How salt water chlorination works in swimming pools

Most people will know that that the best chemical for killing algae and bacteria in swimming pool water is Chlorine.


Most people will know that salt is Sodium Chloride, a Sodium atom bonded to a Chlorine atom. Chemical Symbol NaCl.

Most swimming pool owners know that they have to regularly buy “Chlorine” granules and throw them in their pool to kill algae and bacteria to keep their pool water clean and safe to swim in. It gets used up and you have  to go and buy more.

The more savvy pool owner will know that one of the types of  “Chlorine” he could use is actually Sodium Hypochlorite and when he adds it to water Hypochlorous Acid (HOCl) is formed and that is what actually kills stuff.  Sodium Hypochlorite  sounds a bit like Sodium Chloride – salt. Well it is close, but just putting salt in your pool will not kill bacteria  – you have to “hypo” it. To “hypo” it you have to add an Oxygen atom.  If you mix a lot of water (H2O) and little salt (NaCl) together and pass and electric current through it, greatly simplified, an Oxygen atom and a Hydrogen atom join up with a Chlorine atom and you get H + O+ Cl = HOCl, hypochlorous acid. Bingo! All bacteria dead and lovely clean pool water. That really is just about all there is to salt water chlorination except read on because it gets even better.

Strickly speaking it is not Chlorine that kills bacteria or algae it is the Oxygen that does the killing by oxidising the bacteria, the same reaction as burning something. Burning, setting fire to and oxidising are all the same thing, either way the bacteria is dead. So the Hypochlorous acid gives up its oxygen atom to burn the bacteria and the chlorine atom goes back in to the water. That is until it finds its way back to the electrodes of the salt water chlorinator and re-forms in to HOCl again and goes off looking for more bacteria to kill. Self regeneration, a never ending supply of hypochlorous acid means you never have to add sanitising chemicals to your pool water again.

So it is not hard to see why salt water chlorination is an ever more popular way to sanitise a swimming pool. Just add salt to the water and fit a set of clever electrodes called a salt water chlorinator and you can have pristine clean pool water and no need to add sanitising chemicals ever again. The high electrical current between the electrodes will also kill any algea spores in the water so although the hypochlorous acid would have got them anyway the electrodes killing them off mean you can lower the chlorine level down to about 0.5 ppm.

That is a brief explanation of how salt water chlorination works in swimming pools. Read the next article for the pros and cons of actually buying, fitting and using one.