I know I take for granted that I have indoor plumbing but where does the water pressure that causes water to flow come from when I turn on my sink? Is it a pump. Where does the energy come from?
Sometimes pumps are used to maintain water pressure, sometimes water towers which still needs pumps. The energy to run the pumps almost always comes from the power grid although there is no reason you could have diesel (or some other fuel) engine driven pumps in locations where there's no electricity. uh.edu/engines/epi2578.htm
In most cases for a city, the pressure comes from gravity. A water tank is filled by an electric pump and gravity carries the water to your home. The water is not usually pumped directly unless you have your own well.
The water drains out of the sink because it is a downhill flow from the sink to the sewerage outlet. The actual pressure is due to the height of the water and gravity. This can be stated as a head, in meters. Similarly the water comes out of a tap because it has a downhill flow from the supply reservoir typically. This can be a tank on a hill, or a tower. The water is transferred to the reservoir from its source like a lake by a pump, which gets it uphill to the reservoir. The pump is usually an electric one, though a diesel generator could be used as backup, or even a diesel driven pump could be used. Electricity is cheaper to run, considering the energy involved can be costly. The static pressure comes from the height of the reservoir above the tap outlet. This is also seen as a head. The typical pressure is 40 to 80 psi (equivalent to 30 to 60m of head or water column). Once the water is flowing at full bore the pressure drops, due to the pipe resistance and such effects, typically represented as 10-20psi approx. 8 to 16m of head. This is the pressure before the water leaves the tap (faucet), and is enough to flow through some device like a shower. At a remote property a tank stand may be used to get some height, or a pressure pump, which is equivalent. With a pressure pump the water is pumped into a rubber bladder like a large football bladder, so it can be squeezed out on demand. The pump runs cyclically as required to keep sufficient pressure in the bladder. This arrangement is known as an accumulator. High buildings may have related accumulators or a reservoir on top, and a pump/s to get water up there.
