BOD5 Calculation Steps

The Biochemical Oxygen Demand (BOD) test measures the amount of oxygen consumed by microorganisms in a water sample under specific conditions. To find the BOD5 of the raw sewage, we need to account for the dilution made for the test.

1. Calculate Dilution Factor (DF)

The dilution factor represents how much the original sample was diluted. It is calculated as:

$ \text{DF} = \frac{\text{Total Volume of Diluted Sample}}{\text{Volume of Raw Sewage Sample}} $

Given:

• Volume of Raw Sewage Sample = 2.4 ml
• Total Volume of Diluted Sample = 240 ml

Calculation:

$ \text{DF} = \frac{240 \text{ ml}}{2.4 \text{ ml}} = 100 $

2. Calculate Oxygen Depletion

The oxygen depletion is the difference between the initial Dissolved Oxygen (DO) and the final DO after the incubation period.

$ \text{Oxygen Depletion} = \text{Initial DO} - \text{Final DO} $

Given:

• Initial DO = 8 mg/l
• Final DO (after 5 days) = 6 mg/l

Calculation:

$ \text{Oxygen Depletion} = 8 \text{ mg/l} - 6 \text{ mg/l} = 2 \text{ mg/l} $

This depletion represents the BOD5 of the *diluted* sample.

3. Calculate BOD5 of Raw Sewage

To find the BOD5 of the original raw sewage, multiply the oxygen depletion by the dilution factor.

$ \text{BOD}_5 (\text{Raw Sewage}) = \text{Oxygen Depletion} \times \text{DF} $

Calculation:

$ \text{BOD}_5 (\text{Raw Sewage}) = 2 \text{ mg/l} \times 100 = 200 \text{ mg/l} $

Conclusion

The BOD5 of the raw sewage is 200 mg/l.