OVERVIEW

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 charcoal or sand and gravel. Although a number of modifications have been made in the manner 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, increasing 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 filtration 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 carbon processes, including GAC filters/adsorbers.

MECHANISM OF FILTRATION

Removing suspended solids by high-rate granular media filtration is a complex process involving...