Water Treatment

1. Water Treatment

2. Reflections • What are the two broad tasks of environmental engineers? • What is the connection between the broad tasks of environmental engineers and building a water treatment plant? • Why may the water need to be changed/treated?

3. Simple Sorting • Goal: clean water • Source: contaminated water • Solution: separate contaminants from water • How?

4. 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 • sedimentation • filtration • ... particles dissolved chemicals pathogens

5. Unit Processes Designed to Remove Particulate Matter • Screening • Sedimentation • Coagulation/flocculation • Filtration • slow sand filters • rapid sand filters • diatomaceous earth filters • membrane filters

6. Conventional Surface Water Treatment Raw water Filtration Screening sludge sludge Alum Polymers Coagulation Disinfection Cl2 Flocculation Storage Sedimentation Distribution sludge

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

8. Sedimentation • the oldest form of water treatment • uses gravity to separate particles from water • often follows coagulation and flocculation • occurs in NYC’s __________ reservoirs

9. 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 • Particle-particle forces may prevent further consolidation

10. How fast do particles fall in dilute suspensions? • What are the important parameters? • Initial conditions • After falling for some time... • What are the important forces? • _________ • __________ Gravity Fluid drag

11. Sedimentation:Particle Terminal Fall Velocity Identify forces projected

12. Particle Terminal Fall Velocity (continued) Force balance (zero acceleration) We haven’t yet assumed a shape sphere Assume a _______

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

14. Drag Coefficient:Equations General Equation Laminar flow R < 1 Transitional flow 1 < R < 104 Fully turbulent flow R > 104

15. Example Calculation of Terminal Velocity Determine the terminal settling velocity of a cryptosporidium oocyst having a diameter of 4 mm and a density of 1.04 g/cm3 in water at 15°C [m=1.14x10-3 kg/(s•m)]. Work in your teams. Use mks units (meters, kilograms, seconds). Convert your answer to some reasonable set of units that you understand. Solution Reynolds?

16. 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

17. Sedimentation Basin:Critical Path Horizontal velocity Outlet zone Q = flow rate H Inlet zone Sludge zone A = WH Vertical velocity L Sludge out

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

19. 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

20. Settling zone Outlet zone Inlet zone Sludge zone Design Criteria for Sedimentation Tanks • _______________________________ • _______________________________ • _______________________________ • _______________________________ • _______________________________ Minimal turbulence Uniform velocity No scour of settled particles Slow moving particle collection system Q/As must be small (to capture small particles) This will be one of the ways you can improve the performance of your water filtration plant.

21. 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)

22. 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

23. + + + + + + + + + + + + + + + + + + + + + + + + + + + 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

24. 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

25. 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+]

26. 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)

27. 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

28. 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 • _________ _____________ rates • __________ shears the water Differential sedimentation Turbulence

29. 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

30. Coagulation/Flocculation • Inject Coagulant in rapid mixer • Water flows from rapid mix unit into flocculation tank • gentle stirring • flocs form • Water flows from flocculation tank into sedimentation tank • make sure flocs don’t break! • flocs settle and are removed

31. 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

32. Unit Processes in Conventional Surface Water Treatment • We’ve covered • Sedimentation • Coagulation/flocculation • Coming up! • Filtration • Disinfection • Removal of Dissolved Substances

33. Conventional Surface Water Treatment Raw water Filtration Screening sludge sludge Alum Polymers Coagulation Disinfection Cl2 Flocculation Storage Sedimentation Distribution sludge

34. Filtration • Slow sand filters • Diatomaceous earth filters • Membrane filters • Rapid sand filters (Conventional Treatment)

35. Slow Sand Filtration • First filters to be used on a widespread basis • Fine sand with an effective size of 0.2 mm • Low flow rates (10 - 40 cm/hr) • Schmutzdecke (_____ ____) forms on top of the filter • causes high head loss • must be removed periodically • Used without coagulation/flocculation! filter cake

