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 ___________ • remove __________ ___________ • inactivate __________ • *Unit process: a process that is used in similar ways in many different applications • sedimentation • filtration • ... particles dissolved chemicals pathogens
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 sludge sludge Alum Polymers Coagulation Disinfection Cl2 Flocculation Storage Sedimentation Distribution sludge
Screening • 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 NYC’s __________ 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 projected
Particle Terminal Fall Velocity (continued) Force balance (zero acceleration) We haven’t yet assumed a shape sphere Assume a _______
Drag Coefficient on a Sphere Stokes Law turbulent boundary laminar turbulent
Drag Coefficient:Equations General Equation Laminar flow Re < 1 Use the graph Transitional flow 1 < Re < 104 Fully turbulent flow Re > 104
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?
______ kg/m3 Floc Density and Velocity (Approximate) Based on experimental data for Alum-clay flocs 0.4 mm 1030
Sedimentation Basin:Critical Path Horizontal velocity Outlet zone Q = flow rate H Inlet zone Sludge zone A = WH Vertical velocity L Sludge out (property of the particle) (property of the tank)
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?
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 We can’t do this in our laboratory scale plants! Conventional Sedimentation Basin What is Vc for this sedimentation tank?
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) 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.
L b a Lamella Closeup • Region of particle-free fluid above the suspension • Suspension • Thin particle-free fluid layer beneath the downward-facing surface • Concentrated sediment w = width of lamella
Lamella Design Strategy • Angle is approximately 60° to get solids to slide down the incline • Re must be less than 2000 • Shear doesn’t 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
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)
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
+ + + + + + + + + + + + + + + + + + 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
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 smaller than 1 mm) • _________ _____________ (big particles hit smaller particles) • _______ Brownian motion Differential sedimentation 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 Velocity gradient!
How much water is cleared of particles from stationary particle’s 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
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 (rparticles) • How many particles are in a volume of water?
Volume occupied by a particle Set volume occupied by a particle equal to volume cleared
Collision Time • 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 concentration is low • Higher velocity gradients would decrease the characteristic collision time • Why not design a tiny reactor with huge velocity gradients? • SHEAR
Shear • The tangential force experienced by a fluid in a velocity gradient is proportional to the viscosity of the fluid Fluid viscosity Velocity gradient Shear
Too much shear? • Flocs can be broken by too much shear • Amazingly, we haven’t 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? • 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) 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 • Gq should be at least 20,000 where q is the hydraulic residence time in the flocculation reactor Reactor volume Flow rate
Laminar Flow Pipe Flocculation: for tiny flows! • The max value for G is approximately 50/s • These equations assume laminar flow • Laminar flow requires that the Reynolds number be less than 2000 • See if you can figure out equations for the length of the pipe
Given Gq, Q and d, Find Floc Tube Length True for laminar flow
Laminar Pipe Flow displacement r r Velocity gradient velocity
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 don’t break! • flocs settle and are removed