add a small amount (5 milliliters) of solution at pH 11 to a large amount (20 liters) of water at pH 6-7, the mixture becomes pH 6-7. Thus, when you add the hypochlorite at pH 11 to your water in the SWS, you convert the hypochlorite back into the reactive form, and then it inactivates the disease-causing organisms.

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5. Does the SH inactivate giardia and cryptosporidium?

Giardia and cryptosporidium are both protozoa and are resistant to chlorination because they exist in water in a cyst form. The hard coat of the cysts protects giardia and cryptosporidium from being inactivated by chlorine. Cryptosporidium is more resistant to chlorine than giardia. See this inactivation table for more details. Both protozoa, however, are fairly large. Cryptosporidium is approximately 3-5 times the size of the bacteria E. coli, and giardia is approximately 5-10 times the size of E. coli. Cryptosporidium and giardia can thus be removed by filtration. If giardia or cryptosporidium are a significant health problem in the project area, a filtration step (through ceramic, sand, or other filters) can be added before adding the sodium hypochlorite. The Safe Water System intervention has been proven to reduce diarrhea in children, and this intervention does inactivate many of the ones that cause the most severe disease, like cholera, dysentery, and typhoid fever.

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6. What are disinfection by-products, and are they an issue in the SWS?

Disinfection by-products (DBPs) are chemical compounds formed when chlorine is added to water with organic material in it. All natural waters have some organic material in them, and generally waters that are more turbid (dirty) have more organic material. DBPs are a concern whenever chlorine is added to drinking water, whether in the Safe Water System or in a large-scale water treatment plant in the United States, because some studies have suggested that ingestion of DBPs in water over a lifetime may be associated with a very low risk of cancer. However, this risk is very small (1 in 100,000 people over 70 years is what the WHO considers when establishing their drinking water guidelines (1996)). In areas where many people, and many children, have diarrheal diseases caused by unsafe drinking water, the risk of cancer from THMs is miniscule compared to the risk of death or stunting from diarrheal diseases. The World Health Organization is very clear on the importance of microbiologically safe drinking water (obtained by adding chlorine) as compared to the risk from DBPs in their Guidelines for drinking-water quality: "Where local circumstances require that a choice must be made between meeting either microbiological guidelines or guidelines for disinfectants or disinfectant by-products, the microbiological quality must always take precedence, and where necessary, a chemical guideline value can be adopted corresponding to a higher level of risk. Efficient disinfection must never be compromised" (WHO Guidelines for Drinking Water Quality, Volume 2, 1996). Because the DBP question is often raised, we have prepared two fact sheets on DBPs: one is a short one-page document, and the other is a more descriptive 6 page document.

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7. If you add SH to water already treated in a municipal water treatment plant, is there a risk of a chlorine overdose?

This is very unlikely. If sodium hypochlorite is added to water that is already treated, the water would most likely still be within an acceptable range of chlorine residual. Typically, chlorinated urban water systems have free chlorine levels of around 0.1 to 0.5 parts per million. We calculate our sodium hypochlorite solution dose to give untreated water a free chlorine level of around 1 part per million. So if you add our solution (1 part per million) to treated urban water (0.1-0.5 parts per million), the level of the "overtreated" water would still be in the acceptable range of 0.5-2 parts per million (which is the range that balances disinfection efficacy plus reasonable taste).

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8. What happens if a child accidentally drinks the SH?

A review of health effects from accidental and intentional ingestion of sodium hypochlorite (bleach) in Europe was published in 1994 by Proctor and Gamble in conjunction with European poison control centers (Racioppi, et al. Food and Chemical Toxicity, Volume 32, Number 9, pp 845-861). The results of this review show that "acute accidental exposure to household bleach in use or in foreseeable misuse situations results, in the great majority of the cases, in minor, transient adverse effects on health". The authors cited two studies specifically on children: 1) A study in Chicago showing that of 26 children admitted for accidental bleach ingestion, only one had a moderate health effect (irritation of the esophagus, which healed on its own without intervention), with the remaining children having only "minor transient irritation effects", and 2) A study of 23 cases aged 1 - 3 years, with only one case having "superficial burns in the esophagus", which disappeared two weeks later. Suicide attempts in adults have shown that a lethal dose of sodium hypochlorite varies widely, with lethal results at 200-500 mL of 3-12% strength. It is important to remember that the concentration of the SH used in the SWS is approximately 0.5-1.0 percent. In most countries, the SH is sold in 250 ml bottles, but in some 500 ml bottles are used. The hypochlorite ingested in the majority of the cases mentioned above was full strength household bleach: 5 percent. Several factors make it unlikely that the hypochlorite solutions recommended in the Safe Water System could cause harm. First, it is unlikely that a child would accidentally drink 250 or 500 milliliters of something that tastes as bad as the sodium hypochlorite does. Second, based on the data presented above, it is even less likely, at the low concentration used in this project, that anything harmful would occur. However, we highly recommend that part of the educational materials emphasize the need to keep the sodium hypochlorite solution stored somewhere safe (out of sunlight, sealed, away from children) for health reasons, to protect the sodium hypochlorite from degradation, and to prevent spills in households that have limited disposable income to purchase more solution.

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9. Which hypochlorite generator do you recommend?

A number of companies manufacture hypochlorite generators. Information on some of them can be found in our small projects page. There are several advantages in using a hypochlorite generator. First, local production of the sodium hypochlorite that minimizes transportation costs. Second, in the event there is not a reliable bleach producer in the country, the hypochlorite generator provides that capacity. Third, revenues from the sale of the solution can be used to help support operation and maintenance of the machine and to pay the operator. Considerations that must be taken into account when producing bleach in this way include the need for regular operation and maintenance of the machine, payment of a reliable person to operate and maintain the machine, replacement of the cell of the generator every 5 years, and the need for a reliable electricity supply.

