Large, old cities already have problems with their waste canals clogging because modern toilets and showers use too little water. I wonder how this can be solved in the future, perhaps by coating the canal as well?
Surprisingly, at the same time they often outlaw channeling rainwater (coming down from the roof) into the wastewater system, although that seems to be a solution to several problems at once.
Rainwater often causes the opposite extreme with the same result, i.e. canals overflowing with water (rain = a lot of water in a very short time), which means they're blocked too.
