Germ growth in jug filters: figures and evidence
A study from Freiburg found rising germ counts in 24 of 34 filter jugs, up to 6,000 per millilitre. What the figures mean for this type of filter.
A jug filter is a table-top water filter with loose activated carbon in a cartridge. Studies on germ growth have so far examined almost exclusively this one type. A study from Freiburg measured rising germ counts in 24 of 34 filters in 1996. In 2022, Stiftung Warentest (the German consumer testing organisation) found germs in all 8 jugs tested. This article puts the figures into context. And it separates the type of filter from the technology.
The short answer in 5 figures
So far, reliable germ measurements on household filters are available only for jug filters. They are clear-cut. Five figures from 2 studies outline the situation.
| Figure | Finding | Source |
|---|---|---|
| 24 of 34 | household filters with rising germ counts in the filtrate | Freiburg 1996 |
| 6,000 per ml | highest colony count measured in the filtered water | Freiburg 1996 |
| 10,000-fold | factor compared with tap water in extreme cases | Freiburg 1996 |
| 5 of 13 | brand-new cartridges with bacteria or moulds | Freiburg 1996 |
| 8 of 8 | tested filter jugs in which bacteria were found | Stiftung Warentest 2022 |
The Drinking Water Ordinance's limit value for the colony count is 100 per millilitre, measured at the tap. The highest measurement from Freiburg reaches 60 times that value.
What the 1996 Freiburg study measured
Freiburg University Hospital tested 34 filter jugs from households and 6 more in the laboratory. The study was carried out at the Institute of Environmental Medicine and Hospital Hygiene. It was published in 1996 in the European Journal of Clinical Microbiology and Infectious Diseases, volume 15, page 233.
In 24 of the 34 household filters, the germ count in the filtrate increased. The peak value was 6,000 colony-forming units per millilitre. A colony-forming unit, CFU for short, stands for one germ capable of reproducing. Filtrate is the name given to the water after it has passed through the filter. In individual cases, the colony count in the filtrate reached 10,000 times that of the tap water.
The laboratory part explains the mechanism. In 4 of the 6 laboratory filters, the germ count in the fresh filtrate exceeded that of the tap water after around one week of operation. From this, the authors conclude that growth or biofilm formation takes place in the filter material itself. A biofilm is a slimy layer of microorganisms that adheres to a surface.
The study ends with an appeal to the regulators. Authorities should ensure that household filters maintain water quality.
What Stiftung Warentest found in 8 filter jugs in 2022
Stiftung Warentest tested 8 filter jugs and wrote on 22.06.2022 in issue 7/2022 of its magazine test: ‘Superfluous to harmful’. The quality ratings range from ‘satisfactory’ to ‘poor’. ‘Satisfactory’ was thus the best rating in the field.
The laboratory detected bacteria in all 8 jugs. In one model, moulds were also found. The test thus confirms, 26 years after Freiburg, the same basic finding on newer devices.
A second finding concerns capacity. The test water had 16 to 17 degrees of German hardness. Fresh cartridges initially turned it into soft water. After just the first quarter of the stated capacity, only medium-hard water came out. Softening therefore ends long before the indicator shows it.
The prices put the costs into perspective. Replacement cartridges cost 2.65 to 6.65 euros each. A jug cost around 13 to 22 euros.
Why all 4 sources examine the same filter type
4 key pieces of evidence on germ growth all relate to table-top devices with loose fill. Filters on the tap or under the sink are absent from all 4 sources.
| Source | Type | Date | Subject |
|---|---|---|---|
| Freiburg study | Series of measurements | 1996 | 34 household jugs, 6 laboratory jugs |
| Stiftung Warentest, test 7/2022 | Laboratory test | 22.06.2022 | 8 filter jugs |
| VerbraucherService Bayern | Assessment | 16.11.2023 | Table-top water filters |
| Verbraucherzentrale | Assessment | 22.04.2025 | Table-top water filters |
The common claim that filters are germ breeding grounds thus rests on a narrow basis. It holds true for the type tested. There, it is well documented. On the basis of the available sources, it can only be applied to other types with caution.
This is precisely where the claim reaches its limits. Drawing conclusions from the jug about permanently installed filters would require dedicated series of measurements. Such series are so far lacking in the openly available literature.
The 2 conditions for germ growth
Two causes drive growth in the jug: water stands at 20 to 25 degrees, and the granules offer a large internal surface area. Both are inherent to the design and occur together. The first concerns the container, the second the cartridge. A jug filter combines both. It collects water and uses loose fill to do so.
