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 after superchlorination. If you intend to discharge a significant quantity of swimming pool water for any reason, there would usually be a requirement to inform the local water authority. 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 reduce chlorine levels by simply diluting the swimming pool with fresh water. This is safer and would contribute to less chemical pollution as well.

However, if you need to decrease the chlorine quickly, 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. Since each m3 would have 10g of chlorine, and 300m3 X 10g = 3000g. The simplest thing to do would be to calculate how much sodium thiosulphate you would need 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 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 add 8 jugs of sodium thiosulphate to get the free chlorine down from 10.00mg/l to 2.00mg/l.

Adjusting pH

The chemicals that can be used for hand-dosing of pH correctant are sodium bisulphate powder (dry acid) to reduce the pH and sodium carbonate (soda ash) to increase the pH. 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 due to the buffering effect of total alkalinity. 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.

Decreases pH
Increases pH
16 Comments
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I currently use stabilised chlorine granules but I am going to change to use calcium hypochlorite granules thanks to what I have learnt on this course.

Do I need to dissolve the hypochlorite granules into fresh tap water first at a ratio of 1:33? Or can I add the granules straight into the pool even if there is still say 1ppm of chlorine in the water and I want it at 2ppm. Does it have to be dissolved in fresh tap water?

My pool is 45m3. The calcium hypochlorite is 70% strength.
To increase by 1ppm: 45/0.70 = Add 64g
Is that correct?

Many thanks

Stockwell Safety (Administrator) 24th August 2022 at 3:16 pm

64 grams is correct and you do need to dissolve it in water to make a solution before adding in to the pool. The more water the better as the solution will be less concentrated that way. I’m assuming you don’t have an automatic dosing system installed.

Dayle Fossard (Group Leader) 11th May 2022 at 3:09 pm

To keep this simple, as there seems to be some confusion above. Using the calculation would be: Pool volume in m3 x 5 = how much Sodium Thiosulphate needed to reduce the free chlorine by 1ppm

For example a 19 x 7 x 1.2 = 159m3
159 x 5 = 795grams to reduce by 1ppm?

Stockwell Safety (Administrator) 13th May 2022 at 12:30 pm

Correct.

It’s useful to be able to do these calculations on the fly, but just to let you know – you will always have access to the online calculators in this course for as long as your qualification remains valid.

emma.brennan095@gmail.com 16th June 2021 at 7:15 pm

For hand dosing a bromine pool what percentage would you put for chlorine strength in the calculator as its 61% bromine and 27% chlorine, just put the 27%?

Stockwell Safety (Administrator) 16th June 2021 at 8:09 pm

No, not if you’re trying to increase bromine – use the bromine percentage (61%).

malcolm.heppolette@ludgroveschool.co.uk (Group Leader) 27th May 2021 at 10:16 am

Hi Adam
You mention that a possible reason to superchlorinate the pool would be if diarrhoea was present as this may contain cryptosporidium. Isn’t crypto resistant to chlorine, hence the need for coagulation to filter it out?

Stockwell Safety (Administrator) 27th May 2021 at 8:42 pm

Hi Malcolm,
In their publication ‘Treatment and Quality Standards for Pools and Spas’, PWTAG emphasise superchlorination over coagulation for pools that have high-rate filtration. High rate filters, even with coagulation won’t remove crypto. They recommend 20mg/l of free chlorine for a contact time of 13hrs.

I like the dosing calculators, can access those outside of this course?

Stockwell Safety (Administrator) 19th May 2021 at 8:59 am

Hi Harry,
Sure. We’ve now made the lesson that the topic is included in a sample lesson, so you should be able to access it for free, without needing a user account.

Hi, If you fail the assessment twice, what does this mean?

Astrid (Group Leader) 15th January 2021 at 3:01 pm

Hi Helen,

We can give you another go if that happens but we would advise you to look at the lesson again if your first attempt wasn’t successful, before you have the second attempt.

Kind regards,

Astrid

“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 he 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.”

Is there a mistake here, as initially you calculate 3000g of chlorine in the pool but then only do 300 x 5 to clculate 1500g of sodium thiosulphate? Should this be 15,000g of sodium thiosulphate or is the 300g of chlorine correct?

Stockwell Safety (Administrator) 21st December 2020 at 8:32 pm

Hi Tom,
The idea here is to work out how much sodium thiosulphate it would take to bring the free chorine down by just 1mg/l. Then you can make a measuring jug/scoop to that size. Then, if you need to bring the chlorine down by, say, 8mg/l, you add 8 jugs/scoops (which would equate to 12,000g in the example given).

The idea with this is to have a pre-made jug/scoop that you know will reduce the chlorine by 1mg/l, then you simply add however many jugs/scoops as appropriate. ie, to reduce chlorine by 5, add 5 jugs, to reduce chlorine by 8, add 8 jugs etc. etc.

As per Tom Owen question in the example you show how to work out the 10.00 mg/l chlorine in a 300m3 pool = 3000g chlorine. Where do you get the 300g of chlorine from??? should it be 3000g x 5g = 15000 or 1.5kg of thiosulphate for example purposes.

It’s confusing when your showing calculation of 3000g then all of a sudden it’s 300g. I under stand the bring down of free chlorine and the measuring jug

Stockwell Safety (Administrator) 1st November 2021 at 4:38 pm

Where do you get the 300g of chlorine from???

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.

That’s where the 300g of chlorine comes from, ie, the equivalent of 1.00mg/l (working on decreasing chlorine in multiples of 1 rather than multiples of 10).

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