I've never heard or said that a UV sanitizer prevents the breakdown of chlorine from the sun, only that it (UV-C) does not breakdown chlorine like the sun (UV-A) does.
But that is complete and total B.S. and I linked to the actual peer-reviewed scientific paper in a respected journal showing the actual UV absorption spectrum for hypochlorous acid and hypochlorite ion that shows that they DO absorb and break down in UV-C, not just UV-A. In my previous post I link to a user who reported lower chlorine demand after turning off their UV system. Now only a powerful UV system will show up the higher chlorine demand in a pool, but the claim that it lowers demand would only be true IF chlorine were on the edge killing free-floating algae and that the UV system helped with that (which it does, but only when the FC/CYA level is too low such that there is actual algae to kill). It still does not do anything for algae or pathogens stuck to pool surfaces that do not get circulated.
See
The Use of UV For Dechlorination which is a link to an industry website which of course we detest doing, but since scientific peer-reviewed papers in scientific journals are not enough, you can look at this link instead, though of course it is completely opposite to what other industry links will say depending on what the UV manufacturer is trying to sell!
Between the wavelengths 180 nm to 400 nm UV light produces photochemical reactions which dissociate free chlorine to form hydrochloric acid.
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The usual dose for removal of free chlorine is 15 to 30 times higher than the normal disinfection dose.
Note that though the dose for removal of free chlorine is much higher than the normal disinfection dose, that this is to remove the chlorine IN ONE PASS. In a residential pool, if the UV system is sized for disinfection only, then you might get 5-10% higher chlorine demand which probably isn't noticeable. It's if you get a larger UV system that the increase in chlorine demand can be seen. The point is that the UV people who claim that UV-C doesn't break down chlorine are lying. What is true is that a properly sized UV system set only for supplemental disinfection will have SOME increase in chlorine demand that is not generally high enough to notice.
I was told by one PB (very early on in the process before doing my own research and learning) that with a UV sanitizer, .5ppm chlorine level is all that is needed. That seemed low to me and now I know that FC level depends on CYA level, so that particular PB either didn't understand or was giving me a sales job. The current PB only says it will lower your overall chlorine use because the UV system will do much of the sanitizing that chlorine does.
It takes algae 3-8 hours to double in population under ideal conditions, but it only takes bacteria 15-60 minutes to do so. In one turnover of water, typically 3-6 hours in a residential pool, only 63% of the water gets circulated (some gets circulated more than once, but 37% doesn't get circulated at all). So bacteria can grow faster than UV can kill it even if one ignores pathogens stuck to pool surfaces. Of course, it takes a very low level of chlorine to kill bacteria and also they are NOT a major source of chlorine demand in pools. ALGAE is why higher chlorine levels are needed unless supplemental algaecide or phosphate remover is used. Furthermore, in a pool with sufficient FC/CYA level, there is virtually no chlorine usage for killing algae because algae is killed BEFORE it can reproduce so the only demand is from blown-in new algae spores which are minimal.
The majority of chlorine loss each day in an outdoor uncovered pool exposed to the sun is from breakdown from the UV in sunlight. Obviously, the UV system doesn't do anything to reduce this unless you are able to lower your FC/CYA level.
As far as the algae, do you think that having the infloor cleaning system which "brushes" the sides and bottom with a power waterjet every day aids in keeping the algae suspended so the UV system can kill the algae effectively?
That might help as would regular brushing (daily, not just weekly), but why have all these extra systems and work just to try and "help" a UV system when chlorine alone will prevent algae growth if at the proper FC/CYA level? If you really want to have a lower FC/CYA level, you can do that by using supplemental products (at extra cost) that prevent algae growth -- algaecides or phosphate removers though given the good reports of pools in your area presumably using lower FC/CYA levels, your water may be naturally low in phosphates. If you really want to use Trichlor tabs, then to prevent faster CYA buildup, you need to operate at a lower FC/CYA level. That's fine, but again need to prevent algae growth. UV and ozone systems can help SOME with that because SOME algae is free-floating and gets circulated, but not all algae types do that.
