Page 447 - Water and wastewater engineering

11-20 WATER AND WASTEWATER ENGINEERING

2
Converting to pressure by dividing by the area of the bed and converting units of pressure (N/m )
to units of head (m):

F g ( s )(1 e )( D e )
h (11-16)
Le

a ( )()()
g

11-6 GRANULAR FILTRATION PRACTICE
Filter Type
The selection of the type of filter is strongly influenced by the characteristics of the raw water
and treatment processes that precede and follow the filter. Deep bed anthracite monomedia filters
accumulate headloss at a low rate, and they have a high capacity for accumulating solids. This
leads to long filter runs. However, when the water is not conditioned properly because of high
variability in the raw water quality ( Figure 11-8 ), improper chemical dosage, or lack of opera-
tor vigilance, the performance degrades dramatically. Dual-media filters provide a more robust

2.4
Headloss, m 1.8
1.2
0.6
0
65 55 30 25 300 200 85 80
5

4
Turbidity units 3

2

1

0
0 20 40 60 80 100 120 140
Time, hr
FIGURE 11-8
3
2
2
3
Turbidity values at filtration rates of 117 and 293 m /d
m . In the upper graph, the dotted curve is for 293 m /d
m ; the solid,
2
3
117 m /d
m . In the lower graph, the numbers at the top are the raw-water turbidity values at the time shown during the run; the
3
2
2
3
dashed curve is for filter influent; the dotted for filter effluent at 117 m /d
m ; and the solid for filter effluent at 293 m /d
m .
( Source: G. G. Robeck, K. A. Dostal, and R. L. Woodward, “Studies of Modification in Water Filtration,” Jour. AWWA,
56(2), February 1964: 198.)