Because this test relies in part upon living microorganisms, many things can go wrong, with the outcome being inaccurate results. To avoid this, quality controls are used when running the BOD. These are:
Dilution water blanks
Oxygen Depletion Rules
Sample pH adjustments
Dechlorination of Chlorinated Samples
Dissolved Oxygen Meter Calibration
Careful control of the incubator temperature
Routine analysis of a standard (a stabilized sugar called Glucose/Glutamic acid is used)
Annual analysis of an externally supplied standard
If the water that is used to make up the buffered dilution water contains organic matter or chemicals that will cause an oxygen demand, the test results will be incorrect. To ensure against this problem, two BOD bottles that contain only buffered dilution water are incubated along with the sample bottles and the seed correction bottles. These two bottles are known as “dilution water blanks”. If the difference between the initial D.O. and the final D.O. of the dilution water blanks is >0.2 mg/L, it indicates that something is wrong with the water used to make the dilution water, that the D.O. meter calibration is off, or that the glassware used in the test is contaminated. Whatever the cause, the analyst should work to correct the problem. When over-depletion of the dilution water blanks occurs, it should be highlighted on the bench sheet. If this data is used for permit reporting purposes, a notation should be made on the discharge monitoring report detailing the over depletion problem. However, excessive oxygen demand in the dilution water blanks is not a reason to invalidate (throw out) the data from a BOD test.
Because the BOD test is essentially based on oxygen depletion, we must insure that enough oxygen is in the bottles at the beginning of the test and that enough oxygen remains in the bottle at the end of the test for us to accurately measure. The oxygen depletion rules outline the various aspects of oxygen levels that are acceptable during the test.
The depletion rules are as follows:
1. At least 2.0 mg/L of dissolved oxygen must be consumed in sample bottles during incubation or the results from that bottle are not included in calculating the BOD.
2. At least 1.0 mg/L of dissolved oxygen must remain in sample bottles following incubation or the results are not included in calculating the BOD.
If no bottles containing sample meet these depletion rules, the results can still be used for reporting purposes, but the data is suspect and the results should be recorded in such a way that the problem is indicated. If the data is used for permit reporting purposes (NPDES), a notation should be made on the discharge monitoring reports that explains the problem.
If a sample contains caustic alkalinity or acidity (defined as a pH or > 8.5 or < 6.0 respectively), the sample pH must be adjusted to near neutral. If samples have a pH of > 8.5 or < 6.0, they should be adjusted to a pH of between 6.5 and 7.5 before the analysis.
We have already discussed the need to seed samples that have been disinfected. However, if samples were disinfected with chlorine, any residual chlorine remaining in the sample could kill our seed microorganisms. THIS WILL RESULT IN A BOD MEASUREMENT THAT IS MUCH LOWER THAN THE ACTUAL BOD. In order to avoid this problem, all chlorinated samples must be checked for residual chlorine and dechlorinated with a freshly prepared 0.025 N sodium sulfite solution if any residual chlorine is found. After dechlorination, samples must be checked again to verify that no residual chlorine remains.