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Process Control Phosphorus Measurements
Posted: Wednesday, February 16, 2011 7:16 AM
Joined: 12/31/2009
Posts: 3

   We are operating a 5 Stage BNR process, and we are using a V-2000 Chemetrics meter to measure phosphorus at our wastewater facility. It uses the stannous chloride method for determining the phosphorus content.

   The last few weeks we have had some kind of high strength waste to come in our facility that our "bugs" love, and we know this because we have to dramatically increase our air to the aeration basins. Our problem is that whatever is in the waste is causing an interference with the phosphorus testing.

    Using the V-2000 meter for process control, we get high phosphorus readings, but when our main laboratory runs the standard methods test for phosphorus, they are well below our .5mg/l limit. Does anyone have an idea as to what could be in the high strength waste that would cause an interference with the stannous chloride method we are using?


Thank You.

Posted: Wednesday, February 16, 2011 7:43 AM
I'm posting as anonymous because the stupid forum lost my login, but won't let me recreate because they say it is in use.  One more strike against WEF.
SM says that the following about interferences for the stannous chloride method.  


"Positive interference is caused by silica and arsenate only if the sample is heated.  Blue color is caused by ferrous iron...does not affect results if concentration is less than 100 mg/L."  "If HNO3 is used in the test, Cl- interferes at 75 mg/L."  There is no mention if this is a positive or negative interference.  The ascorbic acid method (which is what I presume you mean when you say the main lab is using 'the standard methods test') lists hexavalent chromium and nitrite as negative interferences. 
I'm not familiar with that particular monitoring meter, but I would go over it once or twice and make sure it is appropriately maintained.  It is quite possible that there is some fouling in it that is contributing to high readings with this particular waste stream.


Posted: Wednesday, February 16, 2011 2:08 PM
Joined: 12/31/2009
Posts: 3

Sorry for the "standard methods test". They run Total Phosphorus using method 665.


One other clue; we had some literature that referenced adding hydrogen peroxide to the sample. We did, and the result then came out to be what we would consider normal for us which was @ .2 mg/l after using the correction factor for the V-2000 meter. What would the hydrogen peroxide have "blocked" to give us a normal reading? Without the hydrogen peroxide we were getting a result of over 1 mg/l.


Thanks Again.

Posted: Wednesday, February 16, 2011 2:52 PM
Peroxide is an oxidizing agent and a bleaching agent.  Does your influent become colored when this particular waste stream is present?  If so, the peroxide may be eliminating sample color from being included in the reading. If it is the oxidation portion that is helping out, I have no idea what specifically could be eliminated in this fashion.  


I'm still not certain what method 665 is, so I can't elaborate any farther on that.

Posted: Friday, February 18, 2011 3:28 PM

After checking the 12th ed. Standard Methods I guess that soluble iron is the cause of the problem as it should not exceed 0.4 mg/L. Peroxide would oxidize the soluble iron and then maybe not cause a problem. Nitrite or too much peroxide would be negative interferences as they bleach the blue color. I have not used this reagent for almost 40 years.