Caballero B. (ed.) Encyclopaedia of Food Science, Food Technology and Nutrition. Ten-Volume Set

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the processed whey, RO operating temperature,

and residence time in the RO equipment. However,

the number of bacteria during RO of whey seldom

multiplies by more than the concentration factor.

When RO cellulose acetate membranes are used, an

operating temperature of 10



C has been recom-

mended for control of the microbiological quality

of RO whey concentrate, but when concentration

is carried out with RO membranes from other

synthetic polymers, higher temperatures can be

used. However, RO concentration of whey must be

carried out at a temperature < 55



C to avoid precipi-

tation of calcium phosphate and fouling of the

membrane. (See Membrane Techniques: Principles

of Reverse Osmosis.)

0009 Concentrating permeate from the ultrafiltration of

whey and milk by RO is widely practiced as the first

step in the manufacture of lactose. RO cannot com-

pletely replace evaporation in lactose manufacture, as

the concentrated permeate must contain 60% total

solids in order to separate lactose crystals. This con-

centration cannot be achieved by RO processing

alone. However, RO can be used for the preconcen-

tration of permeate as a very economic way of remov-

ing the large bulk of the water from permeate. The

concentration of ultrafiltration permeate to 15–18%

total solids followed by evaporation up to 60%

is considered an optimum combination for the

manufacture of lactose. Even in already existing

lactose manufacturing plants using evaporation

only, the inclusion of RO can improve the efficiency

of the plant.

0010The feasibility of concentrating milk by RO has

been established in several studies. However, the in-

dustrial application of RO in concentrating milk is

still limited. Whole milk can easily be concentrated by

RO up to twofold. The presence of milk fat has a very

limited effect on the permeation rate of whole milk

compared to skim milk. The main objective of con-

centrating milk by RO is to reduce the storage and

transport costs, which represent a significant part of

the processing costs in some countries, such as Aus-

tralia. Also, concentrating milk by RO increases the

efficiency of processing facilities in the dairy plant

and as a preconcentration step in the manufacture

of dried milk. Generally all dairy products can be

made from milk concentrated by RO with no marked

effect on the quality of obtained products.

0011RO can also be used in the dairy industry for the

recovery of waste solids from the dairy rinse water.

This process can achieve two purposes: first, a decrease

in the biochemical oxygen demand (BOD) of the plant

effluent, which reduces the costs of waste water treat-

ment, and second, recovery of valuable solids that can

Whey

6% TS

Whey

cheese

Concentrate

12% TS

⫻

Permeate

⫻

Electrodialysis Evaporation

ConcentrateDemineralized

whey

Permeate Retentate

For cleaning

Heating/

acidification

Filtrate

Spray-drying

Permeate

powder

Whey protein

concentrate

Demineralized

powder

Whey

powder

Spray-drying Spray-drying Spray-drying

fig0001 Figure 1 Application of reverse osmosis (RO) in the manufacture of different whey products. TS, total solids; UF, ultrafiltration.

3834 MEMBRANE TECHNIQUES/Applications of Reverse Osmosis

be dried and used for feeding purposes. Both objectives

increase the profitability of the dairy plant.

0012 The RO permeate from the processing of different

dairy fluids has a low BOD, in the range of 30–

300 mg l

1

. Therefore it can be used as a good-quality

supply of water for cleaning purposes.

Fruit and Vegetable Juices

0013 Juices of most fruits and vegetables are characterized

by their high osmotic pressure. However, they can be

concentrated by RO to relatively high solids contents.

The resulting concentrates are of premium quality as

they are subjected to minimum heat damage during

processing. However, early studies on RO concentra-

tion of juices were concerned with two factors:

0014 The rapid fouling of membranes due to the pres-

ence of fibers and pectin in the juices. Therefore,

pretreatment may be needed before concentrating

juice by RO. The development of optimum RO

operational conditions, and the selection of type of

membrane and module configuration were estab-

lished for several juices. In this respect a study

showed that hollow fiber modules performed

better than spiral-wound modules because of

their low concentration polarization effects.

