Figure 1-9. Pressure fiIter.
(4) Diatomite fiIters. Diatomite fiItration is a unique process which was
adopted by the United States (US) Army from the British during World War II and which
is now incorporated in the standard field water treatment process. It is also used
commercially in some small municipal systems and is used extensively in swimming
pool fiItration. It is not used in the reverse osmosis process. The principle of diatomite
fiItration is the remarkable absorptive property of diatomaceous earth, the fiIter medium
used in this process. Diatomaceous earth is a white to gray powder made from the
skeletal remains of algae known as diatoms. The particles making up the powder,
being composed of silica, are high in strength, rigid, and abrasive. They are extremely
small, averaging 10 to 200 microns (0.01 to 0.2 mm) in diameter. This size is
approximately forty times smaller than that of the sand grains used in a rapid sand fiIter
(0.35 to 0.55). Moreover, the diatomaceous earth particles are highly irregular in shape.
This irregularity in size and shape gives the particles an extremely high porosity (about