Now imagine another situation: you are walking on the street and suddenly, you feel a drop on your skin. You look up, but the sky is clear. Oh, wait, you spot an airplane. Have you ever wondered whether the plane has just discharged its waste into the atmosphere and some of that waste landed upon your skin? If so, don't worry; that isn't how it works. Nowadays many means of transportation, air-crafts included, have storage tanks that are emptied once the destination is reached. Nonetheless, I remember when I was young a sign would hang on the toilet door of Italian trains, asking passengers not to use the restroom while the train was in a station. I also remember that I was told this was necessary to prevent the smell and dirt spreading throughout the station since trains were getting rid of the human sewage by flushing it out directly onto the tracks. Rain and time would have eventually washed it out. The same was a common practice in the United States of America, and other parts of the world, until the end of the 20th century. Some decades afterwards, the system is different: there is an onboard holding tank.
Train toilets - A sign inviting passengers not to use the toilet when the train is stationary
Planes operate in a similar manner. They either use a closed water system (onboard tank) or, since the Eighties, a vacuum waste one. Thanks to the air pressure differential between inside and outside the vehicle the latter sucks wastewater into a tank which, in turn, can be opened only once on the ground using a valve located externally on the side. For sure, the handling of sewage on flying transportation means has been an object of curiosity for many people. Or, at least, it was for King George V, according to Leonard Mosley's account in his Lindbergh: A Biography. The story goes that Lindbergh, American pilot, got to meet the sovereign after a historical non-stop flight across the Atlantic. On that occasion the king asked him about the method he adopted to pee up in the air. He explained that in his chair there was a hole connected via a funnel to an aluminium container which he dropped over France. Today this would be impossible: in case of malfunctions liquids may indeed leak out, but they would normally evaporate before reaching the earth.
Airplane lavatory - First class restroom on an Air Canada Boeing
On cruise ships, though, it works differently. Due to the sheer amount of passengers they carry and the distances they cover, they have to empty their tanks into the sea. The problem with such old-fashioned wastewater treatment systems is that harmful materials (fecal matter, bacteria, heavy metals, other contaminants) are left behind in the marine environment. Their quantity is not paltry: statistical data provided by Friends of the Earth show that around 4 billion litres of sewage were released into the ocean in 2014. Stronger regulation is necessary to enforce developments and foster more environmentally-friendly practices. However, onboard tanks are being designed: they should create an advanced wastewater purification system suitable for improving efficiency and purity of discharges.
Golden Princess Cruise ship - A cruise ship leaving the coastal area
Actually, there is still one means of transportation which discharges waste by throwing it off into the atmosphere, although scientists are looking for new recycling solutions: solid waste from astronauts is stored for the return, whilst the liquid is sometimes fired back towards the Earth, but it sublimates and turns into gas. So, no, that drop you felt on the skin was not pee coming from the space either.