WEF Discussion Forums
Laboratory Management and Technical Issues
seeding high strength BOD's
Hello, we have an industrial user which uses waste potato starch in their process. They have very high BOD's. We normally don't seed their samples when performing BOD's and typically get results in the 5,000 to 10,000 range. We recently split a sample with an outside lab who did seed the samples and they got results which were about 2X higher than ours. My question is whether the reason for lower BOD's is that the our BOD dilutions are biology limited. That is there is not enough natural seed to take up the required BOD? We are seeing final DO's greater than 1 mg/l on our dilutions so this dosen't seem real possible, but just wondering.
I would say, yes you need to seed the samples. I think the starch will have some chemical activity of its own and oxidize even without the presence of the bugs. At the very least i would set up a batch or two of the samples with seed and without seed to compare in your facility the differences in the two. I would not put too much stock in a one time doubled result from a different lab. Interlab comparison on anything other than G:Ga (or some other stable chemical) for BOD is about as close to meaningless as you can get without being insulting.
There is a tendency to think that the BOD of a sample is somehow related to the need to seed (e.g., high BOD, no need to seed). You can have very high BOD, and essentially no seed. Consider a GGA sample that you prepared to be somewhere between 5000 and 10000 (more or less what you are testing, Donald, except that your sample contains primarily starch rather than glucose and glutamic acid). Would you seed that GGA sample? Of course you would.
The best rule to follow is that unless you KNOW the sample already has sufficient seed (for example, as a domestic influent normally does), you must seed.
So why do you get ANY oxygen depletion on the potato waste sample? There are always sufficient bacteria floating around where the sample was taken and in your lab to provide some depletion. I once inspected a lab that never seeded the GGA sample and got an average in the 180s with good precision. Doesn't say much for the cleanliness of the lab!
I see your point Perry. My opinion was always to seed these samples because the industrial process which the starch is subjected to is high temp and pressure before it is sent to our sanitary system. I just did not think it was plausible for any bacteria to survive, however because we were getting decent depletions w/o seeding we stoped seeding. Given the importance in getting a representative BOD value for which to bill this industry, I think our practice now will be to seed, however I think for the sake of curriosity I will also do what dsmith suggests and compare some seeded vs unseeded samples.
Thanks for all your help.
Please post the results of your seeded vs unseeded BODs for us. We have an ice cream plant and we never seeded the BOD for their discharge. Reading the responses here I can see that we should have.
Good plan, Donald, to compare seeded to unseeded results. If you would like to put a small wager on the outcome, let me know!
I'd be more than happy to post the results. As for placing any kind of a wager with Perry on anthing having to do with BOD results, I'd have to be nuts