Page 450 - Water and wastewater engineering

GRANULAR FILTRATION 11-23

Example 11-5. In continuing the design of Ottawa Island’s rapid sand filter, determine the area
of each individual filter and the plan (horizontal) dimensions of a filter box. Use the filtration rate
3
2
of 216 m /d · m from Example 11-2 .
Solution:
a. From Example 11-4 , the initial estimate of number of filters is 4.

b. Using Equation 11-18 , with all beds in service, the area of a bed is
3
18 400 m /d
,
2
A 21.2296 or 21 3 m /filter
.
2
3
( 4 filters)( 216 m /dm )

c. Redundancy capacity for the maximum day with one filter out of service must be pro-
vided. The choices are:
1. Increase the number of beds to five and reduce the area.
2. Increase the number of beds to six and reduce the area.
3. Maintain the number of beds at four but make the area larger. This allows for a
lower q during average conditions and meets the design loading rate with one bed
out of service on the maximum day.

4. Switch to a dual-media filter that would allow a higher loading rate.
d. Because of construction and operational considerations, filters are built in pairs. Thus,
alternative (1) is eliminated. Alternative (2) would be acceptable but would have a capi-
tal cost 50% greater than the four-filter system. Without switching to dual media, option
3 offers the most economical alternative.
e. If each filter is increased by 1/3, the filtration rate with one filter out of service would be

3
18 400 m /d 3 2
,

q 21660 . m /dm
.
( 3 filters)( 1 333 21 3 . m)( 2 )
2
Therefore, for a trial calculation assume the area of one filter is (1.3333)(21.3 m )
2
28.4 m .
f. As a trial, select a total width of two cells 5.2 m. Each cell has a width of 2.6 m. The
length of each cell is then
2
28 4 . m
L 55 . m
() )
22 6 . m(
This design meets two design criteria: the cell width is < 6 m and the L:W ratio (2.1:1) is
within the recommended range of 2:1 to 4:1.