Sanitation engineering — a below-ground masonry structure in which wastewater undergoes anaerobic decomposition by bacteria is called what?

Introduction:
On-site sanitation systems often rely on primary treatment under anaerobic conditions. The question describes a masonry structure built below ground where wastewater is held for anaerobic bacterial action.

Given Data / Assumptions:

• Structure: below ground, masonry construction.
• Process: anaerobic digestion of settled solids and scum.
• Application: on-site domestic wastewater treatment.

Concept / Approach:
A septic tank is a watertight underground chamber that provides quiescent conditions for sedimentation and scum accumulation, with anaerobic bacteria decomposing organic matter. Effluent typically flows to a soak pit or drain field.

Step-by-Step Solution:

Verification / Alternative check:
Standard sanitation texts define septic tanks in precisely these terms: underground, quiescent, anaerobic digestion with sludge retention.

Why Other Options Are Wrong:

• Cesspool: essentially a soak pit or leach pit without proper treatment staging.
• Lagoon: large, mostly aerobic or facultative surface ponds, not small masonry chambers.
• Skimming mill: not a wastewater treatment unit.

Common Pitfalls:
Confusing cesspools with septic tanks; cesspools lack the defined retention and treatment stages of septic systems.

Final Answer:
The described unit is a septic tank.