How Swimming Pool Salt Water Chlorination Works

by Pool Builders on 03-06-2009 in Articles

Salt Water Chlorination of pools is now in increasingly common use, especially in the United States. However it is not making the same inroads in Europe and Australia. This article describes the water chemistry of how it works and how it compares with liquid chlorine injection that is becoming the dominantly used system outside the US. One of the questions that we are asked most frequently is this; "Can I have automatic salt water chlorination in my pool?" Our answer is always - Yes, But it may not be the best solution. But first of all - How does it work?

About 150 - 180 kg of water softener type salt is put into the pool when it is first filled resulting in a minimum level of about 3000 parts per million of salt in the pool water. Salt makes the water conductive so that the electricity can pass between the plates in the cell described below. If the salt level goes too low, then the chlorine production simply stops. Salt is just the raw material from which the chlorine is produced. During filtration this passes around two electrodes in a small (normally translucent) cell unit placed after the filter and heater in the pool pump house - to avoid excessive corrosion of these units. The control unit is a device that sends power to the salt cell. The unit controls how much chlorine is produced by regulating how long the power is applied to the cell. If the control knob is turned down the unit will apply power to the cell for less time, thereby producing less chlorine. The control unit will often sense the level of salt in the pool and indicate the need to add more salt. Self cleaning units have a feature built into the unit that reverses the polarity of the voltage through the cell in order to help clean any scale buildup off of the cell plates. For the chemists among you this is the equation; NaCl + 2H2O = HOCl + NaOH + 2H2 What does all that mean? - on the left side we have salt + water - and some electric current in a cell that does the nadgery bit!

On the right hand side of the chemical equation we have the HOCl that is the hypochlorous acid that kills all the germs and bacteria. However Sodium Hydroxide and Hydrogen are also produced alongside the HOCl. The Sodium Hydroxide (or Caustic Soda as it is more commonly known) increases the Ph value of the pool water. This is why salt water chlorination must be installed with Ph measurement and adjustment system that will cater for the need to constantly measure PH and add acid to the pool water to keep it in the correct range. An overly acidic pool will eat the tiles and tile grout as well as your heat pump and be thoroughly unpleasant to swim in.

Hydrogen that is also produced is normally just lost into the atmosphere. But oxygen can also be a side product of the cell when water molecules are split into hydrogen and oxygen there have been a few explosions in the US when the cell has been faulty and produced freak conditions.

There are a lot of other disadvantages in using salt water chlorination and it is very harmful to the environment - but please read my other article on Salt Water Chlorination to find out more about this.

The primary disadvantage of salt water chlorination is that in hard water areas the cells are very prone to the build-up of calcium carbonate deposits. The electronic boxes that come with the system switch the polarity of the probes in the cells to reduce this, but periodically the cell will have to be taken apart and cleaned - a thoroughly nasty little job. This should not be such a problem in soft water areas - but we don't have many of these in France! In Europe a fully automatic liquid chlorine injection and Ph control system can be installed for about the same price. Both systems are very reliable - but slightly more complex to set up. However they control the chlorine and Ph levels very accurately and will last as long as the pool with the replacement of a few parts. The process is quite similar to salt water chlorination in that there are a couple of small in-line cells through which all the pool water is passed in about 6 to 8 hours. As the water passes through the first cell the Ph and chlorine levels are measured, first of all the Ph is adjusted by the addition of caustic soda or acid and then the liquid chlorine is injected a drip at a time in the second cell. All the these parts and the whole installation are normally very reliable and can last for 10 years or more without any attention.

So in Europe and Australia most professional pool installers steer their Customers towards the automatic chlorine injection system simply because it is more reliable and cost effective. Salt Water Chlorination is also very unfriendly to the Environment but read my other article to find out more about this. For more information follow this link Swimming Pools in France.

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