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10. Are there advantages to having a private company manufacture the SH?

There are several advantages to having a company make the solution are: 1) Most likely, all a company would need to do to make the desired concentration of hypochlorite is to dilute an existing bleach product. 2) If demand for the solution grows, a company is better able to expand production. 3) Many companies have certification from Bureaus of Standards for bleach products that can often be applied to the new dilute solution. 4) Most reputable companis have quality control procedures. In Madagascar, Rwanda, Kenya, Malawi, India, Afghanistan, and Tanzania, Populations Services International (a social marketing non-governmental organization that has successfully implemented a number of SWS projects) has opted to have private companies make the bleach for them.

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11. What characteristics are required for the SH?

First, it is important that the concentration is correct (usually 0.5 to 1.0 mg/l). A concentration that is too low requires too high a volume to adequately treat enough water to be practical. A concentration that is too high is difficult to accurately dose, raising the risk of too high a dose (which is unpalatable), or too low a dose (which might not effectively disinfect the water). Second, it is important that pH of the solution is at least 11. This increases the shelf life of the solution.

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12. How much does it cost to manufacture the SH?

In the first year of the country-scale project in Zambia it cost US$78,000 to manufacture 400,000 bottles per year. The labor cost was $23,000 and the materials (salt, vinegar, bottles, and labels) cost was $55,000. The total production cost was thus US$0.20 per bottle, and assuming a family usage of one bottle per month, the production cost for a year's supply for one family is about US$2.40. After the first year, costs are expected to decrease. Costs vary by country, depending on labor, materials, and value added taxes. In small-scale projects using a local hypochlorite generator and reusable bottles, the production cost of the hypochlorite is only the cost of the salt, water, labor, and electricity.

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13. How do we determine the appropriate SH dose?

The hypochlorite dose will depend on the characteristics of the local water. Usually an amount in the range of 5 to 10 milliliters added to 20 liters of water is sufficient to inactivate the disease-causing organisms, but not leave an unpleasant taste. Once the size of the cap for your project has been determined, some simple experiments using the sodium hypochlorite locally available, source water in your area, and a kit that measures the amount of free and residual chlorine can be used to determine the appropriate dose. Please contact us at safewater@cdc.gov for more information on how to complete this testing.

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14. What characteristics do you recommend for the SH bottle?

We recommend the following six characteristics for the sodium hypochlorite bottle that is kept in the home: 1) The size of the bottle should be between 250 and 500 milliliters. This is small enough to be affordable and to ensure that the solution will be used before it degrades, but large enough that it will last a family for approximately a month; 2) The neck of the bottle should be compatible with soda bottle caps, which tend to be mass-produced, are inexpensive, typically have the desired volume of 5-10mL for dosing, and are easily accessed in most locations; 3) The volume of the cap should be between 5 and 10 ml so that it can be used to dose the solution; 4) The bottle should be composed of an opaque plastic, to prevent exposure of the solution to direct UV radiation from sunlight, which will decrease the shelf life; 5) A handle is not necessary. This only increases the cost and decreases the space available for instructions; and 6) The neck and cap should have at least four threads to improve the seal. The cap should have a raised ring inside to help seal the bottle as well.

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15. How can we save money on manufacturing the SH bottle?

One way to save money when designing the solution bottle is to design the bottle so that an already locally available cap will fit it. We recommend a plastic soft drink bottle cap with a volume of between 5 and 10 milliliters. Using a locally available cap will save you from having to purchase a mold for the caps, and soft drink caps are typically mass produced at very low cost. The existing caps must fit tightly and securely on the project bottle design, however.

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16. What do we do if the source water is turbid?

Water that looks dirty or cloudy is called turbid water. Turbidity is a measure of the amount of light that is scattered as it passes through the water sample. If more particles are in the water, more light will be scattered, and the turbidity is thus higher. Water that looks "dirty" will have a higher turbidity than water that looks clear. Turbidity itself is not a problem, however turbidity is often used to represent the amount of total suspended solids and the amount of organic matter in the water. There are two issues with adding chlorine to water that has a high turbidity: 1) Chlorine reacts preferentially with organic matter before reacting with bacteria. Thus, you need to make sure to add enough chlorine to inactivate the bacteria, 2) There is a potential of creating more disinfection by-products if there is a higher concentration of organic matter in the source water. There are three strategies that can be used to make turbid water clear: 1) Filtering the water through a cloth filter will remove some of the organic matter; 2) Letting the water settle for 12-24 hours so the organic matter falls to the bottom and then pouring off the clearer water into a separate vessel will also remove some of the organic matter; or 3) Increasing the dose of sodium hypochlorite solution adde d to the water to make sure there is enough chlorine to inactivate the disease-causing organisms. Because every community is different, experiments to determine which is the most acceptable and appropriate strategy will need to be conducted in the project community.

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17. Why does free chlorine in treated water decline over time?

Chlorine is an extremely reactive chemical. Right after the sodium hypochlorite is added to the water, chlorine levels decline because the chlorine is reacting with organic matter and microbes. After those reactions are complete, chlorine in water will slowly escape into the air as a gas. This is the reason that free and total chlorine levels slowly degrade over time in a covered (but not sealed) container, and also why we recommend that the pH of the hypochlorite solution be raised to over 11 to extend the shelf-life before it is used.

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18. How can you distribute the SH throughout the country?

In many projects, PSI has initiated social marketing campaigns, which include the activation of networks of wholesale and retail outlets, facilitating distribution to communities where vulnerable populations live. For smaller projects, one idea is to purchase space on private delivery trucks that are already going to target locations to deliver goods such as soft drinks and beer, or to request a donation of space by the private companies as a charitable activity.

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