WEF Discussion Forums
Laboratory Management and Technical Issues
Negative BOD calculations
I'm curious as to what I should report for an effluent BOD if the calculations come up with a negative number. I suspect that the effluent wasn't totally free of chlorine, but I've never had this happen before. My blanks are fine, GGAs are fine, and other samples run at the same time had >2 drops. Any ideas?
I would suspect that you report a "Lab Accident" if you think the chlorine was not all removed. You definitely have something wrong if you get a negative value. If it was only slightly negative it could be a bad seed correction value and you are over correcting on a very low effluent BOD.
How negative a result are we talking about here? As James said, if it is only a slight bit negative, just report a non-detect according to your normal reporting criteria. You have to remember that your seed correction factor is just an average of what the seed depletes. You could have more or less on any aliquot of seed. As I often say, bacteria are very bad at arithmetic no matter how often the multiply and divide. This is most likely a very clean sample.
Without knowing the actual values read, we can not conclusively say if there is a problem with your BOD analysis.
Besides variance in the seed correction, there is also variability in the two DO calibrations made 5 days apart. It is my personal opinion that depending upon the calibration method used and the measurement mode used (manual or automated), there could be an error from 0.1 to 0.3 mg/l at each calibration. Thus it could be possible for the two calibrations to be 0.3 and -0.3 mg/l off true value, theoretically giving you -0.6 mg/l depletion.
We do not see negative depletions because all the effluents we test have some Oxygen demand. You would only see a negative result on a very, very clean water unless there is something very wrong with your analysis protocol.
I used to get these occasionally in a surface water that had a lot of algae in it. I used to put it down to the algae having a chance to photosynthesis when the incubator door was open. Do not know if my theory was correct or not but we used to report that it was lest than the detection limit.
From the non member formerly known as Adri
I have seen this happen when the seed correction is too large, i.e., much highigher than usual. Also happens if there is peroxide in the sample (used to test effluent from a semiconductor mfr). Algae and light can also give you highigher O2, but not in the dark! Of course blanks can have negative depletion after five days due to inaccuracies in probe calibration, but these are usually quite small negative values.