Testing CO2 in RO water

[billw]

I have a CO2 testkit (Hach) scheduled to be delivered tomorrow to test the CO2 level in the water produced by my RO membrane. Is theree any special precautions I should take when running this test? My main concern is inaccuracy due to outgassing once the water is exposed to the air.

 

[Randy Holmes-Farley Verified Community Expert]

I think testing when you collect it is adequate. CO2 is actually slow to come out of water. Slower than other gases.

 

[billw]

Thanks, Randy!

 

[Randy Holmes-Farley Verified Community Expert]

You're welcome.

Let us know what you find.

 

[billw]

Here's the procedure:

First I installed a tap at the inlet of the DI resin canister

then I back flushed the membrane for a few minutes to remove any stale water.

I then drew a 23 ml sample into the Hach supplied test bottle as specified by the test instructions for the low range test. The test resulted in 18 drops of reagent to turn the indicater pink. This multiplied by 1.25 results in 22.5 ppm CO2.

I then set up a degassing vessel

and ran the bubbler for an hour. The results surprised me. Running the same low range test resulted in 4 drops or 5 ppm CO2 in the degassed water! I will repeat the tests later today but if the results hold up, this would possibly result in a 4:1 savings of DI resin!

Yea, I know that my prefilters need changing, but they do make a pretty nice ATS heading into my RO membrane .

 

[billw]

I forgot to note that the output of my membrane contains about 2 ppm TDS. In rethinking it I don't know what percentage of DI resin is spent on CO2, so I can't project a savings until I determine that (or someone can give me a quick back of the envelope estimate )

 

[billw]

Trial #2.

I again backflushed the membrane and purged the tap. The RO output measured 15 drops (18.75 ppm, let's say 19ppm). The original degassing water, now bubbling for 6.5 hours measured 4 drops or 5 ppm as before. It might have been slightly less. The indicator on this test is very subtle. It looks like on a small sample that a reduction of at least 3:1 can be expected.

 

[billw]

@Randy Holmes-Farley given 2 ppm TDS output from my membrane and a reduction of 75% CO2 content ( 20 ppm -> 5 ppm ) do you have a SWAG as to how much extended life I'll get out of a DI cartridge?

 

[Randy Holmes-Farley Verified Community Expert]

You can roughly equate the ppm CO2 and the ppm TDS. 1 ppm CO2 depletes a DI a bit more than 1 ppm TDS, but cutting the total ions entering from 22 to 7 ppm TDS should increase life by, very roughly, a factor of 3.

 

[billw]

Thanks, Randy!