What am I really saving by using two RO membranes versus one? | BRStv Investigates

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JoshH

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You kindof answered your own question... Running RO in parallel will produce twice the amount of product water and twice the amount of waste as opposed to a single membrane (while using twice the amount of water as a single membrane). So it will produce water twice as fast but use twice as much water. Running it in series is where you get twice the product and half the waste but as you said you have 350 TDS coming in so it might not be worth it to run them in series
 

ThaChad

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Running RO in parallel will produce twice the amount of product water and twice the amount of waste as opposed to a single membrane. So it will produce water twice as fast but use twice as much water. Running it in series is where you get twice the product and half the waste but as you said you have 350 TDS coming in so it might not be worth it to run them in series

Understood. But running 2x the product water, will fill my bulk container 2x faster (than a single membrane), which means the RO will run 1/2 as long, which means that it will still produce the exact same amount of waste water as the single membrane running 2x as long to fill the same container. So I am not saving any water and I am not making any extra product water, but I am making it 2x faster.

From 1 membrane, I get 2-3 TDS after the membrane. Running 2 in series I am getting about 8-9 TDS after the membranes. Even after my 1 RODI stage, I still have 1-3 TDS. So I would need to add a 2nd DI stage to remove the final few TDS, or I can split the 2 membranes and run them in parallel and still accomplish the same production rate.

Currently, I have a valve as my flow restrictor because I needed see how much flow restriction I would need to create the lowest TDS, with the setting it on now, I'm definitely not saving any water!

-ThaChad
 

JoshH

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If your only goal is to produce water twice as fast than definitely running in parallel will do the trick. And yes running in parallel will produce more than likely a little more product water than in series. All you have to do is have an eye on your DI resin consumption as it will deplete twice as fast as using one membrane...

I guess I worded my last response a little wrong lol
 

ThaChad

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If your only goal is to produce water twice as fast than definitely running in parallel will do the trick. And yes running in parallel will produce more than likely a little more product water than in series. All you have to do is have an eye on your DI resin consumption as it will deplete twice as fast as using one membrane...

I guess I worded my last response a little wrong lol

The DI resin shouldn't deplete any faster than running 1 membrane, because the amount of water running threw it will be the same.

IE: I have a 20g drum that I fill up to do a water change. If I have 1 membrane that makes 150GPD = 6.25gph, It would have to run for 3.2hours to fill my 20g drum. VS If I have 2 membranes that make 150GPD/each = 12.5gph, they would only have to run for 1.6hours to fill my 20g drum. yay! However the amount of water running threw the DI resin, is still only 20 gallons. The amount of waste water should be virtually the same, the amount of water running threw my pre-filters should be virtually the same. The only thing I am getting is 2x faster production.

With my system currently running in series, my DI resin IS being uses up a lot faster due to the higher TDS I am getting running them in series.

-ThaChad
 

dragon99

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Can you guys re-investigate this using a booster pump? I think your final results would be drastically different if you had a booster pump set at 80-100psi. Also if you use the Aquatec 8855 booster pump, in the parallel configuration, It should significantly increase your results as both the pressure and the flow/volume would be increased.

My TDS is over 350, so I am not able to run the Dual membrane in Series. I am going to have to run it in Parallel, correct me if I am wrong, but It's not going to use any more water than a single membrane, but it will make water 2x faster, Correct? Obviously it will only need to run 1/2 as long, but product 2x the waste and 2x the product water. I do have the Aquatec 8855 booster pump and it is set to 90 PSI. So I really shouldn't loose much if any water pressure to the 2nd membrane.

P.S. Do you know if Tannins in the water increase the TDS? We have Tannins and our water while clear coming out of the tap, has a tea color to it if it sits in a glass for an extended period of time and/or you fill up a tub or bucket you can see the tea color. I am wondering if this is why my TDS is soo high, would getting a Tennins filter reduce my incoming TDS or not?

Thanks,

-ThaChad
I run 2 membranes in parallel because I have the same problem with incoming TDS in the 350-400 range. I haven't seen any downsides and it produces water twice as fast. I have good pressure on the tap side (~82psi) so I don't have to bother with a booster fortunately.
 

