If that's the case I have had it all wrong ("Membranes are designed with a specific recovery rate in mind, to prevent fouling... I understand that maple membranes are typically rated at about 15%.
Which means you should take no more than 15% of your feed-water (sap) off as permeate, per pass.")
I run at about 240-260 GPH on my little RO, and I remove about 3x as much permeate as I get concentrate on my first pass. That is 75% if the sap temperature is 37-38 F. I don't know where the figure of 15% came from, but it is not talking about maple sap.
After the 1st pass, I will have taken 2% sap up to 8% concentrate +/- a little depending on the temperature. Then I often recirculate the concentrate thru the RO a second or more time. When I do that I need to back off the operating pressure and then I only remove a smaller portion in permeate per minute. On my RO I often get 3 gpm of permeate and 1 gpm of concentrate if I set the pressure at about 280 PSI, If I go down to 270-275 the ratios change, Then I get slightly more concentrate per minute and slightly less permeate per minute. The total is still essentially the same. My re-circulation then might go to the opposite or nearly opposite the ratio I got the first time. Thus my little basic RO on second pass might just remove 1 gpm of additional permeate. My RO only can handle pressures up to 300 PSI, more expensive ones can be run at higher pressures and thus can remove more permeate on the first pass. Many can do between 12% and as high as over 20% in one pass, the newest ones designed for hi brix can get 35% concentrate in one pass, rather than the 8% I can get.
In my case recirculation is done 2 ways. First, on my "first pass" I open a re-circulation loop, that sends some of the flow out of the membranes back thru the high pressure pump (higher end RO's use another pump to push re-circulation back thru the membranes). The other way I re-circulate is by running my concentrate that was pushed out of the RO, back thru again.
A lot of this is going to be based on your needs and more your pocket book. An RO like mine only cost a very small % of what a high brix RO costs. It's a matter of how much sap do you need to process and how fast do you need to do it. You might think of it as balancing $ with time.
Dave Klish, I recently ordered a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.