MicroFiltration and UltraFiltration Lecture Notes

8/12/2019 MicroFiltration and UltraFiltration Lecture Notes

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Microfiltration (MF) ,

Ultrafiltration ( UF) :theoretical basis , examples of application to water

and wastewater treatment


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Reminder of the last course

The 3 fractions in water

- The different kinds of membrane

technology

- The notion of Permeability and

resistance

- Energy consumption for membrane

operation membrane processes .

-


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The 3 fractions in natural water and

sea water

Part icles( size larger than 1 micron)

Col lo ids ( size smaller than 1 micron ,

typically between 1nm an one micron) Solutes( ions and organic molecules ) :

size typically between 1 Angstrom ( 0.1nm) and 1nm.ween Relation between

Molecular Weight of molecules (MWDalton ) and their size


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REVERSE OSMOSIS

NANOFILTRATION

ULTRAFILTRATION

MICROFILTRATION

CONVENTIONAL FILTRATION

Sands

Algae and protozoans

Bacteria

Colloids

Humic acids

Metal ions

Pesticides

Dissolved salts

Sugars

Molecularweight

Viruses

Angstrm

MICRON

IONSIONS MOLECULESMOLECULES MACRO MOLECULESMACRO MOLECULES MICRO PARTICLESMICRO PARTICLES MACRO PARTICLESMACRO PARTICLES

VISIBLE TO NAKED EYEVISIBLE TO NAKED EYEOPTICAL MICROSCOPEOPTICAL MICROSCOPESCANNING ELECTRON MICROSCOPESCANNING ELECTRON MICROSCOPE

Note : 1 Angstrm = 10 -10 meter = 10-4 micron

Range of Applications


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Membrane characteristics : nomenclature

and methods of measurement.

pore size: pore size distribution and average pore size

MWCO( Molecular Weight Cut-off) : the same as poresize but in terms of Molecular Weight ( MW)

permeability, Pure water Permeability

porosity Chemical characteristics : pH range for normal

use ,chlorine and oxidants resistance , solventresistance, etc

Physical characteristics: temperature of use ,

mechanical strength , hydrophobic or hydrophilic( contact angle) , etc


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Permeability and Resistance

Membrane permeability defined by the so called

DarcyLaw: B =.(Q/A).Z)/P

Hydraulic Resistance R = Z/B = P /.(Q/A)

= viscosity ( 10- 3 ) for water at 20C

Q = flowrate (m3/s )

A = filtration Area (m2 )

P = Pressure drop (pinpout ) (Pascal)

Z is the thickness of the membrane (m)

B is given in m2


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Membrane hydraulic resistance

Hydraulic resistance Rm= Z/B = P /.(Q/A)

Rm is given in m-1


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Resistance during filtration

Rm : initial resistance of the clean

membrane

During the filtration , the resistance

increases due to the fouling phenomena :- internal clogging of the pores ( Rint )

- formation of a deposit (cake) : Rc

Total resistance Rt = Rm + Rint+ Rc


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Energy consumption for membrane

operation

In dead end , only the energy of filtration

IF cross flow , energy for filtration + energy

for cross flow ( generally much more than

the energy for filtration). If submerged membranes : energy of

filtration + energy for bubbling


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Dead-end: MF/UF Cross flow: MF, UF


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Microfiltration

The typical concepts in MF.

The concept of Critical Flux


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The concept of critical flux

Its consequences on membrane operation


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The concept of Critical flux

For defined hydrodynamic conditions ( wall shear

stress) it is possible to define a critical flux : this isthe flux below which the particle is not deposited at

the membrane surface.

The critical flux varies with the particle size and

has a minimum value for particles which size is inthe range : 0.1-1 micron.


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Axial

velocity

profile

Membrane

Permeation

drag

van der Waals

attractionSedimentation

Axial drag

Charge

repulsion Inertial lift

Brownian

Diffusion

Drag torque

Shear induced

migration

Forces Affecting Particle Transport

During Membrane Filtration ( from

Prof. Chung Hak Lee)


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Fa c tor s a ffe c ting p a r tic le tr a ns por t inc r os s flow m e m br a ne m ic r ofiltr a tion *

F actor E xp ression

T o w a r d t h e m e m b r a n eG ravit y v d gg p p

182

V an der W aals at t ract ion v A

sA

36 2

P ermeat ion drag (f lux) J

Aw a y fr om th e m em br an eB uoyancy v d gb p l

182

E lect rical double layerrepulsion

v

sR

23

2

ex p

B rownian d if f usion v kT

dBp

3

S hear-induced d if f usion vu d

hsp 0 0225

2

.

