Septic tank design: Freeboard of about 0.3 m is provided above the top sewage line. Does this freeboard primarily serve to accommodate scum, and is that the correct explanation?

Introduction / Context:
Septic tanks provide primary settling and anaerobic digestion. A freeboard space is left above the liquid level to accommodate operational contingencies. Understanding why freeboard is specified helps ensure reliable and odor-free operation.

Given Data / Assumptions:

• Freeboard ≈ 0.3 m above the operating liquid level.
• Domestic septic systems with typical scum formation.
• Ventilation is provided to avoid gas buildup.

Concept / Approach:

During operation, floating scum (oils, fats, vegetative matter) accumulates on the surface. Freeboard prevents scum from contacting the roof slab or flowing into the outlet tee, and provides buffer for surge, foaming, and gas release. Thus, stating that 0.3 m freeboard is provided and that it accommodates scum is accurate and causally linked.

Step-by-Step Solution:

Verification / Alternative check:

Standard septic tank design guides specify freeboard for scum and foam accommodation and to allow safe ventilation space under the cover slab.

Why Other Options Are Wrong:

Any option denying the truth of A or R, or denying the causal relation, conflicts with standard design rationale.

Common Pitfalls:

Confusing freeboard with working depth; ignoring outlet tee/inlet baffle elevations relative to scum layer.

Final Answer:

Both A and R are true and R is the correct explanation of A