**Core Concept**
Septic tanks are essential in wastewater management, especially in off-grid or rural households. Their design and capacity are based on household size and waste volume to ensure effective solid-liquid separation and prevent overloading or contamination.

**Why the Correct Answer is Right**
The recommended septic tank capacity is 20–30 gallons (approximately 0.75–1.5 cubic feet) per person, which translates to 2.5–5 cubic feet per person. This ensures adequate space for sludge and scum settling, preventing clogging and bacterial overgrowth. The standard is derived from public health guidelines for household wastewater treatment, ensuring sufficient retention time for waste decomposition.

**Why Each Wrong Option is Incorrect**
Option A: A minimum capacity of 100 gallons is excessive for a typical household and not aligned with standard design guidelines; it may lead to inefficient operation and increased costs.
Option C: The length is generally longer than the breadth to allow for proper flow dynamics and settling; equal dimensions are not standard or practical.
Option D: A 3 cm air space is insufficient; the recommended air gap is typically 5–10 cm to allow for gas escape and prevent pressure buildup and tank overflow.

**Clinical Pearl / High-Yield Fact**
Always design septic tanks based on household size—underestimating capacity leads to system failure and environmental contamination. Never use a single tank for large households without proper sizing.

✓ Correct Answer: B. A capacity of 20-30 gallons or 2 & half to 5 c.ft. per person is recommended for household septic tanks

✓ Correct Answer: B. A capacity of 20-30 gallons or 2 & half to 5 c.ft. per person is recommended for household septic tanks