Manual/Hand Dosing


This is a hazardous activity and should not be performed by people who have not received the appropriate training. A proper risk assessment should be conducted before any manual dosing procedure is performed.

ALWAYS wear the appropriate PPE.

ALWAYS add the chemical to the water, NEVER add water to the chemical.

NEVER mix a chemical with another chemical. Only ever mix with water.

NEVER hand-dose chemicals into the swimming pool when occupied.

ALWAYS allow time for thorough mixing and distribution of the chemical into all areas of the swimming pool water.

Increasing chlorine

Here’s a video explainer going through the steps outlined above.

Decreasing chlorine

It may be necessary to decrease the levels of chlorine on occasion and certainly following superchlorination. If you are going to be dumping a significant quality of swimming pool water for any reason, there would usually be a requirement to let the local water authority know and they would almost certainly require you to eliminate all traces of chlorine from the water before they granted permission to discharge (chlorine is harmful to aquatic organisms).

In normal operations, it would usually be better to bring the chlorine levels down by simply diluting the swimming pool with fresh water. This is safer and would contribute to less chemical pollution as well.

If you do need to decrease the chlorine quickly though, the chemical to use is sodium thiosulphate. It takes 5g of sodium thiosulphate to neutralise 1g of chlorine. So if, for example, you had 10.00mg/l of chlorine in a 300m3 pool, that equates to 3000g of chlorine in the pool, since each m3 would have 10g of chlorine in it, and 300m3 X 10g = 3000g. The simplest thing to do would be to calculate how much sodium thiosulphate you would need in order to decrease the free chlorine level by 1.00mg/l. See the worked example below:

300g chlorine X 5g sodium thiosulphate = 1500g

So, in this particular example of a 300m3 pool, it would take 1500g of sodium thiosulphate to reduce the free chlorine level by 1.00mg/l.

  • Alternatively, use the dosing calculator provided.

From here, the same steps can be taken as given in the increasing chlorine section to create a jug for hand-dosing sodium thiosulphate (different jug – NEVER mix chemicals). Then, just add the required number of jugs in the same way as for adding calcium hypochlorite. So, in the example given, we would be adding 8 jugs of sodium thiosulphate in order to get the free chlorine down from 10.00mg/l to 2.00mg/l.

Adjusting pH

The chemicals that are advised to be used for hand-dosing of pH correctant are sodium bisulphate powder (dry acid) in order to reduce the pH value and sodium carbonate in order to increase the pH value. Hand-dosing pH correctants is more problematic. This is because it is difficult to calculate the amount of correctant to add in order to bring about the desired change in the pH value of the pool water due to the pH buffering effect of the total alkalinity levels of the pool water. The more buffered the water (due to higher total alkalinity), the more of a given pH correctant you would need to add in order to get to the desired pH value.

Whatever pH correctant is being hand-dosed, refer to the manufacturer’s instructions, which should be on the label on the side of the chemical container. Bear in mind that the recommended amounts may need to adjusted, based on your own experience.