In the design of household sanitation, what is the minimum internal width typically adopted for a septic tank to ensure maintainability and structural practicality?

Introduction / Context:
The internal width of a septic tank influences hydraulic retention, scum accumulation zones, and maintainability (e.g., desludging access). Standards and practical guidelines recommend minimum dimensions so that the tank functions reliably and can be constructed and maintained without undue difficulty.

Given Data / Assumptions:

• Rural/urban domestic septic tank for household sewage.
• Conventional rectangular masonry or concrete construction.
• Focus is on minimum recommended internal width.

Concept / Approach:
Very narrow tanks become difficult to construct properly (formwork, reinforcement clearance) and hinder maintenance. A practical minimum of 0.75 m (75 cm) internal width is widely adopted to accommodate inlet and outlet fittings, baffle walls, and effective sludge storage while keeping excavation and material costs controlled.

Step-by-Step Solution:
1) Consider space for baffles, inlet/outlet tees, and scum clearances.2) Ensure adequate working room for periodic cleaning/desludging.3) Adopt 0.75 m as the minimum internal width meeting functional and construction needs.4) Larger capacities may increase width proportionally; however, the lower bound remains near 75 cm for typical domestic units.

Verification / Alternative check:
Field manuals and municipal guidelines generally show widths from 0.75 m upwards, with length based on population/flow and depth tailored to cover, freeboard, and sludge space.

Why Other Options Are Wrong:

• 70 cm: Often too narrow for fittings and maintenance clearance.
• 80 cm or 90 cm: Acceptable but not the minimum; they exceed the lower practical limit and increase cost for small units.

Common Pitfalls:

• Confusing external with internal width (wall thickness reduces internal space).
• Ignoring access for desludging equipment or manual cleaning.

Final Answer:
75 cm.