Hi, Rob - glad you liked the Channukah chumor...

As in my reply to Tom, up the thread a ways, the relation between elevation and pressure is 0.43 psi/foot. It works both ways - in other words, most faucets and flush valves are designed for a nominal 20 psi, which can be achieved by storing the water 40 feet above point of use without pressurizing the storage tanks.

The trade-off in size and capacity of a main distribution system (obviously, the tanks need to be refilled, but at a reduced rate) gets handed off to the structural engineers.

Take a typical cluster of public toilets in an office: say, 10 commodes per gender; 20 total. One modern toilet uses 1.5 gallons per flush; a typical public wash basin dispenses about 0.5 gallons per use; a gallon of water weighs 8.33 pounds, so each employee in a typical office environment consumes 2 gallons = 16.66 pounds of water every time he/she uses the bathroom. So, that's 40 gallons x 8.33 = ~333 pounds per use - six full cycles is a literal ton of water, for each theoretical bathroom group on each floor. Regardless of how many usage cycles one wants to include in a storage tank's capacity, the dead weight of the water and its container must be taken into the building's structure.

I ran some "back of the envelope" numbers on fire protection in my post to Tom. What wasn't addressed there, but is critical if you're considering storage as opposed to a continuous-delivery system, is duration. The code mandates a 90-minute duration time for office occupancies. 2500 gallons/min x 90 min x 8.33 lbs/gal = ~ 1.87 million pounds of water.

Since we're talking about an outrageously-high-rise building, that weight has an impact in three dimensions. The building must be allowed to react to horizontal forces - wind, mostly. I don't know what the seismic rating of Lower Manhatten is, but it's not 0; hurricanes are rare but not unheard-of in the Hudson Estuary. How it reacts depends on its overall "stiffness" and how the weight is distributed. And then, as the WTC disaster brought vividly to us all, there's the problem of catastrophic structural failure of intermediate floors and the resultant "pancaking", if you will... now, think about that with a couple million pounds of water added to the load.

None of the difficulties in design are insurmountable, given enough diligence, time and money. It's no surprise the budget estimate has already doubled to $12 billion. If the building "comes in" at any less than $20 billion, then I'll be surprised.