FPE 415 MEMBRANE PROCESSING TECHNOLOGY (1+1) Theory: Introduction to membrane technology- principles, classification& characteristics of membrane process-principles and -types of membranes-membrane processing-principles &characteristics of reverse osmosis-ultrafiltration process-advantages and disadvantages-uses of membrane for processing-difficulties encountered during membrane processing-cleaning of membranes-membrane fouling. Practicals: Ultra filtration-reverse osmosis-cleaning of membranes-operation of ultrafiltration and reverse osmosis process-calculation of flux rate-cleaning process for membrane.

Introduction

Ultrafiltration (UF) is a pressure-driven membrane separation process which has found its way to several industrial applications during the last two decades. UF effects the separation and concentration of suspended colloidal materials from true solutions.

The dairy industry has been a major area for adopting the new technology of UF. Without any doubt, the dairy industry has been one of the pioneers in the development of UF equipment and techniques based on the experience gained from its application in the dairy field. On the other hand, UF offered dairy technologists a powerful and versatile tool for the fractionation and concentration of milk constituents; that inspired their efforts to develop new products and to tailor the properties of the existing dairy products according to market needs.

The interaction between scientific findings and industrial application has been continuously rewarding to both the dairy industry and manufacturers of UF equipment. The application of UF in the dairy industry developed much faster than could be expected for a new technology.

One should recall that reverse osmosis, another member of the membrane processing technologies, had been introduced earlier than UF and received much more basic and developmental efforts. However industrial application of reverse osmosis went slower than that of UF. This has been attributed to the nature of the two processes. Reverse osmosis is purely a concentration process that can find competition from other concentration processes, i.e. evaporation, while UF is unique in bringing about separation and concentration of some compounds in one step.

By UF, molecules in a solution are separated according to the molecular size; thereby molecules of colloidal and bigger sizes are retained. UP comprises two processes:

1. Separation of macromolecules and suspended materials from small molecular weight solutes.

2. Concentration of the separated macromolecules and suspended materials to a small volume of the original solution.

Depending on the degree of separation and concentration achieved by UF, it is possible to obtain concentrates (retentates) and ultrafiltrates (permeates) the composition and properties of which are different from the original fluid and which are suitable for processing into a new generation of diversified products. At the same time, passing disposed effluents through UP plants decreases the pollution and regenerates some valuable ingredients from the waste products.

UP works on the same basis as other filtration processes (i.e. a mechanical separation of molecules) and can be operated at a normal range of temperature, which protects the separated materials from any physical or chemical changes that affect their characteristic properties.

However, the following limitations on current membrane technology have to be taken into account):

1. The UF membranes available have a wide range in pore size, which limits the applicability for the resolution of individual proteins.

2. Membrane fouling has to be considered: periodic cleaning and sanitation leads to a decline in performance, frequent cleaning would reduce the lifetime of membranes.

3. Insufficient resistance against chemicals and temperature.

4. Undesirable effects of drying and rewetting on some membranes.

Historical background

The concept of separating colloidal- materials from their solutions by membrane filtration can be traced back to t4e last century. Separation of enzymes and proteins was practiced on a laboratory scale by filtration through the semi-permeable membrane used for dialysis. However, the techniques used were tedious and time consuming which did not encourage any attempt for their use in large scale fractionation and

concentration of colloidal materials.

The evolution and expansion of UF on an industrial scale became possible after the discovery of asymmetric membranes by Loeb & Sourirajan (1962), although their objective was to prepare a tight membrane for use in desalination of sea water by reverse osmosis. Shortly after that, asymmetric UF membranes of high permeation rate, suitable mass transfer properties and good mechanical resistance were developed on the same basis.

The search for a suitable construction of UF equipment led to a tubular configuration in 1969, DDS developed a plate and frame design for UF equipment and other configurations for UF equipment became commercially available at about the same time. Since then, a rapid development in the equipment and membranes used has been apparent.

Future application of ultrafiltration in the dairy industry

The following possibilities of future applications of UF in the dairy industry can be mentioned:

1. Production of semi-hard and hard cheeses. First trials for the production of these cheese varieties by UF were not encouraging. On the basis of improved' UF equipment and experience gained from pilot experiments, promising results are emerging.

2. Production of new concentrated products. Ultrafiltered milk concentrates can be used as a base for a new generation of concentrated milk products.

3. On-farm UF of milk. Pilot experiments in France and in the United States showed the feasibility of UF application on farm level and the subsequent use of the retentate in cheese manufacture. The permeate obtained can be used in the farm particularly for feeding animals.

4. Preparation of whey protein hydrolysates for pharmaceutical uses.

5. Fractionation of milk proteins. Protein fractions of different functional properties can be prepared for specific uses.

6. Membrane bioreactors (combined microbial fermentation or enzymatic hydrolysis and continuous separation of products from the reaction medium by UP): this trend increases the productivity of fermentation and enzymatic products. With the use of whey or permeate as a fermentation medium, a wide range of products can be produced.

7. Fractionation of protein hydrolysates and preparation of peptide fractions for unique nutritional or industrial uses. For example a morphopeptide and a peptide with high surface activity have been prepared from p-casein hydrolysates by UP techniques.

BASIC PRINCIPLES

Classification and characteristics of filtration processes

Filtration processes are usually classified according to the molecular size of components retained by the filter media. However, two major classes can be .identified: conventional particle filtration and membrane filtration process. Conventional particle filtration is usually used in the separation of suspended particles larger than 10 /lm, while membrane filtration separates substances of molecular size less than 10 microns. In addition, several differences can be recorded between particle and membrane filtration. These differences are:

- The filter media used. A thick, open structure is usually used in conventional particle filtration, while in membrane filtration a thin membrane of controlled pore size is used.

