Wastewater Calculations - Mathematics and Statistics for the Environment - page 668

∆ h

h 1

v

v SC h 2

FIGURE 24.2 Water profile through a screen.

24.2.3 h ead l oss through b ar s Creen C alCulations
Head loss through a bar screen is determined by using Bernoulli's equation (see Figure 24.2):

2 2

v v

sc

h +=++Losses h (24.4)

1 2

2 g 2 g

where
h 1 = Upstream depth of flow.
v = Upstream velocity.
g = Acceleration of gravity.
h 2 = Downstream depth of flow.
v sc = Velocity of flow through the screen.

The losses can be incorporated into a coefficient:

1

( )

2 2

hh h =−= vv − (24.5)

1 2 sc

2

2 gC

d

where C d is the discharge coefficient (typical value = 0.84), a value usually supplied by the manu-
facturer or determined through experimentation

24.2.4 g rit r emoval C alCulations
The purpose of grit removal is to remove inorganic solids (sand, gravel, clay, egg shells, coffee
grounds, metal filings, seeds, and other similar materials) that could cause excessive mechani-
cal wear. Several processes or devices are used for grit removal, all based on the fact that grit is
heavier than the organic solids, which should be kept in suspension for treatment in subsequent unit
processes. Grit removal may be accomplished in grit chambers or by the centrifugal separation of
biosolids. Processes use gravity/velocity, aeration, or centrifugal force to separate the solids from
the wastewater.
Wastewater systems typically average 1 to 15 ft 3 of grit per million gallons of flow (sanitary
systems, 1 to 4 ft 3 /MG; combined wastewater systems, 4 to 15 ft 3 /MG of flow), with higher ranges
during storm events. Generally, grit is disposed of in sanitary landfills. Because of this process, for
planning purposes operators must keep accurate records of grit removal. Most often, the data are
reported as cubic feet of grit removed per million gallons for flow:

3

Grit volume (ft )
Flow

3

Grit removed (ft /MG) = (24.6)

(MG)