I once built a fort on a waterfall and made it so my dwarves would path through the waterfall to get to the fort. Water would splash the dwarves and they would get happy thoughts, and then pools of blood and muck would litter the floor tiles surrounding the grates. Using a statue to designate a room, this muck would get cleaned. Things went swimmingly until a siege camped out above the waterfall and my dwarves couldn't get in or out of the fort without getting scared.
I saw a design for a mist generator that used falling water to clean dwarves walking through a 2-wide hallway, and thought it was brilliant. Please excuse the tileset, hopefully the mspaint stuff i did makes the design clearer.
water is pumped up 2 levels, then falls through grates onto the path on level Z where dwarves walk. After getting it running, I've noticed 2 things. Its good at giving dwarves happy thoughts, however if you k-look at the grates on level z, the contaminants that wash off of the dwarves are left on level z on the 'open space' that has the grates. Dwarves reliably get jobs to clean this, but those jobs are canceled due to dangerous terrain. Furthermore, dwarves and animals can path onto the walkable tile of the pump, evading the cleansing deluge of water. The only way I've found to clean the contaminants is to turn off the pumps, at which point they are cleaned properly, but as soon as I turn them on again, another pile shows up, and I'm spammed with canceled cleaning jobs. If somehow the tile just north of the grates got grime on it, then the dwarves would clean that with no problems, and since clean jobs clean 9 tiles at once, the grates with the falling water would get cleaned as well.
The walkable tile of the pump can be solved by widening the design somewhat and putting more water in the system. The real problem is getting something that knocks grime off and will be cleaned automatically. I believe this can be done by making a separate mist generator and having dwarves path through a little ditch with 3/7 water, but what I really want is a compact design that uses pumps and falling water to reliably clean.