The calculations for BOD take into account the unseeded blanks and the seeded solutions. These values must be subtracted out in order to obtain reasonable BOD results. The following calculations are taken from Standard Methods (Method 507: 1985, p. 531).
A. The BOD of the blanks are calculated by subtracting the final DO from the initial DO:
BOD blank=DO1-DO2;
B. The BOD of the seeds are calculated by subtracting the final DO from the initial DO and multiplying this number by the dilution factor:
BOD seed=(DO1-DO2) * Dilution factor per 300 ml
C. The BOD of sample and standards are calculated by subtracting the final DO from the initial DO and multiplying this number by the dilution factor. The final value is determined by substracting out the BOD blank and the seed blank for each delta DO.
If the testing procedure was carried out correctly and the dilutions of the sample were made appropriately, the analyst should have obtained BOD values that are within a reasonable percent error and relative percent difference. Generally, values are discarded for a specific sample dilution if the final DO of the sample is < 1.0 mg/L of if delta DO is < 1.0 mg/L. This also stresses the importance of using different dilutions for each sample to key in on the appropraite BOD when little is known about how the sample will react and how high its BOD levels are.
For references pertaining to this method, refer to the References section or go back to the Introduction page.