Lat eral migrat ion vu d

hlp p

1 3 8

12 8

2 3

2

.

* T h e t y p e o f m e m b r a n e u n i t : p l at e a n d f r a m e . dp, part icle diamet er; p, part icle de nsit y; ,

dynamic viscosity; A , Hamaker const ant ; s ,

separat ion dist ance; l, liquid viscosity; , D e b y e -Hckel paramet er; , f luid permittivity; ,boundary layer t hickness calculat ed by t he

Lvque equat ion; , zet a pot ent ial;u o, averagefluid velocity; h , half -channel height .


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Total Back-Transport Velocity Including Brownian Diffusion,

Shear-Induced Diffusion, and Lateral Migration as a Function of

Particle Size and Fluid Velocity ( From Prof. Chung Hak Lee)(temperature, 55oC; particle density, 0.99 g/cm3; channel height, 1.0 mm)


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Particle Diameter, m10 -2 10 -1 10 0 10 1

Back-TransportVeloc

ity,m/s

10 -7

10 -6

10 -5

10 -4

10 -3

Flux,

L/m2-h

1

10

100

1000

Fig. 7.7. Particle back-transport velocities as a function of particle size.

55oC, 0.5 m/s

Brownian Diffusion

Shear-Induced Diffusion

Total

Particle Back-transport as a Function of Particle

Size ( From Prof. Chung Hak Lee)


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EFFECT OF PARTICLE SIZE ON CRITICAL

FLUX

(Particle concentration = 200 mg/L; ionic strength = 10 -5 M)

0

50

100

150

200

250

0.E +00 1.E -06 2.E -06 3.E -06 4.E -06

P article size (m)

Jcrit.(L/sqm

C a l c u l a t e d

O b s e r v e d ( T M P c o n c e p t )

O b s e r v e d ( b y D O T M )

O b s e r v e d ( M a s s b a l a n c e

c o n c e p t )


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Effect of Particle Size on critical flux

( from S. Kim et al. 2002)


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The filtration equations


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Microfiltration membrane

- The typical applications in water

and wastewater treatment Polishing step after a conventional treatment for drinking

water production

Direct application on raw water

Pretreatment of RO Tertiary treatment after conventional wastewater plant.

Combined with biological process or physicochemicalprocesses (see next courses on MBR and hybrid

Reversible and Irreversible Fouling : backwash operation.

The Filtration equations


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FOR DRINKING WATER PRODUCTION

UF and MF membranes are being used for

clarification anddisinfectionin place of

conventional settler- deep bed filter.

UF and MF membranes are able toproduce water with low turbidity (less than

0.1NTU) and very low micro-particles

concentration


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Drinking Water

Applications

Disinfection

(crypto, giardia)

Disinfection

+ Pesticides

Clarification

Polishing

NOM, BCOD,

pesticides,

softening)

Desalination

Reverse

Osmosis X

Nanofiltration X X

Microfiltration

Ultrafiltration X(UF for virus) X(PAC hybrid) X X

Hybride Processes(PAC, coagulants)


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DISINFECTION

UF and MF are both able to remove with

the same efficiency Crypto sporidium,

Giardia and bacteria : the efficiency is

more related to the integrity than to the cutsize.

UF is supposed to be more efficient for

virus removal however a real disinfection

efficiency relies on a multi barrier process.


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em rane

Alternative

for Clarificationpumping

coagulation

flocculation

clarification

filtration

chlorination

Microfiltration


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ZeeWeed Operation

AirAir

PermeatePermeatePumpPump

RawRawWaterWater

RejectReject

Treated

Water


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Clarification/Disinfection

Reference

Coliban: 126 MLD in Australia,Worlds largest MF plant to date.

Immersed Membranes (cmf-s) MEMCOR,

Main Objectives: clarification and disinfection


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Largest Microfiltration Plant in the World for

Potable Water Treatment uses MEMCOR CS

Challenge

The specification for the treated water from the treatmentplants was designed to meet existing guidelines and anticipatefuture drinking water regulations.

