Why is permeate flow reduced as I increase pressure?

[Bricklayer]

Thanks all for excellent advice. Bottom line is that I should be giving my RO more love. I should take into account that I use the circulation method. Only have one valve--circulate in and out of my sap tank through the RO until I get up to 8 or 10 brix. So it gets a good workout. With a 350 gallon run, that's a lot of gallons running through that RO in circulation.

I did a complete cycle of rinse--soap wash--rinse--acid wash with 24 hour soak--rinse. It did increase performance. I learned from the new owners of Deer Run that I have a 4x40 MES membrane so now I can do more research into the exact details of care. As far as a baseline goes, I find it difficult to set one because the variables are always changing--temperature especially. There is a huge difference in performance based on temperature. It's quite amazing to take off 50 gallons of permeate per hour when the sap is warm. It's very slow when working with cold sap.

One question about performance ratings. When they rate a membrane or RO system at so many gallons per hour or day, what does that mean? The number I care about is how many gallons of permeate will it take off per hour at a given temperature and brix.

Thanks again. I hope everyone had a good season. Mine was slightly below average.

The way manufacturers size their RO units with gph numbers is based on the hp pumps performance. Basically how much sap it can process per hour at the max PSI they recommend the pump to run at. For example. A CDL 600 gph unit will pump 10 gpm ( 600 gph ) at 500 psi. So it’s a 600 gph.
Basically if you run it at that pressure or any pressure your concentrate + permeate flow would equal what the pump is able to pump at that pressure.
A 250 or 125 is the same thing. Max pump discharge at a max pressure.

[TomorrowRiverMapler]

There certainly seems to be a shroud of secrecy around RO membranes for maple sap. Also pricing seems to be quite variable. For example, I'm thinking of replacing my Leader Micro II 4x40 membranes which are Hydranautics and cost $325. A 4x40 H2O membrane is $420. These are from the same vender selling Leader/H2O products. Is an H20 worth $100 more? Mine are 6 years old and at 85% throughput versus when new. The cost to clean is at least $100 each with no guarantees of getting them back to new condition. Probably will just buy new ones.

[Bricklayer]

Don’t think there is a maple company out there that “ makes “ their own RO membrane. They are just labeled as their own.
I’ve had great results with the MES membranes when I had a 4” unit . The person who bought my 4” RO bought new membranes off Amazon and had good results as well. I would recommend MES though. Never had any issues with those membranes.

[Jgranat]

Compared to the drinking water RO process, the maple sap desired is the "dirty" side of the process, so a cheap membrane would still work, May even seem to work better with a bit of sugar build up. This would not be the first time clever marketing was used to sell the same thing to a consumer at an inflated price.

[spike in wi]

Marc, We had a Deer Run 125 for the last 6 or 7 years. A couple things that we discovered on slower production was on sap that sat for 2 or 3 days would plug the prefilter a couple times on a run. We ran about the same taps and production that you do. We learned to run the sap daily to eliminate that problem. We did a soap wash and acid wash at the end of season and soap wash on season startup. When we first got the RO we ran at the 275 psi but eventually backed down to 250/260 or so. Really didn't notice much difference and the pump ran quieter. What others had mentioned was doing a soap wash and that helped us a couple of times. Our prefilter is 10 micron. I've heard others use 5 micron but those would plug in half the time maybe?? Our main culprit was the prefilter in most cases. My 2 cents worth!! Mike

[TapTapTap]

[QUOTE=Jgranat;418249]Compared to the drinking water RO process, the maple sap desired is the "dirty" side of the process, so a cheap membrane would still work, May even seem to work better with a bit of sugar build up. This would not be the first time clever marketing was used to sell the same thing to a consumer[QUOTE]

I'm not so sure about the cheap membrane - hopefully someone with expertise beyond me will also respond.

However, I do understand that there are some significant differences between the home drinking water RO and the maple sap RO. For one, the acceptable efficiency of the home RO is around 4 gallons of waste water per gallon of clean water. For us maple producers, our permeate can be around 10 times the volume of concentrate (a complete reversal of the home RO need, and then some). Concentrating sap to our expectations requires that we push hard on the raw sap pressure through the membrane (up to 500 psi). We also have a huge loading of impurities on the membrane. Perhaps our membranes don't need to be more efficient or robust, but I'm not sure.

I'm also not sure if the membrane works better with a little bit of sugar build up. I think the opposite - that the membrane efficiency, as measured by output at a constant pump pressure, decreases immediately and continues to decline throughout the process until the membranes are cleaned again. It's like your shop vac filter - filtration might increase with dust on it but it sucks less. In the case of the shop vac, smaller dust particles are discharged as the filtration improves. But - with the RO, filtration doesn't necessarily improve since the filtration is already sized to allow only water molecules through. There's no advantage to having more filtration.

Ken

[Jgranat]

I have some background in industrial filtration, and there are applications where the filter cake improves filtration. If it is merely a size only factor of operation, then there really could be no difference in the membranes, water molecules do not come in different sizes, one size fits all. I am still learning about this process, I would also like to hear a better explanation, hence my first question.