Graphene can be used to filter water if nanopores are introduced into graphene membranes. These nanopores are at the atomic level that they allow the sifting of water through them while preventing passage of all other things like salts and even highly toxic chemicals like arsenic. Thus pure water can be filtered using graphene.
Such a graphene membrane is patented by Mr. Lockheed Martin and would be a research that will ease the future of water desalination and filtration. The membrane is called Perforene. It is perforated with nanosized pores and require only fractions of the energy that is currently used for reverse osmosis in water desalination. Graphene is also a very cheap substance.Thus it would be a very cost effective method.
But the method is yet under development and has not been employed in any desalination plants till date. Perforene is said to be more than thousand times stronger than steel. Still, graphene is very delicate and because of its very thin structure, it might be difficult for transfer. Also, high scale production methods are also not yet deviced. Thus graphene water filtration plants are not a reality yet. In the near future, they are expected to push back the reverse osmosis methods for water desalination and filtration.