In a standard-rate trickling filter followed by secondary settling, by what percentage is the influent BOD typically reduced under normal operation?
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A30 to 40%
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B40 to 60%
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C60 to 80%
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D80 to 90%
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E95%
Answer
Correct Answer: 80 to 90%
Explanation
Introduction / Context:Trickling filters are attached-growth biological reactors. Their performance is often reported as percentage biochemical oxygen demand (BOD) removal after secondary clarification. Knowing typical ranges aids in process selection and compliance estimates.
Given Data / Assumptions:
- Domestic sewage, standard-rate trickling filter.
- Proper distribution, ventilation, and media depth.
- Settled effluent measured after secondary clarification.
Concept / Approach:
Under well-operated conditions, trickling filter systems commonly achieve about 80–90% BOD removal overall. Loadings, temperature, and recirculation affect performance; high-rate configurations may have different ranges but still target secondary-quality effluent.
Step-by-Step Solution:
Identify typical secondary-treatment performance criteria.Match with established removal efficiencies of trickling filter + clarifier.Select 80–90% as the representative range.Verification / Alternative check:
EPA and standard texts list secondary processes (TFs and activated sludge) in roughly the same overall BOD removal bracket when properly designed.
Why Other Options Are Wrong:
Lower ranges (30–60%) correspond to preliminary/primary removal or poorly loaded filters; 95% is typical of tertiary polishing, not standard TF systems.
Common Pitfalls:
Confusing primary settling removal (about 25–35%) with biological secondary removal; ignoring the contribution of the secondary clarifier.
Final Answer:
80 to 90%