What is filtered water? How it works

Reading time: ~11 minutes

What exactly is filtered water?

Filtered water sounds like a type of water, but it is a treatment. You start with water that is already drinking water and then remove one more layer of substances. Exactly what comes out depends entirely on the filter you use: a 20-euro filter jug and an under-sink system do not do the same work, even though the result is called filtered water in both cases.

The difference with bottled water is worth noting too. Spring water and mineral water come from a protected source and may legally be treated only to a very limited extent. Filtered water, by contrast, starts with mains water and is only treated afterwards. Those two routes produce completely different products, and we have put them side by side in our piece on tap water versus bottled water.

Is filtered water better than tap water?

That depends on what you mean by better. On safety, Dutch tap water is hard to beat. RIVM reports that drinking water quality almost always meets the standards, in more than 99.9% of all measurements, and that the requirements are laid down in the Drinkwaterbesluit, the Dutch drinking water decree. So a filter does not make unsafe water safe, because the water was already safe.

What it really comes down to are the substances that remain in very low concentrations, and the substances that only enter your water after the mains network. Two examples make that concrete.

The first is PFAS. For 20 different PFAS, RIVM reports a legal limit of 100 nanograms per litre, which Dutch drinking water meets. On the basis of new knowledge about effects on the immune system, however, RIVM advises a much stricter guideline value of 4.4 nanograms per litre, expressed in PFOA equivalents. The same institute calculated that people take in roughly three times as much PFAS through food as through drinking water, and that the sum of the two exceeds the health-based limit value. Anyone who wants to do something about that has little control over food, but does have control over drinking water. What each technique achieves there is set out in our overview of the water filter against PFAS.

The second example is your own home. The water company is responsible up to your water meter; the pipework beyond it is yours. In older homes there may still be lead pipe there, and copper and brass also release substances into standing water. That is precisely why filtering at the sink can add something that filtering at the water company cannot.

How do you get filtered water at home?

There are five routes, and they differ sharply in what they remove, how much effort they take and what they cost per litre. We explain the technology behind them step by step in how a water filter works.

1. The filter jug

The best-known and cheapest way in. A cartridge with activated carbon and usually an ion exchanger tackles mainly chlorine, taste, smell and part of the limescale. The limitation lies in contact time and in maintenance: the cartridge has to be replaced every four to six weeks, and a forgotten cartridge becomes a breeding ground. We worked out just how strict that interval is in our piece on how often to replace a Brita filter.

2. The tap-mounted filter

A small filter housing that you screw onto the tap outlet, with a lever to switch between filtered and unfiltered water. Practical, because there is nothing to refill, but the housing is small and so is the filter capacity. What it actually achieves in practice is covered in our explanation of a tap-mounted water filter.

3. The dispenser or gravity filter

A tank on the worktop in which the water trickles down through ceramic or carbon candles. No plumbing connection needed, which makes it popular as a stopgap, but it takes up worktop space and you fill it by hand. More on that in our article on the water dispenser with a filter.

4. The under-sink filter

A fixed filter in the cold water line under your sink. The water is under mains pressure and the filter housing is larger, so there is more filter material and more contact time available than in a jug. You notice nothing of it: the water simply comes out of the tap you already have. This is the category the PureFilter Mineral+ belongs to.

5. Reverse osmosis

The heaviest option, with a membrane that holds back almost everything. That is also the objection: minerals go out with it, flush water is needed, and most systems require electricity and a storage tank. What that does to the composition of your water is set out in reverse osmosis water and health. Which of these five routes fits which kind of question is something we line up on our page about which filter technology produces which kind of filtered water.

Which substances come out and which do not?

This is where suppliers part ways, and where you should pay most attention to certification rather than marketing. NSF International draws a distinction that sums up the whole discussion: NSF/ANSI 42 covers aesthetic effects such as chlorine, taste and smell, while NSF/ANSI 53 covers substances with a health effect, such as lead, cysts and volatile organic compounds. A filter that only meets 42 makes your water taste better. A filter that also meets 53 additionally does something about the substances you were actually worried about.

What a filter usually does not do is worth mentioning as well. Limescale does not disappear by itself: hardness calls for ion exchange or a softener, not for carbon. Bacteria are not removed by most household filters, which is not a problem in the Netherlands because the tap water is already microbiologically sound. And nitrate or fluoride need a specific filter stage; a standard carbon cartridge lets them through.

The advantages of filtered water

• Less of the substances you cannot see: a filter tested for health effects tackles lead, PFAS, pharmaceutical residues and microplastics. Those are exactly the substances for which a standard exists or for which RIVM advises a stricter guideline value.
• Taste and smell: activated carbon removes chlorine and taste compounds. You notice it most in tea and coffee, which consist almost entirely of water. What that means for a machine is covered in our piece on a filter in your coffee machine.
• Better for the environment than bottles: according to Milieu Centraal (the Dutch independent information body on sustainability), tap water is the best thirst quencher for the environment. Filtered tap water keeps that advantage, because no bottle, no transport and no deposit-return logistics are involved.
• Cheaper in the long run: a fixed filter costs a fraction of bottled water per litre. The sums are further down.
• No lugging: with a fixed filter there is nothing to refill, nothing to chill and nothing to carry up the stairs.

The drawbacks of filtered water

This is the question most often typed into Google alongside “filtered water”, and the honest answer ultimately sells better than the flattering one.

