The aeroplanes have a special type of toilet called “vacuum toilets”. So, the human excreta or sewage in an aeroplane is disposed of through vacuum toilets.
When we flush the toilet in an aeroplane, then a valve opens in a pipeline which has vacuum inside it.
The vacuum in the pipeline sucks the human excreta from the toilet seat with a great force making a loud noise and carries it into a tank on board the aeroplane.
When the aeroplane lands at an airport, then the tank containing all the human excreta is emptied into a sewer line into the ground through connecting pipes.