Another homemade RO question

Thanks, I’ll plan to boil right after RO.

I’m afraid I’m having a difficult time understanding the math behind recirculating. If we assume one 400 GPD membrane without recirculation there would be about 8 gallons per hour flow generating 4 gallons per hour permeate and 4 gallons per hour concentrate. At these values, recovery rate is 50% and sap is concentrated 2x.

With 50% recirculation, 2 gallons of concentrate are fed back into the system. Does the total system flow remain at 8 gallons (6 new sap and 2 concentrated sap) or does it increase to 10 gallons (original setting of 8 new sap plus 2 concentrated)? How do I calculate the concentration factor and recovery rate?

Thanks!

Your pump performance dictates your total flow thru the system. Pumps flowrates vary by pressure. Lower pressure --> higher flow. Higher pressure --> lower flow.

Membranes are rated for a given pressure. Most home membranes are low pressure and work down to 70 psi, but are given their rating at 100 psi. Going too far above 100 psi might increase performance a bit but runs risk of increasing fouling. Probably best to run in the ballpark of 100 - 110 psi.

So, your pump should be able to provide more than 8 gph at 100 psi. Let's assume 12 to keep it simple. Set needle valve so you are getting 4 gph of permeate and 8 gph of concentrate. Now split that concentrate in half. 4 gph back to pump, and 4 gph out. Your pump will be taking 8 gph from your tank, and 4gph from recirc to push 12gph across membrane. Your SYSTEM recovery rate is 50% (4 gph / 8 gph), but your membrane recovery rate is only 33% (4 gph / 12 gph). You concentration will still be 2x.

I could use a little more clarification about the membrane fouling. Am I reading it right that a higher ratio of concentrate:permeate (I assume based on the psi which, if I do my math right 125 psi = 1:2 ratio but 100 psi = 1:1?). Does a lower PSI equal less fouling albeit at the lesser c:p ratio/ flow rate)/

How does one measure one's sap gph intake?

I do not [yet] have a recirc line back into my 2 400 GPD [series] membranes.

Just when I think i understand recirculation and recovery rate, I get further confused, lol. This is a great example which makes sense to me. 12 gph through the pump, 8 gph new sap plus 4 gph recirc, so the system recovery is 4 gph / 8 gph = 50%, so the output of the system is 4 gph of concentrate at 2x (so 2% sap is now 4%). Membrane recovery is 4 gph / 12 gph = 33%.

If you don't do recirc, there is 12 gph of new sap which still produces 4gph of permeate, but now 8gph of concentrate. So system recovery rate is the same as the membrane, 4 gph / 12 gph = 33%, but the concentrate is reduced to 2% * 12 gph / 8 gph = 3% which is 1.5x instead of 2x, however, you are getting that lower concentration at a higher rate, 8gph, instead of 4gph. In both cases, you remove the same amount of water per hour, which is all that counts. So if you start the day with 120 ga of 2% sap, and you want 60 ga of 4% sap, you have to run the system for 15 hours to get 60 ga of permeate, 4 gph * 15 h. Membrane recovery rate is the same in both cases. What am i missing?

Quote:

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Originally Posted by DRoseum

Your pump performance dictates your total flow thru the system. Pumps flowrates vary by pressure. Lower pressure --> higher flow. Higher pressure --> lower flow.

Membranes are rated for a given pressure. Most home membranes are low pressure and work down to 70 psi, but are given their rating at 100 psi. Going about 100 psi might increase performance a bit but runs risk of increasing fouling.

So, your pump should be able to provide more than 8 gph at 100 psi. Let's assume 12 to keep it simple. Set needle valve so you are getting 4 gph of permeate and 8 gph of concentrate. Now split that concentrate in half. 4 gph back to pump, and 4 gph out. Your pump will be taking 8 gph from your tank, and 4gph from recirc to push 12gph across membrane. Your SYSTEM recovery rate is 50% (4 gph / 8 gph), but your membrane recovery rate is only 33% (4 gph / 12 gph). You concentration will still be 2x.

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Remember, system recovery rate and membrane recovery rate will only be equal in a single membrane setup with no recirculation.

You are correct it would take 15 hrs in theory to reduce 120 gal to 60 gal of 2x concentrate, assuming 12 gph pump performance with recirc and the final outputs being balanced 50/50.

Without recirc, and all other factors / settings identical, you would produce 80 gal of 1.5x concentrate in 10 hrs and 40 gal of permeate.

Depending upon multiple factors, such as when you will be running the RO, and your boiling rate, either one could be more ideal for a person.

Do you plan to RO as you boil? You might want to run it for a few hours, then boil in parallel to "end" both processes at the same time. Some will want the RO to run overnight while they sleep and boil it all the next day.

This really comes down to what works best for each person's overall setup and preferences. Do what makes sense for your operation. For me, I want to get as much water out as quickly as I can via the RO and boil in parallel and finish the RO process just slightly before boil is done - so i match RO processing output to my evap rate.

Thanks for the great explanation of how recirculation works, especially how it reduces recovery rate. Given my small volume, I’ve decided to build a one 400 GPD membrane system with recirculation as you described and scale up later if necessary. I’m sure I speak for many when I say we really appreciate you sharing your expertise!

I have a quick question on recirculation.. is there any difference between running a recirculating line back to the pump inflow vs having your concentrate line running back into your feedtank? You'd know you are done when your tank is half full compared to when you started?

A couple drawbacks on recirculating back to your tank
(1) concentrate sap doesn't uniformly mix with raw sap. You can get layers of sap and be recirculating parts of it.
(2) you are introducing concentrated bacteria back into your tank and increasing microbial activity.
(3) not uniform / consistent concentration flowing across membranes for the entire time and you may see performance drops or need to readjust needle valve throughout.

Quote:

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Originally Posted by DRoseum

A couple drawbacks on recirculating back to your tank
(1) concentrate sap doesn't uniformly mix with raw sap. You can get layers of sap and be recirculating parts of it.
(2) you are introducing concentrated bacteria back into your tank and increasing microbial activity.
(3) not uniform / consistent concentration flowing across membranes for the entire time and you may see performance drops or need to readjust needle valve throughout.

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Hmmm. I guess I thought there was some benefits to recirculating back into the tank: 1) The tank will be refrigerated and the concentrate would be chilled quite quickly after RO processing back into it (although so will the head tank I would pump the concentrate into so perhaps not as much of a benefit, 2) I thought the bacteria would be diluted once it's circulated back to the tank vs all concentrated in a head tank.

I hadn't considered at all the different concentrations developing in the tank and the impact on the performance.

In my desire to kiss this (keep it simple stupid) I realize there isn't really a kiss method.. there is some complexity no matter what configuration is chosen.. Maybe I'll just bite the bullet and use two 400gpd membranes and do one pass and be done with it..

Thanks for the input Dan!
Paul.

With such a small number of taps, I would think your RO time and boil time will be greatly reduced compared to the majority, so the time for the bacteria to work will be relative short. I'm a small back-yarder and made all Golden syrup last year, so I'm hoping the added RO processing will darken things up a bit. That being said, I added a re-circulation loop since it is fairly simple, and I will run the concentrate back to the sap until the permeate flow drops significantly. This is my first year with RO, so I'm hoping for the best...