4-Stage vs 5-Stage Reverse Osmosis Systems

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If you are looking at purchasing a 4-stage reverse osmosis system, you may have thought, ‘Well, why not spend a little more and get a fifth stage for even better water?’

It is a good question, but extra filtration stages may be useless for your particular water, and we don’t want you wasting money.

So, let’s look at 4-stage vs. the 5-stage reverse osmosis systems to determine which is right for you.

Key Takeaways

  • A typical 4-stage reverse osmosis system has a sediment and a carbon pre-filter, a reverse osmosis filtration step, and a carbon post-filter.
  • 5-stage systems vary in their configuration. You may have one with three pre-filters or with two. And you may have 1 or 2 post-filters.
  • Choosing between 4 and 5 filter stages as well as the exact setup should depend on your filtration needs.

What’s the Difference Between 4-Stage and 5-Stage Reverse Osmosis Systems?

First of all, what’s the difference between 4-stage and 5-stage reverse osmosis systems?

A 4-stage reverse osmosis system typically has a sediment filter, carbon filter, RO membrane and post-carbon filtration. A 5-stage can have multiple configurations depending on your individual needs.

4-Stage RO Systems

A typical 4-stage reverse osmosis system consists of:

  1. Sediment Prefilter: This prefilter effectively captures dirt, debris, and other minute particles.
  2. Carbon Prefilter: It eliminates chlorine and other substances that cause unpleasant tastes and odors.
  3. RO Membrane: This membrane filters out up to around 99% of impurities in your water.
  4. Polishing Filter: Particularly useful as a post filter after the RO water has been sitting in the storage tank, it guarantees the water taste is pristine.

5-Stage RO Systems

Under Sink Reverse Osmosis System

You utilize various combinations of filter stages in a 5-stage RO system. These options include:

  1. Three pre-filters (two sediment filters and one carbon filter), an RO membrane, and a carbon post-filter.
  2. Three pre-filters (one sediment filter and two carbon filters), an RO membrane, and a carbon post-filter.
  3. Two pre-filters (one sediment filter and one carbon filter), RO membrane, carbon post-filter, and a remineralization filter.
  4. Two pre-filters (one sediment filter and one carbon filter), RO membrane, carbon post-filter, and a UV lamp.

Here’s an example setup:

  1. Sediment prefilter: Screens out larger dirt and debris particles.
  2. Carbon prefilter: Reduces elements like chlorine, organic chemicals, and other contaminants that RO does not effectively filter.
  3. Second carbon prefilter: Same as above.
  4. RO membrane: Filters out dissolved solids, such as heavy metals, and other potentially hazardous chemicals.
  5. Polishing filter: Delivers “polished” water using activated carbon filtration.

The Different Filter Stages in Home RO Systems

You have a wide range of filtration steps to choose from, particularly if you want to purchase a reverse osmosis system with 5 stages or more.

Sediment Pre-Filters

Sediment filtration is an essential step in the water purification process as it effectively removes debris from the water. By placing it at the beginning of the filtration process, sediment filtration acts as a first line of defense, capturing larger particles that could potentially clog subsequent filtration steps. This ensures that the rest of the system can target and filter out other contaminants, giving it the support it needs to purify water and effectively remove even the most stubborn substances.

Carbon Pre-Filters

Water passes through activated carbon, which removes various impurities and contaminants that cause unpleasant tastes, odors, cloudiness, colors, as well as chemicals, including volatile organic compounds.

As a result, any residual chlorine taste commonly found in municipal water is eliminated. This also benefits the RO membrane, since chlorine in could damage it.

Reverse Osmosis Membrane

This is the main phase of water filtration, and it removes the most contaminants. Water moves through a special membrane with tiny pores, measuring only 0.0001 microns. This incredible membrane eliminates up to 99% of impurities found in the water, including harmful substances like arsenic and chromium-6.

Carbon Post-Filters

After undergoing the reverse osmosis process, water is oftentimes stored in a tank. Carbon post-filtration removes any residual flavor from water that has been sitting in such a tank. By incorporating a carbon filter made from coconut shell, any lingering taste is eliminated.

Remineralization Stage

A remineralization filter is an additional step after RO filtration that adds back natural minerals such as calcium and magnesium to the water. This adds a mineral taste to the water and increases its pH level, which many prefer.

UV Water Purifier

A UV purifier is a component that can be integrated into an RO system, particularly valuable when the source water contains elevated levels of microorganisms such as bacteria and viruses. This stage employs ultraviolet light to eradicate pathogens in the water, preventing their reproduction and, therefore, their ability to spread disease.

Different Water Filter Cartridges and Membranes

4-Stage vs 5-Stage Reverse Osmosis Systems – Which to Choose?

So, 4-stage vs 5-stage reverse osmosis, which one should you choose?

The number of filter stages needed varies based on water quality. If your water has significant contamination or multiple types of contaminants requiring different filter methods, more filter stages may be necessary.

  • For example, if you have water high in dirt and sediment, you may want to consider a 5-stage system with two sediment filters
  • If you have a lot of chlorine, you may wish to have two carbon pre-filters instead of one.
  • Cysts in the water may call for UV disinfection.
  • If you prefer pure water with re-added minerals for taste, a system with a remineralization filter is the ideal choice.

It really depends on the composition of your feed water.

Does a Higher Number of Filter Stages Equal Higher Water Quality?

Manufacturers of RO systems will sometimes add filter stages to their systems, and often, all the excess stages are expensive and unnecessary. The number of steps does not solely determine the performance of a filtration system (i.e., more does not mean it is better). Factors such as filter design, surface area, water flow rate, and types of filter media used also play a role.

While additional stages can enhance contaminant reduction, some use the same steps over and over, which might not boost filtration efficiency unless your water is high in that particular contaminant that the step removes.

What Else to Consider

Before racing off to buy the latest 10-stage RO system, take a minute to figure out precisely what you want and expect from one. You might find that you only need a 4-step, which means significant cost savings!

Your Water Quality and Needs

As mentioned, several factors come into play when considering the quality of the feed water. For example, if your primary concern is eliminating bacteria and nothing else, you might need a UV filter and not an RO system at all! If there is a significant amount of sediment in your feed water, an additional filtration step for sediment could be beneficial. To determine your specific needs, it is recommended to test your tap water and identify the contaminants you want to remove.

Personal preference also influences the selection of stages for your RO system. If you only need to eliminate the taste of residual chlorine, a carbon filter might be sufficient. However, if you want to ensure pure water regardless of contaminants, a reverse osmosis system is an excellent option.

System Installation

The larger the system and the greater the number of stages, the more challenging it can be to install.


With water filtration systems, space becomes a serious consideration. Multi-stage filters can need plenty of room. Similarly, tank systems have more substantial space demands compared to tankless systems.


The maintenance level of an RO system is typically influenced by the number of filtration steps (more steps = more care).


A system becomes more expensive as you increase the number of filtration steps. This applies to the initial purchase and the annual running costs, considering the replacement filters.

If you have any questions about 4-stage vs 5-stage RO 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.
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