On Mon, 20 Apr 2009 04:16:01 +0000, Larry wrote:

AS it was explained to me, these boat RO systems, to be small, use very
high pressure on the membrane, as opposed to large commercial systems
like a public utility would use in a purification plant at low pressure.

Higher pressure is needed to desalinate sea water, much higher. It
depends on the quantity of disolved salts, not membrane size.