To gain further information regarding use of PG with RO membranes I contacted Dow again about their desalination Filmtec (SW) membranes yesterday. Their reply is below:
"Dow does not recommend the use of propylene glycol as an anti-freeze in FILMTEC(TM) Membranes. Alternately, you may store the elements in a 1% sodium bisulfite solution, which will freeze below -4°C (22°F), but the membrane will not be damaged, provided they are thawed before loading and use. Please see the attached Technical Data Sheet for additional information."
Nevertheless, I do know that some other manufacturers do seem to approve PG for certain membranes and as Tellie indicated some (a few) membranes actually come packaged in dilute PG (10%).
So the situation is cloudy at best. It would be good to know real life experiences linked to specific membrane types. Not all RO membranes are
manufactured in the same way from the same materials. It is certainly easier to stabilize modules in their systems with PG antifreeze
if this is an approved/proven option.