3-Stage Water Filter vs Reverse Osmosis System: Which to Choose?

This page may contain affiliate links. If you buy a product or service through such a link we earn a commission at no extra cost to you. Learn more.

Choosing an appropriate filtration setup for your household water can be more complicated than it seems at a glance. You need to start with an overview of your current situation, including a water analysis that tells you what kinds of contaminants you have, and in what concentrations. Then you need to choose a good system that includes all necessary filtration stages.

Reverse osmosis is commonly praised for being the most thorough water purification method available on the domestic market. At the same time, other solutions are also quite popular, including regular 3-stage water filter setups. How does reverse osmosis measure up against those and which one is better in the end?

Let’s find out.

Key Takeaways

  • 3-stage water filters use 3 filter stages. Reverse osmosis systems for home use usually feature 4 or 5 stages; some even more. But there are also 3-stage RO systems.
  • 3-stage water filters may use all kinds of filter media, depending on the type of contamination that needs to be addressed. Most common are sediment and carbon filter stages. Most RO systems use sediment and carbon pre-filters, an RO membrane, and a carbon post-filter.
  • Reverse osmosis diffuses water through a thin, semipermeable membrane. Any impurities are left behind and flushed down the drain. Most regular water filters trap one or more specific type of contaminant.

3-Stage Water Filter vs Reverse Osmosis System | Summary

A 3-stage water filter system basically incorporates multiple filtration stages that can address different types of contaminants. Water passes through each stage in sequence, getting progressively more and more purified. The stages should be specifically chosen to address the contaminants present in the water supply. Most common is a sediment filter combined with 2 carbon filter stages.

A reverse osmosis system, although having multiple filter stages as well, focuses on one stage – the reverse osmosis membrane. That’s because reverse osmosis is a very thorough purification method which effectively removes almost all impurities from water.

Generally speaking, a 3-stage water filter can be anything and everything; it depends on the filter elements used. Some reverse osmosis systems are even 3-stage water filters. Still, most RO systems will probably do a better job at filtering your water than the standard 3-stage unit.

Under Sink Reverse Osmosis System

What Is a 3-Stage Water Filter and How Does It Work?

A 3-stage water filter, as the name implies, is a water filter comprising three separate filtration stages. It’s important to note that there is no fixed setup used across the board. The first stage in one system might be the second stage in another, or might not even be present there.

As such, many different filter combinations are possible. For example, you might have a setup with two sediment filters and one granular activated carbon filter. Or you might have one sediment filter, one carbon filter block, and one catalytic carbon filter. Many other configurations exist. Everything should depend on the types of contaminants you need to address.

Still, some setups are much more common than others. If you don’t have any special requirements and want to go with an off the shelf solution, your 3-stage water filter setup will probably look something like the following:

Stage 1

A sediment filter used as the first stage is very common. This is useful for preventing the overall system from clogging up, especially when you’re using reverse osmosis and other more sensitive filtration methods.

Stage 2

Then, you might have a stage of granular activated carbon. This is useful for removing chlorine and other elements which cause bad taste and odor. This effectively addresses many organic compounds as well.

Stage 3

Finally, in the above setup you would most probably have a carbon block at the end of the chain. This can deal with more difficult contaminants like organic chemicals, pesticides, disinfection byproducts and even potentially heavy metals. This depends entirely on the type of filter used, and this is usually the stage that you would fine-tune to your specific requirements.

What Is a Reverse Osmosis System and How Does It Work?

A reverse osmosis system relies on one main component for its functionality. And that is a thin semipermeable membrane lined with very fine pores. Reverse osmosis works by pushing water against this membrane at a high level of pressure. This forces water molecules to pass through the membrane while everything else gets left behind.

RO is a highly effective purification method which removes almost all types of contaminants from a water supply. There are only a few types of contaminants that a reverse osmosis system cannot deal with, such as dissolved gases. Even then, you can simply combine your reverse osmosis membrane with one or more additional filtration stages to address that problem. In fact, pretty much all RO systems for home use do come with additional pre and post-filter stages (sediment, carbon, and possibly other).

Differences Between Reverse Osmosis and 3-Stage Filters

Let’s have a look at some of the specific differences between RO systems and regular 3-stage water filters.

Filter Stages

All 3-stage water filters feature 3 filter stages, obviously.

Most reverse osmosis systems come with 4 or 5 stages. Some even more.


The way water filtration usually works is very different from reverse osmosis.

Most traditional water filters use some type of filter media that water flows through. Contaminants are trapped, blocked, adsorbed, absorbed, oxidized, reduced, or bound. In the case of activated carbon, for example, the main mechanism is contaminants getting trapped in the small surface pores of the material.

With reverse osmosis, on the other hand, water diffuses through the membrane, but the impurities are being rejected. They are washed off the membrane surface in a second stream.

Contaminant Reduction

Reverse osmosis is aimed at removing many types of contaminants. In fact, it can easily get rid of almost all impurities you can encounter in your household drinking water – including the good stuff like minerals.

In a 3-stage filter system, each stage is usually designed for a specific type of contaminant. You need to combine different stages to ensure that your water gets purified as thoroughly as possible.

Installation and Maintenance

Both systems usually go under the kitchen sink. But while a 3-stage water filter can be installed inline, a reverse osmosis system requires a dedicated faucet and a drain connection.

plumber installing reverse osmosis system under sink

Filter Life

Pre and post-filters aside, the membrane of a reverse osmosis system can last for several years without needing replacement.

