RO water filter: how does it work and do you need one? (2026)

Published on July 19, 2026 | Reading time: ~13 minutes

An RO water filter promises the cleanest water you can make at home. Reverse osmosis is the technology behind bottled water factories and even seawater desalination, and for a few years now it has also been found under Dutch kitchen counters. But is such a reverse osmosis water filter really the best choice, or are you overshooting the mark with a cannon?

In this guide, we explain how an RO water filter works, what it does and does not remove, what the hidden disadvantages are, and for whom a simpler inline filter is more practical. No scaremongering, just concrete figures.

1. What is an RO water filter?

The abbreviation RO stands for "reverse osmosis". Osmosis is the natural process in which water moves through a membrane from a low to a high salt concentration. In reverse osmosis, you reverse that process: with pump pressure, you force the water in the other direction, away from the dissolved substances. Hence the name reverse osmosis.

An RO water filter is therefore not a single filter, but a system with multiple stages. At PureFilter, we see that many people confuse reverse osmosis with a regular carbon filter jug, while in terms of technology and results, it is a completely different category.

2. How does a reverse osmosis water filter work?

A modern RO system purifies water in three to four stages. Most home systems nowadays work without a storage tank and filter directly when you turn on the tap.

Stage 1: Pre-filtration

A sediment and carbon pre-filter captures sand, rust, chlorine, and organic substances. This step is essential because chlorine damages the sensitive RO membrane. Without good pre-filtration, the membrane will break down much faster.

Stage 2: The RO membrane

This is the heart of the system. Under a pressure of typically 4 to 6 bar, the water is forced through the membrane. Pure water goes to the tap, and the retained substances are led to the drain with part of the water. This wastewater is called the "drain."

Stage 3: Post-filter and optional re-mineralisation

An activated carbon post-filter gives the water a fresh, neutral taste. Because reverse osmosis also removes minerals, some systems add a re-mineralisation filter here that restores calcium and magnesium and raises the pH above 7. Without this step, osmosis water tastes flat.

3. What does an RO water filter remove?

Reverse osmosis is the most thorough filtration method for home use. The membrane retains 95 to 99 percent of all dissolved substances, including substances that a carbon filter handles less effectively:

• PFAS and forever chemicals: up to more than 99 percent, one of RO's strongest points.
• Nitrate and nitrite: pass straight through most carbon filters, but are retained by RO.
• Fluoride: difficult to remove with other techniques, RO does this effectively.
• Heavy metals: lead, arsenic, cadmium, and copper.
• Limescale (TDS): calcium and magnesium, so limescale in kettles or coffee machines also disappears.
• Chlorine, medicinal residues, and microplastics.

This thoroughness is also the downside: reverse osmosis does not distinguish between harmful substances and healthy minerals. Everything that is dissolved is removed. A well-certified inline carbon filter removes the same problem substances such as lead, PFAS, and chlorine, but leaves the minerals in.

4. Advantages of reverse osmosis

• Broadest removal: no other home technique removes as many different substances. Ideal for demonstrably high nitrate or fluoride levels.
• Limescale-free water: because almost all minerals disappear, you will no longer get limescale in your kettle or coffee machine.
• Suitable for special applications: aquariums, humidifiers, and some medical situations require water with a very low TDS value.
• Certainty with well water: if you use well water or groundwater with varying quality instead of tap water, RO offers the most certainty.

5. Disadvantages and considerations of an RO water filter

Reverse osmosis sounds impressive, but in practice, many households encounter four concrete disadvantages:

Water waste

An RO system discharges part of the water as wastewater. In many systems, this is 2 to 4 litres of wastewater per litre of drinking water. Economical modern models achieve 2:1 or 3:1, but older systems with tanks sometimes waste even more.

Minerals disappear

Calcium and magnesium are completely filtered out. The RIVM and Milieu Centraal emphasize that tap water in the Netherlands makes a useful contribution to the daily intake of these minerals. Osmosis water also tastes flatter, which many people experience as "dead water." A remineralization filter partially solves this, but adds costs and maintenance.

