03 Conventional Water Treatment

transcript

Monroe L. Weber-Shirk

School of Civil and Environmental Engineering

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 waterSource: (contaminated) surface waterSolution: separate contaminants from waterHow?

Where are we going?

particlesdissolved chemicals

pathogens

Unit processes* designed to remove ___________ remove __________ ___________ inactivate __________

*Unit process: a process that is used in similar ways in many different applications sedimentation filtration ...

Unit Processes Designed to Remove Particulate Matter

ScreeningSedimentationCoagulation/flocculationFiltration

slow sand filtersrapid sand filtersdiatomaceous earth filtersmembrane filters

Conventional Surface Water Treatment

Screening

Coagulation

Flocculation

Sedimentation

Filtration

Disinfection

Storage

Distribution

Raw water

AlumPolymers Cl2

sludge

Screening

Removes large solids logsbranches rags fish

Simple processmay incorporate a mechanized trash

removal system Protects pumps and pipes in WTP

Sedimentation

the oldest form of water treatmentuses gravity to separate particles from wateroften follows coagulation and flocculationoccurs in NYC’s __________reservoirs

Sedimentation: Effect of the particle concentration

Dilute suspensionsParticles act independently

Concentrated suspensionsParticle-particle interactions are significantParticles may collide and stick together

(form flocs)Particle flocs may settle more quicklyParticle-particle forces may prevent further

consolidation

How fast do particles fall in dilute suspensions?

GravityFluid drag

What are the important parameters?Initial conditionsAfter falling for some time...

What are the important forces?___________________

projected

Sedimentation:Particle Terminal Fall Velocity

maF 0 WFF bd

wPDdVACF W

p wgr" velocity terminalparticletcoefficien drag

gravity todueon acceleratidensitywater

density particle

area sectional cross particle

volumeparticle

_______W

________bF =

Identify forces

Particle Terminal Fall Velocity (continued)

bd FWF

gVAC wppt

wPD )(2

4 2 ( )4 3

r rr-=

Force balance (zero acceleration)

4 rp 2rAp

We haven’t yet assumed a shape

Assume a _______sphere

Reynolds Number

tDrag Coefficient on a Sphere

laminar

Re tV d

turbulentturbulent boundary

Stokes Law

Drag Coefficient:Equations

Laminar flow Re < 1

Transitional flow 1 < Re < 104

Fully turbulent flow Re > 104

24ReDC

Re tV d

4 General Equation

Use the graph

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 15°C [=1.14x10-3 kg/(s•m)].

Reynolds?

999 kg/m

9.81 m/swρ

Work in your teams.Use mks units (meters, kilograms, seconds).Convert your answer to some reasonable set of units that you understand.

Solution

0.1 1 10

floc diameter (mm)

floc densityVt (m/day)

Floc Density and Velocity (Approximate)

floc w

Water inlet

36 - 100 m/dayWater inlet

36 - 100 m/day

0.4 mm

______ kg/m3 floc 1030

Based on experimental data for Alum-clay flocs

Sedimentation Basin:Critical Path

Horizontal velocity

Vertical velocityL

captured getsbarely just that velocity terminal cV

Sludge zoneInle

ne Outle

Sludge out

A = WH

Q = flow rate

(property of the particle)

(property of the tank)

Sedimentation Basin:Importance of Tank Surface Area

Suppose water were flowing up through a sedimentation tank. What would be the velocity of a particle that is just barely removed?

H HQ Q QVLW Aq

= = = ="

residence timevolume of tank

A top surface area of tankWHL

Want a _____ Vc, ______ As, _______ H, _______ . small large

Time in tank

small large

Conventional Sedimentation Basin

Settling zone

Sludge zoneInle

ne Outle

Sludge out

long rectangular basins

4-6 hour retention time

3-4 m deepmax of 12 m

widemax of 48 m

We can’t do this in our laboratory scale plants!

What is Vc for this sedimentation tank?

3 24 18 /4c

H m hrV m dayhr day

Settling zone

Sludge zoneInle

ne Outle

neDesign 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.

