[jcranmer]

Having worked at a water treatment plant in the past: if the power is out for a sufficiently long time, then your water is going to go dry. The time it takes is dependent on the distribution system (and time of year, you consume about twice as much water in summer as in winter), but where I worked, it was about 12 hours.

Also, we had no generators. Why? Because a) power outages of that length are extremely uncommon (2 in 70 years, IIRC) and b) the power draw of a large water treatment plant is insanely high. The water company was literally the largest consumer of power in the entire state, and turning on some of the pumps require the power company's permission because it draws that much power. It's not very feasible to keep a backup power system for such a large facility when the need for it is so very low.