- Pressure used. In membrane filtration, the use of pressure is essential as the driving force for separation, while in particle filtration pressure is applied only to accelerate the process. Gravity is the main force affecting particle separation.

- Process design. The design of membrane filtration is basically different from the conventional particle filtration. The flow of feed stream in conventional filtration is perpendicular to the filter media, and filtration can be conducted in an open system. In membrane filtration, a cross flow or tangential flow design is followed, and filtration must be carried out in a closed system. In cross flow design, the feed stream runs parallel to the membrane surface, and the permeate has a perpendicular flow to the filtration membrane.

- Degree of separation. In conventional particle separation, suspended material can be separated completely from the liquid, while membrane filtration can only concentrate the retained materials in smaller quantities relative to the original liquid.

Membrane filtration is subdivided into three classes according to the molecular size of the retained solutes. However, no sharp boundaries exist between these classes and usually there is an overlap between the molecular sizes that can be separated by these processes.

The four classes of membrane filtration are:

1. Microfiltration, which separates particles and suspended materials of sizes in the order of 0.1-10 µm. Therefore, it can effect the separation of bacteria and milk fat globules.

2. Ultrafiltration which separates colloidal materials in the order of 103-106 daltons (molecular size 0.001-0.02 µm) from smaller molecules.

3. Nanofiltration which separates multivalent ions the nominal molecular weight cutoff (MWCO) of NF membranes ranges from 100 to 1000.

4. Reverse osmosis; small molecules and ions (molecular weight less than 1000; molecular size less than 0.001 µm) can be separated from the solvents by this process.

Material used for the preparation of membranes

Polymers

Theoretically all high molecular weight polymers of good film-forming properties can be used for the manufacture of UF membrane. However, it is of utmost importance that the polymer retains its ultra-microporous structure during manufacture and under conditions to which it is exposed when in use. It must be kept in mind, that when a polymer is converted into the form of a UF membrane, it retains an enormous surface area per unit mass to its surrounding atmosphere. Therefore, the susceptibility of the membrane matrix to chemical attack is usually far greater than that exhibited by the polymer in bulk. Consequently ‘super polymers’ suitable for very high temperatures and resistant against corrosion can be considered to be among the best materials for UF membranes. Most of these super polymers are of low polarity and can be used in preparing hydrophobic but water wettable membranes, and retain water wettability as long as they are kept wet. Once dried they are all hydrophobic. They also suffer from rapid scale formation which impairs permeability. Therefore, it is desirable that the membrane should have hydrophilic characteristics which can improve the permeation rate and which, in the meantime, may be stored dry without a deterioration in their performance on reuse. Therefore, a new generation of hydrophilic polymers or suitable blends of hydrophilic polymers or co-polymers are now in use for the preparation of UF membranes. Generally, the requirements in polymer materials for making membranes are:

- to be a good film-former in an extremely thin layer which offers the least resistance to the passage of permeate;

- to posses hydrophilic groups; - to have high swelling ability with high tensile strength.

The following materials have been used successfully in the preparation of UF

membranes

Cellulose acetate

Cellulose acetate containing 38-40% by weight of acetyl groups has been successfully used in the preparation of the first generation of UF membranes. This means that about 2.5 out of 3 free hydroxyl groups of the glucose units in cellulose are substituted with acetyl groups.

Cellulose acetate has favourable properties for the production of strong flexible membranes. However, the use of cellulose acetate for membranes is subject to several limitations:

- low resistance to pH changes (pH range 3-7);

- maximum temperature of 35°C;

- low resistance to chemicals normally used for cleaning and sanitization, especially to chlorine.

Therefore, cellulose acetate membranes are not widely used now in UF equipment applied in the dairy industry.

Synthetic polymers

Polysulphones are widely used polymers in the preparation of UF membranes. The aromatic poly-sulphones are relatively resistant to oxidation because the sculpture atom is in its highest oxidation state and the siphoned group tends to draw electrons from the adjacent benzene rings to stabilize them against oxidation.

Aromatic poly-sulphones provide dimensional stability and good resistance to acids, alkalis, salt solutions and detergents even at elevated temperatures or under moderate pressure. Therefore, membranes made from poly-sulphones are characterized by a high pH and temperature resistance. Poly-sulphones are easy to fabricate in a wide range of configurations and a wide range of pore sizes.

The formulas of poly-acrylonitrile and poly-vinylidenefluoride are presented. Polyvinylidenefluoride has a good resistance against temperature and chemicals and an excellent abrasion resistance.

In the general formula of polyimide, R is a divalent organic group. Polyimides have the unique characteristic that they are not damaged if they get dry.

Polytetrafluoroethylene (PTFE) has extraordinary chemical resistance and can be used in a wide range of temperature.

In chemistry, polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer of tetrafluoroethylene which finds numerous applications. PTFE is most well known by the DuPont brand name Teflon.

PTFE is a fluorocarbon solid, as it is a high molecular weight compound consisting wholly of carbon and fluorine. Neither water and water-containing substances nor oil and oil-containing substances are wet by PTFE, as fluorocarbons demonstrate mitigated London dispersion forces due to the high electronegativity of fluorine. PTFE has one of the lowest coefficients of friction against any solid.

