Water Treatment Monroe L. Weber-Shirk School of Civil
Water Treatment Monroe L. Weber-Shirk School of Civil and Environmental Engineering 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? Simple Sorting Goal: clean water Source: (contaminated) surface water
Solution: separate contaminants from water How? Where are we going? Unit processes* designed to remove ___________ particles remove __________ ___________ dissolved chemicals inactivate __________ pathogens *Unit process: a process that is used in similar ways in many different applications sedimentation filtration ... Unit Processes Designed to
Remove Particulate Matter Screening Sedimentation Coagulation/flocculation Filtration slow sand filters rapid sand filters diatomaceous earth filters membrane filters Conventional Surface Water Treatment Raw water Filtration Screening Alum Polymers
Removes large solids logs branches rags fish Simple process may incorporate a mechanized trash removal system Protects pumps and pipes in WTP Sedimentation the oldest form of water treatment uses gravity to separate particles from water often follows coagulation and flocculation occurs in NYCs __________ reservoirs
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 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 Sedimentation: Particle Terminal Fall Velocity Identify forces F ma Fd Fb W 0 Fb p particle density
Fd w water density pp g W _______ g acceleration due to gravity C D drag coefficient pr wg Fb ="________ Fd C D AP w p particle volume projected Ap particle cross sectional area
Vt 2 2 Vt particle terminal velocity W Particle Terminal Fall Velocity (continued) Force balance (zero acceleration) Fd W Fb C D AP w Vt 2 Vt 2 p ( p w ) g 2
2 p ( p w ) g C D AP w p 2 d Ap 3 2 Vt We havent yet assumed a shape 4 gd p w 3 CD
w 4 p r 3 3 Ap r 2 sphere Assume a _______ 4 gd ( r p - r w ) Vt = 3 CD rw
Drag Coefficient on a Sphere Drag Coefficient 1000 Stokes Law 100 10 1 0.1 Reynolds Number laminar turbulent
V d Re t turbulent boundary Drag Coefficient: Equations Vt General Equation Laminar flow Re < 1 Transitional flow 1 < Re < 104 Fully turbulent flow Re > 10 4
Vt d Re 4 gd p w 3 CD 24 CD Re w Vt d 2 g p w 18 Use the graph
CD 0.4 Vt gd p w 0.3 w Example Calculation of Terminal Velocity Determine the terminal settling velocity of a cryptosporidium oocyst having a diameter of 4 m and a density of 1.04 g/cm3 in water at 15C [=1.14x10-3 kg/(sm)]. w =999 kg/m3 g =9.81 m/s 2 Work in your teams.
Use mks units (meters, kilograms, seconds). Convert your answer to some reasonable set of units that you understand. Solution Reynolds? Floc Density and Velocity (Approximate) Water inlet floc w w floc density
0.4 mm 1 floc diameter (mm) 10 floc terminal velocity (m/day) Based on experimental data for Alum-clay flocs 36 - 100 m/day Horizontal velocity Q = flow rate A = WH
Vertical velocity Vt d 2 g p w 18 Inlet zone Q Vh A Vh Sludge out Vt
Outlet zone Sedimentation Basin: Critical Path H Sludge zone L (property of the particle) Vc terminal velocity that just barely gets captured (property of the tank) Sedimentation Basin: Importance of Tank Surface Area
Time in tank Q W H HQ Q Q Vc = = = = q " LW As residence time WHL volume of tank A s top surface area of tank Vh
H Vc L Want a _____ small Vc, ______ large As, _______ small H, _______ large . Suppose water were flowing up through a sedimentation tank. What Q would be the velocity of a particle that is just barely removed? Vc = A s Conventional Sedimentation Basin What is Vc for this sedimentation tank? Outlet zone
Settling zone Inlet zone long rectangular basins 4-6 hour retention time 3-4 m deep max of 12 m wide max of 48 m long Sludge zone Sludge out
H 3 m 24 hr Vc 18 m / day 4 hr day We cant do this in our laboratory scale Outlet zone Inlet zone Design Criteria for Sedimentation Tanks Settling zone Sludge zone
_______________________________ 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) This will be one of the ways you can improve the performance of your water treatment plant. 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 Lamella Design needs improvement! Need method to transport particles to bottom of tank. Lamella Closeup Q Vc = As Qlamella vc
wL cos wb sin w = width of lamella L vc b Vlamella L cos sin b Region of particle-free fluid above the suspension
Suspension Thin particle-free fluid layer beneath the downward-facing surface Concentrated sediment Lamella Design Strategy Angle is approximately 60 to get solids to slide down the incline Re must be less than 2000 Shear doesnt causing resuspension if flow is laminar Lamella spacing must be large relative to floc size (flocs can be several mm in diameter) Upflow velocity (Q/As) can be as large as 100 m/day Qlamella
N lamella Q N lamella Ltan k L cos b sin Vlamella b Re Sedimentation of Small Particles? How could we increase the sedimentation rate of small particles? Increase d (stick particles together)
Increase g (centrifuge) d g p w Increase density difference Vt (dissolved air flotation) 18 2 Decrease viscosity (increase temperature) Particle/particle interactions Electrostatic repulsion In most surface waters, colloidal surfaces are negatively charged like charges repel __________________ stable suspension van der Waals force
an attractive force decays more rapidly with distance than the electrostatic force is a stronger force at very close distances Electrostatic Layer of counter ions +++ + + +++ + van der Waals 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 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 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 + 6H2O2Al(OH)3 + 6H+ + 3SO4-2 pH = -log[H+]
