Although the post I'm replying to is a couple of years old, the questions it poses are still alive. Here are my thoughts:
I pretty much agree with all of the other replies, although I was surprised to hear about the supposed seven-year life of a membrane. After 14 years as the "cruising evangelist" for PUR/Katadyn and several lengthy visits to the factory, I've never run across that bit of info. I'm not saying it is wrong, I just never heard it before.
Katadyn recommends that a membrane, if stored and unused, should be biocided annually. That is advice that may err on the side of caution. On the other hand, I think it is worthwhile to not only biocide annually but, before doing so, run the watermaker and determine if it is still functioning OK.
I can offer two anecdotes from my own experience. First, during a visit to the factory many years ago, an experienced company technician said that annual biociding is really not necessary, and that he had knowledge of a number of membranes that had sat on a shelf at the factory for several years that were still quite good. Also, I have serviced some old model 35 watermakers, which use the exact same membrane element as the later model 40E (and 80E, for that matter) that had not been biocided for many years, and they were all fine.
Although I can't and won't speculate on the shelf life of an unused membrane, I can say that the main threats to membranes are (1) exposure to water with chlorine in it, (2) exposure to petroleum products and (3) membrane drying out. The last case is more likely with membranes that are located in hot engine
rooms or that have been stored for lengthy periods of time and not sealed carefully in an air-tight container.
Finally, I should add that one is always taking a risk when buying
a used watermaker (e.g., from Craigslist, Ebay, etc.). Personally, I would never do so unless I was able to actually run the unit long enough to determine if the water quality was good. Also, buying
an older unit with the PUR brand name (e.g., 35, 40E, 80E) can be risky. There were a number of problems with those brand units that were corrected by Katadyn when they bought the company. If you want more info on those problems and what to look for, visit my website about them: www (dot) katadyn (dot) ishipaco (dot) com. You may want to send the unit back to the factory if it needs some of the redesigned components I write about there. If you have any questions I don't answer there, please feel free to email
me at powersurvivorhelp (@) ishipaco (dot) com and I'll try to answer them.