1. Monroe L. Weber-Shirk S chool of Civil and Environmental Engineering Water Treatment 

2.  Unit processes* designed to  remove _________________________  remove __________ ___________  inactivate ____________  *Unit process: a process that is used in similar ways in many different applications  Unit Processes Designed to Remove Particulate Matter  Screening  Sedimentation  Coagulation/flocculation  Filtration Where are we going? Particles and pathogens dissolved chemicals pathogens Empirical design Theories developed later

3. Conventional Surface Water Treatment Screening Coagulation Flocculation Sedimentation Filtration Disinfection Storage Distribution Raw water Alum Polymers Cl 2 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. Sedimentation  the oldest form of water treatment  uses gravity to separate particles from water  often follows coagulation and flocculation

6. Sedimentation: Effect of the particle concentration  Dilute suspensions  Particles act independently  Concentrated suspensions  Particle-particle interactions are significant  Particles may collide and stick together (form flocs)  Particle flocs may settle more quickly  At very high concentrations particle- particle forces may prevent further consolidation

7. projected Sedimentation: Particle Terminal Fall Velocity Identify forces

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

9. Reynolds Number for a Floc  Find the diameter of the largest floc that falls with laminar flow characteristics  Density of water 1000 kg/m 3  Viscosity of water 0.001 kg/(m s) What is the coefficient of drag? _____ 45 fluid properties 1 For flocs C D =45/Re because flocs aren’t spheres

10. Laminar Flow Boundary Turbulent Laminar Flocs larger than ≈1 mm have turbulent flow characteristics The flow field around most flocs (water treatment) is laminar

11. Sedimentation Basin Settling zone Sludge zone Inlet zone Outlet zone Sludge out  long rectangular basins  4-6 hour retention time  3-4 m deep  max of 12 m wide  max of 48 m long

12. Sedimentation Basin: Critical Path  Horizontal velocity  Vertical velocity L H Sludge zone Inlet zone Outlet zone Sludge out WH flow rate What is V c for this sedimentation tank? V c = terminal velocity that just barely ______________gets captured

13. Sedimentation Basin: Importance of Tank Surface Area L H W Suppose water were flowing up through a sedimentation tank. What would be the velocity of a particle that is just barely removed? Want a _____ V c, ______ A s, _______ H, _______ . smalllarge Time in tank smalllarge

14. Lamella  Sedimentation tanks are commonly divided into layers of shallow tanks (lamella)  The flow rate can be increased while still obtaining excellent particle removal Lamella decrease distance particle has to fall in order to be removed

15. Settling zone Sludge zone Inlet zone Outlet zone Design Criteria for Sedimentation Tanks Minimal turbulence (inlet baffles) Uniform velocity (small dimensions normal to velocity) No scour of settled particles Slow moving particle collection system Q/A s must be small (to capture small particles)  _______________________________

16. Sedimentation of Small Particles?  How could we increase the sedimentation rate of small particles? Increase d (stick particles together) Increase g (centrifuge) Decrease viscosity (increase temperature) Increase density difference (dissolved air flotation)

17. Particle/particle interactions  Electrostatic repulsion  In most surface waters, colloidal surfaces are negatively charged  like charges repel __________________  van der Waals force  an attractive force  decays more rapidly with distance than the electrostatic force  is a stronger force at very close distances stable suspension

18. Energy Barrier  Increase kinetic energy of particles  increase temperature  stir  Decrease magnitude of energy barrier  change the charge of the particles  introduce positively charged particles + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + Layer of counter ions van der Waals Electrostatic

19. Coagulation  Coagulation is a physical-chemical process whereby particles are destabilized  Several mechanisms  adsorption of cations onto negatively charged particles  decrease the thickness of the layer of counter ions  sweep coagulation  interparticle bridging

20. Coagulation Chemistry  The standard coagulant for water supply is Alum [Al 2 (SO 4 ) 3 *14.3H 2 O]  Typically 5 mg/L to 50 mg/L alum is used  The chemistry is complex with many possible species formed such as AlOH +2, Al(OH) 2 +, and Al 7 (OH) 17 +4  The primary reaction produces Al(OH) 3 Al 2 (SO 4 ) 3 + 6H 2 O  2Al(OH) 3 + 6H + + 3SO 4 -2 pH = -log[H + ]

21. Coagulation Chemistry  Aluminum hydroxide [Al(OH) 3 ] forms amorphous, gelatinous flocs that are heavier than water  The flocs look like snow in water  These flocs entrap particles as the flocs settle (sweep coagulation)

22. Coagulant introduction with rapid mixing  The coagulant must be mixed with the water  Retention times in the mixing zone are typically between 1 and 10 seconds  Types of rapid mix units  pumps  hydraulic jumps  flow-through basins with many baffles  In-line blenders