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) 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 In-line static mixers
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 ________ ________ (effective for particles Brownian motion smaller than 1 m) _________ Differential _____________ sedimentation (big particles hit smaller particles) _______ Shear Mechanical Flocculation Shear provided by turbulence
created by gentle stirring Turbulence also keeps large flocs from settling so they can grow even larger! Retention time of 10 - 30 minutes Advantage is that amount of shear can be varied independent of flow rate Disadvantage is the tanks are far from plug flow Hydraulic Flocculators Types Horizontal baffle Vertical baffle Pipe flow Questions for design How long must the suspension be in the reactor
How should the geometry of the reactor be determined? Velocity Gradient Flocculation With fixed frame of reference With red particle as frame of reference Increase Velocity Gradient du G dy Velocity gradient! How much water is cleared of particles from stationary particles perspective? Volume cleared is proportional to projected
area of stationary particle Volume cleared is proportional to time Volume cleared is proportional to the velocity gradient The velocity of the water flowing past the particle increases with the diameter of the particle 2 d t G d 3 cleared d Gt
How much volume must be cleared before a collision occurs? What is the average volume of water occupied by a particle? Given C mg/L of particles in suspension Need to know particle diameter (d) And density (particles) How many particles are in a volume of water? N C particles number 3 volume particles d 6 Volume occupied by a particle occupied
particles d 3 6 C particles cleared d 3Gt Set volume occupied by a particle equal to volume cleared 3 particles d 6 d 3Gt C particles tcollision particles
G C particles Collision Time tcollision particles G C particles A measure of how long the particles must be in the velocity gradient to double in size A series of collisions must occur for particles to grow large enough to be easily removed by sedimentation Flocculation Reactor Design Critical design is when particle
particles tcollision concentration is low G C particles Higher velocity gradients would decrease the characteristic collision time Why not design a tiny reactor with huge velocity gradients? SHEAR du dy Shear The tangential force experienced by a fluid in a velocity gradient is proportional to the viscosity of the fluid
G Shear N m 2 Fluid viscosity N s m 2 Velocity gradient 1 s Too much shear? du
dy Flocs can be broken by too much shear Amazingly, we havent been able to find good information on the shear level that causes aluminum-clay flocs to breakup fine grained cohesive sediments within estuarine waters were shown to produce smaller flocs when the shear exceeded 0.35 Pa (equivalent to a G of approximately 400/s) Reaction time? tcollision particles G C particles
Low particle concentrations require longer flocculation Goal is to get flocculation to work when turbidity is as low as 10 NTU (equivalent to approximately 20 mg/L of kaolin clay) tcollision g 2650 L 1 g 400 0.020 s
L 331 seconds Reaction time is more complex Aluminum hydroxide polymers significantly increase the number of particles and the probability of collision (and hence decrease tcollision) So for now we have to go with some empirical guidelines G should be at least 20,000 where is the hydraulic residence time in the flocculation reactor Q Reactor volume Flow rate
Laminar Flow Pipe Flocculation: G for tiny flows! max 32Q 3 d The max value for G is approximately 64Q G 3 d 3 50/s These equations assume laminar flow 64Q d 3
3 G Laminar flow requires that the Reynolds number be less than 2000Vd 4Q Re D See if you can figure out equations for the length of the pipe Gmax 1.5G Given G, Q and d, Find Floc Tube Length 64Q G
3 d 3 True for laminar flow d2 L d 2L 4 Q Q 4Q d 2 L 64Q 16 L G
3 4Q 3 d 3d 3dG L 16 Laminar Pipe Flow displacement r r velocity Velocity gradient
Coagulation/Flocculation Inject Coagulant in rapid mixer Water flows from rapid mix unit into flocculation reactor Water flows from flocculation reactor into sedimentation tank make sure flocs dont break! flocs settle and are removed Jar Test Mimics the rapid mix, flocculation, sedimentation treatment steps in a beaker Allows operator to test the effect of different coagulant dosages or of different coagulants Low tech water bottle test Unit Processes in Conventional Surface Water Treatment
Weve covered Sedimentation Coagulation/flocculation Coming up! Filtration Disinfection Removal of Dissolved Substances Conventional Surface Water Treatment Raw water Filtration Screening Alum Polymers sludge
Diatomaceous earth filters Membrane filters Rapid sand filters (Conventional Treatment) 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 filter cake of the filter causes high head loss must be removed periodically Used without coagulation/flocculation! 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 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 Rapid Sand Filter (Conventional US Treatment) Size (mm) Anthracite Influent Drain Effluent Sand Gravel 0.70
Specific Depth Gravity (cm) 1.6 30 0.45 - 0.55 2.65 45 2.65 45 5 - 60 Wash water Particle Removal Mechanisms in
Filters Transport Molecular diffusion Inertia Gravity Interception Attachment Straining Surface forces Filter Design Filter media silica sand and anthracite coal non-uniform media will stratify with _______ smaller 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 Backwash Anthracite Influent Drain Effluent Sand Wash water is treated water!
