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CE 370 Screens. Water Treatment Coarse Bar Racks Coarse Bar Racks Clear space between bars is up to 3 inches (75 mm) Clear space between bars is up to.

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Presentation on theme: "CE 370 Screens. Water Treatment Coarse Bar Racks Coarse Bar Racks Clear space between bars is up to 3 inches (75 mm) Clear space between bars is up to."— Presentation transcript:

1 CE 370 Screens

2 Water Treatment Coarse Bar Racks Coarse Bar Racks Clear space between bars is up to 3 inches (75 mm) Clear space between bars is up to 3 inches (75 mm) Are used to retain large objects into water intake Are used to retain large objects into water intake Traveling Racks Traveling Racks Located behind bar racks Located behind bar racks Have openings of 3/8 to ½ inch (10-13 mm) Have openings of 3/8 to ½ inch (10-13 mm) Are used to prevent small particles Are used to prevent small particles

3 Traveling Water Screens

4 Wastewater Treatment Manually cleaned screens Manually cleaned screens Mechanically cleaned screens Mechanically cleaned screens

5 Mechanically-cleaned Bar Screen

6 Manually Cleaned Bar Screens Space between bars are 1 to 2 inches (25 to 50 mm) Space between bars are 1 to 2 inches (25 to 50 mm) Mounted at 30 to 75  angle to horizontal (typically 30 to 45  ) Mounted at 30 to 75  angle to horizontal (typically 30 to 45  )

7 Mechanically Cleaned Bar Screens Bar spacing is 0.5 to 1.5 inches (12 to 38 mm) Bar spacing is 0.5 to 1.5 inches (12 to 38 mm) Mounted at 45 to 90  with horizontal (typically 60  ) Mounted at 45 to 90  with horizontal (typically 60  ) Cleaning is performed at time intervals or when the head loss reaches a certain value (2 inches or 50 mm) Cleaning is performed at time intervals or when the head loss reaches a certain value (2 inches or 50 mm)

8 Design Parameters The approach channel should be straight for at least 0.6 meter ahead of the screen to produce uniform flow through the screen The approach channel should be straight for at least 0.6 meter ahead of the screen to produce uniform flow through the screen Approach velocity should be at least 1.5 ft/sec (0.46 m/s) to prevent precipitation of debris Approach velocity should be at least 1.5 ft/sec (0.46 m/s) to prevent precipitation of debris Velocity through bars should be less than 2 ft/sec (0.62 m/s) at design flow Velocity through bars should be less than 2 ft/sec (0.62 m/s) at design flow Velocity through bars should be not more than 3 ft/sec (0.91 m/s) at maximum flow Velocity through bars should be not more than 3 ft/sec (0.91 m/s) at maximum flow

9 Design Parameters A stand-by screen should be provided A stand-by screen should be provided Stop-gates slots are provided ahead and behind screens for maintenance reasons Stop-gates slots are provided ahead and behind screens for maintenance reasons

10 Design Equations Head-loss across bar screens can be represented by: Head-loss across bar screens can be represented by: h L = head loss, ft (m) h L = head loss, ft (m) V a = approach velocity ft/sec (m/s) V a = approach velocity ft/sec (m/s) V b = velocity through bar openings ft/sec (m/s) V b = velocity through bar openings ft/sec (m/s) g = acceleration due to gravity g = acceleration due to gravity

11 Screenings Amount Amount For ½ inch spacing = 8 ft 3 (60 m 3 ) per million gallons (million m 3 ) of treated wastewater For ½ inch spacing = 8 ft 3 (60 m 3 ) per million gallons (million m 3 ) of treated wastewater For 1.5 inch spacing = 1.5 ft 3 (11.2 m 3 ) per million gallons (million m 3 ) of treated wastewater For 1.5 inch spacing = 1.5 ft 3 (11.2 m 3 ) per million gallons (million m 3 ) of treated wastewater Management Management Disposed in landfills Disposed in landfills

12 Example A mechanical bar screen is to be used in an approach channel with a maximum velocity of 1 m/s. The bars are 15 mm thick, and the openings are 25 mm wide. Determine: A mechanical bar screen is to be used in an approach channel with a maximum velocity of 1 m/s. The bars are 15 mm thick, and the openings are 25 mm wide. Determine: The velocity between the bars The velocity between the bars The head loss in meters The head loss in meters

13 Solution Assume the channel has a width (W) and depth (D) Assume the channel has a width (W) and depth (D) Net area of screen = WD [25 / (25+15) = (5/8)WD Net area of screen = WD [25 / (25+15) = (5/8)WD Area of screen = WD Area of screen = WD Use continuity equation Use continuity equation V a A a = V b A b OR V b = (V a A a ) / A b V a A a = V b A b OR V b = (V a A a ) / A b V b = 1  [WD / (5/8)WD] = (8 / 5 ) m/s V b = 1  [WD / (5/8)WD] = (8 / 5 ) m/s

14 Solution Head Loss Head Loss

15 Shredding Why ? Why ? Reduce size of particles Reduce size of particles How ? How ? Grinders Grinders Barminutors Barminutors comminutors comminutors Fate of shredded solids Fate of shredded solids Return to wastewater flow downstream from screens Return to wastewater flow downstream from screens

16

17 Comminutor Installation


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