Grit RemovAL

The purpose of grit removal is to remove the heavy inorganic solids that could cause excessive mechanical wear. Grit is heavier than inorganic solids and includes sand, gravel, clay, egg shells, coffee grounds, metal filings, seeds, and other similar materials. Several processes or devices are used for grit removal. All of the processes are based on the fact that grit is heavier than organic solids, which should be kept in suspension for treatment in subsequent processes. Grit removal may be accomplished in grit chambers or by centrifugal separation of sludge. Processes use gravity and velocity, aeration, or centrifugal force to separate the solids from the wastewater.

12.4.1 Gravity-/Velocity-Controlled Grit Removal

Gravity-/velocity-controlled grit removal is normally accomplished in a channel or tank where the speed or velocity of the wastewater is controlled, ideally, to about 1 foot per second (fps), so the grit will settle while organic matter remains suspended. As long as the velocity is controlled in the range of 0.7 to 1.4 feet per second (fps), the grit removal will remain effective. Velocity is controlled by the amount of water flowing through the channel, the depth of the water in the channel, the width of the channel, or the cumulative width of channels in service.

12.4.1.1 Process Control Calculations

Velocity of the flow in a channel can be determined either by the float-and-stopwatch method or by channel dimensions.

■ Example 12.3. Velocity by Float and Stopwatch

Time required (seconds)

problem: A float takes 25 s to travel 34 ft in a grit channel. What is the velocity of the flow in the channel?

Solution:

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