The name of the game is free radicals. If you are looking to kill off any little germ in the water, then free radicals are the ticket. High valence ions and ionizing radiation breaks down the organic molecules and destroy the integrity of germ cells, in fact, all living cells disintegrate in the presence of free radicals, which is why we take vitamins called anti-oxidants, which react with the free radicals we are constantly consuming. Most organic molecules have weak enough bonds that UV radiation of about 10 electron volts is enough to break them, which is why UV exposure is a hazard for us. UV does basically the same thing the free radicals do. The difference is that free radicals can react to break the molecular bonds of one molecule, then move on and do it over and over again without ever binding up unless they encounter something like an anti-oxidant which captures them in a stronger molecular bond so they can then be neutralized.

So bombarding the water with free radicals and ionizing radiation works wonders for killing nasty little germs, and can help reduce other organic contaminants, though probably not enough to make a reliable difference. Chlorine has been the most popular free radical application for a long time, but because the chlorine ions persist long after application (which does have it's benefits, as residual chlorine can help to keep the water from being re-infected), and all that chlorine consumption and eventual release to the environment is not so desirable. Ozone breaks down after application fairly quickly, and UV is containable to the point of application, so although there isn't any residual benefit, there is also no residual debtriment, bioaccumulation etc.

Chronic exposure to Ozone causes emphysema. I know of a couple former dam operators that liked to walk the generator floor a little too much, and developed the disease.