Lamella Closeup

Region of particle-free fluid above the suspension

SuspensionThin particle-free fluid layer beneath

the downward-facing surfaceConcentrated sediment

cos sinlamella

w = width of lamella

cos sin

lamellac

Lamella Design Strategy

Angle is approximately 60° to get solids to slide down the incline

Re must be less than 2000Shear doesn’t causing resuspension

if flow is laminarLamella 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

lamellalamella

Re lamellaV b

tan cossin

klamella

Sedimentation of Small Particles?

How could we increase the sedimentation rate of small particles?

Increase d (stick particles together)

Increase g (centrifuge)

Decrease viscosity (increase temperature)

Increase density difference(dissolved air flotation)

Particle/particle interactions

Electrostatic repulsion In most surface waters, colloidal surfaces are

negatively charged like charges repel __________________

van der Waals force an attractive forcedecays 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

increase temperaturestir

Decrease magnitude of energy barrierchange the charge of the particlesintroduce positively charged

particles

+++++++++

Layer of counter ions

van der Waals

Electrostatic

Coagulation

Coagulation is a physical-chemical process whereby particles are destabilized

Several mechanismsadsorption of cations onto negatively charged

particlesdecrease the thickness of the layer of counter

ionssweep coagulationinterparticle 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 usedThe 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

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 waterThese flocs entrap particles as the flocs

settle (sweep coagulation)

Coagulant introduction with rapid mixing

The coagulant must be mixed with the waterRetention times in the mixing zone are typically

between 1 and 10 seconds Types of rapid mix units

pumpshydraulic 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 collideWe need relative motion between particles

________ ________ (effective for particles smaller than 1 m)

_________ _____________ (big particles hit smaller particles)

_______

Differential sedimentation

Brownian motion

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 minutesAdvantage is that amount of

shear can be varied independent of flow rate

Disadvantage is the tanks are far from plug flow

Hydraulic Flocculators

TypesHorizontal baffleVertical bafflePipe flow

Questions for designHow long must the suspension be in the “reactor”How should the geometry of the reactor be

determined?

Velocity Gradient Flocculation

With red particle as frame of reference

With fixed 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 timeVolume cleared is proportional to the

velocity gradientThe velocity of the water flowing past the

particle increases with the diameter of the particle

3cleared 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?3

particles

numbervolume

Volume occupied by a particle

6particles

occupiedparticles

Set volume occupied by a particle equal to volume cleared

3cleared d Gt

36particles

particles

particlescollision

particles

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

particlescollision

particles

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?

particlescollision

particles

The tangential force experienced by a fluid in a velocity gradient is proportional to the viscosity of the fluid

Fluid viscosity

Velocity gradient

Too much shear?

Flocs can be broken by too much shearAmazingly, 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)

particlescollision

particles

1400 0.020collision

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

Reactor volumeFlow rate

Laminar Flow Pipe Flocculation: for tiny flows!

The max value for G is approximately 50/s

These equations assume laminar flowLaminar flow requires that the

Reynolds number be less than 2000

See if you can figure out equations for the length of the pipe

4Re Vd QD

max 1.5G G

Given G, Q and d, Find Floc Tube Length

64 164 3 3d L Q LGQ d d

316dGL

True for laminar flow

Laminar Pipe Flow

displacement

velocityVelocity gradient

Coagulation/Flocculation

Inject Coagulant in rapid mixerWater flows from rapid mix unit into

flocculation reactorWater flows from flocculation reactor into

sedimentation tankmake sure flocs don’t 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

We’ve coveredSedimentationCoagulation/flocculation

Coming up!FiltrationDisinfectionRemoval of Dissolved Substances

Screening

Coagulation

Flocculation

Sedimentation

Filtration

Disinfection

Storage

Distribution

Raw water

AlumPolymers Cl2

sludge

Filtration

Slow sand filtersDiatomaceous earth filtersMembrane filtersRapid sand filters (Conventional Treatment)

Slow Sand Filtration

First filters to be used on a widespread basisFine sand with an effective size of 0.2 mmLow flow rates (10 - 40 cm/hr)Schmutzdecke (_____ ____) forms on top

of the filtercauses high head lossmust 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 coliformsmuch 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)

