Water Treatment.

Name: Water Treatment.
Uploaded: 2017-06-30T16:59:49+00:00
Duration: PTM28S7
Description: Water Treatment.

Water Treatment

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 particles
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 Screening Filtration sludge sludge Alum Polymers Coagulation Disinfection Cl2 Flocculation Storage Sedimentation Distribution sludge

Screening Removes large solids Simple process
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 Assume a _______ sphere

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?

Floc Density and Velocity (Approximate)
Based on experimental data 0.4 mm ______ kg/m3 1030

Sedimentation Basin long rectangular basins 4-6 hour retention time
3-4 m deep max of 12 m wide max of 48 m long Settling zone Sludge zone Inlet zone Outlet zone Sludge out We can’t do this in our laboratory scale plants!

Sedimentation Basin: Critical Path
Horizontal velocity Q = flow rate Outlet zone H Inlet zone A = WH Sludge zone Vertical velocity L Sludge out (property of the particle) (property of the tank) What is Vc for this sedimentation 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?

Design Criteria for Sedimentation Tanks
Settling zone Sludge zone Inlet zone Outlet 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 Lamella systems are essentially the same as conventional sedimentation tanks, except that a series of flat plates are fitted in the tanks, inclined at about 60o to the base of the tank. The plates are spaced at about 150 mm apart. Solid particles settle on to the surface of the plates and slide down the surface. The plates are scraped when excess sludge accumulates. Effective surface area for settlement is increased by the inclined plates giving such systems a smaller footprint than conventional tanks. Several variations on this theme are being developed by water companies, including tube settlement and mechanically rotating plates. More lamella references Journal paper

Lamella Closeup w = width of lamella L 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 (or lack thereof!)
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 For small designs the ratio of shear to gravity increases and prevents gravity from transporting the solids down the lamella as a density current Lamella spacing must be large relative to floc size (flocs can be several mm in diameter) Small lamella spacing may cause shear that breaks up the floc

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

Energy Barrier Increase kinetic energy of particles
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 + 6H2O2Al(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

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

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

Flocculation tank design goals
Keep velocities high enough to keep flocs in suspension If the flow is turbulent, then average velocity should be greater than 10x terminal velocity of largest floc Keep velocities low enough to prevent shear from breaking flocs

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

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

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

Filtration Slow sand filters 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 ( cm/hr) Schmutzdecke (_____ ____) forms on top of the filter causes high head loss must be removed periodically Used without coagulation/flocculation! filter cake

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)
Specific Gravity 1.6 2.65 Depth (cm) 30 45 Size (mm) 0.70 5 - 60 Anthracite Influent Sand Gravel Drain Effluent Wash water

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

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

Backwash Wash water is treated water! WHY? Anthracite
Only clean water should ever be on bottom of filter! Sand Influent Gravel Drain Effluent 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 ____________ pathogenic microorganisms Ideal disinfectant _______________ inactivating Toxic to pathogens Not toxic to humans Fast rate of kill Residual protection Economical

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

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 Trihalomethanes (EPA primary standard is 0.08 mg/L) Chlorine oxidizes organic matter Pathogen/carcinogen tradeoff

Chlorine Reactions Cl2 + H2O  H+ + HOCl + Cl- HOCl  H+ + OCl-
Charges +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 Hypochlorous acid Hypochlorite ion free chlorine residual low

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?

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 pH 7.5 (mg/L) 2°C 10°C 2°C 10°C Inactivation is a function of _______, ____________ ______, and ___________. time concentration pH temperature

NYC CT? Kensico Delaware Pipeline volume =603,000 m3
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 chlorine can provide a protective _______ residual

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

Summary Backwash Lagoon Clearwell Filtration Flocculation Rapid Mix
Sedimentation Rapid Mix

Cryptosporidium Oocyst

Reynolds Number Check Re<<1 and therefore in Stokes Law range
Re = 1.1 x 10-6 Re<<1 and therefore in Stokes Law range

Diatomaceous Earth Clay DE

Water Treatment.

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