Chlorine is normally added to the water tank:

  • when the germ test shows that germs are present
  • as a routine task to maintain the free residual chlorine level

6.1 Treating water with chlorine

Safety with chlorine


If a water supply requires chlorination and the system does not have an automatic chlorination plant, chlorine chemical will need to be added regularly to the water in the tank. The chemical, usually a form of hypochlorite, normally comes as a solid. When it is dissolved in water it produces chlorine which kills any germs in the water and provides the safety buffer.

Because chlorine comes in different forms there are different instructions for their use. Also, the dose will depend on the amount of water in the tank and the amount of chlorine already in the water. Chlorine powders usually come in plastic buckets or bottles. Instructions for use are always written on the container. Always read and follow these instructions.

Sometimes the instructions for using chlorine are difficult to follow. Always check with the EHO, the EHP supervisor, Program education staff or the water supplier before using chlorine in a water tank for the first time.

Chlorine powders are dangerous chemicals and must be used carefully.

There are two dangers associated with chlorine powder:
  • The powder and its solution give off chlorine gas. Chlorine gas is very poisonous
  • If the powder gets onto the skin or into the eyes it can cause painful and damaging burns
The wet powder will also bleach (take out the colour) in clothing.

The following safety precautions should always be taken with chlorine powder:
  1. When working with the chlorine concentrate avoid breathing in the fumes.
    Always open the concentrate packet and mix the solution outside in the open air. Never lean close to the open part of the packet or the top of the bucket when mixing the concentrate. Always put the lid back on the concentrate container once the required amount has been removed.
  2. Keep chlorine powder away from children and food.
  3. Do not let the chlorine powder get wet before you mix it with water in a bucket.
  4. Do not add the chlorine powder straight into the water tank. The correct amount of powder should first be dissolved in water in a bucket.
  5. Always add the chlorine powder to the water, never the other way around.
  6. When adding chlorine powder to water in a bucket, add it slowly and stir the water all the time. Avoid splashing.
  7. Keep stirring the solution in the bucket until all the chlorine has dissolved.
  8. If any chlorine does get onto the skin or clothes, wash it off quickly with lots of water.
  9. The container used for measuring out the chlorine powder should be used only for that purpose.

Working out how much chlorine to use

Before starting to chlorinate the water in a tank, the volume of water in it must be measured. This is the amount of water in the tank measured in litres or cubic metres.

If the EHP finds these calculations difficult, he/she should check with the EHO, EHP supervisor, Community Nurse or the local school teacher.

It must be remembered that the amount of water in the tank will differ from time-to-time and because of this, the volume of water must be worked out each time the water is chlorinated.

It may also be necessary to allow for any chlorine left in the water from a previous treatment. Before adding any more chlorine, the chlorine level should be measured and taken into account when calculating how much more is needed. For example, if the chlorine level is 0.2 ppm, the amount to be added will be less than if there was no chlorine.

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The volume in litres can be worked out in the following way:
  1. Using a stick marked in metres and centimetres measure the depth of the water. Write this measurement down.
  2. With a measuring tape also marked in metres and centimetres, measure and write down the diameter of the tank. The diameter of the tank is the distance from one side of the tank to the other side measured straight across the middle. Measure the depth of the water and the diameter of the tank in metres. As the diameter of the tank is not going to change, this measurement can be recorded in the office for future use.
  3. Using these measurements do this sum for a round tank:
Fig.  6.35: Depth and diameter of tank.
Fig. 6.35: Depth and diameter of tank.

Volume in litres = 785 x depth x diameter x diameter
Example:Depth of water in the tank = 3.5 metres
Diameter of the tank = 3.0 metres
Volume = 785 x 3.5 x 3.0 x 3.0 = 24 727 litres

Note: The factor of 785 used in this calculation is a simplified approximation of usual formula for calculating volume. The result is sufficiently accurate for a round tank chlorination.

The volume of a square or rectangular tank is easier to work out. Measure the depth of water in the tank as before, and then measure two of the sides. In the case of a square tank which sides are measured is not important. However, in a rectangular tank measure a long side and a short side.
Using these measurements do this sum for a square tank:
Volume = Depth x length of one side x length of the same side.
For a rectangular tank the sum will be:
Volume = Depth x length of a long side x length of a short side
Example for a rectangular tank:
Depth of water in the tank = 2.75 metres
Length of a long side = 3.0 metres
Length of a short side = 2.5 metres
Volume in cubic metres = 2.75 x 3.0 x 2.5 = 20.625
There are 1,000 litres in a cubic metre.
Therefore: Volume = 20.625 x 1 000 = 20,625 litres

After the volume of water in the tank has been worked out, the instructions on the chlorine container will tell how much chlorine powder will need to be added to the water in the tank.

