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Prelim 2 Review Office Hour Special! 5:30 pm on Tuesday in 366

Prelim 2 Review Office Hour Special! 5:30 pm on Tuesday in 366. With a focus on Sedimentation and Filtration. Sedimentation: How do these velocities connect to design?. Settle capture – determines the slowest terminal velocity floc that will settle to a surface

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Prelim 2 Review Office Hour Special! 5:30 pm on Tuesday in 366

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  1. Prelim 2 ReviewOffice Hour Special! 5:30 pm on Tuesday in 366 With a focus on Sedimentation and Filtration

  2. Sedimentation: How do these velocities connect to design? • Settle capture – determines the slowest terminal velocity floc that will settle to a surface • Slide capture – determines the slowest terminal velocity floc that will slide down the plate. Function of velocity gradient at plate settler surface, angle, and floc density • Upflow • Terminal – floc property!

  3. Settle Capture Velocity • Which provides better removal of particles, a low settle capture velocity or a high settle capture velocity? • Suppose you keep plate length constant and decrease spacing between plates. What happens to performance and cost?

  4. AguaClara Sedimentation Tank Entrance Channel Dump poorly flocculated water channel Exit Weir that controls water levels all the way back to the next free fall (LFOM!) Exit Channel Launder Plate Settlers Floc Weir Floc Hopper Inlet Manifold Diffusers Floc Hopper Drain (to remove sludge) Sed Tank Drain

  5. How can you drain and then refill a sedimentation tank?

  6. Questions • Why do horizontal flow sedimentation tanks perform much worse than theory predicts? (eliminate inlet manifold?) • Why do we use plate settlers? • What is the failure mechanism for plate settlers for small spacing? • What determines the size of the entrance channel? • How could the velocity be increased in the entrance channel to reduce sedimentation?

  7. Sed Questions • What determines the depth of the sedimentation tank? • What happens if you make the sedimentation tank 2 m wide or 50 cm wide? • Why does the flow divide evenly between the sed tanks?

  8. Sedimentation Conclusions • Current design strategy is to prevent floc breakup between floc and sed tanks • Plate settlers make it possible to significantly reduce sedimentation tank area • Reduced plate spacing results in shallower tanks • Floc blankets improve performance especially if operated close to the optimal upflow velocity • Flow distribution is VERY important • The first full scale floc blanket was created in Atima, Honduras in mid 2012. The plant is performing well, but we do not yet have a measure of the improvement caused by the floc blanket

  9. Sedimentation Conclusions • 3 processes in one tank: flocculation, sedimentation, consolidation • Eliminate the need for mechanized sludge removal by using hydraulic sludge removal through drains • Provide the operator with options for dumping poorly treated water • Make the sedimentation tank easily accessible (removable plate settlers)

  10. Plate Settlers • Is plate settler flow laminar or turbulent? • What are the two advantages and the two disadvantages of putting plate settlers closer together? (connect head loss and rollup!) • How much bigger would an AguaClara sedimentation tank have to be if it didn’t have plate settlers? (Vc=0.12 mm/s) Vup below the active plate area is about 1 mm/s)

  11. Floc Roll Up • What relationship did we linearize to obtain a simpler analytical solution for the floc roll up failure mode? • What must be true for this approximation to be reasonable? u a gsin(a) g a

  12. Plate Settlers • If AguaClara adopted a design change and doubled the upflow velocity in our sedimentation tanks, then what changes would we need to make in our plate settlers? • Approximately how much longer would they need to be if we keep the spacing the same? • If we keep velocity gradient constant?

  13. Floc Hopper Q's: • What is in the floc hopper before it fills with flocs? • Why do flocs flow over the floc hopper weir? • How are the flocs removed when the blanket is lower than floc weir? • Why does a floc blanket interface slowly rise?

  14. extra Mass Balance in Sed Tank Flocculator Floc blanket Floc hopper QFlocBlanket CFlocBlanket QInfluent - QSludge QReturn QInfluent Inlet Manifold CInfluent QSludge CSludge

  15. Floc Blanket • Why would a floc blanket not work if AguaClara didn’t use plate settlers? • What mechanisms could explain why a floc blanket reduces the turbidity of the settled water?

  16. Filtration • How much of filters will be on the prelim? • What is the point of having different filtration beds with different size sand/gravel particles? • Are they mixed together in a stacked rapid sand filter? • How is water driven into the filter exit pipes up into the exit channel, is it pumped?

  17. Filtration • What is the purpose of the slotted pipes in the SRSF? • What is the size of the slots? (0.2 mm) • How does this connect with the sand size? • How many meters of water does a rapid sand filter process in 1 day? • How many meters of water are required to backwash a filter for 15 minutes?

  18. Begin Filtration cycle Inlet plumbing head loss Outlet box Total filter head loss What causes water to flow through the filter bed? ____________________________ differences in piezometric head

  19. Big picture: Prelim question that didn’t make the cut… An AguaClara plant that includes all of the latest in design innovations (flocculation, floc blanket, floc hopper, plate settlers, stacked rapid sand filter) is built for a community of 15,000 people with a per capita demand of 3 mL/s per person. The facility has 8 sed tanks and 2 filters. During a drought the flow rate through the plant is reduced by 50%. Which unit processes are affected negatively by a reduction in flow rate and what changes in operation would you advise?

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