BOD result is wrong
The first question is "How do you know?" Are your QC values correct (such as GGA, Blank, replicates)? Are the dilutions linearly depleting? Are the calculations correct? If the answer to these questions is yes then the BOD result is probably correct. Often times a high BOD result, especially one that causes an out of compliance situation, is suspected as being incorrect. It is possible though that the BOD actually is high. There are some things that can be done to give that extra assurance that the result makes sense.
- If possible, analyze the COD on all BOD samples so that variations in the COD versus the BOD can be obtained at all times. If the BOD goes through the roof, so should the COD.
- Do periodic "split samples" with other laboratories. Given equal quality QC both laboratories should have the same values.
- Measure the oxygen uptake rate with a standard BOD probe and measure this for all BOD samples. If the oxygen uptake rate goes up, so should the BOD.
- Measure the ORP on all samples that BOD is performed on. If the ORP goes up, so should the BOD.
- Monitor the TSS. Increases in TSS are often indicative of higher BODs.