How a Reverse Osmosis Water Filter Works

Reverse osmosis is sometimes known as ultra or hyper-filtration. Reverse osmosis water filters are commonly used at most commercial water bottling plants. Removing salt from seawater is also a common task for reverse osmosis water filter systems.

Reverse osmosis works by forcing water through a thin membrane filter with pores as small as 0.0005 microns. By comparison, bacteria measure from 0.2 to 1.0 microns and viruses measure from 0.02 to 0.4 microns. These membranes are semi-permeable, allowing water molecules to pass, but not much else.

High-end reverse osmosis water filters use a process called "crossflow" that makes the membrane self-cleaning. Most reverse osmosis water filters pre-treat water by passing it through an activated carbon filter. For increased biological protection, some systems pass filtered water through an ultraviolet system.

Cheap Rejection
Reverse osmosis water filters are cheap to operate. When maintenance costs are included, these systems can produce a gallon of filtered water for as little as 5 cents.

Reverse osmosis water filters have the highest rejection rates of any filter on the market. Rejection rates measure the amount of contaminants that a filter repels. Reverse osmosis systems typically have rejection rates that average 90% or higher.

But Earth Friendly?
Because the pores in a reverse osmosis filter are so small, they can clog easily. If your water supply has a high level of sediments, expect to add sediment pre-filtration. This can drive up the cost of a reverse osmosis water filter system.

The semi-permeable membrane used by reserves osmosis filters only passes a fraction of the water applied. It takes from 3 to 10 gallons of water to produce one gallon of purified water. The resulting wastewater has higher concentrations of contaminants, putting a more stress on downstream treatment plants.

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