Stagnant water at room temperature
Filtered water collects in an open container. It often stands there for hours. Stagnant water is water that stands in a container for a long time. At 20 to 25 degrees Celsius, germs multiply rapidly in it. In summer, it happens even faster.
Tap water already carries individual germs at the tap. As long as it stays cool and keeps flowing, their number remains low. In the warm jug, both protective factors disappear. Which pathogens matter here is explained in the article on bacteria in tap water.
Loose granules as a surface for colonisation
The cartridge contains loose activated carbon. Depending on the model, small synthetic resin beads for ion exchange are added. VerbraucherService Bayern (Bavarian consumer advice service) states: ion exchangers and activated carbon, with their fine cavities, provide an ideal breeding ground for bacteria and germs.
The surface that binds substances through adsorption serves as a habitat for germs. Adsorption refers to the accumulation of dissolved substances on a surface. The structure and key parameters of the carbon are explained in the article What is activated carbon.
The fridge addresses 1 of 2 causes
As of 22.04.2025, the Verbraucherzentrale (German consumer advice centre) advises storing filtered water in the fridge. This advice addresses the first cause. The second remains.
Cold significantly slows down multiplication in standing water. However, the laboratory part of the Freiburg study examined both temperature levels. In 4 of 6 filters, the germ count in the fresh filtrate was higher than that of the tap water after around one week. Measurements were taken at room temperature and at 4 degrees.
So even at 4 degrees, the germ count rose. Cold alone therefore merely slows the increase. If you keep the jug cool, you slow down multiplication in the water. The biofilm in the cartridge continues to grow.
This results in an order of priority. Changing the cartridge has a stronger effect than cold. Both together work best.
Brand-new cartridges put to the test
Of 13 brand-new cartridges, the material already carried germs or mould in 5 cases. The finding comes from the same study and concerns unopened products.
This figure puts things into perspective. According to it, a fresh cartridge starts out already colonised in around 38% of cases. Growth in the jug then starts from a higher level.
Two practical consequences follow from this. Firstly, it is worth rinsing a new cartridge as instructed. One to two fillings are usual. Secondly, a fresh cartridge is only of limited value as proof of hygiene.
More recent figures for the German market are not yet available. The 1996 figure thus remains the most reliable information on new products.
What the replacement indicator measures
A replacement indicator records the time since the last change, as VerbraucherService Bayern states (as of 16.11.2023). It says nothing about the quality of the water.
The difference matters because the two end points do not coincide in time. According to Stiftung Warentest, softening ends after just the first quarter of the capacity. Colonisation, by contrast, depends on temperature and standing time. It is therefore tied to usage.
There is also a second effect. The Verbraucherzentrale writes on this (as of 17.11.2025): filters can break through unnoticed and release retained substances back into the water in concentrated form. The technical term for this is breakthrough.
The article on water filter cartridges compares replacement intervals and types.
Silver in the granules migrates into the water
Some manufacturers add silver to the granules to slow down germ growth. VerbraucherService Bayern points out the downside: over time, the silver is washed out, ends up in the drinking water, and the germs continue to multiply.
Two consequences follow from this. The effect against germs decreases over the service life. The granules still offer surface area. And the dissolved portion ends up in the glass.
There is currently no information from the authorities on the amount of silver released in milligrams per litre. This article therefore only describes the mechanism.
How silver works in the water tank of motorhomes and boats is shown in the article on silver ions in the water tank.
What a jug filter retains
A jug filter reduces hardness, binds chlorine and improves the taste. For 5 other groups of substances, retention remains low, including metals, hormones and germs.
| Substance group | Jug filter performance |
|---|---|
| Limescale and water hardness | Reduction by ion exchange |
| Chlorine | Reduction by adsorption |
| Substances affecting taste and odour | Binding to the activated carbon granules |
| Bacteria and germs | low retention |
| Lead, copper, nickel | low retention |
| Hormones and pharmaceutical residues | low retention |
| Pesticides and nitrate | low retention |
| Microplastics | low retention |
The purpose of this type of filter is taste, for tea and coffee, for example. If you want to tackle heavy metals in tap water or microplastics in drinking water, you need a different method.
Where the Drinking Water Ordinance ends
The Drinking Water Ordinance, TrinkwV for short, applies up to the point of use. From the tap onwards, hygiene is the user's responsibility. What happens in the jug is therefore outside its scope.
Up to the tap, strict values apply, for which the water supplier is responsible. Annex 1, Part I sets 0 colonies in 100 millilitres for both Escherichia coli and intestinal enterococci. Annex 3, Part I lists coliform bacteria as an indicator parameter with the same value. For the colony count at 22 and at 36 degrees, a limit value of 100 per millilitre applies in each case.
