The Anatomy of a Reverse Osmosis System

Today, we will be taking a look at the anatomy of a reverse osmosis system for typical home use. We will explain and debunk some of the terminology involved, before showing you what is each part’s role in the process.

Reverse osmosis system anatomy


Reverse Osmosis systems come equipped with a lot of parts with obscure and confusing names. Today, we take a look at a typical home RO system, and explain some of the terminologies, before explaining each part’s role.

The image we will use is a diagram of a 5-stage reverse osmosis system. This is the standard number of filter stages, but systems are available with more or even fewer stages of filtration. A system with 6 or 7 stages would involve an alkaline or pH filter, while a system with less would not filter the water in such a refined way.

Breaking Down the Anatomy of a Reverse Osmosis System

Let’s look at the process through which a reverse osmosis system does its job. It’s one of the most advanced filtration systems that exist right now, and we will try to break it down as simple as possible.

While a reverse osmosis system brings numerous advantages, we feel the need to remind you that it has some disadvantages as well. Now, with that out of the way, let’s look at how it works:

Pre-filter (1st stage):

This is the first point of contact for the untreated water. The pre-filter’s job is to remove the larger particles and protect the filters that come after it (especially the reverse osmosis membrane). It mainly removes things like sediment, rust, and dirt.

Pre-filter (2nd & 3rd stage):

The carbon filters’ main function is to remove chlorine and other organic chemicals from the water. These chemicals can have bad odors and tastes, so the carbon filters work to improve the taste and smell of the water too.

Automatic Shut-off Valve:

When the storage tank reaches capacity, this valve will close and stop any more water from passing through. This prevents the over-spill of the water tank.

RO Membrane (4th stage):

This is where the system really earns its money, and the reverse osmosis process takes place. The membrane consists of a very fine material, which will typically only allow particles of a size smaller than 0.0005 microns (0.00000005 cm) to pass through.

Flow Restrictor:

This has the job of regulating the water flow. The reverse osmosis process works best at higher water pressures, and this component helps with the efficiency of the system.

Storage Tank:

This is where your water is stored, prior to coming out of the faucet. Sizes range from around 3 gallons to 9 gallons.

Drain Line:

This takes the by-product wastewater away to the nearest drainage area.

Post-filter (5th stage):

This filter serves to refine the taste before the water exits the faucet. The odor and coloration may also be improved here.

Post filter (6th stage):

Though not included in the picture, many RO systems will involve a further stage of filtration. This is an alkaline or pH filter, which works to add healthy minerals back to the water that may have been removed during the filtration process. It also serves to raise the pH to more alkaline levels.

FAQ About the Anatomy of a Reverse Osmosis System

What is not removed by reverse osmosis?

A reverse osmosis system will not remove some pesticides, solvents, and volatile organic chemicals (VOCs), with the most relevant examples being:

  • Ions and metals such as Chlorine and Radon
  • Organic chemicals such as Benzene, Carbon tetrachloride, Dichlorobenzene,
  • Toluene and Trihalomethanes (THMs)*
  • Pesticides such as 1,2,4-trichlorobenzene, 2,4-D, and Atrazine.

How do you test a reverse osmosis membrane?

The easiest way to test a reverse osmosis system’s membrane is to get a TDS meter. The TDS meter, as the name implies, tests how many total dissolved solids are in the water. You can use that to test the effectiveness of your RO system’s membrane since it’s the one that removes minerals like calcium and magnesium.

First, test the unfiltered tap water by filling a glass with it and using the TDS meter. Then get a glass of reverse osmosis water and test that one as well. If the membrane works properly, the reverse osmosis water should have at least 10 times less the amount of total dissolved solids.

For example, if the regular tap water has 100 TDS, the RO water should have 10 or less TDS. If that’s the case, then you don’t need to replace the membrane. Usually, the ideal time to replace a RO membrane is once every three years. But it is best to test your water beforehand to avoid replacing a still-functioning one.

Final Thoughts on the Anatomy of a Reverse Osmosis System

We also have detailed reviews of the best RO systems available. To find out which systems we regard as being the best, then please check out our guide. As ever, if you have any questions about anything water filter related, then don’t hesitate to get in touch, and we’ll do our best to help out in any way we can.

Hi I'm Jamie, teacher and owner/chief contributor at We strive to bring you the most informative water filter reviews in the most intuitive and user friendly way. Besides offering advice on the water filter to suit your needs, we also post topical articles on water pollution and the environment. Stay connected and follow us on, Facebook, Twitter, Google+ and more!

1 Comment
  1. Why does this say the alkaline filter is not included in the picture? It is shown in the picture…between stage 4 & 5. Although, your call it out as a 6th stage filter. Confusing. Does the alkaline filter install “as shown” between stages 4 & 5 or is it truly a 6th stage installed after the post filter?

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