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While the 21st Edition of Standard Methods has not yet been approved for Clean Water Act monitoring, it is worthwhile to know what some of the changes are in the BOD test (SM 5210B). Some of these changes may be allowed or even encouraged by regulators and lab accreditors. This article points out some of the more significant changes in the order they appear in the method. Comments are those of the author.
-Paragraph 3g(2) adds allylthiourea (ATU) as an approved nitrification inhibitor.
-Paragraph 4b(4) and 5b allow sample temperature prior to dilution to be 20 +/- 3° C rather than +/- 1° C (but the incubation temperature remains 20 +/- 1° C.)
-Paragraph 4b(5) adds pretreatment procedures for samples containing hydrogen peroxide.
-Paragraph 4c expands guidance on selection and storage of source water and pinpoints the maximum allowed blank depletion at 0.20 mg/L rather than the 0.2 mg/L.
-Paragraph 5a expands guidance on preparation of dilution water and recognizes the fact that source water problems can lead to blanks exceeding 0.20 mg/L depletion, even in the absence of any "contamination" in the test process.
-Paragraph 5d adds the following in referring to the amount of seed to add to the seeded bottles: "...if 1 mL of seed...is required to achieve 198 +/- 30.5 mg/L BOD in the glucose/glutamic acid check, then use 1 mL in each [seeded] BOD bottle..." (Comment: Reading the entire subparagraph, one would conclude that if 6 mL of seed were used in the GGA bottle, giving a depletion of 4 mg/L DO, the same amount would be used in each seeded bottle, also causing a depletion of 4mg/L. This means the seed and not the sample would be by far the major contributor to DO depletion in seeded samples with low BOD, such as effluents. Because of imprecision inherent to the BOD test, the seed contribution to DO depletion could completely mask the sample contribution for low BOD samples. This seems contrary to good reason.)
-Paragraph 5i allows a +/- 6-hour variance in the "5-day" incubation period (i.e. it is now 5 days +/- 6 hours).
-Paragraph 6b adds "the glucose/glutamic acid check is the primary basis for establishing accuracy and precision of the BOD test..." (Comment: Paragraph 8 confuses this issue by retaining the old statement that "there is no measurement for establishing bias of the BOD procedure." Since "accuracy" as used in 6b contains a "bias" and "precision" component, it appears that paragraph 6b recognizes that the 198 mg/L objective for the GGA test is a bias objective.)
-Paragraph 7b establishes the following circumstances under which results should be "identified" in reports:
-Blank exceeds 0.20 mg/L;
-GGA result is outside acceptance limits;
-Test replicates show more that 30% difference between high and low values (Comment: elsewhere, the term "dilutions" is used rather than "replicates" and it is probably "dilutions" that is intended.);
-Seed control samples do not deplete at least 2.0 mg/L, with a retention of at least 1.0mg/L DO, or
-The minimum DO retained is less than 1.0 mg/L.
(Comment: The method does not say what is meant by "identifying" the results, but it probably means mentioning the result(s) in the remarks section of a report. Also, the method does not say results must be "identified" if no dilution depletes at least 2.0 mg/L. This probably means that reporting such results with the "<" less than symbol is sufficient.)
-Paragraph 8b establishes a lower detection limit of 0.0 mg/L, replacing the 2.0 mg/L lower limit in previous editions. The 0.0 mg/L is possible when all the DO depletion in a seeded, undiluted bottle is contributed to the seed.
There are other, less significant changes in the 21st Edition and, because many of the paragraphs were restructured, it would be difficult to assure that all significant changes are mentioned here. The reader is encouraged to study the new method in detail.
Reprinted with permission from Environmental Express (Volume IV; Number 1; May 2006).
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