0015 The variable retention of the different aroma vola-

tiles during RO concentration. The balance in the

concentration of these compounds is critical to

obtain a concentrate with similar characteristic

flavor of the fresh juice. In this connection, thin-

film composite RO membranes show better reten-

tion for aroma volatiles than cellulose acetate

membranes.

Different studies have shown the technical feasibility

of concentrating the juice and pure

e of different fruits

and vegetables, including orange, grapefruit, pine-

apple, apple, pear, passionfruit, apricot, cherry,

strawberry, sugar beet, tomato, roasted coffee beans,

and green tea extract. However, industrial processing

by RO has been limited to the juices described below.

Tomato Juice

0016 Tomato juice is a very variable material depending on

the fruit variety, soil, and climatic conditions and

processing conditions. However, the quality of the

tomato juice has little effect on its behavior during

RO processing. Early attempts at concentrating

tomato juice by RO were based on removal of the

suspended material (mainly dietary fiber) by centrifu-

gation or ultrafiltration, concentrating the clear

serum by RO, followed by mixing the concentrate

with the insoluble precipitate or concentrate. How-

ever, this method has not been applied commercially.

0017The advent of the new generation of RO thin-film

composite membranes enabled food processors to

concentrate tomato juice using RO on a commercial

scale. The different parameters for RO of tomato

juice have been optimized. It is of interest to note

that changing feed velocity has a small affect on

flux. This unusual behavior has been attributed to

the rheological properties of tomato juice. Several

plants are now concentrating tomato juice by RO

from 4.5–5.0 to 8 Brix. The different tomato juice

concentrates – sauces (8–12 Brix), passata (16 Brix),

and pastes (27–28 or 37–38 Brix) – can then be made

from RO concentrate by evaporation. The rejection

of the tomato juice solids is little affected by the

quality of the fruit and the operating conditions.

However, a rejection of 99% or more of tomato

juice solids has been recorded. The RO tomato juice

concentrate is characterized by improved color and

flavor.

Citrus Juices

0018Unlike tomato juice, the retention of aroma and

flavor is much more critical for the quality of the

concentrate of most fruit juices. It is estimated that

the various volatiles which give the characteristic

flavor of fresh orange juice are removed with the

first 15–20% of water removed by evaporation.

These volatiles are partially recovered by appropriate

condensation and stripping and added back to the

concentrate prepared by evaporation. However,

the flavor of the concentrate is different from that

of the fresh juice. Orange juice can be directly concen-

trated by RO up to 30% solids at acceptable flux

and with good-tasting product. An ultrafiltration/

RO system has been developed for the preparation

of orange juice and grapefruit juice concentrate that

avoids the limitations of the traditional method for

concentrating the juice. In this method, the fresh

juice is first fractionated by ultrafiltration into con-

centrate and ultrafiltration permeate. The ultrafiltra-

tion concentrate is concentrated by evaporation and

sterilized, while the ultrafiltration permeate is con-

centrated by RO. The RO concentrate is then mixed

with evaporated ultrafiltration concentrate before

aseptic packaging. When the prepared concentrate

(double strength) is diluted, it is of similar quality to

the fresh juice.

0019When lemon juice was concentrated by RO, 79.9%

of volatiles were recovered. Loss of the volatile

compounds has been attributed to adsorption in the

polymeric matrix of the membrane.

Apple Juice

0020The presence of pectin in apple juice affects the be-

havior of the juice during RO concentration and the

MEMBRANE TECHNIQUES/Applications of Reverse Osmosis 3835

quality of the concentrate. Therefore, apple juice is

usually depectinized either by enzymatic treatment or

by removal of pectin by microfiltration. Depectinized

apple can be concentrated by cellulose acetate RO

membranes from 10 to 20–25 Brix at 20



C, with

good retention of volatile flavor compounds. Using

noncellulosic membranes, depectinized apple juice

can be economically concentrated to 20–25 Brix at

30–50



C, with better retention of soluble solids and

flavor compounds. Economic evaluation showed that

a RO unit used to concentrate apple juice to reduce

shipment costs would pay for itself in 4 months.

0021 The retention of aroma volatiles during RO con-

centration of apple juice varied between different

compounds, with trans-2-hexenal showing the

highest and hexyl acetate the lowest retention.

