What is the use of Reverse Osmosis (RO)?It depends on your water quality.
Water contains ions like Calcium, Magnesium and other essential minerals which our body requires for our day to day activities. I mentioned it as essential as mineral free water should not be drunk by us. In some cases the minerals will be found in excess and might cause some hazards like kidney stone which caused when calcium ion and oxalate ion( found in tomatoes) precipitate. So it is very clear that we need ions, but not in excess. So to remove ions we are opting for RO technology where the excess ions will be removed, make it safer to drink.
Why UV treated water is safe to drink?
UV technology is something where Ultra Violet rays are passed in the water which kills the harmful micro organisms present in it. Remember it can only kill micro organisms and cannot purify or remove ions in water. If you need a water purifier just to kill micro organisms, then you can opt for Pure It or Aqua guard. Both the water purifiers just removes the suspended impurities and kills micro organisms. In Pure It instead of UV, Chlorine tablets are placed to kill the microbes, which is what you see as white piece in battery. Once the Chlorine tablet is over the battery turns red. But both cannot reduce ions.
How to choose a water purifier?
It depends on the consumer (Kid or Old person etc.,)
The choice of water purifier should solely be decided based on the raw drinking water available in your locality. If you really want to check the quality of water, you can give a sample of the water to some chemical lab near you. They will charge you between Rs.200 to Rs.1000 depending the parameters analysed. If the water contains more ions, then you have to go for RO and if the ions are normal then Pure It itself is sufficient.
Remember when water is treated with RO, the minerals are removed. If the RO which you are going to buy is removing more minerals, it might cause problem particularly to kids who require more minerals. Consuming mineral free water will drain even the minerals present in your body.
What is the best water purifier?
I am not going to suggest you any particular brand, but when you choose a RO, buy the one which has UV attached/ inbuilt with it and also has the feature of TDS control. TDS control gives option to the consumer about how much minerals need to be present in the water. So depending on your needs you can control the TDS yourself.
Brand Suggestion I personally used Kent RO which is so compact, noise free, has inbuilt UV and TDS control. If the mineral content is normal in your drinking water, just go for Pure It. Its economical as well apt.
Source : I am a Chemist and analysed these water samples in person.