In wastewater engineering, what are “clarigesters”? Choose the option that correctly describes the construction and working form of a clarigester unit used for combined sedimentation and digestion.

Introduction / Context:
Clarigesters are process units used in municipal sewage treatment that combine primary settling (clarification) with sludge digestion in one structure. They are often deployed when land is limited or when a compact plant layout is desired. Understanding their configuration helps operators and designers distinguish them from septic tanks, primary clarifiers, and separate sludge digesters.

Given Data / Assumptions:

• Treats raw or settled sewage in a primary stage.
• Combines clarification and anaerobic sludge digestion.
• Geometry is typically circular with internal hoppers for sludge collection.

Concept / Approach:

A clarigester is essentially a circular, two-storey Imhoff-type arrangement. Settling occurs in the upper zone; heavier solids pass through slots into a lower, sealed digestion compartment. Hopper-shaped bottoms facilitate gravity movement and compaction of sludge, while gases are vented without disturbing the settling flow above. This configuration differentiates a clarigester from a simple septic tank (single compartment, rectangular or circular) or from separate clarifier–digester systems.

Step-by-Step Solution:

Verification / Alternative check:

Textbook descriptions of clarigesters emphasize their circular form and the presence of bottom hoppers to store and digest sludge. Plant schematics show scrapers directing solids to hoppers and gas vents protecting the settling zone from turbulence.

Why Other Options Are Wrong:

(a) Lacking hoppers limits efficient sludge concentration. (b) and (d) septic tanks are not two-storey Imhoff arrangements. (e) is unnecessary because a correct description exists.

Common Pitfalls:

Confusing clarigesters with standard primary clarifiers (no digestion zone) or with stand-alone Imhoff tanks of different geometry; overlooking the need for gas vents to prevent carryover.

Final Answer:

circular Imhoff double tanks with bottom hoppers