Two RO membranes in series?

[cds333]

After watching this video I have some questions.

He mentions there are two ways to run dual membranes- in parallel or in series, however the "series" he describes is not actually in series- it is the "wastewater staging" setup designed to take the wastewater from one membrane and filter that through the second.

As you can see the series setup does nothing to improve the TDS, which is actually worse than a single membrane.

Why did he not mention a true series setup- where the output of the first membrane is run through a second membrane?

Lets say your first membrane is not giving you sufficient rejection- so you want to filter that water with the second membrane to achieve the desired output TDS.

For example your first membrane is putting out 15 tds at 80% rejection, and this is unacceptable. Thus you use a second membrane to achieve another 80% rejection of the original 15, giving you 3 tds.

Why did he not mention this? or why would this not work?

(I am aware that this method would have a much higher waste/product ratio- that alone would not be a reason why this would not work)

Thank you!

 

[laverda]

I didn't watch the video, but running the waste through the second membrane is the proper way to running them in series. The second membrane will not be as low TDS as the first due to the water going in is higher TDS than the first membrane
There would not be enough pressure for the second membrane to work your way. I would expect you would get very little water out of the product line unless you added a boster pump. I suggest you buy a better membrane if yours is 80%. It's going to be much cheaper any way.

 

[Randy Holmes-Farley]

Agree. It's the lack of pressure drop across the membranes that becomes a problem in a true serial use. Water only crosses a RO membrane at a reasonable rate because its pushed by sufficient pressure.

This article describes the parts of an RO/DI and what they do and how:

Reverse Osmosis/Deionization Systems to Purify Tap Water for Reef Aquaria by Randy Holmes-Farley - Reefkeeping.com

 

[cds333]

The membrane is not rated at 80%- it is rated at 99% with a minimum of 96%, but that's another thread...

So I would probably have to make a large batch of water, and then swap over the input line from the well to the RO tank I just made, and then run that water through the membrane again, with a pump, to achieve the desired result?

 

[lakai]

I used to run two membranes with the whole separate cation, anion di resins. it burns through a lot more resin that way and sometimes get some weird tds problems. I switched back to one membrane and regular mixed resin and things are a lot better

 

[outhouse]

I run 3 membranes in series, I can get away with it because my water is generally 36ppm to start with, so I save water. I also use a pump tp increase pressure

Agreed with above, with dirty water you dont get a choice to save water

 

[Mike konesky]

The membrane is not rated at 80%- it is rated at 99% with a minimum of 96%, but that's another thread...

So I would probably have to make a large batch of water, and then swap over the input line from the well to the RO tank I just made, and then run that water through the membrane again, with a pump, to achieve the desired result?

Yes you will have to filter it twice. My well water sucks and I need to do it. Run regular RO to container, then pump through RO again and di for final product. I have it hooked up to do both at once. 2-100gpd and 2-75gpd making regular RO, and 2-75gpd making final product. Water is 9 out of first sets, 0 after second set and before di. Resin lasts forever

 

[Randy Holmes-Farley]

The membrane is not rated at 80%- it is rated at 99% with a minimum of 96%, but that's another thread...

So I would probably have to make a large batch of water, and then swap over the input line from the well to the RO tank I just made, and then run that water through the membrane again, with a pump, to achieve the desired result?

Yes. Sounds like a big effort for not that much benefit (IMO).