When people use the toilet on planes, the design of the toilets uses so much less water than a toilet on the ground, in the airport, and how much a flush of water takes.

So if you assume that people were going to use the bathroom regardless, they just saved the processing of a lot of water by doing it on the plane. Probably ounces versus gallons. (~100 mL vs. 1-2 L)

(parenthetical note, sometimes I wonder if a vacuum toilet would become available for home use, it's so effective!)

Now certainly, flying around to use the bathroom used up a lot more energy, but regardless... Also, you might ask whether people tend to drink more than average because they're offered drinks on a plane.

I used to wonder, considering the hundreds of pounds of water/soda/etc loaded on a plane, as people drink and consume it all, wouldn't it be efficient for the plane to dump that water in flight and save the weight and extend the range. Turns out (of course) that is so negligible compared to the overall weight of the plane...

Edit : Writer of the article also thinks that.

My rule is to eat and drink as little as possible before a flight, ideally not eating at all. If I do eat, hopefully it's high protein and low in fiber. As a result I usually don't need to use the lavatory, which is nice because it has made me much less apprehensive about getting a window seat.

I wish more people had this mindset. On so many flights I've had, seemingly half the plane is ready to take a shit. My only explanation is that they're overnourished on the Standard American Diet. Imagine how much less waste would have to be pumped out of airplanes.

15 tons equates to over 10k liters, if you are interested

https://www.thecalculatorsite.com/conversions/common/liters-...

A lot of folks poop on a schedule: Making sure you don't poop on the plane might very well involve adjusting food for a few days before a flight, depending on how often you poop. Some folks poop multiple times a day, some folks poop a few times a week. Some women poop more during their monthly cycle, too - which, by the way, does not happen at the same time every month nor do you always have notice.

You'd be able to eat not too long before the flight, though, if you are healthy simply because most folks don't process food too terribly fast (but the timing varies). Adjusting what you eat for days just isn't worth it, and not everyone can do this.

Also, not everyone on the flight is going to be American, but the chances are higher if you are taking in-country flights in the US. I personally don't encounter many Americans on flights - I'm often the only one. But then again, I don't live in the US either.

Second, pilots can trim the plane while in flight.

Third, no safety margin is needed here because this is not a safety problem.

The amount of pee that can be deposited into the tanks is limited by the amount of pee they can physically hold at the start of the flight plus the amount of drinks they can consume during flight.

But more realistically, on a long flight, it is just the amount of drinks they can consume assuming they drinks are mostly used to replenish whatever they have peed. Most people try to empty their bladder before flight and are willing to have full bladder when they leave the plane (to relieve themselves afterwards) so I think even this is already a little pessimistic.

On 747-400 you have max 660 passengers. 1135L divided by 660 passengers gives ~1.7L per passenger on average. Which is a lot. Consider this is an average -- some people may drink way more than that but most people drink less than that in a day.

So it seems it might be possible to run out of the tank -- if you have a very long economy flight in a large plane converted to densest possible seating arrangement and if you are willing to believe people will be buying over 1.7L of fluids per person on an economy flight.

Of course, all that calculation goes out the window if you plan to fly an economy flight full of Australian Oktoberfest fans directly from Germany to Sydney.

747s can fly with an extra engine under the wing. Compared to that, a full waste tank is nothing.

https://www.qantasnewsroom.com.au/roo-tales/that-time-when-w...

From the article: 1 liter (747) to 2 liters (A380) per passenger. More than enough, even if a dozen people manage to catch the runs and eject more liquid.

> how would an almost full tank affect flight behavior

Not that much, pilots can (and have to, as part of weight redistribution caused by the fuel tanks emptying) "trim" the airplane - basically, applying an offset to the neutral position of control surfaces (see https://en.wikipedia.org/wiki/Trim_tab). And all the content of the toilet waste tank was already in the plane before (either in the bodies of the passengers, or in the freshwater tank), so it's only a minor distribution change.

Worse problem: cleaning up un-capped, tipped-over bottles that are no longer full of pee after deboarding.

There's a reason why even Ryanair, otherwise the leader in borderline unethical airline behavior, quietly buried their 2010 attempt to introduce a loo charge [1].

[1]: https://www.theguardian.com/business/2009/mar/05/ryanair-toi...

Eliminate the bathrooms and make everyone wear Depends. I would not be surprised this happens. The bathrooms could be replaced with a few seats and they save money by forcing people to clean up themselves :)

I now refuse to fly, I would rather drive or take a train or even a bus. Of course with covid driving is probably the better option if you can spare the time.

Of course I heard many people now drive instead of fly, which is making medium travel distances (say under 1000 miles) not enjoyable. Never mind the CO2 being pumped out.

Again the US needs a good public transportation system, which of course will never happen. If pols just stop talking about this, it would probably lower CO2 emissions from all their hot air they are expelling.