Enterococci and E. coli in tap water

Reading time: ~11 minutes

Few messages cause as much alarm as a letter or push notification from the water company: bacteria have been found in the drinking water, boil your water for three minutes before use. Usually it concerns enterococci or E. coli, the bacterium colloquially also known as the "poop bacterium".

This article explains what these two bacteria are, why drinking water companies specifically watch for them, what you can and cannot do during a boil-water advisory, and what filtration does and does not solve in this story. That last point is important, because that is where most things go wrong online: a water filter is not a disinfectant. If you are looking for general background, start with our explanation of whether tap water is healthy.

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1. What are enterococci and E. coli in tap water?

Drinking water companies cannot test for every possible pathogen. That is why they look at these two. Vitens explains it like this: enterococci and E. coli do not make people ill themselves, but if they are found in the drinking water, that is a warning that other, disease-causing bacteria may also be present in the water.

The standard is strict and leaves no room. Under the Drinkwaterbesluit, according to the glossary from Vitens, a maximum of 0 CFU per 100 millilitres applies to both bacteria, meaning literally zero colony-forming units in a sample of one tenth of a litre. There is no permitted amount: a single detection is already an exceedance and leads directly to measures.

The difference between the two lies mainly in the measurement speed. For E. coli, the culture result is known after one day; for enterococci this takes two days because that bacterium grows more slowly. That is why a boil-water advisory involving enterococci often lasts longer in practice than people expect.

2. How do these bacteria get into tap water?

Dutch drinking water is purified and delivered under controlled conditions, so a bacterium never enters it spontaneously. According to the RIVM, it involves contamination from outside: during works or with a broken pipe, faecal matter from humans or animals can end up in the drinking water. Vitens names the same two causes, a pipe burst or works on the network, and always looks for the source first whenever bacteria are found.

In practice, this comes down to the following:

• Pipe burst: in the event of a burst, contaminated groundwater can enter the pipe network, especially if the pressure briefly drops.
• Works: when a pipe is being laid or repaired, the network is opened up and contamination can enter.
• Pressure loss: if the pressure drops away, water from the surrounding area can be drawn in. More on this in our explanation of what to do when no tap water comes out of the tap.
• Inside the home: an incorrectly connected appliance or a pipe that has stood still for a long time inside the house can also affect the water quality at the point of use.

That last point is exactly why filtration at the tap exists: the quality the water company delivers is not automatically the quality that comes out of your own tap. If you first want to know what types of water filters there are and where they differ, you will find the complete overview there.

3. Do you get sick from enterococci or E. coli in drinking water?

Usually not, and that is important to know before the alarm sets in. The RIVM states that healthy people generally do not become ill from E. coli or enterococci themselves. The risk lies in what may have come along with them: other bacteria, viruses and parasites from faecal matter that can cause illness, with gastrointestinal complaints as the most common consequence.

For four groups, the consequences of contaminated drinking water can, according to the RIVM, be serious:

• Young children and babies: a small body loses too much fluid more quickly during diarrhoea. See also our explanation of water for baby formula.
• Older people: a less strong immune system and a higher risk of dehydration.
• Pregnant women: extra caution is standard advice.
• People with a weakened immune system: for example during or after treatment.

Vitens puts the risk into perspective without dismissing it: most customers notice nothing, and anyone who swims in a recreational lake or eats unwashed vegetables probably takes in more bacteria. The boil-water advisory exists so as to take no risk at all for the people who are indeed sensitive to it. It is a precautionary measure, not an alarm.

4. Boil-water advisory: what you can and cannot do

During a boil-water advisory, the RIVM's advice is to first boil water you want to drink for three minutes. After boiling, all pathogens are gone from the water. Using unboiled water for showering, brushing your teeth or washing up is not dangerous. A water quality advisor at Vitens sums it up concisely: boiled water is just as safe as bottled water.

Three minutes is not an arbitrary number, nor is it just "warming it up a bit". It means three minutes of actual, continuous boiling, counted from the moment the water starts boiling. A kettle that switches off automatically as soon as boiling point is reached will not reach that time: use a pan instead, or switch the kettle back on a few times. If in doubt, always follow the advice of your own drinking water company, as it knows exactly what has been found.

No water company can say in advance how long a boil-water advisory will last. Vitens only lifts the advisory after two clean test results in a row, and because enterococci need two days to be counted reliably, this takes time.

5. How often does this really happen in the Netherlands?

Rarely, and this can be substantiated with figures. According to Vewin, the drinking water companies carried out a total of 565,689 measurements in 2023, with 99.9 percent of the samples meeting the statutory standards. In 601 cases the standard was not met, and in 24 of those cases it involved enterococci or E. coli.

Twenty-four microbiological exceedances out of well over half a million measurements: that is the context that rarely comes across in news reports. The drinking water is checked against more than 60 different parameters, according to Vewin. Vitens alone already takes around 200,000 water samples a year, at extraction wells, in storage cellars, in pipes and at thousands of people's homes, 365 days a year.

