Frequently Asked Questions (FAQs) about Sodium Hypochlorite Solution(SH)
FAQ about Sodium Hypochlorite Solution (SH)
Investigations have shown sodium hypochlorite to be an effective disinfectant having broad applications. Although a number of other disinfectants (calcium hypochlorite, ozone, UV, solar disinfection) and treatment processes (filters, slow sand filtration) have been investigated, sodium hypochlorite appears to offer 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. 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. The other disinfection methods noted above also effectively disinfect water and are useful in a number of settings.
Chlorine tablets and/or HTH (also named calcium hypochlorite) are widely available in some areas. A number of potential users of the SWS know that these tablets are used to disinfect water. Unfortunately, we have also found that many people have different levels of knowledge regarding appropriate dosing instructions, which is a concern because the tablets vary significantly in strength. In Haiti, a small saran wrap bag of approximately 100 HTH pellets is widely available and inexpensive. However, the pellets vary in size, the quality of the pellets is unknown, and, depending on impurities in the manufacturing process, they can degrade quickly. In other countries, very high strength tablets may be sold which, when added to water for disinfection, leave a strong, unpleasant taste. It is important for users to know the quality and strength of HTH and/or chlorine tablets and understand the appropriate dosing strategy before attempting to use them for drinking water treatment; in most instances, however, this is impossible for users to do. For these reasons, hypochlorite solution is likely to be a better option.
First, it is important that the concentration of the SH solution produced is correct (usually 0.5 to 1.0%). 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 the pH level of the solution be at least 11. This increases the shelf life of the solution.
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 can be used to determine the appropriate dose. To conduct the experiments, you will need locally available SH, source water in your area, and a kit that measures the amount of free and combined chlorine. Please contact email@example.com for more information on how to complete this testing.
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, which requires that the hypochlorite remain at a 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 repeat trial-and-error testing. The exact pH is not important in this context—you simply need to ensure that the pH level is above 11.
No, because when the sodium hypochlorite solution is added to water, the water decreases the pH and the sodium hypochlorite becomes 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 is a mixture of 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.
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 (to achieve 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 and reasonable taste).
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 inorganic and 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 it is recommended that the pH level of the hypochlorite solution be raised to over 11 to extend the shelf life of the solution before it is used.
It is important to remember that the concentration of the SH used in the Safe Water System (SWS) is approximately 0.5-1.0%. A review of health effects from accidental and intentional ingestion of full strength bleach (sodium hypochlorite), which is 5-6%, in European poison control centers1 showed 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 also 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. As mentioned above, the hypochlorite ingested in the majority of the cases in the review was full strength household bleach: 5-6%. Several factors make it unlikely that the hypochlorite solutions recommended in the Safe Water System could cause harm remembering that in most countries, the SWS SH is sold in 250 ml bottles; in some countries, the 500 ml bottles are used. 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, it is even less likely that, at the low concentration used in this project, anything harmful would occur. Despite these safety data, it is highly recommended 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, due to limited incomes, would be unable to purchase more solution.
Giardia and Cryptosporidium are both protozoa and are highly tolerant to chlorination because they exist in water in a cyst or oocyst form. The hard coat of the cysts or oocysts protects Giardia and Cryptosporidium from being inactivated by chlorine. Cryptosporidium is much more resistant to chlorine than Giardia (see pathogen inactivation table for more details). Both protozoa, however, are fairly large, which means that they can 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.
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 is often used to represent the amount of total suspended solids and the amount of organic matter in the water. Bacteria and other pathogens may also stick to particles in the water so high turbidity may increase the chance that there are pathogens in the water. There are two issues associated with adding chlorine to water that has a high turbidity: 1) Chlorine reacts equally with all the organic material in the water as well as with the bacteria and other pathogens. If there is a great deal of organic material then it will take more chlorine to fully react with all the dissolved solids and organic material as well as inactivate the bacteria and other pathogens, 2) There is a potential for 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 treat turbid water: 1) Filter the water through a cloth filter to remove some of the organic matter and then chlorinate; 2) Let the water settle for 12-24 hours so the organic matter and solids fall to the bottom and then pour off the clearer water into a separate vessel where it is then chlorinated; or 3) Increase the dose of sodium hypochlorite solution added to the turbid water to be sure there is enough chlorine to inactivate the disease-causing organisms. Because every community is different, experiments to determine which is the most acceptable, appropriate, and effective strategy will need to be conducted in the project community.
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 suggest 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. In areas where many people, and many children, have diarrheal diseases caused by unsafe drinking water, the risk of cancer from DBPs is very small compared to the risk of death or stunting from diarrheal diseases. In their Guidelines for Drinking-water Quality, the World Health Organization states: “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” 2. For more information, please see our detailed page on DBPs.
A number of companies manufacture hypochlorite generators. There are several advantages in using a hypochlorite generator. First, local production of the sodium hypochlorite minimizes transportation costs. Second, in the event there is not a reliable bleach producer in the country, the hypochlorite generator can provide the 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, the need to test the concentration and pH of the SH solution produced, 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.
There are several advantages to having a company make the solution: 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 companies have quality control procedures. In countries around the world, Populations Services InternationalExternal (PSI–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.
In the first year of the country-scale project in Zambia, it cost US$78,000 to manufacture 400,000 bottles. The labor cost was $23,000 and the materials (salt, vinegar, bottles, and labels) cost was $55,000. The total production cost was therefore US$0.20 per bottle. 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 usually 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.
CDC recommends 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 one month. 2) The neck of the bottle should be compatible with soda bottle caps, which tend to be mass-produced, inexpensive, typically have the desired volume of 5-10mL for dosing, and are available 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 its shelf life. 5) 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. 6) A handle is not necessary. This only increases the cost and decreases the space available for instructions.
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 5–10 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.
In many projects, PSI has initiated social marketing campaigns, which include activating networks of wholesale and retail outlets and 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.