Water Treatment

1. Water Treatment

2. Where are we going? • 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 Particles and pathogens dissolved chemicals pathogens Empirical design Theories developed later

3. Conventional Surface Water Treatment Raw water Filtration Screening sludge sludge Alum Polymers Coagulation Disinfection 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. 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. Sedimentation:Particle Terminal Fall Velocity Identify forces projected

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

9. fluid properties Reynolds Number for a Floc • Find the diameter of the largest floc that falls with laminar flow characteristics • Density of water 1000 kg/m3 • Viscosity of water 0.001 kg/(m s) 1 45 What is the coefficient of drag? _____ For flocs CD=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. long rectangular basins 4-6 hour retention time 3-4 m deep max of 12 m wide max of 48 m long Settling zone Outlet zone Inlet zone Sludge zone Sludge out Sedimentation Basin

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

13. Sedimentation Basin:Importance of Tank Surface Area Time in tank W H L Want a _____ Vc, ______ As, _______ H, _______ q. small large small large Suppose water were flowing up through a sedimentation tank. What would be the velocity of a particle that is just barely removed?

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 Outlet zone Inlet zone Sludge 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/As 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) Increase density difference (dissolved air flotation) Decrease viscosity (increase temperature)

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. + + + + + + + + + + + + + + + + + + + + + + + + + + + Electrostatic 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

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 [Al2(SO4)3*14.3H2O] • 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 Al7(OH)17+4 • The primary reaction produces Al(OH)3 Al2(SO4)3 + 6H2O2Al(OH)3 + 6H+ + 3SO4-2 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. These values were obtained empirically, so even though the theory was wrong the values might be right! Flocculator Design (Prior to 1992): The “shear is dominant” assumption Drag coefficient = 2 for flat plate perpendicular to flow • Velocity gradient (G) • P is power input to • mechanical paddles • Hydraulic (head loss) • Recommended G and Gq values • G – 20 to 100 /s • Gq – 20,000 to 150,000 • Based on the (incorrect) assumption that the primary collision mechanism was fluid shear

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 Number concentration of size i particles [1/cm3] Collision frequency between two particles of sizes i and j[cm3/s] We double counted the formation of these particles

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

29. Differential Sedimentation Rectilinear model Curvilinear model aj Use trajectory analysis to get Critical path ai xc ai + aj

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. Differential Sedimentation Fluid Shear Brownian Motion Dominant Collision Mechanisms Plot conditions G = 10/s T = 20°C rp = 1.1 g/cm3 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 mm and the large particle is less than about 20 mm • Differential sedimentation is important if one of the particles is greater than 20 mm

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

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 V h force of drag will vary with time Fd Fg  Grains: usually we mean incoherent sands, gravels, and silt, but also sometimes we include cohesive soils (clays) that form larger particles (aggregates) 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) 1 Suspension Saltation 0.1 0.056 Threshold of movement No movement 0.01 1 10 100 1000 Laminar flow of bed Turbulent flow of bed

42. G and t and biggest particles Assume floc density is 1100 kg/m3 How large were the flocs that are kept in suspension given empirical design for G? m = 0.001 kg/(m s) Dr = 100 kg/m3 g = 9.8 m/s2 PShields = 0.05

43. Recommended G and Gq values:Turbidity or Color Removal * 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. d Vertical-flow Baffled Flocculator H V K180 bend= 2.5 - 4

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 • 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 Research13(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