Case Study: Essex swimming school fined for chemical burns to 3 year old girl
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.
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. 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 page.
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.
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.
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.
- 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|
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 page.
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.
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.