Chem Geek, if you'll forgive me, I'll translate and simplify a bit.

Chem Geek is pointing out that UV lamps may emit more UV in lower wavelengths, than the sun does. UV wavelengths are simplified as UV-A, UV-B, and UV-C (see wikipedia, here for more: http://en.wikipedia.org/wiki/Ultraviolet ). Only UV-C (100 - 280nm) is specifically germicidal on its own. The Trident unit mentioned may have either UV-C *or* UV-B. If the lamps are UV-B, they do nothing that sunlight doesn't do much more of. But, even if they are UV-C, the effect is usually functionally insignificant . . . for outdoor pools.

The reason is, sunlight UV (UV-A + UV-B) converts some of the oxidizing chlorine present into extremely powerful hydroxyl radicals, which oxidizes just about everything -- bacteria, viruses, chloramines, and more.

It's true that UV-C units can provide additional protection against giardia and the like, but only if the rather complex operating requirements (below) are met. The simpler solution, followed by most pool owners, is to keep people from pooping in the pool . . . and to evacuate the pool in the case of failure.

---------------------------------------------------------------------



By the way, I have personally seen, many, many times, that solar UV *does* have a very substantial effect on even heavily used outdoor pools: chloramine build up is NEVER a problem on such pools, if adequate chlorine levels are maintained. By contrast, even less heavily loaded indoor pools, often have a problem. Chloramine control on indoor pools seems to me to be the ONLY valid application for UV units.

Controlling the risk from giardia and crypto can be accomplished more easily and effectively by increasing circulation rates (necessary for UV sanitation, too!) and utilizing DE filtration to filter out such large pathogens. Apparently, very substantial improvement can be achieved even with sand filters, by make sure media is correctly sized, keeping rates below 15 gpm/sft and utilizing a 'filter aid' like polyquat. Of course, an even MORE effective step is to exclude fecally incontinent swimmers (infants, older adults with fecal incontinence, all swimmers with diarrhea).

It is true that in specific situations, UV-C systems *can* provide supplemental protection against infection from pathogens, like giardia or cryptosporidium that can be transmitted as oocysts (eggs!), and which are resistant to both chlorine and UV-formed hydroxyl radicals. But -- and this is a VERY big BUT -- providing this protection is very conditional:
=> the circulation has to be continuous and high rate, since the UV units ONLY affect water that is currently passing through the pipes, and have ZERO effect on water in the pool itself.
=> the UV units must radiate in the UV-C range, and must be sufficiently powerful;
=> the glass or quartz tubes in the UV unit must be clean, and the water itself must be clear, since dirt, whether on the glass or in the water, blocks the UV.

Only if all these conditions are met, will the UV unit have a significant effect. But, for residential outdoors pools, and for most other pools, there are simpler and more effective methods of managing the risk.

In certain special cases, such as hydrotherapy pools, working with specific client populations, it may well be worth the effort to provide additional protection, utilizing high circulation rates and DE filtration and UV-C irradiation.