90 percent) due to the high void space between particles. It also gives the particles
property of clinging together to form a "bridge" across an opening larger than the
particles themselves, but through which water passes (see figure 1-10).
Figure 1-10. "Bridging" of diatomaceous earth particles across septum.
(a) Construction. Figure 1-11 illustrates the construction of a diatomite
fiIter. It consists of a metal tank enclosing five or six fiIter elements (depending on the
model). Each fiIter element is made up of a series of plastic cups fastened to a central
tie rod. The cups are enclosed in a perforated metal tube that is covered by a plastic
membrane sleeve (see figure 1-12). The plastic membrane provides a porous surface
(septum) upon which the diatomite coat forms. This diatomite coat is usually called the
(b) Operation. The diatomite fiItration process involves three separate
operations: precoat, fiItration, and backwash. The precoat operation is necessary to
prepare the fiIter element with a coat of diatomite (also called fiItraid). A slurry of water
and diatomite are introduced to precoat the fiIter. The diatomite is held on the fiIter
element by a pressure difference between the inside and outside of the fiIter elements.
Unfiltered/raw water enters through the funnel, mixes with slurry water, and passes
through the filtering element system (see figure 1-12). This liquid keeps the fiIter cakes
porous as the rigid particles of diatomite mix with a small amount of continuous flow of
slurry. The precoat fiIter allows water to pass but prevents passage of schistosome
larvae, amoebic cysts, and about 90 percent of the bacteria. The flow rate of a
diatomite fiIter is about 5gpm/ft2 (see figure 1-13 for the flow).