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| | | Forum Newbie
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Last Login: 3/20/2008 11:48:52 AM
Posts: 2, Visits: 43 |
| | We have an advanced secondary plant. We run both BOD and CBOD on a number of samples, and just CBOD on other samples. CBOD is actually used as a billing parameter. We use the CBOD to BOD ratio of our influent to the plant as a tool to help us determine whether the tests are being performed properly. Normally, the CBOD/BOD ratio is less than one (i.e. the CBOD is less than the BOD), which would be expected. In a certain percentage of the samples, the ratio is one or slightly higher-this is not unexpected. However, over the last year, the percentage of samples having a ratio greater than one has increased significantly. When we split samples with an outside lab, the always get ratios less than one. This leads us to conclude that we may have some problem. We have conducted a wide range of experiments to try to get to the bottom of this, but have come up emptyhanded. Any thoughts? |
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| | | Supreme Being
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Last Login: Today @ 9:37:52 AM
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| | I'm assuming you've eliminated the simple errors (yes, I know what happens when I assume..) and don't have any recurring calculation errors...there was a brief instance when the seed factor was calculated wrong for CBOD for a particular customer and CBOD was consistently 25% higher than BOD. This has happened to us on two occassions in the last five years. The first time, we cleaned our DI system and this eliminated the problem (for whatever reason, the CBOD results were more adversely affected than the BOD results). After our DI system was cleaned and put back up, everything straightened out. The second time, it turned out there was something wrong with the batch of inhibitor we were using (we use TCMP). We bought a new lot of inhibitor and compared the two and the new lot gave the expected result. Good luck...... Stressed Organism |
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| | | Forum Member
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Last Login: 4/17/2008 11:45:01 AM
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| | Do you run BOD and CBOD blanks? We do and often have depletion in the inhibited blanks due to breakdown of the TCMP. We have had better results using allylthiourea in some trial runs, however I don't believe the 21st edition of SM where ATU is allowed is approved yet. For years we reported slightly higher CBOD than BOD for EPA PE samples due the contribution from the n-serve. Luckily the acceptance ranges were fairly liberal. |
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| | | Forum Newbie
Group: Forum Members
Last Login: 3/20/2008 11:48:52 AM
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| | could you explain what you mean by "cleaning your DI system"? What steps did you take specifically? |
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| | | Supreme Being
Group: Forum Members
Last Login: Today @ 1:37:46 AM
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| A few questions that might help coming up with a cause...
1. What do you use for seed? Has that changed recently?
2. Do you do separate seed control bottles for BOD and CBOD (i.e., a seed control bottle with inhibitor, and a seed control bottle without the inhibitor)?
3. Do you correct for the blank (i.e., subtract the amount for the blank from the depletion for each bottle)? (You shouldn't, but some labs do.)
Perry |
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