Airports Had to Deal With Over 2B Litres of Airplane Toilet Waste in 2019

I wonder if it has enough liquid to cause a tank overflow. Let's say just dump drinking water into the tank...

It seems very likely to me that they carry more than 1L of fluid per passenger, plus there's also the crew. So yes, they could :)

Of course, if they dump all their drinking water, it'll be hard to reach the maximum pee capacity per passenger. But you could always resort to dumping food.

First of all, all that pee is on the plane when it lifts off, in one form or another. The only thing that changes is distribution of mass (depending on where the tank is and where they keep bottles with drinks). A tonne of pee gathered from all over the plane is most likely less effect than 3-4 people moving from one end to another end of the plane. So a stewardess with help and a cart.

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.

Solution: Pay toilets in the coach cabin.