In an aerated grit chamber, what causes heavier grit particles to settle?

• A. Injected air creates a spiral flow of wastewater and slows the flow, allowing heavier grit to settle.
• B. Raw wastewater enters at the end of the chamber and is mixed vigorously.
• C. The chamber disinfects the wastewater with chlorine.
• D. Grit is removed by dissolving it chemically.

Answer: (A)

Heavy solids settle mainly by gravity when the flow velocity is reduced. In an aerated grit chamber, injecting air creates a spiral, or swirling, flow that slows the overall movement of the wastewater. That slower flow gives dense grit particles time to sink to the bottom, where they can be collected. The air helps keep lighter organic matter in suspension so it doesn’t settle with the grit. The other options don’t promote this gravity-driven separation: disinfection with chlorine, vigorous mixing of raw wastewater, or chemically dissolving grit don’t cause heavier particles to settle.