Sedimentation

1. Sedimentation

2. Topics • Floc resuspension • Floc Hoppers • Delivering flocs • Minimum channel width • Scour velocity • Introduction • Capture velocity • Plate settlers • Floc rollup • Entrance region • Floc blankets

3. Conventional Surface Water Treatment Raw water Filtration Screening, grit removal, flow measurement sludge sludge Rapid Mix Disinfection Coagulant Cl2 Flocculation Storage Sedimentation Distribution sludge

4. Screening • Removes large solids • logs • branches • rags • fish • Simple process • may incorporate a mechanized trash removal system • Protects pumps and pipes in WTP

5. Evolution of Sedimentation • Oldest form of water treatment • Uses gravity to separate particles from water • Often follows flocculation • Traditionally a big tank where particles settle • Combined processes including • Enhanced flocculation (colloids attach to flocs) • Sedimentation • Sludge consolidation (reduce waste stream)

6. Sedimentation:Particle Terminal Fall Velocity Identify forces projected

7. Drag Coefficient on a Sphere Stokes Law turbulent boundary laminar turbulent

8. Floc Terminal Velocity Optimal velocity for floc blankets Capture velocity for AguaClara plate settlers Why flocculation is necessary! 1 mm DFractal= 2.3 and d0 = 1 mm The model takes into account the changing density of flocs

9. Horizontal Flow Sedimentation Tank W • How much time is required for water to pass through the tank? _____ • How far must a particle fall to reach the bottom of the tank (worst case)? _____ • How fast must the particle fall? q H H L exit entrance Yes Will it remove any smaller particles? ___ Settle Capture velocity: A property of the sedimentation tank plan view area. The slowest settling particle that the sedimentation tank can reliably capture.

10. Vertical Flow Sedimentation Tank exit • How much time is required for water to pass through the tank? _____ • How far must a particle fall relative to the fluid to not be carried out the exit? _____ • How fast must the particle fall (relative to the fluid)? q L W H H Will any smaller colloids be captured? ______________________________ ______________________________ Only if they collide and attach to a larger floc entrance

11. Settle Capture Velocity Guidelines • Based on tube settlers • 0.12 – 0.36 mm/s • Based on Horizontal flow tanks • 0.24 to 0.72 mm/s • AguaClara adopted 0.12 mm/s in an effort to reduce effluent turbidity as much as possible because AguaClara wasn’t using filters • We’d like to know the performance curve. How does settled water turbidity change with the capture velocity? More research is needed! • We could save plastic if we were willing to increase the capture velocity http://www.brentwoodprocess.com/tubesystems_main.html Schulz and Okun

12. extra Settling zone Outlet zone Inlet zone Sludge zone Sludge out Conventional Horizontal Flow Sedimentation Basin • long rectangular basins • 4-6 hour retention time • 3-4 m deep • max of 12 m wide • max of 48 m long • Ratio of surface area to cross sectional area must be less than 18 • What is Vc for conventional design?

13. extra Settling zone Outlet zone Inlet zone Sludge zone Design Criteria for Horizontal Flow Sedimentation Tanks • _______________________________ • _______________________________ • _______________________________ • _______________________________ • _______________________________ • Vc of 0.23 to 0.7 mm/s* • Residence time of 1.5 to 3 hours* Minimal turbulence (inlet baffles) Uniform velocity (small dimensions normal to velocity) No scour of settled particles Slow moving particle collection system Q/As must be small (to capture small particles) * Schulz and Okun And don’t break flocs at inlet!

14. extra Turbulence in Horizontal Flow Sedimentation Tanks • Question: If the flow through a sedimentation tank turbulent and if it is, is vertical transport by turbulent eddies significant? • Given • Ratio of surface area (width*length) to cross sectional area (width*depth) must be less than 18 (length/depth < 18) • Vc of 0.23 mm/s (use conservative design)

15. extra Turbulence in Horizontal Flow Sedimentation Tanks • What is the horizontal flow velocity (VH)? • Vc*Width*Length=VH*Width*Depth • VH = 18*Vc = 4.2 mm/s • Re Therefore turbulent • Turbulent eddies often have velocities that are order 10% of the mean velocity – 0.42 mm/s. This is higher than Vc and thus the high horizontal velocity could prevent flocs from settling

