Asymmetric nanotube containing membranes

A water filtration method, comprising passing water through a membrane to filter the water, wherein the membrane comprises a porous polymer and terminating on one side in a skin, with open-ended nanotubes embedded in said membrane, at least a portion of the nanotubes each having two open ends protruding through opposite surfaces respectively of said skin to provide fluid communication through each of said nanotubes through said membrane, and said skin forming a substantially impermeable barrier around said nanotubes, said porous polymer and said skin comprising the same polymer.