6. Does a water filter help against bacteria in tap water?

Here we deliberately deviate from what you read elsewhere. A household drinking water filter is not a disinfection installation and is not a replacement for a boil-water advisory. If there is a boil-water advisory in your area, boil for three minutes, even if you have a filter.

There is a technical reason for this. The PureFilter Mineral+ is certified to NSF 42, 53, 61 and 372. As NSF International itself describes, NSF 42 covers substances without a direct health effect, such as chlorine, taste and odour, and NSF 53 covers health-related chemical contaminants; 61 and 372 cover the safety of the materials themselves. These are not certifications for rendering bacteria and viruses in a contaminated pipe network harmless. Anyone who promises that a carbon filter or a reverse-osmosis system solves a contamination is selling a certainty that cannot be given.

Why boiling is unbeatable here

Boiling works on precisely the problem a boil-water advisory points to: micro-organisms. It does nothing against chemical substances, because those do not disappear through heat. Filtration works on precisely the opposite: chemical substances that the standard does not fully cover, but not on an acute microbiological contamination. The two are not competitors, they solve different problems.

At PureFilter we see that most questions about bacteria come in during a boil-water advisory in the region, while the customers who eventually buy do so for a completely different reason: they want cleaner and softer water every day, not an emergency solution for a disruption lasting a few days.

7. Boiling, bottled water or a filter: the comparison

In short: boiling is the right choice during a boil-water advisory, bottled water is an expensive stopgap, and a filter is the daily solution for the substances boiling does nothing about. If you want to see that trade-off more broadly, our comparison of the best water filter per situation will help.

8. After the boil-water advisory: what to do in your home

The advisory has been lifted, so now what? Water companies only lift a boil-water advisory once all samples are clean in the laboratory, so the water from the street mains is fine at that point. In your own home, at that moment, there is still water in pipes and appliances that entered during the contamination. These four steps take little time and remove that doubt:

• Flush the pipes: let the cold water tap run freely for a few minutes, so the standing water is flushed out.
• Empty appliances: throw away the contents of the kettle, coffee maker, ice machine and water jug and refill them.
• Discard ice cubes: cubes made during the boil-water advisory are best thrown away. Freezing does not render bacteria harmless.
• Assess the filter cartridge: if you have a filter jug or a filter that water flowed through during the contamination, consider a fresh cartridge. You do not clean a saturated cartridge, you replace it.

The same applies to the hot water side. Hot water from the tap is not intended for drinking for other reasons, which we explain in our blog about drinking warm tap water.

9. Where a water filter does make a difference

A boil-water advisory lasts days. The substances people filter for have been there for years. That is the real difference in time horizon, and it is the reason filtration at the point of use makes sense in a country where, according to the ILT figures, drinking water meets the standards in 99.9 percent of measurements.

The PureFilter Mineral+ is a compact under-counter system that you connect to the cold water pipe. In five steps it removes more than 100 contaminants, up to approximately 99 percent, including PFAS, microplastics, heavy metals such as lead and copper, fluoride, nitrate, medicine residues and chlorine. Swedish Mineral Rock™ then adds magnesium, calcium and potassium back in, so the water is not only pure but also soft and full of flavour. This sets it apart from an osmosis system, which strips out everything including the minerals.

Two themes come up most often here. The first is PFAS, for which we have a dedicated page about the water filter against PFAS. The second is substances that barely feature in the regular monitoring programme, such as hormone-disrupting substances in tap water. The same principle applies to both as to bacteria: knowing what the technology does and does not do is worth more than a big promise.

10. Buying advice

Do not buy a filter to solve a boil-water advisory. Buy one if you want pure, soft water from your own tap every day, without refilling jugs or hauling bottles. That is the question an under-counter filter answers.

The PureFilter Mineral+ costs €149.00 including the first filter and is installed under your counter within 5 to 10 minutes, without a plumber and without tools. The water comes from your existing tap. A replacement filter lasts up to 12 months and costs €65.00, less than one cent per litre. By our own calculation, that is a factor of ten lower than the €1 to €2 per day a household spends on spring water. You get 40 days of free returns and a 5-year warranty. Feel free to compare the range of water filters for the home, or go directly to the page where you can buy a water filter.

11. Frequently asked questions

12. Conclusion

Enterococci and E. coli in tap water are not an everyday contamination but a warning sign: they should not be there, the standard is zero, and if they are found a boil-water advisory follows. Boiling for three minutes resolves that, and the figures show how exceptional it is, with 24 microbiological exceedances out of 565,689 measurements in 2023.

The honest conclusion about filtration is therefore twofold. During a boil-water advisory, you rely on boiling, not on a filter. Outside of that, filtration at the point of use is exactly the place where you catch the substances boiling leaves behind and that the standard does not fully cover: PFAS, lead from your own pipes, medicine residues and microplastics. Anyone who understands both no longer needs to doubt what to do the next time the water company sends a message.