We all know that getting good food and good air is very essential for fundamental well being. This amplifies its importance in space, particularly in the International Space Station and other spacecraft. Like oxygen, water is a precious resource in space. Hence water management in spacecraft is extremely important. There have been rumors that astronauts on the International Space Station have to drink water recycled from their own urine. Well, it’s true! Onboard the ISS, wastewater, be it urine or greywater from cleaning and cabin humidity, is carefully collected, treated, and reused. How do they get water? Well, they have a choice, they can, of course, bring water with them which they expect to need on the entire mission with the idea that we’re not going to reuse it, or we can try to recycle the water. Generally, for long-duration missions, it’s advantageous to recycle the water.
There are two types of water which they recycle. There is what we call greywater, and that comes from recapturing the cabin humidifier. They have a heat exchanger where the air passes over blades of cold water and the humidity condenses out of the cabin atmosphere. That’s how they control the humidity of the cabin. But that water is then separated from the air centrifugally, and it’s captured and sent into the recycling. This is the water that comes both from the breath and perspiration. There’s water that they use for washing hands and any other personal hygiene. Together, the cabin humidity and the washing water are put into a wastewater tank, and that’s called greywater. For a long time, that was the only water that was recycled on the International Space Station.
However, recently, they have started also processing urine, which is not greywater. It’s much dirtier. And so need extra processing on the urine in order to turn the dirty urine into greywater which can then be processed along with the rest of the wastewater. There’s a lot of contaminants, solid contaminants in urine, urea, salt, and many others. The other problem with urine is that the urea eventually decomposes into ammonia and carbon dioxide which gives the typical bad odour of unprocessed urine. When we start to take the clean water out of the urine, we concentrate the solids and produce sludge, which can clog up pumps and valves. Thus we have to be careful at the handling of urine, which is why it’s taken a while to get the system working.
Water recovery system for ISS.
So this is the big picture of the combined water system in the International Space Station. We take the grey water, the humidity, and hygiene wastewater that goes directly into the water processor. The urine, on the other hand, has to first go through a urine processor. Most of it goes then to the water processor, but a little bit of it has to be removed as waste brine, essentially, very highly concentrated solids in the liquid. These are two racks in the US segment of the International Space Station. One of them handles all of the water and urine processing.
There’s another rack where the oxygen is generated and where the air processing is done. Just like all the other processes, we separate gas and liquids. This is one of the major challenges in any sort of physical and chemical operations in space because it doesn’t have gravity-driven convection. As an example, if we want to boil water on Earth, we heat water and the little bubbles that form move upward which is basically convection and thus causes water to boil. What happens in weightlessness?
Well, the problem is that the gas bubbles don’t move upward to the surface. This is because they are not any lighter than the liquid surrounding them, as neither has any weight. So instead of moving upward the bubble just keeps getting bigger and bigger. Eventually, you’re left with just gas on the bottom surface. We no longer get the same heat transfer as we did into the liquid, and this might halt the process. To fix this, we have to periodically stir the material. The easiest way of separating gas and a liquid is centrifugation. Hence we take the urine, do a little bit of pre-treatment to control the pH so that it doesn’t harm the interior materials, and put it into a vapor compression distillation unit. The unit is turning around because it has to be moving centrifugally so that they can pull the liquid off the outside. The aim is to take the pre-treated urine and evaporate it so that pure water goes off and leaves a much more concentrated solution. Now, the problem with trying to evaporate it, a standard distillation technique, is that it takes a lot of energy as we have to heat it to a boiling point.
So here’s where we can take advantage of being in space. As we reduce the atmospheric pressure on the surface of the liquid, it will boil at a lower temperature. This is the problem we face when we’re in the mountains, that as you get to higher and higher altitudes, the water boils at lower temperatures so that if we’re trying to cook rice or potatoes we have to cook them for longer periods as we’re at higher altitudes. However, as we are in space, we have a ready source of vacuum. Hooking this unit into the vacuum of space produces a vacuum over the urine, and thus, evaporates at a much lower temperature than if it were fully pressurized. This allows lower energy consumption than if done without a vacuum. Using the rotating drum, we evaporate the urine and drive off the wastewater, which is now greywater. This is put into the wastewater tank along with the other sources of greywater. The liquid that’s left is the highly concentrated brine.
People have been working for a long time trying to figure out ways that we could further recycle the brine, but it’s very difficult as it contains many contaminants in it. It is possible that someday we’ll probably figure out how. However, for the time being, this brine is discarded. This is how water is recycled in space.
Astronauts drinking recycled water from the urine and other wastewater.
– Article by Surya Kant, 3rd year Department of Chemical Engineering