Gravel

Influent

DrainEffluent Wash water

Anthracite

Size(mm)0.70

0.45 - 0.55

5 - 60

SpecificGravity

Depth(cm)30

Particle Removal Mechanisms in Filters

Transport

Attachment

Molecular diffusionInertiaGravityInterception

StrainingSurface forces

Filter Design

Filter media silica sand and anthracite coalnon-uniform media will stratify with _______ particles

at the top Flow rates

2.5 - 10 m/hrBackwash rates

set to obtain a bed porosity of 0.65 to 0.70 typically 50 m/hr

smaller

Gravel

Influent

DrainEffluent Wash water

Anthracite

Backwash

Wash water is treated water!

WHY?Only clean water should ever be on bottom of filter!

Ways to Improve Filtration

Filter to wasteExtended Terminal Sub-fluidization WashAlum feed directly to filter?Potato starch?

Disinfection

Disinfection: operations aimed at killing or ____________ pathogenic microorganisms

Ideal disinfectant_______________ _____________________________________________ _______________

inactivating

Toxic to pathogensNot toxic to humans Fast rate of kill Residual protectionEconomical

Disinfection Options

Chlorine chlorine gas sodium hypochlorite (bleach)

Ozone Irradiation with Ultraviolet light SonificationElectric CurrentGamma-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 USTypical dosage (1-5 mg/L)

variable, based on the chlorine demandgoal of 0.2 mg/L residual

Trihalomethanes (EPA primary standard is 0.08 mg/L) Pathogen/carcinogen tradeoff

Chlorine oxidizes organic matter

Chlorine Reactions

Cl2 + 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 7.5

free chlorine residual

+1 -2 +10Charges -1

Hypochlorous acid Hypochlorite ion

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 TimeEnumerating Giardia is difficult, time-consuming and costly.How would you ensure that water treatment plants meet this criteria?Where are Giardia removed/inactivated?

Safe Drinking Water Act

EPA Credits for Giardia Inactivation

Treatment type CreditConventional Filtration 99.7%Direct Filtration* 99%Disinfection f(time, conc., pH,

Temp.)

* No sedimentation tanks

Disinfection CT Credits

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 2541 159 94 228 134

To get credit for 99.9% inactivation of Giardia:

Inactivation is a function of _______, __________________, and ___________.

concentrationtimepH temperature

NYC CT?

Kensico

Hillview

Delaware Pipeline21.75 km long5.94 m diameterpeak hourly flow = 33 m3/s

3.4 x 106 m3

volume =603,000 m3

5 hour residence time!

NYC CT Problem

Hillview Reservoir is an open reservoirShould the chlorine contact time prior to arrival at

Hillview count?

Giardia contamination from Upstate Reservoirs will be decreased, butrecontamination at Hillview is possible

Widely used in EuropeO3 is chemically unstableMust 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

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+2

usually not necessary with surface waters

Removal of Dissolved Substances (2)

Activated Carbon (between filtration and disinfection) extremely adsorbentused to remove organic contaminants spent activated carbon can be regenerated with superheated

steamReverse 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

Screening

Coagulation

Flocculation

Sedimentation

Filtration

Disinfection

Storage

Distribution

Raw water

AlumPolymers Cl2

sludge

Summary

Cryptosporidium Oocyst

kg1.14x1018

kg/m 999kg/m 1040m/s 189.m 4x10 3

m 4x10

m/s 189.

kg/m 999

kg/m 1040

m/s1014.3 7 xVt

cm/day 7.2 tV

Reynolds Number Check

Re<<1 and therefore in Stokes Law range

3.14 10 m/s 4 10 m 999kg/mRe kg1.14x10

Re = 1.1 x 10Re = 1.1 x 10-6-6

Diatomaceous Earth

lamella lamellalamella

QQ v wbN

cos sinlamella cLv vb

tan cossin

klamella

sincos sin

QbLvb wbL wbL

cossin

sincosc k

v wL wL

cos sinlamella

cos cos sinc

LwL wLb

cos sinc

lamella

Qv LwbNb

03 Conventional Water Treatment

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