other solids. Under no circumstances may screenings be disposed with garbage that is
to be used for hog feeding or to be plowed under in cultivated land.
c. Comminutor. Grinding or shredding is usually done with a device called a
comminutor. A comminutor is usually installed in conjunction with a bar screen (see
Figure 2-2). In such an arrangement, the screen is available for use when the
comminutor is being required. The comminutor consists of a vertical, cylindrical screen
with openings of 1/3 to 3/8 inch that is revolved rapidly by an electric motor. The
cylinder is equipped with sharp teeth which pass between teeth in a comb (see Figure
2-3). The waste stream flows through the openings in the cylinder, but solids larger
than the holes are thrown repeatedly between the comb and cylinder teeth until the
solids are shredded fine enough to go through the cylinder holes. The shredded
material later settles with other suspended matter in the sedimentation basins.
Maintenance includes cleaning, lubricating, and keeping the cutting edges in proper
alignment and at the proper clearance.
The power switch should be locked in the open (OFF) position when
inspecting or performing maintenance on all shredding equipment.
Replace worn or nonrealigning cutting edges (blades, shear bars, and
2-3. GRIT REMOVAL
The purpose of grit removal is to remove as much grit as possible without
removing enough organic material to cause odor and disposal problems. Grit removal
helps to keep the moving parts of the wastewater equipment free from damage. If this
material is not removed from the wastewater, troublesome deposits occur in other parts
of the treatment plant where removal is laborious and expensive. Grit accumulations
from grit chambers not having mechanical removal and washing equipment contain
sufficient organic matter to create a nuisance condition. Such grit is usually carted
away and buried. Grit that is continuously removed from grit basins by mechanical
equipment, and which has been thoroughly washed is not objectionable. It is commonly
used for filling or leveling of low places in the wastewater plant area. The two grit
removal units (the flow rate controlled and the aerated units) are noted below. As with
any equipment, it will need to be cleaned, adjusted, and lubricated according to the
manufactured instructions or standing operating procedures (SOP).
a. Grit Chamber (Flow Rate Controlled). A grit chamber (see Figure 2-4) is an
enlarged channel or long tank commonly located immediately following the screening
apparatus and immediately preceding the sedimentation basin. The chamber is used
primarily to remove suspended inorganic matter from wastewater. It is designed to
reduce or slow the flow velocity of the wastewater just enough to let suspended
inorganic material gravel, and cinders settle to the bottom for removal. Grit, except very
finely divided material, settles quickly when the flow velocity is reduced to about one
foot per second. The flow is not retarded further because slower velocities would let
organic solids settle.