Among my friends it is a sort of 'common wisdom' that you should throw away water after a couple of days if it was taken from the tap and stored in a bottle outside the fridge, because it has 'gone bad'. First of all, the couple of days is not very well defined, which already makes me a bit suspicious. Second, I cannot think of anything in tap water that would make the water undrinkable after a couple of days already.

Can someone clarify this issue for me? Does tap water really 'go bad' after a couple of days outside the fridge? Why?

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    $\begingroup$ I suspect that "going bad" has more to do with changes in taste than actual health hazards. I believe chilled water tastes different from room-temperature water due to the lower amount of dissolved gasses in the latter. If water is left in an open cup over a few days, then it starts gathering matter which floats in the air, which may impart a disagreeable taste if a sufficient amount accumulates. Tap water probably isn't a very good culture medium, as it is relatively poor in organic nutrients. Of course some microorganisms will develop in exposed water, but they shouldn't multiply too much. $\endgroup$ Mar 23, 2014 at 13:57
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    $\begingroup$ Tap water outside the fridge but in a closed container should be perfectly fine for months though. After a very long time, eventually some photosynthetic organisms will make it inside and very slowly reproduce by creating organic matter out of $\ce{CO2}$ in the air and whatever residual light they can gather. $\endgroup$ Mar 23, 2014 at 14:01
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    $\begingroup$ I'm wondering if part of the origin of this idea is due to reusable water bottles (you didn't say if they were sterile or not) that are often not properly cleaned between uses as 'they've only had water in them' and are contaminated with bacteria and microscopic food particles from your mouth, the bacterial colonies then produce the same compounds that cause bad breath and other unpleasant tastes and the water in the bottle starts to taste and smell bad. Refreshing the water temporarily solves this problem, though washing the bottles properly would be better. $\endgroup$
    – aml
    Jan 6, 2016 at 14:57
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    $\begingroup$ Pseudomonas aeruginosa seems to always find a way, even in bottles of deionized water that are left out. In such an environment, this microbe can produce fluorescent green toxins. $\endgroup$
    – CCovey
    Jan 21, 2016 at 20:46

5 Answers 5


First of all, it depends on how the tap water was treated before it was piped to your house. In most cases, the water was chlorinated to remove microorganisms. By the time the water arrives at your house, there is very little (if any) chlorine left in the water. When you fill you container, there is likely to be some microorganisms present (either in the container or in the water). In a nutrient rich environment, you can see colonies within 3 days. For tap water, it will probably take 2 to 3 weeks. But that doesn't mean that the small amount of growth doesn't produce bad tasting compounds (acetic acid, urea, etc.).

BTW Nicolau Saker Neto, cold water dissolves more gas than hot water. Watch when you heat water on your stove. Before it boils, you will see gas bubbles that form on the bottom and go to the surface (dissolved gases) and bubbles that disappear while rising to the surface (water vapor).

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    $\begingroup$ In the Netherlands, where I live, the chlorination of water completely stopped in 2005. At the moment they use ozone or UV-light for the disinfection. So it is safe to say that there is no chlorine (left) in the water ;) $\endgroup$
    – Michiel
    Mar 24, 2014 at 6:19
  • $\begingroup$ @Michiel Even if primary disinfection is done without chlorine I doubt there is no residual chlorine dripped into the water before it leaves the treatment facility. This is not meant to clean the water but to keep it clean. The amount of chlorine put into the water is a function of how long the operators expect the water to be in a pipe before coming out of your tap. $\endgroup$
    – Brad
    Mar 24, 2014 at 20:01
  • $\begingroup$ @Brad It's apparently really the case but here is a critical view. $\endgroup$
    – Gala
    Oct 1, 2014 at 18:45
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    $\begingroup$ Also in Germany tap water is not chlorinated at all. Chlorination is actually forbidden by law since 25 years. A reasonably well maintained pipe system in a country with median temperature around 10°C never fouls. $\endgroup$
    – Karl
    Mar 30, 2016 at 23:29

I cannot think of anything in tap water that would make the water undrinkable after a couple of days already.