The federal report on drinking water quality of February 2025 measures compliance for 2020 to 2022. Suppliers met the Escherichia coli requirement in 99.97% of tests and the intestinal enterococci requirement in 99.89%.
Legally, the jug and cartridge count as consumer goods. Food law applies to them. The water that runs into the jug complies with the Ordinance. For the water that goes into the glass hours later, there is no testing obligation at all.
Risk of germ growth in the 6 methods
Six common methods differ markedly in their germ risk. Two points are decisive: whether the device stores water, and whether it stands idle between draws.
| Method | Risk of germ growth | Cause |
|---|---|---|
| Jug filter | high | Stagnant water at room temperature, loose fill |
| Ion exchanger | elevated when idle | Synthetic resin as a surface for colonisation |
| Reverse osmosis | elevated at the membrane | Suspended solids collect on the membrane |
| Distillation | low | Bacteria remain in the residue during evaporation |
| Activated carbon block filter | low | solid block, flow-through instead of storage |
| Ultrafiltration | low | Membrane retains germs mechanically |
An ion exchanger swaps dissolved ions for others, for example calcium for sodium. Its medium usually consists of synthetic resin. This serves germs as a surface.
In reverse osmosis, pressure forces the water through a fine membrane. Suspended solids collect in front of it. This creates a zone in which germs grow. All methods are presented in the article on filtration technologies and filtering methods.
What activated carbon block filters do differently
An activated carbon block filter consists of sintered carbon in a solid block and works in 3 ways. Sintered means bonded under heat and pressure. Compared with loose fill, 2 things change. During operation, the block only holds its own volume. And every draw flushes it through. The 3 ways in detail:
- Mechanical: The fine channels retain larger particles, such as sand, rust and asbestos.
- Catalytic: In the pores, the block converts certain substances, including chlorine.
- Adsorptive: Substances accumulate on the internal surface, including lead, copper and hormone residues. Van der Waals forces are at work here.
A block filter therefore covers a much wider range than loose fill, including hormones, pesticides, pharmaceutical residues, microplastics, asbestos, metals and chlorine. How well it retains germs depends on the pore size. Only a membrane provides reliable retention. The different designs are explained in detail in the article Activated carbon filters for water.
Which filter type blocks germs mechanically
Ultrafiltration retains germs mechanically. Its membranes have pores of 2 to 100 nanometres, while bacteria measure 200 nanometres or more. Adsorption alone only partly achieves this. It binds substances instead of blocking them.
Combining the two closes the gap. The activated carbon binds chlorine, metals and organic residues. The membrane stops the germs. This is exactly how the Nano cartridge from Alb Filter works. According to the manufacturer, its hollow-fibre membrane has a pore size of 0.1 micrometres and retains 99.999% of germs.
As an under-sink filter or directly on the tap, the cartridge is permanently fitted in the line. Between draws, it only holds its own volume. This eliminates both of the causes that characterise the jug filter.
For everyday use, an activated carbon cartridge such as Active Plus is sufficient. If legionella is the concern, the article on legionella filters goes further. The page on drinking water filters gives an overview of all types.
Stiftung Warentest and Alb Filter
To date, there is no Stiftung Warentest rating for Alb Filter. The test in issue 7/2022 of test magazine covered 8 filter jugs. Permanently installed filters on the tap or under the sink were not included.
On 26.02.2009, the foundation published a general assessment of filter and water softening systems in the home. A product test of permanently installed drinking water filters is therefore still outstanding.
The testing basis behind the manufacturer's information therefore remains decisive. When comparing, look at the test standard, testing institute and test date.
If you want to know the water quality in your own home, take a sample at the tap. The procedure is explained in the article How do you measure water quality.
6 steps for hygienic use
If you stick with a filter jug, you can reduce the risk with 6 steps. The order follows their effectiveness. The most effective step comes first.
- Change the cartridge earlier than indicated. Softening ends after the first quarter of the capacity. The indicator merely counts time.
- Empty the jug every day. Filtered water should be drunk on the day it is filtered.
- Store filtered water in a cool place. At 4 to 7 degrees in the fridge, the germ count grows much more slowly.
- Rinse the jug with hot water and leave it open to dry. Germs grow poorly on dry surfaces.
- Flush a new cartridge. Discard one to two fillings, as specified by the manufacturer.
- Remove the cartridge before going on holiday. Put a fresh one in when you return.
Permanently installed cartridges follow a different logic. Alb Filter specifies a change after 4 months at the latest for its own cartridges. The deciding factor there is performance. It decreases over the service life.
