- Producing Chlorine Gas by Electrolysis

Salt-water Chlorinators use electricity to break down common salt to produce Chlorine Gas and Caustic Soda.

An explanation of how the system works is detailed below but, as far as the swimmers are concerned, the main difference is that there is almost no chlorine in the pool and what little there is is in the form of Sodium Hypochlorite which breaks down quickly in sunlight.

The resulting experience is totally odour-free water, with a soft 'silky' feel to it. This gives a much pleasanter swim, and there is no need to shower off afterwards to rinse chlorine compounds from the skin!

How it works

Despite what your parents told you - water is not a particularly good conductor of electricity - until you dissolve something in it 

The addition of common salt (Sodium Chloride) turns pool water into an electrolyte: - the water will now carry an electric current easily

This type of sanitizing equipment produces Chlorine Gas, and applies it directly in to the pool, from the low concentration of salt dissolved in the pool water 

Electrolysis takes place in an electrolytic cell installed in-line in the circulation system, usually after the water has been filtered: - 

• Layers of electrically charged plates within the cell pass a low DC current which breaks the salt down and produces millions of bubbles of Chlorine Gas and liquid Caustic Soda

• Many Chlorinators have a transparent chamber or cell that contains the electrolysis plates and a ‘smoke’ of gas bubbles can be seen leaving the plates and being carried away by the flow of water

• The Chlorine Gas sanitizes only the volume of water in which it is produced

• By the time that volume of water arrives at the pool the chlorinator products have re-combined to form salt

• Hence there is very little Chlorine in the pool water

• A salt concentration of 2500-6000 ppm in the pool is needed for effective operation, depending on the manufacturers instructions for each Chlorinator

• This is achieved by the addition of 125-300 kg of salt per 50,000 litres or 13,000 US Gals of water

• As the salt water passes through the cell it is superchlorinated (preventing the build up of chloramines) and, because caustic soda is produced at the same time and is not removed or separated, it neutralises the acidic condition produced by the Chlorine Gas

• This means the pH of the water is not affected

• When choosing a Saltwater Chlorinator to install to your pool try to buy one that also controls the pH of the pool; they are a tad dearer but absolutely worth the difference in price

• To prevent scale build up on the plates the control unit may periodically reverse the charge on the plates to repel any build-up that the opposite charge attracted

• Once the Chlorine Gas generated from the water does its job of sanitizing, it recombines with the other by-products of electrolysis and reverts back to salt - to be used over and over again

• Therefore it is only necessary to add salt to replace losses due to bather splash-out and filter-backwash

Most modern Chlorinators measure the salinity of the water and advise you when to add more salt

Important items

- that the Chlorinator salesman sometimes forgets to mention . . .

• Running the electrodes will increase electricity use

• This may be up to several hundred watts, depending on the model, all the time that the pool pump is running

• Saltwater can cause corrosion of metal pool fittings - not usually a problem in new pools as no metals are used but in older pools it may be a big problem

• Salt can degrade and weaken the mortar joints of pool surround paving slabs or cause damage to cast-concrete surrounds

• Salt forms a heavy scale build-up above the waterline in tiled pools, which looks ugly and needs to be removed periodically

• The costs of periodic replacement of electrodes may be high

• Salt Water Chlorinators remove ammonia and nitrogen compounds from pool water by a process called Oxidization

• The strength of Oxidation is measured in millivolts and is called an ORP or REDOX potential

• Salt Chlorinators are not always sold with an ORP control system

• This means that they will not generate extra Chlorine on hot days or when there is a high bather load, resulting in potentially unhealthy water

• On cool days with no bather load, they are wasteful as they still generate the same amount of Chlorine and use the same amount of electricity

• Total dissolved solids (TDS) in water create a corrosive situation even when the pH is in its ideal range.  Pools should be partially or completely drained when the TDS level reaches 1500 ppm above the salt level recommended by the Chlorinator manufacturer

• The pH must be properly controlled as suggested by the manufacturer - that's why it's a good idea to buy a Chlorinator with added pH control; they've taken care of the pH aspect/problem for you

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Ken Walker - MyPoolGuru©