03 Chapter 1

I n t r o d u c t i o n

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Chapter-1 Introduction Membrane separation processes such as reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF) and microfiltration (MF) have received great importance in recent years for a variety of separation applications. 1-3 The scope of applications for polymeric membranes encompass desalination of brackish and sea water to obtain potable water, purification of drinking water for the removal harmful pathogens, colloidal materials, etc., water reclamation from industrial effluents, and product recovery recover y & pollution control in the chemical, electronic, food and biotechnology industries. For all these applications, the performance limits are clearly determined by the membrane itself in terms of pore size and its distribution and surface properties like surface hydrophilicity or hydrophobicity. Nevertheless, the physical (molecular size and shape) and chemical characteristics (charge and charge density) of the solutes also have a large effect on the membrane separation performance. In the case of UF and MF membranes, the separation occurs mainly due to sieving effect i.e., difference in solute size. The separations in NF and RO membranes takes place through the difference in solubility and diffusivity of the solutes solutes in the membrane. membrane. In general, membrane membrane processes require minimal temperature changes and chemical addition, operate in either continuous or batch modes, use significantly less energy than traditional separation processes, do not alter the chemical structure of the processed materials, and are easy to integrate into existing processes due to their modular nature and compact size


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1. Overview of membrane 1.1 Membrane description

Membrane is defined as a thin barrier that allows the passage of one component and restricts the transport of certain other components of a mixture in a selective manner. A membrane can be homogenous or heterogeneous, symmetric or asymmetric in structure, can carry a positive or or negative charge or be neutral. The The transport of the species through a membrane occurs by either convection flow or by diffusion of individual molecules under some external driving force such as pressure, concentration, electrical field or temperature gradient. The driving force used for a particular separation is dictated by the type of the membrane and specific properties of the species to be separated. The species that are allowed to pass through and rejected by the membrane depend on the morphology (pore size) and nature (hydrophilic or hydrophobic) of the membrane, and the physical (molecular size and shape) and chemical characteristics (charge and charge density) of the solutes. The key property that is usually exploited is the ability of a membrane to control the permeation rate of a chemical species.

1.2 Principle of membrane membrane transport transport phenomenon phenomenon

When two solutions having different concentration are separated by a semipermeable membrane, there exists osmotic pressure difference. Under normal conditions, the solvent from dilute side permeates through the membrane towards the concentrate side until the osmotic pressure reaches the equilibrium, and this phenomenon is called “osmosi s’. However, if an external pressure exceeding the osmotic pressure is applied on the concentrate solution side of the membrane, the solvent flow occurs through the membrane towards the dilute solution side, as shown schematically in Figure 1.1. This phenomenon phen omenon is referred as “reverse osmosis”.


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Figure: 1.1: Conceptual diagram of Osmosis and Reverse Osmosis phenomenon

Depending upon the chemical and morphological nature of the membrane used and the physico-chemical properties of the constituents of a given solution to be processed, separation, concentration and fractionation can be effected by employing suitable operating conditions of the membrane process. In fact, the field of membrane technology is like a keylock system, where suitable membrane material with good thermal, chemical and mechanical stability, good selectivity and productivity are the important and often decisive key to the successful use of membrane methods. Hence, the key to the success of any membrane process lies in understanding: a) the mechanism of transport for a given application on hand, (b) factors involved in the tailor-making a membrane from a variety of polymeric materials available to suit a given purpose, and (c) the optimum operating conditions. Therefore, the principles of membranes processes encompasses the already established and well tested theories and principles in diverse filed, like polymer chemistry, organic chemistry, physical chemistry, transport processes, thermodynamics, chemical engineering, etc. As the principles of membrane processes are derived from a variety of diverse areas so are the applications of


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membrane processes in diverse fields like desalination, chemical processing, food processing, pollution control, gas separations, medicines, etc.

