Environmental Engineering Reference
In-Depth Information
fact that grit is heavier than the organic solids, which should be kept in
suspension for treatment in subsequent processes. Grit removal may
be accomplished in grit chambers or by the centrifugal separation of
sludge. Processes use gravity and velocity, aeration, or centrifugal force
to separate the solids from the wastewater.
5.3.3.1 Gravity-/Velocity-Controlled Grit Removal
Gravity-/velocity-controlled grit removal is normally accomplished
in a channel or tank where the speed or the velocity of the wastewater
is controlled to about 1 foot per second (fps), ideally, so the grit will
settle while organic matter remains suspended. As long as the velocity
is controlled within the range of 0.7 to 1.4 fps, the grit removal process
will remain effective. Velocity is controlled by regulating the amount of
water flowing through the channel, the depth of the water in the chan-
nel, the width of the channel, or the cumulative width of channels in
service.
5.3.3.1.1 Cleaning
Gravity-type systems may be manually or mechan-
ically cleaned. Manual cleaning normally requires
that the channel be taken out of service, drained, and
manually cleaned. Mechanical cleaning systems are operated continu-
ously or on a time cycle. Removal should be frequent enough to prevent
grit carryover into the rest of the plant.
Key Point:
Before and during
cleaning activities always ven-
tilate the area thoroughly.
5.3.3.1.2 operational Considerations
As noted above, gravity-/velocity-controlled grit removal
normally
occurs in a channel or tank where the speed or the velocity of the waste-
water is controlled to about 1 fps (ideal), so grit settles while organic
matter remains suspended. As long as the velocity is controlled in the
range of 0.7 to 1.4 fps, the grit removal remains effective. Velocity is
controlled by the amount of water flowing through the channel, by the
depth of the water in the channel, by the width of the channel, or by the
cumulative width of channels in service. During operation, the opera-
tor must pay particular attention to grit characteristics for evidence of
organic solids in the channel, grit carryover into the plant, or mechani-
cal problems, as well as to grit storage and disposal (housekeeping).
5.3.3.2 Aeration
Aerated grit removal systems
use aeration to keep the lighter organic
solids in suspension while allowing the heavier grit articles to settle out.
Aerated grit removal may be manually or mechanically cleaned; however,
the majority of the systems are mechanically cleaned. During normal
operation, adjusting the aeration rate produces the desired separation.
