Which apparatus is used for the settable solids test?

• A. Imhoff cone
• B. Beaker
• C. Hydrometer
• D. Flask

Answer: (A)

The settable solids test measures how much solid material in a wastewater sample settles out by gravity in a defined time, usually about one hour. An Imhoff cone is designed exactly for this purpose. Its conical bottom concentrates the settled solids so they form a distinct layer that can be read clearly against calibrated markings, giving an accurate measure of the volume of settleable solids in the sample. The cone is typically used with a known sample volume (often 1 liter) and the settled solids are read as a volume after the settling period, yielding a value in milliliters per liter.

Other vessels like a beaker, a hydrometer, or a flask aren’t suited for this measurement. A beaker is a general lab container with no specialized geometry or calibration for reading settled solids. A hydrometer measures liquid density, not sedimentation. A flask is also a generic container without the required conical sedimentation shape or precise graduate markings needed for this specific test.