Perhaps you should ask whomever suggested the UV and ozone systems how algae and bacteria stuck onto pool surfaces (walls and bottom) are going to be handled by these UV and ozone systems? Oh yeah, that's right, they don't do a darn thing since they only kill what is free-floating and gets to their system. While some algae may be free-floating, some are not and most bacteria prefer to grow on surfaces, not free-float. Furthermore, even for free-floating algae or pathogens it takes 7 turnovers (usually 7 days or more) to have 99.9% of the water pass through the system for 3-log reductions; with a single turnover (usually 1 day or more) only 63% of the water has been circulated through the system. Bacteria can double in population every 15-60 minutes while algae can double every 3-8 hours. The claim of high kill rates through the system is irrelevant since it is limited by circulation.
So you can then ask them exactly how one can have a lower chlorine level if chlorine is the only thing preventing algae and pathogens from growing on pool surfaces or in any areas of pool water that don't get circulated.
If they then give you some baloney about how the ozone will oxidize bather waste so that chlorine doesn't have to, ask them exactly how much chlorine demand is created by such bather waste. They will probably stare blankly into space at which point you can tell them that every person-hour of swimming in your 18,120 gallon pool uses up only 0.06 ppm FC. Then tell them that losses from sunlight will be at least 1 to 2 ppm FC per day so ask them how the UV or ozone systems are preventing losses of chlorine from sunlight since that is how most chlorine gets lost. Oh yeah, more blank stares.
Then ask them if the UV or the ozone will deplete the chlorine in the pool. If they say no, ask them why Figure 3 in
this paper that is peer-reviewed and published in a respected journal shows both hypochlorous acid and hypochlorite ion absorption of UV in the UV-C range of their UV lamps (including 254 nm), not just the UV-A and UV-B range in sunlight (300-400 nm). More blank stares. Then show them the paper
Chemistries of Ozone for Municipal Pool and Spa Water Treatment where the section on "Reactions of Ozone With Chlorine Species" shows how ozone reacts with hypochlorite ion to produce chloride and chlorate ions.
Now in practice many UV and especially ozone systems are undersized for residential pools so there may be little chlorine loss seen, but if they are that undersized then what exactly are they doing? They may tell you that the UV and ozone will inactivate the protozoan oocsyst
Cryptosporidium parvum which is highly chlorine-resistant and that is true, but then ask them where this comes from. If they tell you the truth they will say that it is not like algae spores that are blown into pools or like soil bacteria that may also get blown in or like fecal bacteria or bacteria on your skin that you add to the water or like viruses you add to the water. Crypto comes from people with the parasite AND that have diarrhea and release it into your pool from a discharge. As noted in the
CDC 2011-2012 Cryptosporidiosis Surveillance Report, the incidence rate of cryptosporidiosis peaked in 2007 at around 4 per 100,000 population and the number of outbreaks in commercial/public pools in 2007-2008 was 58 (see
CDC Outbreaks of Illness Associated with Recreational Water — United States, 2011–2012) which out of 300,000 such pools represents 0.02% over a 2-year period. What are the odds that you invite someone over to your pool who knowingly has diarrhea caused by Crypto? Even if they did, the UV or ozone would not prevent person-to-person transmission of the disease. It would only prevent it continuing to infect over the next weeks (remember it takes 7 turnovers for 99.9% inactivation/kill), but don't you think you would know if someone did have diarrhea in your pool? And don't you think a 0.02% commercial/public pool risk likely translates into more like a one in a million or lower risk for your residential pool given the far fewer people using the pool?
Let them know how unhappy you are that they are either ignorant (nicer word than incompetent) or deceitful and then ask yourself if you really do want to proceed with buying these unnecessary systems for your outdoor residential pool.
. At this point I do not want to know 