Reverse Osmosis, Its a filter that only lets through very small particles, i.e pure water, everything else is filtered out, its too pure to use as is so you have to mix it with a little tap water or add minerals to it, the advantage is you dont get any impurites at all and its the only real way you can control ph and hardness of the water.
You can buy it from shops or you can get a filter yourself that costs about £100 and lasts 2 years