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CHAPTER 8

HIGH-RATE

GRANULAR MEDIA

FILTRATION

Kevin Castro
Stearns & Wheler, LLC
Cazenovia, New York

Gary Logsdon
Retired
Lake Ann, Michigan

Stephen R. Martin
CDM
Cambridge, Massachusetts

Filtration, as it applies to water treatment, is the passage of water through a porous medium
to remove suspended solids. According to Baker (1948), the earliest written records of
water treatment, dating from about 4000 B.C., mention filtration of water through char-
coal or sand and gravel. Although a number of modifications have been made in the man-
ner of application, filtration remains one of the fundamental technologies associated with
water treatment.
Filtration is needed for most surface waters, to provide a second barrier against the
transmission of waterborne diseases. Although disinfection is today the primary defense,
filtration can assist significantly by reducing the load on the disinfection process, in-
creasing disinfection efficiency, and aiding in the removal of precursors to disinfection
by-product (DBP) formation. The Surface Water Treatment Rule (SWTR) and Enhanced
Surface Water Treatment Rules (ESWTR) recognize three categories of granular filtra-
tion techniques:
• Rapid sand
• Slow sand
• Diatomaceous earth
This chapter covers the design of the first category of filters. However, in this instance
the term rapid sand includes not only sand, but also other types of filter media such as
crushed anthracite coal and granular activated carbon (GAC). Chapter 9 covers the other
two categories of granular filtration techniques. Chapter 14 further discusses activated car-
bon processes, including GAC filters/adsorbers.

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