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8.4 CHAPTER EIGHT

surface clogging. Filter aids are often fed at low dosages in dilute liquid form to allow
dispersion without mechanical agitation just before filtration.
Filter aid polymer doses to gravity filters are usually low (0.02 to 0.05 mg/L). Doses
required for pressure filters may be higher because of the higher operating head losses
normally employed. Because water viscosity increases with decreasing temperature, break-
through as a result of floc shearing is more likely at lower water temperatures. Conse-
quently, increased polymer doses and a longer contact time before filtration may be re-
quired in cold weather.
Assuming that adequate coagulation is feasible, the designer must decide whether clar-
ification is desirable. In the past, settling has been provided before high-rate granular me-
dia filtration when turbidities exceeded roughly 10 ntu (Culp and Culp, 1974). The in-
creased storage capacities of dual- and mixed-media filters have made filtration of water
with higher turbidities practicable. The primary advantage of providing direct filtration is
the elimination of capital and operating costs associated with clarification. (For the pur-
pose of this chapter, direct filtration is defined as high-rate granular media filtration di-
rectly following flocculation, without a clarification process.) The higher solids load on
the filter will, however, shorten run times and increase the portion of product water re-
quired for filter washing. Although the point at which advantages outweigh disadvantages
varies with local conditions, a number of investigators have suggested conditions that
would justify consideration of direct filtration (Conley, 1965; Cleasby, 1972; Hutchison,
1976; Culp, 1977). Development of packaged or preengineered filtration systems in the
1970s has led to a hybrid process hereafter referred to as two-stage filtration. Two-stage
filtration combines traditional high-rate granular media filtration (gravity or pressure) pre-
ceded by a high-rate clarifier or roughing filter, generally in an upflow configuration. Di-
rect filtration and two-stage filtration are generally employed for higher-quality waters
with lower and more consistent turbidity and lower organic content. It is imperative that
pilot studies be conducted to determine the feasibility of direct filtration or two-stage fil-
tration for each application.

Filter Media

Although the selection of filter media type and characteristics is the heart of any filtra-
tion system, selection is usually based on arbitrary decisions, tradition, or a standard ap-
proach. Pilot plant studies using alternative filter media and filtration rates can determine
the most effective and efficient media for a particular water.
In drinking water applications in North America, the most commonly used filter me-
dia are natural silica sand, garnet sand or ilmenite, crushed anthracite coal, and GAC. Se-
lecting appropriate filter media involves a number of design decisions concerning source
water quality, pretreatment, and desired filtered water quality. Filter media cleaning re-
quirements and underdrain system options depend on the filter configuration and filter
media selected.
Media variables the designer can control include bed composition, bed depth, grain
size distribution, and, to a lesser extent, specific gravity. In addition to media design char-
acteristics, media quality can be controlled to some extent through specifications cover-
ing, where applicable, hardness or abrasion resistance, grain shapes, acid solubility, im-
purities, moisture, adsorptive capacity, manner of shipment, and other such factors.
Suggested criteria and a discussion of the applicability of these parameters can be found
in the AWWA Standard for Filtering Material (BIO0) and Standard Granular Activated
Carbon (B604).
In the United States, granular media have been traditionally described in terms of ef-
fective size (ES) and uniformity coefficient (UC). The ES is that dimension exceeded by