Basically, not a good idea. Roots tend to seek water. Even normally shallow root trees.
JMHO, but you might want to check with a nursery for their thoughts.

Willow trees are known to send roots 50 feet down to sources of water.

I would not recommend planting any trees or bushes above the sewer line. Roots will eventually puncture your sewer line and then you with either have to up root the tree to repair the line or you will have to reroute the line around the tree.