16. extra Problems with Horizontal Flow Tanks • Perform 3 times worse than laboratory sedimentation tanks* • Are incompatible with floc blanket • Generate eddies that prevent flocs from settling if the horizontal velocities are too high • Cause preferential flow through plate settlers at the end of the tank (where the horizontal velocities decrease and the pressure below the plates increases) • AguaClara uses vertical flow sedimentation tanks *Schulz and Okun page 140

17. Vertical Flow Sedimentation Tanks • Have lower velocities and hence turbulence levels might be lower (plan view area is larger than width x height • Require careful attention to delivery and extraction of water • AguaClara uses channels at the ends of the tanks that are connected to pipes to deliver and extract water – other geometries would be possible

18. extra Stagnant Water (or Ripe for Innovation?) • State of the art in sedimentation… • Empirical guidelines • No understanding of scaling effects • Last significant paper on tube settlers was published in 1978 • No significant revisions to Ten State Standards in the past 30 years

19. extra Stagnant Water (or Ripe for Innovation?) • There aren’t any textbooks or design books that have a coherent design approach for sedimentation tanks based on the physics of the process! • Regulation, Liability concerns, and Buzz Word science have successfully stopped research and innovation in a core technology for a sustainable society • The problem is extremely complex!

20. 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

21. Sedimentation Tank Geometry Manifold Plate settler Inlet VPlate↑· S = VActive↑· B LSedActive WSed· LSedActive·VActive↑= QSed WSed· LSedFloc·VSed↑= QSed Resuspension Floc hopper Floc blanket

22. 3 Steps to Sedimentation Success • The floc must be able to settle unto the surface of a plate or tube settler • Settle capture velocity • It must slide down the incline to reach the lower section of the sedimentation tank • Slide capture velocity • Finally the floc must be removed from the lower section of the sedimentation tank • Floc hopper or sludge drain

23. Settle Capture Velocity for Plate (and Tube)Settlers Path for critical particle? How far must particle settle to reach lower plate? hc L h S a What is total vertical distance that particle will travel? a VPlate ↑ Va What is net vertical velocity? Resultant particle velocity

24. Compare Times Time to travel distance hc Time to travel distance h = 5 parameters… how do we choose?

25. extra Comparison with Q/As As is horizontal area over which particles can settle hc L h S a a Va VPlate↑ Same answer!

26. extra L S a Performance ratio (conventional to plate/tube settlers) • Compare the area on which a particle can be removed • Use a single plate settler to simplify the comparison Conventional capture area Plate/tube capture area

27. extra Settle Capture Velocity Confusion Surface Water Treatment for Communities in Developing Countries, Schulz and Okun (1984) Consistent, but no one uses this geometry except in labs Water Quality and Treatment (1999) inconsistent Weber-Shirk Assume that the geometry is

28. extra Thick Plate Settlers S Distance between plate settlers B Center to center distance T Plate settler thickness Vertical velocity component beneath the plate settlers Vertical velocity component between the plate settlers hc L h S a a Va

29. extra Thick Plate Settlers Mass Conservation Geometry

30. extra Plate Settler Design Strategy (Conventional approach) • Angle is approximately 60° to get solids to slide down the incline • Plate Settler spacing of 5 cm (S) • L varies between 0.6 and 1.2 m • Vc of 0.12-0.36 mm/s • Find V↑ through active area of tank • Find active area of sed tank • Add area of tank required for angled plates: add L*cos(a) to tank length Why not 0° or 90°?

31. Plate Settler Design (AguaClara approach) VSed↑ • Upflow velocity (determines size of tanks) (1 mm/s) • If floc blanket is a goal then needs to be approximately 1 mm/s • Capture velocity (0.12 mm/s) • target turbidity • particle size distribution after floc blanket • Plate angle (60 deg) • self cleaning (60 deg works well) • Spacing (2.5 cm) • Clogging (not a problem at 2.5 cm) • floc roll up: Will the floc slide down? (next topic in the notes!) • Length of the plate settlers • will be the parameter that we calculate Needs research! 2011 AguaClara design

32. Plate Settler Spacing Constraints: Floc Rollup

33. Floc Roll Up Solution Scheme: Another failure mode • Find the velocity gradient next to the plate • Find the fluid velocity at the center of the floc • Find the terminal velocity of the floc down the plate (for the case of zero velocity fluid) • Set those two velocities equal for the critical case of no movement • Find the floc sedimentation velocity that can be captured given a plate spacing (VSlide)

34. u t S Infinite Horizontal Plates: Boundary Conditions y No slip condition u = 0 at y = 0 and y = S x let negative be___________ What can we learn about t?

