sump pumps

I currently have a sump pump system with a battery back-up. I noticed at the store there is a "gravity-fed" sump pump and there is no back-up pump needed with it. Does anyone know about this-how well they work? If my basement floods it is due to the city system overloaded and backing up not a leaky basement. Thanks. Oh, my main pump is connected to the street piping and the back-up goes outside thru the wall and into the yard. What do you think would be the best way to pipe this other system?
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