• A filter you forget is worse than no filter at all. Saturated filter material holds nothing back, and a damp cartridge left in place for weeks is a place where bacteria grow. This is the biggest practical objection, and it weighs more heavily with jugs than with fixed filters, simply because the interval is shorter.
• Minerals can disappear. With reverse osmosis, the vast majority of calcium and magnesium goes out along with everything else. Why that is not harmless for taste and composition is set out in our article on minerals in drinking water.
• It costs money. Purchase price plus replacement filters. With a cheap jug the entry price looks low, but the short replacement interval actually pushes up the cost per litre.
• You solve a problem you have not measured. Filtering makes more sense in an older house with lead or copper pipework than in a new-build home supplied from groundwater. Anyone who does not know what is in their water is buying on gut feeling.
• Water loss with osmosis. A membrane system flushes part of the water away as concentrate. With the other four routes that does not apply.

An honest weighing-up also means this: if the only thing bothering you is the taste of chlorine, a jug is enough and you do not need a fixed system. What individual brands in that category do and do not live up to is something we tested in our review of what Aquaphor water filters do and do not do.

Filtered water, tap water and bottled water

The row about lead is the most interesting one. It is the only point where a filter at the sink can do something the water company fundamentally cannot, because the problem only arises after the water meter. If you want to know which type of filter scores best on the other rows, you will find the trade-off on our page about the best water filter.

What does filtered water cost per litre?

Start with the basics. According to Vewin (the association of Dutch water companies), the weighted average drinking water tariff in 2026 is €2.71 per cubic metre including taxes, of which 24.9% consists of taxes. One cubic metre is 1,000 litres, so tap water costs roughly 0.27 cents per litre. We go through those sums in more detail in our piece on what a cubic metre of water costs.

On top of that comes the filter. A replacement filter for the PureFilter Mineral+ costs €65.00 and delivers 15,000 to 20,000 litres of filtered water. That works out at €0.0033 to €0.0043 per litre, in other words less than one cent. Add the tap water to that and you are at roughly half a cent per litre of filtered water, against €0.40 to €1.00 for a litre of bottled water.

With a filter jug those sums work out differently from what you might expect. The cartridge is cheap but lasts only four to six weeks, and the capacity per cartridge is a fraction of that of a fixed filter housing. Per litre, a jug is therefore almost always more expensive than an under-sink filter, even though the entry price is lower.

Which filter suits your situation?

Three questions decide the answer: what do you want taken out, how many litres do you use per day, and are you allowed to connect something permanently? If you only want to get rid of the taste of chlorine and you drink a litre a day, a jug will do. If you also want to tackle the substances with a health effect and the whole household is drinking along, a fixed filter is the logical choice. If you are in doubt, our decision guide to which water filter suits you will talk you through it.

What we supply ourselves falls into that last category. The PureFilter Mineral+ is an under-sink filter that removes more than 100 contaminants: PFAS, microplastics, heavy metals such as lead and copper, fluoride, nitrates, pharmaceutical residues and chlorine. It is NSF-certified to 42, 53, 61 and 372, so on taste as well as on health effects. It is emphatically not reverse osmosis.

What sets it apart is in the final step. Where most filters only take things out, the Mineral+ with five filter stages adds calcium, magnesium and potassium back in through Swedish Mineral Rock™. Purify first, then remineralise, in that order. That is precisely the opposite of what a membrane system does to your mineral balance, and it is why the water tastes softer and fuller instead of flat.

The filter costs €149.00 including the first cartridge. A replacement filter costs €65.00 and lasts about twelve months for 1 to 3 people; for 4 to 6 people, about six months. The duo with two cartridges costs €115.00. If you would rather not have to think about it, the Filterplan subscription sends the cartridges out automatically and on time for €48.00 or €86.00 per delivery. You install it yourself in 5 to 10 minutes on the cold water line. You do not need a plumber; for the installation you use a single pair of water pump pliers, and you change the cartridge later with the key supplied. A 3/8 connection is supplied as standard. The full steps are set out in our guide to installing a water filter, and an overview of our fixed filters for drinking water can be found in the webshop.

Frequently asked questions

Conclusion

Filtered water is tap water with one extra treatment, and its value lies not in safety but in refinement. Dutch drinking water meets the standards in more than 99.9% of measurements, so a filter repairs nothing. It does lower the concentrations of substances for which stricter guideline values are advised, such as PFAS, and it captures what is only added to the water in your own household pipework.

After that, the choice comes down to two things. First, certification: NSF/ANSI 42 is about taste, NSF/ANSI 53 about health effects, and that difference determines whether you are buying a taste improver or a genuine filter. Second, maintenance, because a filter left in place too long is worse than no filter at all. Take both seriously and you pay less than one cent per litre and never have to lug a bottle again. Which type scores highest on that count is something we compare in our overview of the healthiest water filter.

The PureFilter Mineral+ costs €149.00 including the first filter, removes more than 100 contaminants, is NSF-certified to 42, 53, 61 and 372, and gives back calcium, magnesium and potassium through Swedish Mineral Rock™. The replacement filter costs €65.00 and lasts about twelve months for 1 to 3 people, and about six months for 4 to 6 people. With 40 days of free returns and a 5-year warranty, you can take your time at home to taste whether you notice the difference.