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.)