No filter removes everything, and not every household needs one. We described the principles behind the individual technologies in our article on water technologies. Here we take a practical look: which system to choose, where to install it and how to maintain it so that it improves your water rather than making it worse.
How to read the numbers: Target = the value we aim for
in drinking water
under the EU directive
limit applying from 2026
Start with a water analysis
If you are on the public supply, your water utility publishes test results – hardness, nitrate, iron and microbiology. Public water must meet legal limits, so a filter mainly addresses taste, chlorine, hardness or substances from the internal plumbing, such as lead from old pipes.
With a private well, you are responsible for quality yourself. Have the water tested by an accredited laboratory – at least for microbiology, nitrate, iron, manganese and hardness – and repeat the test, for example after flooding. Only the results show whether you need disinfection, nitrate removal or just a sediment filter.
What certifications mean
A promise such as “removes 99% of impurities” means little without independent testing. The most widely used standards are NSF/ANSI:
- NSF/ANSI 42 – chlorine, taste, odour, coarse particles.
- NSF/ANSI 53 – health-related contaminants, such as lead or protozoan cysts.
- NSF/ANSI 58 – reverse osmosis; depending on the claim, also nitrate or arsenic.
- NSF/ANSI 401 – selected pharmaceutical and pesticide residues.
- NSF P473 – the original protocol for PFOA and PFOS, now verified mainly under standards 53 and 58.
Tests by independent European laboratories can be equivalent. A certificate, however, applies only to the substances explicitly listed – if you care about PFAS or microplastics, look for exactly that claim.
Filters on the main water inlet
Whole-house filters are installed after the water meter and treat all the water in the home.
Sediment pre-filter
It catches sand and rust that damage taps, valves and the water heater. It is a sensible foundation in almost any home, but it does not remove chemicals.
Whole-house carbon filter
It reduces chlorine, odour and some organic substances throughout the house. Flow rate is decisive: an undersized filter causes a noticeable pressure drop. Without chlorine, water also stands in the pipes with no residual disinfectant, so replace the media on time.
Softeners: softening is not purification
An ion exchanger swaps calcium and magnesium for sodium. It protects appliances from limescale but does not purify water – it removes neither bacteria, nitrate nor PFAS. According to the WHO, hard water is not a health risk, so keep at least one drinking tap bypassing the softener.
UV disinfection for wells
A UV lamp inactivates bacteria, viruses and protozoa without chemicals, but only in clear water – it needs a sediment filter upstream and, if necessary, iron removal. It does not remove chemicals. The lamp is usually replaced once a year.
Installation and maintenance
Work on the main inlet is a job for a professional, who will provide a service bypass, backflow protection according to EN 1717 and correct sizing. Sediment cartridges are typically rinsed or replaced every few weeks to months, carbon media according to the manufacturer, and reverse osmosis membranes usually after several years.
Lead is often released from old pipes within the building itself. Where this is a risk, a certified filter belongs right at the tap you drink from.
Under-sink systems
They treat only water for drinking and cooking, so they can be more thorough. A carbon block certified to 42 and 53 reduces chlorine and, depending on the claim, lead or some PFAS, while leaving minerals in. Reverse osmosis removes the most dissolved substances, including nitrate, but produces wastewater and strips minerals, so it is paired with remineralisation. Both connect to a separate tap.
Free-standing options
- Filter jugs – a cheap entry point that improves taste and reduces chlorine. A cartridge lasts only a few weeks, though.
- Gravity filters – a larger reservoir with no plumbing or power; cartridge quality is what counts.
- Countertop units – including small RO systems, suitable for tenants.
- Faucet-mounted filters – short contact time with the carbon, hence limited effect.
- Filter bottles – for travel, with very small capacity.
Compare the cost per litre: divide the cartridge price by its capacity. An under-sink system costs more to buy but, at family consumption, often works out cheaper per litre than a jug.
Activated carbon traps impurities, but it is also an ideal surface for bacteria. An overdue cartridge makes water worse – schedule replacements by the calendar, not by taste.
A sensible combination
For most households a tiered approach works: a sediment pre-filter at the inlet and whole-house carbon if needed, a softener for hard water and UV for a well. For drinking, one tap with a carbon block or reverse osmosis is then enough. If your public water is good and you only care about taste, a certified jug will do.
- Strong evidence Reverse osmosis and certified filters reduce the listed substances (lead, nitrate, PFOA, PFOS); UV reliably disinfects clear water.
- Moderate evidence Carbon filters for PFAS and microplastics – performance depends on design and cartridge age.
- Weak evidence Health benefits of filtering water that already meets limits.
- No evidence Claims that a softener “purifies” water or that a filter “revitalises” it.
Who should be cautious
- Families with infants and a well – nitrate is a risk when preparing infant feeds; neither carbon nor boiling removes it.
- People restricting sodium or with kidney disease – softened water contains more sodium, or potassium with potassium chloride salt. Do not change your treatment; talk to your doctor.
- People with weakened immunity – with a well, do not rely on a filter alone.
- Tenants – changes to plumbing require the owner’s consent.
What to look for when buying
- A filter chosen according to your water analysis, not advertising.
- NSF/ANSI certification or an equivalent test covering your target substances.
- Cartridge capacity, replacement interval and cost per litre.
- For whole-house systems: flow rate, pressure drop and a bypass.
- For reverse osmosis: the wastewater ratio and remineralisation.
- Red flag: “100% pure water” or “structured” water.
Conclusion
The best filter is not the most expensive one but the one that matches your water. Start with an analysis, choose an independently verified system, protect the house at the inlet and your drinking water at the tap – and change cartridges on time. A neglected filter is worse than none.