"If it became common practice to do that you could indeed have a watercourse problem."
The Victorians found that the amount of horse traffic in cities was a major pollution problem. Nowadays one could ban horses from streets because they pollute the watercourses with the run-off. How much oil and rubber goes into the drains from motor vehicles?
At least coffee is organic and presumably biodegradable.
Given the number of rats in cities it would be interesting to calculate how much urine they generate in watercourses.
Biting the hand that feeds IT
