Page 181 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

136 Fundamentals of Water Treatment Unit Processes: Physical, Chemical, and Biological

are 0.23–0.38 (0.75–1.25 ft=s). The 0.3 m=s (1.0 ft=s) criteria
BOX 7.1 CAMP AND GRIT CHAMBER has been well established since the 1909 experiments of N.A.
PRACTICE Brown in Rochester (reported by Metcalf and Eddy, 1916) who
related grit size, horizontal flow velocity, and percent of grit
Camp’s 1942 paper on grit chamber design provided
retained for each size. Camp (1946) in analyzing the math-
definitive design guidelines. Like most of his papers,
ematical relations and experimental work of A. Shields formu-
this one also was comprehensive, that is, characterized lated an operational equation to describe the critical mean
by theory, criteria for practice, and application channel velocity, v c , for incipient motion of particles of size,
examples. This chapter was constructed mostly from d, and specific gravity, SG, given here as Equation 7.1,
Camp’s 1942 paper.
Other than Camp’s paper, the literature on grit has 0:5
8b
been sparse, with only a few papers appearing over the g(SG 1)d (7:1)
f
v c ¼
decades; for example, Morales and Reinhardt (1984)
who investigated the performance of grit chambers in
where
several plants; Londong (1989), who developed a
v c is the critical mean horizontal velocity to start incipient
rationale based on pilot plant studies; Hirano et al.
motion of particles (m=s)
(1998), who reported on the problems of grit over-
d is the size of particle (m)
whelming the San Francisco WWTP; and a series of
SG is the specific gravity of particle
four papers by Wilson et al. (2007a–d), who revisited
b is the some unknown function of d=d b (where d b is the
some of the premises of grit characteristics and removal.
boundary film thickness) which can be assumed a con-
Principles are constant, however, and so Camp’s work
stant (see Figure 11 in Camp, 1946), varying from 0.04
has remained the primary reference.
for smooth sand beds to 0.10 for a bed of sand ripples
f is the Darcy–Weisbach friction factor, taken as 0.03
2
2
the lighter organics from the bed while leaving the grit. Thus, g is the acceleration of gravity 9.81 m=s (32.2 ft=s )
control of horizontal velocity is necessary.
The ideal horizontal velocity, v H , is 0.3 m=s (1.0 ft=s) for all The equation will work for any units desired as long as g is in
flows (from minimum to maximum). Accepted operating limits those same units.
Length

v =0.30 m/s Path of organics
Depth v =21 mm/s Path of 0.2 mm grit particle
s

FIGURE 7.1 Path of subsidence of the smallest particle of grit to be completely removed. (Adapted from Rex Chainbelt, Grit Collectors,
Product Manual, Sanitation Equipment, Conveyor and Process Equipment Division, Rex Chainbelt, Inc., Milwaukee, WI, Data Sheets 315-
4.001-315-4.531, 1965.)

Length

Depth Path of organics Grit Path of organics
Path of organics
Path of organics
Grit Path of 0.2 mm grit particle
Grit

FIGURE 7.2 Path of lighter organics relative to grit. (Adapted from Rex Chainbelt, Grit Collectors, Product Manual, Sanitation Equipment,
Conveyor and Process Equipment Division, Rex Chainbelt, Inc., Milwaukee, WI, Data Sheets 315-4.001-315-4.531, 1965.)