Are all water treatment plants somehow *lower* than all the drains/houses/buildings/sewer lines that go to it? (I.e., to make sure all water drains to the treatment plants.) Surely this can’t be the case…?
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Drains go to the wastewater treatment plant (not the water treatment plant). Pumps are used. There are lift stations strategically placed throughout the city that lift (pump) the sewage water to a point where it can flow by gravity to the treatment plant where it is once again pumped to the various treatment processes.
Source: I’m a City Planner and married to a Civil Engineer.
Sewage mostly flows by gravity because it’s cheaper and easier to maintain the pipes, however it can be pumped. Your city likely has a bunch of small sewer pump stations (sometimes called lift stations) to boost the sewage and help it along its merry way.
If they *can* be built lower I imagine that designers would try to do so, but more often than not sewer lines have pumping stations periodically along their length.
E.g. an underground sewer pipe is positioned to flow downhill for some number of miles, and when it gets too deep to be convenient a pumping station brings it back up, and it flows downhill some more. If you imagine a cross section view along the whole length, it would look like a sawtooth.
> Are all water treatment plants somehow lower than all the drains/houses/buildings/sewer lines that go to it?
Generally yes. If not, then pumping is required, but pumping sewage is expensive. It’s usually less expensive to just select the piece of land necessary to get all pipes to go downhill and use eminent domain to purchase the land.
Another alternative is forcing everyone that’s below the treatment plant to be on septic, but that’s not good either.
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