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In cases where tap water is not drinkable by default, does simply boiling it make it drinkable? And provided the water does look clear.

(For example I'm in an apartment in Pridnestrovi where the tap is connected to both a 'cold water supply' and a 'lukewarm hot water supply'. I asked the host who said to boil the water from the 'cold water supply' to drink. But would the water not be drinkable if I boil the water from the 'lukewarm hot water supply' instead?)

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    This is not a 'The Great Outdoors Stack Exchange' question, but on our sister site you will find more 'cleaning water' questions. outdoors.stackexchange.com/questions/tagged/water-purification
    – Willeke
    Jul 23, 2021 at 14:05
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    I think it depends on why the water of the hot water supply is not safe to drink. If it is because of for instance lead pipes you get a different answer than if it is because of an open to the air water tank on the roof.
    – Willeke
    Jul 23, 2021 at 14:07
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    See Tetsujin's comment on his own question re Legionella. A hot water system not designed to address Legionella can be lethal. Jul 26, 2021 at 1:29
  • Did you truly not realise, it could never matter whether the water looked clear? Either way, where tap water is not drinkable by default, simply boiling it should remove any biochemical pollutant. However, boiling should have no effect on simple physical or chemical pollutants, including acids, alkalis or poisons like chlorine or, for instance, most of what's listed at kansashealthsystem.com/care/centers/poison-control-center/hazardous-chemicals Jul 26, 2021 at 19:54

6 Answers 6

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You can not see whether water needs boiling, filtering or other ways of cleaning.

Clear water can have a chemical contamination which you cannot get out with 'in house' cleaning methods.
Lead pipes do add tiny specks of lead which you also cannot see, or not see with your bare eyes.

While things you can see may not be nasty, just visible. Like many kind of algae.

So check with the people who told you that you can not use the hot water supply, and have to boil the cold water supply, and do what they tell you or buy bottled water in the mean time.

It can be that the hot water supply is no worse than the cold water supply, but it can also have additional problems. And until you know what those problems are, you can not know what you need to do to make that water drinkable.

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    +1 for mentioning lead - this is not eliminated by boiling, and lead might lead to really bad effects on the brain. Jul 24, 2021 at 17:34
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    @PerAlexandersson - I think for the OP, lead is a red herring. The OP states the water is declared to be non-potable. Lead pipes could be in a building whether or not the water is declared potable by the authorities. That's something you are almost never going to know for certain, unless the building is less than maybe 50 years old.
    – Tetsujin
    Jul 24, 2021 at 18:07
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    I do not see where you see 'authorities' in the question. Someone told OP to boil water from the cold tap for drinking water. (I am not even sure which part of the world, even less which country and what the standards are there.) Sometimes buildings get checked for led pipes, that happened in schools in the Netherlands fairly recently, resulting in kids having to bring water from home due to lead pipes in the schools. (And I would have been certain we did not have any of those left.)
    – Willeke
    Jul 24, 2021 at 19:45
  • 'The authorities' are whoever declares a territory's water to be potable or not. Presumably the people who already live there already know this information & are passing it on to the visitor. The OP's question is 'If it's OK do do this from the cold tap, why not also from the hot tap?'. Th part of the answer that covers whether this is safe from a biological point of view is fine, the lead is a red herring. We don't know why boiling rather than any other method is recommended by the locals, but that's the best info we have.
    – Tetsujin
    Jul 25, 2021 at 7:52
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    @Tetsujin Not a red herring. This answer doesn't say that lead is definitely the problem, just that it's an example of a class of problems that are not obviously visible and not solved by boiling. It's a direct answer to the question: "In cases where tap water is not drinkable by default, does simply boiling it make it drinkable?" Jul 26, 2021 at 13:48
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As an addendum to the existing answers...

No matter where in the world you are, or whether the water is considered potable by the very highest standards… never drink from the hot tap, whether you boil it or not.

The hot supply could be tanked. Standards for header tanks vary from unreliable to none. Frequently, the tank will be in an attic or roof space, often open-topped & at the mercy of every insect, bird, rodent etc that can get to it or die & fall in it. This will often be accompanied by an inch of unidentifiable sludge in the bottom of the tank.

