In a standard-rate trickling filter followed by secondary settling, by what percentage is the influent BOD typically reduced under normal operation?

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:

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%