Biochemical oxygen demand is a measure of effluent strength in terms of the amount of dissolved oxygen utilized by microorganisms during the oxidation of organic components in the effluent. BOD is determined by incubating a suitable dilution of effluent in a standard dilution water or specific composition at 68°F. After incubation, the amount of dissolved oxygen consumed is obtained by titration and the results expressed as parts per million of BOD. Complete biological oxidation generally requires about 20 days at 68°F. However, the test has been standardized to be completed in 5 days, hence the term BOD5. If no inhibiting substances are present, the oxidation of milk solids is approximately 68-70% complete in 5 days.
Knowledge of BOD5 in parts
per million of dissolved oxygen and the volume of effluent permits the
calculations of pounds of BOD. the pounds of BOD can be calculated by
the formula:
-
Pounds of BOD5 = gallons of
water x 8.34 x ppm BOD5 .
If the exact composition of the effluent is known, the pounds of BOD5 can be calculated quite easily by applying the factors 1.031, 0.890, and 0.691 for protein, fat, and carbohydrate, respectively. The exact composition of effluent is rarely known. However, the amount of BOD5 possessed by common processing materials can be calculated from known composition. For example, the BOD5 of 100 pounds of milk containing 3.25% protein, 3.6% fat and 5.0% lactose would be 10.00.
The calculation is made as follows:
| Pounds of Component | Pounds of Milk | % Component | Factor | Total | |
| Protein Fat Lactose |
100
100 100 |
3.25
3.60 5.00 |
3.25
3.60 5.00 |
1.031
0.890 0.691 |
3.35075
3.204 3.455 |
| Total BOD5 | 10.00975 | ||||
This type of calculation can be used to determine the amount of BOD5 in any product. The calculation is an approximation and will differ somewhat from BOD5 levels determined in the laboratory. However, they are sufficiently accurate to estimate the potential contribution of a processing ingredient to the total waste load.