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I have a friend who tells me that the non-drinking water sign you see in cafes, supermarket toilets in the UK is just to mitigate responsibility. Is this true, or what about the water could be unsafe?

An example of this sign found on the web: example not drinking water sign

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Commonly in the UK, cold water may be stored in open (or roughly covered) tanks in the roofspace.

Such water, while it will have been drinkable mains water when it arrived is considered non-potable as there is no knowing what might have contaminated it while sitting in the tank.

Contrary to the other answer, it is very unlikely that grey water is coming out of a tap, though untreated water is another possibility in rural areas.

See https://www.theguardian.com/notesandqueries/query/0,5753,-2404,00.html for thoughts on why this system exists. (A classic Fawlty Towers episode sees the hotel inspectors find a dead pigeon in the hotel water tank.)

Whether this is the case in every instance is debatable.

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    (+1) This also applies in households in some areas where the cold water supply is tanked in the loft and only one tap, typically in the kitchen, is a direct feed. – mdewey Mar 18 '18 at 16:52
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    When taps for handwashing are fed by a blend valve combining hot and cold to make warm, that's also regarded as non-potable for most if not all hot water systems. the tap may run cold initially so it's not immediately obvious that there's a blend valve tucked away – Chris H Mar 18 '18 at 21:20
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    My previous house was at the top of a hill. The water company (Severn Trent) had an electric pump at the bottom of the hill to provide our supply. Obviously this pump would fail when the power went out so we were required to have 100 gallons of cold water storage in the house so that the toilets etc continued to work. Only the kitchen sink cold tap was connected directly to the mains supply. This was the local water board regulation. At the time (1968 to ~1980) the power went out regularly and once we were without water for a week when the pump failed. – uɐɪ Mar 19 '18 at 8:38
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    The situation @ʎəʞouɐɪ describes used to be common; the only difference at the top of the hill was a bigger tank. Supposedly the mains couldn't keep up when everyone got home from the same factory at the same time and wanted a bath, unless there was local storage. In newer houses it's common for the cold water tank to be used only to supply the hot water system. Modern tanks are plastic with decent (though not airtight) lids. Older tanks were often open and galvanized (so the water would also taste horrible). – Chris H Mar 19 '18 at 9:04
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    ... dead pigeon... whether this is the case in every instance... LOL – Joel Coehoorn Mar 19 '18 at 17:46
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It is a requirement of the Health and Safey Executive that water is marked as "Not Drinking Water" unless it meets the requirements to be considered drinking water.

Drinking water

An adequate supply of high-quality drinking water, with an upward drinking jet or suitable cups, should be provided. Water should only be provided in refillable enclosed containers where it cannot be obtained directly from a mains supply. The containers should be refilled at least daily (unless they are chilled water dispensers where the containers are returned to the supplier for refilling). Bottled water/water dispensing systems may still be provided as a secondary source of drinking water. Drinking water does not have to be marked unless there is a significant risk of people drinking non-drinking water.

Should water in a particular supermarket toilet come not directly from the mains supply but via a storage tank of some kind between the mains and the toilet it would not be considered drinking water.

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    The text you quote is irrelevant - it says nothing about marking any supply as "non-drinking water". – Mark Pattison Mar 18 '18 at 18:35
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    NEVER drink the water from a supermarket toilet, or any other toilet for that matter, use the sink instead. – RyanfaeScotland Mar 18 '18 at 21:26
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    @RyanfaeScotland I suspect that when they use the word "toilet" they are referring to the entire bathroom/washroom area rather than the literal toilet itself. The water in the bathroom/washroom sinks may also be considered non-drinking water. Naturally drinking water out of a toilet bowl is a bad idea. – stanri Mar 19 '18 at 7:01
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    @stanri I think it was a joke.. – Trotski94 Mar 19 '18 at 8:51
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    The quoted text is guidance to employers about providing drinking water to their staff. This isn't relevant to the question. – David Richerby Mar 19 '18 at 10:30
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The important thing to know about drinking water, in the UK and everywhere else, is that, while it leaves the water treatment works in a disinfected, safe, state, it then has to travel through many miles of pipework before being consumed. It is impossible to keep this pipework completely clean and so the water is treated with a disinfection agent such as chlorine (traditional solution, but can affect the taste of the water) or chloramine (used more often nowadays since it has less taste) that stays in the water and keeps it safe for a period of time, typically 5-7 days depending upon the strength and chemical used. The water distribution systems (the pipes under the streets) are designed such that the water will reach the consumer well under this time period and so be safe at the point of use.

