Handbook of Civil Engineering Calculations

2. Find the required filter surface area
   Filter surface area required is calculated using

TT-U f • ,!/A^ peak flow (gal/min)

Filter surface area required (fr) = ——————-—-—. ^7x

Hydraulic loading (gal/mm-ft

2

)

Using values from above:

Filter surface area required (ft^2 ) =^6 ^^44 ^"^ = 1736 ft^2 (161.3 m^2 )

3. Determine the number of filters required
   Standard filter widths available from various manufacturers are 8, 12, and 16 ft (2.44,
   3.66, and 4.88 m). Using a width of 12 feet (3.66 m) and length of 50 feet (15.2 m) per fil-
   ter, the area of each filter is

Area of each filter = (12')(50') = 600 ft^2 (55.7 m^2 )

The number of filters required is 1736 ft^2 /600 ft^2 per filter = 2.89. Use 3 filters for a total
filter area of 1800 ft^2 (167.2 m^2 ).
It must be kept in mind that most state and local regulations stipulate that "rapid sand
filters shall be designed to provide a total filtration capacity for the maximum anticipated
flow with at least one of the filters out of service." Therefore, 4 traveling bridge filters
should be provided, each with filtration area dimensions of 12' wide x 50' long (3.66 m x
15.2m).
The media depth for traveling bridge filters ranges from 11" to 16" (27.9 to 40.6 cm).
Dual media may be used with 8" (20.3 cm) of sand underlying 8" (20.3 cm) of anthracite.

4. Find the hydraulic loading under various service conditions
   The hydraulic loadings with all filters in operation is

Average flow: ^oof^ =

L16

gal/min-ft

2

(0.79 L/s-m

2

)

Peak flow:^694 ^^mm = 2.89 gal/min-ft^2 (1.96 L/s-m^2 )

T^uUU Tt J

Hydraulic loading with 1 filter out of service (3 active filters) is

Average flow:^2 ™J^^ = 1 -54 gal/min-ft^2 (1.05 L/s-m^2 )

3(600 it )

Peak flow:^694 ^g^mm = 3.86 gal/min-ft^2 (2.62 L/s-m^2 )

3(600 rr)

These hydraulic loadings are acceptable and may be used in specifying the traveling
bridge filter.
The amount of backwash water produced depends upon the quantity and quality of in-
fluent to the filter. The backwash pumps are usually sized to deliver approximately 25
gal/min (1.58 L/s) during the backwash cycle. Backwash water is generally returned to
the head of the treatment facility for reprocessing.