Emergency treatment of drinking-water at the point of use
Technical note 5 on drinking-water, sanitation and hygiene in emergencies
Disinfection of water
Disinfection destroys all harmful organisms present in the water, making it safe to drink.
Boiling is a very effective method of disinfecting water, but it is energy consuming. The water should be brought to a ‘rolling’ boil and held there for between 1 minute at sea level and 3 minutes at high altitudes. Apart from the high cost of the energy involved in boiling, the other disadvantage is the change in taste of the water. This can be improved by aeration, by vigorously shaking the water in a sealed container after it has cooled.
Many chemicals can disinfect water but the most commonly-used is chlorine. When used correctly, chlorine will kill all viruses and bacteria, but some species of protozoa and helminths are resistant to chlorine. There are several different sources of chlorine for home use; in liquid, powder and tablet form. They vary in size and strength (i.e. in how much chlorine they contain) so different quantities are required depending on the formulation. Always follow the manufacturer’s instructions for use. To prevent misuse, clear instructions must be given to all users (see Figure 5.6).
Chlorine compounds should not be given out to users outside of the container they are supplied in by the manufacturer. People cannot tell how much of the product to use or how to use it simply by looking at it.
- Meet with community leaders and ask them which wells serve each section of the community.
- Select the most commonly used wells as a source for drinking- water that provided a plentiful supply.
- Check there are no obvious sources of contamination from nearby latrines, ponds or surface water. Also map livestock areas (pig pens, cattle sheds, chicken coops) as potential sources of contamination by animal waste.
- Assess the type and extent of damage to the top of the well and the lining.
- Ask the community about the original depth of the well. Use this to estimate the amount of silt and debris in the well.
- Test the pump (if there is one) to see if it is still working. If not, determine the repairs necessary.
- Estimate the resources needed for repairs (personnel, equipment, time and materials).
Solar disinfection (SODIS)
Ultra-violet rays from the sun will destroy harmful organisms present in the water. Fill transparent one- or two-litre plastic containers with clear water and expose them to direct sunlight for about five hours (Figure 5.7), or for two consecutive days under 100% cloudy sky). Cool the water and shake vigorously before use.
Combined treatment systems
A few large companies have developed compounds that both remove suspended particles and disinfect the water. One such compound contains a chemical that helps suspended particles join to make larger, heavier ones that will settle to the bottom of the container. It also contains chlorine that disinfects the water after settlement has occurred. The compounds have been proven to be effective but not all relief agencies approve their use because they are expensive and it can be difficult to ensure that they are used correctly.