> I can’t even imagine how to go about fixing that.
One segment at a time, I guess. But like you said, it gets really complicated in older cities with many hundreds of years of things having been put in the ground. Anyway they can probably process that just fine, it just means that at rainy days there will be higher water content; it means they'll have to scale up. Or if there's overflow, dump it directly if they're already doing it now; if they can reduce the amount of sewage going into surface water, it's already an improvement.
One segment at a time, I guess. But like you said, it gets really complicated in older cities with many hundreds of years of things having been put in the ground. Anyway they can probably process that just fine, it just means that at rainy days there will be higher water content; it means they'll have to scale up. Or if there's overflow, dump it directly if they're already doing it now; if they can reduce the amount of sewage going into surface water, it's already an improvement.