Penalties are imposed for excursions from any of the 25criteria specified. The water treatment challenge can besummarized as:

Continuous 2 to 5 micron particle removal and 4-log reductionfor Cryptosporidium.

Reliable organics removal (algal toxins, color, taste and odorcompounds).


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Solution

The Coliban Water Region Water Authority engaged USFilter and Veolia Water-Australia to commission the AQUA 2000 Project, which is a build-own-operate-transfer (BOOT) project. It includes the construction and operation for 25 yearsof a water treatment scheme for the Coliban Water Authority in Victoria,Southeastern Australia. This will comprise of three water treatment plants, thelargest of which, at Sandhurst, will use MEMCOR CS microfiltrationtechnology.

The plants use a combined process of microfiltration, ozonation and biologicalactivated carbon (BAC) to deliver water that far surpasses World HealthOrganization standards.

Microfiltration membranes provide a physical barrier, removing particles downto 0.2 micron. The MEMCOR CS plant consists of eight cells (6 duty cells, 2stand-by cells), each containing 576 submerged membrane modules. Waterenters each cell and is drawn through the outside of the porous membranes tothe inside by a filtrate pump, producing filtered water. These cells are

backwashed intermittently using filtrate and air to scrub the fiber surface.Periodic chemical cleaning is performed when the maximum transmembranepressure (TMP) is reached.


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Results

Installing the MEMCOR CS system has increased economies ofscale for the Sandhurst WTP. Chemical costs are significantlyreduced, and so are related maintenance, storage and disposal.

Membrane integrity, minimal mechanical repairs and the CMF-Ssystems ability to filter high and variable turbidities and algae

loads without chemicals or operator intervention also reducesoperational costs by 10-15% over the systems life cycle, whencompared to those of conventional filtration systems.

The MEMCOR CS systems smaller footprint has reducedcapital costs at the Sandhurst WTP by roughly 20%, allowingColiban Water to pack greater filtration capacity into a limitedspace while expanding its potable water production.

All three plants are currently achieving levels well in excess of7-log removal/inactivation of pathogens.


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Second case study : WW reclamation.

Tertiary treatment Challenge

The desert community of Scottsdale, Arizona had no natural surfacewater sources and a decreasing groundwater supply. Scottsdale hadhistorically treated and disposed of its wastewater. As the city hasgrown, disposal of wastewater presented several problems, such as:

The city was paying money to give away reclaimed water a commodity

The sewerage system would need upgrading, at a similar cost to the Water

Campus Water lost from the city would have to be replaced, at a further treatment

cost

In 1980, the State of Arizona passed the Groundwater Management Act(GMA), whereby facilities are given withdrawal credits whenrecharging groundwater an attractive alternative for Scottsdale, asgroundwater requires only disinfection for potable use. The ScottsdaleWater Campus was developed as a water resources management

facility.


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Solution

USFilter supplied a MEMCOR CMF (ContinuousMicrofiltration) system as a pretreatment to RO. Thissystem consists of 24 units containing 90 moduleseach, with a capacity of 18.5 MGD.

The Water Campus contains a 50 MGD water

treatment plant, a 12 MGD water reclamation plant,and an advanced water treatment facility, whichconsists of CMF, RO and recharge systems.

The CMF units are designed to achieve a minimum14-day cleaning frequency. They were also designed

to run at a flux of 17.9 GFD, treating Colorado Riverwater, and a 24.3 GFD when treating effluent.


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Results

Since commissioning, the CMF units haveconsistently exceeded their performance target of a14-day cleaning frequency.

CMF units on Colorado River water run at effluent flux,and when running on effluent, have been cleaning at

least monthly. All CMF units are operating with a Pressure Decay Test

of less than 0.2 psi/min.

Operating costs are approximately half of the pilot trialestimated costs.

The CMF plant does not require a full-time operator.


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UF is slightly different

The retention of molecules gives birth to a

new phenomena : polarization

concentration


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Ultrafiltration membranes

The specifity of UF membranes : the

retention of macromolecules and viruses

The gel layer model


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Mass balance on the solute

dz

dcDjc v

11

Boundary condition Z = 0, c 1= c10

Z = l , c 1= c1

1

10lnc

c

l

Djv

Integrating

Figure 11.Conc. polarization.

