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During the 2003 Northeast blackout I had the misfortune of being in Manhattan. After about 12 hours municipal water pressure was gone – I assume because the pressure is supplied from water towers or reservoirs that are fed by electric pumps.

In such a condition, what would happen if someone determined to get water from a tap connected a (perhaps gas-powered) pump to a municipal water supply line?

I know that older municipal systems may often lack functional backflow check valves everywhere. In such an event, it seems likely that the pump would suck air into the system through any other consumer taps that were left open. So it would get whatever water is between it and the closest open taps, and then it would pull get air.

Therefore, for purpose of this question let's assume that all outlets are either closed or else protected with working backflow valves. Will the pump be able to pull water all the way from the raw water supply, through whatever treatment facilities and supply pumps normally feed the mains, and finally out the service tap to which it is connected?

I suspect not, but I have no idea of where it would stop. (For example, if municipal water is always fed by gravity from a reservoir of treated water, then presumably when the reservoir runs dry the utility would lock it to prevent air from entering the supply system. But I have no idea what facilities or mechanisms are upstream in a major water utility system.)

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  • $\begingroup$ This questions seems clear and specific to me and I am quite curious to the answer. @feetweet : (sorry layman) As far I know, the water supply goes mainly by gravity, if it is needed, there are pumps. The pipes have a standard pressure, which isn't very high (probably below 1-2 atm), if it is not enough (high buildings), there are also pumps. In infrastructurally not very well developed countries/regions it is quite common that there is an annoying low water pressure around the top. Furthermore, the pipe system can be filled with air, the pumps handle this. $\endgroup$
    – peterh
    Jan 30 '17 at 4:28
  • $\begingroup$ If water goes away, of course there is no water even if you open it, and you can feel that it sucks air. If there will be water soon, you can feel that air is coming out from the pipe (and we can hear the sound of the pumps). $\endgroup$
    – peterh
    Jan 30 '17 at 4:29
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It isn't just a question of air. When you put suction on a system that's normally under pressure, you're going to draw in contaminants through every available opening — leaks in underground pipes, garden hoses left in buckets, heating systems, etc. Generally a very bad idea.

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  • $\begingroup$ Often there is some sort of backflow prevention in place where there is a chance for back-contamination if suction were to come up. Maybe I'm just thinking of on drain lines; but I assume situations like this are fairly well accounted for in modern plumbing. I thought there were codes to minimize backflow contamination in these types of situations. $\endgroup$
    – JMac
    Jan 24 '17 at 13:48
  • $\begingroup$ @JMac: Nothing is perfect. Check valves leak under the best circumstances, and poorly maintained ones even more so. Pipes develop leaks; the loss when under pressure is usually too minor to justify repairing them. Water outlets normally protected by an "air gap" might not be if drains start to back up. $\endgroup$
    – Dave Tweed
    Jan 24 '17 at 13:54
  • $\begingroup$ Well a check-valve isn't a real back-flow prevention device; but I definitely agree with what you're saying; there will be potential points of failure. I'm mostly trying to point out that it's not like things like garden hoses and heating systems should ever be able to tie into the suction except in cases where something failed. Plus the kind of suction you would need to actually see the backflow could be pretty extreme depending on how far away the failure is. $\endgroup$
    – JMac
    Jan 24 '17 at 14:04
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    $\begingroup$ @JMac: And I'm just pointing out that a system as old, large and complex as the Manhattan municipal water supply has literally millions of tiny preexisting failures in it at any time. Under normal conditions, they are not a problem, but if you change an assumption as fundamental as the sign of the pressure, they become problems. $\endgroup$
    – Dave Tweed
    Jan 24 '17 at 14:59
  • $\begingroup$ I get that, but wouldn't it require a pretty extreme amount of work to actually generate enough suction to start a serious problem? To create a vacuum big enough to get fluid from an undesirable source you're probably running a pump that you really wouldn't want to run against suction anyways. I just think it might be useful to frame the OPs question in a more likely scenario along with worst case. I guess his premise is pretty extreme though (high powered pump with all residential air gaps plugged). $\endgroup$
    – JMac
    Jan 24 '17 at 18:32

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