Coffee

0022 Roasted coffee extract contains about 13% total

solids. This extract can be concentrated by RO up to

36% solids content without appreciable loss of solids

or aroma compounds. The RO concentrate of coffee

extract can be used as a base for the preparation of

instant coffee. (See Coffee: Roast and Ground.)

Alcoholic Beverages

Wine

0023 Grape must with a low sugar content of < 17%

makes table wine of inferior quality and insufficient

alcohol content to meet legal requirements. The

traditional method of combatting this problem is

called chaptelization, in which sucrose or glucose

is added to the must before fermentation. This

method can be replaced by RO concentration of

must. The wine results from RO concentrated must

have improved flavor without any adverse effects on

general quality.

0024 Removal of bitterness has been reported as another

application of RO in the wine industry. Red wine of

poor quality can be subjected to RO concentration

using cellulose acetate membrane, with an approxi-

mate molecular cut-off of 300 Da. The bitterness-

causing compounds (probably ketones) pass in the

permeate while the concentrated wine has improved

quality. This wine concentrate has much shorter cold

stabilization time to remove tartrate. (See Wines: Pro-

duction of Table Wines.)

Low-Alcohol Beer

0025 The demand for beer with low alcohol content has

increased recently. This can be achieved either by

controlling the fermentation conditions to develop

less alcohol or the partial removal of alcohol from

normal beer. The first trend has the disadvantage of

impairing the development of flavor compounds in

addition to alcohol.

0026With the use of special RO membranes, it is pos-

sible to remove the alcohol partially from beer. This

has been applied commercially in many breweries in

different parts of the world. A thin composite RO

membrane with a molecular cut-off of 100 Da has

been used to remove 25–30% of alcohol content in

beer. The beer is processed by RO while diluting the

retentate with water to a constant volume under

carbon dioxide. Therefore, the alcohol content of

the beer can be changed to the desired level. The

quality of water used for diluting the concentrated

beer determines the quality of the final product.

Corn Wet Milling

0027Corn wet milling is a process that gives starch as the

main product output in addition to several other

products, namely, oil, protein, and fiber. This process

is a water-intensive technology as 1.5 m

of fresh

water per ton of corn is needed in modern corn wet

milling. RO has been used to recover water from the

light middlings (the overflow from the hydrocyclone

starch system). The recovered water is then used to

wash starch. Application of this process in a corn wet

milling plant reduced the load of the steep water

evaporator to 30% of the original value. The use of

RO in corn wet milling had no adverse effect on the

quality of the produced starch.

Concentration of Dextrose Syrup and

Sweet Water

0028In the manufacture of dextrose syrup, the syrup

is usually concentrated by evaporation from 30–

34% to 40–45% total solids before decolorization

with carbon. Evaporation can be successfully

replaced by RO concentration, with marked savings

in energy.

0029Also, during the manufacture of the high-fructose

syrup, an effluent called sweet water results from

washing the ion exchange and carbon columns and

storage and transportation facilities. The sweet water

contain 3–8% sugar content. This effluent can be

preconcentrated more economically by RO compared

to evaporators.

Removal of Solvent from Micella

0030The edible oil industry is based on oil extraction from

seeds with suitable solvent, degumming and bleach-

ing of the extract, followed by removal of the solvent

from the micella by evaporation.

3836 MEMBRANE TECHNIQUES/Applications of Reverse Osmosis

0031 Using special RO membranes, it is possible to

remove large amounts of the solvent from the micella,

and removal of the residual solvent can be completed

by evaporation. However, most of the material used

in RO modules does not resist the destructive action

of the solvent.

See also: Alcohol: Properties and Determination; Apples;

Beers: Biochemistry of Fermentation; Citrus Fruits:

Oranges; Processed and Derived Products of Oranges;

Lemons; Grapefruits; Limes; Coffee: Roast and Ground;

Milk: Processing of Liquid Milk; Milling: Principles of

Milling; Syrups; Tomatoes; Vegetable Oils: Oil

Production and Processing; Whey and Whey Powders:

Production and Uses

Further Reading

Darnes P and Jordanian P (1987) How el-goods reverse

osmosis to brew low alcohol beer. Brewing and Distil-

ling International 17: 47–49.

