On-Site Sanitation – What Is a Septic Tank? Choose the correct characterization of a household/community septic tank process for primary on-site sewage treatment.

Introduction / Context:
Septic tanks are widely used in decentralized sanitation for households and small communities. They provide primary treatment of domestic sewage before effluent dispersal to a soak pit, leach field, or further polishing units. Understanding the biological process inside a septic tank is essential for design, operation, and health protection.

Given Data / Assumptions:

• Enclosed tank with limited oxygen ingress.
• Domestic wastewater with settleable and floatable solids.
• Effluent directed to subsurface infiltration for further attenuation.

Concept / Approach:

Septic tanks operate primarily under anaerobic conditions. Settleable solids form sludge, floatables form scum, and the supernatant undergoes anaerobic digestion by microbes, reducing BOD and volatile solids. Aerobic processes require free oxygen and are characteristic of trickling filters or activated-sludge systems, not sealed septic tanks.

Step-by-Step Solution:

Verification / Alternative check:

Field performance data show typical effluent BOD reductions of the order of 30–50% within septic tanks, consistent with anaerobic degradation and long detention times.

Why Other Options Are Wrong:

• Aerobic method: contradicts sealed, oxygen-limited environment.
• Physical only / physicochemical: while settling is physical, the core treatment is biological (anaerobic), not chemical addition or aeration.

Common Pitfalls:

Expecting septic effluent to be fully treated; neglecting desludging schedules; ventilating the tank in ways that disrupt anaerobic conditions or cause odor issues near habitations.

Final Answer:

An anaerobic method of on-site treatment