said by LazMan:
It's typically illegal to discharge into the sewer system.
It's illegal where I live but we did it anyways. The problem is the houses are so close together, there is no drainage ditch! Only other option would be to discharge near the street, but then that creates lots of ice which is a hazard to both pedestrians and vehicles. And it's not like we can just throw a bit of salt on it to melt it away, the ice builds up in thick layers everytime the pump runs.
So into the sewer it goes. We do however have some valves so we can direct the output of the pump outside if necessary. Typically we'll discharge to the street in the summer time and into the sewer when the freezing weather starts.