WEF Discussion Forums
Laboratory Management and Technical Issues
High CBOD/Low COD
Is there any substance that could deplete the CBOD entirely but not show up in the COD test? I had normal COD values ( 32 and 37 mg/L)for final effluent a few weeks ago and set up CBOD as usual (200, 250 300 mls) and all depleted. This happened 2 of the 3 sampling days that week. I reran those 2 samples and had results of 7.4 and 13.4. Of course these were run 5 to 6 days outside of holding time. This is the only time I have ever observed this phenomenon. Anybody have any thoughts?
The BOD gremlins are at it again, Gregg! Usually, Santa's elves are able to keep them under control, but the elves are pretty busy this time of years and might not have caught the gremlins causing this problem that seems to have no reasonable explanation.
If you kept those two samples at 4° or less, the 5 or 6 days beyond allowed holding time should not have affected the results very much and certainly not enough to lower the CBOD values to 7.4 and 13.4. However, the excessive holding time leaves you with no legally acceptable results to report on the DMR (that is, you cannot report the 7.4 or 13.4). The only value to report, in my opinion, is "greater than" the value you get by multiplying the dilution factor by the oxygen depletion. For example, the 300 mL bottle would have a dilution factor of 1, and if the depletion was, say, 8.0, your reported value would be >8 mg/L. Permit managers don't like such values, but since you have nothing better they would have to live with it.
David is right in saying that a defective inhibitor (or failure to add the inhibitor? Is a new person doing the test??) is one thing that might have been the cause. However, a defective inhibitor or lack of any inhibitor in the bottle would mean that you ran a BOD rather than a CBOD, and now you're left with explaining why an influent would have such a high BOD that all DO would be depleted. The only explanation I can think of is that COD was run on the effluent as intended, but that the CBOD was somehow run on a sample other than effluent. I would hope you can exclude that as a possibility.
As a footnote that has nothing to do with the problem at hand, I hope you use the special nutrient/buffer pillows that are added directly to the bottle when sample size is 200 mL or greater. Bottles are topped off with reagent grade water and not dilution water in those cases. Hach, USABlueBook, Environmental Express and other suppliers carry those special pillows. Failure to use them would have caused low depletion, not high depletion as you have experienced.
Gregg, It was probably some contaminated inhibitor, or maybe a set of contaminated BOD bottles. We always set a 20% sample, 60 ml, so that we can get a value at the excursion level. If the 20% depletes out then the value is greater than the COD and that would indicate that it is not a valid result.
You could keep a preserved sample for COD and then recheck the COD and neutralize for BOD, seeded, to try and explain any big discrepencies in results.
Thanks for all the replies. QC work was all acceptable for both runs. Blanks 0.06, GGA 206 as usual (something else I don't understand because it is inhibited also, so should be lower), and .60 and 0.77 seed correction factor. I think the gremlins hypothesis is the best one! I did start running 100, 200, and 300 mls of final so I wouldn't get that same surprise again.
Last 20 GGA values average 209 with 6.1 SD. It seems to me that it should be lower if nitrification inhibitor is used. I have 10 years of results in the 200-220 range and my DMRQA CBOD rsults have always been on the high side of the acceptable range.
On the subject of seeding, I have never understood how it is valid to adjust seed volumes up or down to get a GGA value into a certain range. That makes it seem like the BOD value is a flexible number instead of a particular number. We don't adjust any other tests in that way. I know, it is a biological test and living things don't behave in tests as the chemical do, but still it has always seemed hoaky to me.
When a lab's precision is good for the GGA test (and yours is excellent, Gregg, as indicated by the 6 mg/L standard deviation for the GGA test), the average BOD for GGA is almost always a function of the strength/viability/health...whatever you want to call it...of the seed. For the CBOD test, there is another factor, and that is the inhibitor. If the CBOD GGA results are higher than expected, it could mean the inhibitor is not working. If you haven't done so already, I suggest running some BOD GGAs in the same batches with your required CBODs. If the results are essentially the same, either your inhibitor is not working, or there are no nitrifiers in the seed and therefore nothing for the pyridine to inhibit. I predict that the BOD average will be even higher.
As you know, I agree with you completely on the "adding additional seed to increase GGA CBOD results" issue. During an EPA-initiated teleconference with Standard Methods BOD Committee, EPA, and a few state regulators including myself, I asked the SM BOD Committee rep why the suggestion to overseed was in the method. The answer? Because it works. Maybe so, but it's never been demonstrated to me. Then I asked why one would want to do that because, 1) you are supposed to analyze standards (e.g., the GGA) exactly like you do other samples, and you don't overseed environmental samples, and 2) if there are nitrifiers in the seed, you expect the CBOD GGA results to be lower than the BOD results because if they aren't, the pyridine isn't doing its job. The answer? None! To say that it is a "hoaky" requirement lets them off easy!
From a regulatory standpoint, I don't think you have a problem, Gregg. There are labs that would kill to have that 209 average and 6 standard deviation because most regulators consider 198 to be the goal for the CBOD of GGA. If those regulators finally realized that the CBOD average should be lower...I consider it to be in the vicinity of 172...you might be in trouble. But cross that bridge if you come to it!
Now that I have retired maybe I can respond to this issue more quickly.
You need a good seed material that has no nitrifiers and a significant number of bacteria so that any additional amount of seed material will not increase the BOD or CBOD result. Additional seed material will increase the seed correction factor as there are organics in the seed material along with the bacteria. Any nitrifiers in the seed will oxidize nitrogen from the glutamic acid or ammonia from the dilution water.
Running BOD and CBOD on GGA should give the same result as the organic concentration is the same in both analysis.
Lower results for CBOD, in my opinion, can be due to some nitrification in the BOD test or toxicity of the inhibitor in the CBOD test.