PTFE is used as a non-stick coating for pans and other cookware. It is very non-reactive, partly because of the strength of carbon–fluorine bonds, and so it is often used in containers and pipework for reactive and corrosive chemicals. Where used as a lubricant, PTFE reduces friction, wear and energy consumption of machinery.

History

PTFE was accidentally invented by Roy Plunkett of Kinetic Chemicals in 1938. While Plunkett was attempting to make a new CFC refrigerant, the perfluorethylene polymerized in its pressurized storage container, with the iron from the inside of the container acting as a catalyst. Kinetic Chemicals patented it in 1941 and registered the Teflon trademark in 1945.

By 1950, DuPont had acquired interest in Kinetic Chemicals and was producing over a million pounds (450 tons) of Teflon per year in Parkersburg, West Virginia. In 1954, French engineer Marc Grégoire created the first pan coated with Teflon non-stick resin under the brand name of Tefal after his wife urged him to try the material he had been using on fishing tackle on her cooking pans. In the United States, Kansas City, Missouri resident Marion A. Trozzolo, who had been using the substance on scientific utensils, marketed the first US-made Teflon coated frying pan, "The Happy Pan," in 1961.

An early advanced use was in the Manhattan Project as a material to coat valves and seals in the pipes holding highly reactive uranium hexafluoride in the vast uranium enrichment plant at Oak Ridge, Tennessee, when it was known as K-25.

Properties:

PTFE is often used to coat non-stick frying pans as it is hydrophobic and possesses fairly high heat resistance.

PTFE is a white solid at room temperature, with a density of about 2.2 g/cm3. According to DuPont its melting point is 327 °C (621 °F), but its properties degrade above 260 °C (500 °F). PTFE gains its properties from the aggregate effect of carbon-fluorine bonds, as do all fluorocarbons.

The coefficient of friction of plastics is usually measured against polished steel. PTFE's coefficient of friction is 0.1 or less, which is the second lowest of any known solid material (diamond-like carbon being the first). PTFE's resistance to van der Waals forces means that it is the only known surface to which a gecko cannot stick.

PTFE has excellent dielectric properties. This is especially true at high radio frequencies, making it suitable for use as an insulator in cables and connector assemblies and as a material for printed circuit boards used at microwave frequencies. Combined with its high melting temperature, this makes it the material of choice as a high-performance substitute for the weaker and lower melting point polyethylene that is commonly used in low-cost applications. Its extremely high bulk resistivity makes it an ideal material for fabricating long life electrets, useful devices that are the electrostatic analogues of magnets.

Because of its chemical inertness, PTFE cannot be cross-linked like an elastomer. Therefore it has no "memory" and is subject to creep. This is advantageous when used as a seal, because the material creeps a small amount to conform to the mating surface. However, in order to keep the seal from creeping too much fillers are used, which can also improve wear resistance, and reduce friction. Sometimes metal springs apply continuous force to PTFE seals to give good contact, while permitting a beneficially low percentage of creep.

Gore-Tex is a material incorporating fluoropolymer membrane with micropores. The roof of the Hubert H. Humphrey Metrodome in Minneapolis is one of the largest applications of Teflon PTFE coatings on Earth, using 20 acres (81,000 m2) of the material in a double-layered, white dome, made with PTFE-coated fiberglass that gives the stadium its distinctive appearance. The Millennium Dome in London is also substantially made of PTFE.

Powdered PTFE is used in pyrotechnic compositions as oxidizer together with powdered metals such as aluminium and magnesium. Upon ignition these mixtures form carbonaceous soot and the corresponding metal fluoride and release large amounts of heat. Hence they are used as infrared decoy flares and igniters for solid-fuel rocket propellants.[9]

PTFE is also used in body piercings, such as a sub-clavicle piercing, due to its flexibility and biocompatibility.

In optical radiometry, sheets made from PTFE are used as measuring heads in spectroradiometers and broadband radiometers (e.g. illuminance meters and UV radiometers) due to its capability to diffuse a transmitting light nearly perfectly. Moreover, optical properties of PTFE stay constant over a wide range of wavelengths, from UV up to near infrared. In this region, the relation of its regular transmittance to diffuse transmittance is negligibly small so light transmitted through a diffuser (PTFE sheet) radiates like Lambert's cosine law. Thus, PTFE enables cosinusoidal angular response for a detector measuring the power of optical radiation at a surface, e.g., in solar irradiance measurements.

PTFE is also used to coat certain types of hardened, armor-piercing bullet, so as to prevent the increased wear on the firearm's rifling that would result from the harder projectile.

PTFE's low frictional properties have also been used as 'feet' for computer mice such as the Logitech G5, Logitech G7 and Logitech G9 series and most Razer gaming mice (e.g. the Deathadder, Lachesis, ...). The low friction provided by PTFE allows the mice to glide across surfaces more smoothly and with less effort.

PTFE's high corrosion resistance makes it ideal for laboratory environments as containers, magnetic stirrer coatings, and as tubing for highly corrosive chemicals such as hydrofluoric acid, which will dissolve glass containers.

PTFE is also widely used as a thread seal tape in plumbing applications, largely replacing paste thread dope.

PTFE grafts can be used to bypass stenotic arteries in peripheral vascular disease, if a suitable autologous vein graft is not available.

PTFE can be used to prevent insects climbing up surfaces painted with the material. PTFE is so slippery that insects cannot get a grip and tend to fall off. For example PTFE is used to prevent ants climbing out of formicaria.

SAFETY

The pyrolysis of PTFE is detectable at 200 °C (392 °F), and it evolves several fluorocarbon gases and a sublimate. Animal studies indicate that it is unlikely that these products would be generated in amounts significant to health at temperatures below 250 °C (482 °F) although birds are proven to be much more sensitive to these decomposition products.

