Cleaning and disinfecting wells
Technical note 1 on drinking-water, sanitation and hygiene in emergencies
Steps 3 and 4: Disinfection of wells
WHO endorses the disinfection of drinking-water in emergency situations. There are various ways of doing this but the most common is chlorination as it leaves a residual disinfectant in the water after chlorination.
Chlorine has the advantage of being widely available, simple to measure and use, and it dissolves easily in water. Its disadvantages are that it is a hazardous substance (to be stored and handled with care) and that at commonly applied concentrations it is not effective against all pathogens (e.g. cysts and viruses, which require higher chlorine concentrations).
The chlorine compound most commonly used is high strength calcium hypochlorite (HSCH) in powder or granular form which contains 60-80% chlorine. Also used is sodium hypochlorite in liquid bleach or bleaching powder form. Each chlorine compound has a different amount of usable chlorine depending on the quantity of time the product has been stored or exposed to the atmosphere and the way it is made. Box 1.2 outlines methods for calculating appropriate chlorine doses for HSCH granule chlorine.
Stir the water in the well thoroughly with a long pole and then allow the water to stand for at least 30 minutes. Further details on chlorination are given in Technical note 11 below.
Step 4 - Dewater the well
Following the contact period, remove all water in the well using a pump or bucket. When the well has refilled, wait a further 30 minutes and measure the chlorine concentration. If the residual chlorine concentration is less than 0.5mg/l the well is safe to use. If the concentration is greater than 0.5mg/l, remove all the water from the well again and repeat the process.
Two issues need extra care when dewatering the wells:
- water with high concentration of chlorine should not flow into streams or wetlands;
- when dewatering on coastal areas salt water intrusion should be avoided (see Technical note 15 for more information).
Do not allow anyone to use the well during the cleaning process. The water will have a strong concentration of chlorine that will give it a bad taste and smell and could be dangerous.