In a municipal trickling filter used for secondary treatment, which process is fundamentally responsible for the removal of organic matter?

Introduction / Context:
Despite the name “trickling filter,” the dominant removal mechanism in this unit process is biological. Wastewater is sprinkled over a media bed that supports biofilm growth, effecting oxidation of dissolved and colloidal organics.

Given Data / Assumptions:

• Trickling filters are fixed-film biological reactors.
• No chemical oxidants are normally dosed for core BOD removal.

Concept / Approach:
The attached biomass utilizes organic substrates and oxygen supplied by natural draft or forced aeration. While some incidental straining can occur, sustained BOD and ammonia removal arise from microbial metabolism across the biofilm thickness with periodic sloughing that is captured in secondary clarifiers.

Step-by-Step Solution:
Identify primary mechanism → biological oxidation by biofilm.Acknowledge minor filtration effects but note they are not dominant.Therefore, select biological action as the best answer.

Verification / Alternative check:
Performance correlations (e.g., NRC/Metcalf & Eddy equations) relate loading and biofilm kinetics, not straining capacity—further evidence that biology is primary.

Why Other Options Are Wrong:

• Pure mechanical filtration does not explain soluble BOD removal.
• “Neither” conflicts with decades of operating experience.
• “Both in equal measure” overstates the role of physical straining.
• Chemical oxidation is not the basis of standard trickling filter operation.

Common Pitfalls:
Taking the word “filter” literally and assuming granular filtration is the key mechanism; it is an attached-growth bioreactor.

Final Answer:
Biological action by attached microbial biofilm.