Wastewater engineering — classifying unit operations A trickling filter in a sewage treatment plant is categorized under which stage of treatment?

Introduction / Context:
Municipal wastewater treatment is commonly divided into primary (physical), secondary (biological), and tertiary (advanced polishing) stages. Recognizing where a process fits supports correct process design and regulatory compliance.

Given Data / Assumptions:

• Trickling filters are fixed-film biological reactors.
• Primary processes remove settleable and floatable solids physically.
• Tertiary processes polish effluent (nutrient removal, filtration, disinfection).

Concept / Approach:
In a trickling filter, wastewater is distributed over media (rock, plastic) supporting a biofilm that oxidizes dissolved and colloidal organics, reducing biochemical oxygen demand (BOD). This is a biological oxidation step characteristic of secondary treatment, often followed by secondary clarification to remove sloughed biomass.

Step-by-Step Solution:
Identify process type: biological oxidation of organics by biofilm.Map to treatment stage: biological BOD removal = secondary treatment.Exclude primary (physical settling) and tertiary (advanced polishing).

Verification / Alternative check:
Design manuals classify trickling filters alongside activated sludge and rotating biological contactors as secondary processes.

Why Other Options Are Wrong:
Primary treatment uses screens and clarifiers; tertiary focuses on nutrient removal/filtration; “none” and “preliminary” are not accurate.

Common Pitfalls:
Confusing the secondary clarifier (a physical settler) with primary treatment; it follows the biological reactor but is still part of secondary treatment.

Final Answer:
Secondary treatment.