The only time you could even think to drink from the hot supply is if you know for absolute certain is it on a direct 'instant' heater, straight from the regular cold supply.

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    Even sealed tanks are not guaranteed to be hygienic; Legionella is still a risk in a sealed system. Commercial ones must be certified & monitored. Domestic, you'll probably never know unless you trace the pipework right round the building - waterhygieneexperts.co.uk/index.php/cold-water-storage-tanks My dad was a plumber. He instilled this into me at a very early age.
    – Tetsujin
    Jul 25, 2021 at 10:04
  • How do you know that the cold water supply isn't tanked too?
    – jamesdlin
    Jul 25, 2021 at 10:24
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    @jamesdlin - You don't. I never said you did. The only info we have is that the OP was told by existing residents that the cold needed to be boiled. We have no idea why or how they accessed that information, but that's all we have to go on. It is far more common for hot to be tanked, unless you're in a high-rise.
    – Tetsujin
    Jul 25, 2021 at 10:35
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    This is greatly exaggerated. In the Netherlands, hot water is typically heated ad hoc by a gas heater. The source is exactly the same as the water from the cold tap. Jul 25, 2021 at 22:09
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    At least in Germany, it often pays to peek into the cabinet underneath the kitchen sink, as direct instant heaters (called "Boiler" in German) will often (albeit not always) be installed there.
    – Sixtyfive
    Jul 26, 2021 at 9:07
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"Don't drink from the hot water tap" is universal advice

You probably know sugar dissolves a lot easier in hot water than cold. That's because hot water really wants to absorb minerals a whole lot more than cold water.

Which means, hot water leaches chemicals from pipes much, much worse than cold water. We try to use low-leach materials for pipes, like PEX, but the amount is never zero. the point is, it's much worse with hot.

It's not just the hot water transiting through the pipes, it's also the hot water sitting in the tank. At the least, most tanks have a zinc "sacrificial anode" to stop corrosion of the tank proper, so that zinc oxide is getting in the hot water for sure.

So, the clever person thinks "If I use from the hot water tap, I'm that much closer to boiling and it'll save energy/time". That doesn't actually save energy (because hot water isn't free), and the saved time isn't worth the added leachate.

So water you are boiling for consumption should be drawn from the cold, always.

Further, the water which has been sitting stopped in the pipes has had time to leach. You are better off running the water flow for as long as it takes to remove the cylinder of water which has been standing in the pipe for hours. Unless you're in the American West...

Separate from that, hot water at the wrong temp breeds bacteria

such as legionella. This was something discovered in fairly recent science, particularly during the Flint water crisis. As such, it's currently considered best-practice to hold tanked hot water at 60C/140F.

This in turn requires anti-scald mixing valves on all water outlets - fortunately most modern "joystick / 2-axis" style faucets include those.

Needless to say, if 60C kills the bacteria in hot water, 100C will kill the bacteria in cold water.

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    The legionella discovery isn't exactly recent science. The disease is named after the 1976 outbreak. And it didn't take long to establish that even moderate heat kills the bacteria. But yes, the leaching is a real risk with older pipes. I don't know why you bring up PEX. What minerals would it leach? It's polyethelene.
    – MSalters
    Jul 26, 2021 at 10:37
  • How do you avoid this on a mixer tap?
    – Weckar E.
    Jul 26, 2021 at 10:56
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    Mixer taps with only one handle usually have an area where only cold water will stream out and an other area where only hot water will come out (although it might be protected so you can not open it to hot by accident.) The area in between is mixed water. (The ones with two knobs to turn or two handles are even easier, you select the one you need.) If you are not sure which tap was used last, let the cold water run a bit to clean the pipe.
    – Willeke
    Jul 26, 2021 at 15:51
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    @WeckarE. By going "full cold" on the tap. Note that mixer taps certified for sale in the first world are required to have that "don't scald the customer" thermostatic control. Jul 26, 2021 at 16:54
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As Willeke already pointed out in her comment, it depends on why the tap water is not safe to drink.

