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8.18 CHAPTER EIGHT

plied. After the water level in the filter has been lowered to the top of the washwater col-
lection trough, the wash starts. It usually lasts from 3 to 15 min. A low-rate water wash
is used at the end of the wash cycle in multimedia filter beds to restratify the filter
media.
Upflow Water Wash with Air Scour. Three variations of air and water wash were dis-
cussed previously. The first--air scour alone followed by low-rate water wash--is com-
monly applied in Great Britain to single-medium sand filters with 0.6- to 1.2-mm ES me-
dia. After the water level in the filter is lowered to below the washwater overflow, air is
injected at 1 to 2 ft3/min/ft 2 (0.3 to 0.6 m3/min/m 2) for 3 to 5 min. Water wash of 5 to
7.5 gpm/ft 2 (12 to 18 m/h) follows. Bed expansion and stratification are not achieved, al-
though relatively cool water temperatures may result in fluidizing upper sand layers. Prob-
lems with gravel disruption have not been experienced if air and water are applied sepa-
rately (Cleasby et al., 1977).
Air scour alone followed by high-rate water wash can be applied to dual-media or mul-
timedia filters, because bed stratification occurs during water wash. This method has been
used in the United States with air scour at 2 to 5 ft3/min/ft 2 (0.6 to 1.5 m3/min/m 2) fol-
lowed by high-rate water wash at 15 to 23 gpm/ft 2 (37 to 56 m/h).
Concurrent air scour and water wash are generally limited to the deep, coarse-grained
filters common in Europe. For 1- to 2-mm ES media, air scour rates of 2 to 4 ft3/min/ft 2
(0.6 to 1.2 m3/min/m 2) are used with a water flow of 6.3 gprn/ft 2 (15.4 m/h). For 2- to
6-mm ES media, 6 to 8 ft3/min/ft 2 (1.8 to 2.4 m3/min/m 2) and 6.3 to 7.5 gprn/ft 2 (15.4 to
18.3 m/h) are used. Concurrent air and water wash typically lasts 5 to 10 min and is fol-
lowed by water wash alone for another 5 to 10 rain. The rate of final water wash is gen-
erally 1 to 2 times that used with air scour. In some installations, concurrent air and wa-
ter wash is used to improve transport of solids to the washwater collection troughs rather
than to increase scour in the bed (Harris, 1970).
Upflow Water Wash with Surface Wash. Combined surface and water wash usually
involves three phases. After the water surface level is lowered in the bed, surface wash
is activated and operated alone for 1 to 3 min. Low-rate water wash is then applied si-
multaneously for an additional period of roughly 5 to 10 min. Termination of surface wash
precedes a final phase (1 to 5 min) during which a higher washwater rate is used to ex-
pand the bed 20% to 50%. This usually requires a washwater rate of 15 to 23 gpm/ft 2 (37
to 56 m/h). Washwater flow during surface agitation is usually limited to that required to
expand the bed only slightly. If anthracite makes up the top filtering layer, bed expansion
above the surface-wash system may be desirable to reduce the likelihood of media loss.
Rotary surface-wash systems typically add 0.5 to 0.7 gpm/ft 2 (1.2 to 1.8 m/h) to the wash-
water flow. Fixed-nozzle systems typically deliver 2 to 4 gpm/ft 2 (5 to 10 m/h).

Filter Arrangements

Filters can be configured in a number of ways in the overall plant layout. It is important
to develop a layout that is the least costly and is operationally optimized (Begin and Monk,
1975). Amirtharajah (1982) shows how a minimum-cost filter design is obtained by dif-
ferentiating the cost function in terms of length-to-width ratio as a variable.

Configuration of Filters. Filters are normally placed next to one another along one or
both sides of a pipe gallery. This approach provides the most compact arrangement and
also simplifies filter operation and maintenance. If possible, areas for future expansion
should be provided at one end of the row (or rows) of filters, and piping in the gallery
should be installed with blind flanges at the ends to make future filter additions easier.