Gestoli C, Brandini S, di Francesca R and Zerdie G (1995)

Concentrating fruit juice by reverse osmosis. The low

retention–high retention method. Fruit Processing 5:

183–187.

Glover FA (1985) Ultrafiltration and Reverse Osmosis for

the Dairy Industry. Technical Bulletin 5. Reading, UK:

NIRD.

Kane L, Braddock RJ, Sims CA and Matthews RF (1995)

Lemon juice aroma concentration by reverse osmosis.

Journal of Food Science 60: 190–194.

Kollacks WA and Rekers CJN (1988) Five years of experi-

ence with the application of reverse osmosis on light

middlings in a corn wet milling plant. Starch/Starke 40:

88–94.

Koseoglu SS and Egelgau DE (1990) Membrane applica-

tions and research in the edible oil industry: an assess-

ment. Journal of American Official Chemists Society 67:

239–249.

Koseoglu SS, Lawhon JT and Lusas EW (1990) The use of

membranes in citrus juice processing. Food Technology

44: 90–97.

Kosikowski FV (1986) Membrane separation in food pro-

cessing. In: McGregor WC (ed.) Membrane Separations

in Biotechnology, pp. 201–253. New York: Marcel

Dekker.

Paulson DJ and Wilson RL (1987) Cross flow membrane

technology: its use in food industry: recent innovations.

In: Kroger M and Shapiro R (eds) Changing Food Tech-

nology, pp. 85–123. Lancester, PA: Technomic.

Rao MA and Vitali AA (1999) Fruit juice concentration and

preservation. In: Shafier Rahman MS (ed.) Hand Book

of Food Preservation, pp. 217–258. New York: Marcel

Dekker.

Robe L (1983) Hyperfiltration methods for preconcentrat-

ing juice save evaporation energy. Food Processing 44:

100–108.

Singh N and Cheryan M (1998) Membrane technology in

corn refining and bioproduct processing. Starch/Starke

50: 16–23.

Principles of Ultrafiltration

D Pal, National Dairy Research Institute, Karnal,

Haryana, India

Introduction

0001Though very rapid progress has been made in all the

membrane processes during the last two to three

decades, ultrafiltration, because of its unique nature

and diverse uses, has received more attention world-

wide. This article discusses the principle of the ultra-

filtration process, the membrane materials, and their

configurations. Some major limitations and operating

conditions are also included.

What is Ultrafiltration?

0002A pressure-driven, cross-flow membrane system that

can simultaneously purify, concentrate, and fraction-

ate organic molecules of a feed stream is known as

ultrafiltration. The process is very simple (Figure 1).

The pressurized influent stream is passed across a

semipermeable membrane which separates it into

two effluent streams known as permeate and reten-

tate (concentrate). Permeate is that fraction which has

passed through the semipermeable membrane and

contains some small dissolved molecules. Retentate

is that stream which has been enriched in the solutes

or suspended solids not passed through the mem-

brane. The physical and chemical nature of the

membrane itself controls which components perme-

ate and which are retained.

Feed

Membrane

Concentrate

Permeate

Macromolecules

(such as proteins, lipids, etc.)

Low-molecular-weight solutes

(such as lactose, salts, etc.)

Water molecules

fig0001Figure 1 Principle of ultrafiltration process. P, pump. Repro-

duced from Membrane Techniques, Principles of Ultrafiltration,

Encyclopaedia of Food Science, Food Technology and Nutrition,

Macrae R, Robinson RK and Sadler MJ (eds), 1993, Academic

Press.

MEMBRANE TECHNIQUES/Principles of Ultrafiltration 3837

Principle of Ultrafiltration

0003 The mechanism of separation in ultrafiltration is essen-

tially a sieving process in which constituents of a feed

stream are separated according to their molecular

weight. The ability of the membrane to retain the

majority of the defined macromolecules of known mo-

lecular weight is generally used to specify the porosity

of the membrane. The term used is molecular weight

cut-off (MWCO), which should be the molecular

weight of the smallest test macromolecule that is

largely rejected by the membrane. Most ultrafiltration

membranes reject the constituents having a molecular

weight in the range 1–1000 kDa. Table 1 shows the

molecular weight and relative diameter of major milk

constituents. Since the majority of ultrafiltration mem-

branes have pore sizes ranging from 1 to 50 nm, essen-

tially all milk constituents except water, lactose, ions,

and some water-soluble vitamins are rejected. Ultrafil-

tration membranes are completely impermeable to

lipids and proteins and substances bound to them.

