17 DEC 2003
Figure 8-1. Solids Loading Rate Versus Rate of Head Loss
8-188.8.131.52 Frequency of backwash will depend on the type of solids, solids loading rate,
filter length, and acceptable head loss. Frequency of backwash is calculated using Fig-
ure 8-1. The increased head loss may be determined from the solids loading rate. The
time required to reach terminal head may be determined by dividing the operating head
by the rate of increased head loss. The time required for the carriage to transverse the
filter during the backwash cycle is available from the manufacturer. The total time re-
quired from completion of one backwash cycle to the completion of the next backwash
cycle is the sum of the traversing time and the time required to reach terminal head. Di-
viding 24 hours by the total time will provide the number of backwash cycles per day.
8-184.108.40.206 The percentage of backwash water required per day is determined by:
Backwash water required [gal. (L)] = backwash flow rate [gpm (L/min.)] traversing time [min.]
Percent backwash is the backwash water required divided by the total throughput.
Backwash water requirements below 24% are common. The requirements are incorpo-
rated into design as hydraulic loading.
8-3.4.3 Continuous Backwash Filters. Because all continuous backwash filters are
proprietary, relatively little flexibility is available in selecting the unit once a single
manufacturer has been chosen. Similar to the approach for sizing a traveling bridge fil-
ter, the manufacturer will determine filter size based on flow rate and hydraulic loading.
Hydraulic loading rates may vary between 1.48.15 L/s m2 (212 gpm/ft2), with a typical
rate of 3.4 L/s m2 (5 gpm/ft2). Table 8-2 gives acceptable continuous backwash filter
loading rates for particular water treatment applications. The manufacturer can provide