In a domestic situation, where the kitchen tap, the primary source of drinking water, typically runs direct off the water mains, this is safe. In large commercial buildings however, there is a large amount of additional pipework, extra storage tanks, pumps, etc. and in order for the water to be guaranteed safe (it's not ok to be alright most of the time) all of this additional infrastructure must be periodically flushed, disinfected, and tested in order to ensure that no pathogens can be introduced into the water. This is an expensive hassle and so the simplest thing to do is to have a separate, simple, drinking water supply within the building, which does get cleaned and tested regularly, and to label all other water points as non-potable.

You probably won't get ill drinking from such water points (due to the presence of existing disinfection agents and engineering safety factors), but is it really worth a dose of the squits?

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I have lived in the UK since 1974, and have never seen a sign marking tap water as unfit for human consumption. At what commercial premises, and in which town, are you specifically alleging that you have seen such a notice?

It would be highly unlikely that commercial premises in England would use (untreated) rainwater instead of the (treated) water mains, for any purpose. I know the UK has a bad reputation for it always raining here, but this is largely a myth. In reality you could not expect to meet the water demands in a commercial building, such as a restaurant or supermarket, just by putting an open water tank on the roof. Even if there is only moderate demand, you would be bound to end up sooner or later with an empty tank, and thus non-flushing toilets, especially in the (relatively dry) summer months.

The newspapers would have a field-day with such a story. But that never happens. What business wants bad publicity of that sort, especially one which sells food?

The 'non-potable' notice is often encountered in France, and is a recognised hazard of holidaying in popular areas such as Brittany or Provence; and I have even seen the notice in Paris. But British public health laws are of very long standing, and water treatment plants are fairly common: I live within two miles of one! So untreated water is rare as hen's teeth here.

The term 'non-potable' is actually French in origin, presumably because this condition is only encountered by the English when travelling, notably in France, and so was imported into our language. But even so it's not a commonly used term, and is mostly only encountered in travel guides. The word 'potable' on its own I have never seen used - not in conversation, nor in a newspaper. It's only ever used as part of 'non-potable', and (not being an English word) wouldn't be understood separately from that term. The French term has survived simply because there isn't an English equivalent: the Victorian public health laws have been in force so long that the concept of tap water being unsafe is unknown here.

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    Potable has been an English word since the 15th Century. dictionary.com/browse/potable has a number of examples of it being used. Half of English are of French origin, so your suggestion that it is caused by travellers encountering poor standards is preposterous. Neither of the upvoted answers suggest that untreated water is the cause of this notice. – Mark Perryman Mar 19 '18 at 8:58
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    I live in the UK and have seen this in plenty of places. Most typically it's at places like motorway service stations. – WhatEvil Mar 19 '18 at 9:17
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    I have also seen these signs including just a few days ago in a motorway service station. I have wondered about them but they make more sense now. – badjohn Mar 19 '18 at 9:51
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    I've never seen a kangaroo, but you don't find me going on the internet asserting that they don't exist... – AakashM Mar 19 '18 at 11:10
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    I've seen these signs often enough, typically in publicly accessible buildings like supermarkets, town halls and motorway service stations. I've even seen it alongside another tap marked "Drinking water". – Valorum Mar 19 '18 at 23:36

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