Humans need water to survive, and hence a reliable water supply is essential for Space Settlements. Fortunately Near Earth Objects consist for a substantial part of water. However human and industrial consumption of water produces waste water. Though we could “dump” this into space and replace it with fresh water from NEOs, that would be quite inefficient, and it also limites the growth of Space Settlements in the long run. Consequently we need to recycle water in Space habitats.
Basically we need a closed loop in which waste water is turned back into drinkable water. Hence a reliable and preferably cheap method of water purification is essential for the successful humanization of space.
A potentially promising method seems to be found. According to this article in the Science Daily, graphene is an excellent water filter. Not only is this method fast, it is also not energy intensive, and scientists hope to be able to turn seawater into freshwater just by using a hand pump with this graphene filter in the near future.
Nowadays graphene can be produced easily, and it is an allotrope of carbon, an element very abundantly in Near Earth Asteroids.
On a side note, we could wonder whether graphene filters can be used for extracting uranium from seawater. Though the concentration of uranium in seawater is low, the total amount of uranium in the oceans far exceed to total known reserves on land. But due to the concentration of uranium in seawater many scientists believe, extraction from seawater will be impracticable, at least with current market prices.
According to the Science Daily article, graphene filters can accurately distinguish between difference in atomic seizes. This would be important if we want to filter out uranium atoms from seawater.
Probably we would need two filters: one the filter out atoms and molecules larger than uranium, and a second filter which on let smaller atoms and molecules through. Consequently the uranium atoms will get trapped between the two filters, and with each cycle the concentration of uranium will increase.
If this procedure will be feasible, then most countries will be able to become self-sufficient in their uranium supply. Currently only a few countries dominate the world market.