Transcript
Chapter 9 - Basic Principles of
Ultrafiltration & Microfiltration
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Principle of Membrane Filtration
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Feed Permeate
MEMBRANE Solvent (water) Solute
DRIVING FORCE
What is a membrane?
Membrane filters are thin sheets or tubes made from organic polymers.
A membrane has the ability to transport one component more readily than the other because of differences in physical and/or chemical properties between the membrane and the solute.
• Transport through the membrane occurs as a result of a driving force (pressure) & the permeation rate is proportional to the force.
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MF/UF Polymeric Membranes
• Symmetric: membranes have pores of uniform size
throughout. Their thickness is ca. 10-200 μm (0.01 – 0.2
mm). Resistance to mass transfer is determined by the total
membrane thickness - the thinner the membrane the higher
the permeation rate.
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MF/UF Polymeric Membranes
• Asymmetric: Very dense top layer with a thickness of 0.1-
0.5 µm (0.0001 – 0.0005 mm) supported by a porous sub-
layer with a thickness of 50-150µm. The pores change in
size over the depth of the membrane. These membranes
combine the high selectivity of a dense membrane with the
high permeation rate of a thin membrane.
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Cross-section of an asymmetric
polysulphone UF membrane
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Composite Membranes
• Composite membranes are ‘skinned’ asymmetric membranes. However, the top-layer and the support layer originate from different polymeric materials. The support layer is usually already an asymmetric membrane on which a thin dense layer is deposited (of another material).
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MF Membranes
Polymer Materials
Polycarbonate
Polyvinylidene fluoride
Polytetrafluoroethylene
(PTFE, Teflon)
Polypropylene
Polyamide
Cellulose esters
Polysulphone
Poly(ether-imide) 8
UF Membranes
Polymer Materials
Polysulphone/
Polyethersulphone
Polyacryonitrile
Cellulose esters
Polyimide/polyetherimide
Polyamide
Polyvinylidene fluoride
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MF & UF
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- Macromolecules, salt - Virus, Proteins, sugars
- S.S, colloids - Bacteria
Water
- Particulates - MW>1000
Water
- salt - Low MW organics (MW<1000)
• MF
• Pore Size: 0.05-10µm • Pressure: <2 bar
• UF
• Pore Size: 1-100nm • Pressure: <3 bar
What is removed by MF & UF?
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Nanofiltration
Salt
Viruses
Colloids
Ultrafiltration
Microfiltration
Sand
Bacteria
Dissolved Organics
Micron
Scale
(10-3 mm)
Typical Size
Range of
Selected
Water
Constituents
Membrane
Process
Scale
0.001 0.01 0.1 1.0 10 100 1000
Media Filtration
Reverse Osmosis
Applications of MF & UF
• Disinfection of surface water & ground water
under the influence of surface water
(combined with chlorination).
• Production of industrial water from surface
water (removal of suspended & colloidal
matter). 12
Removal of suspended & colloidal matter
including algae, cysts, bacteria and viruses
Applications of MF & UF
• Pre-treatment of feed water (surface water & sea
water) for nanofiltration & reverse osmosis
systems (usually combined with chlorination).
• Production of industrial water from domestic/
industrial WWTP effluent (in combination with
reverse osmosis & ion exchange).
• Final disinfection in the production of drinking
water from domestic waste water. 13
Membrane Performance (I)
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Membrane
Type
Suspended
Matter
(e.g. algae)
Bacteria,
Giardia,
Crypto
Viruses Natural
Organic
Matter
Microfiltration
Ultrafiltration
Nanofiltration
Reverse Osmosis
+
+
+
+
+
+
+
+
+/-
+
+
+
-
+/-
+
+
Membrane Performance (II)
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Membrane
Type
Hardness &
Sulphate
Chloride
Nitrate
Sodium
Pesticides Assimilible
Organic
Carbon
Microfiltration
Ultrafiltration
Nanofiltration
Reverse Osmosis
-
-
+
+
-
-
+/-
+
-
-
+/-
+
-
-
+/--?
+/-?
