Reverse osmosis water filters function by passing water through a membrane-type filter that leaves impurities on the other side. Clean water is deposited in a reservoir, to be pumped up to a separate drinking water faucet, and the contaminants on the other side are flushed out of the system later. This type of water filter is among the best ways to clean your water, and it will remove most contaminants: most bacteria and viruses, pesticides and other VOCs, hydrogen sulfide, nitrates, sediments, arsenic, chlorine, fluoride, heavy metals like lead and mercury, iron, and even bad tastes.

Some call a reverse osmosis water filter a hyperfiltration system because it's so good at eliminating contaminants. When water goes through a standard reverse osmosis water filter, water from your supply will move through several reservoirs. The filter allows clean water to move to the clean water side of the system and leaves contaminants behind to be washed away later. It is a passive system - that is, water is not moved through with pressure but rather via a chemical process called osmosis. It's a very slow but very thorough filtration system, and will produce about 15 gallons a day of bottled-quality water, which it holds in a reservoir. If you're a big bottled water drinker, you can imagine how much money that can save you in the long run!

A reverse osmosis water filter may have a thin film composite (TFC) or a cellulose triacetate (CTA) membrane, each of which has strengths and weaknesses. The TFC is best by far at contaminant removal, but it is easily damaged if you have a chlorinated water supply going directly into it. For TFC filters, it's a good idea to install an activated carbon filter upstream of the TFC filter. The CTA filter is not as good at removing contaminants, but it is also tougher and will not be so easily damaged. Because a damaged film can ruin your reverse osmosis water filter, choose carefully between these two options.

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If you have a problem with biological contaminants, you should consider using not just a reverse osmosis water filter by itself, but also using an ultraviolet filter, as this filter is the most effective by far in removing biological contaminants without otherwise contaminating your water supply. So your perfect reverse osmosis water filter may need to have a total of three filters to be completely effective at cleaning your water: the activated carbon filter (which needs to be changed regularly), the osmosis filter, and the ultraviolet filter (which needs an electrical power outlet). With these three filters in your system, however, you can be assured of a tasty, reliable, and clean water supply for your home and family.

A concern to consider with a reverse osmosis water filtration system: the amount of water used in producing your clean water. Each one gallon of clean water requires up to ten gallons of water, the remainder of which is flushed from the system. If this is a problem in your area, make sure that your contaminated water is redirected to your gray water reservoir, where it can be safely used to water your garden.