1.3 Types of membrane separation processes

Membrane processes are broadly classified on the basis of driving force that is used to enable the separation into the following types: A. Pressure driven processes a) Microfiltration (MF) b) Ultrafiltration (UF) c) Nanofiltration (NF) d) Reverse osmosis (RO) e) Gas separation (GS) f) Pervaporation (PVP) B. Concentration driven processes a) Dialysis (DS) b) Liquid membrane (LM) C. Voltage driven processes a) Electrodialysis, (ED) 1.3.1 Pressure driven processes

Membrane separations where the driving force is a pressure difference across the membrane. a) Microfiltration (MF)

A pressure driven membrane operation in which particles in the solvated size range of 0.02 to 10 m are separated from a fluid mixture. It has many applications such as separation of emulsions (oil - polluted industrial effluents), concentration of various colloidal suspensions (pigments, metal hydroxides etc.) and as a pretreatment for reverse osmosis.

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b) Ultrafiltration (UF)

A membrane separation process in which the membrane fractionates the components (solutes) present in a solution predominantly according to their size and shape. It is used for the separation of compounds with molecular weights of higher than

1000 daltons. The

applications of UF includes separation of proteins, concentration of emulsions and enzyme solutions, etc. The applied pressure is usually in the range of 1.54.0 kg /cm2. c) Nanofiltration (NF)

Low pressure membrane separation process that is useful for the separation/ concentration of low molecular weight organic compounds, for example, glucose, lactose, sucrose, amino acids, etc., from aqueous/organic liquid mixtures at the operating pressures of 410 kg/cm2. The membranes are usually having the pore sizes in the range of 1 10 nm. d) Reverse osmosis (RO)

A pressure driven membrane operation in which solvent is transported through a membrane while most of the solutes with molecular weight of higher than 100 daltons are rejected . The well known applications of reverse osmosis are: 1) desalination of sea and brackish water; 2) treatment of process effluents in the chemical, food, biological, textile and paper industries; 3) water reuse and wastewater reduction, etc. e) Gas separation (GS)

A pressure driven membrane separation process in which gases are separated by molecular size and solubility. Both porous and dense membranes can be used for gas separation applications. Gas permeation has been applied for: 1) hydrogen separation from synthesis gas; 2) recovery of hydrogen for hydrotreaters (refineries); 3) treatment of sour gas in order to produce a pipeline quality product, etc. f) Pervaporation (PVP)

Pervaporation is a pressure driven process in which the pressure is generally atmospheric on the feed side and low on the permeate side as vacuum is applied. As a result,

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permeate is collected in the vaporous from, contrary to all other membrane processes (except gas permeation) where permeate is in the liquid form. Pervaporation involves phase transition from liquid to vapour. It is considered to be potentially useful in cases where distillation is not applicable, for example, separation of close boiling components, fractionation of mixtures containing components that decompose easily at temperatures close to the boiling point, separation of aqueous-organic liquid mixtures such as water-ethanol. Typical membrane pore sizes, pressure ranges, types of the rejects and applications for pressure driven membrane processes are given in Table 1.1. High pressure processes namely RO and NF have a relatively small pore size as compared to low-pressure processes such as MF and UF. Reverse osmosis primarily remove constituents through chemical diffusion, whereas, MF and UF separate the constituents through physical sieving.

Table 1.1: Characteristics of pressure driven membrane processes Parameter

Pore Size (A0) Solute size (MW) Rejects

Application

Pressure (kg/cm2) Membrane Type

Reverse Osmosis (RO) 5-10

Nanofiltration (NF) 10-20

Ultrafiltration (UF) 20-100

Microfiltration (MF) >100

<300

<1000

>1000

>100000

Low MW organics, Amino acids Proteins, Inorganics, NaCl, Na2SO4

Low MW organics, Antibiotics, Mono- and oligosaccharides, Polyvalent ions.

Microparticles, Bacteria

Desalination, Water & effluent water treatment, Concentration of process streams

Removal of colour, hardness and organics, Separation of organics from inorganics.

Macromolecules Proteins Antibiotics Enzymes Polysaccharides Colloids Bacteria Food processing, Pharmaceutical

25 - 60

5 - 25

Composite

Composite

Disinfection, Colloidal & suspension removal

Dairy Oil removal, Drinking water 2-8

<2

Composite (or) Asymmetric

Symmetric Asymmetric

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1.4. Classification of pressure driven membranes Based on physical structure and chemical nature, pressure driven membranes can be classified into two types: dense (nonporous, homogeneous) and asymmetric (porous, heterogeneous) membranes, as shown in Figure 1. 2.

