What kind of sewage system was built? Are we imagining that the real-world restrooms are reasonably sanitary pit latrines that are periodically emptied, rotated, or converted to composting? Are we imagining that there are ditches or piping that dump sewage into the stream? (I like the first version because it’s closer to wysiwyg: pretending there are underground pits that we don’t see is easier than pretending piped or open sewage or stream contamination.)
I’m not sure if this is an “ask staff” question, “ask the engineers who built it”, or “find out when you get to game”.
It is very similar to the London sewer systems of the 18th century that Connor posted - in other words, the real-world restrooms drop into a grate that goes into some tunnels that lead the sewage far out away from the town where it is dumped into the aether. There is no stream contamination (that’s the point of the sewer), and while there are underground tunnels you don’t see, that won’t always be true. We may run things in the sewers from time to time as long as you have them.