We do not use or endorse the use of UV systems for outdoor pools. UV systems may benefit indoor pools by burning off CCs.
UV manufacturers that push UV systems say you need to maintain a minimal amount of chlorine in the water. Here at TFP we group them together as an "alternative" system that is potentially unsafe to the swimmers in the water.
Additionally, UV systems are impossible to test to see if they are working or not.
I'd like to comment on what has been posted and hope that it brings some closure to the discussion. Not that I believe in using "appeals-to-authority" argumentation, but I should at least put forth that I do have significant experience with UV systems - I spent the better part of my engineering career working on and with UV lithographic system for semiconductor manufacturer as well as direct experience with high-powered UV excimer lasers, both chlorine and fluorine based UV lasers. Nothing will scare you more than having layers of skin ablated off your arm when you accidentally pass it through the open beam path of an excimer laser.
So far we have had arguments from the extremes which, while interesting, tend to fall off the mark a bit.
Here's my summary and thoughts - A UV system is by no means detrimental to being added to your equipment pad and UV will kill certain pathogens that chlorine has a harder time with, BUT the systems that are designed for residential pool use suffer from both inherent weaknesses in the UV source power as well as overall design flaws that render their performance sub-optimal compared to commercial UV systems.
In short, there's lot of expense incurred with the addition of a UV source for relatively little gain in water quality.
So let's look at Cost-Benefit Analysis:
UV-C light, also known as UV Germicidal Irradiation (280-200nm, with 260nm being the optimal wavelength), does effectively kill a whole host of bacterial pathogens that chlorine either has a hard time with or can not kill at all (ex, protozoan oocysts).
While UV-A and UV-B light come from the sun and hit a pools surface, very little UV-C makes it through the atmosphere due to the ozone layer absorbing it. UV-C light will also kill algae (planktonic, or free floating, algae) but so too does chlorine.
UV light does not, on it's own, generate disinfection by-products like combined chloramines; it simply inactivates or kills the pathogen which degrades its ability to reproduce.
Standard online pricing search yields that a Paramount UltraUV system can range from ~$490 for a 230V, 1-lamp unit and up to ~$730 for a 230V, 3-lamp unit.
I have not found any 4-lamp units online even though the manufacturers user manual claims that 4 lamps is the max number for these systems. Each lamp has a replacement cost of ~$100 per lamp. I have yet to find pricing for the exact replacement model of the fused quarts inner tube, but a check of cached on-line sales pages shows the 2-3/8" x 21" tube replacement assembly to be ~$75. While lamps are rated for a 13,000hr lifetime (continuous use), most recommendations seem to point to a 2-yr replacement schedule for the bulbs and no scheduled replacement for the quartz tube outside of physical damage.
So, if one had a 3-lamp unit, the replacement cost of the lamps would exceed the system cost within 4 years. None of this assumes any plumbing reconfiguration that might have to occur if this unit is added to an existing pool as opposed to installed upfront with the pool equipment.
While we can all argue in the abstract about a "perfect" system, one must take into account the inherent design flaws of these UV disinfection systems. I will simply list a few here without going into great detail and this should not be considered an exhaustive list:
- Volume treated/turnover rates - because of the low internal volume of the treatment tank and the fact that water is flowing through it at a considerable rate, a long treatment time is necessary. Assuming a pool pump operates at 50GPM and the internal volume of the UV tank is 15 gals, the water only resides inside the unit for about 18 secs. Because of these volume differences, the operating instructions specify the use of large turnover rates of pool water to ensure that as much of the pool volume as possible gets treated. This means one must run their pumps longer and at lower speeds to improve the efficiency of the UV treatment. Also, without overstating the obvious, only planktonic pathogens (free floating algae and bacteria) are treated by these methods.
- No Residual Disinfection - I think this states the obvious, but once water exits the UV system, there is no "residual" disinfection. Any pathogen that might have made it through the tank without being inactivated is now free to go on it's way.
- Bulb Output - The UV bulb types used here are low pressure mercury vapor lamps that typically operate around 60W or so electrical power. While this type of lamp is efficient at producing UV-C light, the power densities are fairly low (1 W/cm). No bulb will operate perfectly over it's lifetime and so, as the bulb ages, it's UV-C output will diminish with time. Mercury lamps should never be discarded in general trash but sent back to manufacturer for proper disposal as mercury is both a human and environmental hazard.
- Quartz tube housing - The quartz tube that houses the lamp will, over time, build up scale on the surface that is in contact with the water as well as develop what are known as "color-centers" in the volume of the fused quarts. These color-centers are caused by the UV irradiation damaging the crystalline structure of the quartz which cause greater absorption over time, in effect, reducing the ability of the quarts tube to transmit UV light. Routine cleaning of the quartz tube surface is a must to ensure proper operation of the unit and replacement is required should the tube become frosted or damaged.
- Water quality - The water in our pools is not pure. It contains plenty of dissolved solids, particulates and chemicals that will enhance the absorption of UV light and scatter it thus reducing the efficiency of the UV germicidal action. Hypochlorous acid does, in fact, react with UV-C light but not to an extent to cause significant chlorine demand. However, any UV light that converts hypochlorous acid to chloride is lost to disinfection of pathogens.
In current practices, UV disinfection finds it's most efficient use in drinking water treatment. We have at least a few TFP members who work in the water treatment industry so they can certainly respond better.
However, in that context, UV disinfection systems are typically deployed inside surge-tanks that hold 10,000-15,000 gallons of water at a single time and the UV sources are allowed to operate for longer periods of time on those volumes. UV systems are also used in conjunction with Ozone & Peroxide treatment tanks (referred to as peroxone in the industry) to act as another layer of disinfection.
Once again, these are static volumes of water that are fully treated before entering a closed loop distribution system; this is a far cry from the way residential pools operate.
Anyone interested in reading more about this can look up the term "Advanced Oxidation Processes" on the internet.
Residential Pool Context
Finally, let's put a lot of this into context. For the standard residential, low bather-load pools, the biggest nuisance by far is algae. Algae is not considered a human pathogen (it's exceedingly difficult to get algal infections in the human body), it is a nuisance. Yes, there are some bacteria and other pathogens that can inhabit algae and use them as a disease vector, but those instances are rare. What is also quite rare is bacterial loads in residential pools.
While we all sometimes think that our pools are the breeding grounds for the next Black Plague outbreak, the simple truth is that most residential pools (when properly chlorinated) are quite free of any significant bacterial loads.
Also, the really nasty bacteria and parasites that keeps us up at night (p. legionella, cryptosporidium, giardia, etc) are just not that common is a properly chlorinated swimming pool.
UV systems used in the context of swimming pools find their most efficient use in high bather-load public/commercial pools where the risk of disease transmission via person-to-person contact is much greater. If I swim with my kids in our pool, there is no pathogen from them that I am not already exposed to on an on-going basis. If I jump in the public pool at the local YMCA, I am basically swimming with dozens, if not hundreds, of people that I do not know and who can easily expose me to the diseases they may be carriers of.
I am neither strongly for or against UV systems. I think they have a lousy cost/benefit ratio but others may put a higher price on adding layers of protection. I feel that their marginal increase in pool water sanitation is fully offset by their up-front and recurring financial costs.