I'm now not so sure about this fresh water business.
In the Dessalator Duo 60 manual it says:
Remember: The biggest enemy of the membranes is fresh water.
Fresh water should be always used with no pressure when going through the system
(pressure dial turned all the way anti clockwise) and the system should always run with no
pressure after a fresh water flush to dump all the fresh water that are in it, before making
freshwater from sea water (also with the pressure dial all the way anti clockwise).
When running the watermaker with the dial all the way anti clockwise, it will shut it self
down automatically after 1 minute. Only then, the watermaker is ready for use.
I have the Filmtec Membrane Technical Manual and I can find no discussion of this at all anywhere.
In fact a number of systems are illustrated that
1. Treat Brackish water
2. RO the RO water (double RO) to make ultrapure water.
So I don;t know where this freshwater enemy story comes from......nothing on a Google search.
If it really was a problem I would think Filmtec would warn about it.