Thank you Reg and heathcliffe for your kind words. These are pretty simple calculations any bright high-schooler could do but I'll take all the praise I can get.

It turns out that Kevin Costner has been bankrolling development of an oil/water separation system which might be used for a piece of the cleanup. His device uses a centrifuge rather than membranes but the result is much the same. Costner claims an effectiveness of 97% in this article:

http://www.csmonitor.com/USA/2010/0520/Kevin-Costner-oil-spill-cleanup-idea-interests-BP

His business partner claims an effectiveness of 99% on page 2 of this one:

http://articles.latimes.com/2010/may/21/nation/la-na-oil-spill-hollywood-20100521

The downside is that the largest of his machines can only treat 200 gpm and only twenty of them exist. It might be possible to protect a limited bit of coastline with these but they're far from being a large scale solution.

By the way, centrifuges are an even more proven technology than membranes for this sort of thing. They've been used for a very long time to separate oily butterfat from watery skim milk in dairy cream separators. Grandma's version was hand cranked and modern ones are powered by electric motors but the principle is the same.