Wastewater laboratory practice: an Imhoff cone is used to measure which parameter during primary settling assessment?

Introduction / Context:
The Imhoff cone is a simple volumetric device used in wastewater treatment plants to estimate the volume of solids that will settle out under quiescent conditions. It supports process control of primary sedimentation and grit removal operations.

Given Data / Assumptions:

• Standard cone volume graduations (typically up to 1000 mL).
• Settling time commonly 1 hour for settleable solids.
• Mixed liquor or raw sewage sample.

Concept / Approach:

Settleable solids are those that settle by gravity in a specified period. Measuring their volume provides a fast field indicator of solids loading and clarifier performance without lab filtration or drying steps required for total or fixed solids tests.

Step-by-Step Solution:

Verification / Alternative check:

Compare settleable solids with suspended solids measurements to evaluate the fraction removable by gravity vs requiring biological treatment.

Why Other Options Are Wrong:

(a), (b), and (c) require standard gravimetric procedures (filtration/ignition), not the Imhoff cone.

Common Pitfalls:

Agitating during settling; reading at incorrect time; failing to note floating scum which may bias interpretation.

Final Answer:

Settleable solids (volume after 1 hour)