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.

Case Study: Essex swimming school fined for chemical burns to 3 year old girl

A three-year-old girl suffered first-degree burns after sitting in corrosive cleaning fluid at a swimming lesson, a court heard. The chemicals were spilt by maintenance staff at a teaching pool run by First Strokes Swim Schools in Stanway near Colchester.

The company admitted three breaches of the Health and Safety at Work Act at Chelmsford Magistrates’ Court. A district judge fined the firm £10,500 and ordered it to pay £2,350 costs.

The girl was sitting at the edge of the pool waiting for her lesson to start on 22 May last year when she began to complain of pain in her leg, the court was told. When her mother took her to Colchester General Hospital, doctors said she had suffered first and second-degree burns.

First Strokes admitted a contractor had undertaken maintenance work earlier in the day using the company’s own supply of sodium hypochlorite to treat the pool water. A small amount of the chemical spilt on to the side of the pool, the court heard.

Prosecutors said the company had failed to assess the pool water treatment and cleaning chemicals used at the site, and failed to supervise contractors properly. The company also admitted it had failed to comply with two improvement notices for its lack of hazardous substance control assessments, and to not having a safety system for dosing the pool by hand.

Mike Lilley, a councillor at Colchester Borough Council, who brought the case, said: “The public should be reassured that we take a very dim view of any business that fails to put health and safety first and whose actions lead to personal injury.”

IOSH Coverage | PWTAG Coverage | BBC Coverage

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.

10 thoughts on “Manual/Hand Dosing”

  1. 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%?

  2. 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?

    • 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.

    • 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 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,


  3. “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?

    • 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.


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