Living on board the International Space Station
 

Water recycling on board the ISS
 

Sweating astronauts

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After collecting the condensed water, the water needs to be purified by removing bacteria and unwanted ions and molecules. This is necessary for the crew’s health. The unit that performs this task is the water processor and the purification is done in several steps:

1.  When wastewater enters the water processor a liquid separator removes the gas bubbles from the liquid. Then gas and water can be treated separately, which simplifies the equipment and processes needed for the following steps.
2.  Once the gas is removed the water is filtered like coffee in a coffee filter. All particles with a diameter larger than 0.5 micron are trapped in the filter, like the grinded coffee in the coffee filter. (For comparison: the average thickness of a human hair is about 10 micron.)
3.  After this step the water is forced across a surface containing sorbent and ion exchange material. In this step most of the contaminants are removed from the water.
4.  Then, there are only some tiny molecules left that need to be removed before the astronauts can re-use the water. These molecules are removed by heating the water to more than 100ºC and then flown across a catalyst.

If there are still particles left after having been through this filtering process, the water goes through it again after cooling down. When the water gets out of the water processor on board the ISS, it is cleaner than the water most of us drink on Earth!

The sorbent material takes in (absorbs) the molecules we want to remove (it works like a sponge).

Ion exchange material attracts unwanted ions (in the same way a magnet attracts metal) and releases the wanted ions.

A catalyst is a chemical that promotes certain chemical reactions in a way so that less energy is needed to gain the wanted reaction.

If a catalyst is present, it can split a chemical reaction into two partial reactions that will need lower activation energy. In other occasions reactions occur only if a catalyst is present. A common example of this is the catalyst in every modern car. It reduces the amount of exhaust gases by breaking the fuel leftovers down to mostly water and carbon dioxide.

Water management on the ISS