Cleaning and disinfecting wells
Technical note 1 on drinking-water, sanitation and hygiene in emergencies
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.
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.
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.