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8.16 CHAPTER EIGHT

Advantages of auxiliary surface wash include proven effectiveness in alleviating dirty
filter problems, improved cleaning (when compared with water wash alone) without a
great change in system complexity, and possibly lessened danger of gravel upset if the
quantity of washwater introduced through the underdrain is reduced.
Because surface wash systems constitute a possible connection between filtered and
unfiltered water, backflow prevention devices must be provided in supply lines.
Continuous Backwash. An alternative to the automatic control of standard filters is
the use of continuous backwashing filter beds, which eliminate the need to remove the
beds from service for washing. Beds are divided into a series of narrow, contiguous cells,
each containing its own underdrain system that allows it to be washed independently from
remaining cells. Washing is accomplished by means of a traveling hood suspended above
the bed. As the hood travels across the bed, each cell is isolated, and a small backwash
pump draws clean water from the filter effluent and reverses the flow through that par-
ticular cell. Water is removed by a second washwater pump located in the traveling hood
and discharged to waste.
The wash cycle time is controlled by preset adjustable timers to permit optimization
of the automatic operation feature. Media depth varies with each application but is typi-
cally 30 to 36 in. (0.8 to 0.9 m).
In addition to automatic washing features, these filters have the capability of produc-
ing relatively constant washwater flow. In a properly sized system, this constant flow can
eliminate the need to provide washwater equalization facilities and can permit direct re-
cycle to the plant headworks. Consult with local regulators as to the feasibility of recy-
cling washwater.

Wash Rates. In the United States, wash rates are expressed as volumetric flow per unit
surface area (gpm/ft2). In Europe, wash rates are expressed as the equivalent water rise
velocity (ft/s, ft/min, in./min, mm/s, or m/h). Wash rates are generally variable and de-
pend on washwater temperature, filter media characteristics, and washing method. Water
viscosity decreases with increasing temperature. Consequently, as washwater temperature
rises, drag forces on media grains are reduced, and higher wash rates are required to
achieve bed expansion. Each degree Celsius increase in water temperature requires roughly
a 2% increase in wash rate to prevent a reduction in bed expansion. Filter wash systems
should be designed for the warmest washwater temperature that will be encountered.
Filter media characteristics also affect washing rate. Rate requirements increase with
increasing grain size and density. Also, angular grains are more easily expanded than
round grains. In filters using more than one type of filter medium, sizes of each type of
medium must be selected carefully to ensure proper positioning after water wash. Rec-
ommended size ratios for dual- and mixed-media beds were discussed previously. Figure
8.8 displays the effect of media size on the water wash rate required to achieve 10% bed
expansion for three common filter media. Figure 8.9 shows the effect of water tempera-
ture on the viscosity of water and on the wash rate for silica sand and anthracite coal.
Specific backwash rate curves can be provided by media suppliers. A minimum rate of
15 gpm/ft 2 (37 m/h) is recommended, with typical rates ranging from 15 to 23 gpm/ft 2
(37 to 56 m/h). Rates as low as 10 gpm/ft 2 can be utilized for full-depth GAC or anthracite
filters.
Characteristic washing rates and durations vary for each washing method discussed
previously. The suitability of a washing method is related to influent water quality, filter
media characteristics and bed configuration, and underdrain design. Consequently, not all
washing methods are applicable in all cases, and different methods may or may not yield
similar results in a particular case.
Upflow Water Wash without Auxiliary Scour. When water wash is used alone, a high-
rate wash is employed. Generally a wash rate of 15 to 23 gpm/ft 2 (37 to 56 m/h) is ap-