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Thread: Re-circulation Explained

  1. #1
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    Default Re-circulation Explained

    Hi guys,
    While all of us rookie RO enthusiasts are trying to squeeze every bit of knowledge out of you experienced operators, can we get a bit more in depth about re-circulation? We have a general idea (I think) about it but can we get more specific about how it benefits our systems and why we would opt to buy a bigger pump to be able to utilize it. I know it's been mentioned in other posts but I thought it might be good to have its own thread as an easier way for folks (like myself) to access the information. At some point I'll have a 3 ring binder full of RO information with reference tabs! Thanks everyone for your participation in all the RO threads, it's been awesome reading all this and many of us appreciate the wealth of knowledge and explanation.
    Mead Maple "It's for the kids..."
    Paul Cerminara
    2019 - First season ever
    -Homemade 275 gallon oil drum arch
    -75 taps, all on traditional buckets and drops into 5 gallon buckets
    -Goal: 3 gallons
    -Season Total: 7.5 gallons - pulled taps after running out of firewood and time
    -Fear: Becoming a member of MT.com and getting bit

  2. #2
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    Sorry to hijack.... I am also a rookie... but I will take a stab at answering... Take it with a grain of salt.. Hoping one of the experts will take me to task if I'm off base....

    Re-circulation will aid in increasing your output concentration rates, while operating in a continuous flow fashion, rather than batching... while adhering to membrane recovery recommendations.

    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.
    There's a pretty good explanation of why fouling happens at the link below... (It is describing Water filtration... but the concept will be the same with sap..). Read the Feedwater Chemistry section.
    https://blog.harnrosystems.com/how-t...eatment-system
    My simpleton way of looking at it is..... I picture the membrane like a perforated screen filter. Lots of flow across the surface of the screen will keep it wiped clean and keep the sugar moving fast and liquidized, and prevent it from landing in the holes and plugging them up as it crystallizes....

    So... the other 85% of the flow needs to go somewhere... Choices might be...
    ... into your concentrate tank at low brix concentration, which you could boil... or run it back through the RO a second or 3rd time later. (basically recirculating in batches)
    ... some or all of it, back to your sap tank, in which case you are still recirculating... and will eventually bring your entire stock of sap up in Brix... as a batch.
    ... or reintroduce most of it back into the piping loop, immediately before your pressure pump... in which case most of the sap will get several passes through the membrane while keeping your flow high and recovery rate good, and producing a higher concentration in one operation. No batching. This can be done with the one high pressure pump... while you bleed off small volume of higher concentrated product.
    ... or reintroduce most of it after your pressure pump.... Same as above but--- you will need to add a dedicated recirculating pump. Haven't heard of anyone on here using this on a small rig... but might be an option if you are looking for high flow rates that a typical procon wont produce.... or just because you want to.. This system could theoretically run on less horsepower, because you'd be recirculating inside of the pressure system (where there will be very little pressure drop across the recirculating pump. This would use less energy than re-pressurizing all of your recirc volume... Smaller procon required just for the concentrate volume and a small HP circulator to move sap around in the loop.)
    .... or Dont recirculate - Just keep adding more membrane housings in series... Essentially creating several "membrane passes" in one straight through operation... ending up with finished brix and not needing to recirculate at all. I think the cost of membranes and housings is what prevents this one from happening a lot.??

    OK - Give it to me..... I can take it...

  3. #3
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    Default

    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 about 1320 taps in '15, doing fewer each year, about 450 planned for 2020 (and after?)
    2012 Mahindra 36 HP 4x4/ loader/cab/heat/AC:-)
    added a gooseneck equipment trailer and F350 to tow it to haul more sap
    3x8 raised flue evaporator
    250 GPH converted to electric, RO by Ray Gingerich
    6.32 KW solar system, 1.48KW is battery backed up, all net metered
    http://s1041.photobucket.com/albums/...anssugarhouse/
    website: www.cnymaple.com

  4. #4
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    15 percent rejection rate means 85 percent pure water gets through and 15 percent of feed water is wasted, rejected to drain, in a water purification system. In maple the reject waste is our concentrate. Re-read your article that you linked.

  5. #5
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    Default

    This is great, thanks guys.