JoshH

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The DI resin shouldn't deplete any faster than running 1 membrane, because the amount of water running threw it will be the same.

IE: I have a 20g drum that I fill up to do a water change. If I have 1 membrane that makes 150GPD = 6.25gph, It would have to run for 3.2hours to fill my 20g drum. VS If I have 2 membranes that make 150GPD/each = 12.5gph, they would only have to run for 1.6hours to fill my 20g drum. yay! However the amount of water running threw the DI resin, is still only 20 gallons. The amount of waste water should be virtually the same, the amount of water running threw my pre-filters should be virtually the same. The only thing I am getting is 2x faster production.

With my system currently running in series, my DI resin IS being uses up a lot faster due to the higher TDS I am getting running them in series.

-ThaChad

Sorry Chad, you're right, been an off day for me lol mine's set up in series so my minds kind of stuck on those benefits/downsides
 

ThaChad

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Does anyone run 150GPD membrane? I am trying to decide if I should run a 1000ml (2.5x) restrictor, or a 1200ml (3.0x) restrictor.

Honestly I'd like to try both and see what the results are, but the darn things are about $12-$15 a pop & I will need 2 for running in parallel.

Thanks,

-ThaChad
 

dragon99

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I'm only running a single flow restrictor with 2 membranes. Just using a Y-fitting in front of it. I think I'm using the 650ml with (2) 75gpd membranes.
 

ThaChad

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I'm only running a single flow restrictor with 2 membranes. Just using a Y-fitting in front of it. I think I'm using the 650ml with (2) 75gpd membranes.

In a Parallel setup? or a Series setup? In series you would only run 1 flow restrictor.. But in Parallel you would need to run 2..

-ThaChad
 

ThaChad

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I wonder if running a second sediment filter would be of use in this case. Id be curious to see the tds at each stage of the rodi system.

Ya I'll check it out later (maybe tonight). I have one .5 micron sediment filter. I'll just fill up a cup of water before and after the filter and use my handheld TDS to see what the difference is.

I'd be interested also, Because I have 350+ TDS coming out of my 3 Pre-filters.

I am wondering if tannins in the water causes TDS? I know we have tannins and I am curious of I get a tannins filter, would that lower my TDS of the infeed water?

I have a old Culligan water softener system that can be converted to a tannins filter by just changing the media in the tank. It's sitting in my garage..

-ThaChad
 
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Clittrell

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you only use the restrictor on the second one : )

I have a RODI that auto flushes. All of that would go after second membrane? I have 90psi with a booster pump with a Dow 75 what restrictor should I be running if I go to a in series solution.
 

Whiskeyboy84

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I realize this is an older thread and all but I am getting ready to order my RO/DI system probably on the first, my question is with running the dual membranes in series if I added a third would I see an additional savings on my waste water production as I live in the southwest and water at times is a precious commodity. Just wondering what the diminishing gains would look like running three membranes in series at say 75 psi with a booster pump and then run two or three stages of DI. I am building a 250g system so initial plus water changes after cycling I figure I need a decent RO/DI setup.
 

Maurizio

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Thank you for the great information.
In my place in Italy I recover the rain water, with a very low TDS around 120 ppm. during the winter.
So I can put two mwmbranes in series and alot of saving in the waste water.
But during the summer I have to switch to the well water, that have a TDS around 1500 ppm,
the well water has a hardnes arounf 80 French Degrees but the water get soft before the membranes because I have a Ionic resin softener before the water reach the membranes.
But the TDS obviusly remain High.
What do you suggest in this case?
Usually I change the membranes avery 2 years.

Thank you very much
 

Maurizio

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you only use the restrictor on the second one : )
Thank you so much for all your effort.
Im writing from italy where I leave and follow you guys.
I have question:
I recover rain water during the winter with very low TDS around 120 ppm and with this I feed my two membranes in series, also to reduce the amount of waste water.
But during the summer I have to use my well water, that have a total hardnes around 80 French Degrees and a TDS around 1300 ppm.
regarding the hardness I treat the water before the membranes with a cationic resin softener lowing the hardness around 7 french degrees, but as you imagine the TDS steel remain the same.
What do you suggest in this case?
Before the membranes I have BB 20" sediment filter 10 Micron, BB 20" sediment Filter 1 micron, BB 20" Matricx C Carbon Block filter and an UV Lamp of 45 W
then another 10" stage of 1 micron sediment filter, one Dulton Ceramic Imperial Filter , and a Carbon block filter, then I go in to the membranes.
Thank you so much.
m.
 

