Which statement is true about Imhoff cones?

• A. They measure settleable solids
• B. They measure suspended solids
• C. They measure dissolved solids
• D. They measure total solids

Answer: (A)

Imhoff cones are used to quantify the portion of solids in a wastewater sample that settle out by gravity under quiet, time-limited conditions. You fill the cone with a known volume of sample (typically 1 liter) and let it stand for about an hour. The settled sludge at the bottom is then read as a volume, usually expressed as milliliters of settled solids per liter of sample. This measurement specifically target the solids that settle quickly, hence “settleable solids.”

This is why the statement is true: it directly measures the solids that settle out, not the solids that remain suspended in the water (which would be measured by another method), not the dissolved solids (which are those dissolved substances not removed by filtration and require evaporation/drying to quantify), and not the total solids (which would include both dissolved and suspended material and require drying the entire sample).