The Anatomy of a Reverse Osmosis System

Today, we take a look at a typical home reverse osmosis system, and explain and debunk some of the terminology involved, before explaining 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 terminology, 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 less 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.


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 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 systems efficiency.

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 waste water 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.


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 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.


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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