From a discussion in another thread, where I was told that my UV sanitizer didn't do anything but burn chlorine, I decided to do some research into the matter.
My research points to this result: The extinction of chlorine from a UV-C sanitizer is very small - about 0.06PPM per day for my setup.
The point of confusion for most people is the wavelength of light involved.
Let me break it down like this:
Type of UV light Wavelength Where it comes from Effect on chlorine UV-A 315-400nm The sun Burns chlorine fast UV-C 100-280nm UV Sanitizer (254nm) Little effect
I completely agree with everyone who says that UV-A light - the wavelength that we get most from the sun - absolutely does burn chlorine significantly. However, the 254nm wavelength of light from a UV Sanitizer, being well in the UV-C band, affects chlorine differently from UV-A from the sun.
My personal result is that an outdoor pool in Central Texas burns about 0.5ppm of chlorine per week in winter with the UV sanitizer turned on 12 hours per day (pumping 6 hours at low speed, 6 hours at medium-high speed). Given that the traditional school of thought says that a similar pool should burn 0.1-0.2ppm of chlorine per day, my own personal evidence says that the net effect of adding a UV sanitizer on chlorine consumption is net positive.
Here is my research:
J. Chem. Phys., Vol. 120, No. 24, 22 June 2004, page 11553, Figure 5, in the paper, "Theoretical study of the UV photodissociation of Cl2 : Potentials, transition moments, extinction coefficients, and Cl*’Cl branching ratio" http://web.chem.ucsb.edu/~devries/ch...ssociation.pdf
You can see from the paper that the extinction coefficient for chlorine is near zero in the UV-C wavelength band and is highest in the UV-A (sunlight) band.
J. Environ. Eng. Sci. Vol 6, 2007, page 277, "Photolysis of aqueous free chlorine species (HOCL and OCL-) with 254nm Ultraviolet Light" Photolysis of aqueous free chlorine species (NOCI and OCI-) with 254 nm ultraviolet light (PDF Download Available)
You can see from this paper again, particularly in figure 3, that the absorption of 254nm UV-C is quite low compared to the UV-A band.
Their final conclusion, referring to the 254nm wavelength produced by UV sanitation bulbs: "At fluence of 400J/m2, the decomposition of free chlorine is very slight (~1%) in DI water when the concentration of chlorine is not very high (~20 mg Cl/L)".
The manual for my UV unit says it produces 30mW/cm2 (Source: http://codypools.com/owners/sanitati...Ultra%20UV.pdf ). If the chamber holds 1 gallon of water and I have a 16000 gallon pool and have the light on for 12 hours, then the time spent in the chamber for each gallon of water is a little under 3 seconds, corresponding to a fluence of 90mJ/cm2 or 900J/m2.
So I could expect to lose about 2% free chlorine per day - 0.06PPM per day in a pool with 3PPM FC- by running my UV sanitizer 12 hours per day. Also note in their experiments there is no CYA involved and that 20mg CL/L should be close to 20ppm chlorine, so I would expect even less extinction at lower chlorine concentrations.
It's well known that UV sanitation is used in the sanitation of drinking water and waste water and that the 254nm wavelength used in UV sanitation seriously messes with the DNA of micro-organisms. I'm going to go out on a limb here and say that the UV light is doing some of the sanitation that the chlorine would otherwise do, and thus protects the chlorine.
1) The chlorine consumption by a 254nm UV sanitizer is negligible and
2) The germicidal effect of the 254nm UV light may reduce the work that the chlorine has to do
I welcome debate on these points, however, please at least glance at the research I have provided, and provide some solid data of your own.