1.4.1 Homogeneous/Dense/Nonporous membranes

This membrane consists of a closely packed polymer chains with a uniform, continuous packing density throughout the system. Separation of solutes is directly related to their transport rates within the membrane phase, which is mainly due to diffusion. Major applications of dense membranes are in gas separ ation and pervaporation. 1.4.2 Asymmetric/Porous/Heterogeneous membranes

Asymmetric membranes consists of two or more structural planes of non-identical morphologies. They usually contain a thin (0.1 t o 0.2  thickness), relatively dense layer that is supported on a porous and relatively thicker support (120-160

 thickness).

The skin layer

contributes towards the separation ability of the membrane. The bottom porous layer, by virtue of its thickness and porous nature serves as mechanical support to the skin and at the same time offers minimum hydraulic resistance to the flow of the liquid. In this structure, membrane pore size varies continuously in one direction. The size and distribution of the voids depends on the membrane preparation procedure. Asymmetric membranes can be classified further into two types namely, integrally skinned membranes and composite membranes. If the skin and support layer are of the same material and produced in a single step such membranes are referred to as integrally skinned membranes. Membranes that have two or more chemically or structurally distinct layers, i.e.,

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the skin layer and porous support consist of different materials and produced in different steps, are called as composite membranes.

Dense skin layer 0.1-0.3  Porous support >100 

Asymmetric membrane

Dense membrane

Figure 1.2: Schematic structures of asymmetric and dense membranes.

1. 5 Membrane preparation methods

The most generally followed techniques in the preparation of different types of membranes are described below. 1.5.1

Phase inversion method (Asymmetric membrane preparation)

Phase inversion process is the most commonly used method for preparation of asymmetric membranes from different types of polymers that are soluble organic solvent like dimethylformamide, dimethylacetamide, N-methyl-2-pyrrolidne, dimethylsulfoxide, etc. Phase inversion is a process in which a polymer solution film on a support is converted in to a polymer solid film under controlled conditions to obtain a membrane with pore sizes in the range of microfiltration to ultrafiltration3. The change in phase can be initiated in a number of ways, such as solvent evaporation, thermal precipitation, immersion precipitation and vapor precipitation. Among these, immersion precipitation is the most widely used technique for preparation of

microfiltration and ultrafiltration membranes. In this method, appropriate

concentrations of a membrane forming polymer and a low or high molecular weight pore forming

additive

are

dissolved

in

a

suitable

solvent

like

dimethylformamide,

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dimethylacetamide, N-methylpyrrolidone, dimethylsulphoxide, etc. Then, polymer solution film of 100-300

m

is cast on a suitable support (glass plate or non-woven fabric) and then

immersed in a non-solvent bath that is usually water with little amount (0.1%) of surfactant like sodium laurylsulphate. This causes polymer precipitation and separation into solid polymer film with porous structure containing a network of more or less uniform pores. By varying the type of polymer, concentrations of polymer and additive, type solvent, the precipitation medium (gelation bath) and precipitation temperature, etc., it is possible to make membranes having different pore sizes and thus different separation properties.

1.5.2

Interfacial polymerization

This method was developed by Cadotte and co-workers of Film Tech in the 1980s. 4 This is the most widely used current method to prepare high performance reverse osmosis and nanofiltration membranes. In this method, a thin selective polymer layer (0.1 0.2 µm thickness) is prepared on top of a porous membrane by in situ interfacial polycondensation of suitable monomers like diamines, dialcohols, di-/ tri-carboxylic acid chlorides, diisocyanates, etc. The preparation of composite membrane consists of two steps. In the first step, fabric reinforced porous support of a suitable polymer like polysulfone, polyethersulfone, etc., is prepared

according

to

phase

inversion

process.

