I learned that the waste water from the lavatory sink in most commercial airliners is dumped outside the aircraft while in flight. Is this really the case? If so, why is the waste water dumped outside instead of being stored in the septic tank together with the waste from the toilet bowl?
“Grey” water from the lavatory and galley sinks is dumped overboard through electrically heated drain masts.
It is much cheaper and simpler to drain it overboard rather than store it. Dumping it also makes the aircraft lighter and more fuel efficient as the flight progresses……….. (better carbon footprint?)
Between flights it drains onto the ground or unsuspecting ramp workers under the aircraft. In cruise flight the outside air is cold enough for the water to freeze instantly and form small contrails.
There are many photos online showing these mini contrails that form briefly while the water is actually draining:
The lav sink is what is known as "grey water" in a house water drain system. Not sewage, just water with dirt and soap, and maybe toothpaste and mouthwash in it. It goes overboard through a heated drain mast, and just evaporates, the solids in it becoming dust to the wind.
Don't go peeing in the sink, and you won't be creating any problems.
First, it is not just lavatory sinks, it is also (as per this Airbus patent):
the condensate of the cargo spaces and the outer doors, as well as the washwater of all of the hand wash basins and accordingly, has contaminants (such as soap, for example) as well as colored contaminants, such as kitchen waste, like coffee, tea, or juices
If you think about it there's only so much room in an aircraft. The aircraft already stores:
- Clean/Potable water
- Black water (i.e. human excrement from the toilets)
The amount of clean water on-board is larger than the capacity of the black water tank. There is a reason why aircraft toilets flush using vacuum and not water !
Also storing it on-board means more weight, which means more fuel required. You also have to remember that whilst aircraft can take-off "heavy", they cannot land "heavy".
If you think about a very large aircraft like an A380, the amount of additional storage capacity would be enormous !
Finally don't forget airlines are a commercial business. Taking more space for grey water storage menas less space for cargo means less $$$ for the airline.