I live in a century old 3 flat in Chicago, I'm in the ground level unit that is about 12 to 18 inches below grade. During a heavy rainstorm the other week I noticed the catch basin in my laundry room which usually has a water level 3 or 4 feet below the floor level filled all the way to the very top, just inches from flooding my condo! I've lived at my place for a year and never had flooding issues. Although my gutter system feeds into the catch basin I'm almost positive the sewer system backed up from the street level based on how quickly the water level rised and then fell after the rain. When the water was just inches from flooding my laundry room I walked outside and noticed the sewers were overflowing and the streets were flooded. Questions: 1) Am I just lucky to have a catch basin that happens to be positioned inches above the street level sewers and therefore during heavy floods the street sewers overflow instead of my condo? Or did I just get lucky this time? 2) When that water backs up and fills my catch basin is that raw sewage mixed in? Or is that a totally seperate system?