Related to this topic
Alb Filter FUSION – permanently installed instead of an open jug
The FUSION under-sink drinking water filter sits directly in the line: no standing water, no open tank and therefore without the typical risk of germ growth associated with jug filters. The activated carbon block and Nano ultrafiltration filter in a single pass.
Frequently asked questions
Do jug filters really become contaminated with germs?
Yes. A 1996 study from Freiburg found rising germ counts in the filtrate in 24 of 34 household filters, peaking at 6,000 colony-forming units per millilitre. In 2022, Stiftung Warentest detected bacteria in all 8 filter jugs tested, and moulds as well in one model.
Does a jug filter remove bacteria and germs?
Retention remains low. Loose granules bind taste-affecting substances and chlorine. They reduce hardness. Retaining bacteria requires a membrane, such as one used in ultrafiltration with pores of 2 to 100 nanometres.
How often do you need to change the cartridge in a jug filter?
Depending on the model, manufacturers specify around 100 to 150 litres or 4 weeks. In 2022, Stiftung Warentest measured a drop in softening after just the first quarter. An earlier change is therefore closer to the actual performance.
Does it help to put the filter jug in the fridge?
Partly. Cooling slows down multiplication in standing water. However, the laboratory part of the Freiburg study measured the increase in the filtrate at room temperature as well as at 4 degrees Celsius. The authors attribute this to growth in the filter material.
Are brand-new filter cartridges free of germs?
Of 13 brand-new cartridges, the material carried bacteria or moulds in 5 cases. That is around 38%. It is therefore worth flushing the cartridge before using it for the first time.
What does silver in the filter cartridge do?
Silver compounds inhibit germ growth in the granules. VerbraucherService Bayern points out that silver is washed out over time, gets into the drinking water and that germ multiplication then continues.
Do permanently installed water filters become contaminated with germs in the same way?
The four key pieces of evidence relate exclusively to table-top devices. A block filter at the tap consists of solid material. Every time water is drawn, it is flushed through. The openly available literature contains no separate measurement series for permanently installed filters.
What does the Drinking Water Ordinance say about germs in filter jugs?
The Ordinance applies up to the point of use. At the tap, Annex 1 Part I permits 0 colonies each of Escherichia coli and intestinal enterococci per 100 millilitres. For the jug beyond that point, the rules for food contact materials under food law apply.
What is the limit value for the colony count in drinking water?
Annex 3 of the Drinking Water Ordinance sets a value of 100 per millilitre for the colony count at both 22 and 36 degrees Celsius. The highest Freiburg measurement of 6,000 per millilitre is 60 times higher.
Has Alb Filter been tested by Stiftung Warentest?
Stiftung Warentest has not yet issued a test rating for Alb Filter. The test in issue 7/2022 of test magazine covered 8 filter jugs; permanently installed filters were not included.
Does a jug filter remove microplastics or heavy metals?
Retention is low in both cases. A block filter retains lead, copper, nickel, hormones, pesticides, nitrate and microplastics much more reliably. It works both mechanically and by adsorption.
How can you tell if a filter jug is contaminated with germs?
Only a laboratory sample provides reliable evidence. In the quantities in question, germs remain invisible and cannot be tasted. A film at the bottom of the jug, a musty smell or cloudiness indicate advanced colonisation.
Sources
- Daschner et al., Microbiological contamination of drinking water in a commercial household water filter system — 1996, with the Freiburg measurements used in this article
- Stiftung Warentest, “Wasserfilter im Test” (water filters tested), test magazine issue 7/2022 of 22.06.2022, with the bacterial findings and the capacity measurements
- “Wasserfilter aus hygienischer Sicht” (water filters from a hygiene perspective), Verbraucherzentrale — As of 22.04.2025, on storage in the fridge
- “Wasserbehandlung im Haushalt” (water treatment in the home), Verbraucherzentrale — As of 17.11.2025, on filter breakthrough and the particle filter at the house connection
- “Tischwasserfilter” (table-top water filters), VerbraucherService Bayern im Katholischen Deutschen Frauenbund — As of 16.11.2023, on breeding grounds for germs, silver and the replacement indicator
- Trinkwasserverordnung (German Drinking Water Ordinance), full text — of 20.06.2023, Annex 1 Part I and Annex 3 Part I
- “Bericht über die Qualität von Wasser für den menschlichen Gebrauch 2020 bis 2022” (report on the quality of water for human consumption, 2020 to 2022) — Bundesministerium für Gesundheit (Federal Ministry of Health) and Umweltbundesamt (German Environment Agency), February 2025
