Difficulty: Easy
Correct Answer: 30%
Explanation:
Introduction / Context:
Primary treatment removes settleable and floatable solids through sedimentation and skimming. Because most dissolved organics are not removed at this stage, the percentage reduction in BOD is modest. This question checks recall of the typical BOD removal in primary treatment alone.
Given Data / Assumptions:
Concept / Approach:
Primary clarifiers mainly remove suspended solids (about 50–70%) associated with particulate organics. The corresponding BOD removal generally falls around 25–35%, commonly remembered as about 30%. Much higher BOD removals (e.g., 65–85%) require biological treatment (secondary) or chemically enhanced primary treatment.
Step-by-Step Solution:
1) Identify stage: only primary settling → limited removal of dissolved organics.2) Recall typical performance: BOD removal ≈ 30% for municipal wastewater.3) Compare options and select the closest standard value.4) Conclude that 30% is the representative figure.
Verification / Alternative check:
Design texts and exam references consistently quote BOD removal around 25–35% in primary clarifiers under normal conditions.
Why Other Options Are Wrong:
65% and 85% reflect secondary/tertiary treatment levels; “Zero” is unrealistic because particulate-associated BOD is partially removed in settling.
Common Pitfalls:
Confusing TSS removal percentages with BOD removal; assuming chemical addition without being stated.
Final Answer:
30%
Discussion & Comments