Difficulty: Easy
Correct Answer: biodegradable and non-biodegradable organic matter
Explanation:
Introduction / Context:
Oxygen demand tests quantify the oxidizable load in water/wastewater. Biochemical Oxygen Demand (BOD5) measures the oxygen consumed by microorganisms to oxidize biodegradable organics over 5 days, whereas Chemical Oxygen Demand (COD) uses a strong chemical oxidant to oxidize a broader suite of compounds.
Given Data / Assumptions:
Concept / Approach:
COD ≥ BOD because COD includes both readily biodegradable and non-biodegradable (or slowly biodegradable within 5 days) organic compounds that the microbial test does not fully oxidize. Inorganic reducing species (e.g., sulfides, ferrous iron) can also contribute to COD, further widening the gap.
Step-by-Step Solution:
1) Define BOD5: oxygen used by microbes to oxidize biodegradable organics over 5 days.2) Define COD: oxygen equivalent of chemical oxidant required to oxidize the total oxidizable content.3) Recognize the coverage difference: COD captures both biodegradable and non-biodegradable organics.4) Conclude: COD values are typically greater than BOD5 for the same sample.
Verification / Alternative check:
Typical municipal wastewater shows COD/BOD ratios between ~1.5 and 2.5, reflecting the presence of slowly degradable and non-biodegradable organics.
Why Other Options Are Wrong:
(a) describes BOD, not COD; (c) and (d) provide only partial or incorrect coverage of what COD measures.
Common Pitfalls:
Assuming COD equals total organic carbon (TOC); ignoring the influence of inorganic reducers on COD; misusing BOD for rapid process control where COD is preferable.
Final Answer:
biodegradable and non-biodegradable organic matter
Discussion & Comments