I observed that my dupes are constantly bringing polluted water to the bottle emptier.
But since very very long It did not assign any mob-task.
Thus I wonder where the polluted water is coming from.
One possibility could be the metal refinery just above (running permanently). As there is always a bottle of polluted water visible, However the plumbing for dirt water is correct: there is a pipe leading down to the pool below. The refinery is very hot. Pipes and itself build out of Abys