Boiling the tap water will only kill most of the bacteria and viruses in the water. They are probably not common in tap water, but there are also so called thermophile bacteria and viruses, which can easily survive the temperature of boiling water, some species even start to thrive well above the boiling point of water.

Poor tap water may however not only contain bacteria or viruses which can be killed by boiling the water, but also all other kind of dissolved filth, sewer, agricultural (fertilizer, manure) or industrial waste.

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    Thermophiles tend not to do well in cold water: either they simply can't survive, or they get out-competed by non-thermophilic bacteria. If you're getting your drinking water from a hot spring, you need to worry about thermophiles, but not if you're getting it from the tap.
    – Mark
    Jul 23, 2021 at 22:48
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    @Mark -- yeah, thermophiles generally don't operate well at human body temperature either :P Jul 23, 2021 at 23:23
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    Known thermopholes (those that survive more than few minutes in a boiling water) are pretty much harmless to humans.
    – fraxinus
    Jul 26, 2021 at 6:49
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Water can safely be drunk after boiling it if the contamination is microbiological. Chemical pollution typically cannot be removed that way.

Since you are mentioning Pridnestrovi: Apparently that is an autonomous region of Moldavia along the river Dniester that is trying to gain independence.

I looked up sources of pollution and other water issues there and found an official report of the environmental ministry of Moldavia. On page 33 it states that

the quality of groundwater abstracted for drinking purposes is particularly poor in the central and the southern region of the country, where half of the groundwater samples do not comply with quality standards for chemical parameters. Microbiological pollution is also widespread [...].

The natural and man-induced pollutants mentioned include "nitrates, pesticides, sulfates, etc." against which boiling will not help. Some of the substances like nitrates are more harmful to infants than adults, and pesticides and arsenic accumulate over time so that ingesting small amounts may not be acutely harmful: Drinking water requirements are making sure that drinking large amounts daily over long periods of time is not harmful.

But the standards are there for a reason, and the bottom line is: The pollution is likely not only biological but also chemical, with an unknown mix and concentration of of pollutants. Don't drink it. Boil or otherwise disinfect it it even for teeth brushing or personal hygiene. Probably try not to shower, or keep it short and ventilate the bathroom.

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  • I think abstracted is correct. At least it is used in the UK exactly for this purpose.
    – mdewey
    Jul 25, 2021 at 12:46
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Boiling is not guaranteed to be sufficient for a couple of reasons. The big one is that simple boiling does not remove many chemical contaminants from the water (it may cause some complex ones to break down, but some really nasty stuff, such as botulinum toxin or assorted lead salts, will survive it just fine). If the water is suspect, a combination of active filtration and distillation will get you the cleanest water possible with ‘common’ tools (any cleaner requires chemical treatment, which has it’s own problems, or industrial scale desalination and distillation). However, if you’ve got actual municipal water to work with, simple boiling is usually sufficient.

As far as the hot water, there are two issues there. The first is that it’s only lukewarm. Additionally though, the water heating system is probably tank-based, which means you have a large amount of water in the dark sitting around at that temperature for potentially a very long time. Combined, that means that the hot water system is a breeding ground for microorganisms, which means it’s not exactly safe to drink without boiling. Those circumstances though lead to some risk of microorganisms that naturally produce toxins to grow, and as mentioned above boiling will usually not completely eliminate those toxins.

All together, given that you probably don’t have filtration and distillation equipment available, your safest option is to do as they suggest and use only the cold water supply, boiled, for drinking and cooking.

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    "Despite its extreme potency, botulinum toxin is easily destroyed. Heating to an internal temperature of 85°C for at least 5 minutes will decontaminate affected food or drink" (emergency.cdc.gov/agent/Botulism/clinicians/control.asp). Poisoning typically comes from unheated, unsalted, non-acid foods like stuff preserved in oil, or traditional preserved fish from arctic peoples. Jul 26, 2021 at 18:45

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