When sewage is slowed to a velocity of __ or slower, grit will settle out and collect in the grit chamber

• A. 2.0 ft per second
• B. 0.5 ft per second
• C. 3.0 ft per second
• D. 1.0 ft per second

Answer: (D)

Grit removal relies on letting heavier inorganic particles settle out when the flow is slowed enough. Grit chambers are designed to keep the wastewater at a low enough velocity so that sand and other dense solids have time to fall to the bottom rather than being carried into downstream processes. About 1.0 foot per second (roughly 0.3 meters per second) is the commonly used threshold: at or below this velocity, grit will settle and accumulate in the chamber. If the flow is faster than this, the grit tends to stay entrained and move on with the wastewater. Therefore, slowing to 1.0 ft/s or slower is the effective condition for grit settling.