Manual Dosing – Increasing and Decreasing Free Chlorine Residual

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The following method will outline how to add a hypochlorite disinfectant to the swimming pool. If you’re using a chlorinated isocyanurate disinfectant, follow the manufacturers’ instructions as the method will be different.

Increasing Chlorine

The first thing you need to do is calculate how many cubic metres of water you have in your swimming pool. Do this by multiplying the length by the width by the average depth. See the worked example below:

Length (20m) X Width (10m) X Average Depth (1.5m) = 300m3

The next thing to do is calculate how much calcium hypochlorite/sodium hypochlorite you need to add in order to increase the free chlorine reading by 1mg/l. As an example, if using 65% chlorine strength calcium hypochlorite granules begin by dividing the pool volume figure (from step 1.) by 0.65. The reason you need to divide by 0.65 is because calcium hypochlorite is typically only 65% chlorine. Some products are 70% chlorine, in which case you would divide by 0.70 etc. See the worked example below:

300m3 / 0.65 = 462

The figure obtained provides you with the amount of grams of calcium hypochlorite granules you need to add to the swimming pool in order to increase the free chlorine reading by 1.00mg/l.

  • Alternatively, use the calculators provided at the bottom of the section.

Use a set of kitchen scales to measure out 462g of calcium hypochlorite granules into a clear plastic jug.

It might be useful to mark a line on the jug to indicate the level of calcium hypochlorite granules at 462g, or carefully cut the jug to size. See below.

For example, if you have zero free chlorine in the pool and you would normally operate at 2.00mg/l, then you need to increase by 2.00mg/l. This equates to the number of jugs of calcium hypochlorite granules you need to add to the swimming pool, i.e. 2 jugs.

Now you need to add the granules to the swimming pool water.

For a deck-level pool: carefully deposit the granules into the overflow channel, near to the balance tank intake.

For a skimmer basket pool: carefully deposit the granules into each of the skimmer baskets. From here, the granules will be drawn into the suction-side pipework of the circulation system.

Allow some time for the granules to dissolve and make their way around the system and into all areas of the swimming pool. How long this will take will be dependent on a number of factors, such as the efficiency of the system hydraulics. Time how long it takes, so you will know for next time.

Carry out a set of pool tests, taking the sample from a point in the swimming pool as far as possible from the inlets. This is to help you determine whether the chlorine you have introduced has been distributed to all areas of the swimming pool. If necessary, carry out further tests in order to be sure that all areas of the swimming pool have a sufficient level of disinfectant. Once you are satisfied of this, you can open the pool again to bathers.

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. The principle to bear in mind is that 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 calculators provided at the bottom of the section.

From here, the same steps can be taken as given above in order to create a jug for the purposes of 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.

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