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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?

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    $\begingroup$ Consider what happens when half a litre of slightly unclean water is dropped from an aircraft travelling at Mach 0.85 and 35000ft. What hazard would you be concerned about? $\endgroup$
    – Frog
    Commented Aug 29, 2022 at 4:31
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    $\begingroup$ Where did you learn this? $\endgroup$
    – RetiredATC
    Commented Aug 29, 2022 at 4:55
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    $\begingroup$ As a side-note, I dimly recall an article in the New Scientist dating back probably to the late '70s. It seems that until that point it was routine for raw sewage to be dumped overboard, but questions were raised as to whether diseases could be transmitted that way and the practice was stopped, although I'm not aware of any direct evidence that it was a problem. $\endgroup$
    – Frog
    Commented Aug 29, 2022 at 5:56
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    $\begingroup$ @RetiredATC I found some possibly pirated PDFs of training manuals for the Airbus A320 and Boeing 737 which described their water and waste system (ATA chapter 38). The diagrams appear to show that the water from the lavatory sink goes out of the airplane without being collected in some kind of storage tank. $\endgroup$
    – Flux
    Commented Aug 29, 2022 at 15:39
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    $\begingroup$ Every now and then there's a Blue Water leak and "sanitary" waste freezes and falls to the ground like a cannonball: en.wikipedia.org/wiki/Blue_ice_(aviation). Traditionally, trains just flushed out onto the tracks (I don't know if they still do). There were signs telling you not to flush when the train was stopped, even a well known song (well, known to the train-going generation before me): lyricstranslate.com/en/… $\endgroup$
    – Flydog57
    Commented Aug 30, 2022 at 21:44

3 Answers 3

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“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?)

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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:

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    $\begingroup$ +1 These photos also show clearly that the amount of water involved in the grey water disposal is insignificant compared to the amount of water vapour produced by the actual engines. $\endgroup$ Commented Aug 29, 2022 at 21:25
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    $\begingroup$ Are you sure that none of those photos are showing the contrail from the APU, rather than from a gray water drain mast? $\endgroup$ Commented Aug 30, 2022 at 1:00
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    $\begingroup$ The Emirates 380 could possibly be an APU but it is highly unlikely a 4 engine aircraft would need to operate an APU at cruise altitude. The other 2 are very obviously not APU as you can see where the mini contrail exits from. $\endgroup$ Commented Aug 30, 2022 at 2:05
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    $\begingroup$ @MikeSowsun on the dot - weight is fuel efficiency. With no negative effect of dumping the water overboard.... why not dump it? $\endgroup$
    – MishaP
    Commented Aug 30, 2022 at 12:05
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    $\begingroup$ Reverting to my 13-year-old self, the plane is peeing. $\endgroup$
    – Barmar
    Commented Aug 30, 2022 at 15:39
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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.

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    $\begingroup$ Even urine will evaporate and disperse into the atmosphere. A turd is another story altogether.... $\endgroup$ Commented Aug 29, 2022 at 5:43
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    $\begingroup$ Yes but you wouldn't want to be the one that had to work on the drain mast to fixe the heater or something lol. $\endgroup$
    – John K
    Commented Aug 29, 2022 at 12:48
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    $\begingroup$ A little biochemistry remindment: urine happens to be a sterile water-saline solution with some sulphur-based water-soluble compounds solved in it, and some acids for getting the right pH. All of the forementioned make it a perfect environment for grow of bacteria and fungi which both happen to be everywhere, which will soon start spoiling this solution. There's no lipids (fats) in urine, though. So if any other, less microbe-growth-friendly water solution will dilute the urine, it'll be just like... any other water solution, with nothing special on it both biologically as well as chemically. $\endgroup$ Commented Aug 29, 2022 at 23:18
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    $\begingroup$ @JohnK Read my previous comment (ran out of char limit), and also keep in mind that the greywater would have pretty big amounts of detergents like soap, mothwash and so on, making the mixture requiring far far less other water to deprive the urine all biological urine proprieties. Planes are engineered very carefully and precisely, you know? :D $\endgroup$ Commented Aug 29, 2022 at 23:21
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    $\begingroup$ @MobileDevelopment urine is not actually sterile, it just turns out that what is there is generally not picked up by standard culture techniques. There was a lot of papers 2012-2015 or so about it. $\endgroup$
    – mbrig
    Commented Aug 31, 2022 at 0:05
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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.

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    $\begingroup$ I'm not sure you're correct about aircraft not being able to land heavy, at least in the context of water storage. Burning fuel will make aircraft light enough for landing to be possible. If an aircraft was borderline too heavy to land again right after takeoff, most of the weight of consumables would be fuel, not water. With a full load of water but only a partial load of fuel for a short hop, the plane wouldn't be close to its max takeoff or landing weights. (Correct me if I'm wrong). $\endgroup$ Commented Aug 30, 2022 at 14:17
  • $\begingroup$ So the important point here is efficiency; smaller tank needed, and getting to dump some water weight during flight. (Or not needing a separate tank if they wanted to keep gray and black water separated.) As you say, a tank would take space, and its walls would have some mass, and would still need a way to be drained after landing. $\endgroup$ Commented Aug 30, 2022 at 14:20
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    $\begingroup$ @PeterCordes The other factor in play is balance. With only so many places you could put a bigger/extra waste tank, on a closed system (i.e. no grey discharge) you could end up with a severely out of balance aircraft as a result of a substantial amount of weight being shifted from one location to another. You don't have the luxury that you do with the fuel where the fuel can be rebalanced between tanks. Rebalancing waste tanks would likely lead to blockages ! $\endgroup$ Commented Aug 30, 2022 at 16:17
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    $\begingroup$ Good point. And the weight of pumps is hardly justified for something that isn't safety critical like fuel management. So it would become an extra engineering constraint. Although perhaps you could design fuel tankage to balance it, if you already have a mechanism for fore/after CG shift by pumping fuel. Except you don't want that fuel pump to become necessary for safety if you can help it... So yeah, clearly some secondary downsides if you couldn't just vent graywater. $\endgroup$ Commented Aug 30, 2022 at 16:35
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    $\begingroup$ @PeterCordes You could use the grey water to flush the toilets. Some experimental residential water preserving systems do that. One typical problem with that is microbial growth in the grey water, and of course the overhead for grey water storage on the plane. $\endgroup$ Commented Aug 31, 2022 at 7:54

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