Price and installation

A home RO system costs 270 to 1,300 euros, depending on capacity and certification. Installation requires a connection to the cold water supply, a drain connection, and a hole in the countertop for the separate drinking water tap. Allow 30 to 60 minutes of work or hire a plumber.

Power and maintenance

The pump runs on electricity, and the multiple filters plus the membrane each have their own replacement schedule. The membrane lasts an average of 2 to 3 years, the pre-filters often every 6 to 12 months. This makes maintenance more complex than with a single inline cartridge.

6. Do you need an RO water filter in the Netherlands?

The Netherlands has one of the strictest drinking water standards in the world. According to industry association Vewin, tap water almost always meets all legal requirements and is continuously monitored. For most households, the complete desalination of reverse osmosis is therefore overkill.

Where tap water in the Netherlands is vulnerable, the problem usually lies in the last few metres: lead internal pipes in houses built before 1960, chlorine taste, limescale, or specific contaminants such as PFAS and medicinal residues. These substances can be perfectly removed with a certified inline carbon filter, without having to sacrifice healthy minerals.

7. Comparison table: RO versus inline filter, Brita and Berkey

8. The alternative: PureFilter Mineral+

The PureFilter Mineral+ is designed for exactly the situation most Dutch households find themselves in: safe tap water that deserves a little extra care in the last few metres. The inline filter sits under your sink and removes more than 100 contaminants, including PFAS, lead, chlorine, medicinal residues, microplastics, and nitrate.

The difference with reverse osmosis lies in the approach. Where an RO water filter removes everything and then possibly re-adds minerals, the PureFilter retains dissolved minerals and even adds extra calcium, magnesium, and potassium through its Swedish Mineral Rock™ technology. This way, you maintain a fresh taste and a useful mineral intake, without a drop of wastewater.

At PureFilter, we see that customers who previously had an RO system switch precisely because they felt they were drinking "dead water": technically clean, but tasteless. The PureFilter combines the purity they sought with the taste and convenience they missed.

• Installation: 5 to 10 minutes, no plumber needed.
• Capacity: 15,000 to 20,000 litres per cartridge, usually one replacement per year.
• Certification: NSF 42, 53, 61, and 372.
• No water waste and no power outlet.

9. Buying advice: which filter is right for you?

The choice between an RO water filter and an inline filter depends on your situation:

• Demonstrably high nitrate or fluoride levels, or well water? Then reverse osmosis is the most thorough solution. Choose an NSF 58-certified system and consider a remineralization filter for taste.
• Regular Dutch tap water that you want purer and tastier? Choose a certified inline filter such as the PureFilter Mineral+. You remove problem substances and retain minerals.
• Rental property where you're not allowed to drill or connect? An RO system is difficult then. Check our page under-sink water filter for compact options.
• Small budget to start? A Brita is better than nothing, but its capacity of 150 to 200 liters and limited certification make it more expensive and less effective in the long run.

Still in doubt? On our page best water filter, we compare all options side by side, and in the frequently asked questions, you'll find answers to the most common questions.

10. Frequently asked questions about an RO water filter

11. Conclusion

An RO water filter is the most thorough filtration technique you can use at home. Reverse osmosis removes 95 to 99 percent of all dissolved substances, from PFAS and nitrates to fluoride and heavy metals. For well water or a demonstrably high nitrate or fluoride level, this is a strong choice.

But that thoroughness comes at a price: the system wastes 2 to 4 liters of wastewater per liter, removes healthy minerals, costs €270 to €1,300, and requires electricity, installation, and maintenance of multiple filters. For regular Dutch tap water, which already meets strict standards, this is often more than necessary.

For most households, a certified inline filter is the smarter balance. The PureFilter Mineral+ removes more than 100 contaminants including PFAS and lead, retains and enhances minerals via Swedish Mineral Rock™, wastes no water, and installs in 5 to 10 minutes. Purity and taste, without the drawbacks of reverse osmosis.

Want to know more? Visit purefilter.nl or read our guides on the under-sink water filter and PFAS in tap water.