Page 180 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

7 Grit Chambers

This chapter summarizes the horizontal-flow grit removal velocity of the grit chamber should be adequate to suspend
process (Camp, 1942). Aerated grit chambers are included, deposited organics but not the grit particles larger than 0.2
an innovation on the scene since the 1950s. Proprietary grit mm, that is, 0.23 v H 0.38 m=s (0.75 v H 1.25 ft=s).
removal processes are not included, in keeping with the prin- Removal principle: Grit removal is a sedimentation process,
ciple-oriented theme. that is, the force of gravity acting on particles. Alternatively,
Grit removal is a sedimentation process and the chapter the force acting could be ‘‘generated’’ by centrifuging, in
could have been included in Chapter 6, but was felt to warrant which case proprietary equipment is involved.
being dealt with separately in order to focus on grit as a
special case.
7.2 HORIZONTAL FLOW GRIT CHAMBERS
The classic grit removal process is the horizontal flow grit
7.1 GRIT
chamber.
Not much has been published on grit removal as compared to
other unit processes in water treatment. Perhaps the topic has
lacked‘‘glamour,’’ or perhaps there was not much to be said after 7.2.1 THEORY
Camp’s 1942 paper (Camp, 1942), and became an established Grit removal is a sedimentation process, with principles delin-
process (Box 7.1). Ancillary technologies, that is, to make the eated in Chapter 6. The essential elements are summarized in
process work, include scrapers, lift devices, and grit washing. this section. Scour of organics is an added factor.
Purposes of grit removal: The two purposes of grit removal
are: (1) to protect pump and other metal surfaces from an 7.2.1.1 Ideal Basin
excessive rate of abrasion wear, and (2) to prevent nuisance The theoretical design of a grit chamber is based upon Camp’s
conditions. concept of the ideal settling basin (1946), outlined in Section
Nuisance conditions: Two categories of nuisance include: 6.3.3, and described first in his 1942 article on grit chambers.
Key assumptions of ideal settling theory, for a horizontal flow
1. Grit accumulations in digesters, and in pipelines and basin, are: (1) the horizontal flow velocity is uniform over
channels. Grit in digesters causes sluggish circula- depth, (2) a discrete particle subsides at a constant fall
tion and accumulates at the bottom. Grit in pipes, for velocity. Combining these two velocities will define the path
example, from primary clarifiers to digesters, may of a subsiding grit particle, shown in Figures 7.6 through 7.8.
cause clogging; such a problem is not uncommon The path shown is for a 0.2 mm particle of grit to be 100%
and has been reported frequently by operators. removed. The corresponding fall velocity, calculated by
2. Excessive organics in grit-to-be-disposed (Camp, Stoke’s law, for a 0.2 mm particle with specific gravity 2.65
1942). The ‘‘quality’’ of grit refers to its organic is 21 mm=s (0.069 ft=s).
fraction; the approximate range is 0.05 organic- The function of a grit chamber is to remove silica sand
fraction and 0.50 g-organics=g-inert suspended- particles of 0.2 mm, while passing putrescible organic par-
solids. The lower fraction is considered a relatively ticles. Since organic particles have a slower fall velocity than
‘‘clean’’ grit. The higher value is not an uncommon grit, the subsidence path has an angle with the horizontal
value for some grit removal operations and would be much less than the grit; thus, as indicated in Figure 7.1,
considered unsuitable for land disposal due to its most organic particles will be carried out with the flow.
putrescible nature. At the entrance to the grit chamber, however, organic
particles as well as other particles are distributed uniformly
Objectives of design: The design of a grit chamber requires over depth. Therefore, the path of some of these organic
two processes be operative: (1) settling of grit, and (2) scour particles will intercept the bottom of the grit chamber, as
of organics. As recommended by Camp (1942), the settled grit indicated in Figure 7.2.
should have a diameter of 0.2 mm and larger, with specific
gravity of 2.65. Camp asserted that most of the grit that causes 7.2.1.2 Scour
problems has larger size. Removal of 0.2 mm grit with If the grit chamber is to function effectively, any deposited
0.05 organic fraction has been accepted as objectives for organics should be resuspended. This is accomplished by
practice. To accomplish scour of organics, the horizontal maintaining a sufficient scour velocity which will remove

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