In wastewater analysis, what does an Imhoff cone directly measure during the settleability test (typically reported in mL/L after 60 minutes)?

Introduction / Context:
An Imhoff cone is a simple tapered vessel used in wastewater laboratories and field settings to evaluate how much material will settle by gravity in a set time. The result helps operators anticipate primary settling performance and sludge production rates.

Given Data / Assumptions:

• Measured parameter is the settled volume after a defined time (often 60 minutes).
• Reported typically as millilitres per litre (mL/L).
• Sample is raw sewage or primary effluent as appropriate.

Concept / Approach:

Settlable solids are the fraction that will settle out under quiescent conditions. The Imhoff cone allows direct reading of the settled volume at the bottom after the test period. This differs from suspended/dissolved solids which require filtration/evaporation and gravimetric analysis.

Step-by-Step Solution:

Verification / Alternative check:

Process control charts in primary treatment commonly track mL/L of settleable solids via Imhoff cones.

Why Other Options Are Wrong:

Suspended and dissolved solids require filtration/evaporation and drying; volatile/fixed solids are subdivisions based on ignition, not directly observed as a settled volume.

Common Pitfalls:

Shaking or disturbing the cone; misreading the meniscus; confusing settleable with suspended solids.

Final Answer:

settlable solids