Reef Chemistry Question of the Day #37 RO/DI

Randy Holmes-Farley

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Reef Chemistry Question of the Day #37

You are using RO/DI water to replace evaporation in your aquarium. In each scenario below, assume the same total amount of evaporation replacement water is used.

Which of the following is the least efficient way to run an RO/DI in terms of DI resin usage in a month, and why. If you know it, give not just the common name for the reason, but describe the mechanism.


A. Making just as much RO/DI water as you need each day.
B. Making four big batches of RO/DI water and using them each over the course of a week.
C. Making one, even bigger, batch of RO/DI water and using it all over the course of a month.



Good luck!
























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DFW

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A, because each time that you start running water through the system, there is a brief period where there are a higher amount of dissolved solids passing through to the di filter, therefore the resin will be used up faster. I still make my water in 5, and 10 gallon batches anyway as this is most convenient for me.
 

NeverlosT

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I guess I will go with A as the least efficient for the reason that DFW posted above.

Not sure what is the next least efficient. C probably deals with evaporation more and the water sitting stagnant may get less pure in an uncontrolled environment over a long period of time?

So I think B is the best.
 

CastAway

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I think A is least efficient, and C is most efficient. Tagging along
 

DRThompson

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A is least efficient for DI resin because of TDS creep. Better to make large batches at once and flush the membrane before each use.
 
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Randy Holmes-Farley

Randy Holmes-Farley

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The answer for the least efficient way is....A. Making just as much RO/DI water as you need each day.

Many of you had the correct answer, and DRThompson had the words I was looking for: TDS Creep!


Here's an intuitive sense of what TDS Creep is.

What is happening is that ions are slowly diffusing across the RO membrane all of the time, whether there is water being pushed through by pressure or not.

So the space on the downstream side of the membrane becomes salty (with diffused ions) and eventually has salinity matching the tap water side, for at least the small space there by the membrane. The ions don't generally diffuse much far down the system, so it basically fills up a little area with some salty water and then slows down greatly.

That’s no problem if you make 100 gallons, because 25 ml of salty water is easily taken out by the DI.

But if you make that 100 gallons in 50 different batches, and each has some of that salt creep salt, then the water getting into the DI has significantly more ions to remove, and you'll be consuming the DI faster.

How much faster it is will depend on the tap water salinity (TDS), and in some cases it may be trivially low, but in others it may be significant.

This has more on RO/DI systems and how they work:

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

H
appy Reefing. :)
 
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gdemos

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will a Higher TDS RO/DI Batch require more dry salt per gallon to reach a given salinity level (say 1.025), when compared to a Lower TDS RO/DI Batch?
 
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Randy Holmes-Farley

Randy Holmes-Farley

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Not really, unless the TDS is super high.

The TDS of seawater is something like 35,000 ppm, depending on exactly what TDS scale is being used. So whether the source water is 0 or 10 or 100 ppm TDS won't really matter for how much it takes to reach 35,000 ppm = 35 ppt. :)
 

JOKER

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I have the brs 6 stage with tds meter and you can really see this when starting up. My tap water has a tds of around 200, after the sediment and carbon filters it is down to around 3-4. When I first start my unit to run my water I will see around 100 or so before the di filters. I now run my water in the sink for a few minutes until reaches 0
 

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