The sewage on planes does not get dumped out of the plane and out of the sky.
When flying the planes sewage remains on the plane until they land and then they remove the sewage using sewage pumping trucks.
The planes just like RV's have sewage tanks known as black water tanks and gray water tanks that the water goes into.
The toilet water and poop, pee etc all go into a black water tank underneath the plane and the gray water from sinks etc goes into a gray water tank in the same location as the black water tanks.
Then when the plane lands a truck will come around and hook a hose to the planes sewage tanks and pump out and suck out the sewage from the planes sewage tanks.
Then the sewage gets sent off to a location to be dumped such as on fields etc or gets dumped into a sewage line to go to a sewage treatment plant.