Appendix 16. Batch chlorination

16.1 Determination of the amount of chlorine required for a given volume

– Prepare 1 litre of 1% chlorine solution or “mother solution” (Appendix 15).

– Measure the pH of the water to be treated.

– Take 4 non-metallic containers of known volume (e.g. 20 litre plastic buckets with lids or jerrycans) labelled 1 to 4. Rinse them 3 times with the water to be treated. Fill all 4 containers with the same amount of water to be treated.

– Add a dose of 1% mother solution to each bucket using a syringe, progressively increasing the dose:
• Container 1: 1 ml
• Container 2: 1.5 ml
• Container 3: 2 ml
• Container 4: 2.5 ml

– Mix with a clean stirrer and cover the buckets or close the jerrycans.

– Wait for the chlorine to react: 30 minutes if the pH is < 8 and 60 minutes if the pH is > 8. Then measure the FRC concentration (Appendix 17) in each bucket.

– Choose as reference the container with a FRC concentration of 0.5 mg/litre if the pH is < 8 and 1 mg/litre if the pH is > 81 .

– Calculate the dose of mother solution necessary to chlorinate the total volume of water to be treated in the reservoir.

 • If the FRC concentration is below the recommended level, start the test again from the beginning increasing the dose of mother solution to obtain the required level of FRC (e.g. 2.5 ml, 3 ml, 3.5 ml, and 4 ml).
• Higher or lower doses of mother solution may be necessary if the volume of water used for the test is over or under 20 litres.
• Pre-treatment may be necessary to reduce the turbidity if it is > 5 NTU.

16.2 Water treatment

– Prepare 1% chlorine mother solution (Appendix 15).

– Pour the necessary volume of mother solution to chlorinate the volume of water to be treated into the reservoir while it is being filled, so that it mixes well with the water.

– Wait the required amount of time (30 or 60 minutes depending on the pH) then check the FRC concentration before distributing the water.

Example: 2000 litre reservoir of water

FRC concentrations recorded in four 20 litre buckets of water (pH < 8) after 30 minutes were:

BucketVolume of 1% solutionFRC
11 ml0 mg/litre
21.5 ml0.1 mg/litre
32 ml0.5 mg/litre
42.5 ml1 mg/litre

Bucket 3 has therefore desired FRC concentration. 2 ml of mother solution were necessary to chlorinate 20 litres of water.
100 times more mother solution are necessary to chlorinate 2000 litres, i.e. 200 ml of mother solution (100 x 2 ml).

Note: check pH and water turbidity once a week. If these parameters change then the dose of mother solution to be added to the water in the reservoir to obtain the desired FRC concentration needs to be re-determined.

Ref Notes
1 The standard recommended values for FRC levels are 0.2-0.5 mg/litre if the pH is < 8 and 0.4-1 mg/litre if the pH is > 8. As a precaution, it is better to take the highest concentration levels of these ranges.