Water filters for the kitchen: the 4 designs and their limits

A kitchen water filter treats the water at the last point before the glass. Activated carbon binds chlorine, odours and organic substances to its inner surface. A hollow-fibre membrane with a pore size of 0.1 micrometres retains bacteria. An ion exchanger swaps calcium and magnesium for other ions. This reduces the hardness.

Adsorption, membrane filtration and ion exchange are handled by separate components. No filter medium does it all. Activated carbon does not retain dissolved salts, a membrane does not bind chemicals, an ion exchanger does not remove pesticides. It is the cartridge, not the design, that determines the filtration result.

The Trinkwasserverordnung (German Drinking Water Ordinance) sets its requirements for the water at the outlet of the draw-off point. An under-sink filter sits just upstream of it, a tap filter right at it, a jug downstream of it. All 3 alter water that already complies with the limit values. More on the legal situation can be found in our article on the German Drinking Water Ordinance.

The “TrinkWas” guide published by the Umweltbundesamt (German Environment Agency) in February 2020 contains a clear recommendation. It reads verbatim: “Do not install devices in the drinking water installation that are intended to additionally purify or treat the water.”

The same guide names exactly 2 exceptions. Both concern cases other than drinking water in the kitchen.

The first exception is very hard water with 2.5 millimoles of calcium per litre or more. In this case, DIN 1988-200 provides for softening or stabilisation if hot water is heated to above 60 degrees. The second exception concerns highly corrosive materials in the domestic plumbing. In that case, dosing approved treatment substances can be useful.

The reasoning behind this caution is given in the same chapter. Bacteria can multiply in filters that are not changed regularly. Overloaded filters release retained substances again, according to the authority “in this case even in higher concentrations”.

The recommendation on post-treatment of 15 December 2021 is more explicit. It holds that installation “purely as a precaution” is “to be rejected in principle”. Anyone who installs a device thereby takes on an obligation to maintain it. That is the yardstick against which every purchasing decision has to be measured.

The same recommendation names one case in which a point-of-use filter is justified. If contamination with microorganisms has been proven, the use of “bacteria-tight point-of-use filters” may be necessary as an immediate measure until the system has been remediated. The authority requires 2 things here: tracing the cause and eliminating it. A filter does not replace remediation, it bridges the gap until then.

Both sources consider one filter necessary, and it sits far away from the kitchen. The Verbraucherzentrale (German consumer advice centre) calls the particle filter at the house connection the “only sensible and also mandatory exception”. It sits after the water meter and retains rust particles from the pipes. DIN 1988 requires it. The Umweltbundesamt specifies backwashing it every six months. The particle filter has little to do with the water quality at the tap. It protects the pipes.

4 designs share the market: under-sink, countertop, tap and jug. They differ in the work required on the plumbing, the space they need and the flow rate.

The under-sink filter disappears into the cupboard and filters continuously. The tap filter sits visibly on the spout. It can be changed without tools. The table-top water filter needs no connection and filters in batches.

One point carries weight with permanently installed devices: they reduce the flow rate. Filtered water is unnecessary for the cooking pot and vegetables. Devices with a switch-over solve this. They route the water past the filter.

An under-sink filter needs 3 things. These are a free angle valve on the cold water supply, space for a housing around 122 millimetres high and a route upwards. The third point determines how much work is involved.

There are 2 options for the water outlet. A separate drinking water tap requires a second hole in the sink or worktop. A 3-way tap carries filtered and unfiltered water through the same tap, kept separate right up to the spout. If you want to keep your tap, choose a tap filter.

A stainless steel housing made of V4A 1.4404 measures around 122 millimetres in height with a diameter of 69 millimetres. The water pressure should be 2 to 3 bar. In German households, it is usually 3 to 4 bar.

Work on the drinking water installation is not a DIY job. The German Ordinance on General Conditions for the Supply of Water (AVBWasserV) governs who may carry it out. The water supplier and registered installation companies are authorised to do so. Details on installation can be found in our installation guide, and the housing variants in the overview of under-sink filters.

A tap filter is screwed onto the thread on the spout, where the aerator usually sits. Fitting takes less than 5 minutes. No tools are needed. The drinking water installation remains untouched. A tap filter does not require a specialist installer.

The thread should be measured before buying. M16 to M28 are common, both male and female. An outside diameter of 21.8 millimetres indicates M22, as does an inside diameter of 20.9 millimetres. For other sizes, an adapter helps.

