WHy do you suppose RODI systems are so inefficient?
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twentyleagues
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It take a lot to filter out all the bad. Ro membranes do 95% of the job its also where all of the waste comes from. You can decrease the waste by adding another membrane run off the waste water of the first with little detriment to the product. Ive run them in the past its basically a 1:2 ratio instead of a 1:4. I just got done plumbing a rodi of the same type they say at optimal performance is 1:1.5. We will see im going to run the first 2-3g into buckets.
Tuan’s Reef
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Add another membrane. It will reduce waste water by half.
EeyoreIsMySpiritAnimal
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Get a booster pump. Mine's almost 1:1 now.
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Randy Holmes-Farley
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If you want to know the actual reason, it's because of osmotic pressure.
If you use any sort of semipermeable membrane to separate a salty solution from a less salty solution, there's a strong force of water wanting to travel from the lower salt to the higher salt solution. That natural travel reduces the energy of the system overall.
In order to offset that pressure, and in fact to get water to flow from the high salt to the low salt, one has to put a lot of pressure on the salty side.
The higher the pressure you can generate, the bigger saltiness differences you can overcome with that pressure. Commercial systems trying to desalinate seawater run up to 1000 psi (but, of course, need appropriate membranes).
If you use any sort of semipermeable membrane to separate a salty solution from a less salty solution, there's a strong force of water wanting to travel from the lower salt to the higher salt solution. That natural travel reduces the energy of the system overall.
In order to offset that pressure, and in fact to get water to flow from the high salt to the low salt, one has to put a lot of pressure on the salty side.
The higher the pressure you can generate, the bigger saltiness differences you can overcome with that pressure. Commercial systems trying to desalinate seawater run up to 1000 psi (but, of course, need appropriate membranes).
twentyleagues
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My new system has a booster pump. Im on a well this time though so my pressure is not great 35-40psi. I had to get one. I dont think ill get better then the recommended 1:1.5 we will see. At the last house it was city water, had around 70psi. I know higher pressure and temp help with ratio.
Gtinnel
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Do you get that from a single membrane?
Randy Holmes-Farley
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FWIW, you can get lower waste water amounts (all the way to zero) just by increasing the flow restrictor back pressure, the question is at what trade off of increased TDS in the water getting through the RO membrane.
Gtinnel
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And it would have to greatly reduce the lifespan of the membrane.
EeyoreIsMySpiritAnimal
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Yes
twentyleagues
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So I'm home from work and finished the ro/di. Few leaks lines not sat all the way didn't check brs' work. Lol. It's in the basement so not to bad. Pressure pump was at 90 psi. Turned that down to 75. Set up 2 5g buckets and set a timer for 15 minutes. It's very close. 7" in the product bucket. 8" in the waste bucket. I'm no mathematician I'm sure someone could figure it out. Not quite 1:1 probably like stated 1:1.5. but at 7" thats very close to 2.5g of water in 15 min. I'm pretty happy with that.
I did something similar a few years back. Instead of using 2 membranes connected in parallel, I hooked them in series (product water from one went into the inlet of the 2nd). The only issue was setting the proper flow restrictions theough both membranes. It was tricky.
The only filters that I've replaced since it's inception are the pre-filters. Making over 200g a week for those 2 years +, the tds out of the 2nd membranes is 7 and the final tds is 4. I just purchased a new max cap (spectrapure) and plan to put that on in the next month or so... but my color changing di are still blue.
I documented it on another forum. You can read all about it here:
Untraditional Dual Membrane System (RODI)
Throughout the years in this hobby I've accumulating a lot of knowledge that's not traditionally in my field of expertise and because of that I've always tried to associate concepts with what I know, just so that I can make sense of it and create a foundation on which to build upon. More times...
humble.fish
