Wastewater quality — what does BOD measure? Biochemical oxygen demand (BOD) of wastewater is primarily a measure of which constituent class?

Introduction / Context:
BOD is a key indicator of oxygen-depleting potential in receiving waters. It quantifies the dissolved oxygen consumed by microorganisms while stabilizing biodegradable organic matter over a specified period (commonly 5 days at 20°C). Understanding what BOD represents helps distinguish it from related metrics like COD and TOC.

Given Data / Assumptions:

• Standard test BOD5 at 20°C unless otherwise stated.
• Biological oxidation under controlled laboratory conditions.
• Nitrification inhibition may be used to isolate carbonaceous BOD.

Concept / Approach:
BOD measures only the biodegradable fraction of organics that microbes can oxidize within the test period. Non-biodegradable organics, inorganics, and suspended solids that are not readily biodegradable do not contribute substantially. Chemical oxygen demand (COD) instead measures chemically oxidisable matter, often exceeding BOD due to oxidising non-biodegradable organics as well.

Step-by-Step Solution:
Define BOD: oxygen consumed biologically by microbes on biodegradable organics.Differentiate from COD and TOC: COD is chemical oxidation (e.g., dichromate); TOC measures carbon irrespective of biodegradability.Conclude that BOD corresponds to biodegradable organic matter concentration.

Verification / Alternative check:
Typical municipal wastewater shows COD/BOD ratios of 1.5–2.5, reflecting non-biodegradable fractions; this supports BOD’s focus on the biodegradable component.

Why Other Options Are Wrong:
Total organics (b) includes non-biodegradable matter; COD (d) is a chemical test; biochemicals (a) and TSS (e) are not measures of oxygen demand by biodegradation.

Common Pitfalls:

• Assuming BOD equals COD; they differ in mechanism and magnitude.
• Ignoring nitrification oxygen demand in extended BOD unless inhibited.

Final Answer:
Concentration of biodegradable organic matter