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SLOW SAND AND DIATOMACEOUS EARTH FILTRATION 9.17

precoat of the filter leaves that initiates every operating cycle. Although DE is the most
common precoat material used, other precoat material such as ground perlite performs as
well in many applications.
For many years, the type of equipment available limited the use of DE filters for mu-
nicipal drinking water treatment. The use of stainless steel and plastics in the fabrication
of equipment has significantly changed the performance capability of the filters by im-
proving their ease of operations and maintenance.
DE is mined from the fossilized remains of microscopic plants called diatoms, de-
posited in what were the beds of ancient oceans. A powdered medium is manufactured
from the diatomite deposits that is almost pure silica. One of the more common diatomite
media used for drinking water treatment has a mean particle size of 22.3/xm with 80%
of the particles ranging in size from 5 to 64 p~m. This medium, when deposited on the
filter septum, has an average pore size of about 7.0/xm. Specifications for DE material
is covered in AWWA Standard B101 (2001).

DE Filter Operation

As illustrated in Figure 9.7, DE filter operations occur in three steps:
1. A precoat of about % in. (3 mm) is deposited on the filter.
2. After the precoat has been deposited, filtering begins, and at the same time a small
amount of DE material (called body feed) is added to the source water to maintain the
porosity of the media.
3. Particulates in the source water are trapped in the precoat layer until maximum head
loss is reached, at which time the filter run is terminated and media material is cleaned
from the septa.

Porosity Control The principal requirement for maintaining effective DE filter runs is
to maintain the porosity of the filter cake. Source water solids generally vary in size and

O

A

O

O
0~

Precoat Formation Filter Cake Formation Filter Cake Removal
a. b. C.
FIGURE 9.7 Precoat filtration cycles.