In septic tanks used for on-site sanitation, the principal biological decomposition of organic matter is carried out by which type of microorganisms?

Introduction / Context:
Septic tanks are closed, non-aerated sedimentation-cum-digestion units commonly used for individual houses and small communities. Understanding the dominant biological regime informs maintenance and expected effluent quality.

Given Data / Assumptions:

• Septic tanks are covered tanks without aeration equipment.
• Scum and sludge layers form; gas vents exist.
• Objective: identify dominant microorganisms responsible for digestion.

Concept / Approach:

In the absence of oxygen, organic matter is decomposed by anaerobic bacteria producing biogas (chiefly methane and carbon dioxide). Aerobic processes are limited to thin layers near inlets/outlets if any air enters, but the overall tank digestion is anaerobic.

Step-by-Step Solution:

Verification / Alternative check:

Septic tank gas composition (methane and CO2) confirms anaerobic activity.

Why Other Options Are Wrong:

Aerobic bacteria require oxygen; simultaneous aerobic-anaerobic action with aeration does not occur in sealed septic tanks; algae do not drive digestion in the dark, enclosed tank.

Common Pitfalls:

Assuming aerobic stabilization like in activated sludge; overlooking the role of scum/sludge blanket and gas evolution in mixing.

Final Answer:

anaerobic bacteria