36. Diatomaceous Earth Filters • Diatomaceous earth (DE) is made of the silica skeletons of diatoms • DE is added to water and then fed to a special microscreen • The DE already on the microscreen strains particles and DE from the water • The continuous DE feed prevents the gradually thickening DE cake from developing excessive head loss • Was seriously considered for Croton Filtration Plant

37. Membrane Filters • Much like the membrane filters used to enumerate coliforms • much greater surface area • Produce very high quality water (excellent particle removal) • Clog rapidly if the influent water is not of sufficiently high quality • More expensive than sand and DE filters

38. Rapid Sand Filter (Conventional US Treatment) Depth (cm) 30 45 45 Specific Gravity 1.6 2.65 2.65 Size (mm) 0.70 0.45 - 0.55 5 - 60 Anthracite Influent Sand Gravel Drain Wash water Effluent

39. Particle Removal Mechanisms in Filters Transport Molecular diffusion Inertia Gravity Interception Attachment Straining Surface forces

40. Filter Design • Filter media • silica sand and anthracite coal • non-uniform media will stratify with _______ particles at the top • Flow rates • 2.5 - 10 m/hr • Backwash rates • set to obtain a bed porosity of 0.65 to 0.70 • typically 50 m/hr smaller

41. Backwash • Wash water is treated water! • WHY? Anthracite Only clean water should ever be on bottom of filter! Sand Influent Gravel Drain Wash water Effluent

42. Disinfection • Disinfection: operations aimed at killing or ____________ pathogenic microorganisms • Ideal disinfectant • _______________ • _______________ • _______________ • _______________ • _______________ inactivating Toxic to pathogens Not toxic to humans Fast rate of kill Residual protection Economical

43. Disinfection Options • Chlorine • chlorine gas • sodium hypochlorite (bleach) • Ozone • Irradiation with Ultraviolet light • Sonification • Electric Current • Gamma-ray irradiation Poisonous gas – risk of a leak

44. Chlorine • First large-scale chlorination was in 1908 at the Boonton Reservoir of the Jersey City Water Works in the United States • Widely used in the US • Typical dosage (1-5 mg/L) • variable, based on the chlorine demand • goal of 0.2 mg/L residual after 10 minutes • Trihalomethanes (EPA primary standard is 0.1 mg/L) Chlorine oxidizes organic matter Pathogen/carcinogen tradeoff

45. Chlorine Reactions Charges 0 +1 -2 +1 -1 Cl2 + H2O  H+ + HOCl + Cl- HOCl  H+ + OCl- • The sum of HOCl and OCl- is called the ____ ______ _______ • HOCl is the more effective disinfectant • Therefore chlorine disinfection is more effective at ________ pH • HOCl and OCl- are in equilibrium at pH 7.5 free chlorine residual low

46. EPA Pathogen Inactivation Requirements • SDWA requires 99.9% inactivation for Giardia and 99.99% inactivation of viruses • Giardia is more difficult to kill with chlorine than viruses and thus Giardia inactivation determines the CT Concentration x Time

47. EPA Credits for Giardia Inactivation Treatment type Credit Conventional Filtration 99.7% Direct Filtration 99% Disinfection f(time, conc., pH, Temp.)

48. Disinfection CT Credits To get credit for 99.9% inactivation of Giardia: Contact time (min) chlorine pH 6.5 pH 7.5 (mg/L) 2°C 10°C 2°C 10°C 0.5 300 178 430 254 1 159 94 228 134

49. NYC CT? Kensico Delaware Pipeline 21.75 km long 5.94 m diameter peak hourly flow = 33 m3/s volume =603,000 m3 5 hour residence time! Hillview 3.4 x 106 m3

50. NYC CT Problem • Hillview Reservoir is an open reservoir • Should the chlorine contact time prior to arrival at Hillview count? Giardia contamination from Upstate Reservoirs will be decreased, but recontamination at Hillview is possible