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Digesting A Wastewater Sample

I have not been to this great site in a while,hope all is well with everyone. Now to the question...We run a sample of our outgoing treated wastewater everyday,what we do is grab a sample,then run it through a gleckman cylinder pump with a paper filter,normally we do this twice to the same sample. We do not digest our sample in house,due to lack of equipment & lack of time,once a month EPA requires us to send a sample out to an independent lab to be analyzed. On a normal day we usually average around .105 on out Ni reading about .08 on the Cu reading and about .050 on the Cn reading. We use a HACH DR/380 colorimeter to read our samples with,it get's calibrated one time per week. Our environmental guy that handles the samples grabs so many ml per hour for 8 hours then takes it to the independent lab and they do an 8 hour composite. I failed to mention that our environmental person puts a few ml of nitric acid in the sample to preserve it. Anyway when we get our results back the lab reads alot more on our Ni sample then we get,like for the month of September we got a reading of .114 and the lab got a reading of .867 (for Pennsylvania that is still well within limits). Would digesting the sample make that much of a difference? The Cu & Cn that we read are very close to the labs numbers it is just our Ni reading that is way off?

Brian C. Gaylets
- Scranton, Pennsylvania, U.S.A.

First of two simultaneous responses --

Are you using Hach's Heptoxime method (the one with the chloroform extraction) or the PAN method? The PAN method is kind of a short cut method. There's lots of interferences, both positive and negative.

What I used to do for digestion is; put 250 ml sample in a 600 ml beaker [beakers on eBay or Amazon], add enough ACS grade hydrochloric acid to bring the pH under 2, then 5 ml more, and then heat the sample for 8 - 10 hours. I had a special "simmer" hotplate that I built that I would use. (You do not want the sample to boil. Nor do you want it to go to dryness.) This heating would reduce the volume of the sample about 80%. Then, I'd add back distilled water to make the sample volume back up to 250 ml, and run my tests.

For special samples that had "killer" interferences, I had a more elaborate technique. I'll explain it if you like, just ask. See how you make out with the digestion. For reconfirmation, I'd suggest you run a "spiked" sample - borrow a few mls of 1000 ppm nickel standard, add 1.00 ml to a liter of your wastewater, then run it and see if the result goes up as it should - by 1 ppm. If it doesn't, you know there's something wrong.

Good luck!

David Wichern
- Plainview, New York

Second of two simultaneous responses -- 2006

Hach has a wonderful section near the back of the ops book called interferences. It is possible that you have something in the solution that interferes with the reading. I assume that this is a colorimeter based instrument.
It is possible, but doubtful that the analytical firm has a boo boo in their bringing it back up to volume or a wrong fudge factor in the calculation. If you had not said that it was filtered, I would have said finely suspended metal.
I would ask them for a sample of the digested material back to see what your instrument reads.
Also, your calibration standard may be bad.

James Watts
- Navarre, Florida

Thanks for the help I really appreciate it. I did lack a little information in my question. It is a colorimeter instrument. We ordered a bunch of new reagent's for our sample's and we are going to try the Heptoxime method rather than the PAN method that we are currently using. Is it possible not to digest the sample and get an accurate reading? Like I said we really don't have the time to digest it,we run samples every day to record it,they look at our paper work when they do inspections,and we are short on manpower besides due to cutbacks. But again thanks for the help guys, ya bailed me out!

Brian C. Gaylets
- Scranton, Pennsylvania, U.S.A.


Are your samples clear? Then the answer is 'maybe.' Or, do your samples have a lot of suspended matter? Then, the answer is 'probably not.'

I know the digestion sounds like a big deal, but it's not. You put the sample on a hotplate so that it heats up to about 175 F, then leave it overnight. Then, analyze it in the morning. That's what I used to do.

dave wichern
Dave Wichern
Consultant - The Bronx, New York

Our samples are quite clear actually, very seldom do we see any suspended matter in the grab. I have taken all the suggestions and I am going to try them and see what works better for us. Again thanks for the help, I really appreciate it.

Brian C. Gaylets
- Scranton, Pennsylvania, U.S.A.

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