35. Navier Stokes Flow between Plates Integrate to get average velocity Va is average velocity between plates We have velocity gradient as a function of average velocity

36. extra Laminar Flow through Circular Tubes: Equations no gravity R is radius of the tube Max velocity when r = 0 Velocity distribution is paraboloid of revolution therefore _____________ _____________ average velocity (V) is 1/2 vmax VpR2 Q = VA =

37. extra Velocity gradient at the wall Where dp/dx is the pressure gradient in the direction of flow NOT due to changes in elevation Tube geometry Plate geometry Average velocity

38. Floc Rollup Constraint Velocity at center of floc floc diameter vα Linearized (plates) a gsin(a) g Failure point (floc stationary) a Terminal velocity a a

39. Spacing as a function of floc terminal velocity But terminal velocity and floc diameter are related! Sedimentation velocity solved for diameter This is the smallest spacing that will allow a floc with a given settling velocity to remain stationary on the slope (and not be carried upward)

40. Minimum Plate Settler Spacing (function of Floc Vt) Fewer, but longer plate settlers due to higher upflow velocity in the sed tank Current AguaClara design for What happens if a floc forms from organic matter rather than clay? Plate settler spacing of 5 mm or larger should be adequate if the capture velocity is 0.12 mm/s

41. Slide Capture Velocity VSlide is the terminal sedimentation velocity (Vt) of the slowest-settling floc that can slide down an incline. Flocs with this terminal velocity (the slide velocity) will be held stationary on the incline because of a balance between gravitational forces and fluid drag. Flocs with a terminal velocity lower than VSlide will be carried out the top of the tube (i.e., “roll up”) even if they settle onto the tube wall. Thus, the slide terminal velocity represents a constraint on the ability of plate settlers to capture flocs. VSlide↑ What happens if the primary particles are less dense? _____

42. Experimental Evidence that the Slide Capture Velocity Matters All experiments were performed at the same settle capture velocity Floc Roll-up and its Implications for the Spacing of Inclined Settlers Matthew W. Hurst, Michael J. Adelman, Monroe L. Weber-Shirk, Tanya S. Cabrito, CosmeSomogyi, and Leonard W. Lion. Journal of Environmental Engineering, submitted (2012)

43. Plate Settler Spacing effects • Plate settler spacing has a strong influence on sedimentation tank depth • Diminishing effect for small S • Reduced spacing results in increased pressure drop through plate settlers* • Which plate settlers will have more uniform flow distribution? ___________ 2.25 m deep 5 cm 1 cm 1.42 m deep 0.5 cm 1.32 m deep Small S *proof coming up…

44. Velocity Shear (wall on fluid) Pressure drop (from head loss)through plate settlers Force balance Viscous shear Change L to maintain capture velocity

45. Plate Settler Head Loss Head loss is tiny! We need some head loss to get reasonable flow distribution between (and within) plates. This lack of head loss may be one of the reasons for poor performance of full scale plate settlers. The velocity of any turbulent or mean flow eddies needs to be less than ______ to achieve uniform flow through plate settlers. The floc blanket will end up helping us here! 4 mm/s

46. extra Reynolds Number in Plate Settlers Re is laminar for typical designs Reducing S reduces Re further

47. extra Entrance Region Length • The distance required to produce a velocity profile that then remains unchanged • Laminar flow velocity profile is parabolic • Velocity profile begins as uniform flow • Are tube and plate settlers long enough to get to the parabolic velocity profile? a

48. extra Entrance Region Length for Tube Distance for velocity profile to develop Shear in the entrance region is _______ than shear in long pipes for laminar flow. higher laminar turbulent

49. extra Entrance Length Region for AguaClara Designs * • Entrance length is shorter than plates *This equation applies to tubes. The coefficient may need to be changed to apply it to plates.

50. Plate Settler Conclusions… • Laminar flow • Parabolic velocity profile is established • Very low head loss (and thus flow distribution between plates is difficult to ensure) • Designed to capture flocs with sedimentation velocities greater than the settle capture velocity • Spacing determines the ability of the flocs to roll down the incline (slide capture velocity)