How Does Reverse Osmosis Work

If you’re here, you’re probably wondering “How does reverse osmosis work and what is special about it?” This post walks through what reverse osmosis (RO for short) is and how it works.

Water is an essential natural resource. In its natural state, water may be unusable – dirty or in some cases salty. For these reasons, dirty water has to go through a purification processes such as filtering to be safe to drink.

Reverse Osmosis Water Filter Systems ensure that you have clean and uncontaminated drinking water. For a comparison of different units, see our Best Reverse Osmosis Filters page.

Water is disinfected by most public water distribution authorities. Still, there is room for contamination when the water is in transit to your home. To ensure you are drinking 100% safe water, consider installing an RO system in your home.

To learn how reverse osmosis works, keep reading…

What is reverse osmosis?

Reverse Osmosis is a process of removing contaminants, such as dirt and minerals from water by passing water under pressure through a filter (called a semi-permeable membrane).

Think of the semi-permeable membrane like a piece of skin. Not much can filter through the skin without pressure.

The RO process results in pure water on one side of the membrane and the impurities (minerals, bacteria, etc) on the other side of the membrane.


Yep, that’s a lot of technical talk. Basically we’re forcing water through a filter that has small holes that only the water can pass through. The impurities can’t go through so they’ll stay on the other side of the filter.  These impurities will need to go somewhere so they’re typically flushed down the drain as waste.

The principle of reverse osmosis

The principle of osmosis states that molecules will migrate from regions of low concentration to regions of high concentration through a semi-permeable membrane.

Osmosis explains the absorption of water by plants via the roots.

Um, yeah. But that is osmosis, not reverse osmosis…

Reverse Osmosis or RO is the opposite of osmosis.

In RO the  membrane filter allows water molecules to pass through but prevents the mineral and other solvents molecules from passing through. But this process cannot occur naturally.

The water has to be forced through the reverse osmosis membrane. The pressure of the water has to be higher than the pressure resulting from the naturally occurring osmosis.

How does Reverse Osmosis Work in the Home?

The typical residential Point-of-Use Reverse Osmosis system has four stages. Four stages (or sets of filters) are responsible for the removal of 99% of more than 65 possible contaminants in water.

Some of the contaminants include fluoride, chlorine, lead and other dissolved salts.

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How RO Water Filters Work


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Stage 1 – The Sediment Filter

Made of Melt Blown polypropylene, the sediment filter removes dirt, silt, rust and large sediments of up to 5 microns. This filter is crucial in removing sediments that would otherwise damage or plug up the more delicate filters that follow.

Stage 2 – The Carbon Filter

Chlorine, a common compound in water, will go through the sediment filter. More importantly, chlorine damages the reverse osmosis filter. So we need to filter that out before it gets to the RO membrane. That’s the job of the carbon filter which has slightly smaller holes than the sediment filter.

Chlorine also affects the taste and odor of water.  The carbon filter isolates chlorine, reduces the amount of lead and removes harmful bacteria such as Giardia and Cryptosporidium.

Stage 3 – The Reverse Osmosis Membrane

The Reverse Osmosis membrane handles the isolation of 99% of contaminants. This filter allows water to pass through and stops contaminants as small as 0.0001 microns.

Stage 4 – The Post Carbon Filter

The post carbon filter is the final stage of the water filter. This carbon filter will remove any remaining odor and improves the taste of the final drinking water.

What does Reverse Osmosis remove from water?

As mentioned earlier the process removes contaminants. These contaminants are both organic and inorganic impurities.

Common water contaminants include:

  • Chlorine
  • Chromium
  • Asbestos
  • Arsenic
  • Fluoride
  • Lead
  • Mercury
  • THMs (TriHaloMethanes)
  • Harmful bacteria such as Giardia and Cryptsporidium.

Reverse Osmosis systems also remove odor and bad taste.

Factors that will change the effectiveness of the RO system

  • The pressure of the tap water. Normally, the pressure of tap water is 40-80psi. Pressure lower than that will reduce the effectiveness of the system. Pressure is needed to force the water through the RO filter.
  • The contaminants in the tap water. Some types of impurities reduce the effectiveness of the system. Also, the number of different contaminants determine the effectiveness of the system. Over time the filters will get plugged up and need to be replaced.
  • The quality of the membranes and filters. Always ensure your RO system is using the best filters in the market.
  • The temperature of the water. The temperature is not a big issue if the pressure is okay. However, cold water tends to filter out slower than hot water.

Things to consider when purchasing a residential RO system

Reverse Osmosis systems differ so when buying such a system consider these factors:

  • Number of filters – most systems have four filters (discussed above). Some systems have five filters. The extra stage is usually a carbon pre-filter that comes before the water passes through the reverse osmosis filter (so there are 2 carbon filters before the RO membrane).
  • Quality of the filters and other components – a little research will help here. Check the reviews of the RO water filter system. Also, the costlier a system the more likely it has quality components.
  • Input Water Quality – If the input water is very contaminated, the RO filter will not last as long. Also, hard water tends to plug up RO filters. Consider softening the water before reverse osmosis filter.  When considering water softeners, our Best Water Softeners Reviews page is helpful.
  • Gallons per day – RO systems have a specified amount of water they can produce each day. A huge household uses many gallons per day, so choose carefully.

Summary of RO system benefits

This article answers the question, how does Reverse Osmosis work? Now that you understand the inner workings behind these filters, consider going to our reverse osmosis systems reviews page to see RO Water filter comparisons and reviews.

Drinking contaminant-free water reduces the risk of water-borne diseases.  Water purified through an RO system is safe for whatever purpose, be it drinking, cooking or washing.

Priced at less than $1000 and in many cases below $500, RO systems are affordable and worthwhile investments.


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