Cya Indoors

[Tom Kirshbaum]

I understand the function of CYA is to keep chlorine from disappearing in sunlight. But what about an indoor spa in a room with very little natural light and which is generally opened only at night? Why does such a pool need any CYA at all?

Follow up: if CYA isn't needed, why bother with the dichlor in the dichlor/bleach method? Why not just use enough bleach to get the CH up to the right level and forget the dichlor?

[chem geek]

CYA doesn't just protect chlorine from breakdown by the UV in sunlight. It also significantly moderates chlorine's strength. When CYA is present, most of the chlorine is bound to it and only a small percentage remains unbound and "active" as hypochlorous acid for disinfection as well as hypochlorite ion which also participates in oxidation. Fortunately, it takes a fairly small amount of chlorine to disinfect quickly so without CYA in the water the chlorine is "too strong" and will oxidize swimsuits, skin and hair and will also outgas more quickly oxidizing hot tub covers. This is why you need to start out with some CYA in the water, either added initially as pure CYA or added cumulatively via initial Dichlor use.

If you use only Dichlor, however, the CYA will continue to build up and the active chlorine level will drop as a result unless you proportionately raise the FC level to keep the FC/CYA ratio constant. As the active chlorine level drops, the rate of disinfection and oxidation drops so that chlorine is no longer able to keep up with oxidizing bather waste (especially if the bather load is higher, such as every day soaking, and if there is no ozonator) so the water turns dull/cloudy and the risk of bacterial biofilm formation also increases. This causes one to have to change the water sooner than if one managed the CYA level as with the Dichlor-then-bleach method. Because CYA does slowly break down in spas (usually at a rate of around 5 ppm per month), one can use Dichlor about once a month to bring back the CYA level so that the chlorine does not become "too strong".

This is all known science since at least 1974 as described technically in this paper but the pool and spa industry has tended to ignore this because this knowledge would affect sales of the most popular and convenient pool chlorine product, Trichlor tabs, as well as spa products such as Dichlor and would also interfere with the much more profitable sale of clarifiers, flocculants, algaecides, phosphate removers, enzymes, etc. that are used to make up for the fact that chlorine becomes less effective as the CYA level builds up.