The filter life of a 3-stage system varies from one stage to another. Some filters need to be replaced very often, down to once every few months. Others can last for a longer time, up to a year

Water Flow

One of the main downsides of reverse osmosis systems is that they are pretty slow. That’s why a water tank is typically used as a buffer zone.

3-stage filter systems also impact the water flow rate, but depending on the types of filters used, the difference might not be that significant. Still, having water pass through multiple filter cartridges will always slow it down to some extent. In the end though, you can easily get 90% of your original flow rate with a 3-stage filtration system.


One major disadvantage of reverse osmosis is that it wastes water in the process. This is unavoidable, and it’s something you have to get used to. You can reduce the amount of water that gets wasted, but you cannot eliminate this factor completely. This is not a problem with your standard 3-stage water filtration system.


A cheap under sink reverse osmosis system may cost $150. Higher priced models can come in at around $500.

The cheapest 3-stage under sink water filter we could find costs $70. If you invest $500, you already get the very best.


The main similarities between reverse osmosis and 3-stage water filters boil down to their intended purpose and to some extent, their external appearance. In both cases, you have a system designed to remove contaminants from water. And in both cases, you have filter cartridges connected to one another running in sequence. A sediment filter often come first, followed by 1 or 2 activated carbon filters.

Beyond that, there are not too many similarities between reverse osmosis and 3-stage water filters.

Pros and Cons of Reverse Osmosis Systems

Here are the most important advantages and disadvantages of reverse osmosis systems.

Pro: Very Thorough Contaminant Removal

Reverse osmosis is as thorough as it gets when it comes to removing water contamination. Almost nothing can pass through the membrane of an RO filter.

Pro: Improved Water Smell and Taste

RO water is exceptionally pure and that’s what it tastes and smells like.

Pro: Low Maintenance Requirements

You don’t need to replace the filter elements of a reverse osmosis system that often. Once or twice a year is usually enough for the pre-filters. Most RO membranes and post-filters last even longer.

Aside from that, there’s not much maintenance work required.

Con: Wastes Water

Some water will always be wasted in the process. That’s unavoidable, and the best you can do is to minimize the amount of wasted water by adding a pressure pump.

Con: Works Slowly

Reverse osmosis is slow, and takes a long time to work. That’s why you often need a storage tank as part of your setup.

Con: Removes Some Useful Minerals

The thorough purification capabilities of reverse osmosis can also be seen as a downside. The process also removes various minerals which are useful to have in your diet. The solution: Using a remineralization filter.

Pros and Cons of 3-Stage Water Filters

Pro: Diverse Range of Designs

3-stage water filters are highly customizable. You can pick any number of combinations for the different stages, allowing you to precisely adjust the overall setup to your current requirements. This lets you deal with some potentially tricky situations involving difficult-to-remove contaminants.

Pro: Better Flow Rate

The typical 3-stage filter system doesn’t obstruct the flow rate of water as much as a reverse osmosis system does. You can enjoy having access to fresh water without the need for a storage tank in many cases.

Pro: Doesn’t Waste Water

Unlike a reverse osmosis filter, 3-stage water filters don’t waste any water in the process. This might not matter much to some people, especially those used to reusing their reject water for other purposes, but it’s still a factor worth considering.

Con: More Frequent Maintenance

You will have to replace the filter cartridges of your system more frequently. How often exactly depends on the specific types of filters you’re using. In some cases, you will have to anticipate replacing the filters every few months.

water filter cartridges and membranes

Con: May Not Remove All Contaminants

Depending on what kinds of contaminants you are dealing with, a 3-stage system might not be as effective as reverse osmosis. Even if you’re careful in picking the specific components to ensure that you cover as many contaminants as possible, you might still be left with some blind spots.

3-Stage Water Filter vs Reverse Osmosis: Which to Choose?

So, 3-stage water filter vs reverse osmosis – which one should you choose for your home? As usual in these cases, the answer depends. You need to make sure that you’re addressing the types of contaminants that you’re dealing with, and that you’re also meeting your basic requirements.

Test Your Water Supply!

The best way to start off on the right foot is to have your water professionally tested. You can do that by submitting a sample to a testing laboratory and waiting for the results. This will tell you exactly what kinds of contaminants you’re dealing with and in what concentrations. You may also be able to buy a DIY desk kit from your local hardware store, but we don’t recommend going with that approach. This will not provide you with the full picture. Water quality reports can also be lackluster.

What Do You Want?

You must also consider your personal requirements, of course. Here are some questions to ask yourself:

  • Do you want to filter your entire household’s water, or just individual access points?
  • How many people do you live with, and what are their water consumption habits?
  • Do you only want to improve your water’s aesthetics?
  • Do you care about wasting water?
  • Do you mind performing frequent maintenance?

If you have any questions about 3 stage filters vs reverse osmosis systems please don’t hesitate to leave a comment below!

About the Author Gene Fitzgerald

Gene Fitzgerald is one of the founders of BOS and currently head of content creation. She has 8+ years of experience as a water treatment specialist under her belt making her our senior scientist. Outside of BOS, Gene loves reading books on philosophy & social issues, making music, and hiking.
Learn more about .


Information provided on BOS is for educational purposes only. The products and services we review may not be right for your individual circumstances.
We adhere to strict editorial guidelines. Rest assured, the opinions expressed have not been provided, reviewed, or otherwise endorsed by our partners – they are unbiased, independent, and the author’s alone. Our licensed experts fact-check all content for accuracy. It is accurate as of the date posted and to the best of our knowledge.

Leave a Comment: