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) surface water Solution: separate contaminants from waterHow?
4 Where are we going? Unit processes* designed to particles remove ___________remove __________ ___________inactivate __________*Unit process: a process that is used in similar ways in many different applicationssedimentationfiltration...particlesdissolved chemicalspathogens
5 Unit Processes Designed to Remove Particulate Matter ScreeningSedimentationCoagulation/flocculationFiltrationslow sand filtersrapid sand filtersdiatomaceous earth filtersmembrane filters
6 Conventional Surface Water Treatment Raw waterScreeningFiltrationsludgesludgeAlumPolymersCoagulationDisinfectionCl2FlocculationStorageSedimentationDistributionsludge
7 Screening Removes large solids Simple process logsbranchesragsfishSimple processmay incorporate a mechanized trash removal systemProtects pumps and pipes in WTP
8 Sedimentation the oldest form of water treatment uses gravity to separate particles from wateroften follows coagulation and flocculationoccurs in NYC’s __________reservoirs
9 Sedimentation: Effect of the particle concentration Dilute suspensionsParticles act independentlyConcentrated suspensionsParticle-particle interactions are significantParticles may collide and stick together (form flocs)Particle flocs may settle more quicklyParticle-particle forces may prevent further consolidation
10 How fast do particles fall in dilute suspensions? What are the important parameters?Initial conditionsAfter falling for some time...What are the important forces?___________________GravityFluid drag
11 Sedimentation: Particle Terminal Fall Velocity Identify forcesprojected
12 Particle Terminal Fall Velocity (continued) Force balance (zero acceleration)We haven’t yet assumed a shapeAssume a _______sphere
13 Drag Coefficient on a Sphere Stokes Lawturbulent boundarylaminarturbulent
14 Drag Coefficient: Equations General EquationLaminar flow R < 1Transitional flow 1 < R < 104Fully 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.SolutionReynolds?
16 Sedimentation Basin long rectangular basins 4-6 hour retention time 3-4 m deepmax of 12 m widemax of 48 m longSettling zoneSludge zoneInlet zoneOutlet zoneSludge outWe can’t do this in our laboratory scale plants!
17 Sedimentation Basin: Critical Path Horizontal velocityQ = flow rateOutlet zoneHInlet zoneA = WHSludge zoneVertical velocityLSludge outWhat is Vc for this sedimentation tank?
18 Sedimentation Basin: Importance of Tank Surface Area Time in tankWHLWant a _____ Vc, ______ As, _______ H, _______ q.smalllargesmalllargeSuppose water were flowing up through a sedimentation tank. What would be the velocity of a particle that is just barely removed?
19 LamellaSedimentation tanks are commonly divided into layers of shallow tanks (lamella)The flow rate can be increased while still obtaining excellent particle removalLamella decrease distance particle has to fall in order to be removed
20 Design Criteria for Sedimentation Tanks Settling zoneSludge zoneInlet zoneOutlet zoneDesign Criteria for Sedimentation Tanks_______________________________Minimal turbulence (inlet baffles)Uniform velocity (small dimensions normal to velocity)No scour of settled particlesSlow moving particle collection systemQ/As must be small (to capture small particles)This will be one of the ways you can improve the performance of your water treatment 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 repulsionIn most surface waters, colloidal surfaces are negatively chargedlike charges repel __________________van der Waals forcean attractive forcedecays more rapidly with distance than the electrostatic forceis a stronger force at very close distancesstable suspension
23 Energy Barrier Increase kinetic energy of particles ElectrostaticEnergy BarrierIncrease kinetic energy of particlesincrease temperaturestirDecrease magnitude of energy barrierchange the charge of the particlesintroduce positively charged particlesLayer of counter ions+++van der Waals
24 CoagulationCoagulation is a physical-chemical process whereby particles are destabilizedSeveral mechanismsadsorption of cations onto negatively charged particlesdecrease the thickness of the layer of counter ionssweep coagulationinterparticle 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 usedThe chemistry is complex with many possible species formed such as AlOH+2, Al(OH)2+, and Al7(OH)17+4The primary reaction produces Al(OH)3Al2(SO4)3 + 6H2O2Al(OH)3 + 6H+ + 3SO4-2pH = -log[H+]
26 Coagulation Chemistry Aluminum hydroxide [Al(OH)3] forms amorphous, gelatinous flocs that are heavier than waterThe flocs look like snow in waterThese flocs entrap particles as the flocs settle (sweep coagulation)
27 Coagulant introduction with rapid mixing The coagulant must be mixed with the waterRetention times in the mixing zone are typically between 1 and 10 secondsTypes of rapid mix unitspumpshydraulic jumpsflow-through basins with many bafflesIn-line blenders
28 FlocculationCoagulation has destabilized the particles by reducing the energy barrierNow we want to get the particles to collideWe need relative motion between particlesBrownian motion is too slow_________ _____________ rates__________ shears the waterDifferential sedimentationTurbulence
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 minutes
30 Coagulation/Flocculation Inject Coagulant in rapid mixerWater flows from rapid mix unit into flocculation tankgentle stirringflocs formWater flows from flocculation tank into sedimentation tankmake sure flocs don’t break!flocs settle and are removed
31 Jar TestMimics the rapid mix, coagulation, flocculation, sedimentation treatment steps in a beakerAllows operator to test the effect of different coagulant dosages or of different coagulants
32 Unit Processes in Conventional Surface Water Treatment We’ve coveredSedimentationCoagulation/flocculationComing up!FiltrationDisinfectionRemoval of Dissolved Substances
33 Conventional Surface Water Treatment Raw waterScreeningFiltrationsludgesludgeAlumPolymersCoagulationDisinfectionCl2FlocculationStorageSedimentationDistributionsludge
35 Slow Sand Filtration First filters to be used on a widespread basis Fine sand with an effective size of 0.2 mmLow flow rates ( cm/hr)Schmutzdecke (_____ ____) forms on top of the filtercauses high head lossmust be removed periodicallyUsed without coagulation/flocculation!filter cake
36 Diatomaceous Earth Filters Diatomaceous earth (DE) is made of the silica skeletons of diatomsDE is added to water and then fed to a special microscreenThe DE already on the microscreen strains particles and DE from the waterThe continuous DE feed prevents the gradually thickening DE cake from developing excessive head lossWas seriously considered for Croton Filtration Plant
37 Membrane FiltersMuch like the membrane filters used to enumerate coliformsmuch greater surface areaProduce very high quality water (excellent particle removal)Clog rapidly if the influent water is not of sufficiently high qualityMore expensive than sand and DE filters
38 Rapid Sand Filter (Conventional US Treatment) SpecificGravity1.62.65Depth(cm)3045Size(mm)0.705 - 60AnthraciteInfluentSandGravelDrainEffluentWash water
39 Particle Removal Mechanisms in Filters TransportMolecular diffusionInertiaGravityInterceptionAttachmentStrainingSurface forces
40 Filter Design Filter media Flow rates smaller Backwash rates silica sand and anthracite coalnon-uniform media will stratify with _______ particles at the topFlow ratesm/hrBackwash ratesset to obtain a bed porosity of 0.65 to 0.70typically 50 m/hrsmaller
41 Backwash Wash water is treated water! WHY? Anthracite Only clean water should ever be on bottom of filter!SandInfluentGravelDrainEffluentWash water
42 DisinfectionDisinfection: operations aimed at killing or ____________ pathogenic microorganismsIdeal disinfectant_______________inactivatingToxic to pathogensNot toxic to humansFast rate of killResidual protectionEconomical
43 Disinfection Options Chlorine Ozone Irradiation with Ultraviolet light chlorine gassodium hypochlorite (bleach)OzoneIrradiation with Ultraviolet lightSonificationElectric CurrentGamma-ray irradiationPoisonous gas – risk of a leak
44 ChlorineFirst large-scale chlorination was in 1908 at the Boonton Reservoir of the Jersey City Water Works in the United StatesWidely used in the USTypical dosage (1-5 mg/L)variable, based on the chlorine demandgoal of 0.2 mg/L residualTrihalomethanes (EPA primary standard is 0.08 mg/L)Chlorine oxidizes organic matterPathogen/carcinogen tradeoff
45 Chlorine Reactions Cl2 + H2O H+ + HOCl + Cl- HOCl H+ + OCl- Charges+1-2+1-1Cl2 + H2O H+ + HOCl + Cl-HOCl H+ + OCl-The sum of HOCl and OCl- is called the ____ ______ _______HOCl is the more effective disinfectantTherefore chlorine disinfection is more effective at ________ pHHOCl and OCl- are in equilibrium at pH 7.5Hypochlorous acidHypochlorite ionfree chlorine residuallow
46 EPA Pathogen Inactivation Requirements Safe Drinking Water ActSDWA requires 99.9% inactivation for Giardia and 99.99% inactivation of virusesGiardia is more difficult to kill with chlorine than viruses and thus Giardia inactivation determines the CTConcentration x TimeEnumerating Giardia is difficult, time-consuming and costly.How would you ensure that water treatment plants meet this criteria?
47 EPA Credits for Giardia Inactivation Treatment type CreditConventional Filtration 99.7%Direct Filtration* 99%Disinfection f(time, conc., pH, Temp.)* No sedimentation tanks
48 Disinfection CT Credits To get credit for 99.9% inactivation of Giardia:Contact time (min)chlorine pH pH 7.5(mg/L) 2°C 10°C 2°C 10°CInactivation is a function of _______, __________________, and ___________.timeconcentrationpHtemperature
49 NYC CT? Kensico Delaware Pipeline volume =603,000 m3 21.75 km long5.94 m diameterpeak hourly flow= 33 m3/svolume =603,000 m35 hour residence time!Hillview3.4 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, butrecontamination at Hillview is possible
51 Ozone Widely used in Europe O3 is chemically unstable Must be produced on siteMore expensive than chlorine (2 - 3 times)Typical dosages range from 1 to 5 mg/LOften followed by chlorination so that the chlorine can provide a protective _______residual
52 Removal of Dissolved Substances (1) Aeration (before filtration)oxidizes iron or manganese in groundwateroxidized forms are less soluble and thus precipitate out of solutionremoves hydrogen sulfide (H2S)Softening (before filtration)used to remove Ca+2 and Mg+2usually not necessary with surface waters
53 Removal of Dissolved Substances (2) Activated Carbon (between filtration and disinfection)extremely adsorbentused to remove organic contaminantsspent activated carbon can be regenerated with superheated steamReverse Osmosissemi-permeable membrane allows water molecules to pass, but not the larger ions and moleculesprimarily used for desalinationalso removes organic materials, bacteria, viruses, and protozoa
54 Conventional Surface Water Treatment Raw waterScreeningFiltrationsludgesludgeAlumPolymersCoagulationDisinfectionCl2FlocculationStorageSedimentationDistributionsludge