While PTFE is stable and non-toxic, it begins to deteriorate after the temperature of cookware reaches about 260 °C (500 °F), and decompose above 350 °C (662 °F).

These degradation by-products can be lethal to birds, and can cause flu-like symptoms in humans.

Meat is usually fried between 200–230 °C (392–446 °F), and most oils will start to smoke before a temperature of 260 degrees is reached, but there are at least two cooking oils (Safflower oil and Avocado oil) which have a higher Smoke point than 260 degrees. Empty cookware can also exceed this temperature upon heating.

A 1959 study (conducted before the U.S. Food and Drug Administration approved the material for use in food processing equipment) showed that the toxicity of fumes given off by the coated pan on dry heating was less than that of fumes given off by ordinary cooking oils.

A wide range of aliphatic and aromatic polymides have been used in preparing UF membranes. Typical structures of two polyamides used are presented.

Preparation of ultrafiltration membranes

UF membranes are prepared by the following methods:

Phase inversion refers to the process by which a polymer solution is inverted into a swollen three dimensional network gel through an intermediate step of a heterogeneous metastable solution. This is the most widely used process in preparing asymmetric UF membranes.

Thermal inversion is based on the use of polymers which are only soluble at high temperatures in the solvent used. On cooling the solvent loses its solubilizing power and the polymers form a gel.

Dynamic membranes are formed by deposition of a solute component (colloidal suspension) on microporous support under appropriate pressure and cross-flow conditions. This method has been used in preparing inorganic zirconium oxide membranes on porous carbon.

Ultrathin composite membranes. This method is based on preparing ultrathin films of the required rejection properties and then laminating these films on a porous support membrane .of high permeability and tensile strength. However, use of this process is limited until now to the preparation of reverse osmosis membranes.

Track-etched membranes. Seamless membranes are irradiated in a nuclear reactor. Charged particles pass through the membrane leaving tracks. Damaged materials are then removed by etching, leaving clear uniformly shaped pores.

Membrane structure

Microscopic examination of commercially-available membranes provides the basis for elucidating the structure of these membranes. There can be seen two levels of the structure: the macro and microstructure. Generally the macrostructure of membranes can be observed by ordinary microscopes; usually the membrane is made of two sections with different characteristics and composition.

1. The separation membrane, the total thickness of which is in the order of 0.1-0.25 mm.

2. A porous support that provides the mechanical resistance for the membrane against destruction or rupture by the pressure applied. This section is not directly involved in the filtration process. The porous support is usually characterized by sufficiently large pores to minimize its resistance to the flow of the filtrate. The thickness and nature of the porous support are dependent on the nature and properties of the supported membrane but usually it consists of polymeric materials of high resistance to chemicals, pressure and temperature used during filtration.

Required membrane characteristics

In order to understand the requirements needed in UF membranes, the main performance parameters should be considered. These are flux, permeability and mechanical strength which ultimately determine the durability of the membrane.

Membrane structure

In order to obtain a membrane having optimum performance it is necessary to have the skin layer as thin as possible. This will increase the flux and at the same time retain the permeability characteristics of the membrane. However, this leads to a decrease in the durability of the membrane as a result of:

- decrease in the flux due to compaction of the membrane, - increase in permeability of solutes during prolonged operations due to

cracking and stretching of the upper skin layer. It is also necessary that the membrane has a uniform pore size distribution and, therefore, a sharp molecular cut-off.

Pressure resistance

Generally, UF membranes are subjected to a pressure ranging from 1.5-8 bar. Under these conditions, the permeability of pure water decreases with time depending on the membrane material used. Consequently, membranes should resist mechanical compression (compaction) otherwise their morphology, will be deformed and their performance characteristics will be changed.

Temperature resistance

The processing temperature is a major factor affecting thermoplastic materials used for the membrane manufacture. Since many processes are performed at elevated temperatures, careful material engineering and characterization is needed. For example, UF of milk is usually carried out at 50°C to control bacterial growth during processing and to avoid the increased viscosity of concentrated milk at lower temperatures. Therefore, the membrane should resist the maximum temperature which is expected during operation and cleaning.

pH resistance

The chemical composition and properties of the solution which has to be treated affects the membrane durability. Historically, alkaline solutions were a problem to cellulose acetate membranes as the rate of hydrolysis of cellulose acetate increases exponentially with increasing pH. The pH limits of cellulose acetate have been one of the major driving forces to find new 'non-cellulosic' membranes.

Chemical compatibility

The chemical compatibility denotes the ability of a membrane to withstand the chemicals used in the cleaning solutions (acids, alkalis and detergents), oxidizing agents and disinfectants used for the sterilization of membranes. In addition, interactions between certain constituents of the treated solution and the membrane can accelerate flux reduction and membrane fouling.

Resistance to biodegradation

Biological fluids have variable loads of micro-flora that may be active against the membrane material during UF processing. Therefore, it is necessary to select membrane materials that can resist biodegradation by a contaminating micro-flora.

Testing and evaluating membranes

Testing and evaluation of membranes is an important subject for both manufacturers and users of these membranes. One major concern for membrane

manufacturers is to make their membranes according to the specifications issued by regulatory authorities. For the membrane user, testing of membranes provides precise reproducible means assessing the variability among different membranes, their performance and suitability for the intended use.

