Question about fouling of membranes

[SmellsLikeSyrupNH]

I have taken my single post RO..... Series 4 Procon, 3/4HP motor, Dow XLE 4040 Membrane and added a 2nd membrane to the system and I plumbed them in series.

I've been told that a single pass through a single 4040 should of taken me from 2% to 6-8%. I never got that sort of performance at 200psi. So now adding the 2nd membrane and again using a single pass I am hoping to finally approach those number or even a bit higher.

I've been told to watch out for membrane fouling on the 2nd membrane. What exactly does that mean? How would I avoid that? Is it a real risk simply by adding the 2nd membrane will the sugar content be too high once it passes through? Can I avoid it by running at a different pressure?

Just want to see everyone's thoughts or experience running a 2 membrane 4040 setup.

Thanks!

[ddociam]

I’m doing the same thing only I have a 330 gph pump and two hp motor. I would love to hear some discussion on this. One thought I had was keeping the flow higher on the concentrate side I think would slow down the fouling.


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[Clinkis]

I started with a single post, then a 2 post and now a 3rd post run on a 330gph procon. It will be tough to go from 2%-6-8% in one pass unless you have some form of recirculating. Each membrane should remove approx 15% of water so, at best, a 2 post will remove 30% of your water per pass. You can push it higher but your membranes will foul quickly. With my 3 post I can remove 50-60% in a single pass which takes 2% sap to 4%. I recirculate my sap and take it up to 10-12%.

[mol1jb]

This past season was my first operating a similar sized single 4x40 membrane RO. I was operating mine at pressures around 280psi. Running at a higher pressure will help in removing more water per pass. My unit has a re circulation line as well. When I opened my re circulation line a fair bit I could take 2.5% to 6% but my flow rates slowed down 100gph to 70gph total flow.

I would think that your current pump and motor are on the small side for operating 2x membranes. Most using procons as pressure pumps run the 330gph with atleast a 1.5hp motor for 2x membranes. Did you check with NEXTgen maple to get their input on this?

[SmellsLikeSyrupNH]

I did he hasn’t tried this sort of setup yet and wants me to report back on my results.

I was running 2 passes last season on the single membrane, I was hoping that adding the 2nd would reduce the need to run it multiple times.

Thanks

[JoeJ]

You cannot stop the membranes from fouling. All you can do is clean them properly after use.

I have never owned a small RO unit and do not know the suggested protocol for cleaning their membranes. However,I do have a 2 membrane, 1220 gph RO and would think that you would clean any membrane using the same methods. My experience has been that you need to first of all, know what the membranes are capable of producing before you put any sap through the machine. If you don't know that benchmark number with a clean membrane,you a kind of working blindly. At the start of the season, after rinsing the storage solution out of the membranes, I do a benchmark test. I put some clean, low iron and low manganese soft water into my wash tank (the same water I use to rinse out the storage solution) and let the pump circulate the water until it reaches 75 degrees with 200 psi on the pressure meter. I then take a flow meter reading from each membrane and use that a my season benchmark. After each days running of the RO, I rinse, wash, rinse and then before I use the RO next, I do the benchmark test to check for cleanliness of the membranes and compare the flow meter reading to make sure that they are the same as the first of the season reading. If the daily benchmark reading is less than the first of the season benchmark, you can repeat the wash rinse cycle to clean up the membrane. Then redo your benchmark test. At some point if the benchmark test drops too low, you can do a citric acid wash to clean the membranes

[bowhunter]

The key to keeping the membranes clean for as long as is practical is by keeping the flow out of the last membrane above 3 gpm for a 4 inch membrane and keeping the recovery below 15% on each membrane. The recovery is the per cent of the flow into the membrane that is removed as permeate. You do this by sizing the pump accordingly and running with concentrate recirculation back through the membranes. These are recommendations made by DuPont in their RO technical manual for FilmTec membranes. They have worked well for me. I have a 2 1/2 inch membrane that I rinse after each run with permeate for about an hour. I generally do not have to do a soap wash until the season is over. Frequent soap washes can damage a membrane and reduce it's life substantially. Secondarily heavily fouled membranes cannot be effectively cleaned.