Ryanbrs

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Thank you so much for all your effort.
Im writing from italy where I leave and follow you guys.
I have question:
I recover rain water during the winter with very low TDS around 120 ppm and with this I feed my two membranes in series, also to reduce the amount of waste water.
But during the summer I have to use my well water, that have a total hardnes around 80 French Degrees and a TDS around 1300 ppm.
regarding the hardness I treat the water before the membranes with a cationic resin softener lowing the hardness around 7 french degrees, but as you imagine the TDS steel remain the same.
What do you suggest in this case?
Before the membranes I have BB 20" sediment filter 10 Micron, BB 20" sediment Filter 1 micron, BB 20" Matricx C Carbon Block filter and an UV Lamp of 45 W
then another 10" stage of 1 micron sediment filter, one Dulton Ceramic Imperial Filter , and a Carbon block filter, then I go in to the membranes.
Thank you so much.
m.

Hi,

A water softener won't remove TDS just exchanges the calcium and other hardness for sodium which is less likely to cause issues in your homes plumbing, water heater, equipment, sinks and fixtures. To remove TDS with resin you need a mixed bed resin, most often based on hydrogen and hydroxide exchange.

With 1,300 TDS and ideal performance, you will see 13 to 26 TDS in the product water coming out of the membrane. If it is higher than that I would check the pressure feeding the membrane and make sure it is over 50 psi. With well water, it is pretty common for it to be as low as 35 which will be an issue and require a booster pump, getting the pressure up will have the biggest impact on performance. If that's not the issue I would guess the membrane isn't seated/sealed properly. I would remove it and put it back in place.
 

Ryanbrs

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Does anyone run 150GPD membrane? I am trying to decide if I should run a 1000ml (2.5x) restrictor, or a 1200ml (3.0x) restrictor.

Honestly I'd like to try both and see what the results are, but the darn things are about $12-$15 a pop & I will need 2 for running in parallel.

Thanks,

-ThaChad

Higher flow rates are mostly based on adding more material/surface area and there is only so much that fits inside a standard membrane housing. The 150 is pushing it and because of that, requires higher pressures and often has lower rejection rates which increase DI resin consumption and the membrane will likely scale the membrane faster reducing it's useful life.

So a 150 gpd membrane is cheaper and easier to install than dual 75's but will likely not perform as good and will eventually cost more.
 

Ryanbrs

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I realize this is an older thread and all but I am getting ready to order my RO/DI system probably on the first, my question is with running the dual membranes in series if I added a third would I see an additional savings on my waste water production as I live in the southwest and water at times is a precious commodity. Just wondering what the diminishing gains would look like running three membranes in series at say 75 psi with a booster pump and then run two or three stages of DI. I am building a 250g system so initial plus water changes after cycling I figure I need a decent RO/DI setup.

You can run 3 or even 4 in series but there will be diminishing returns. The membranes at the end of the chain will scale faster in many cases that may mean it needs to be replaced in 18 months rather than 36 but it could go in either direction depending on other system performance factors.
 

Whiskeyboy84

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Thanks for the response @Ryanbrs that is what I was figuring with the point of deminishing returns I am using y’alls BRS dual membrane 150gpd system and love it took me almost three days to fill my 250g system of course we had a couple of cold fronts and I noticed water production at 50-60 degree temperatures is somewhat slower lol ! Now that I have everything up and running I am keeping 40g on hand but with my 250g reef a 32g nano for my daughter and my 60g brackish for my wife’s Mollie addiction that one will likely grow to 100-150g by summer, a class room goldfish tank for my wife’s special needs class and my newly acquired 5g Beta tank I go through RO/DI water pretty fast the LFS is gunna be hurting now that I have my own RO/DI system lol!
 

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