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 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.”
Increasing chlorine
Step 1: Calculate Pool Volume
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
Step 2: Calculate Substance Quantity
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 (if you’re using stabilised chlorine, follow the manufacturers’ instructions as the method will be different).
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 dosing calculator provided.
Step 3: Measure Out
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.
Step 4: Decide how much you need to increase the free chlorine reading by
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.
Step 5: Add substance to pool
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.
Step 6: Allow to mix
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.
Step 7: Test
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.
Here’s a video explainer going through the steps outlined above.
“I need to ‘shock-dose/superchlorinate’ the pool. How do I do that?”
Superchlorination is not recommended as a routine or even occasional method of shock dosing to compensate for inadequacies in pool treatment. It is generally bad practice, and can generate unwelcome byproducts. But if something has gone wrong – poor results from microbiological testing perhaps, or a catastrophic breakdown in treatment – it may be necessary to superchlorinate.
It can also be a way to deal with contamination by diarrhoea, as some intestinal pathogens (eg.: Cryptosporidium) are resistant to normal levels of chlorine. In this case it may be needed where filtration is inadequate (high-rate for example, or regular coagulation not practised). Superchlorination can also deal with other organisms should the need arise.
Preparation
Two pool parameters are needed as a starting point:
- the capacity of the pool in litres (1m3 = 1,000 litres, 1 gallon = 4.54 litres)
- the pool turnover period (the number of hours for a volume of water equivalent to the entire water volume of the pool to pass once through the water treatment plant)
The following chemicals and equipment will be required to undertake the procedure (which of course must include subsequent dechlorination):
- a suitable chlorine donor – sodium or calcium hypochlorite (not any cyanurate-based disinfectants as they are not effective enough)
- a dechlorinator – normally sodium thiosulphate pentahydrate
- a pool water test kit together with a dilution pot
- one or more clean 10-litre plastic buckets
- a cold water supply.
Operators must be confident that the pool plant (valves, seals etc) will withstand superchlorination.
Superchlorination procedure
- Close the pool to swimmers. If more than one pool uses the same filtration system, all pools will have to be closed to swimmers and superchlorinated. Do not allow anyone to enter the pool(s) until superchlorination and subsequent dechlorination is completed.
- Isolate automatic dosing controllers to avoid damage to the sensors.
- Raise the free chlorine concentration as required to deal with the problem, based on the table shown below.
- Add the total amount of calcium hypochlorite to tap water in the bucket(s) until fully dissolved/mixed (the ‘Manual/Hand Dosing’ lesson explains how to do this).
- Then spread evenly around the pool surrounds and mix well by agitation. Failure to dilute and spread evenly can result in the precipitation of hardness scale.
- Superchlorination will raise pH, so acid will be needed to reduce the pH value to 7.5 or less.
- In the case of contamination by diarrhoea ensure the water temperature is 25°C or higher.
- Ensure that the filtration system is operating while the water reaches and is maintained at the chlorine level required for superchlorination (see table).
- With spas, all aerators, sprays etc should be operating throughout.
- Test the free chlorine concentration 15 minutes after the initial addition to ensure that the correct concentration has been achieved (see table). This may necessitate dilution of the sample with chlorine-free water to give an accurate measurement.
- Leave for the desired contact time (see table).
- Check every two hours to ensure the concentration is being maintained. If necessary re-dose toreinstate the required free chlorine residual, again checking pH.
- Backwash the filter thoroughly after the given contact time and top up pool to the required water level. Be sure the backwash effluent is discharged directly to waste (and not to a septic tank or water course). As usual, rinse the filter before resuming filtration.
| Reason for superchlorination | Concentration required | Contact time |
| Diarrhoea (possible contamination with Cryptosporidium) | 20 mg/l | 13 h |
| Algal growth | 10 mg/l | 24 h |
| Legionella (spas) | 50 mg/l | 16 h |
| Raised colony counts, coliforms, E. coli | 5 mg/l | 1 h |
| Raised P aeruginosa | 5 mg/l | 12 h |
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.
in the example gien for reducing Chlorine, the last 300g/Cl, did you ean to put: 300m3? Otherwise, it doesnt make sense.
Its very confusing to understand where that 300g/cl comes from suddenly.
In the example, the 300 refers to the metres cubed volume of water.
No reference is made to 300g/Cl that I can see.
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
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.
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?
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.
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%?
No, not if you’re trying to increase bromine – use the bromine percentage (61%).
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.
I like the dosing calculators, can access those outside of this course?
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?
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?
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
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).