Wastewater Treatment – Purpose of a trickling filter Identify the primary function of a trickling filter in sewage treatment.

Introduction / Context:
Trickling filters are attached-growth biological reactors used in secondary treatment of wastewater. The process reduces biochemical oxygen demand (BOD) by supporting aerobic biofilms on media over which settled sewage is distributed and aerated.

Given Data / Assumptions:

• Influent is primary-treated sewage with most settleable solids removed.
• Filter medium provides surface for microbial growth.
• Air flows through voids to maintain aerobic conditions.

Concept / Approach:

The rotating distributor spreads wastewater as a thin film over media. Biofilms metabolize dissolved and colloidal organics, converting them to biomass, carbon dioxide, and water. Sloughed biofilm and remaining suspended solids are captured downstream in secondary clarifiers.

Step-by-Step Solution:

Verification / Alternative check:

Typical BOD removal across trickling filters ranges from moderate to high depending on loading (standard vs high-rate designs), confirming its role as a biological oxidation unit.

Why Other Options Are Wrong:

(a), (c), and (d) describe physical filtration in water treatment, not the biological action in wastewater secondary treatment.

Common Pitfalls:

Confusing trickling filters with rapid sand filters, or ignoring the need for secondary clarification after the filter.

Final Answer:

Biological oxidation process to remove BOD from sewage