Having had several experiences with sewage: bubbling up into my bathtub, over the edge and into my hallway and closet, when I lived on the bottom floor of a four-story condo, and the time at another house, that the city was jet-washing the line, encountered a clog, and blasted through it sending sewage into my tub, toilet, kitchen and bath sinks, washing machine, and all of them, but the kitchen sink, overflowed the edge onto the floor...
Not remotely funny.
Sewage tanks are usually built below ground level for a reason. Gravity better be on your side (that's how I knew to blame the city; it wouldn't normally flow uphill).
What do you think? http://consequeries.com/