A gravity grit removal system operates best with wastewater velocity of:

• A. 0.5 ft/sec
• B. 1.5 ft/sec
• C. 1.0 ft/sec
• D. 2.0 ft/sec

Answer: (C)

In gravity grit chambers, the flow velocity should be set so that heavy inorganic grit can settle by gravity while lighter organic solids remain in suspension and are carried downstream. About 1.0 ft/sec (roughly 0.3 m/s) provides that balance: it’s fast enough for grit particles to settle out but not so fast that settled grit is re-entrained or carried out of the chamber. If the velocity is too low, organic solids may begin to settle in the chamber, reducing grit removal efficiency and creating other operating problems. If the velocity is too high, grit will stay suspended or be flushed out, defeating the purpose of the grit removal step. Thus, the best operating velocity for a gravity grit removal system is around 1.0 ft/sec.