    So if I wanted to add a re-circulating pump to the system, how do I go about sizing that? Would it be the same as my primary HP pump or could I get away with something smaller? Also, does that line get plumbed directly into the original line with a "Y" fitting that would go into the membrane inlet?
    Mead Maple "It's for the kids..."
    Paul Cerminara
    2019 - First season ever
    -Homemade 275 gallon oil drum arch
    -75 taps, all on traditional buckets and drops into 5 gallon buckets
    -Goal: 3 gallons
    -Season Total: 7.5 gallons - pulled taps after running out of firewood and time
    -Fear: Becoming a member of MT.com and getting bit

  6. #6
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    SW Ontario
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    Quote Originally Posted by arcticmaple8 View Post
    15 percent rejection rate means 85 percent pure water gets through and 15 percent of feed water is wasted, rejected to drain, in a water purification system. In maple the reject waste is our concentrate. Re-read your article that you linked.
    I am just learning myself... and I may have it all wrong.. but I was refering to "recovery rate" and not rejection rate.
    This Lapierre Link defines Recovery as "The percentage of the water removed from the raw sap through the membrane."
    http://www.sugaringequipment.elapierre.com/down/191.pdf


    The typical "15% Recovery Rate" specifications is something that I have read on this forum several times...
    I have not found a lot of data to show what happens if you exceed this rating.... So... at this point I am just assuming it to be valid.. I haven't looked up a lot of different membrane spec to confirm the information... But below is a link to a MES maple membrane..... You can see that its test condition rate is 15%. To be honest, I don't know why they choose 15% for testing maple membranes... When I look at Saltwater membranes they tend to test at much lower percentages.

    Before I head too far down the wrong path.... I have sent an email to a membrane manufacturer, asking for some clarification on Recovery Rate. I will pass it on if they reply.

    https://static1.squarespace.com/stat...eet+4+inch.pdf

  7. #7
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    Default

    I found something of interest to the RO enthusiast and student.

    Below is a link to a Maple membrane training presentation from H2O Innovations.... Good resource....
    Took me a while to find an english version... It doesnt seem to be on their website...
    and I've attached a snip of one slide of particular interest... for this "Recovery Rate" / "re-circulation" conversation
    They are discussing things that influence Membrane Clogging.... The first one being "Non-observance of the membranes recovery rate"

    recovery slide.jpg

    https://www.scribd.com/document/1147...g-2011-ENG-pdf
    Last edited by wmick; 05-03-2019 at 12:02 PM.

  8. #8
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    Quote Originally Posted by maple flats View Post
    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.
    On my RO I often get 3 gpm of permeate and 1 gpm of concentrate.
    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). .
    Hey Maple Flats
    Just trying to clarify if we are on the same page with the terminology - When they talk about the "Recovery" rate on a membrane, it is not talking about total system output... but rather... what is happening with that particular membrane at any moment in time... (total flow across the membrane includes anything that was reintroduced by recirc)... (and if you have more than one membrane in series.. then recovery is divided between them)

    You mention that you are recirculating.... Do you know how much? Do you know what the total output of your pump is in GPM?
    Do you have more than one membrane in series? Thats a big factor...
    Your total system recovery rate is 75%.. (at 3Perm+1Conc). but your recovery % on the membrane is less if you are recirculating and/or have more than one membrane.
    Curious - What does your rig entail? for pump gpm and membrane configuration ???
    Last edited by wmick; 05-03-2019 at 12:54 PM.

  9. #9
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    Wmick: i read your link to lappiere. Good read. I see it recomends mot concentrating past 70% in beginning and uses 75% as an example in the testing procedure. This is very typical in maple ros. I didnt see anything about 15% recovery.

  10. #10
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    Mead maple: hope you didnt give up on your question. The pressure pump builds your high pressure but at a lower flow rate while the recirc pump moves sap in a loop through membrane while under that high pressure but at a high flow rate. An example of a single 4 x 40 inch ro would be a high pressure pump procon 240gph(4gpm) which would require approximately 1hp to build 250psi at 4gpm. Adadd a recirc within the pressure loop and you dont need any additional pressure so a 15gpm centrifugal pump with low pressure high volume will only require approximately a 1/2 hp motor but this pump must be capable of being under 250psi... sizing your recirc is about the max flow through your membrane per manufacturer spec. An xle4040 is 15gpm i believe. With no recirc your membrane gets 4gpm from pressure pump minus the permeate flow of 1gpm maybe. So without recirc membrane will plug faster and require rinsing/wash sooner

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