Disinfecting a well after pollution

Wells must be disinfected:

- after construction (before they come into service)
- after repair or maintenance
- after a period of non-use (before being put back into service)
- after accidental pollution (after a landslide, if an animal falls in, or after floods, etc.).

Comment

Open wells must be maintained once a year (drained, the entire lining must be examined, repaired if necessary, any sediments accumulated on the bottom must be removed). The best time to carry out this maintenance is at the end of the dry season.

Technique

- Brush the walls above the water line with a solution of 200 ppm (1)(5) active chlorine obtained by diluting 10 times a solution at 2000 ppm (see the "Products for generating chlorine" instructions on page 24)

- Determine the volume of water contained in the well:

Volume of a cylinder: V = p ´ r² ´ h

where V= Volume in cubic metres
p = 3.14
r = radius of the well = 1/2 the diameter in metres
h = height of the water in the well in metres

Example

A well of 140 cms in diameter and with a water height of 3.5 metres:

Volume = 3.14 x (0.7 x 0.7) x 3.5 = approx. 5 cubic metres.

- Determine the quantity of chlorinated product to be used to chlorinate all the water at a dilution of 100 mg of active chlorine per litre (100 ppm).

For one cubic metre, 140 g of calcium hypochlorite will be needed at 70% or 66 dichloro-isocyanurate (NaDCC) tablets with 1.5 g active chlorine; with another chlorine-generating product, calculate the dose according to the percentage of chlorine.

Comment:

In countries were water is rare, chlorinate at only 50 mg per litre (50 ppm) to avoid having to throw away too much water at the end of the operation.

- Dilute the chlorinated product in buckets of water (without exceeding 250 g of product per 10 litre bucket).

Attention: do not use metal buckets because the metal reduces the chlorine (plastic, rubber or earthenware would be suitable).

- Empty the buckets into the well and mix the water with a large (clean) stone, at the end of a (clean) rope.

- Leave for 12 hours to take effect. Do not allow any access to the wells (this water is not suitable for consumption).

- Draw off the water and discard it until it no longer smells of chlorine, then continue using the well as normal.

Comment

If the well is equipped with a pump, proceed in the same manner but, after having poured the chlorinated product into the well and stirred the water, pump until the water which comes out smells of chlorine. Leave it for 12 hours to take effect, then pump and discard the water until it has no further smell of chlorine.