Then,

polyamide,

polyurea,

poly(amideimide) or polyester skin layer is prepared on the surface of a reinforced porous polysulfone or polyethersulfone membrane by in situ interfacial polymerization of multifunctional diamine or diol in water with a multifunctional acid chloride or isocyanate in hexane. This involves: a) immersion of porous support in aqueous solution of diamine/diol of appropriate concentration; b), draining of the excess aqueous solution from the support; c) contacting the properly drained support with a multifunctional acid chloride or isocyanate solution in water immiscible solvent like hexane; d) curing the nascent composite membrane at the required temperature for selected time; and finally e) post treatment. The performance,

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i.e., solute separation and permeate flux, of the composite membrane is mainly determined by the chemical nature and morphological properties (pore size and its distribution) of the top skin layer. The porous membrane acts as a passive support for the skin layer. The unique advantage in the preparation of composite membranes is the possibility to tailor made individually both the porous support and the skin layer for their specific function. Membranes suitable for a wide range of separation processes such as RO, NF, UF, pervaporation and gas separation can be prepared by judicial selection of the monomers for skin layer formation and by optimizing its formation conditions.5-15

1. 6 Membrane materials

The salient features of polymers that are suitable as barrier materials are the inherent chemical, mechanical and thermal properties and their processability. The appropriate chemical structure combines polar and non-polar functional groups with a polymeric network. Polymers with adequate molecular weight are known to ensure tough membranes. Processability of the polymers in terms of solubility and tractability is also one of the major requirements for membrane preparations. The family of nitrogen-containing polymers are envisaged as suitable polymeric materials for a broad spectrum of membrane applications. This class of polymers is characterized by amide/imide (NH CO, NCO,

((C=O)2 N)

linkages between aromatic or heterocyclic structural units. 16-18 Aromatic polyamides, poly(amide-hydrazide)s, polybenzimidazoles, etc., are reported to be excellent barrier materials for high pressure and high temperature service conditions with an inherent chemical and bacteriological stability.19 Poly( p-phenylene - terephthalamide),20 poly(piperzineamide),21 poly(amide-hydrazide)22-24, polyamides,29-37 cellulose

poly(amide-imide)s,25-26

poly(benzimidazole),27

nylon-4,28

and cellulose esters (particularly cellulose diacetates, cellulose

triacetate and their blends),38 aromatic polysulfones, 39 fluoropolymers (PVDF, nafions,


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PTFE)40-41 are the most widely used barrier materials for a variety of applications because of their physicochemical and film forming properties and immediate availability.

1.7 Membrane configurations

Membranes are generally prepared in flat sheet (1 m width 100-200 µm thickness), hollow fibers with OD of 1-2 mm and ID of 0.70-1.0 or tubular form. For practical utility, flat sheet membranes and hollow fibers are made into useful configuration known as modules of different size, as shown in Figure 1.3. Membrane module is a manifold assembly consisting of membrane and other spacer material in order to separate the streams of feed, permeate, and retenate. Flat sheet membranes are used in the form of spiral wound modules or plate and frame system. In general, because of their low cost and high packing density, spiral wound membranes are the most common membrane design for ultrafiltration, nanofiltration and reverse osmosis systems from very small to very a large scale applications. Hollow fiber membranes are commonly used in microfiltration and ultrafiltration applications and have the advantage of using them in either of the out-to-inside or inside-to-outside permeation mode. Both the spiral and hollow fiber membrane module offer a compact design and a smaller treatment plant footprint.

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Flat sheet membrane

Hollow fibers

Spiral modules

Hollow fiber modules

Figure 1.3: Flat sheet and hollow fiber membranes and their modules

1.8 Membrane characterization

Membranes are generally characterized in terms of: 1) morphological structure (pore size and pore size distribution), and 2) performance for the selected separation application. Membrane morphology is usually determined by various instrumental techniques like scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), and also by permeation measurements. The performance of the membranes are usually measured by carrying out the separation experiments for any selected application. In the permeation experiments, the fluid entering the membrane module is referred as feed, the portion of the feed passing through the membranes is called as permeate , and the portion of the feed not passing the membrane is known as retenate . The fluid volume or mass passing through the membrane per unit area   and unit time is referred as flux (J), usually the units of which are kg.m 2s 1. The ability of a

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membrane to hinder a component to pass through it or to retain a component in the feed solution is termed as retention or rejection .