Determination of pore sizes

The bubble-point test is the most widely used method to determine the maximum average pore size. The method is based on the relationship between the height of a water column and the capillary diameter: the smaller the capillary diameter the higher the water column. Water is held up in the capillary by surface tension forces, but can be pushed back down by a pressure which has the same equivalent height as that of the water column. Thus by determining the pressure necessary to force water out of the capillary, the diameter of the capillary can be calculated. This principle has been used in determining the pore size of membranes where it is assumed that the pores are equivalent to capillaries and water is held in these pores by capillary forces. The test is carried out by connecting the membrane from the bottom to a source of regulated air pressure, while the membrane surface is wetted with water. At the pressure where bulk flow of gas begins, bubbles will be seen. This pressure is called the bubble-point pressure which is related to the large pore sizes in the membrane.

Pore size distribution

This is a statistical concept expressing the variance of the pore sizes around the mean. The method is based on measuring the air flow rate at different pressures through wet and dry membranes. The rate of air flow through the wet and dry membrane would be equal at bubble-point pressure if all the pores in the wet filter were of the same diameter. If the membrane has pores of different diameters, only the largest pores are emptied of water at bubble-point pressure. With increasing pressure, smaller pores are emptied and the rate of air flow increases. Therefore, the difference in air flow through the wet and dry membrane is dependent on the relative distribution of pore size in the membrane and the pressure applied.

The mean pore size is equivalent to that pressure at which air flow through the wet filter is one half of the flow through the dry filter.

Physical and mechanical characteristics

The thickness is measured with a precise micrometre thickness gauge. The tensile and compressive properties are usually determined by measuring changes in flow rate of a membrane subjected to variable pressures and periods of time.

The anisotropy is measured by using an ink-filled hollow point pen placed in

contact with both sides of the membrane. Circular stains of unequal diameter on the two sides of the filter will indicate anisotropy. The degree of anisotropy can be determined as the ratio of the area of the two circles.

The filtration and clogging rates indicate the water flow rate drop in fluid flow rate, respectively, with time at constant pressure.

Chemical compatibility

This test is carried out by subjecting series of membranes to the desired solvent or solution under the temperature conditions and for the period of time that will be used in the application. The membrane is then examined by visual observation, measurement of water flow rate or scanning electron microscopy. In general, the following responses of a membrane to a solvent or chemical can be noted:

- no chemical effect,

- slight swelling or distortion: the membrane may be compatible with .the solvent for a short period,

- extensive swelling and slow dissolution of the filter,

- complete dissolution or disintegration of the filter.

Extractable material and ash contents

The water extractable materials are determined by weight losses after extracting the filter with boiling water. The ash content of the membranes is very small but it may need to be determined as a background figure.

Toxicity

One important test that has to be conducted is an actual toxicity test for plastics. The filter sample (120 cm2 surface area) is divided into small pieces and extracted in 20 ml physiological saline solution at 50 or 70ºC for 24h. The extract is then injected intravenously in five white mice. The mice are observed at frequent intervals for 72 h for signs of toxicity or death. The membrane sample is considered non-toxic if none of the test animals shows any sign of toxicity.

ULTRAFILTRATION HARDWARE

Ultrafiltration modules

UF modules cover four membrane configurations. These are tubular, plate and frame, hollow fiber and spiral wound designs.

Tubular design

The tubular design offered by Abcor consists of the membrane material being casted on the inside walls of an epoxy-reinforced fiberglass backing materials. The tubes are either enclosed individually or in a bundle of 5-7 tubes in PVC or stainless steel shells; each can be considered as a module. The tubular modules available are listed in.

In the pilot plant (UF 44/180 S) a single module tube is used. In industrial scale modules, the outer permeate collection shells are removed and the permeate is collected in an enclosed cabinet housing several dozens of tubes. The tube ends are connected with U bend tubes, allowing for serial flow of the treated fluid. The Abcor tubular system is designed for cleaning the permeate and feed side of the membrane.

The Carbosep modules offered by SPEC are based on dynamically formed zirconium oxide membranes on the inside walls of the porous carbon tubes. The membrane tubes are 120 cm long, they have 1 cm external and 0-6 cm inside diameters. The tubes are assembled in a stainless steel housing, where the tube ends are retained by silastic gaskets at the end opening of the outer stainless steel case. Carbosep modules are available in five standard sizes depending on the number of tubes per module.

Advantages

1. Capable of handling feed stream with fairly large suspended particles (maximum particle size 10% of the tube diameter).

2. Easy to clean by standard CIP systems. 3. Individual membrane tubes can be easily replaced on site.

Disadvantages

1. Need efficient pumping to generate the high velocity required (>10,000 Reynolds number).

2. High pressure drop. 3. High energy consumption. 4. Low surface area/volume ratio. 5. High floor space required to install the equipment. 6. High hold up volume.

Plate and frame design

These are three major manufacturers of the plate and frame design. These are

the De Danske Sukkerfabrikker (DDS), Dorr-Oliver and Rhone-Poulenc. These systems consist of membranes sandwiched between membrane support plates which are arranged in stacks. The feed material is forced through very narrow flow channels that consist of parallel flow (Dorr-Oliver) or a combination of parallel and series flow channels (DDS and Rhone-Poulenc).

Advantages

1. Economic in energy consumption, intermediate between spiral wound and tubular systems.

2. Operating mainly in laminar flow; needs no special pumps. 3. Fairly good performance with viscous solutions. 4. Fairly high concentrations can be attained with some modules of this

configuration. 5. Low hold up volume 6. Minimum floor space is required. 7. Permeate from each membrane element can be examined separately. 8. Faulty membrane elements can be stopped without need for shut down of the

operating line. 9. Replacement of membranes on site is relatively easy.

