Which step is when water moves very slowly to allow coagulated and flocculated particles to settle out?

• A. Screening
• B. Sedimentation
• C. Filtration
• D. Disinfectant added

Answer: (B)

Sedimentation is the step where water is kept moving very slowly in a calm basin so the coagulated and flocculated particles can settle out by gravity. After coagulation and flocculation, particles form larger clumps (flocs). With slow, gentle flow, these heavy flocs fall to the bottom, and clearer water exits from the top. The key idea is allowing enough detention time and minimizing turbulence so the solids can separate from the water.

If the flow were fast, the flocs wouldn’t settle and could be carried along; and this step is typically followed by filtration to remove any remaining fine particles, while disinfection is done later to kill microbes.