Microbiology of sewage treatment: The majority of bacteria naturally present in raw domestic sewage belong to which nutritional class, based on how they obtain energy and carbon?

Introduction / Context:
Sewage treatment exploits the natural ability of microorganisms to degrade organic matter. Understanding which microbial groups dominate in influent sewage helps explain why biological processes like activated sludge and trickling filters are effective.

Given Data / Assumptions:

• Typical domestic sewage composition with high biodegradable organics (BOD).
• Ambient conditions support mixed microbial consortia.
• Focus is on broad trophic classification: parasitic vs saprophytic vs strictly anaerobic pathogens.

Concept / Approach:
Saprophytes feed on dead or decaying organic material, exactly the substrate provided by sewage. While pathogens can be present, they are a small fraction compared with the vast community of saprophytic heterotrophs that drive biochemical oxidation and stabilization of wastes.

Step-by-Step Solution:
Identify the dominant energy source in sewage: abundant dissolved and particulate organics.Match to microbial lifestyle: saprophytic heterotrophs thrive on these substrates.Select the option that names saprophytic organisms.

Verification / Alternative check:
Process performance correlations (e.g., sludge yield, oxygen uptake rate) align with active saprophytic biomass that metabolizes organics to CO2, water, and new cells.

Why Other Options Are Wrong:
Parasitic: require living hosts; not the primary degraders in sewage.Obligate anaerobic pathogens: may exist but are not the majority; sewage contains both aerobic and anaerobic saprophytes.Phototrophic: require light; not dominant in opaque sewage.

Common Pitfalls:
Overstating pathogen prevalence relative to the robust decomposer community responsible for treatment.

Final Answer:
Saprophytic (decomposers of organic matter)