The Water Cycle
Water in a Pool
Chlorine reacts with organic nitrogen compounds from proteins in bather pollution, these include creatine from sweat and urine. These compounds are stable, and promoted by excess chlorine, which causes a problem for disinfection. They are removed however, by exposure to ozone, or by dilution. The most common procedure is to remove with ozone, and then filter through the carbon filter afterwards.
Trihalomethanes are formed by reaction between the disinfectant and compounds in source water such as humic acids, or those introduced by swimmers. The UK standard of the four, is 100 micrograms per litre as a three monthly average. To reduce the event of these compounds it is recommended to promote pre-swim hygiene, to not overload the pool, and to use the minimum amount of disinfectant. Humic acid can be reduced through coagulation, as can it all be avoided by not shock dosing the pool, which is adding large amounts over a short period of time.
Chloroform is the most common chloramine, it makes up around 86% by weight. It is a potential carcinogen, it will be absorbed by the skin, by swallowing or by inhalation.
Bromine is another danger, it is why bromine is used more infrequently to chlorine based compounds. This is because bromine is a carcinogen and a mutagen.