What is reverse osmosis?

Sometimes shortened to the acronym RO, these systems force water, under pressure, into a module that contains a semipermeable membrane and a number of other filtration steps. A typical RO system has a prefilter designed to capture larger particles, chlorine, and other substances; a semipermeable membrane that captures more contaminants; an activated carbon filter that removes residual taste, odor, and some organic contaminants; and a storage tank to hold the treated water for use.

How does it work?

This process is called "reverse" osmosis because the pressure forces the water to flow in the reverse direction (from the concentrated solution to the dilute solution) to the flow direction (from the dilute to the concentrated) in the process of natural osmosis. RO removes ionized salts, colloids, and organic molecules down to a molecular weight of 100.

You can get a whole-house RO, but more commonly, a point-of-use RO system would be on your counter top or installed under the sink. They’re great for treating water for cooking and drinking, but they don’t usually produce large amounts of treated water — more like 3 to 10 gallons a day. For that reason, typically people choose to install RO-treated faucets in the most popular areas of the home such as kitchens and bathrooms, as opposed to installing it for every drinking tap. Just like any other kind of filter technology, reverse osmosis systems require regular maintenance. That includes periodically replacing the unit’s prefilters, postfilters, and membrane modules.