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8.22 CHAPTER EIGHT

European-type deep-bed filters use relatively coarse and uniformly graded media• As
a result, bed stratification is not required, and air scour presents less of a hazard to proper
bed operation. Also, the use of strainers is more likely to be feasible because of the larger
permissible openings• Consequently, false-bottom underdrains with nozzles designed for
both air and water distribution and without support gravel are commonly used in deep-
bed filters.
Pipe Laterals. Pipe,lateral underdrains were once popular because of their relatively low
cost and adaptability for use in pressure filters. Problems with relatively high head loss
and poor washwater distribution resulted in a general decline in their use. They are still
encountered, however, when older filters are upgraded.
Pipe underdrain systems generally consist of a centrally located manifold pipe to which
smaller, equally spaced laterals are attached. Lateral pipes usually have one or two rows
of 1A- to 3A-in.-diameter (6- to 19-mm) perforations on their bottom sides. The lateral
pipes may be fitted with nozzles as illustrated in Figure 8.11. Guidelines for lateral de-
sign include the following ratios:

Filter me~a

• • , .. .•, • . , . ,

• .., ~

• . . . .-scour supoJy pipe

• ~ ¢ ~ ~ ,,-
'o ' -. , ' -

Lateral pipes
0' " ,. ,.''~'-.
NOZZle busfl " " '" . 0 "-0 • "
Colleclion channel
"" , :.' " '0'
i . t " '" '." " ~ Floor of filler tank
t~ .' , "'o 7

FIGURE 8.11 Pipe lateral underdrain with nozzles. (Courtesy of Paterson Candy Ltd.)