Re-circulation Explained

[needmoremaples]

Is there a type of valve you can use to set your back pressure? In a recirculation setup I need to throttle back the pressure as sap % builds. I have a 4x40 with procon pump and 1 hp feed pump. I'd like to just set the pressure to 225 and check it like every hour. Of course I'd prefer foodgrade or at least for potable water.

[Sappy Camper]

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.

Hello Dave

I am relatively new to the Trader but have lurked for a long time. I grew up in the maple biz, but we boiled raw sap from our 4000 buckets on a 5x16, sometimes working 3 shifts, with no RO. In the olden days. I went to college, had a career, and have recently gotten back to sugaring on a much smaller scale, 275 taps this year, with ambition for no more than 500. I make candy with every bit of syrup and have a small business selling it.
I have read many posts of yours and you have imparted a great deal of thoughtful advice over the years, a very helpful resource to many. Thanks. I read the post quoted above with great interest, as I begin to think about building an RO. I was searching for info on recirculation and came across this post. I am wondering if you have any advice or modifications you would make on that system? I would like to make one similar, and would love to see a schematic (sketch) if you have one. I have questions about other parts of the system like gauges, interlocks, and a feed pump if you have one. It seemed like yours was low tech and manual operation, which for the cost of things now is what I plan to do. I don't know if you are even on the Trader anymore, as your tagline implies you have retired from maple. I hope you get this message and that you are well

Tim Bishop
Palermo, ME

[maple flats]

Sorry Tim, I'm not an engineer, I just observe what I see in my meters and gauges. Afraid I can't help.