Role of septic tanks in onsite wastewater treatment Septic tanks primarily promote which process for stabilising deposited solids?

Introduction / Context:
Septic tanks are widely used for onsite sanitation where centralized sewers are absent. They provide primary treatment by settling and biological stabilisation of solids. Correctly identifying the dominant biological regime informs design (e.g., tank sizing, retention time) and maintenance (e.g., desludging frequency).

Given Data / Assumptions:

• Septic tanks are closed vessels with limited oxygen ingress.
• Organic solids accumulate as sludge and scum.
• Residence times are long enough to support biological breakdown.

Concept / Approach:
Because oxygen transfer into a sealed tank is minimal, the decomposition of settled solids occurs under anaerobic conditions. Anaerobic bacteria hydrolyse and ferment the organics, producing methane, carbon dioxide, and other reduced compounds. Clarified effluent then flows to a soakaway or further treatment units.

Step-by-Step Solution:

Verification / Alternative check:
Field operation experience shows gas generation (biogas), characteristic of anaerobic digestion, and slower, long-term stabilisation of solids.

Why Other Options Are Wrong:

Common Pitfalls:
Assuming septic tanks provide complete treatment; effluent still needs soil absorption or further polishing to remove nutrients and pathogens.

Final Answer:
anaerobic decomposition