Yep, checked with the engineers this morning, and 400 psi would be the absolute minimum pressure it would take to desal using RO. We are running about 300 psi to get the flow rate up high enough on our recycled (brackish) water system, and the desals typically run at 50 atm, or about 800 psi to generate sufficient flow.

At this point, I think the only manual method that would generate the amount of water required would have to be centrifugal.