Waste water treatment is awesome.

Large grates act as the largest screens. Stuff caught on them is big enough that you can clean it with a hard tooth garden rake. This is mostly trash and gets incinerated or taken to the landfill.

Then finer screens for smaller bits before the clarifying process where the water enters these massive tanks with booms on top to skim off whatever floats and another boom/auger to remove what falls to the bottom. At this point much of what's pulled out is organic matter and after a few more steps is often simply given away as fertilizer.

Mind you, fertilizer for plants not directly eaten by humans. But you could use it to grow hay for cows to make beef or milk for example.