Encyclopedia of Environmental Science and Engineering, Volume I and II

WATER TREATMENT 1317

a plant is designed to operate at two gallons per square foot

per minute, but provided for an overload when necessary

(Figure 8).

The cleaning of the rapid sand filter, instead of throw-

ing the filter out of service, is accomplished by simply

backwashing. This is accomplished by passing clean water

backwards through the filter at a high velocity. This velocity

should not be greater than the terminal settling velocity of

the smallest particle of sand which is in the filter which is

not to be washed over the side. Through this mechanism the

sand bed is expanded and the sand is lifted and floated while

the particles rub mechanically against one another and wash

off the foreign material. The dirty water is washed away in

drains. After this has been conducted for a few moments, the

filter is allowed to go back into service and the head loss is

now smaller so the rate of flow through the filter is increased

once more.

Pressure filters Whereas the rapid sand filter is indeed a grav-

ity filter, a pressure filter is somewhat the same type of system

only pressure is applied to the water to pass it through the

filter. The most common household unit nowadays would be

the swimming pool filter, where the water is pumped vertically

through the sand and the filter, and when the head loss through

the filter becomes excessive as registered on the pressure

gauge, the operator will reverse the flow through the filter,

accomplishing the backwash described above (Figure 9).

Diatomaceous earth filter Diatomaceous earth is the silicious

residue of the bodies of diatoms which were deposited in past

geological ages and now form extensive beds where they are

mined. The earth is processed and ground, and the silica par-

ticles are extremely irregularly shaped and thus provide a very

good porous coating. The diatomaceous earth filter was devel-

oped by the army for field use to remove certain chlorine-

resistant organisms responsible for dysentery.

BACKWASH

WATER

OUT

RAY WATER

IN

SAND

GRAVEL

FILTERED

BACKWASH WATER OUT

WATER

IN PRESSURE FILTER CUTAWAY

FIGURE 9

WASH WATER

TROUGHS

EXPANDED

SAND

GRAVEL

FILTERED EFFLUENT BACKWASH WATER

GRAVEL

SAND

RAPID SANDFILTER

(a) FILTERING (b) BACKWASHING

FIGURE 8

FILTER

CAKE

WASTE

FOR WASH

WATER

PRECOAT

POT

FILTERED

WATER

BODY

FEEDER

RAW WATER

FEEDER

DIATOMACEOUS EARTH FILTER

FIGURE 10

HEAD

LOSS

CLARIFIED

WATER

OUTLET

SAND

5 FEET

3 FEET

SLOW SAND FILTER

FIGURE 7

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