1.9 Major advantages of membranes

i) Energy efficient by virtue of ambient temperature operation. ii) No phase change- Possibility to process thermally labile chemicals at room temperature while retaining flavour and texture. iii) Low capital & operating costs. iv) Simplicity of operation and therefore minimum training requirement to operators. v) Availability of different configurations. vi) Modular in nature-over a wide range of capacities can easily be fabricated. vii) Short start-up and shut down times.

1.10 Ultrafiltration: state of the art

Ultrafiltration is a membrane filtration technique for concentration, fractionation and purification of macromolecules chemicals or

solvents.

in solutions without phase change or addition of

The document "Terminology for Pressure Driven Membrane

Operations approved and issued in June 1986 by the European Society of Membrane Science and Technology42 defines an ultrafiltration (UF) membrane as a barrier which fractionates the components of a liquid predominantly according to their size and shape". The pores of an 9

ultrafiltration membrane are on the scale of 10

meter or one nanometer and more. Average

pore diameters are in the range from less than a nanometer to about 10 nanometers. Hence, virtually

all

suspended

particles

are rejected by ultrafiltration membrane,

including

bacteria, viruses, which have a minimum size of some 20 nanometers. Ultrafiltration is in essence molecular filtration; it discriminates between molecules of different sizes and roughly speaking between those with different molecular weights.

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Water, with a molecular weight of 18 Da and an effective diameter of about 0.2 nanometer can pass freely through all ultrafiltration membranes. Simple sugars, such as glucose, which has a MW of 180 Da and a size of about one-half nanometer, can pass through most of the ultrafiltration membranes. Proteins and other large biological molecules, on the other hand, are generally excluded, since they often have a molecular weight of >20,000 Da and a size of several nanometers. By choosing a membrane with carefully controlled pore size it is even possible to separate molecules with modest differences in size, such as sucrose, MW 342, and cyanocobalamin, or vitamin B-12, MW-1300. The transport of solutes through ultrafiltration membranes depends on: 1) the pore size of the membrane and 2) interactions between UF feed components and membrane matrix.

1.10.1 Ultrafiltration Membrane Materials

There are a variety of ultrafiltration membranes, each with its advantages and disadvantages. UF Membranes can be prepared from inorganic materials (Ceramics, Glasses, Metals) as well as from polymers43-44. Although the former are very attractive owing to their superior chemical and temperature resistance, polymeric materials offer the possibility to obtain membranes covering a large spectrum of pore sizes and configurations of interest for industrial applications.

Besides the typical configurations such as, flat sheets, tubes and

hollow fibers, polymeric membranes can be closely packed into spiral-wound module to give high membrane surface to volume ratio. Cellulose polymers which have long been used for making asymmetric

UF membranes are progressively

chemically stable polymers such as

polysulfone,

substituted by thermally and

polyvinylidenefluoride

(PVDF),

polyacrylonitrile (PAN), polyamides, polyimides etc.45-49 The structure and porosity of the membrane can be controlled to some extent by Correlations between

varying the preparative parameters.50-56

preparative parameters such as polymer concentration, exposure

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conditions, etc., and ultimate membrane properties have been

reported

by

several

researchers for a wide variety of polymer-solvent-nonsolvent systems.57-63 Thin film composite membranes are a new family of ultrafiltration membranes. They are formed, like their RO and NF counterparts, by interfacial polymerization of the barrier layer on the surface of a porous polysulfone or polyethersulfone support material. These type of membranes provide a wide range of separation characteristics

with high

temperature stability, excellent chemical resistance and mechanical ruggedness.

Besides,

the hydrophilic nature of the barrier layer minimizes the effects of foulants. TFC membranes having the barrier layer of the reaction product of TDI with PEI

and dialcohols

are in

market.64-65

1.10.2 Ultrafiltration membrane applications

In recent years, ultrafiltration (UF) membrane technology has received tremendous importance for drinking water purification for the removal of pathogens, colloidal materials, turbidity, and for water reclamation through membrane bioreactor (MBR) systems. UF membranes are also extensively used

for concentration, purification and fractionation of

various products in diverse fields such as food, medical, and biotechnological industries, paper industry, dairy industry.66-72 Commercial applications of ultrafiltration are numerous and are found in

many

different areas because it offers unique separation

possibilities.

