Ch 9 - Principles of UF MF

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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