Disadvantages

1. Fairly difficult to clean. 2. Susceptible to plugging.

Spiral wound

Abcor and Osmonic/Ladish are the main manufacturers of UF spiral wound modules for use in the dairy industry. These modules are characterized by their low costs compared to other designs.

The spiral wound modules consists of two flat sheets of membranes separated by porous membrane material. The three sides of the two membranes and the supporting porous sheet are sealed together, while the fourth side is sealed separately to a perforated permeate collecting tube. The membrane element (the two membranes and their supporting sheet) is sandwiched between two mesh feed spacers which allow free flow of the treated fluid over the membrane surface and also act as turbulence promoters to keep the membrane clean at relatively low velocities. The membrane and the mesh spacers are then wrapped around the permeate collecting tube to form the spiral wound membrane. An antitelescopic device is fixed at the downstream end of the

membrane element to prevent the possible sliding of the layers brought about by the velocity of the treated fluid. The whole assembly is placed in a tube cover. With this design, the feed material travels length-wise along the spiral, while the permeate spirals to the perforated collection tube. The feed channel height is controlled by the thickness of the mesh-like spacer. Currently, the most common spacers used are:

- 0.75 mm spacer for low channel height, - 1.1 mm spacer for large channel height.

Advantages

1. Compact, high surface area per volume; minimum floor space required. 2. Minimum energy consumption. 3. Low capital and operation costs. 4. Low hold up volume.

Disadvantages:

1. Difficult to process fluids which are high in suspended solids.

2. High pressure drop; difficult to operate with high viscous solutions.

3. Mesh spacer creates dead spots in the flow bath; may retain particles.

4. Relatively difficult to clean.

5. Faulty membrane elements require changing the whole module.

Hollow fibre

Modules of this configuration are available from Romicon and Asahi Kasei (Japan). The Alfa-Laval UF plants are equipped with Romicon cartridges. Hollow fibre modules are cartridges which contain bundles of 45 to over 3000 hollow fibre elements per cartridge. The fibres are orientated in parallel, all are potted in a resin at their ends and enclosed in the permeate collecting tube. The feed stream flows through the inside of these fibres and the permeate is collected outside and removed at the top of the collection vessel.

The hollow fibre elements are self-supporting capillary tubes with an inside dense skin and inside diameters .ranging from 0.5 to 1.4 mm.

A special feature of this configuration is the back flushing capability. Because of the homogeneity of the fibre, the cleaning solution or permeate can be pressurized to

flow back through the membrane to dislodge fouling materials on the feed side of the fibre.

The following summarizes the advantages and disadvantages of the hollow fibre configuration:

Advantages

1. High surface area per volume. 2. Low hold up volume. 3. Fair resistance to blockage of the flow channel. 4. Improved cleanability by back flushing. 5. Low energy consumption.

Disadvantages

1. Fibres are susceptible to plugging at the cartridge inlet. 2. Low maximum pressure allowed. 3. Connection of several elements is limited to parallel configuration. 4. Difficult to maintain high flow with high viscous solution in long cartridges. 5. Damage of a single fibre requires the replacement of the entire cartridge. 6. Isolation of a damaged element is difficult without shutdown of the entire

system.

DESIGN OF ULTRAFILTRATION PLANTS

Batch operation

In batch operation the fluid is pumped from a tank through the UF module and the retentate is recycled to the feed tank. As the permeate is removed, the tank contents become more concentrated. The operation is stopped when the tank contents reach the required concentration.

A variant of the basic batch process consists of keeping a constant level in the retentate tank by pumping into the system a quantity of feed stream equivalent to the removed permeate.

The batch operation mode has several advantages and disadvantages:

Advantages

- simplicity, - flexibility, - multi-purpose plant

Disadvantages

- long residence time of processed fluid in the plant, - retentate with the required concentration is obtained only at the end of

operation, - high energy consumption.

Semi-continuous operation

In the semi-continuous (single feed and bleed) operation, the feed stream is pumped from the tank to a circulation loop by means of the feed pump. The main recirculation is made directly through the modules(s) and only a smaller part is recycled to the feed tank. A sensor controls the opening and closing of automatic valves and enables a part of the retentate at the required concentration to be drawn off periodically and replaced by a corresponding quantity of feed stream from the storage tank. These operations are performed without stopping the plant and at controlled intervals. It is, therefore, like a continuous process, but feeding of processed fluid and drawing off (bleeding) of retentate are carried out at periodic intervals.

This mode of operation can be evaluated by the following advantages and disadvantages:

Advantages

- outstanding flexibility, - short average stay of the processed fluid in the plant, - increased output, - less energy consumption than with the batch operation.

Disadvantages

- induces more rapid fouling than the batch mode, - retentate is not obtained continuously, - lower flux compared to the batch mode, - higher membrane area required.

Continuous mode operation

In the continuous mode (multi-stage feed and bleed) operation the total

membranes are of the UF is divided by a number of stages connected in series. Each stage represents a separate feed and bleed mode. Therefore, the feed stream is recirculated into the first stage loop until it reaches a certain concentration. The retentate drawn off the first stage is recirculated into the second stage until it reaches a certain higher concentration. The same is repeated in the successive stags until the retentate coming out from the final stage has the required final concentration. This design overcome the disadvantage of low flux in the feed and bleed operation and simultaneously retain its continuous nature.

Advantages

- short average stay of fluid in the system, - retentate obtained continuously, - better energy utilization.

Disadvantage

- not flexible; a single purpose plant.