Ultrafiltration is used for product recovery and pollution control in the chemical industries. The common

features

of different

ultrafiltration applications are concentration &

fractionation.

a) Ultrapure water for electronic industry

UF is a highly reliable and quite inexpensive water treatment method for the removal of trace concentrations of

colloidal

or

macromolecular

impurities

which

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include microorganisms, pyrogens and carbon and resin fines in water for the production of ultrapure water for pharmaceutical industry and semiconductor manufacturing. In ultrapure water system, UF serves two purposes. It is used as a pretreatment stage to ion exchange to remove colloids that reduce the efficiency of the ion exchange process. An average of upto four times the previous life of the resin beds before requiring regeneration may be achieved in an ultrapure water system when the water is pretreated with ultrafiltration.73 Western Electric in Allentown, USA, installed a 1300 gpm hollow fiber membrane based ultrafiltration system for post D.I. treatment of water for production of ultrapure water required for making miniature electronic integrated circuits. The schematic diagram of water purification system for electronic industry is shown in Figure 1.4. The cartridges contain 2940 polysulfone hollow fibers (HF 53-20-GM80). A midwestern (USA) manufacturer of precision components for the electronic industry installed a post-DI UF plant designed and manufactured by Osmonics, Inc. of Minnetonka, Minn., USA. The SEPA-O

spiral-wound

module plant contains

polysulfone membrane of pore size of 15 angstrom (0.0015 micron) and is

designed to remove organic and other particulates over 1000 molecular weight.

Water

Deionised Water

Alum Pretreatment

Sedimentation/ Filtration

Chlorination

Ultrafiltration

Filtration and UV sterilization

Mixed bed exchanger

Figure 1.4: Schematic diagram of ultrapure water production in electronic industry

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b) Biotech and Pharmaceutical Industries

UF is playing a critical role in successful development of many new products and processes with dramatic cost savings and processes. In

improved

yields

compared

to

traditional

biotechnological downstream processing the product is obtained at

low

concentration in a complex liquid. The product thus has to be separated from a mixture of cell mass, substrate components, additives and byproducts. In downstream processing, UF can be used for primary separation, which include broth clarification, cell harvesting and cell debris removal, concentration of diluted product streams and the final purification.74-75 UF is used on an industrial scale for the fractionation of bacterial cells from fermentation broths in the production of amino acids 76 and antibiotics.77 These products include penicillanic acid, clavulanic acid,

cephalosporin,

streptomycin, neomycine, tylosine, polymyxine and

bactiracine. These are all intermediate molecular weight products and hence UF is not used to concentrate the antibiotics but to remove high molecular weight (HMW) products from the fermentation broths. UF is also used industrially in the production of HMW products such as enzymes and growth hormones, 78 and concentration of proteins, antibiotics, enzymes, vaccines or polypeptide hormones. Concentration factors ranging from 10 to as high as 100 can be achieved with additional benefit of simultaneous removal of electrolytes and low molecular weight metabolites.79 Eurolysin, France, installed a UF plant of membrane area of about 900 m 2 for the production of amino acids. The feed flow is 45,000 l/h and operating temperature is 30oC. Merck UF plant from Dorr-Olive with 684 m2 membrane area is used for the production of antibiotics. In the final purification, UF is also more cost effective than disposable 0.2 micron filters for the removal of pyrogens

from products intended for infusion, and to produce

ultrapure water used in product formulations and injection in pharmaceutical industry. UF membranes for the concentration/purification of bovine hemoglobin has been reported.80

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These membranes were prepared by copolymerizing styrene and DVB in presence of a PVC base film, followed by sulfonation to obtain a cation exchange membrane. The membrane was further treated with aqueous H2O2 solution which resulted in a microporous membrane with no cation exchange capacity because of the complete decomposition of the cation exchange resin component. A Japan patent describes the production of heat and chemical resistant UF membranes which show 100% albumin removal with a water permeability of 8.7 m3/m2-day (kg cm2). These membranes are prepared by graft copolymerization of hydrophilic monomers onto fabric reinforced polyethersulfone (victrex 500 3p) and polysulfone (victrex 5200 p) membranes. Toray industries,

