The pool filter does the majority of the work to keep the water mechanically clean, but chemicals are required to maintain the pool in optimum condition. The chemical balance of pools is important for the following reasons:
- Dangerous pathogens, such as bacteria, thrive in water. A pool filled with untreated water would be a perfect place for disease-carrying microorganisms to move from one person to another.
- Water with the wrong chemical balance can damage the various parts of the pool.
- Water that is not properly balanced can become murky or even irritate the skin and eyes.
In order to get rid of pathogens in the water, a disinfecting/sanitizing agent is required. The most common sanitizer is chlorine, which is a chemical compound available in a solid state, like calcium hypochlorite, or a liquid, like sodium hypochlorite. When the compound is added to the water, the chlorine reacts with the water to form various chemicals, most notably hypochlorous acid. Hypochlorous acid kills bacteria and other pathogens by attacking the lipids in the cell walls and destroying the enzymes and structures inside the cell through an oxidation reaction. Alternative sanitizers, such as bromide, do basically the same thing with slightly different results.
Chlorine is typically prepared in liquid, powder or tablet form (though some professionals use gaseous chlorine), and it can be added to the water anywhere in the cycle. Generally, chlorine is added to the return line after the filter by means of chemical feeder. It can also be added directly into the pool by placing tablets in the skimmers or inside floating chlorine-dispenser / chlorinators, but sometimes the chlorine tends to be too concentrated in those areas when done in this way. One problem with hypochlorous acid is that it's not particularly stable. It can degrade when exposed to ultraviolet light from the sun, and it may combine with other chemicals to form new compounds. Pool chlorinators often include a stabilizing agent, such as cyanuric acid, that reacts with the chlorine to form a more stable compound that does not degrade as easily when exposed to ultraviolet light. Even with a stabilizing agent, hypochlorous acid may combine with other chemicals, forming compounds that are not very effective sanitizers. For example, hypochlorous acid may combine with ammonia, found in urine, among other things, to produce various chloramines. Not only are chloramines poor sanitizers, but they can actually irritate the skin and eyes and have an unpleasant odour. The distinctive smell and eye irritation associated with swimming pools are actually due to chloramines, not ordinary hypochlorous acid -- a strong smell usually means there is too little free chlorine (hypochlorous acid), rather than too much. To get rid of chloramines, the pool needs to be shock-treated i.e. add an unusually strong dose of chemicals is added to clear out organic matter and unhelpful chemical compounds. The formation of chloramines is related to the second major element in pool chemistry, maintaining the right pH in the pool.