My RO froze! - (also Q&A and info on building my RO and blower) Off Grid Maple Syrup 2024 

Excellent question - and one that is impossible to put an exact number on; the membranes can only tolerate so much ‘stuff’ before water can’t pass through - I’ve maxed out at 1250 ppm (this number includes sugar and minerals and the more minerals the lower the max sugar % that can be reached). For birch sap I maxed at brix 6.3 because of the extremely high mineral content. With maple, using this years data on ppm the theoretical max would be 22%, but 15-18% is probably more realistic as a max because as the % rises membranes fowl much faster and eventually flushing every 5 hours would drop to maybe even hourly. Warmer liquid may also increase efficiency but may not result in a higher % in the real world. Yes - I boil match. If I slowed the boil I would match concentrate production (GPH); which would give a higher sugar % but fowl membranes faster and decrease the amount of water removed each hour. For example I recorded about 4 GPH conc and 10 GPH perm at 7.5 brix vs. 10 GPH conc and 15 GPH perm at 5.8 brix I could run the RO overnight but I’d need an auto-off in case something fails and also temperatures would need to be above freezing. So far I can’t justify the risk of an unsupervised RO in my setup.

you can increase to 120-130 psi, add more membranes, or put both permeate and concentrate in the same barrel and run until you reach the desired brix. When I do birch sap I dump the conc. line back into the sap barrel and run until I get a fairly high brix, then move the conc. line into a pot until the pot is full and barrel empty.