My sewer lines drain into the city sewer system, not a septic tank. The closet toilet is just on the floor above the cellar (one story house) and up-stream of the cellar about 20'. There is another fixture (sink) directly above the cellar/pump pit. It is a rather small house with just one bathroom (with one sink, shower, toilet) and the kitchen which only has a kitchen sink (no other drainage needs). The sewage line "starts" at the bathroom and runs down towards the kitchen and my new cellar area, then takes a 90 degree turn and heads out of the crawlspace to the street. It is 3" ABS and other than the bathroom, has two other 1.5" lines sweeping into it. One is the sink above that I already mentioned, and the other is futher down stream and is from the kitchen sink.
This pump is a Zoeller M-57 effluent pump that is capable of handling up to 1/2" solids. It's main purpose is to pump out water that will run into the pit from a sink that I plan to install and anything that gets washed into the floor drain (mostly just soapy water with very few if any solids).
Thanks for the advise. From your reply, it sounds like you recommend I just tie directly into the main line and trust the check valve to do it's job in case of a sewage back up.