Another homemade RO question

[Rick M]

I’ve been running a homemade 3 x 150 gpd RO with an aquatec 8852 pump the last few seasons and it has been great. Rough calculations put me at being able to pass through 6 gallons of raw sap per hour and run the RO at 95-100 psi. At that rate I end up with equal volume of permeate and concentrate.

I increased my number of taps to about 120 last year. During the big runs I found myself running the RO for over 16 hours straight. So I’d like to increase the rate of my RO to cut my processing time of 100 gallons of raw sap down to about 4-6 hours while getting the same results.

I’d appreciate input on whether or not it would be more beneficial (while being cost conscious…or as some would say cheap) to either:
1. Add a 4th 150 gpd membrane
2. Change out my 3 150’s to either 2 or 3 400’s

I’m also thinking about cutting in a recirculation line after the last membrane and teeing in back in before the 1st membrane. From what I gather it keeps the membranes from fouling and it’s a simple upgrade. I just don’t know if it’ll make much difference in the overall performance of the unit.

Thanks in advance

[eustis22]

well, you have 50 or so more taps than i do and when I ran 4 150 gpd membranes off an aquatec 8852 I still only got about 8 gph which, when you're processing 120 gallons of sap, makes for a pretty long day. A lot of times the below freezing temps caused me to basically stockpile sap until I could get the shack warmed up enough to RO. Ideally I am aiming to process 120 gallons in 8 hours so I this year I am going with 2 400 gpd membranes and a bigger pump recommended in DR Roseum's thread. I don't think I am quite ready for a Procon until I relocate to Maine. Just my .02

[DRoseum]

You could also just add a 3012 housing with a 600 gpd membrane in it if you want to limit expense. Put it first and then the 3 150s in series after it. Agreed on bigger pump helping as well.

Maxflow sells 600gpd membranes that fit on the 3012/400 gpd housings.

[Sourwood Sugarbush]

Hi,

I am new to the forum and a novice syrup maker. I’ve read many previous posts about DIY RO systems for hobby producers and want to build one for the 2023 season. With my limited number of maple trees I don’t expect to have more than 30 taps. If I want to remove 1/2 to 2/3 of the water, how many membranes should I use? I was planning an Aquatech 8852 pump and 150 GPD membranes from Membrane Solutions as I found a discount code graciously provided by Roseum Maple Syrup.

Thanks in advance for any advice!

[wobbletop]

For DIY membranes, the number and size of the membranes are more about processing time than water removal. Given enough time, the membranes will eventually all remove about the same amount of water. Using a 400 or 600 gpd membrane instead of the 150 means you'll reach that point sooner. I'd start with a 400GPD and then add additional ones when you need to process more in the same amount of time.

[DRoseum]

Hi - glad you found the discount codes! Your setup really depends upon your boiling approach. Do you want to boil all the sap you get each day? Or are you saving sap up for weekend boils? Or concentrate over night and boil the following day?

Most days you will average 1 gal /tap. 30 taps = 30 gal of sap. That's roughly 2 -3 hrs of running the RO to cut it in half. Then you still have 15 gallons to boil down to roughly a half gallon of syrup. Maybe 2 times during the season you will get a bigger run of 2 gal or more per tap (60 gals total). How much time to you want the RO process to take, and how much sap do you want to boil for each "batch"?

For your size, you might want to start with a single 400 gpd or 2 - 3 of the 150 gpd membranes. You can also introduce recirculation to up your concentrate while keeping recovery rates low. Split the concentrate outlet in half and feed one back to the intake of the pump and the other will remain as your concentrate discharge.

I always try to fully process each days sap to completion - letting sap sit too long can darken the syrup or even spoil the sap due to microbial activity, and its even worse/faster when its concentrated and not boiled. So for me, matching the speed of the RO concentrate output to my boiling rate was a key factor. Below i will outline some maximized setups for that pump which i believe gets about 16 gph at 100psi if i remember correctly. Might be less, but let's use that as a starting point.

First, recovery rate = (permeate out / flow in) for each membrane and LOWER is better to reduce fouling.

