Thanks to Brilliant for supporting this SciShow video! As a SciShow viewer, you can keep building your STEM skills with a 30 day free trial and 20% off an annual premium subscription. Did you know you can start fires with water? Well, not exactly the same water that firefighters use to put out fires. To start a fire with water, you need supercritical water. This stuff burns organic material that we’re trying to get rid of, like poop. And scientists think it can be used to get rid of even more persistent waste, too. Like forever chemicals. Supercritical water has the potential to burn all sorts of waste, and might get us over more than one hurdle on the way to clean drinking water.
If water is liquid, ice is solid, and steam is gas, supercritical water is the fourth option. It’s water that’s been compressed and heated into something that’s not quite a liquid and not quite a gas. Instead, it looks like a gas and in some ways behaves like a gas, but in other ways also behaves like a liquid. To make that happen, you have to heat it under high pressure, to temperatures well above boiling. Imagine putting water on the stove. Normally, when water reaches approximately 100 degrees Celsius it starts to boil. At that temperature, the pressure inside the water equals the pressure of the air outside the water, so the water can become steam. But if you raised the air pressure using a pressure cooker, it would also raise the internal water pressure needed to start boiling. So by raising the air pressure, you can create an environment where water doesn’t boil at 100 degrees. Instead, water inside a conventional pressure cooker boils at about 120 degrees. That’s why pressure cookers can cook food so quickly.
By keeping water at a higher pressure, they can heat food to temperatures hotter than the boiling point. If you can increase that pressure even more than a pressure cooker, you can raise water’s boiling point to even higher temperatures. But you can’t keep that up indefinitely. At a certain point, liquid water just can’t keep itself together anymore, no matter how much pressure you put on it. That point is called the critical point, and it happens at 373 degrees Celsius. It also requires a pressure of 217 bars, or more than 200 times the air pressure you’re most likely feeling right now.
Once water reaches the critical point, it has to change states. But because of the pressure, it can’t turn itself into ordinary steam. Instead, it becomes something that looks like water vapor, but has some very different properties. Like a gas, supercritical water can make its way through tiny gaps in solid objects. But like a liquid, it can dissolve materials. That last bit is what makes supercritical water so special. Because it’s already really hot, more than three times hotter than ordinary boiling water, it can dissolve substances that wouldn’t normally dissolve in room temperature water.
See, when water that hot hits organic material like the stuff that makes up our waste, it breaks the bonds between molecules. So those molecules are now free to react with oxygen in what’s called oxidation. The short and sweet of it is that the waste molecules lose electrons and become more positively charged. And that process of oxidation is what’s chemically happening when you start a fire, too. What we see as burning is actually fuel being oxidized, which is why fires go out when you take away their source of oxygen. That means fires burn away organic waste in a very similar way to supercritical water. When supercritical water encounters a molecule of waste, it breaks up the bonds between the atoms, letting them react with oxygen. That leads to oxidation that burns away what’s left of it. This process is known as supercritical water oxidation. It allows for burning without the same danger as fire. Supercritical water reactions have been found to be a great method of getting rid of sewage, as it burns organic material cleanly without flames. This makes it particularly useful for spacecrafts, as the byproducts of the reaction - water and carbon dioxide - can be recycled back into the spacecraft for use. The water can be used to sustain plants, which can then provide oxygen and food to astronauts, allowing them to stay in space for longer.
However, this process still has a few hurdles to overcome before it can be widely applied. One of these is the salt that is produced as a byproduct, which can cause corrosion and damage delicate machinery.
Supercritical water can also be used to clean contaminated water, by breaking down per- and polyfluoroalkyls (PFAS) molecules. However, this process produces carbon dioxide, which is a pollutant on Earth and can worsen the climate crisis. Researchers are looking into ways to eliminate the carbon dioxide emitted by the reaction before it reaches the atmosphere, such as injecting it deep into the ground where it is locked away.
With these innovations developing fast, we could be seeing a lot less waste and a lot more fiery water in our future!