Ultrafiltration

0004 Nearly all ultrafiltration membranes are asymmetric

in morphology. It means they have a thin dense layer

or skin (about 0.2 mm) on top of the membrane which

defines the degrees of separation affected, and a

spongy support layer (about 100 mm thick) under-

neath. These membranes are usually homogenous in

material, in that they consist of the same polymer or

copolymer throughout their structure. The following

materials have been used successfully in the prepar-

ation of ultrafiltration membranes.

Cellulose Acetate (CA)

0005 These were perhaps the first commercially developed

ultrafiltration membranes. The CA membranes, how-

ever, are not used so widely now in ultrafiltration

equipment because of the following limitations:

0006poor thermal (up to 35



C) and chemical (pH

range 3–7) stability;

0007poor tolerance to chlorine;

0008more susceptible to microbial spoilage than other

types;

0009undergo creep or compaction to a relatively

greater extent.

Polysulfone (PS)

0010Ultrafiltration membranes made of the tough, rela-

tively inert polysulfone polymer are perhaps the most

widely used currently in the food industry. In fact,

polysulfone is considered a breakthrough for ultrafil-

tration principally due to its following advantages:

0011It can be operated at higher temperatures (up to



C), with some manufacturers claiming that

their PS membranes can withstand temperatures

up to 125



0012It is compatible with a much wider pH range

(0.5–13), and thus their cleaning is easy.

0013It is available in a wide range of pore sizes, ranging

from 0.001 to 0.02 mm.

0014It has a better resistance to chlorine.

0015The only limitation of the PS membranes is that

they can only be operated in a very narrow pressure

range (170–700 kPa).

Mineral or Ceramic Membranes

0016The latest membranes on the market for ultrafiltra-

tion applications are mineral membranes. These are

formed by the deposition of alumina, zirconia, carbon,

or silver on to a microporous support of the same

material. Since these are made up of inorganic mater-

ials, they are free from the disadvantages of polymeric

membranes, such as narrow range limitations for

temperature, pH, and operating pressure. The manu-

facturers claim that the ceramic membranes can with-

stand pressures up to 2.0 MPa without creep or

compaction, and tolerate the whole pH range and

temperatures up to 400



C. The cleaning of these

membranes is very easy. They can be sterilized by

autoclaving. The limitations of ceramic membranes

include their availability only in tubular configur-

ation. The large pore sizes restrict their applications.

Also, these membranes are very expensive.

Ultrafiltration Membrane Configuration

0017The commercial ultrafiltration plants are available in

four designs, namely tubular, hollow fiber, plate and

frame, and spiral wound. The general characteristics

of each module are discussed here. Their advantages

and drawbacks are listed in Table 2.

tbl0001 Table 1 Molecular weight and relative diameters of some milk

constituents

Milkconstituent Molecular weight (Da) Diameter (nm)

Water 18 0.3

Chloride ion 35 0.4

Calcium ion 40 0.4

Lactose 342 0.8

a-Lactalbumin 14 500 3.0

b-Lactoglobulin 36 000 4.0

Blood serum albumin 69 000 5.0

Casein micelles 10

–10

10–600

Fat globules – 200–10 000

Data from Kessler HG (1981) Food Engineering and Technology. Freising:

A Kessler.

3838 MEMBRANE TECHNIQUES/Principles of Ultrafiltration

Tubular Module

0018 Tubular modules were perhaps the earliest design

of industrial-scale ultrafiltration equipment using

synthetic membranes. In this module, the membrane

itself may be cast directly on to a fiberglass tube, or

they may be cast on to a separate paper tube which is

then inserted into a perforated stainless-steel tube

(Figure 2). Bundles of several individual membrane

tubes are housed together in a stainless-steel shell.

