TM 5-814-3/AFM 88-11, Volume III
(6) Design procedure. Select bar size and spacing and determine efficiency factor. Determine number
of units desired. Divide total maximum daily flow or total maximum storm flow by the number of screens
desired to obtain maximum flow per screen. The procedure is then as follows:
Maximum daily flow in mgd 1.547 Maximum daily flow in cfs.
Maximum storm flow in mgd 1.547 = Maximum storm flow in cfs.
cfs = Net area through bars for maximum daily flow.
cfs = Net area through bars for maximum storm flow.
Whichever of the above gives the larger value shall be used for design.
Net area in sq.ft.
' Gross area or channel cross§ion wet area.
Efficiency coefficient for bars
Minimum width of bar rack = 2 ft; maximum width = 4 ft.
Channel cross§ion wet area
' Corresponding depth or width.
Maximum desired width or depth
The figures are based on recessing channel walls 6 inches each side for chain tracks and screen frame. The
overall width of screen frame is 12 inches greater than width of bar rack. If it is not possible to recess the
wall, the channel should be made 1 foot wider than calculated above.
a. Description and function. Comminuting devices are shredders which incorporate mechanisms that
cut the retained material without removing it from the sewage flow. Comminutors are protective devices for
the plant and also provide a means for reducing odors, flies and unsightliness often found around other
coarse-screening devices. However; solids from a comminutor produce more scum at the digesters.
Comminutors are generally located between grit chambers and the primary settling tanks.
b. Design basis. Comminutors will be required in locations where the removal of screenings will be
difficult. Comminutors are available commercially; their design consists basically of screening device and
cutting device (fig 10-3). More recently, "in-line" comminutors have been used to reduce the cost of
structures for shredding solids. Each comminuting device must have a bypass for maintenance and repair
purposes. The bypass will include a bar screen, described as coarse screens in paragraph 10-2a.
Manufacturer's data and rating tables for these units will be consulted for recommended channel dimensions,
capacity ranges, upstream and downstream submergence, and power requirements. Figure 10-4 illustrates
a typical manufacturer's design data.