WHY? Only clean water should ever be on bottom of filter! Gravel Wash water Ways to Improve Filtration Filter to waste Extended Terminal Sub-fluidization Wash Alum feed directly to filter? Potato starch? Disinfection Disinfection: operations aimed at killing or inactivating pathogenic microorganisms ____________ Ideal disinfectant
_______________ Toxic to pathogens _______________ Not toxic to humans _______________ Fast rate of kill _______________ Residual protection _______________ Economical Disinfection Options Chlorine chlorine gas Poisonous gas risk of a leak sodium hypochlorite (bleach) Ozone Irradiation with Ultraviolet light Sonification
Electric Current Gamma-ray irradiation Chlorine First large-scale chlorination was in 1908 at the Boonton Reservoir of the Jersey City Water Works in the United States Chlorine Widely used in the US oxidizes organic Typical dosage (1-5 mg/L) matter variable, based on the chlorine demand goal of 0.2 mg/L residual Trihalomethanes (EPA primary standard is 0.08 Pathogen/carcinogen tradeoff mg/L)
Chlorine Reactions Charges 0 +1 -2 +1 -1 Cl2 + H2O H+ + HOCl + ClHypochlorous acid HOCl H+ + OCl- Hypochlorite ion The sum of HOCl and OCl- is called the free chlorine ____ ______ residual _______ HOCl is the more effective disinfectant Therefore chlorine disinfection is more effective at ________ low pH
HOCl and OCl- are in equilibrium at pH 7.5 EPA Pathogen Inactivation Requirements Safe Drinking Water Act 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 Enumerating Giardia is difficult, time-consuming and costly. How would you ensure that water treatment plants meet this criteria? Where are Giardia removed/inactivated? EPA Credits for Giardia Inactivation
Treatment type Credit Conventional Filtration 99.7% Direct Filtration* 99% Disinfection f(time, conc., pH, Temp.) * No sedimentation tanks Disinfection CT Credits To get credit for 99.9% inactivation of Giardia: Contact time (min) chlorine pH 6.5 pH 7.5 (mg/L) 2C
10C 2C 10C 0.5 300 178 430 254 1 159 94 228 134 Inactivation is a function of _______, time ____________ concentration pH temperature ______,
and ___________. 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 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 Ozone Widely used in Europe O3 is chemically unstable Must be produced on site More expensive than chlorine (2 - 3 times) Typical dosages range from 1 to 5 mg/L Often followed by chlorination so that the residual chlorine can provide a protective _______ Removal of Dissolved Substances (1) Aeration (before filtration)
oxidizes iron or manganese in groundwater oxidized forms are less soluble and thus precipitate out of solution removes hydrogen sulfide (H2S) Softening (before filtration) used to remove Ca+2 and Mg+2 usually not necessary with surface waters Removal of Dissolved Substances (2) Activated Carbon (between filtration and disinfection) extremely adsorbent used to remove organic contaminants spent activated carbon can be regenerated with superheated steam Reverse Osmosis semi-permeable membrane allows water molecules to pass,
but not the larger ions and molecules primarily used for desalination also removes organic materials, bacteria, viruses, and protozoa Conventional Surface Water Treatment Raw water Filtration Screening Alum Polymers sludge Coagulation sludge Cl2
Disinfection Flocculation Storage Sedimentation Distribution sludge Summary Cryptosporidium Oocyst Vt p 1040 kg/m 3
w 999 kg/m g 9.81 m/s 2 d 4x10 6 m 3 Vt 4x10 6 d 2 g p w 18 2 m 9.81 m/s 2 1040 kg/m 3 999 kg/m 3
3 kg 18 1.14x10 s m Vt 3.14 x10 7 m/s Vt 2.7 cm/day Reynolds Number Check Vd Re 7 6
3 3.14 x 10 m/s 4 x 10 m 999kg/m Re 3 kg 1.14x10
s m Re = 1.1 x 10-6 Re<<1 and therefore in Stokes Law range Diatomaceous Earth Clay DE Qlamella Q N lamella N lamella vlamella
L vc cos sin b Q 1 vc wbN lamella L cos sin b Ltan k L cos b sin Qb sin L vc cos sin
b wbLtan k wbL cos b Qlamella vc wL cos wb sin vlamella wb L cos Q sin sin vc wLtan k wL cos vc wLtan k
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