These modules have relatively large open-feed chan-

nels with internal diameters of 12–25 mm and lengths

of 0.6–6.4 m. The feed solution, under turbulent con-

ditions, flows inside each tube, whose inner wall con-

tains the membrane. The permeate passes through the

membrane and support material and is collected in

the housing.

Hollow Fibers

0019 These are in the form of self-supporting tubes with the

dense ‘skin’ layer on the inside of the tube, the diameter

ofwhich generallyrangesfrom0.5to1.1 mm(Figure3).

Hundreds of these fibers, depending on fiber diameter

and size of the housing, are sealed into a cartridge in a

‘shell-and-tube’ arrangement and bonded at each end

into an epoxy tube sheet. The feedstream flows

through the inside of the fibers and the permeate is

collected on the outside. Each industrial cartridge may

contain from 0.7 to 2.8 m

of membrane area. These

are operated in laminar flow using high shear to con-

trol concentration polarization.

Plate and Frame

0020In this configuration, flat sheets of the membrane

are placed between support plates which form flow

channels over the membrane with heights in the range

0.5–2.5 mm. This arrangement is very similar to a

conventional plate and frame filter press. The mem-

brane and their supports are sandwiched together in

tbl0002 Table 2 Advantages and limitations of different ultrafiltration configurations

Configuration Advantages Limitations

Tubular Handles suspended solids with larger particles Highest energy consumption per unit volume of permeate

Able to predict membrane performance using

simple fluid dynamics

Possible to replace individual membrane on site,

so replacement cost is less

High pressure drop

Lowest surface area-to-volume ratio, so needs

maximum floor space

Hold-up volume per unit area high

Cleaning easy

Hollow fiber Energy consumption lowest among the modules

Highest surface area-to-volume ratio, lowest

hold-up volume

Only module that allows backflushing

Since no support for the hollow fibers, operates in

narrow pressure range (170–270 kPa)

Fibers are susceptible to plugging

Handling of large particles and suspended solids

problematic

Complete cartridge needs to be replaced in case of

leakage, so replacement costs high

Plate and frame Energy consumption moderate; less than tubular Cleaning of membrane more difficult

In case of leakage the particular membrane is

replaced; so replacement cost is lowest

Initial capital costs relatively high

Surface area-to-volume ratio and hold-up volume

intermediate between tubular and spiral wound

Spiral wound Allows very high applied pressure without

damage to the membrane

Relatively difficult to process fluids having high

suspended solids or fibrous matter

Very economical in terms of energy consumption

and membrane replacement

Large particles may hang up in the mesh spacer, so

causing cleaning problems

Capital costs very low High pressure drop

Surface area-to-volume ratio very high

Low hold-up volume

Feed in

Membranes

Perforated steel

supporting tubes

Permeate

Stainless-steel shell

Concentrate out

fig0002Figure 2 Tubular ultrafiltration module. Reproduced from

Membrane Techniques, Principles of Ultrafiltration, Encyclopaedia

of Food Science, Food Technology and Nutrition, Macrae R, Robinson

RK and Sadler MJ (eds), 1993, Academic Press.

MEMBRANE TECHNIQUES/Principles of Ultrafiltration 3839

larger numbers (e.g., 180 spacer plates and 360 mem-

brane sheets totalling 27 m

of active membrane area)

to form one module. The plates containing membrane

sandwiches on either side are generally horizontally

stacked together in ultrafiltration plants, and the feed

is pumped between the plates (Figure 4). The perme-

ate may be collected either from each sandwich or

together from the whole stack. Most plate units tend

to be operated in laminar flow, although some wide-

channel configurations could be under turbulent flow.

Spiral Wound

0021 Like plate and frame, the spiral wound module also

makes use of economical, flat sheet membranes. Two

membrane sheets with a mesh-type spacer in between,

to form the permeate channel, are pasted along three

sides (appearing like an envelope). The fourth side is

pasted to a perforated tube through which the perme-

ate passes. Another mesh-type spacer is placed on

top of the envelope and the whole arrangement is

rolled around the perforated central permeate tube

(Figure 5). The feed channel height is controlled by

the thickness of the mesh-type spacer placed between

two envelopes; spacers of 0.75–1.55 mm are common.

