126. A wastewater sample has COD of 1 g/L of which 80% is the BOD. During partial aerobic oxidation only 200
mg of COD was converted to CO2 and H2O. The remaining BOD (mg/L) is:
1. 800
2. 200
3. 600
4. 1000

Question Recap

A wastewater sample has:

• COD (Chemical Oxygen Demand) = 1 g/L = 1000 mg/L

• 80% of COD is BOD → BOD = 80% of 1000 mg/L = 800 mg/L

During aerobic oxidation:

• 200 mg/L of COD was converted to CO₂ and H₂O.

Step-by-Step Explanation

1. Initial BOD = 800 mg/L

2. COD Converted = 200 mg/L

   • This represents organic material that has been completely oxidized.

   • Assuming this 200 mg/L comes entirely from the BOD portion, the amount of BOD removed = 200 mg/L.

3. Remaining BOD = Initial BOD – BOD removed
   = 800 mg/L – 200 mg/L
   = 600 mg/L

✅ Correct Answer: 3. 600

Understanding COD and BOD in Wastewater Treatment

• COD measures the total amount of oxygen needed to chemically oxidize both biodegradable and non-biodegradable organics.

• BOD measures only the biodegradable portion that microbes can oxidize.

• During treatment, BOD is typically reduced first, making it a key indicator of organic pollution.

Conclusion

In wastewater management, understanding how COD and BOD interact is essential. In this case, 600 mg/L of BOD remains after partial oxidation, highlighting the need for further biological treatment. Efficient management ensures environmental safety and regulatory compliance.