Biological treatment in trickling filters During biological treatment of sewage in a trickling filter, the predominant microorganisms are:

Introduction / Context:
Trickling filters are attached-growth biological reactors where wastewater is distributed over a medium (stones or plastic media) supporting a biofilm. Oxygen transfer occurs by natural or forced ventilation. Knowing whether the process is primarily aerobic or anaerobic guides expectations for BOD removal, odour formation, and nitrification potential.

Given Data / Assumptions:

• Wastewater is distributed intermittently or continuously over the medium.
• Air flows through voids, supplying oxygen to the biofilm.
• Normal domestic or municipal sewage loadings.

Concept / Approach:
Trickling filters are designed to maintain aerobic conditions in the active biofilm, enabling aerobic heterotrophs and nitrifiers to remove organic matter and oxidise ammonia when design allows. While micro-zones deeper in the slime layer can be anoxic, the bulk mechanism relies on oxygen diffusion and is classified as aerobic treatment. “Saprophytic” simply means decomposer, which could be aerobic or anaerobic; the more precise and correct classification here is aerobic bacteria. Anaerobic systems, such as digesters or septic tanks, operate without oxygen and are not representative of trickling filters.

Step-by-Step Solution:

Verification / Alternative check:
Design texts specify air requirements and ventilation for trickling filters to ensure aerobic operation and prevent odour generation typical of anaerobic processes.

Why Other Options Are Wrong:

• Anaerobic bacteria: Not predominant in well-aerated trickling filters.
• Saprophytic bacteria: Too general; does not specify oxygen requirement.
• Parasitic bacteria: Not relevant to wastewater treatment performance.
• Photoautotrophic cyanobacteria: Require light and are not the principal degraders in trickling filters.

Common Pitfalls:
Assuming any biofilm reactor is mixed redox; while micro-gradients exist, the process is fundamentally aerobic in design and operation.

Final Answer:
Aerobic bacteria