The feed, under laminar flow, is pumped lengthwise

along the unit, while the permeate is forced through

the membrane sheets into the permeate channel and

spirals its way towards the perforated central tube.

Operating Conditions

0022 The processing parameters during membrane

processes are optimized with a view to attaining

maximum possible flux, which is the volumetric rate

of flow of the permeate through the membrane. Four

vital parameters that affect the flux during ultrafiltra-

tion processing are discussed here.

Pressure

0023For a particular membrane, the flux is directly pro-

portional to pressure of the feed and inversely propor-

tional to viscosity (the Hagen–Poiseuille’s pore

theory) and can be expressed as

J ¼ AðP

 Þð1Þ

where J is the flux (l m

2

1

), A is the diffusivity

constant, D P

is P

P

(P is the hydraulic pressure)

and Dp is the osmotic pressure. The subscript f refers

to feed and p to the permeate. Since the osmotic

pressure of the retained macromolecules in ultrafiltra-

tion is negligible, the flux is directly proportional to

applied transmembrane pressure, i.e., J ¼ A D P

. This

relationship, however, holds true under ideal condi-

tions only. When concentration polarization, associ-

ated boundary layer, and/or fouling takes place the

flux becomes pressure-independent. The membrane

itself also provides some resistance to the flux.

Feed Concentration

0024In the mass transfer-controlled region, the flux can be

expressed employing the film theory as:

J ¼ k log

ðc

Þð2Þ

Backflush out

Permeate

Backflush in

Drain for backflushFeed

Concentrate

fig0003 Figure 3 Romicon hollow fiber ultrafiltration module. Repro-

duced from Membrane Techniques, Principles of Ultrafiltration,

Encyclopaedia of Food Science, Food Technology and Nutrition,

Macrae R, Robinson RK and Sadler MJ (eds), 1993, Academic

Press.

Permeate

Concentrate

Feed

Membrane

Lock ring

Membrane

Section

plateSupport plate

Passage

ring

Support

plate

(a)

(b)

fig0004Figure 4 Plate and frame ultrafiltration module: (a) internal

flow of product; (b) oval-shaped plate membranes and support

plate. Reproduced from Membrane Techniques, Principles of

Ultrafiltration, Encyclopaedia of Food Science, Food Technology

and Nutrition, Macrae R, Robinson RK and Sadler MJ (eds),

1993, Academic Press.

3840 MEMBRANE TECHNIQUES/Principles of Ultrafiltration

where k is the mass transfer coefficient, c

is the

concentration of gel at the membrane surface, and

is the concentration of solutes in the bulk.

According to this theory, the flux should decrease

with increasing feed concentration, and J ¼ 0 when

the concentration of solutes at the membrane and in

the bulk are the same.

Temperature

0025 A higher operating temperature reduces the viscos-

ities of the feed and permeate and so increases the

diffusivity. Thus it helps increasing the flux in both

pressure-controlled and mass transfer-controlled

regions. In general, it is best to operate at the highest

possible temperature compatible with the feed and

the membrane.

Feed Velocity

0026 Higher turbulence has a major beneficial effect on

flux in the mass transfer region. The higher the feed

velocity (flow rate) across the membrane, the faster is

the ‘sweeping’ of the built-up solutes away from the

membrane surface, thereby reducing the thickness of

the concentration boundary layer. Turbulence during

the operation of ultrafiltration can be generated by

stirring or by pumping of the feedstream.

Flux-Limiting Factors

0027 The performance of membrane systems is generally

gauged by the flux behavior. During ultrafiltration

processing, the flux is lower than that of pure water.

Concentration polarization and fouling are perhaps

the main factors for this decline.

Concentration Polarization

0028 During ultrafiltration, the solution is brought to the

membrane surface by hydraulic transport. Because of

the semipermeable nature of membrane, a portion of

solvent, with or without some solutes, permeates

through the membrane. This leads to a higher concen-

tration of solutes at the membrane surface compared

to the bulk. The building-up of the solutes on the

membrane surface, known as concentration polariza-

tion, causes a steep concentration gradient within

the boundary layer and results in a backdiffusion of

solutes into the bulk. Eventually, a steady state is

reached when the movement of the solutes towards

the membrane by convective transport and their

return to the bulk by diffusive transport balance each

other. An increase in applied pressure will not help in

this situation.

