How Does Reverse Osmosis Work?

Simply put, Pressure from your water supply forces water molecules through very tiny pores in a membrane filter, leaving contaminants behind. The purified water is then stored in a holding tank until you need it.

Reverse Osmosis Membrane

The pores in a reverse osmosis membrane are between 0.0001-0.0005 µm (one-ten-thousandth to five-ten-thousandths) microns in diameter, which is only slightly larger than individual water molecules. Using your house water pressure, water molecules are forced through these microscopic pores, leaving behind many contaminants, too large to pass through. This purified water or "permeate", is stored in a holding tank, while the water that didn't pass through the membrane, containing the contaminants, is flushed down the drain.

Reverse Osmosis Illustration

Stages of Filtration- What is Reverse Osmosis?

Reverse Osmosis systems for home drinking water are units that consist of a combination of several filters. Depending on the system you chose, there is usually one or two prefilters that will filter out larger particles and chemicals from the incoming water. The water is then sent through the reverse osmosis filter to remove many other harmful contaminants. After the RO filter, the purified water is then stored in the pressurized holding tank.

Every R.O. system has at least one pre-filter (before the RO membrane) in the system - an activated carbon filter, which removes chlorine and other chemicals. This pre-filter helps to prevent fouling of the RO membrane filter. On some RO systems, there are sometimes two pre-carbon filters.

Note: "Activated" carbon   - means that the carbon filter has been treated to improve its adsorption properties and filtration capacity by adding pores and cavities to the carbon material. This increases the surface area of the chemical-absorbing carbon, and greatly improves its effectiveness.

Some RO systems are adapted for water wells. In these Reverse Osmosis Systems, the first filter is a sediment filter. The sediment filter removes rough particles, sand, dirt, rust, and silt. This helps keep the subsequent filters from fouling too quickly. The second stage will then be an activated carbon pre-filter, and the third stage will be the reverse osmosis filter.

Any RO system can be adapted for well-water use. If your water source has particulates in it, such as water from wells, you should add a sediment filter housing, with replacable filter cartridge, on your incoming water supply main, upstream of your drinking water filter system.

These whole-house sediment filter housings are available at local hardware store for about $50., and a replacement two-pack of sediment filters is around $11. As my water comes from my own well, and it has particulates in it, I need to change my filter out once a week. I've learned how maximize these filters. By purchasing the rolled paper filters (instead of woven string type), I can score and tear off one revolution of paper, to expose new, unclogged portion of it. By doing this, I can get many weeks use out of them.

After passing through the pre-filters (before the RO membrane), and also passing through the RO membrane, the purified water is then stored in the holding tank.

When someone opens the counter-top tap, the pressurized holding tank forces the stored water through yet another carbon filter. This filter is called the post-filter (after the RO membrane). It's also known as the "polishing" filter. This activated carbon filter removes any tastes or odors the RO water may still have. This last filter gives your drinking water a final polish, removing any lingering impure smells or tastes, and improves the efficiency of the system.

The result is pure, healthy, and fresh-tasting water, free of chemicals and other contaminants!


Reverse Osmosis Flow Chart


Use Reverse Osmosis Filtration at Home for Pure Water - Video



Note: If you have a refrigerator with an ice-maker and water dispenser, it's highly recommended that you tee into the supply line between the polishing filter and the counter-top tap of your Reverse Osmosis System. This way you can have your 'fridge dispensing ice-cubes and water with the same crisp and clean RO filtered water!

Visit our Particle Size Chart page for more discussion on the sizes of contaminant particles, and understanding the filters that will successfully eliminate them.

Click to visit our Drinking Water Filter Systems page to see the highest-rated systems.