REVERSE OSMOSIS MEMBRANE PROCESS

Reverse osmosis (RO) occurs when the difference in the solution pressure across the membrane (∆P = PR - Pp ) exceeds the corresponding difference in the osmotic pressure (∆π=πR-πP), e.g., (∆P=∆π). In this case, the solvent (water) will pass through the membrane. Reverse osmosis is a separation process of small (monovalent) ions and molecules (M < 300) on the dense membranes. The range of the molecular size that is separated with RO is within the range 1-10A. The transmembrane pressure extends from 10 to 100 bars depending on the concentration difference of the separated species on both sides of the membrane. Membranes used for RO can separate either non-organics or organics from water. It was found that the RO separation mechanism for organics was different from and more complicated than that for inorganic ions. The separation properties of organics were mainly controlled by the physicochemical properties of organic molecules, the structure, and physicochemical properties of the membrane, and the interaction between the membrane material and organic solute.

RO Membranes

The early RO membranes used for desalination were characterized by very low Permeability. In these early years, RO membranes were classified as cellulosic and non-cellulosic. The former were made of either cellulose acetate (CA), Triacetate or a blend of the both. They have a relatively low cost, but feed water must be pre-chlorinated to prevent their biodegradation and they should be maintained in a narrow pH range to avoid hydrolysis.

Nowadays, non-cellulosic membranes are formed from a variety of chemical polymers and have relatively wide pH compatibility. The membranes used in RO processes are dense or composite. The RO separation properties of these composite membranes for organic solutes have been studied numerous times to find that some polymer membranes had more favorable separation properties for organics than those of cellulose membranes. The solute rejections of NS-100, HR-95, HR-98, SC-200, SC-1000, and PEC-1000 for an aqueous ethanol solution could reach above 60%, especially the PEC-1000 membrane for which the rejection of the aqueous ethanol solution could reach 97%.

The first synthetic polyamide was developed by John Cadotte, who mixed together two monomers, trimesoyl chloride (TMC) and meta-phenylene-diamine (MPD), at Northstar Research in 1977. He created a fully aromatic, cross linked polyamide membrane that exhibited an excellent chemical and mechanical durability with desirable flux and rejection transport properties. The membrane was unique since they used a monomeric aromatic amine instead of the polymeric aliphatic amines used previously. In additioI1, the use of a trifunctional acid chloride allowed for a membrane with some degree of cross linking. This invention would become patented under the U.S. patent number 4277344 (called as 344), and it soon became the premiere membrane for RO, nanofiltration (NF), and ultrafiltration (UF) applications. The chemistry of the "344" patent is the basis for almost all high rejection spiral wound RO membranes that are currently commercially available. The high-performance RO and NF membranes in production today are primarily condensation polymers, whose origin began with the first synthesis of nylon. Today, the thin layer composite polyamide membrane is widely accepted as the optimal system, and recently it has been found possible to further improve its performance.

The new fouling resistant membranes were invented in the early 1990s by DuPont and are protected by two U.S. patents. Nevertheless, they did not develop it into a commercial product. In 1992, TriSep Corporation purchased DuPont's spiral wound assets, which included exclusive patent rights to the X-20 membrane. TriSep continued the development of X-20 and offered X-20 as a commercial product in 1995. The original transport properties of the X-20 membrane obtained. In a preliminary set of

experiments, the pH of the feeding solution was 9.0. The rejection of the sulfate ion was high in all membranes tested (96-99.4%) regardless of the working pressure. Ammonium rejection values were between 72.3% and 83.9%, while acrylonitrile reiection was low (10.5-28.8%) compared with the results obtained for the other pollutants. Cyanide rejection was negative for all membranes tested except for HR95PP, which produced a rejection percentage of16.5%. Cyanide and ammonium could not be acceptably eliminated in a single-step operation when they are simul-taneously present in the industrial wastewater. The same wastewaters contained an organic product (acrylonitrile), and four inorganic species (sulfate, ammonium, cyanide, and sodium) were also treated with RO membranes SEPA SSlC from Osmonics, a thin layer composite CA membrane with a high selectivity toward salts that can be used in a relatively wide range of temperatures, pressures, and pH values. It was found that the pH value of the solution plays an important role in the ionization of the different species and, subsequently, in their retention. The rejection percentage of the sulfate ion was high in the two treatments assayed, regardless of the operational pH and sequence of steps followed. The degree to which NH: and CN- was eliminated was strongly dependent on the pH of the feed stream. The ammonium ion was also strongly eliminated regardless of the sequence of the steps, while the best results with CN - were obtained when the first step had a nearly neutral pH and the second a pH of 11.0.

RO Modules

The primary task for the module design was to increase the mass (water) transfer coefficient and to squeeze as large a membrane surface area as possible into a module. In practical applications, several membrane modules in the series (usually held inside a pressure vessel) were necessary to achieve a reasonable recovery. The spiral wound membrane module invented by Bray is the most widely used because it has a high membrane surface area to volume ratio, it is easy to replace, can be manufactured from a wide variety of materials, and is sold by several manufacturers. Owing to the large diffusion coefficient of salt, the narrow channel height, and the presence of spacers to promote mixing, concentration polarization is unlikely to develop significantly in a spiral wound membrane module at a low range of feed salt concentrations. Dow invented hollow fibers from CA, whereas DuPont contributed in the development of successful aromatic polyamide hollow fibers "Permasep".

