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8,28 CHAPTER EIGHT

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FORM I

FIGURE 8.17 Underdrain layout. (Courtesy of Ondeo Degremont.)

are constructed of plates mounted on concrete or steel piers or on clay tile saddles to form
blocks. Very small pore sizes make porous-bottom underdrains susceptible to plugging
and therefore unsuitable for use in softening or iron and manganese removal plants or
other plants where plugging by chemical deposition may occur. They may also clog with
rust or debris during backwashing.
Additional problems that may occur include breakage because of the brittle nature of
the porous material and failure of caulked joints between plates. Porous bottoms have
been used successfully in a few locations, but they are less widely accepted than the block
or false-bottom underdrain systems discussed previously.

Filter Performance Monitoring
Filter design should include instrumentation to monitor filtered water turbidity (individ-
ual and combined), filtration rate, head loss, backwash rate, and filter run length. If aux-
iliary air scour backwash is used, airflow monitoring should also be included. Particle
counting may also be afforded for an additional operational tool. Pilot filters may help
determine primary coagulant, coagulant aid, and filter aid dosage.
Turbidity. Turbidity is the most common measure of filter performance. Turbidity is an
indication of the amount of suspended material in a water sample. The nephelometric tech-
nique, the current standard in the United States, compares the intensity of light scattered
by a water sample with a standard reference suspension under the same conditions. Tur-
bidimeters can be either discrete sample or on-line type. On-line turbidimeters include a
light source to illuminate the water sample, one or more photoelectric detectors, and an
analog indicator of turbidity based on the intensity of light scattered 90 ° to the path of
the incident light. Turbidity is reported as nephelometric turbidity units (ntu).