Many cities have old fashioned combined sewers and have severe problems becuase storm and ground water is requireing huge expenditures in capacity expansion just to treat storm-related water. This expansion pumps up the utility bills where sewerage is related to water purchases, so people end up with out of line water/utility bills. The old timers were not too smart!!!
Where possible, the separation of the sewers is being done despite the high construction cost since it is cheaper than new treatment plants.
What it boils down to is either banning storm related water from going to the treatment plants or pay the higher cost of building new plants and paying more for treatment because of storm water.
Some cities have been able to put off new sewage plant construction for 10 or 20 years by separating storm-related water from sewerage and reduced operation costs and utility bills.
Some progressive countries like India now separate sanitary water(sewage) from gray water(sinks) by having two separate systems in the house in an the ground.