Wastewater treatment — Secondary treatment primarily uses which mechanism to consume and remove organic wastes?

Introduction:
Municipal and many industrial wastewater plants are organized into primary, secondary, and tertiary stages. Secondary treatment targets dissolved and colloidal biodegradable organics using biological processes. This question checks recognition of the core mechanism used in secondary treatment.

Given Data / Assumptions:

• Conventional activated sludge, trickling filters, oxidation ditches, or aerated lagoons.
• Influent contains biodegradable organics measured as BOD/COD.
• Goal is to reduce BOD and suspended solids.

Concept / Approach:
Secondary treatment relies on microbial consortia to oxidize organic matter to CO2 and water, and to assimilate nutrients into biomass. Processes include suspended-growth (activated sludge) and attached-growth (trickling filters). Physical and chemical steps can assist but biology is the primary engine of secondary treatment.

Step-by-Step Solution:
Identify the stage: “secondary” implies biological conversion of organics.Map to unit processes: aeration basins plus clarifiers (activated sludge) or biofilm reactors (trickling filters).Select the choice that explicitly references micro-organisms.

Verification / Alternative check:
Performance metrics (BOD removal, sludge yield) depend on microbial kinetics and oxygen transfer, not solely on chemicals or sieving.

Why Other Options Are Wrong:

• (b) Pure chemical oxidation is tertiary or advanced treatment in many designs.
• (c) Filtration alone is primary/tertiary clarification, not the heart of secondary treatment.
• (d) and (e) omit the fundamental biological aspect.

Common Pitfalls:
Confusing primary clarification with secondary biology; assuming disinfection belongs to secondary (it is typically post-secondary).

Final Answer:
Micro-organisms (biological oxidation and assimilation)