Japan,

has

developed UF membranes from

polyvinylidenefluoride (kynar 400), poly(TFE-co-VDF) (VTO 6008) and the blends of these two polymers for medicinal use and for the concentration of aqueous solutions. 81 c) Food Processing Industry

The applications of UF in food processing industries are

very promising and

innumerable, due to the fact that the ratio of energy savings are very much higher when compared to conventional techniques. The largest membrane area in food sector is installed in the dairy industry for ultrafiltration of milk and whey. 82 The important applications of UF in food

processing industry are: i) Treatment of whey and milk; ii)

Fractionation

and

concentration of egg albumin, proteins, extracts such as vanilla, lemon, peel extract, etc., and animal, fish and vegetable oils; iii) to recover valuable products from soya whey, corn steep liquor, and other dilute waste streams; and iv) purification of glucose, sucrose and other saccharides.

About 2/3 of membrane area installed in the dairy industry is used for the treatment of whey

and about 1/3 for milk.

Ultrafiltration of milk is used, for

example, in the

manufacture of youghurt, cheese making and for preconcentration of farms. The concentrated milk could be used for later conversion into fluid products like flavour milk, ice cream, high

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protein milk, dried products like whole milk and skim milk powders, etc. The maximum concentration obtained is reported to be 52% total solids for whole milk and 39% for skim milk. One of the well known food processing applications of membrane is cheese whey processing. Whey, a byproduct of

cheese making, contains about 0,8% protein

(lactalbumin), 5% lactose, 1% of minerals and large amount of BOD (30,000 to 50,000 ppm). Intensive work has been and is being done, on UF for the manufacture of fresh, soft, semi-hard, and hard

cheeses.83-85

The whey proteins, having excellent functional

properties, lactose and salts can be recovered and concentrated simultaneously by ultrafiltration. Cheddar cheese is successfully made commercially from UF retenates by a process developed in Australia.86 New

Zealand dairies have produced a large variety of

WPC's with different functional properties.

d) Purification and concentration of products

There is a great scope for the use of UF in simultaneous

purification and

preconcentration of juice. In juice processing, the process stream contain compounds such as pectins, cellulose, hemicellulose, starch and proteins, which cause an undesired turbidity when stored. Since the late 70's, UF has been used commercially for the clarification of fruit juice such as apple, grape, pear, pineapple.87 Similarly sugarcane juice, a popular drink in India, can be clarified to a good quality sparkling clear juice by a single step UF process. A pilot scale process has been successfully developed for purification and concentration of betalaines.88 Polyamides, polysulfone, polyethylene, etc. which have wide pH stability (0.5 to 13.0), high temperature tolerance ( 90oC), and withstand high pressures (80 bar) have been used as membrane materials.89 Ultrafiltration of wine gives crystal-clear permeate, without any addition of chemicals, by removing cloudy precipitants like mold, yeasts and bacteria. UF can also be used for producing alcohol-free beer or wine.

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UF is a promising technique for the recovery of protein concentrates and isolates from cakes of oil seeds like soya, groundnut, coconut, etc. UF has been found to yield 15% more protein than the traditional acid coagulation.90 Besides, the protein obtained by UF had superior functional properties and higher chemical store due to better amino acid profile. UF is finding increasing applications in the simultaneous purification and concentration of food enzymes like pectinases, alpha-amylase, protcolytic enzymes, etc.91

e) Egg-white concentration

Egg- white is useful for baking industry because it can support foams carrying large quantities of flavour and sugar. UF/RO can be used for the removal of part of the water from egg-white and whole-egg without browning reaction and with improved functional properties of the protein.92 - 93 Polysulfone and polyolefine (NTU-20100) based ultrafiltration membranes were in use for egg-white concentration.94 - 95

f) Textile industry

UF is playing a vital role in textile industry in the economical operation of plants. Valuable organic chemicals and process water

can be reclaimed for reuse resulting in

substantial savings. UF process can be used to: i) concentrate and recover valuable dyes and latexes and limit discharges to accepted levels, ii) reclaim caustic wash water and iii) recover sizing agents (PVA) from the effluent.

g) Paper and pulp industry

UF has been used in paper and pulp industry for the treatment of effluent and to recover the valuable chemicals.96 - 98 The main constituents present in waste stream of wash plant

effluent from a pulping mill are pulping chemicals, fine fibers

technique can be applied to produce recyclable

water

and water.

