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CHAPTER 9

SLOW SAND AND

DIATOMACEOUS EARTH
FILTRATION

Michael V. Broder
Hazen and Sawyer, PC
New York, New York

John C. Byron
Stearns & Wheler, LLC
Cazenovia, New York

Although rapid sand filters predominate, the two types of filters covered in this chapter
may be effective in many applications where source water quality permits. In general, the
combined costs of constructing and operating slow sand and diatomaceous earth (DE) fil-
ters may be considerably less than the cost of rapid sand filtration plants for the same
capacity.
The principal mechanisms for separation of solids in all porous media filters are at-
tachment and straining or entrapment. Because of the relationship of the somewhat large
pores in rapid sand filter media compared with particulates, the primary mechanism for
separation is attachment. In the case of slow sand and DE filters, however, the pore-par-
ticulate size relationship results in more substantial separation by entrapment.
During the initial operation period of slow sand filters, the separation of organic mat-
ter and other solids generates a layer of biological matter on the surface of the filter me-
dia. Once established, this layer is the predominant filtering mechanism. Solids are re-
moved from water by a DE filter as the source water flows through a precoat layer of
powderlike DE.
For all practical purposes, most solids are separated at the surface of the media in both
actions. Because of the small pore size of the media, particulate separation is ideal for re-
moving the cysts of Giardia and Cryptosporidium. In most situations, neither type of fil-
ter requires previous conditioning of the raw water.
In general, application of slow sand and DE filtration should be limited to source wa-
ters with turbidity levels less than 5 ntu. Where particulates are dominant (rather than or-
ganic matter) both types of filters may be used with water of up to 10-ntu turbidity. If ei-
ther type of filter is used on water with higher turbidity, filtrate quality will generally be
acceptable, but the more rapid buildup of solids on the filter results in rapid loss of head
and shortened length of filter runs.

9.1