Second, bigger rated membranes get more water out per minute, and more membranes in series get more water out per minute and can reduce fouling and maximize membrane life.

Many people use the aquatec 8852 with 150 gpd membranes. I used a single 400 gpd membrane with the 8852 pump for about 36 taps at one point and it served me quite well. You can use up to 2 of the 400 gpd membranes in series with that pump and get even better performance. The 400 gpd membranes will take water out of the sap at about ~4 gph each. (800gpd total --> 16 gallons sap/hr, rejecting 8 gallons of water with 50% system recovery and ~25% and ~33% membrane recovery rates). Adding a 3rd membrane will remove even more but it will be running at 50% recovery rate and experience more fouling. I did this and it worked but was probably hard on that last membrane.

Or you could also use up to 5 or 6 of the 150 gpd membranes to roughly get the same system performance (total would be 750 or 900 gpd for that system, 50% recovery). One of the benefits of the 5 or 6 membrane setup is that you will be able to have each membrane operating at a lower recovery rate while still having the overall system recovery rate be 50%. That is better for overall life of the membrane and minimizing fouling. With 6 of the 150 membranes, the worst individual recovery rate would be in the last membrane and be only ~18% . However, there is pressure loss across each membrane so your performance of each subsequent membrane in series may decline slightly (pros and cons!!). I believe most people have found 5 to be about the max for that pump.

[Sourwood Sugarbush]

Thanks wobbletop & DRoseum for taking the time to help me with my first RO build!

I’ve been working to understand Recovery Rate and Permeate Removal. For example, I attempted the three 150 GPD scenario. Would you please review and comment if I’m on the right track?
450 gallons per day
18.75 gallons per hour
Derate for cold sap: 9 gallons per hour
50% Recovery rate
4.5 gallons permeate per hour
4.5 gallons concentrate per hour (2x sugar)
First membrane: 17% recovery rate
Second membrane: 20% recovery rate
Third membrane: 25% recovery rate

For the one 400 GPD setup you mentioned, is there an issue with fouling as it seems the recovery rate is 50%?

Given how severe the membrane derating is for temperature (3-4% per deg C), do you recommend using an immersion heater to raise the sap temperature?

Thanks again for your help, I really appreciate it!

[DRoseum]

No problem. Your math looks good for the 3 x 150 gpd membrane setup.

For the single 400 gpd membrane, you are right. Recovery rate would be 50%. I did this for 1 season and flushed /washed membrane frequently and am still using that membrane (4 full seasons and over 5500 gallons of sap thru it) and haven't seen major performance decline.

To avoid the higher recovery rate, you can do recirc. Split the concentrate output after the needle valve in half and run half back to a tee at the inlet of the pump and the other half becomes your concentrate outlet. Adjust needle valve to still produce 50% outlet of permeate and concentrate. This will keep your membrane recovery rate lower while still giving you a 50/50 final output for your system performance. You should be able to get it down under 30% this way.

[wobbletop]

Given how severe the membrane derating is for temperature (3-4% per deg C), do you recommend using an immersion heater to raise the sap temperature?



Heat is not your friend... it leads to spoiling your sap quicker. The pump will also heat up your sap and combined with it being concentrated, you need to boil it as soon as possible.

[DRoseum]

Agreed - don't heat it and boil it asap.

[Sourwood Sugarbush]

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!

[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 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.

[eustis22]

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.

[HowsItRunning]

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?


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.

[DRoseum]

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.

[Sourwood Sugarbush]

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!

[paulslund]

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?

[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.

[paulslund]

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.

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.

[HowsItRunning]

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...

[eustis22]

This recirc line interests me....does anyone have any pics of how to connect it?

[DRoseum]

This recirc line interests me....does anyone have any pics of how to connect it?

see my 2-1-2023 post at 5:04 pm in this other thread:

http://mapletrader.com/community/showthread.php?37676-Higher-Brix-RO

I drew a rough sketch of how to plumb it, the various options for operating it, and have a video where I show one configuration on a unit I built.