WEF Discussion Forums
Laboratory Management and Technical Issues
HELP! I have been doing cBOD's for over 2o years, and this is a first! Have had high GGA's in BOD and cBOD for the past 3 months. Blanks, seed blanks all look good-no change to procedure, reagents, dilution water, glassware....normally, my cBOD GGA is around 172. I have had 220, 188, 178, aditional 5 in the 180's, last time was 215! Just not sure what to change, or check at this point.
I suspect your nitrification inhibitor is losing potency. We are in a humid environment and it will occasionally degrade due to (we suspect) that humidity. Try changing that out and see if your performance improves. Your first sentence says that you are getting high results in the BOD as well. Are you getting the high results on each from the same day? Is everything, i.e. seed and dilution water, the same between the batches
There's that log-out function again. That last post was me.
I do not see that you have a problem. All your values are between 178 and 220 mg/L CBOD. This is well within the 198 +/- 30 mg/L that Standard Methods requires for the analysis. I understand that they may be higher than the 172 you were getting but 172 is on the low end of the known value.
Our range is 195 to 212 mg/L TBOD and 183 to 204 CBOD on GGA and a range of 189 to 218 TBOD for a 200 mg/L standard of KHP. The KHP value is using a 0.70 factor for BOD/COD making a BOD standard of the COD standard. This helps evaluate any nitrification interference with the GGA standard.
I can certainly try that, but that would not explain the high BOD values as well. BOD standard should be about 198ppm. cBOD standard should be about 172ppm- that is my understanding.
The BOD test is a bioassy rather than a chemical analysis. Thus the expected value will be a certain percentage of the total oxidation value of the organic compounds in the sample. The BOD test is a wet oxidation and will follow the same chemical reaction as COD just not to completion during the 5 day incubation period. The average published data for the 12th ed. was 218 mg/L +/- 11 using fresh settled sewage. We now have an assigned value of 198 mg/L which is an average of many different laboratory results where purchased seed material is used ensuring no nitrification interference.
The CBOD should have the same value as BOD as the sample has the same organic content at the same oxidation state utilized for both tests. The reason that CBOD has a lower value is that many laboratories get lower values that was averaged. Based on case studies published in "Third Century of BOD" by Baird and Smith, 2002, TCMP is not toxic to carbonacous BOD and that lower results are most likely due to improper test setup and insuffecient seed amounts.
In our lab we have seen almost no difference using purchased seed ensuring no nitrification interference.
How does one know if the seed has nitrifiers in it at all? Unless one is using commercial seed that is guaranteed to contain these, one should expect seasonal variation when using natural sources.
If your CBOD GGA is the same or more as the BOD GGA, then there are no nitrifiers in your seed, but your samples, particularly raw sewage samples, normally will contain nitrifiers.
The effects of nitrification is the most perplexing thing about the BOD analysis. I think this is the cause of the most variation of results that we see from lab to lab.
We do not want any nitrfication in our BOD test as we want to evaluate the organics in the sample. This has been the case since the 9th ed. of Standard Methods in 1946 when ammonium ion was added to the dilution water to standardize it. The thought then was that there are few nitrifiers in the samples so there will be no problem. But after more treatment nitrifiers are a problem.
So we want seed material with no nitrifiers or at least as few as possible.
I know, and I'm sure that Luis and many others know, that scientifically it is better not to have nitrifiers in the seed. But from a practical stand point, there is no problem with them being there when doing BOD. If the regulator (i.e., a permit writer for a wastewater treatment plant) doesn't want nitrifiers to be a part of the test, the permit would be written for the plant to run carbonaceous BOD rather than BOD.
When a WWTP lab runs an effluent BOD sample, their objective is to make sure the plant is meeting its discharge permit limit. If they can do that with seed containing nitrifiers, they and their regulator are happy. When they run a GGA sample, their objective is to get results in the 198 mg/L range so they can keep their regulator/accreditor off their backs. The lab is more likely to do that with a seed containing nitrifiers, because if there are no nitrifiers in the seed, they are in effect doing a CBOD, and the CBOD test is known to give considerably lower GGA results than BOD.
Perhaps the fight to be fought is to get all regulatory agencies to require CBOD rather than BOD run on plant samples. However, because the vast majority of historical data for WWTP plants is BOD and not CBOD data, that change is probably not going to happen soon.