Enrico - Generally maples and willows are bad about sending roots into sewers. A rough rule of thumb is the roots will reach out as far as the branches. Since these trees, when mature, reach out 30 to 40 feet from the trunk, plant large trees at least this far from the sewer. Most shrubs and flowers reach down a foot or so. Just think what would be ripped out if you had sewer work when you're planting. I usually plant smaller flowers over the sewer line so I won't lose much if they dig a foot or so wide trench to fix a leaking sewer.

If your on the Island of long and have the sewer line installed properly and NOT Orangeburg piping or clay then roots should not be a problem

One of my accounts in Riverdale ( Bronx) has this amazing home was build prior to the civil war with origional sewers and waste piping with lots of big trees all around and there is no root infiltration.

This picture was taken this summer in Manhattan a building over 10 stories high note the roots under the roofing membrane as I am preparing to install a new rooof drain