Years and years ago, I went on a tour of the Paris Sewer (museum). It was billed as a cool and interesting thing to visit, and while there was a lot of neat stuff to see and learn, we rushed through it as fast as possible to get out of the smell.
When I was there in 2000 it definitely had a smell, but it wasn't terrible.
Some years later I got around to reading Les Miserables, and it goes off at one point on a long digression about the construction and history of the Paris sewers, which was really interesting.
In my city they use organisms to process wastewater and the smell is quite remarkable. There is one section right at the beginning that smells like sewage, but the rest of the processing plant smells like freshly turned dirt in your garden, albeit stronger. Many years ago before they modified the process the smell was horrible.
The smell in the secondary tanks - aeration - is surprisingly inoffensive. The smell for the tertiary tanks can be even a little sweet smelling. In the digestion tanks it's pretty much odorless.
It depends on the input of course. I presume it's mostly food scraps and human waste. Nothing industrial.
