The main biological process responsible for sewage treatment inside septic tanks is predominantly which of the following?

Introduction / Context:
Septic tanks are onsite primary treatment units where wastewater is detained to allow solids to settle and undergo biological stabilization.

Given Data / Assumptions:

• Enclosed tank with no aeration system.
• Typical detention times of 24–48 hours or more.
• Sludge undergoes slow stabilization.

Concept / Approach:
Without aeration, the dominant process is anaerobic digestion: hydrolysis, acidogenesis, and methanogenesis convert organics to biogas (methane and carbon dioxide) with volume reduction of sludge.

Step-by-Step Solution:
Identify septic environment: low oxygen, quiescent conditions.Conclude anaerobic pathways dominate.Therefore, select anaerobic decomposition.

Verification / Alternative check:
Design manuals describe septic tanks as anaerobic primary treatment units; aerobic mechanisms require aeration which is absent.

Why Other Options Are Wrong:

• Aerobic decomposition: Not supported in a sealed, unaerated tank.
• Parasitic decomposition: Not a primary engineered mechanism.
• Chemical oxidation/Photolysis: Minor or negligible in septic conditions.

Common Pitfalls:
Assuming septic tanks provide complete treatment; effluent typically needs soil absorption or further treatment.

Final Answer:
Anaerobic decomposition (anaerobic digestion)