7.33 The backwashing rate used to clean the filter is 0.4(10 ) m/s. If the bed is expanded to 150% from an unexpanded depth of 0.8 m resulting in an expanded porosity of 0.7, calculate the unexpanded porosity of the bed.

7.34 Pilot plant analysis on a mixed-media filter shows that a filtration rate of 15 m3/m2-h is acceptable. If a surface configuration of 5 m x 8 m is appropriate, how many filter units will be required to process 100,000 m /d of raw water?

7.35 The results of a Buchner funnel experiment to determine the specific cake resistance of a certain sludge are as follows:

Volume of

25 48 1.92

100 520 5.2

_AP = 51 cm Hg, filter area = 550 cm2, ¡i = 15(10_4) kg/m • s, and c = 0.25 g/cm3. Determine a.

7.36 A pilot study was conducted using a dual-media filter composed of anthracite as the upper 30-cm part and sand as the next lower 30-cm part of the filter. The results are shown in the table below, where co (= 20 mg/L) is the concentration of solids at the influent. If the respective sizes of the anthracite and sand layers are 1.6 mm and 0.5 mm, what is the length of the filter run to a terminal head loss of 3 m at a filtration rate of 120 l/m -min? Assume the clean water head loss is 0.793 m.

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