You know, this to me is just one of those "TMI" topics that I don't like associating with Middle-earth.
After all, if Minas Tirith followed 14th Century practice, they'd likely have been channeling their effluent into the Anduin.
Not an image Tolkien would have been likely envisaging.
Since we're accepting talking trees, undead wraiths, and spiders that live for thousands of years, I can deal with just going with the idea that bodily waste in Arda just magically disappears, leaving a "sweet scent of flowers".