23. Flocculation  Coagulation has destabilized the particles by reducing the energy barrier  Now we want to get the particles to collide  We need relative motion between particles  Brownian motion is too slow (except for tiny particles)  _________ _____________ rates  __________ shears the water Differential sedimentation Turbulence

24. Flocculation  Turbulence provided by gentle stirring  Turbulence also keeps large flocs from settling so they can grow even larger!  High sedimentation rate of large flocs results in many collisions!  Retention time of 10 - 30 minutes

25. Flocculator Design (Prior to 1992): The “shear is dominant” assumption  Velocity gradient (G)  P is power input to  mechanical paddles  Hydraulic (head loss)  Recommended G and G  values  G – 20 to 100 /s  G  – 20,000 to 150,000  Based on the (incorrect) assumption that the primary collision mechanism was fluid shear Drag coefficient = 2 for flat plate perpendicular to flow These values were obtained empirically, so even though the theory was wrong the values might be right!

26. Improved Model Development  Transport mechanisms  Diffusion  Shear  Differential Sedimentation  Monodisperse vs Heterodisperse suspensions  Rectilinear models ignored near field effects of hydrodynamic and electrostatic repulsion and van der Waals attraction  Curvilinear models incorporated these near field effects

27. Heterodisperse, Rectilinear Flocculation Change in number concentration of size k particles Collision frequency between two particles of sizes i and j [cm 3 /s] Number concentration of size i particles [1/cm 3 ] We double counted the formation of these particles

28. Rectilinear Collision Frequency – Transport Mechanisms Brownian motion Shear Differential Sedimentation Add them all up k is Boltzmann’s constant 1.38 x 10 -16 G is the average velocity gradient 1/s Assumes laminar flow

29. Differential Sedimentation Rectilinear model a i + a j Curvilinear model ajaj aiai Critical path xcxc Use trajectory analysis to get

30. Herterodisperse, Curvilinear Flocculation  Hydrodynamic interactions prevent collisions - water between particles must move out of the way  Van der Waals attractive force promotes collisions - become significant at small separation distances  Electrostatic repulsion prevents collisions – diffuse layer of ions rich in those with charge opposite to that of the surfaces is induced in the fluid surrounding each particle

31. Dominant Collision Mechanisms Differential Sedimentation Fluid Shear Brownian Motion Plot conditions G = 10/s T = 20°C  p = 1.1 g/cm 3 This model assumes floc density is independent of floc size

32. Curvilinear Simplified Conclusions  Shear is only important for particles within a factor of 5 of the same size  Diffusion is important if the small particle is less than 1  m and the large particle is less than about 20  m  Differential sedimentation is important if one of the particles is greater than 20  m

33. Application of Results  Increasing the concentration of large particles will increase the collision rate  Turbulence can be used to keep large particles in suspension  Need high fluid velocities at bottom of tank  Could use grid or jet turbulence  Recirculate large particles by providing upflow zone  No need to try to optimize the fluid shear mechanism since the differential sedimentation mechanism is more efficient

34. Mechanical Flocculators  Mechanical flocculators are preferred in the Global North  Speed of the mechanically operated paddles can be varied (but do operators vary this?)  Disadvantages  Motors, speed controllers, gear boxes (to reduce speed), and bearings may not be maintainable  Require electricity

35. Hydraulic Flocculators  Flocculation parameters are a function of flow and thus cannot be adjusted independently  Head loss is often significant  Cleaning may be difficult, but appropriate designs can accommodate cleaning  Types  Vertical flow  Horizontal flow  Tapered (to reduce shear as flocs grow larger?)  Gravel bed flocculators (related to filters!)

36. Potential Research Project  I don’t know if anyone has designed a better flocculator based on this new understanding of the importance of keeping large particles in suspension  But there is at least one existing design that keeps particles in suspension

37. Water inlet 36 - 100 m/day Sludge Blanket Flocculator/Sedimentation Tank Overflow channel Desludging valve Water Outlet Sludge blanket High velocities keep particles moving up Raw water with alum (Brownian motion) Differential sedimentation causes large flocs to collide with small Brownian particles Retention time 1-3 hours Turbidity less than 900 NTU Critical velocity

38. Flocculator Design  Keep particles suspended  requires tanks designed to resuspend particles that settle  Keep shear levels low so that particles don’t break apart  We need some data for appropriate shear levels  Shear =_________________________  Use the velocity gradient G that is recommended for flocculator design and calculate the shear!  The existing designs were based on the wrong theory, yet they work.  How can we reconcile this? velocity gradient x viscosity

39. Basic Mechanism of Bed Load Sediment Transport  drag force exerted by fluid flow on individual grains  retarding force exerted by the bed on grains at the interface  particle moves when resultant passes through (or above) point of support Grains: usually we mean incoherent sands, gravels, and silt, but also sometimes we include cohesive soils (clays) that form larger particles (aggregates) FdFd h force of drag will vary with time V FgFg point of support 