Tagging the question with probably points in the right direction. The only effect I can think about is growth of anaerobic microbes, e.g. Escherichia coli, causing diarrhea.

There's good chance to keep the water from 'going bad' and even 'turn it good' again when keeping the water in PET bottles in the sunlight, letting SODIS (solar disinfection) work for you.

  • $\begingroup$ Ok, seems logical that it needs to be anaerobic when the bottle is closed, but how does the E. coli. end up there in the first place? $\endgroup$
    – Michiel
    Mar 23, 2014 at 14:39
  • $\begingroup$ E. coli grows in the lower intestines of most mammals, leaves the body via defecation. Sewage contamination of drinking water will most likely lead to elevated concentrations. Lack of proper water treatment will keep it there, giving it a chance to grow. $\endgroup$ Mar 23, 2014 at 14:48
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    $\begingroup$ @Michiel Unless something has been recently gamma irradiated or heavily boiled you can pretty much assume a very small amount of all common bacteria is in everything $\endgroup$ Mar 23, 2014 at 15:11

A few lazy weeks of summer cottages, camping far from the beaten track or boat excursions in the archipelago: When summer comes, we seek to be closer to nature. Sometimes that means we may waive some of everyday life's certainties, like drinking water straight from the tap.

Do this!

Fill the drinking water of good quality, such as municipal drinking water in sterilized bottles or cans. Keep the water dark and as cool as possible. That keeps drinking water fresh for at least one week.

Smell and Taste

  • Putting a limit on how long drinking water can be stored has more flavoring to it than health risks, says Torbjörn Lindberg, State inspector at NFA.
  • The risk of getting sick of water is small in Sweden. But you should of course do not drink water that tastes or smells bad, is turbid or highly colored.

Get microorganisms in drinking water

Good drinking water does not contain pathogenic microorganisms, such as viruses and bacteria. It is also a nutrient-poor environment that makes it difficult for such microorganisms to survive. They are simply ill-suited to the aquatic environment. If they are there from the beginning, it is therefore likely that they will disappear over time, and the risk of getting sick is reduced.

Drinking water can be bad

Sometimes, bacteria, micro-sponges and algae that are adapted to the aquatic environment are found in the drinking water from the start. They can also multiply over time. It is completely natural and gives no health risks. However, drinking water can start to smell or taste badly, especially if kept long and hot, for instance in the sun. It can also be colored and can form a slimy layer on the inside of the container.

  • $\begingroup$ Please link to the sources that you used or indicate if this comes from your own experience. $\endgroup$
    – tschoppi
    Mar 24, 2014 at 15:43

I am not an expert and its been a long time since I last studied chem and bio. The best answer will probably only come from a biologist. But out of interest, I will try to answer. Also, I suggest that you post this question on websites such as http://www.askabiologist.org.uk/ . Its likely that you will get some good answers there as well.

Assuming that the water in the bottle is thoroughly "purified" at the water plant , there has been absolutely no contamination (pipes, faucet etc.) on the way, and the bottle and handler are clean, then it is unlikely that water will go bad. Example that suggests contamination at faucet - http://www.journalofhospitalinfection.com/article/S0195-6701(98)90295-X/abstract

I don't think that most bottles are 100% air-tight. So, the next thing I would check is if there are any water-borne pathogens that can spread by air. If such pathogens exist, then is it possible for them to enter the bottle through small "air spaces" ? That seems unlikely to me.

So, if water is microbe free and can be kept that way by a bottle, then it should be good for a "long" time.


It depends on the water. If the water is already contaminated, then after a couple of days, there are more chances of water to go bad.

But if the water is mineral, already purified, then it takes too many days.

Also the other fact about the sunlight, if the water is directly faced with sunlight, then sunlight causes formation of the algae in water.

In simple words, you can drink a water, if it is a mineral water, then after couple of days, if it is a tap water, 2-3 days normally.

This is my individual opinion, I am not a doctor. You follow what you want. :)


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