A. Well Disinfection

After a well is built, the whole structure should be carefully disinfected. Disinfection is needed to kill any possibly harmful bacteria that could be transferred from the pipe or concrete lining to the water and then on to the people who consume the water.

The well can be disinfected by adding enough chlorine to the well water to produce a strong chlorine solution. This solution can then be used to rinse off the rest of the well and so disinfect it.

1. First, the volume of the water in the well will have to be determined.

NOTE: The volume of water in a circular well can be easily computed by measuring the depth of the water and the diameter of the well. Multiply (water depth) x (1/2 diameter) x (1/2 diameter) x (3.1416). Expressed another way this becomes: Volume = (depth)(radius)² (3.1416).

2. From Table VIII-1 find the amount of chlorine that will have to be added to the computed volume of water to produce a strong chlorine solution.

3. Dissolve the required amount of the chemical in a bucket of water before adding it to the well. Add no more than 100 g of bleaching powder or calcium hypochlorite to each bucket of water.

4. Pour the solution into the well. It is best to agitate the water to insure that the chlorine is evenly mixed.

5. The strong chlorine solution should be left in the well for at least 12 hours and preferably for 24. It must be stressed that this strong chlorine solution is not suitable for humans or animals.

6. After the 12 to 24 hour contact time, the strongly chlorinated water should be pumped from the well until the residual chlorine level is below 0.7 mg per liter of water. (See below.) The pumping equipment to be installed on the well can be disinfected by using it to remove the excess chlorine. Choose a disposal place for the chlorine solution where it will have as little contact with plant and animal life as possible.

B. Water Disinfection

Water can be easily disinfected by adding to it one of several commonly available chemicals which contain chlorine. The most common type of household bleach is a mild chlorine solution which can be used to disinfect water.

The amount of chemical or solution needed to disinfect water will depend on the degree of contamination of the water and the amount of chlorine present in the chemical. However, in most cases where the water is clear with no suspended solid particles, the following disinfection procedure can be used.

1. Determine the volume of water to be disinfected.

2. Find the amount of chlorine compound that will be needed to disinfect that volume of water. (See Table VIII-2.)

3. Dissolve the required amount of chemical in a bucket of water before adding it to the water to be disinfected. Add no more than 100 g of bleaching powder or calcium hypochlorite to each bucket of water.

4. Pour the bucket of chlorine solution into the water to be disinfected. Agitate the water to ensure good mixing.

5. When the chlorine residual (See below.) in the water drops below 0.2 mg per liter, this disinfection procedure should be repeated.

C. Chlorine Residual

The chlorine residual is the amount of chlorine that is left in treated water. Chlorine is used up as it disinfects. Add enough chlorine to the water so that there is a little left over (the residual) after the chlorine has had at least 30 minutes to react with and kill all the living organisms in the water. This assures that all the disease causing bacteria have been destroyed and that there is still some chlorine available to kill other contaminants which might enter the water at a later time.

The recommended chlorine residual is 0.5 mg per liter. A higher residual will cause an obvious chlorine taste in the water. Above 3.0 mg per liter chlorine concentration can cause diarrhea.

Chlorine residual is easily checked with any of the commercially available color comparators. Most of these use an "orthotolidine solution", which turns progressively more yellow at higher chlorine residuals.

TABLE VIII-1 - AMOUNTS OF CHEMICALS REQUIRED FOR A STRONG CHLORINE SOLUTION CAPABLE OF DISINFECTING WELLS AFTER THEIR CONSTRUCTION*

TABLE VIII-2 - AMOUNTS OF CHEMICALS NEEDED TO DISINFECT A KNOWN QUANTITY OF WATER FOR DRINKING*