40. Threshold of Movement Force on particle due to gravity Force on particle due to shear stress We expect movement when  important dimensionless parameter

41. Shields Diagram (1936)     Threshold of movement Turbulent flow of bed Laminar flow of bed Suspension Saltation No movement 0.056

42. G and  and biggest particles Assume floc density is 1100 kg/m 3 How large were the flocs that are kept in suspension given empirical design for G?  = 0.001 kg/(m s)  = 100 kg/m 3 g = 9.8 m/s 2  Shields = 0.05

43. Recommended G and G  values: Turbidity or Color Removal Type Velocity gradient (G) (1/s) GG  (s)  0 * (Pa) without solids recirculation 20-10020,000- 150,000 1000-15000.020-0.1 With solids recirculation 75-175125,000- 200,000 1100-17000.075-0.175 * Estimated from G assuming viscosity of 0.001 kg/(m s)

44. Suggested Design Process  residence time of about 30 minutes  Maximize vertical mixing to keep heavy flocs in suspension  Keep shear levels less than 0.2 Pa * to avoid floc breakup  Or perhaps 0.2 Pa is required to keep heavy floc in suspension *We need some research to see if this shear level is correct!

45. Vertical-flow Baffled Flocculator d H K 180 bend = 2.5 - 4 V

46. Scaling floc sed down to POU  Need to reduce fluid velocities to avoid turning the sedimentation tank into a CMFR (____________ ___________________)  Batch may work best to ensure good sedimentation  Consider recycling flocs from previous batches  Continuous flow could take advantage of big flocs (upflow flocculation/sedimentation) Completely mixed flow reactor

47. Coagulants  Inorganic  Aluminum Sulfate (alum)  Ferric chloride  Organic  Chitosan Chitosan  Moringa oleifera Moringa oleifera  Dosage – 10 to 100 mg/L based on “jar” tests

48. Jar Test  Mimics the rapid mix, coagulation, flocculation, sedimentation treatment steps in a beaker  Allows operator to test the effect of different coagulant dosages or of different coagulants  Suggests a batch technique for POU turbidity removal

49. Upflow Flocculator/Sedimentation Tank particle capture  What is the size of the smallest floc that can be captured by this tank with critical velocity of 100 m/day?  We need a measure of real water treatment floc terminal velocities  Research…

50. Physical Characteristics of Floc: The Floc Density Function  Tambo, N. and Y. Watanabe (1979). "Physical characteristics of flocs--I. The floc density function and aluminum floc." Water Research 13(5): 409-419.  Measured floc density based on sedimentation velocity (Our real interest!)  Flocs were prepared from kaolin clay and alum at neutral pH  Floc diameters were measured by projected area

51. Floc Density Function: Dimensional Analysis!  Floc density is a function of __________  Make the density dimensionless  Make the floc size dimensionless  Write the functional relationship  After looking at the data conclude that a power law relationship is appropriate floc size

52. Model Results  For clay assume d particle was 3.5  m (based on Tambo and Watanabe)  a is 10 and n d is -1.25 (obtained by fitting the dimensionless model to their data)  The coefficient of variation for predicted dimensionless density is  0.2 for d floc /d particle of 30 and  0.7 for d floc /d particle of 1500  The model is valid for __________flocs in the size range 0.1 mm to 3 mm clay/alum

53. Additional Model Limitation  This model is simplistic and doesn’t include  Density of clay  Ratio of alum concentration to clay concentration  Method of floc formation  Data doesn’t justify a more sophisticated model  Are big flocs formed from a few medium sized flocs or directly from many clay particles?  Flocs that are formed from smaller flocs may tend to be less dense than flocs that are formed from accumulation of (alum coated) clay particles

54. Model Results → Terminal Velocity Flocs aren’t spheres

55. Floc Density and Velocity (Approximate) 0.4 mm ______ kg/m 3 1030

56. Flocculation/Sedimentation: Deep vs. Shallow  Compare the expected performance of shallow and deep horizontal flow sedimentation tanks assuming they have the same critical velocity (same Q and same surface area) More opportunities to ______ with other particles by _________ ____________ or ________________ Expect the _______ tank to perform better! deeper collide differential sedimentation Brownian motion

57. Flocculation/Sedimentation: Batch vs. Upflow  Compare the expected performance of a batch (bucket) and an upflow clarifier assuming they have the same critical velocity  How could you improve the performance of the batch flocculation/sedimentation tank?

58. 0.01 0.1 1E+031E+041E+051E+061E+071E+08 Re friction factor laminar 0.05 0.04 0.03 0.02 0.015 0.01 0.008 0.006 0.004 0.002 0.001 0.0008 0.0004 0.0002 0.0001 0.00005 smooth Frictional Losses in Straight Pipes