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Adding the chlorine

Always add the chlorine to the bucket holding the water ready for mixing. Never add the chlorine before the water.
  1. Check the level of water in the tank and work out the volume of water, to find how much chlorine powder will be needed.
  2. Measure out this amount of chlorine in the special container.
  3. Partly fill a plastic bucket with water and get a flat paddle to use as a stirrer.
  4. Slowly add the chlorine powder to the water and stir until it has all dissolved. Do this out in the open air.
  5. Take the bucket of chlorine solution to the water tank and slowly pour it into the tank. If possible add small amounts of the solution to different parts of the tank.
    It may be necessary to climb a ladder to get to the opening of the tank. If this is the case, the EHP will need a ladder.
  6. If possible stir the water in the tank.
  7. Wait for 2 hours and then test the chlorine level in the tank water to make sure it is near the 0.2 - 0.6 level.
Fig.  6.36: Measuring chlorine powder.
Fig. 6.36: Measuring chlorine powder.
Fig.  6.37: Adding chlorine solution to tank.
Fig. 6.37: Adding chlorine solution to tank.
Fig.  6.38: Mixing in the tank.
Fig. 6.38: Mixing in the tank.

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If the chlorine level is less than the range of 0.2 - 0.6 ppm more chlorine will need to be added.

Should the chlorine level be above the 0.6 ppm level, it does not present a health problem. The high level of chlorine may make the water taste of chlorine and some people in the community may not like this. If the water is needed for drinking, let it stand in an open container or boil it. This will allow the chlorine to escape. However, if it is going to stand in the open the water must be protected from contamination.

The community may wish to take steps to correct a high chlorine level by topping-up the storage tank if there is room. Another solution to the problem is to allow some of the water to run to waste and then top-up the tank. Check the chlorine level after each of these actions.

Always remember that there should be a free residual chlorine level of 0.2 - 0.6 ppm after chlorination has been finished. Sometimes however, because of the level of contamination in the water, it may be necessary to add extra chlorine to kill all the germs and to get the required free residual chlorine level.

6.2 Tank cleaning

Occasionally the inside of the community water tank will need to be cleaned out. This would be necessary if anything happened in the tank to contaminate the water supply. For example, a dead animal may be found in the tank, dust and dirt might be washed into it or slime may have built up on the sides.

These are the steps involved in cleaning out a water tank:

Before the cleaning day

  1. Let the community people know well beforehand that the tank is to be cleaned and that the water will have to be turned off for a few hours. This will allow them to collect enough water to keep them going whilst the water is turned off.
  2. Discuss the tank cleaning job with the water supply agency before commencing the job. The agency can provide any technical assistance especially if the system has an automatic chlorinator. There may be special precautions which need to be taken when the pump is switched off.
    If you have any problems contacting the water supply agency, the EHO or the EHP supervisor can help.
  3. Try and plan the tank cleaning job when the tank is nearly empty so that a lot of water will not be wasted.
  4. Organise at least 2 people for the cleaning job. One person to get inside the tank and do the cleaning, the other to watch from the outside as a safety precaution and to assist with the cleaning job.
  5. Make sure all the necessary equipment will be on hand to do the job. For example, a broom with hard bristles, a scrubbing brush, bleach powder, and a shovel. A bucket on a rope may be needed to lift the dirt out of the tank.
  6. Make sure there will be enough water available to rinse and refill the tank after it has been cleaned.
Fig.  6.39: Tank cleaning.
Fig. 6.39: Tank cleaning.

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On the cleaning day

  1. Turn off the pump which fills the tank.
  2. There should be no need to turn off an automatic chlorinator. However, follow any instructions given by the water supplier.
  3. Turn off the main tap to cut off water supply to the houses.
  4. Disconnect the pipe which takes the water to the houses. This will allow the water in the tank to run out.
  5. It may not be necessary to disconnect any pipes if the tank has a draining pipe and a valve which can be used to let the water out of the tank.
  6. Empty the tank.
  7. Make sure that the second person is outside the tank all the time the cleaner is inside.
  8. Thoroughly clean all of the inside of the tank. It may be necessary to use a scrubbing brush. Bleach powder may help get rid of dirt and slime which has built up inside the tank.
  9. Thoroughly rinse out the tank with fresh water and allow this water to go to waste.
  10. Reconnect any disconnected pipes and turn on the pump to refill the tank. Turn on the main tap supplying water to the houses.
  11. Make sure the automatic chlorinator is working properly as the tank fills or add the correct amount of chlorine when the tank is full.