The polarization phenomena


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Transport Equations

Ultrafiltration : the species transported - solventChief force - pressure

Solvent velocity force on solvent

PLj Pv

typermeabiliL

timeperareapersolventofvolumethej

P

v

:

:

P

l

kJv

: Darcys L a w

thicknessl

typermeabililawsDarcythek

:

':Figure 3.Ultrafiltration from a pressure

difference.


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Gel Polarization

When the water contains macromolecules

retained by the membrane ,

Due to the concentration at the membrane

wall , the concentration may reach acritical value which results in the formation

of a gel

From this moment, the flux cannot

increase anymore , whatever the pressure


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Application to water and

wastewater treatment

In water treatment , the normal conditions

of operation are far from these critical

conditions

In waste water treatment , this phenomenacould occur for effluent with high organic

concentration


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Disinfection - Reference

Clay Lane: 160 MLD in England, Three Valleys,one of the largest UF plants ever built

Principal objectives: cryptoporidia removal


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Stability of the operation

0 200 400 600 800 1000 1200 1400

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

(bar)

Flux (m /m .h)3 2

Temps (heures)

p (bar)

Time (hours)


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Raw & Treated Water Turbidity

at Bernay Ouest during Rain Events

(Aquasource UF membranes)

Sampling Date

Parameter Units Dec. 21, 93 Jan. 27, 94

Raw water

Treated

water Raw water Treated

water

Turbidity NTU 32.0 0.3 7 0.1Total Fe g/l 8,920 < 20 115 < 20

Total Mn g/l 410 < 10 < 10 < 10

Organic matter mg O2 /l 12.8 3.3 1.1 0.9

Total coliforms #/100 ml 126,000 0 1,300 0


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In existing waterworks

Anicheapplication for membranes:

the treatment of the backwash water from

sand filters

Possibility of increasing the production by3 to 4% and reducing the size of sludge

treatment facilities.


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Reverse Osmosis Pre-Treatment Removal of suspended solids larger than

0.035 microns and fouling organicmolecules

Typical advantages of ultrafiltration for ROpre-treatment:

Absolute filter at 0.035 - 0.1 micron Lower chemical consumption - no need to settle the coagulated

organics

Lower sludge volume to be disposed

Easy to operate

Produces a high quality water (SDI < 3), allowing for easy

operation of RO Plant (ie: lower power requirement, longercleaning intervals, longer membrane life)


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Why a pretreatment is needed

Mechanical damage

Membrane degradation

Particulate fouling

Organic fouling

Coagulant fouling

Biofouling

Silica fouling

Other inorganic scaling

FOULING : 77%


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Local conditions close to the

membrane wallOrganics are rejected by the membrane and thus

concentrated

Concentration depending on the recovery ratio : trend tohigher recovery results in higher average concentration .

Local concentration at the membrane wall depending too

on hydrodynamic conditions : concentration at themembrane wall may be several times higher than theaverage concentration.

Targeting less than 1ppm TOC (or BDOC)in the feedlooks realistic for ensuring the absence of bioactivity at

the membrane wall


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Why a pretreatment is needed

Proper pre-treatment is the most critical factor

for successful long-term performance of reverse

osmosis seawaterdesalinationplant. Brehant

et al.,Desalination144: 353-360, 2002.

optimization of the pretreatmentis one of the

most critical aspects ofRO.Van der Bruggen

and Vandecasteele,Desalination,143: 207-

218, 2002.


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Basic Eqn for Ultrafiltration

)( PLj Pvtcoefficienreflection:

If the membrane rejects all solutes, then = 1 .

If the membrane passes both solvent and solute, then = 0


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Typical application of UF membranes in

water and wastewater treatment

Direct filtration of sewage

The same as MF


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Typical modules used for UF and

MF

Hollow fibre modules ( in cartridge or

submerged ) are mostly used.

Ceramicmonolithmodules are

becoming competitive.


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UF or MF ?

Theoretically depending upon the

comparison between pore size distribution

and particle size distribution

UF if virus removal is needed MF + coagulation and/or adsorption may

be equivalent to UF (see Hybrid

membrane processes)

Preliminary tests are useful.

Documents

MicroFiltration and UltraFiltration Lecture Notes