That's my water tank. When I moved out here from the city, I had no idea how a water system worked. Then one day a contractor hit a water pipe and I had to get on the fast track to learn how to take down, and then bring up, an entire water system. With that knowledge, I can say that I'm head of my own private water department. I think it's helpful to understand how water systems, and the water in general, work when trying to come to grips with issues like bottled water and industrial uses of water.
Here's how a private water system works . . . .
Elsewhere on the property—far from this tank— there is an underground water pump that takes water from the water table (300 feet below) and fills this gigantic tank. There is a floating switch in the tank that turns on the underground pump whenever the level in the tank lowers to a certain point. (I think it's around 8,000 gallons.)
I got water, but the problem is how to get it out of the tank. There is a giant outlet at the bottom. I doubt I could turn it, but if I did, the pressure of all that water would cause it to burst out. That outlet is reserved for the Fire Department. There is actually a hookup for a fire hose. One reason for this quantity of water in the tank is for fire fighting.
The water tank has to serve 12 outdoor taps scattered on the property and a few outbuildings. The pressure in the tank is not enough to push the water throughout the property, so that brings me to pump number 2.
There is another pump, in a pump house, that is connected to a pressurization tank. The pump automatically maintains the pressure in the pressurization tank so that it's between 40 and 60 p.s.i. (pounds per square inch). It's the pressure in this tank that causes water to flow from each tap smoothly. It all works great unless there is a power failure. Without power to the pressure pump, water won't flow.
Water treatment? Not much really. The water comes from 300 feet below the earth to the water tank. On it's way to a tap, the water travels through a "whole-house" filter. The cold water for the kitchen tap first goes through an under-the-sink filtration system.
The result is delicious, safe, pure water.