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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.
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