Handbook of Civil Engineering Calculations

will be removed. If the velocity is too great, grit will be carried out of the chamber. If the

velocity is too small, organic material will be removed with the grit. The quantity of air is

easily adjusted by throttling the air discharge or using adjustable speed drives on the

blowers. With proper adjustment, almost 100 percent grit removal will be obtained, and

the grit will be well washed. Grit that is not well washed will contain organic matter and

become a nuisance through odor emission and the attraction of insects.

Wastewater will move through the aerated grit chamber in a spiral path as illustrated

in Fig. 4. The rolling action will make two to three passes across the bottom of the tank at

maximum flow and more at lesser flows. Wastewater is introduced in the direction of the

roll.

At peak flow rate, the detention time in the aerated grit chamber should range from 2

to 5 minutes. A detention time of 3 minutes will be used for this example. Because it is

necessary to drain the chamber periodically for routine maintenance, two redundant

chambers will be required. Therefore, the volume of each chamber is:

ft3 _ (peak flow rate, gal/day)(detention time, min)

( }

~ (7.48 gal/ft

3

)(24 h/d)(60 min/h)

Using values from above, the chamber volume is:

FIGURE 4. Aerated grit chamber. (Metcalf & Eddy, Wastewater Engineering: Treatment,
Disposal, and Reuse, 3rd Ed., McGraw-Hill._)

Trajectory of

grit particles

Inlet

Helical liquid,

flow pattern

Outlet weir