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seabass

RO water and phosphate

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seabass

I was about to test some dry rock to see if it's leaching phosphate or not, so I partially filled a clean 5 gallon bucket with RO/DI water. I expected to see 0.00ppm of phosphate using my Hanna Phosphate Checker; but it read 0.08ppm (about 3 times more than the recommended about of phosphate in a reef tank).

Unfortunately, my dual inline TDS meter has been giving me an error message so I could only check the RO water (which varies between 1 and 2ppm TDS), and not the DI output. So I just recently purchased a hand held TDS meter which I got out and tested the water that registered 0.08ppm of phosphate. It read 1ppm TDS (basically the same as my RO water).

So I assumed that the DI resin was spent and replaced it. I also took apart the inline TDS meter and saw that the wire from the probe had become disconnected from the circuit board (so I plugged it back in). So my inline meter is working (confirmed with the hand held meter), and my RO/DI water is now testing at 0ppm. Also, the phosphate level is testing at 0.00ppm.

I realize that water passing through spent DI resin can be more contaminated than water coming directly from the RO membrane; however, I still thought I'd share this long story with you to reemphasize how important the DI stage is to reef keeping. It is possible for RO water with only 1ppm TDS to contain three times the recommended level of phosphate.

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AZDesertRat

I wouldn't rely on the checker for testing RO/DI ir ultrapure waters, they are not very accurate since RO/DI has so few ions and is very volatile. A good handheld TDs meter is much more reliable. While I have two dual inline TDS meters, I rarely even turn them on anymore since they never agree with a handheld so are more of a "litmus paper" type test in my experience.

 

DI starts releasing stored up contaminants even before it is exhausted, especially weakly ionized substances like phosphates, silicates and nitrates. I would highly recommend a good handheld TDS meter such as the HM Digital COM-100, TDS-3, TDS-4TM or their new AP line, all of which are truly temperature compensated unlike the inlines and they have a built in digital thermometer function too so serve dual purpose.

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seabass

Thanks for the response.

 

I wouldn't rely on the checker for testing RO/DI ir ultrapure waters, they are not very accurate since RO/DI has so few ions and is very volatile.

I knew this about pH, but not about phosphate. It's interesting that after I switched the DI resin, I'm reading 0.00ppm phosphate and 0ppm TDS. Anyways, it's the best test I have to check pure water for phosphate, so I guess I'm stuck with it.

 

A good handheld TDs meter is much more reliable. While I have two dual inline TDS meters, I rarely even turn them on anymore since they never agree with a handheld so are more of a "litmus paper" type test in my experience.

I did just recently purchase a hand held meter (the TDS-3) and used it to confirm 1ppm TDS before switching the DI resin, and 0ppm with the new resin. For me anyway, the TDS-3 does seem to agree with my inline meter. At least now I can check it against something considered reliable.

 

DI starts releasing stored up contaminants even before it is exhausted, especially weakly ionized substances like phosphates, silicates and nitrates.

That explains the high phosphate readings. I might just have to check what the phosphate reading of the regular RO water is before it enters the DI stage.

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seabass

OK AZ, I think you are right. It seems to be hard to get reliable results with purified water. Although the results might still indicate relatively higher or lower levels of phosphate in the water. I used up a bunch of phosphate reagent, but here is what I got.

Phosphate from DI while the TDS from the RO water was still high (after just starting to make water): 0.25ppm
It's a little strange that my TDS meters read 0ppm for the same water that tested at 0.25ppm phosphate. Although now I see what AZ was talking about.

Phosphate from RO after running for awhile: 0.08ppm
Phosphate reading DI after running for awhile 0.07ppm

Apparently it would be better to mix salt in the water, and test that. Also, my original shock at 0.08ppm phosphate wasn't likely as bad as I thought (but the TDS was still 1ppm, so the DI resin needed replacing anyway). It also tells me that more stuff actually gets through the DI stage during the release of RO TDS creep; so I'm reinstalling the DI bypass so I can discard the first minute of water, when I start up the RO/DI unit (not only will this prolong the life of the resin, but it might improve the quality of water in my RO/DI storage container).

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SantaMonica

Good pointer on the hand-held vs. inline. I'll have to dig up mine.

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