WEF Discussion Forums
Laboratory Management and Technical Issues
Dear GGA, We need to talk
I've been communicating with Perry about this problem, and I thought I'd throw it out there for others to ponder.
For seed material, I use rehydrated Polyseed spiked with WWTP raw influent. Historically, this has worked well for BOD and CBOD in the sense that 198 ppm was achievable for both. About two months ago, however, ONLY my CBOD values began to fall drastically into the 150-170 range. TCEQ auditors tell me that I have to use 198ppm as a target for both BOD and CBOD, pointing to SM21 authors who say that overseeding is the answer to raise CBOD GGA values. However, I have not been able to elevate the GGA values by overseeding or by concentrating the Polyseed. I can run a same-day batch of BODs and CBODs using the same equipment, reagents and standards and get 200ppm for BOD and 160 for CBOD. If there are no nitrifiers in Polyseed, how is it that my numbers are so different. I have just recently acquired some seed capsules from BioSystems and will have some results on those next week. No nitrifiers in that seed either, though.
Has this ever happened to anyone else out there? I realize that many/most/all WWTPs may not have to run both BOD and CBOD, so maybe I'm barking up the wrong tree. But, any help is appreciated.
I analyze samples for a WWTP and I use rehydrated Polyseed, with no additions. I routinely analyze both BOD and CBOD with spike values averaging 210 ppm. My CBOD values are higher than my BOD values on average. I use the same amount of seed for both.
We have been having the same poblem with our GGA numbers for the last couple of months. We had been using NSI GGA and decided to try the Environmental Express GGA. So far our numbers are looking good. What brand GGA are you using?
We have had the same problem for last few weeks. We use polyseed, so no nitrifiers. For us, a couple of weeks after the CBOD dropped, the BOD also had a sudden drop. We had 21 GGA's in a row average 200 with a std dev of 7.89 with a couple of outliers at 186 and a 222 (no trending). The previous 102 GGA's averaged 197 with a std. dev of 28.6 (we were adjusting our protocol during this time as well having some trouble with outliers). After the 21 stellar GGA results, we had a sudden drop to 142, back up to 170 then down into the mid 140's again. We have seen this before and ruled out GGA, Polyseed itself , glassware, change in procedure. I am down to that the polyseed is sensitive to a change in our DI water that is exacerbated by the nitrification inhibitor. Grasping at straws really.
I would be very interested in your biosystems results.
I'm sad to report that the Biosystems seed behaved exactly the same. In other words, I had no problem hitting 198 for BOD but that same value for CBOD is elusive. I have overseeded, hyperseeded, insanely seeded the GGAs for CBOD all with the same results: 155-175ish.
I talked with Polyseed yesterday, and they tell me this is quite common and that they do not recommend using polyseed for CBOD. They're telling me that the Hach nitrification inhibitor is toxic to the seed. I've heard all this before. The question is, why can some labs use polyseed and Hach nitrification inhibitor and get identical values (19 for BOD and CBOD? If there truly is a toxic effect, nobody should be able to successfully use these two products together for CBOD analysis.
Over the past six months, I've played with: rehydration volumes, adding WWTP raw influent (from three different cities) to the rehydrating seed, overseeding, Biosystems seed, and different water sources. No matter what I change (one thing at a time), I see the same trend with depressed CBOD values. Everything points to the inhibitor changing the value. So, my next step is to try ATU instead of TCMP. If anybody has any helpful tips for using this form of inhibitor, please let me know.