Wastewater quality: the oxygen required by aerobic bacteria to biologically oxidize biodegradable organics in sewage is called

Introduction / Context:
BOD is a foundational parameter in wastewater engineering, indicating the amount of oxygen microorganisms will consume while stabilizing biodegradable organic matter. It is central to plant sizing, discharge permitting, and river oxygen-balance studies.

Given Data / Assumptions:

• BOD test typically conducted over 5 days at 20°C (BOD5) unless otherwise noted.
• Measures biologically oxidizable organic load (carbonaceous stage primarily).
• Aerobic bacterial activity is assumed.

Concept / Approach:

BOD quantifies the oxygen demand exerted by biodegradable organic matter. It differs from COD, which measures oxygen equivalent of total oxidizable matter via chemical oxidant, and from DO, which is the concentration of oxygen present in the water at a given time. High BOD indicates strong sewage requiring more treatment capacity.

Step-by-Step Solution:

Verification / Alternative check:

Compare BOD with COD and TOC; BOD/COD ratios indicate biodegradability. Secondary effluents typically show significant BOD reduction relative to raw sewage.

Why Other Options Are Wrong:

(b) DO is oxygen present, not consumed; (c) COD is a chemical oxidation measure that includes non-biodegradable fractions; (d) is incorrect since BOD is the standard definition.

Common Pitfalls:

Assuming BOD equals COD; neglecting nitrification stage unless inhibited; mishandling incubation conditions.

Final Answer:

Bio-Chemical Oxygen Demand (B.O.D.)