UF

and a concentrated stream of

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pollutants which can be recovered to achieve zero discharge. UF also has been found to be very effective in reducing COD and colour load to the extent of

60-70% and 80-90%

respectively. Two commercial UF plants have been in operation in Japan since 1981. These plants contain charged polysulfone membrane from Nitto, the NTU-7410 membrane. process from spent liquor can be used to fractionate,

UF

concentrate and recover

lignosulphate from spent sulphite liquors, and alkali lignin from kraft black liquors. In general spent sulphate liquors contain about 60% lignin, 30% reducing sugars and 10% inorganic materials. The spent pulping liquors may be separated into purified fractions, of, in particular lignin compounds and sugars by ultrafiltration. 99 – 102 A retenate stream of about 25% total solids containing 95% lignosulphate is produced.103 A full scale UF plant with a membrane area of 1,120 m2 has been installed in Norway in 1981. Polysulphone based UF membrane (nominal MWCO of 20,000) and PVDF membrane (nominal MWCO of 100,000) (PCI FP 100) are widely used for the treatment of spent liquors.

h) Other UF applications

The most important UF application in the chemical and mechanical industries is the recovery of paint in electrodeposition painting process.104 It has been reported that an UF plant can pay for itself within six months, primarily by reducing the consumption of paint.105, 106 Emulsified oily waste water in metal working industry contains oil (mineral, vegetable or synthetic), fatty acids, corrosion

emulsifiers

(anionic

and

nonionic

inhibitors (amines), bactericides and other chemicals designed to

surfactants) provide a

long-lasting and effective fluid. The discharge of oily waste waters as such is subject to increasingly severe environmental regulations. UF can be used for the concentration of waste water, and the maximum attainable oil concentration ranges from 25 to 65% oil,107 from the mixed waste water which contain oil concentration in the range of 1 to 10%.

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1.11 Membrane fouling

The commercial ultrafiltration membranes are mainly based on hydrophobic engineering polymers like polysulfone (PSF), polyethersulfone (PES), polyvinylidenefluoride (PVDF) and polyacrylonitrile (PAN) owing to their excellent chemical resistance, thermal stability and mechanical properties. However, these polymeric membranes are hydrophobic and are prone for rapid fouling during the ultrafiltration process. Fouling results in severe decline in permeate flux with operation time and thus necessitates frequent chemical cleaning with different types of chemicals like surfactants, acids, alkali, etc., which often shortens the membrane life. It has been generally acknowledged that membranes with hydrophilic surface are less susceptible for fouling. Different approaches were adopted to impart surface hydrophilicity to conventional hydrophobic membranes to improve their fouling resistance properties. Surface modification of the ceramic-supported PES membrane with PVA  polyamide composite thin layer by interfacial polymerization technique has been reported.108 Atom transfer radical polymerization of different types of methacrylate monomers on the surface of polyvinylidenefluoride membrane for imparting antifouling properties has been described. 109 Preparation of UF membranes based on PES PAN blends and their chemical modification to convert the nitrile group of PAN to carboxylic acid functionality for imparting fouling resistant properties to the membrane were reported. 110 Surface modification of polysulfone and polyethersulfone UF membranes with polymers having hydrophilic functional group like SO3H

or

COOH

by either pre adsorption or free radical polymerization technique to

impart fouling resistance property has been reported.111 UVassisted graft polymerization of hydrophilic monomers to increase surface wettability and to reduce the interactions between natural organic matter (NOM) and the membrane surface has been reported. 112 A few reviews have also been published on various methods employed for control of fouling in MF, UF, RO


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and other types of membranes. 113, 114 These surface modifications have improved the fouling resistance nature of the membranes but, in majority of cases, have altered the original pore size or the molecular weight cut off value of the membrane towards lower values due to the formation of an additional layer. Thus, membranes that have inherently good chemical stability and resistant to fouling are utmost importance in the quest for improvements in membrane technology for different separation applications.

03 Chapter 1

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