Originally Posted by
MSPeconomist
If I may please ask a dumb question: When someone flushes a toilet during flight (or while sitting at the gate on the ground), is the used toilet water then stored on the aircraft in separate sewage tanks or is it somehow expelled from the aircraft (like old fashioned trains where one wasn't supposed to use toilets or flush while the train was in a station)? This would obviously impact whether the weight of all the water would be carried the entire distance to the destination.
Waste water that is used during toilet flushing is held in black water tanks, and empties by the "honey wagon" on arrival at destination. It is NOT expelled from the aircraft like the old fashioned trains.
Water from hand washing in sinks and galley sinks is expelled through super heated drain lines into the outside air.