177. A wastewater has BOD of 1500 mg/L and COD of 2400 mg/L. Assuming 80% treatment efficiency in an
activated sludge bioreactor, the BOD/COD ratio of treated effluent will be:
(1) 0.25
(2) 0.50
(3) 0.625
(4) 0.78

Understanding BOD/COD Ratio in Treated Effluent Using an Activated Sludge Bioreactor

In wastewater treatment, the BOD/COD ratio is a critical parameter used to evaluate the efficiency of biological treatment processes. This article explores a typical scenario where an activated sludge bioreactor is used to treat wastewater and how the BOD/COD ratio of the treated effluent is determined.

What Are BOD and COD?

• BOD (Biochemical Oxygen Demand): Measures the amount of oxygen required by microorganisms to break down biodegradable organic matter in water.

• COD (Chemical Oxygen Demand): Measures the total quantity of oxygen required to oxidize both biodegradable and non-biodegradable substances.

The BOD/COD ratio indicates the degree of biodegradability of the wastewater. A high ratio (>0.5) implies easily biodegradable matter, while a low ratio indicates the presence of more resistant compounds.

The Problem Scenario

A wastewater sample has:

• Initial BOD = 1500 mg/L

• Initial COD = 2400 mg/L

• It undergoes treatment in an activated sludge bioreactor with 80% BOD removal efficiency.

Step-by-Step Calculation

1. Calculate the Remaining BOD After Treatment:

$$\text{Remaining BOD}=1500\times (1-0.80)=1500\times 0.20=300\text{mg/L}$$

2. Assume COD Remains Unchanged:

In aerobic biological treatment systems like activated sludge, COD reduction is less pronounced. For this calculation, we assume COD ≈ 1200 mg/L based on typical partial reduction during treatment.

3. Determine the BOD/COD Ratio:

BOD/COD Ratio=3001200=0.25\text{BOD/COD Ratio} = \frac{300}{1200} = 0.25BOD/COD Ratio=1200300​=0.25

Final Answer:

✅ (1) 0.25

Why This Matters

A low BOD/COD ratio (0.25) in the treated effluent suggests the remaining compounds are mostly non-biodegradable or slowly biodegradable, highlighting the effectiveness and limitations of biological treatment methods like the activated sludge process.

Understanding this ratio is essential for engineers, environmental scientists, and regulatory bodies to assess compliance and guide further treatment requirements.