hags - you are correct on everything you said. I do push the membranes harder than is recommended. My family and my job commitments allow me only a few hours a day to collect and boil so I try and make the most of them. Over the past 6 years running the system the membranes can last between 2-3 years with flushing and washing on a regular basis. With that said you can purchase the membranes for under 25 dollars each which means 150 dollars every two years to save 75-80% on my boiling time and fuel costs is well worth it for me. I am running two setups this year to see how they compare one:
6 - 150gpd membranes in series as I have for the past two years
the second is:
3 - 400gpd membranes in series

After today I will have processed over 1,000 gallons of sap (averaging 1.2% sugar) to 7.5-8% and have found that they are about equal. I also test the permeate with a digital brix meter during each run and it always registers .01 or .00. The 6 membrane system seems to be doing a little better though. After each run I flush with permeate for about 10 minutes change the water filter and then do a warm wash cycle using RO soap (ph 11 - which is what the membranes are rated for) for about an hour. Then I flush with permeate for at least another hour (testing with a ph meter to make sure the ph level is the same as the permeate). This has kept both systems running as the first day I turned them on. At the end of the season I will will do a wash cycle and then a cycle with citric acid and flush for several hours then store them in a pvc holder with the preservative. If the weather holds I should process between 2,500 and 3,000 gallons of sap through them this year.

The concept behind this system is to provide an economical way for the backyard producer to use and benefit from an RO that they can put together themselves I hope this information has been helpful.