Whatever gets to the sewer plant came through some small pipes. Thats the largest size they have to deal with, So they can scale the equipment so clogs don't happen.

They pick the person who can hold their breath the longest to go down there and unplug it.

Nothing is big enough to clog the main sewer digestion tank, and what does go there gets disintegrated even more.

actually they dont care if someone can hold their breath the longest. They have people do it or fire them.

Super Mario Draino

There are more than one pipe to a real plant. The whole process takes several hours and sometimes days to clean and filter sewerage. So any and all processes are done in multiple tanks.