Needing some more information on UV

[PoolDoc]

Hi F;

[ EDIT: just noticed your SWCG and tree worries. Salt water from pools is a potential problem ONLY when it's not diluted, but is concentrated by evaporation. While that can happen in Texas, the situation you describe -- overflows from rains -- is the EXACT opposite of the problem scenario. If there are heavy rains, any salt runoff will be so diluted that it's a non-issue. In fact, even when undiluted water from salt pools has gotten on lawns, it doesn't seem to be a problem. However, I don't think I'd recommend repeatedly discharging the backwash from a salt pool filter at the base of a tree you wanted to keep. ]

I have limited time this weekend. I did check your links, before removing them, and you are correct that most of the solar radiation that reaches the surface is UV-A. (The ozone layer apparently stops most of the UV-C). It's not quite true that UV-A is not germicidal, but it's certainly not as effective as UV-C. However, it's not at all clear to me that UV-C is in any way effective against algae, or algal spores, which are a huge issue on OUTDOOR pools.

I deleted your "Ask Alan" link, because the page appared to be simultaneously providing information about UV in pools and doing paid ads and links to Delta UV, AND because there were some very serious errors in the statements made. I hope I'll have time to get back to this, but I'm not promising anything.

It's sort of a 'niche' question, and a rather tiny niche at that. Even if I take at face value the 10,000 pool number, that would be 10,000 out of about 10,000,000 pool owners in the USA, based on stats I've seen over the years. In other words, 99.9% of the pool owners out there are NOT interested. But, I'd bet those numbers from DeltatUV are (a) rounded up, (b) aggregates of ALL UV purchases over the last decade, (c) 90% indoor pools, and (d) not adjusted for the folks who bought, and then abandoned the UV units. If there are 100 *active* users of UV on *outdoor* pools in the USA, I'd be surprised.

UV does have some reasonable applications on INDOOR pools, and is being used commercially to resolve some specific problems. But to use UV on an outdoor pool on the theory that it might save money is wishful thinking.

However, Chem_Geek actively investigates all sorts of alternative systems, so I'll email him and see if he wants to take a look at this.

Good luck!

[chem geek]

A UV system for an outdoor residential (low bather-load) pool exposed to sunlight makes no sense. As Ben noted, there is reasonable use for UV in indoor pools mostly to help break down or prevent combined chlorine formation, but UV is not a general oxidizer and will not reduce chlorine demand. In fact, depending on the type of UV system you get, your chlorine demand could go up somewhat. It is not a bulk-water sanitizer so you can't lower your FC level (FC/CYA ratio, to be precise) as a result. No circulation-based disinfection system, be it UV or ozone, does anything against pathogens or algae growing on pool surfaces; this is why a bulk-water disinfectant such as chlorine is essential.

It is true that the wavelengths used for UV systems are different than that from the sun (whose UV drops off rapidly below 300 nm due to atmospheric absorption), though again how different depends on the type of UV system you get. This link gives some useful basics about the different spectrums of UV. Note that hypochlorous acid is degraded by UV from <200-350 nm with a peak at 236 nm while hypochlorite ion is degraded by UV from 250-350 (and <200 to 230) with a peak at 292 nm. So medium/high pressure (broad spectrum) UV tends to degrade more chlorine than low pressure (single wavelength) UV.

In an outdoor pool exposed to sunlight, the UV in sunlight breaks down chlorine and in doing so produces hydroxyl (and chlorine) free radicals that are powerful oxidizers that help to oxidize bather waste that is otherwise slow to get oxidized by chlorine alone and the CC level is typically very low (usually <= 0.2 ppm and almost always <= 0.4 ppm). Also, bather load is low in residential pools. For these reasons, UV doesn't make sense for outdoor residential pools.

For indoor pools, the lack of sunlight exposure can lead to a buildup of combined chlorine (CC) so in this situation the use of a supplemental UV system can be helpful.

As for the disinfecting capability of UV, there isn't any need for that in residential pools since chlorine alone is effective against most pathogens, the most notable exception being the protozoan oocyst Cryptosporidium parvum. This pathogen is only introduced from the diarrhea of an infected person so is very unlikely to get introduced into your own residential pool. However, for commercial/public pools where one infected person can infect dozens if not hundreds of others, the use of UV to help reduce the likelihood of a major Crypto outbreak is reasonable.

There are also side effects of UV where it destroys some disinfection by-products, but creates others. I won't get into that here as this is still emerging science.

[fchapma]

Those were two excellent responses and I appreciate the time committed to both. I recently spoke to a gentlemen here in town and he didn't go quite to the length and detail as you two did but nonetheless changed my mind about the UV option. Again, thanks for the responses.