The visible set-up is the price of the tap filter. The housing sits at eye level above the sink. In return, changing the cartridge takes only seconds. A lever separates filtered from unfiltered water. The designs for the spout can be found in the overview of tap filters.

Activated carbon binds substances to its inner surface through adsorption. 4 groups of substances are bound in this way, while 3 others require a different process. The dividing line runs between organic molecules and dissolved salts.

This division is already reflected in the type designations. The manufacturer lists the activated carbon cartridge as a pollutant filter and the membrane cartridge as a germ filter. On limescale, the manual states that the cartridge filters “bound limescale flakes, but no dissolved calcium carbonate”. The germ filter, in turn, captures “exclusively germs and bacteria down to a size of 0.1 μm”, according to the manufacturer. For microplastics, the product description names the activated carbon, but without a retention rate in %.

A separate restriction applies to lead. The Umweltbundesamt writes: “The use of filters to remove lead is not advisable.” Anyone who suspects lead pipes should have the water tested. According to the Verbraucherzentrale, houses built after 1973 do not have lead pipes. How water quality can be tested is explained in our article on measuring water quality.

An activated carbon cartridge gives no signal when its surface is saturated. The flow rate stays the same. The water looks the same. The taste changes slowly. 4 independent sources describe the same gap.

The manufacturer's manual puts it most directly: “The flow rate only decreases in rare cases. It is therefore important that the cartridges are changed at regular intervals, after 4 months at the latest.” With a germ filter, it is the other way round. There, the membrane visibly clogs up.

The Verbraucherzentrale describes the consequence. Filters “can, for example, break through unnoticed and release retained substances back into the water in concentrated form”. The Umweltbundesamt names the same mechanism and adds the microbial side. Stiftung Warentest measured the decline in filter jugs. There, the softening performance dropped after a quarter of the stated capacity.

This leads to a simple rule. The filter's effectiveness ends before any visible wear, and after that the re-release begins. The replacement interval is therefore not a recommendation by the manufacturer, but the condition under which the device fulfils its purpose at all. Anyone who postpones the change is not operating a weaker device, but a different one. It collects substances and releases them again. That is why the calendar comes before the design.

The purchase price is the smaller half of the bill. An under-sink unit with activated carbon costs CHF 118.90, the replacement cartridge CHF 25.90. With 3 changes a year, that comes to CHF 77.70. Over 5 years, the total is CHF 507.40. Of this, CHF 388.50 goes on operation.

Jugs cost significantly less to buy. Stiftung Warentest quotes 13 to 22 euros for the jug and 2.65 to 6.65 euros per replacement filter. Jug filters are changed more often. In terms of follow-up costs, the two designs are therefore closer together than the purchase price suggests. A pre-filter with a filtration rating of 40 micrometres can extend the service life of the main cartridge if the incoming water carries particles.

Compared with bottled water, the filter wins as soon as the household drinks regularly. Against unfiltered tap water, it loses. Tap water costs a fraction and already complies with the limit values.

3 reasons can support the decision to use a filter. None of them is “just in case”. The first is taste. Activated carbon reduces the smell of chlorine and a metallic aftertaste. That is a comfort argument, not a health argument. As such, it holds up.

The second reason is a specific measured value. Every water supplier publishes a drinking water analysis for its supply area. If you have this analysis or your own laboratory result, you can make a targeted choice. A conspicuous value points to the process needed. If levels of per- and polyfluoroalkyl substances (PFAS) are elevated in the supply area, activated carbon is effective; with nitrate, it is not.

The third reason lies in the domestic plumbing. Old pipes, long periods of stagnation or a building with an unclear history raise questions. The waterworks cannot answer them. The ordinance ends at the point of use. From there, the owner is responsible.

Some of the installation cases are solved by the tap rather than a filter. The Deutscher Verein des Gas- und Wasserfaches (German Technical and Scientific Association for Gas and Water) describes a graded scale in its Kompaktinfo fact sheet of March 2024. After one night of stagnation, the concentration of heavy metals may be elevated. From 72 hours up to 7 days, microorganisms also multiply. The recommendation is to replace the water “every seven days, preferably every three days”. Letting the water run costs nothing and works immediately.

3 points taken together speak against a filter: a clean analysis, short pipe runs and the prospect of forgetting to change the cartridge. An unmaintained filter is worse than no filter. Analysis, pipe run and maintenance discipline come before the purchase decision.