Until, of course, it becomes completely clogged.

Ultra-fine filters are going to get locked up by debris quickly.

Fortunately, you can probably replace a graphene filter quickly with plenty more freely available/easily producible graphene (quality and practicality of diy graphene might be dubiuous), but, considering the clean-room level of filtration, I'd figure you might need LOTS of graphene, and several layers of coarse filtration, before filtering at the nano-scale.