TriChlor is a great sanitizer for swimming pools but its use needs to be properly understood – otherwise the water quality can become poor and the pool unsanitary! When chlorine is dissolved in water Hypochlorous Acid (HClO) is formed and this is the sanitizer, which kills bacteria and helps control algae growth. There are different ways of dissolving the chlorine in the water and the chlorine compound most commonly used for sanitizing swimming pool water is TriChlor (Trichlorocyanuric Acid) and this comes in two forms: granules and tablets. When TriChlor is dissolved in water Hypochlorous Acid and Cyanuric Acid (CNOH)3 are produced. The Cyanuric Acid (CYA) acts as a stabiliser and, without the CYA, the chlorine would disappear quickly from the water, especially in strong sunlight. It is vital to have a certain amount of CYA because otherwise the chlorine would disappear in a few hours. - but real problem can occur if there is too much. It is easy enough to understand how things can go badly wrong. The chlorine gets used up, dealing with the bacteria and debris, but the CYA remains. Over time the CYA level increases. It quickly reaches the stage where the majority of the chlorine is “locked up” and stronger doses of chlorine are required. Eventually the chlorine stops working completely. One 200-gram TriChlor tablet = 2 ppm (parts per million) chlorine and 2.4 ppm CYA. The actual “legal” amount in most areas is 75 ppm (just 31 tablets). The “practical” amount, where chlorine levels can be controlled without any real problem, is reckoned to be 150 – 200 ppm (approx 80 tablets). Now there is a certain amount of water changeover and the CYA become diluted by backwashing, splashing out and leakage. Evaporation does not dilute the CYA – it concentrates it! Most certainly, after just two years, the CYA levels become impractical or impossible.
What happens past this point is that “normal” chlorine levels cease to work properly, more and more TriChlor is required, the water has a “dull” look and frequent shocking is required to stop the water going green.
Normal Chlorine Levels: A public swimming pool has 2.1 ppm - never more than that! Most pool cleaners keep pools around 3 – 4 ppm.
With high CYA levels even 5 ppm is often not enough and here is the real danger – pool test kits will not measure any more than 6 ppm and the chlorine level could be 20, 30 40 ppm – or even more than that!
So what is the solution!
There are several work-a-rounds but it is important to be able to measure the CYA. Most pool chemical suppliers will provide an analysis if you take in a sample and a dedicated CYA test kit can be purchased from Ju-Ju-Ju in Benissa (La Pedrera Complex).
1. Partial Draining: This is the way it is supposed to be done. By draining part of the water each year the CYA level can be kept between 30 and 100 ppm.
The real problem with this solution is that water is expensive.
2. Controlling the Amount of TriChlor used: This is also “doing it by the book”. The chlorine level is maintained between 2 and 3 ppm, the
Free and Combined Chlorine carefullymonitored and the pool shocked regularly with Liquid Chlor (Sodium Hypochlorite) – which does not contain CYA!
This keeps the consumption of TriChlor to a minimum, also the CYA level and reduces dramatically the amount of water required for the annul draining.
The problem here is that the pool requires more visits by the cleaner and swimming time is reduced whilst shocking. Explaining Shocking: Chlorination at normal levels does not remove the debris entirely but combines with it forming nitrates (chloramines).
These chloramines have to be oxidised (burned off) and volatile liquid chlorine is added to raise the chlorine to shock-point (10 – 12 ppm).
Chlorine actually has very little odour. The characteristic pool smell is caused by the chloramines.
3. Convert the Pool to Bromine: Bromine is not affected by CYA and also has lots of other advantages but I will deal with this in another article.
Wikipedia: Swimming Pool Sanitation Ask Alan Schuster: Chlorine Pools
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ARTICLE: Chlorine for Swimming Pools 
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