Difficulty: Medium
Correct Answer: Typical BOD for potable water is about 2.5 mg/L.
Explanation:
Introduction / Context:Biological Oxygen Demand (BOD) indicates the amount of dissolved oxygen that aerobic microorganisms require to oxidize biodegradable organic matter in water over a specified period (commonly 5 days at 20°C, termed BOD5). It is a key indicator of organic pollution and treatment performance.
Given Data / Assumptions:
Concept / Approach:Evaluate each statement against standard definitions and typical values. The incorrect one will contradict accepted ranges for drinking water quality.
Step-by-Step Solution:
1) Option A: Correct—BOD is a measured characteristic, not a substance.2) Option B: Correct—BOD5 is oxygen demanded by microbial decomposition over 5 days at 20°C.3) Option D: Correct—BOD is expressed in mg/L.4) Option C: Incorrect—BOD for potable water is typically < 1 mg/L (often near zero). 2.5 mg/L is high for drinking water and indicates contamination.Verification / Alternative check:Regulatory guidance and design manuals consistently place potable water BOD very low; secondary effluent may be around 20 mg/L, raw sewage 150–300 mg/L, underscoring the scale difference.
Why Other Options Are Wrong:
A: Matches definition of a water quality parameter.B: Accurately describes BOD5 procedure conceptually.D: Correct unit convention.E: Invalid because we have identified one incorrect statement (C).Common Pitfalls:Confusing BOD with COD; thinking BOD is a substance that can be filtered out; assuming drinking water may have multi-mg/L BOD.
Final Answer:Typical BOD for potable water is about 2.5 mg/L.
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