RODI System Question

[benni]

Hello Everyone,

I brought water samples to my local store and had him test the phosphates for me. We had some interesting results. The freshwater coming out of my RODI unit has a reading a bit less than .5, while the water I prepared for a water change (same water, just added Oceanic salt) and my tank water have a reading near zero?? These tests confirmed the results from the phosphate test I have. Anyone have an explanation for 1-why I would have phosphates coming out of my 6 stage RODI unit? All the filters are new, I bought the unit from Bulkreefsupply at the beginning of the year and it still has the original membrane. I would say the amount of water I filter out is 10-15 gallons per week. 2-why would the phosphate level be lower in the same water with just salt added? Any information would be appreciated.

 

[Anonymous]

Damn good question Benni, but chemistry isn't my strong suit. Hopin someone else will pop in and help you out.

 

[Len]

I would think either the salt mix is binding some PO4 (like kalkwasser does) or the test results are not accurate for your RO/DI (though I'm not sure why this is the case). I'm inclined to believe the latter since new RO/DI water should not have 0.5ppm PO4.

 

[benni]

I am going to order a new test kit, when I get it I will post the results.

 

[AZDesertRat]

Testing RO/DI wate ris extremely hard since there are very few ions to measure in the water. A hobbyist grade test kit just does not cut it, you need lab grade stuff for low range accuracy.
This is whey we use TDS, resistivity or conductivity meters as the primary way to test RO and RO/DI, if you have TDS someting is present, if you register 0 or very near or even better somewhere close to 18.2 megaohms/0.055 microsiemens on the conductivity/resistivity meter, the water is as pure as it gets.

 

[benni]

thanks for the information-my TDS reads a zero, so that would mean my water is good to go!

 

[Anonymous]

TDS meter have limited resolution, so a "0" ppm reading can mean 0.49 ppm, for example. That is around 1 MOhm.

Anyway, back to your question, I suspect it is the ionic effect that gives you different reading. Certain type of water test can only be used in freshwater, while others are formulated to be used in salt water. Is your PO4 test kit for freshwater, saltwater, or both? How sensitive is it to sodium, chloride and other major ions in the sample?

 

[AZDesertRat]

Hobbyist grade TDS meters do have a +/-2% accuracy so dupaboy1992 is correct, 0 may not really be 0. But, for our uses, if you are confident your RO/DI is in proper operating condition and your DI is fresh I would not be overly concerned, fretting about fractions of a part per million isn't worth the time. I am a little anal personally so I purchased the HM Digital COM-100 TDS meter which is more sensitive and reads in tenths but its really not necessary. The key is maintain your RO/DI as recommended, replace the prefilter and carbon block at regular 6 month intervals using high quality low micron replacements, disinfect the system while you have it down, and monitor your RO rejection rate or removal efficiency and RO/DI TDS frequently. Take the three necessary TDS readings and write them down to trend your units performance, tap water TDS, RO only TDS (these two are used for the rejection rate calculation) and the final RO/DI TDS.

 

[Bathtub]

Along with the previous comments, ro and especially rodi water is difficult to test with home kits. With the slightest additives however you will typically get a strong reading as the additives provide the only ions the tests need- e.g. Ph corrective additives, you can literally add a trace and that's the result you will test.
Intrestingly my ro unit has Tds in and out readings, I think such meters can be back fitted (tmc)- you may find this useful ...