Septic tank design statements: Identify the incorrect statement about common detailing and dimensions for household septic tanks.

Introduction / Context:
Septic tanks require careful internal geometry to separate scum, sludge, and clarified effluent, and to protect the outlet from scum carryover. Exam questions often test which dimensional statements align with accepted practice and which do not.

Given Data / Assumptions:

• Domestic septic tank serving a single dwelling.
• Typical design targets: adequate freeboard, proper inlet/outlet inverts, and practical plan dimensions.

Concept / Approach:
Accepted practice includes maintaining a small head difference with outlet invert lower than inlet invert (to drive flow), providing a modest clear gap under the cover slab for scum and ventilation, and ensuring minimum width for constructability (often about 0.9 m). However, depth is not fixed to equal the width; effective liquid depth is generally in the range of roughly 1.0–1.8 m depending on design flows and retention time.

Step-by-Step Solution:
Check statement (a): ~7.5 cm gap under the slab above scum is common practice.Check statement (b): Outlet invert set 5–7.5 cm below inlet invert is typical to prevent backflow and maintain gradient.Check statement (c): Minimum width ≈ 90 cm aligns with standard small-tank practice.Check statement (d): Equating depth to width is not a rule and generally incorrect; depth depends on hydraulic detention and sludge storage, not plan width.

Verification / Alternative check:
Practical design guides and building codes for on-site sanitation reflect similar numeric guidance, confirming which statements are acceptable and which are not.

Why Other Options Are Wrong:
Options (a)–(c) are consistent with widely used detailing. Option (e) is false because one statement is indeed incorrect.

Common Pitfalls:

• Assuming a square proportion rule; septic tank depth is set by detention time and sludge storage needs.
• Omitting baffles/tees, which are essential regardless of the plan proportions.

Final Answer:
Depth of the tank should be kept equal to its width