is it done mechanically or by people?
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We first try to separate the raw waste into liquids and solids. Incoming wastewater is typicaly 99+% water. We slowly settle out the ~1% solids using high-tech gravity in large tanks called clarifiers. From there, the solids, which are still 90+% water, go on to even larger tanks where bacteria are bred to break down much of the material into gasses. Mainly methane, which we burn for process heat and electricity. After a few weeks, the remaining sludge is sent off to a mechanism that squeezes even more of the water out. The “cake” that produces is sold to farmers to fertilize non-food crops. The squeezed water is returned to the beginning of the process.
The liquid stream previously separated in the clarifiers goes on to reaaaaly big tanks called aeration basins. Here, air is injected to encourage specific bugs that break your pee and harmful pathogens down into less harmful things like nitrogen gas. These bugs are the true workforce of most modern plants- mine has literal tons of them for every human worker.
From the basins, the liquid stream is clarified again. Solids(mostly bugs) go back to the basins to work some more. Liquids go on to filters. At this point, you can expect the liquid to look almost like tap water. It’s not done, though. We still have…
Disinfection. We inject the water with chlorine gas and push it through concrete mazes. Chemistry happens. 20 minutes later the water is tested for residual chlorine. If the result is high enough, we can be confident most of the nasty bits have been destroyed. Then we inject sulfur gas into the water. More chemistry happens. The sulfur reacts with the remaining chlorine, and the water is clean and safe for the fishes in the river we release it into.
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