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Copyright 2008 SRI International

MEMBRANEFILTRATION

Reverse Osmosisand Nanofiltration

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What is filtration?

Filtration is a process of removingparticulate matter from water by forcingthe water through a porous media. This

porous media can be natural, in the caseof sand, gravel and clay, or it can be amembrane wall made of variousmaterials.

The size of materials that can be removedduring filtration depends upon the size ofthe pores of the filter.

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Water Filtration Methods

There are simple and

cheap ways to filtercontaminants out of

water

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Sand, Gravel, and Charcoal Filtration

Pouring waterthrough sand,

gravel, orcharcoal aresimple andinexpensive

methods ofcleaning water Sand and gravel filtration

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Small Contaminants Pass Through

Sand, gravel, and charcoaldont filter out somecontaminants, like

bacteria viruses

industrial pollutants

agricultural pollutants

salt

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How Can We TrapSmaller Contaminants?

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Membrane Filter Technology

A membrane is a thin material that has pores(holes) of a specific size

Membranes trap larger particles that wont fit

through the pores of the membrane, lettingwater and other smaller substances through tothe other side

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Membrane Classification

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RIGIN

MATERIAL

MORPHOLOGY/STRUCTURE

MEMBRANE

Synthetic Biological

SolidLiquid

Organic Inorganic

Non-porous Porous

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Non Porous Membranes

Discrimination according to chemical affinitiesbetween components and membrane materials.

Mass transport through these membranes isdescribed by the solution-diffusion model asfollows:

y Sorption of a component out of the feed mixture andsolution in the membrane material.

y Transport through the membrane along a potentialgradient.

y Desorption on the second side of the membrane.

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Porous Membranes

Discrimination according to size of particles ormolecules. The mechanism on which separationis based is sieving or filtrating. A gradient in

hydraulic pressure acts as the driving force.

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MEMBRANE CLASSIFICATION

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SYMMETRIC

(HOMOGENOUS)

Constructed by a single material and because

of this reason, the membrane is uniform in

density and pore structure throughout the

cross-section.

ASYMMETRIC May be either homogeneous or heterogeneousand are characterized by a density change given

by the membrane material across the cross

sectional area.

COMPOSITE

(HETEROGENOUS)

Constituted by different (heterogeneous)

materials, the membranes have a thin,

dense layer that serves as the filtration

barrier. But, unlike skinned membranes,

is made of different material than the

porous substructure onto which it is cast.

Skinned type: consist of a dense skinned layer

used as primary filtration barrier and, a thick

and more porous understructure that serves assupport structure.

Graded density type: the porous structure

gradually decreases in density from thefeed to the filtrate side of the membrane.

According tothe PhysicalStructure

(trans-wall

symmetry)

This quality

describes

the level of

uniformity

throughoutthe cross-

section of

the

membrane.

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Membrane Filter Technology

Four general categories:

Microfiltration

UltrafiltrationNanofiltration

Reverse Osmosis

The hierarchy of the processes is identified by thetypes of materials rejected, operating pressures,and nominal sizes on an order-of-magnitude basis

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Water Filtration Diagram

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Membrane Pore

Size (Qm)

Pressure

(bar)

Reverse

Osmosis

(RO)

Ultrafiltr

ation

(UF)

Nanofiltr

ation

(NF)

Microfilt

ration

(MF)

30-

60

20-

40

1-

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