Biochemical oxygen demand (BOD): The BOD exerted during the first 20 days of incubation is generally referred to as which stage of oxygen demand?

Introduction / Context:

BOD measures the oxygen required by microorganisms to oxidize biodegradable organic matter in wastewater. The oxygen demand has distinct phases: carbonaceous BOD (CBOD) and nitrogenous BOD (NBOD).

Given Data / Assumptions:

• Standard BOD test is 5 days at 20°C (BOD5), but long-term BOD can be observed over 20 days or longer.
• Carbonaceous oxidation dominates early periods before nitrification (NBOD) becomes significant unless inhibited.

Concept / Approach:

The first 20 days generally capture the carbonaceous stage, especially if nitrification is inhibited or delayed. CBOD reflects oxidation of organic carbon compounds to CO2 and water, which is the principal oxygen demand in municipal sewage during the initial period.

Step-by-Step Solution:

Verification / Alternative check:

Analytical protocols often report CBOD5 (carbonaceous BOD at 5 days) to exclude nitrification via inhibitors (e.g., allyl thiourea), underscoring that the initial BOD is carbonaceous.

Why Other Options Are Wrong:

• Initial demand/First-stage demand: Vague descriptors; the precise term is carbonaceous demand.
• All of these: Overly broad; only the specific term is correct.
• Nitrogenous demand: Occurs later as ammonia is oxidized to nitrites/nitrates.

Common Pitfalls:

• Confusing CBOD with NBOD and misinterpreting BOD5 versus long-term BOD.

Final Answer:

Carbonaceous demand