What about ozone and/or UV? Can't that reduce or eliminate the need for chlorine? In a word, No, and for a very simple reason. Ozone and UV have NO residual effect so again, a residual sanitizer is still needed. The only place these will kill pathogens is in the contact chamber (where the water is exposed to the ozone or UV), not in the pool. Ozone will destroy chlorine but will oxidize organics so it's a two edge sword. You will generally have higher chlorine consumption with ozone than without and it does not allow you to run lower chlorine levels because there is no residual effect from the ozone. The higher bather to water ratio in a spa makes ozone much more useful there than in a pool because it does oxidize organics. Ozone is also toxic in large enough quantities to actually sanitize pool water. For many, ozone is more useful in bromine systems since it will activate the bromide into bromine sanitizer so it works with bromine instead of against it. However, it will cause bromates to form and they are a suspected carcinogen in drinking water!
UV light can kill pathogens and some of the units also produce some ozone but this only occurs in the contact chamber and there is no residual effect in the pool. Once again, more useful in a spa than a pool. UV might be of some value in an indoor pool in conjunction with chlorine to help destroy persistent chloramines.