What does the settable solids test evaluate?

• A. Settling efficiency of a sedimentation tank
• B. Filtration rate through a membrane
• C. Turbidity reduction
• D. Oxygen demand of organics

Answer: (A)

This test focuses on how well suspended solids settle out of water by gravity. It directly measures the settling behavior of the solids, which determines how effectively a primary clarifier or other gravity-settling device can separate solids from the liquid. The key idea is to gauge the efficiency and speed at which solids fall to the bottom, informing whether the clarifier design and operation will handle the expected solids load.

This is different from filtration rate, which concerns pushing water through a barrier or membrane; turbidity reduction relates to how clear the water becomes after treatment (a result influenced by settling but not a direct measure of settleability); and oxygen demand of organics (BOD) measures the amount of oxygen consumed by organic matter, a chemical/biochemical property rather than the physical settling of solids.