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Airliners have potable water tanks and distribution systems for galley functions and such; of course, this water is used for any non-potable functions as well as it's what's available on a modern airliner.

However, this question makes me wonder if there are water uses on an airliner that could be served by a separate non-potable water system, or if all the water needed is potable. If there are non-potable water uses, what would they be?

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    $\begingroup$ Potable water systems need special requirements to contain drinkable water. They need to be periodically sterilized, piping, etc. It is actually a lot easier to consider all water onboard non-potable and just have people drink bottled water or use bottled water for things like coffee. $\endgroup$ – Ron Beyer Dec 7 '16 at 3:12
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Aircraft travel in countries were pathogen bacteria or viruses are carried by insects which like to lay their eggs in stagnant water. Aircraft water systems must be designed carefully to prevent spreading diseases.

On a typical airliner like the Boeing 737NG, you won't find any non-potable water source that would contain chemically untreated water. Equipment is fed from the potable water tank. Used water is either dumped overboard or stored. From the aircraft familiarization manual, the water system:

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and the toilet waste system:

enter image description here

Condensation water is a by-product of many systems cooling air (including by expanding it). This water is collected and drained overboard. Jet engines (including APU) are the main generators of water, as fuel combustion generates essentially water (in mass) and CO2.

It's worth noting that at cruise altitude, moisture in atmosphere can be insufficient for humans well-being. The water extractors present in air-conditioning packs then reincorporates a part of extracted water into conditioned air.

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Non-potable water = undrinkable. Lavatory facilities such as toilets and sinks, and showers such as in the Emirates A380s could be a use of non-potable water on aircraft. That said, why complicate the matter by having two separate water systems? Plus, dirty water would clog up pipes and build up deposits... so why use it?

Now if we're talking about aircraft that DO use non-potable water then look no further than fire fighting aircraft that pick up water from lakes and rivers and dump it on wildfires.

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  • $\begingroup$ The water that goes down the drain has to go somewhere. I'm not sure if they separate it out, but stuff from sinks/showers may go into a grey water tank and the stuff from the toilets would go in a black water tank. $\endgroup$ – Ron Beyer Dec 7 '16 at 6:03
  • $\begingroup$ @RonBeyer it's stored in waste tanks which get emptied after landing and either dumped or sent to a waste water processing facility (depending on the laws and available facilities in effect).. $\endgroup$ – jwenting Dec 7 '16 at 9:33
  • $\begingroup$ It's not just complication, aircraft are plenty complicated :) Main problem is weight (and to a degree size). Adding the extra tanks and piping (plus the valves, access hatches, etc. etc.) would add a lot of dead weight to the aircraft, meaning less cargo/passenger capacity and/or reduced range. Operating cost goes up, fuel burn goes up, which in turn means even higher operating cost (and environmental impact). Far more effective to use the same water for all operations. $\endgroup$ – jwenting Dec 7 '16 at 9:35
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    $\begingroup$ If I were an aircraft designer, I would most definitely not supply non-potable water to the sinks & showers. That would be a lawsuit waiting to happen should someone contract something from drinking from either. I would simply assume that someone would. $\endgroup$ – FreeMan Dec 7 '16 at 18:30
  • $\begingroup$ @FreeMan I agree - both a waste of weight and a lawsuit waiting to happen. I did say "could" and the question was asking for hypotheticals. I suppose the real question ought to be "would the Russians do it?" :-) $\endgroup$ – Pugz Dec 7 '16 at 18:34

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