Biochemical Oxygen Demand (BOD): The magnitude of BOD in wastewater primarily reflects the amount of which component?

Introduction / Context:
BOD is a cornerstone parameter in water quality, indicating the oxygen required by microorganisms to biologically oxidize biodegradable organic matter over a specified period (commonly 5 days at 20°C, BOD5). Understanding what drives BOD helps in interpreting pollution loads and sizing treatment units.

Given Data / Assumptions:

• BOD correlates with biodegradable organic carbon and nitrogenous compounds.
• Higher organic loads mean more oxygen demand during microbial metabolism.
• Inorganic solids do not directly increase BOD unless they inhibit/affect biology.

Concept / Approach:
Since BOD measures biological oxidation of organics, its magnitude chiefly depends on the concentration of biodegradable organic material. While microbial counts grow in response to organic load, BOD is not simply a headcount of bacteria; it is an oxygen consumption measure tied to substrate availability and biodegradability.

Step-by-Step Solution:
Clarify definition: BOD is oxygen uptake due to oxidation of biodegradable organics.Link cause and effect: more organics → more microbial respiration → higher O2 demand.Exclude inorganic load: non-biodegradable inorganics do not contribute to BOD.Select “Amount of organic material.”

Verification / Alternative check:
Sampling data show strong correlation between chemical oxygen demand (COD) and BOD for many wastewaters; both track organic content, with BOD focusing on the biodegradable fraction.

Why Other Options Are Wrong:

• Bacterial count: secondary consequence, not the primary determinant.
• Inorganic material: largely unrelated to BOD magnitude.
• All of the above: incorrect because only organics directly define BOD.
• DO saturation only: BOD is a demand, not a measure of initial DO saturation.

Common Pitfalls:
Confusing BOD with total suspended solids; assuming all organics equally contribute (biodegradability matters).

Final Answer:
Amount of organic material.