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Article taken from the NELAC Website
Follow-up information provided by Terry Mills, Standard Methods BOD Task Group
Follow-up added October 2, 2004
I would like to remind your readers that "Seeded Blanks" are not used in
the controls for determining a seed
comparison. Also, they cannot be used as some sort of comparison to determine
if the calculated seed correction from the seed controls and the actual
depletion of the seeded blank is within a certain level of mathimatical
variance that is acceptable. The problem with seeded blanks is that when 2, 3,
or 4 mL of seed material and the DO uptake is virtually lost though dispersion
as the dilution water has little if any organic carbon content.
As a result of the inefficiency of a seeded blank, I would have preferred to have
seen some type of caution in using it for BOD seed depletion purposes. In the
statement at the bottom of the section it states the following definitions were to
help clarify the components and requirements of the CBOD test. I believed that
NELAC should have identified which was a "component" which were requirements. A majority
of the analysts who perform the BOD test do not understand many of the
purposes of why certain controls are required, so when they read this, many
of them will start to include "seeded blanks" as a part of their controls.
Article from NELAC Website
Interpretation / Decision
If a laboratory is conducting the analysis for CBOD following method 5210B found in the 20th
Edition of Standard Methods, then the analyst may over-seed the Glucose-Glutamic Acid (GGA)
standard as allowed by the method, but the results must be within the range 198 +/- 30.5 mg/l to
be considered acceptable. The laboratory may either meet the above criterion as the acceptance
range for GGA recovery, or has the option of developing its own acceptance criteria for GGA
recovery under the conditions described below:
- The dissolved oxygen uptake from the seed contribution should be between 0.6 - 1.0 mg/l.
- In establishing in-house GGA control acceptance limits, the laboratory must use accepted statistical treatments of in-house data for no less than 25 GGA checks over a period of weeks or months (Standard Methods 5210B 6.a.).
- The control limits should target the mean value of 164 mg/l, with a range of +/- 26 mg/l, as derived from USEPA’s DMRQA/WP performance evaluation database.
- The control limits established by the laboratory must be set at three standard deviations from the derived mean, and must not exceed +/- 26 mg/l from the mean as the acceptance range. If the laboratory’s calculated acceptance range exceeds +/- 26 mg/l, the laboratory may default to +/- 26 mg/l as its control limit range from the derived mean.
- The mean GGA value for CBOD determined by the laboratory cannot be less than 150 mg/l, and should be higher.
Clarification Comments
The 18th, 19th and 20th Editions of Standard Methods all allow for the laboratory to establish their
own limits for BOD and CBOD, but only the 20th Edition addresses the quality control criteria for
GGA in CBOD in Section 6 of method 5210B.
The laboratory must treat both the GGA standard and all related samples
(including QC samples such as seed blanks and PT samples) in the same
way. Evaluation of the various components under CBOD is a check on the
inhibitor capacity and its effectiveness. The following terms are
defined to help clarify the various components and requirements of
the cBOD analysis.
- CBOD Dilution Water Blank – bottle containing only the buffered dilution water and the nitrification inhibitor.
- CBOD Seed Blank – bottle containing the same amount of seed that is added to the buffered dilution water for each sample plus the nitrification inhibitor.
- CBOD Seed Controls – bottle containing larger amounts of seed added to the buffer dilution water plus the nitrification inhibitor, which gives at least 2.0 mg/l depletion.
- CBOD Seed Contribution – the calculated amount of depletion from the CBOD Seed Control that has been ratioed back to the amount of seed added to each sample.
Question from APG eNewsletter Reader
I have a problem with CBOD analyses, and not just with GGA. My CBOD is usually higher than BOD on the same GGA
and WW samples, which theoritically should not be possible. And, for example, my BOD
blanks (no seed) never deplete more than 0.2, however my CBOD blanks (no seed) lose
from 0.4 to 1.0 ppm. (FYI, I seed the BOD bottles, not the diln water carboy.) Something
is causing the CBOD blanks and sample dilutions to deplete more, as if there is
a demand within the Nit Inib itself. Hach offers very little help.
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Response from Wade DeLong, APG Lab Manger
I hope the following information will be helpful. In response to your
questions regarding the depletion in your cBOD blanks being greater
than your BOD blanks. Your suspicion of the nitrification inhibitor
may be correct. I spoke with Dan McElhatten (2nd Vice President of
the Ohio Water Environment Association – Northwest section) about
this issue. Mr. McElhatten had the same problem in the past and after
a very lengthy troubleshooting process of numerous variables in his
laboratory determined the culprit is the nitrification inhibitor. After
arriving at this conclusion he contacted the state laboratory to get
some guidance on how to avoid this problem. The state lab had the
same problem and their course of action was to not add the nitrification
inhibitor to the cBOD blank. Mr. McElhatten has tried to find other
suppliers for the nitrification inhibitor and found, of the suppliers
he called, all were traceable back to Hach.
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