Science of Water : Concepts and Applications

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Water Treatment Calculations 371 Solution: BACKWASH RATE 2 UFRV (filtration rate, gpmft )(filter run, h)(60 minh) 2 (3.2 gpmft )(61.0 h)(60 minh) 11,712 galft 2 In fi lter backwashing, one of the most important operational parameters to be determined is the amount of water in gallons required for each backwash. This amount depends on the design of the fi lter and the quality of the water being fi ltered. The actual washing typically lasts 5–10 min and uses amounts to 1 to 5% of the fl ow produced. Example 10.80 Problem: A fi lter has the following dimensions: Length = 30 ft Width = 20 ft Depth of fi lter media = 24 in. Assuming a backwash rate of 15 gal/ft2 /min is recommended, and 10 min of backwash is required, calculate the amount of water in gallons required for each backwash. Solution: Given: Length = 30 ft Width = 20 ft Depth of fi lter media = 24 in. Rate = 15 gal/ft 2 /min Find the amount of water in gallons required: Step 1: Area of fi lter = 30 ft × 20 ft = 600 ft2 Step 2: Gallons of water used per square foot of fi lter = 15 gal/ft 2 /min × 10 min = 150 gal/ft2 Step 3: Gallons required = 150 gal/ft2 × 600 ft2 = 90,000 gal required for backwash Typically, backwash rates will range from 10 to 25 gpm/ft2 . The backwash rate is determined by using Equation 10.71: Example 10.81 2 flow rate, gpm Backwash rate, gpmft (10.71) 2 filter area, ft Problem: A fi lter 30 ft by 10 ft has a backwash rate of 3120 gpm. What is the backwash rate in gpm/ft2 ? Solution: 2 flow rate, gpm Backwash rate, gpmft 2 filter area, ft 3120 gpm (30 ft)(10 ft) 10.4 gpm ft 2
372 The Science of Water: Concepts and Applications Example 10.82 Problem: A fi lter 20 ft long and 20 ft wide has a backwash fl ow rate of 4.85 MGD. What is the fi lter backwash rate in gpm/ft2 ? Solution: flow rate, gpm Backwash rate 2 filter area, ft 4,850,000 gpd 1440 minday 3368 gpm 3368 gpm (20 ft)(20 ft) 8.42 gpm ft 2 BACKWASH RISE RATE Backwash rate is occasionally measured as the upward velocity of the water during backwashing— expressed as inches per minute (in./min) rise. To convert from gpm/ft2 backwash rate to in./min rise rate, use either Equation 10.72 or 10.73: Example 10.83 2 (backwash rate, gpmft )(12 in. ft) Backwash rate, in. min 7.48 galft 2 Backwash rate, in. min (backwash rate, gpmft )(1.6) Problem: A fi lter has a backwash rate of 16 gpm/ft2 . What is this backwash rate expressed as in./min rise rate? Solution: 2 (backwash rate, gpmft )(12 in. ft) Backwash rate, in. min 7.48 galft 2 (16 gpmft )(12 in. ft) 3 7.48 galft 25.7 in. min Example 10.84 Problem: A fi lter 22 ft long and 12 ft wide has a backwash rate of 3260 gpm. What is this backwash rate expressed as in./min rise? (10.72) (10.73)
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Second Edition THE SCIENCE OF WATER
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Cover Image: Falling Spring at Fall
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Contents Preface ..................
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Contents ix Velocity Head .........
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Contents xi Specifi c Conductance .
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Contents xiii (5) Beetles (Order: C
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Contents xv Chlorine Residual Testi
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Contents xvii Water Filtration Calc
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To the Reader In reading this text,
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1 Introduction When color photograp
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Introduction 3 On second look, we s
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Introduction 5 As mentioned earlier
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Introduction 7 This scream of lost
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Introduction 9 Metcalf & Eddy, Inc.
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12 The Science of Water: Concepts a
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3 Water Hydraulics Anyone who has t
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Water Hydraulics 47 Another relatio
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Water Hydraulics 49 • • 1 ft 3
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Water Hydraulics 51 √ Important P
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Water Hydraulics 53 FIGURE 3.4 Thru
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Water Hydraulics 55 Example 3.8 Pro
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Water Hydraulics 57 FIGURE 3.6 Lami
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Water Hydraulics 59 With regard to
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Water Hydraulics 61 or hydraulic gr
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Water Hydraulics 63 E 1 smaller fl
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Water Hydraulics 65 √ Note: In de
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Water Hydraulics 67 • Cone of dep
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Water Hydraulics 69 • • • •
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Water Hydraulics 71 The Darcy-Weisb
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Water Hydraulics 73 Of course, pipe
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Water Hydraulics 75 In this section
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Water Hydraulics 77 Slope (S) The s
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Water Hydraulics 79 and important c
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Water Hydraulics 81 and the depth o
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Water Hydraulics 83 TYPES OF DIFFER
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Water Hydraulics 85 √ Important P
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Water Hydraulics 87 Meter backpress
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Water Hydraulics 89 VELOCITY FLOWME
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Water Hydraulics 91 The positive-di
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Water Hydraulics 93 Solution: 1200g
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Water Hydraulics 95 Water and Waste
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100 The Science of Water: Concepts
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6 Water Ecology The subject of man
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Water Ecology 157 The environment i
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Water Ecology 159 FIGURE 6.2 Notone
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Water Ecology 161 H 2 O O 2 FIGURE
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Water Ecology 163 FeS FIGURE 6.6 Th
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Water Ecology 165 Algae FIGURE 6.8
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Water Ecology 167 2. Likewise, biom
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Water Ecology 169 Population densit
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Water Ecology 171 succession can be
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Water Ecology 173 dark winter month
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Water Ecology 175 In rain events wh
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Water Ecology 177 long profi le, cr
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Water Ecology 179 THE FLOODPLAIN A
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Water Ecology 181 It is important t
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Water Ecology 183 Specifi c Adaptat
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Water Ecology 185 serve to indicate
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Water Ecology 187 UNITS OF ORGANIZA
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Water Ecology 189 FIGURE 6.20 Stone
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Water Ecology 191 FIGURE 6.22 Midge
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Water Ecology 193 FIGURE 6.25 Riffl
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Water Ecology 195 They push their m
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Water Ecology 197 FIGURE 6.32 Damse
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Water Ecology 199 Darwin, C., 1998.
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