experience with Ultra Violet systems ??

SWGs, salt water chlorine generators, chlorinators,
ozone generators, UV systems, . . .
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Joined: Tue 09 Oct, 2007 17:09
Location: Haarlem The Netherlands

experience with Ultra Violet systems ??

Postby Jerry » Wed 10 Oct, 2007 13:38

Hi all,

I already found a lot of valuable knowledge about salt chlorinator systems on this forum, but here in Europe (The Netherlands) I also see much UV systems advertised. However I can't find any experiences. So I try it on your continent. Has anyone experience with this Chlorine alternative. I understand you still need to add Chlorine, but at a much lower level. Does that mean you will have a lower PPM level than in a pool without a UV lamp (this would make the testing difficult) or is it simple that you need to add less chlorine to get to the normal level. If so, I wonder if you can combine a UV lamp with a salt chlorinator cell.

Thanks for sharing your experiences,


chem geek
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Postby chem geek » Wed 10 Oct, 2007 14:57


I cannot tell you about experience, but I can tell you about the technical pros and cons of using UV systems as well as ozone systems since both have somewhat similar issues.

Both the UV and ozone systems vary in their effectiveness depending on the types of systems that are used -- the intensity of the UV and whether it is pulsed, the generation method for ozone (corona discharge vs. UV ozone generation), etc.

The main con for both UV and ozone systems is that they can deplete chlorine. Ozone is a strong oxidizer that can oxidize chlorine to chlorate (via chlorite). UV can breakdown chlorine, depending on the specific wavelengths of its radiation and exposure time though it is easier to tune the UV to reduce this effect (one cannot tune the ozone to reduce its effect). So for both ozone and UV the claim of using less chlorine (from "how much chlorine you need to add each day") is not necessarily true -- due to the possible depletion of the chlorine itself you may end up needing more chlorine.

This all depends on the amount of organics that are in the pool since in pools with heavy bather loads, such as commercial and public pools, the organics are what overwhelm chlorine consumption. In such pools, a UV or ozone system can oxidize or break down organics so that chlorine does not have to and that can lead to lower net chlorine consumption even with the destruction of some chlorine by the UV and ozone itself. In a residential pool, however, the bather load is typically very low so the organic load on the chlorine is also very low so the destruction of chlorine by UV and ozone may outweigh the benefit of oxidation of organics.

As for the argument that lower Free Chlorine (FC) levels may be used, that is only partially true. Saltwater Chlorine Generator (SWG) pools superchlorinate a portion of the water passing through the cell so have similar characteristics in terms of oxidizing organics and killing pathogens, BUT all three methods (SWG, ozone, UV) only destroy what passes through their cells, so only free-floating organisms will get killed. These systems do nothing for pathogens in biofilms stuck to pool surfaces. They also do nothing to pathogens newly introduced from bathers until the water gets cycled many hours (or even days) later. Only the residual disinfectant in the bulk water can handle these situations.

Fortunately, most pathogens need a very, very small amount of chlorine to be killed. The same is not true for algae, however, so it is the prevention of algae that is the limiting factor that determines the required disinfecting chlorine level. There are many ways to inhibit algae so use of an algaecide is another way to be able to reduce disinfecting chlorine levels -- that is, you don't just have to use an SWG, UV or ozone to have lower disinfecting chlorine levels. You can simply use an algaecide to achieve the same effect.

It is NOT true, however, that you can use a very low Free Chlorine (FC) level below around 1-2 ppm since you need at least some chlorine in reserve to prevent it from getting used up even locally in the pool water (via oxidation of organics or breakdown from sunlight). Also, the actual disinfecting chlorine level is mostly a function of the ratio of Free Chorine (FC) to Cyanuric Acid (CYA) as well as the pH. By disinfecting chlorine level I am referring to the hypochlorous acid concentration as that is the specific chemical that kills pathogens and inhibits algae to any great extent. It is also the primary oxidizer (of the chemicals; obviously ozone oxidizes and UV breaks down organics as well).

A pool with 3.5 ppm FC and 30 ppm CYA has the same disinfecting chlorine level as 0.1 ppm FC with no CYA. Yes, you read that correctly. So next time you go into an indoor pools that has 1 ppm FC and no CYA, you are swimming in water that has 10 times the amount of disinfecting chlorine in it. This is why my wife's swimsuits degrade (rubber deteriorates) over just one winter season of use in a community indoor pool with no CYA while in our own outdoor pools over multiple summer seasons of use there is virtually no degradation whatsoever. Essentially, indoor pools are over-chlorinated because CYA is not used in these pools because the industry believes that CYA is only needed to protect chlorine from breakdown from sunlight and doesn't consider the pool water chemistry that has been known since 1974 (see this link for more technical details including a spreadsheet that computes all the standard pool water chemical species concentrations). This over-chlorination may also contribute to the higher levels of dichloramine and nitrogen trichloride and other disinfection by-products in indoor pools, but that has yet to be proven in the field (I'm in contact with several people and organizations about this) though breakpoint chlorination models predict this.

So if you want to minimize the amount of chlorine in the pool, you have several alternatives. The chlorine level in a manually dosed chlorine pool that has CYA in it is already very low and if properly maintained should not be a problem -- minimal chlorine smell, no noticeable degradation of swimsuits or skin or hair, etc. The amount of chlorine needed to prevent algae is roughly a minimum FC of 7.5% of the CYA level. This amount of disinfecting chlorine kills 99% of heterotrophic bacteria in around 2-3 minutes (assuming a CT value of 0.08 for hypochlorous acid). Most SWG pools are able to run with an FC of 4.5% of the CYA level and not get algae, but much below that and algae can develop and I would estimate that a UV or ozone pool would be similar in this regard. If you use an algaecide, such as PolyQuat 60, then you can probably go as low as an FC of 3% of the CYA level, though I wouldn't go lower than that or else disinfection times get too long (and in any event, don't go below 1 ppm FC, so adjust CYA level accordingly). You may need a higher CYA level to protect the chlorine from breakdown from sunlight in an outdoor pool so focus on the FC/CYA ratio, not on the FC level itself.

Remember that part of the purpose of a residual disinfectant is to kill pathogens that are newly introduced into the water, so figure on the length of time it can take for something introduced from the fecal route to take to splash into your mouth through the oral route, either from yourself or from another swimmer. The risk in a residential pool is low, but this scenario is why the standards are strict in public and commercial pools since bather loads are so much higher so one person with infection can spread a lot of disease relatively quickly. You also don't want to run out of chlorine in the main body of pool water because bacteria can double in population (under ideal conditions) every 15-60 minutes which is far faster than the pump circulation turnover time in most pools.


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