Superchlorination - The Correct Way

Posted by on Thursday, July 21, 2016 Under: Disinfection

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Following on from our previous post regarding hand-dosing of chlorine, here is some guidance on superchlorination.

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 by-products. 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 residual. 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. Note: 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.

Step 1. 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.

Step 2. Raise the free chlorine concentration as required to deal with the problem, based on the chart below.

Reason for superchlorination

Concentration required mg/l

Contact time

Diarrhoea (possible contamination with Cryptosporidium


13 h

Algal growth


24 h

Legionella (spas)


16 h

Raised colony counts, coliforms, E. coli


1 h

Raised P aeruginosa


12 h


Step 3. Add the total amount of calcium hypochlorite to tap water in the bucket(s) until fully dissolved/mixed. 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.

Step 4. 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.

Step 5. 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.

Step 6. Leave for the desired contact time (see Table). Check every two hours to ensure the concentration is being maintained. If necessary re-dose to reinstate the required free chlorine residual, again checking pH.

Step 7. 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.

In : Disinfection 

Tags: "superchlorination"