Be careful when buying bottled water, it might be no better than the water coming out of your tap. Bottled water only has to meet the same minimum standards required of municipal tap water. There is little scientific evidence to suggest that bottled water is healthier than tap water. Carbon filters are not effective at removing radioactive particles, nitrates or bacteria. Consequently, carbon filters shouldn't be used on un-chlorinated water supplies, such as wells.
Filters also lose their effectiveness with continued use.
Unfortunately, there is also no convenient method of determining exactly how much water has passed through the filter. This process involves forcing contaminated water through a synthetic, semi-permeable membrane. The purity of the water depends on fluctuating water pressure, age and subsequent breakdown of the membrane, and clogging of the membrane pores. Bacteria growth can also be a problem. RO systems use a large amount of water - anywhere from 10 to 20 gallons to get 1 gallon of treated water.