1. Cleaning and disinfecting wells
Technical Notes on Drinking-water,
Sanitation and Hygiene in Emergencies
Flooding, earthquakes, civil unrest and other natural and man-made disasters often cause damage to hand-dug wells. This technical note sets out the actions needed to repair and rehabilitate a hand-dug well so that it can be returned to its former condition. The emergency repair and rehabilitation measures proposed are temporary and should be followed by measures for permanent rehabilitation.
Steps for cleaning and disinfection
Figure 1.1 outlines a four-step approach to cleaning and disinfecting wells after natural or man-made disasters. It is an emergency approach designed to rehabilitate wells so that they produce water of a similar quality to that supplied before the disaster (see Box 1.1). Technical Note 15 gives further information on wells contaminated by seawater.
Figure 1.1. Steps for cleaning and disinfecting wells
Box 1.1. Water quality from hand-dug wells
Water taken from hand-dug wells is often of poor quality, mainly due to the poor construction of the above-ground elements and unhygienic methods of collecting water. The steps described here will not overcome these problems as they are designed to return the well to its original condition. Sources of further information on improving and upgrading wells are given at the bottom of the page.
Step 1: Inventory of existing wells
The disaster may have contaminated or damaged a large number of wells. The first step must be to select which wells should be repaired first. They are the ones that are used most and that are easiest to repair. The following actions should help you to make an informed selection.
- 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).
Step 2: Rehabilitation and cleaning of wells
The amount of rehabilitation and cleaning required will depend on the amount of damage caused by the disaster. Typically it will include the following steps:
- Remove and repair/replace the pumping mechanism or lifting device.
- Remove polluted water and debris from the well using either buckets or pumps. Special care must be taken when using a pump to remove water from wells contaminated with seawater. (See Technical Note 15 for more details.)
- Repair/reline the well walls to reduce sub-surface contamination.
- Clean the well lining using a brush and chlorinated water (see Box 1.2).
- Place a 150mm layer of gravel in the base of the well to protect it from disturbance.
- Seal the top of the well using a clay sanitary seal (Figure 1.2).
- Construct a drainage apron and head wall around the well to prevent surface water, insects and rodents from entering the well. Provide a cover for the well.
Box 1.2. Calculating the chlorine dosage for disinfecting a well using High Strength Calcium Hypochlorite (HSCH)
HSCH and bleach give off chlorine gas which is a serious health hazard. Try to clean the well lining from outside the well using a long-handled brush. If you must enter the well, wear full protective clothing and a breathing apparatus and provide a strong air flow inside the well to carry away the chlorine gas.
Figure 1.2. Sealing the top of a well
Check turbidity and pH
Following cleaning and repair, allow the water level in the well to return to its normal level. Measure the turbidity and pH levels to check whether chlorination will be effective. This can be done using a simple method described in Box 1.3. Never chlorinate turbid water because suspended particles can protect micro-organisms. Table 1.1 outlines the reasons why pH and turbidity are important and what can be done to ensure guideline levels are met. If the turbidity of the well water is greater than 20NTU after the cleaning and rehabilitation stage, remove all water in the well once again and scrub the well lining with a strong concentration of bleach in water (Box 1.2).
Allow the well to refill with water and test the turbidity levels again.
If the water is still turbid, it is probably due either to:
- the failure of the filter pack in the bottom and around the side of the well; or – more likely –
- to poor protection of the top of the well allowing surface water contamination.
Neither of these problems can be solved immediately. However, it is probably safe to allow the local community to begin using the well as the water quality should be at least as good as it was before the disaster.
|Parameter||WHO GDWQ*||Why?||Corrective action|
|pH||6-8||pH of 6.8-7.2 is required to reduce level of chlorine required.||If pH is less than 6 add hydrated lime (calcium hydroxide) to raise pH before chlorination.|
(20NTU emergency limit)
|High turbidity requires more chlorine to oxidise organic matter.||Check the turbidity of the water entering the well through the walls and base.
Make sure there is no contamination from the surface.
Table 1.1. Physico-chemical parameters
*GDWQ: Guidelines for drinking water quality
Box 1.3. Measuring turbidity and the pH level of water
Turbidity is the cloudiness or haziness of a fluid caused by individual particles. The measurement of turbidity, therefore, is a key test of water quality. Specialist laboratory or field equipment (a nephelometer) is required to measure turbidity accurately in Nephelometric Turbidity Units (NTU). If you do not have access to such specialist equipment, then a reasonable NTU estimate can be made using locally available materials as shown below.
Step 3: Disinfection of the well
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.
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)
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.
- CDC (Undated) Disinfection of wells following an emergency. Centre for Disease Control and Prevention. USA.
- Collins, S. (2000) Hand dug wells. Series of Manuals on Drinking Water Supply Vol. 5.
- Godfrey, S. (2003) 'Appropriate chlorination techniques for wells in Angola', Waterlines, Vol. 21, No. 5, pp 6-8, ITDG Publishing, UK.
- OXFAM (Undated) Repairing, cleaning and disinfection of hand dug wells.
- SKAT: St Gallen http://www.rwsn.ch/documentation/
- WHO (2004) Guidelines for drinking water quality – Volume 1. Geneva.
- WHO (2010) 'How to measure residual chlorine in water'.
Technical Note 11
- WHO (2010) 'Cleaning wells after seawater flooding'.
Technical Note 15
Prepared for WHO by WEDC. Authors: Sam Godfrey and Bob Reed. Series Editor: Bob Reed.
Editorial contributions, design and illustrations by Rod Shaw
Line illustrations courtesy of WEDC / IFRC. Additional graphics by Ken Chatterton.