The salinity of the water determines the pressure required, not the size of
the membrane. The process is called reverse osmosis so the pressure has to
be higher than the normal osmotic pressure. That means you need over 800
PSI. For drinkable but not totally salt free product the pressure can be a
little lower. The PUR hand pump does that with a very small piston on a
very long lever. The PUR 35 is rated for 1.2 gallons an hour at 30 strokes
a minute but it is very hard to keep up that rate for more than 10 or 15
minutes. That is fine for an emergency but for only a little more than the
$1,500 price you can build a powered system that will produce 22 gallons of
very pure water an hour.

In theory you could build a manual system with a 20" membrane, a length of
stainless tube and a $100 manual hydraulic pump but the basic materials
would cost about $500 and it would require a lot of machining. Also high
pressure salt water is very corrosive a steel pump intended for hydraulic
fluid will not last very long.

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
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