In the United States, reverse osmosis water filters are some of the most common household filtrations systems available. Reverse osmosis water filters are an extremely effective form of filtration. They are used in military installations where clean water isnt readily available. They are able to filter a wide range of contaminants, including lead. The system consists of a sediment filter to trap particles like calcium carbonate and urea. It also filters silt, rust and sand that affect the taste and appearance of water.

A second sediment filter with smaller pores helps trap smaller impurities. A carbon filter traps organic chemicals, chlorine, heavy metals, asbestos and fluorides. A wide range of volatile organic compounds including many pesticides and herbicides are also removed. A reverse osmosis filter is made up of a semi permeable membrane. The membrane is a thin film composite. The microscopic pores in the membrane allow only pure water to pass through. Crossflow allows the membrane to clean itself. As the fluid passes through the membrane some of it continues downstream, sweeping the contaminants away from the membrane.

Portable reverse osmosis units are gravity powered and need neither a pump nor electricity. The water is pushed through the filters due to the pressure of gravity.

As reverse osmosis systems are more sophisticated than other types of filters, they are significantly more expensive. Another drawback to using them is that at least two gallons of tap water is wasted for every useable gallon that is filtered.

Reverse osmosis systems are either fitted to a single tap or plumbed into the main water inlet pipe, delivering filtered water throughout the house. Reverse osmosis filters are also used by people living in rural areas who do not have access to clean water. They are also used by travelers on long trips to countries where the local water supply is polluted or substandard. In developing nations, where people die because of drinking bacteria-infested or contaminated water, reverse osmosis water filters can help save lives.