0029The solutes rejected by the membrane are usually

deposited on the membrane surface in the form of a

fairly viscous and gelatinous layer. This gel layer is

considered to be mainly responsible for the lower

flux, due to hydrodynamic resistance. When the con-

centration of solutes in the gel layer reaches the super-

saturation point, fouling of the membrane begins.

0030Formation of the gel layer can be minimized by

proper fluid management, such as (1) lowering the

pressure, (2) lowering the feed concentration, and/or

(3) increasing the feed velocity.

Membrane Fouling

0031The fouling of membranes is another major limiting

factor in ultrafiltration processing. In contrast to

concentration polarization, which is considered a time-

independent, reversible process, fouling is an irreversible

phenomenon. During fouling, flux drops with operat-

ing time, usually rapidly in the initial stages and slowly

in later stages. Fouling is generally attributed to:

0032the accumulation or absorption of macromol-

ecules or colloidal particles, such as proteins,

lipids, microorganisms and/or inorganic salts, on

the membrane surface;

0033the precipitation of permeable solutes, such as

sugars and salts, due to overcrowding within the

membrane pores.

Feed

Concentrate

Permeate

Permeate collector

Membrane

Mesh

Membrane

Permeate collector

Permeate

fig0005 Figure 5 Spiral wound UF module. Reproduced from Membrane Techniques, Principles of Ultrafiltration, Encyclopaedia of Food Sci-

ence, Food Technology and Nutrition, Macrae R, Robinson RK and Sadler MJ (eds), 1993, Academic Press.

MEMBRANE TECHNIQUES/Principles of Ultrafiltration 3841

0034 The fouling of membranes not only reduces the

flux but also makes the cleaning operation more dif-

ficult and expensive. The rejection properties of the

membrane are also altered due to the formation of

this secondary layer.

0035 The fouling problem can be alleviated by proper:

(1) selection of the membrane and plant design; (2)

pretreatment of the feed; and (3) cleaning of the

membrane. In fact, the pretreatment of the feed

plays a most important role in reducing fouling prob-

lems, and differs from feed to feed.

0036 In addition to concentration polarization and

fouling, changes in membrane properties, either due

to compaction or chemical deterioration, are also

responsible for a decline of flux. The compaction of

membranes generally takes place due to faulty oper-

ating conditions (high pressure and temperature), and

the use of strong cleaning solutions is responsible for

chemical deterioration.

Cleaning and Maintenance of

Ultrafiltration Membranes

0037 The most important consideration in cleaning and

maintenance is never to allow the feed constituents

to dry out on the membrane surface. While deciding

on the cleaning schedule for an ultrafiltration mem-

brane, the nature of the feed constituents that foul the

membrane, and the recommendations of the mem-

brane and detergent manufactures should be kept in

mind. The following procedure can be adopted for

routine cleaning of a membrane:

0038 After the processing is complete, the feed stream is

flushed out from the whole plant with water. The

highest temperature of wash water compatible

with the membrane should be used. Flushing is

stopped when clear water starts coming out of

the system.

0039 Then a commercial detergent solution is circulated

through the plant. For PS membranes, alkali (1%)

and acid (0.5%) detergents are circulated one after

the other. The addition of a sequestering agent,

e.g., hexametaphosphate, to cleaning detergents

is very beneficial. In the case of CA membranes

and excessive fouling of PS membranes, the use of

a proteolytic enzyme is essential.

0040 After each circulation of detergent, the whole

system is thoroughly rinsed with hot water.

0041 To insure proper cleaning, the flux rate with water

under standard conditions is tested. If the water flux

is lower than the initial flux, step (2) is repeated.

0042 The membrane is sanitized and stored in 0.5%

formaldehyde solution. It is never allowed to

remain dry. (See Cleaning Procedures in the Fac-

tory: Types of Detergent; Types of Disinfectant.)

See also: Cleaning Procedures in the Factory: Types of

Detergent; Types of Disinfectant