UV Light to reduce fc to 0.5 ppm?

Spectralight claims you can reduce free chlorine levels to 0.5 ppm if you use their equipment, which is quite expensive. I'd like to know more about this - anyone here experienced this approach? As a newbie to this forum, I'm not permitted to view the posts about Spectralight (such as 'CAUTION Leaf Pool Pumps or Eco Pool Pumps or...') that Google is suggesting (all I get is the 'you do not have permission to access this page' message). I'm seriously considering switching to the BBB method and want to use something like the Spectralight or other method to lower chlorine levels in our 10k gallon in-ground pool. Suggestions?

There's not much in those posts; Spectralight got lawyered up, after I published the information I found, so I removed most of it from public view.

What I can tell you is this: neither I, nor anyone on the PF Support Team, would buy any product from that company, for reasons that have nothing to do with their threats to sue.

I would also point out that you can reduce the FC on your pool to 0.5 ppm for any reason, or for no reason at all. But that's not to say it will be sanitary or work well.

'Nuff said.

There is no need for a UV system in an outdoor residential pool exposed to sunlight. The only thing that UV will do better than chlorine is inactivate the protozoan oocyst Cryptosporidium parvum, but that is virtually unheard of in a residential pool unless you invite strangers over who have diarrhea.

As for a low chlorine level, you appear to be under the mistaken belief that the Free Chlorine (FC) level represents the active chlorine level. That is simply not true. When Cyanuric Acid (CYA) is present in the water, most of the chlorine is bound to it and effectively inactive. At the recommended FC/CYA levels in the Best Guess Swimming Pool Chlorine Chart, the active chlorine (hypochlorous acid) level is roughly the same as with only 0.1 ppm FC and no CYA.

Commercial/public pools with no CYA, such as many indoor pools, are essentially vastly over-chlorinated. That leads to people thinking that they need a low FC level to avoid problems, but the rate of oxidation of swimsuits, skin and hair, the rate of corrosion of equipment, and the rate of creation of disinfection by-products are all related to the active chlorine level so are lower by an order of magnitude (a factor of 10 or more) than in pools managed according to the principles of this forum.

Two corrections to my post above. First, UV is also useful in some indoor pools to reduce chloramines since there is no UV from sunlight. Second, the last part of my last paragraph said the opposite of what I meant. It should have said the following:

Commercial/public pools with no CYA, such as many indoor pools, are essentially vastly over-chlorinated because there is no CYA to moderate chlorine's strength. That leads to people thinking that they need a low FC level to avoid problems, but the rate of oxidation of swimsuits, skin and hair, the rate of corrosion of equipment, and the rate of creation of disinfection by-products are all related to the active chlorine level so are lower by an order of magnitude (a factor of 10 or more) in pools managed according to the principles of this forum because we understand the chlorine/CYA relationship.

By the way, my wife has personal experience with this difference when she swam over the 5-month winter season in an indoor commercial pool and her swimsuits would degrade (elasticity gets shot) after just one season and her skin was flakier and hair frizzier. In our own outdoor residential pool over the 7-month summer season, the swimsuits would last for 7 years and the effects on skin and hair were far less noticeable. The difference in experience is due to the indoor commercial pool using 1-2 ppm FC with no CYA compared to our own pool with 3-6 ppm FC and 40 ppm CYA which is equivalent to only around 0.1 ppm FC with no CYA so a factor of 10 lower active chlorine level.

Finally, having a UV or ozone system does not let one lower the active chlorine level in the bulk pool water below that which prevents algae growth because algae (and bacteria) can grow on pool surfaces and NEVER get circulated through the UV or ozone system. It also takes multiple turnovers of water, so many hours, to get most of the pool water circulated through these systems. The following chart shows how much of the pool water passes through the UV or ozone system for the number of turnovers indicated (and this assumes perfect mixing):

Turnovers . % Treated
------------ . ------------
.... 1 ............ 63.2%
.... 2 ............ 86.5%
.... 3 ............ 95.0%
.... 4 ............ 98.2%
.... 5 ............ 99.3%
.... 6 ............ 99.8%
.... 7 ............ 99.9%

Typical residential pools only have 1 or sometimes 2 turnovers per day. Algae doubles in population (under ideal conditions) in 3-8 hours while bacteria doubles in population (under ideal conditions) in 15-60 minutes.

Thanks, Richard.