Assimilable - able to be absorbed and incorporated into
body tissues
Modes of Operation of MF/UF
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concentrate
Dead-End Operation Cross-flow Operation
feed
Permeate
feed
Permeate
High energy consumption due to the high cross flow velocity 1-4 m/s used in cross flow systems
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time
Yield
Cake Thickness
Cross-Flow Filtration
Dead-End Filtration
low energy consumption as a high cross flow velocity is not required - the cake grows during filtration!
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time
Yield
Cake Thickness
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MF/UF Operating Modes
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Cross-flow
Dead-end
MF/UF Flow Regimes
Outside-in
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Filtrate out
Feed water in
Backwash out
Backwash in
MF/UF Flow Regimes
Inside-Out
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Backwash out Backwash in
Feed water in
Filtrate out
RECYCLE OUT
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MF/UF Performance Parameters
• Flux : is defined as the volume flowing (Q) through the
membrane per unit area (A) and time (t). Flux is expressed as l/m2.hr, l/m2.day, m3/m2.s
• Selectivity: The selectivity of a membrane is expressed as
rejection or retention. If a solute is completely rejected by a membrane, the retention (R) is 100% and if no solute is rejected, retention is 0%.
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R = Cfeed - Cpermeate = 1 - Cpermeate Cfeed Cfeed
Pore size/MWCO
• MF membranes are usually rated according to their pore sizes (0.05-10 m), which can be measured directly by scanning electron microscopy (SEM)
• UF membranes are rated according to their Molecular Weight Cut-Off (MWCO), as the pores are too small (0.1 - 0.001 m) to be measured directly.
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MWCO - measured in ‘Daltons’
• The MWCO of a membrane is equal to the molecular weight of globular proteins (e.g. albumin, pepsin, cytochrome C) that are 90% rejected by the membrane. Polysaccharides (e.g. dextran) or linear flexible polymers (e.g. polyacrylic acid) can also be used
• A significant difference can exist in terms of ‘retention’ between UF membranes with the same MWCO but originating from different manufacturers as a result of the use of different molecular weight markets and test conditions (pH, ionic strength, pressure, temp. etc.)
• Globular proteins, or sphero proteins are one of the two main protein classes, comprising "globe"-like proteins that are more or less soluble in aqueous solutions (where they form colloidal solutions). This main characteristic helps distinguishing them from fibrous proteins (the other class), which are practically insoluble.
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Choose the MWCO • Once sample volume is determined, the next step is to select the
appropriate MWCO (for ultrafiltration) or pore size (for microfiltration). MWCOs are nominal ratings based on the ability to retain > 90% of a solute of a known molecular weight (in Kilodaltons). For proteins, it is recommended that a MWCO be selected that is three to six times smaller than the molecular weight of the solute being retained.
• 1 kg = 6.o221415 x 1026 dalton (Da). Da = 1.6609 x 10-27 kg
• It is important to recognize that retention of a molecule by a UF membrane is determined by a variety of factors, among which its molecular weight serves only as a general indicator. Therefore, choosing the appropriate MWCO for a specific application requires the consideration of a number of factors including molecular shape, electrical charge, sample concentration, sample composition, and operating conditions. Because different manufacturers use different molecules to define the MWCO of their membranes, it is important to perform pilot experiments to verify membrane performance in a particular application.
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Choose the Molecular Weight Cutoff (MWCO)
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Water Flux
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Flux (J) = Driving Force
Viscosity . Total Resistance
Driving Force : Pressure, temperature, concentration
Viscosity : Depends on feed water temperature
Resistance : Membrane resistance is a function of thickness, pore size, porosity, tortuosity
Total membrane resistance comprises: The resistance of the membrane (Rm) The resistance due to particles
deposited inside pores or blocking the pore entry (Rb)
The resistance due to particles forming a cake (Rc)
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TIME
FLUX
Total Resistance
Clean Water Flux
cbmtotal RRRR resistance Total
cbm RRR
P
R
PJ
total flux Hence,
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Factors Affecting Flux
FLUX
FLUX
FLUX
FLUX
Pressure
Feed Flow Rate
Feed Concentration
Temperature
P Rm
J = clean water
Recovery
Videos
• http://www.youtube.com/watch?v=MEfFq_SJ0Pk&feature=related
• http://www.youtube.com/watch?v=rK7UVY_7K8w&feature=related
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