DYI Watermaker best to use 1 - 4040 or 2 - 2540 membrains

[FishyBiz]

Building DYI watermaker 3GPM @ 800psi pump.

Question should I use single 4040 pressure vessel sw30-4040 membrane or go with dual 2540 membranes in series, what will give more / best product water output?

Single 4040 will save me about $400-500

Plus and minus, real world experience info appreciated

- Thanks

[billknny]

The water output is strictly a function of the surface area of the membrane. Nothing else.

If the single membrane housing fits, and gives you the capacity number you want at the flow and pressure you generate, there are zero benefits to a dual membrane setup.

Be 100% sure you are running the membranes as close to the center of their pressure/flow spec as possible. Not doing so is a common cause of early failure of home brew systems.

(Chemical engineer, designed and ran commercial RO systems.)

[Tellie]

Single 4040 will save me about $400-500", ...Will it? Usually 4"membranes and membrane vessels are a bit spendy.

[FishyBiz]

Thank you for your response Tellie.



I do have a question and few observations.


Observations
Basic DYI manually operated water maker systems seam to use a single 2540 membrane, reportedly producing about 18 GPH. Many cruisers have added a second 2540 membrane in series, using same pump reportedly producing approximately 35 GPH output.


Since 2540 membranes have max feed rate 6 GPM and 4040 membranes 16 GPM. Appears to me 2540 membranes in series is ideal with midpoint feed at 3 GPM for maximum product water output with my small 3GPM pump. This series arrangement should work very well for me, filling water tank from 2/3 empty in under two hours.


But being cheap, always over thinking and looking to improve any design. I have found 4040 pressure vessel and membrane for just about 15% more money than single 2540 setup. The 4040 has an advantage of 35% more surface area than two 2540 membranes, easier to source and less plumbing needed etc..


Questions
Will membrane have longevity issues running 4040 membrane at only 3GPM feed water, understanding membrane is always flushed with carbon filtered fresh water after each run?


Are the reported 2540 single 18GPH and dual 2540 35GPM membrane outputs typical?



https://www.dupont.com/content/dam/dupont/amer/us/en/products/water-solutions/documents/609-00377.pdf


Odd question when looking at Filmtec specs both 2540 and 4040 have 8% recovery rate my calculator says single 2540 @ 3GPM = 14.4GPM and single 4040 @ 3GPM = 14.4GPM; seams very odd since surface area is 2.3 times greater for 4040 vs 2540 membrane and when doing series calculation 2540 is 2x single membrane = 28.8GPM


Where did I go wrong?


Thanks

[FishyBiz]

I have found vessel and membrane at about $50 each extra, for 4040 size, also 4040 is produced much often and available world wide.