WEF Discussion Forums
Biosolids and Residuals
I am just curious about how many replicates of each sample dilution other laboratories are preparing for their CBOD tests?
It has been suggested that we run more replicates of our samples because of differences we are getting with a commercial lab. Since replicates will measure precision, this probably will not explain the differences we are seeing with the results. Our G/GA results are in the acceptable range and our blanks are good. Both labs are using Polyseed but the commercial lab uses the NX version. We are adding Hach TCMP to each bottle.
I would suggest that you run different dilutions so that you have several BOD results that fall in the 2.0 mg/L DO depletion with 1.0 mg/L DO remaining. This will give you several BOD values to average and check for differences such as toxicity.
I prefer the TCMP inhibitor but the ATU has been approved in the latest Standard Methods. As for the polyseed NX, it has ATU but its use will add additional problems. When you set up dilutions to calculate the seed correction you will also have more ATU inhibotor per BOD bottle than for the BOD bottles for sample analysis. A change in the amount of seed material per BOD bottle will also chang the amount of inhibitor per BOD bottle.
I do not know that Standard Methods has approved this.
By 'replicates' are you referring to several bottles of the same dilution or several bottles of different dilutions? We typically will do 4 bottles per sample, with the intention of getting one bottle that under depletes, one that over depletes and two that are acceptable. One sample in 10 we will set up two separate bottles of each of the 4 dilutions. As James suggested, I think you should shoot for a few more bottles in the acceptable range to average out.
I am not a fan of the Polyseed NX. I tried one container of it, and had problems with every batch of samples I used it with. I much prefer adding the inhibitor myself. Chris Fair went into a lenghty discussion on the previous forum version talking about the solubility of the inhibitor powder and how it wasn't such a big deal with the non-precise dispenser cap's variability. I don't know how that applies to the Polyseed stuff. Since it is already in solution, I can see how different volumes of seed could cause problems.
SD, you didn't say why someone suggested you do more replicates. If it was to check precision, looking at the standard deviation of repeated GGA results is perhaps a better indicator of precision because it looks a total precision, whereas looking at results for duplicates/replicates only looks at within-batch precision.
If their reason for suggesting more replicates is so you can average the results in hopes that the average would somehow be closer to the "true" value, that's not good science...the average could still be biased, missing the "true" value by a mile.
Was there any particular reason those who evaluated the situation assumed your results were in error and not the results of the outside lab? If so, how did they come to that conclusion? If the other lab is in the area, you might want to pay them a visit and see how their blanks are, what the average and standard deviation are for their GGA results, and the same for replicates if they do them.
If the person evaluating your work DID give a reason for suspecting your results are problem, that's what I think you need to concentrate on...not whether or not your results agree with some other lab. But you are already aware of that as indicated by your statement, "this [looking at results from replicates] probably will not explain the difference..." You are absolutely correct!
The suggestion was made by an engineer not an analyst. The results from the contract lab are apparently more to their liking (lower). To answer the earlier question, we are preparing 3 dilutions for each sample. The suggestion was to prepare triplicates of each dilution (9 bottles for each sample). The practice of doing a duplicate every ten samples and the GGA in triplicate should be appropriate.
Unless that engineer is from your regulatory agency (e.g., your plant's permit manager), I would tell him/her, "Thank you very much, but I trust our results and here's why." And then talk blank results, GGA results, bias, and total precision. And IF your bias and precision are good, there is no reason for you to run samples in triplicate, and ESPECIALLY no reason to run the GGA in triplicate...you know what the GGA value should be, so you can track precision by analyzing the collective results from the GGA in each batch.
If the engineer IS from your regulatory agency, if your state has a lab accreditation program, seek their advice and, if appropriate, their assistance in convincing the engineer he/she should stick to engineering. If the engineer detected something that suggested your results are biased on the high side, he/she should have requested a lab person from the state accreditation program or state lab to check your procedure.
If you are getting BOD values that vary from what the commercial lab is reporting then I would check with another laboratory for another comparison to determine who's values are correct. Compare methodology and QC data to select another lab.
In checking the 21st ed. Standard Methods which includes the ATU as an acceptable nitrification inhibitor it spells out the correct amount of ATU to add to each bottle. It does not allow for additional ATU per bottle which will occur if using the Polyseed NX version of seed. During seed correction additional seed material will have to be added to get proper depletion of DO. I would not use a lab that uses Polyseed NX as their seed material until excess ATU has been studied. You could try the NX version yourself to see if that is the difference you see compared to the commercial lab.
Perry Brake and James Royer have got it right on the nailhead on this.
Running additional replicates will not change anything. You already run duplicates every ten samples anyway and if these show similar results you can assume that the rest of your analyses have similar precision.
Remember the reason why we run multiple dilutions: to make sure we get at least one bottle with acceptable result. We have found that some sampling sites, such as our plant primary, are so stable that we only run 2 bottles.
Other sampling points like our effluent are consistently clean and we theoretically could run just one 300 mls bottle all the time. However, we once had an plant upset incident a week after we changed to setting up three 300mls for the effluent. The plant was then unable to report a BOD value except as a >6 mg/l, so we immediately went back to setting up 75, 150 and 300 and will stay that way.
Whatever differences there are between your lab and the other lab are probably systemic and due to the different way the test is run or different reagents and/or seeds.