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Thread: UV Sanitizers burn very little chlorine - with data from research

  1. Back To Top    #1

    Join Date
    Oct 2015
    Austin, TX

    UV Sanitizers burn very little chlorine - with data from research

    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:

    Citation 1:
    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"

    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.

    Citation 2:
    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: ). 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.

    In summary:
    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.
    Pool: 16,000 gallons, 16x31, 500SqFt gunite pool, White Pebbletec interior, built by Cody Pools. Pentair Intelliflo VS-3050 pump, Pentair Clean & Clear 420 Cartridge Filter, In-line chlor with Ultra UV, PV3 Infloor cleaning, Pentair Easytouch-P4 control. TF-100
    Spa: 525 Gallons, Coast fiberglass spa of unknown vintage and suspect previous care with a 2-speed main pump and a blower motor, both 4HP. 3-step bromine sanitized.

  2. Back To Top    #2

    AUSpool's Avatar
    Join Date
    Sep 2015
    Sunshine Coast, Australia.

    Re: UV Sanitizers burn very little chlorine - with data from research

    That is a very compressive post and I'm afraid I can't do it justice. I was involved in an assessment of a UV system as part of my Bsc undergrad several years back. Unfortunately the report was lost with a broken hard drive but in brief summery we found no real difference in bacterial assessment of water samples taken both before and after a bank of commercial grade UV sterilisers on an overstocked aquaculture system.

    The report however was inconclusive as we did not know how old the bulbs were. It's my understanding that UV bulbs loose their effective output quite quickly and there is no way to easily monitor that. It's easy to size a system with dwell times and flow rates etc. but in my opinion their a bit hard and expensive to maintain at peak efficiency particularly when you compare them to a SWCG system. My SWCG uses about 180 watts, runs for about 4 hours a day and a simple chlorine test tells me it's doing its job with nothing to replace for almost ten years.

    Cheers, Steve.

    Edit: I found the manufacturers bulb replacement times, every three years when used for 12 hours a day and they suggest that the bulbs will be down to 80% effectiveness at that time which I would be a bit sceptical about. I could not find a wattage for the bulbs. They state that the units are not to be used to treat potable water, what's the reason for that. It is my understanding that UV efficiency in terms of microbial kill rate is linked in part to the distance from the bulb, the reaction chamber looks a bit wide, much wider than the ones I've played with.
    30,000L (8,000g) Pebblecrete | Davey 3sp Eco pump | Poolrite sand filter & SWCG |
    Waterco solar panels & Astral E140 pump| K2006, BlueDevil pH, Salt meter & K1766 | Town water - pH 7.2, TA 50, CH 60 | Esky full of coldies |

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    bdavis466's Avatar
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    Aug 2014
    Riverside, CA

    Re: UV Sanitizers burn very little chlorine - with data from research

    This is a repost from the other thread found HERE. The post pertained to this topic more so than the original thread.

    Quote Originally Posted by marcgr View Post
    So I'm thinking more about this. In a non-UV pool, how clean is the water coming out of the return jets? You're taking nicely chlorinated water and sucking it through a teabag of decomposing leaves in the skimmer basket, and another teabag of decomposing goo and bather waste in the filter (DE filters down to 3-5 micron, e.coli is 0.5x2 micron so will pass through), then returning it to the pool... in my case, right before it returns to the pool, the water gets a dose of UV-C radiation good enough to kill 99.99% of anything living within...

    The chlorine is still needed for getting the remaining micro-organisms that are at large in in the pool, and for when the pump is off.

    Regardless of what the water flows through, a sufficient amount of chlorine is added to bring the entire pool volume to a level of sanitation that will inhibit/prevent the growth of organics and kill bather waste. This amount of chlorine is easily measurable, and that coupled with a CYA level can give the actual level of chlorine available for disinfection (that is not bound to CYA).

    The issue I have with the UV system is how do you know its working? Can you take water samples before and after the light and measure the difference? If you could do that, what would that prove? With a sufficient amount of chlorine in the water, it really doesn't matter what else is also in the water (teabags of decomposing matter) since we are able to measure the chlorine level, and as long as that level is above 7.5% of the CYA level, then it is near impossible for anything to grow or spread (there are few exceptions, like cryptosporidium, but that would be an issue in any pool with any form of sanitation).

    I looked up the literature on the Clear03, and the turnover requirements are insane. For my pool, I would need to run the pump 24/7 at nearly the maximum speed to turn 100,000 gallons in a day (3 turnovers recommended by the MFG), or have an additional UV light in place. Why not just add a sufficient amount of chlorine (regardless of the method) to kill anything in the pool?

    Even with three turnovers a day, how can one be assured that every gallon of water in the pool has made it through the UV lamp? Certainly dead spots and areas of less than adequate circulation would effect this.

    I understand the premise of the UV light and I can see how it could be a method of sanitation in addition to chlorine, but the light does nothing for the bather waste or organics that enter the bulk pool water prior to being run through the pump. Bathers would certainly be at risk of exposure to bacteria if the chlorine level was low (in comparison to the CYA level as the MFG recommends) since the light has done nothing for the water that has yet to pass through it.

    What is really gained by running a low chlorine level? One of your previous posts stated that the study for the UV lamp was done on a pool with no CYA in it with a FC level of 2. If that was the case, it certainly makes sense because a FC of 2 with no CYA is nearly 10x higher than maintaining 7.5%FC/CYA. The chlorine level in that pool was not low, it was very high in comparison.

    At any rate, if UV works for you and you are happy with it, then keep doing what you are doing. You have done far more research than I have on the topic and only you can decide what is best for you and your pool.
    33K Pool/Spa, Pentair Equipment
    Davis Custom Construction - Home Page

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    duraleigh's Avatar
    Join Date
    Apr 2007
    Sebring, Florida

    Re: UV Sanitizers burn very little chlorine - with data from research

    Some years ago, many really knowledgeable folks on TFP came to the conclusion that since you need chlorine anyway (because there is no residual sanitation out to the pool), there is no point in trying to justify the cost of a UV system.

    That said, I hope none of us here would assume to tell you how to manage your own pool. The forum wide goal here is to provide factual information and let you decide what works best for you.
    Dave S.
    42k vinyl and concrete pool, 1.5hp pump, 140gpm filter
    TFTestkits , PoolMath , Pool School

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    Patrick_B's Avatar
    Join Date
    Jun 2011
    Midland TX

    Re: UV Sanitizers burn very little chlorine - with data from research

    And I have moved this to the A to D forum where it really belongs. As Dave states above, you can do what you wish Marc, but your post strongly suggests to me that you are here merely for the argument. This subject has been discussed Ad-nauseam on the forum for years. UV of any kind does not address bulk water concerns nearly as well as Chlorine, and that's the bottom line.
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