RO Market and Applications

According to a Market Report from The Mcvaine Co. - RO, UE MF World Markets - the world market for crossflow membrane systems reached US$ 5 billion in 2003. The first major breakthrough in the commercial application of RO came in 1975 when Dow Chemical, Du Pont, and Fluid Systems developed large-scale RO modules for the Office of Water Research and Technology, USA. Seawater and brackish water desali-

nation systems using RO membranes enabled us to "open" the biggest source of drinking water and industrial waters. Reverse osmosis system purchases have grown from US$ 212 million in 2000 to US$ 274 million in 2006. The structure of the supplier industry is changing from formerly smaller companies (Betz, Osmonics, and Glegg) to large companies such as ITT, G E, Pentair, and Flowserve. China, Taiwan, and Korea represent the fastest growing geographical segments of the market. In the past, Japan's market was larger than the rest of the Asia combined. Soon the ultrapure water hardware purchases in China, Taiwan, and Korea will exceed those in Japan. The United States will probably remain the largest geographical market in this decade. Over the next 10 years, the semiconductor industry will have to expand its RO capacity by over 500% to meet the projected demand for ultrapure water to dean the wafers used in manufacturing. Taiwan is currently the largest market for RO systems for the electronic industry, and industries operating within the country will purchase more semiconductor RO systems this year than Japan. The United States is the larges' market for power applications of RO.

The RO investment for a coal-fired plant is several times higher than that for a gas turbine plant. The market for the desalination of seawater continues to expand as more than 50%'of the world's population finds itself with inadequate supplies of drinkable water. The continued developments of energy reduction are making RO economically viable throughout the world.

There are many beneficiaries of this escalating market. Membrane suppliers such as Dow and DuPont have the potential to individually achieve sales in excess of USS 100 million~ System suppliers, such as lonics and Osmonics, can set even higher goals. Multimillion-dollar markets are available to the suppliers of netting, substrates, housings, and instrumentation. The RO pump market is large enough such that a supplier with a 25% market share could sail past the US $ 100 million sales level.

Reverse osmosis separation of solutes from aqueous solutions is of interest from the point of view of Clean Technologies. Reverse osmosis is increasingly used in the chemical, textile, petrochemical, electrochemical, food, paper and tanning industries as well as in the treatment of tap water and waste waters. The application of RO in water processing is of a great importance for water management, recovery, recycling, and reuse. Reverse osmosis contributes to a wide extent in production of all classes of water purity from ultrapure water, tap water, industrial process water to the treatment of surface (lakes, rivers) and ground water, land fillieachates, and wastewaters. Water desalination is the most advanced application of RO. Presently, the world production of desalted water by means of RO achieved 12 million tons/day, which is 25% of the total amount of the desalted water in the world.

The main applications of RO that are overviewed in this book are ultrapure water production; water treatment; microorganisms removal; wastewater reuse; desalination; landfill leachates treatment; water purification and reuse; pickling baths; in dyeing step; the recovery of the salts from filter pressing; water recovery from treated tannery wastewater, from acid bleaching effluents, electroplating industry, steel industry; the removal of antibiotics from wastewaters; concentration of antibiotics; removal of estrogenic hormones; concentration of juices; fruit juice processing; natural aroma processing; the recovery of apple juice aroma; marine flavors; wine clarification; the application for oily wastewater treatment; TDS and chemical oxygen demand (COD) removal wastewater from a vegetable oil industry; effluent treatments from olive processing; water recycling from palm oil mill effluent; effluent treatment from abattoirs; skim milk concentrating; the treatment of process water in the dairy industry; dairy process water reuse, for the concentration of valuable products; antimony and arsenic removal from drinking water; boron removal from brackish water, seawater, and groundwater where multistage systems are applied; RO or removal of hexavalent chromium from spent tanning effluent; RO with complexation for the removal of zinc; HF recovery from the etching solution in the electronic industry; reinforced glass production, for phosphoric acid solutions.

Although water processing is the major RO application, the separation of organics in different systems is emerging that IS very important for the recovery of valuable substances or removal undesired, toxic components, as well for their purification and concentration in many sectors. Reverse osmosis is used for the separation of non-aqueous solvent systems; the separation of linear hydrocarbons and carboxylic acids from ethanol and hexane solutions; for the separation of methanol, ethanol, isopropanol from nonpolar pentane, hexane, octane; and removal of trihalomethanes.

Matsuura and Sourirajan studied such separations for alcohols, phenols, organic acids, and hydrocarbons in aqueous solutions using porous CA membranes. With the development of new polymer membranes, especially the composite membranes, the RO technique has been used in a great number of fields. As the importance of the separation for organic solutions in chemical, environmental, and food industry increases, the preparation and studies of a suitable RO membrane for these separa-tions are very necessary.

In industrial wastewater treatment systems, RO is used to bring the wastewater to "zero". The treatment of industrial wastewaters by RO reduces the high levels of dissolved salts, but has certain limitations when used for the removal of organic compounds from the effluents of the chemical industry. Reverse osmosis can be applied for the removal of toxic components from industrial wastewaters in chemical,

petrochemical, textile, paper, printing, leather, and food industry. Reverse osmosis may be used for the recovery of particularly valuable components as silver from photographical works and heavy metals. The recovery of uranium from radwastes exceeds 98%. Reverse osmosis in the petrochemical industry enables the reduction of soluted species up to 70% when the concentration is 1200 ppm, COD can be reduced to 51%. Arsenic, beryllium, antimony, cadmium, chromium, molybdenum, nickel, vanadium, platinum, tungsten, lead, mercury, copper, selenium, tall, and cyanides can be removed to more than 90% and phosphoric acid to 98%. The paper industry uses RO membranes for the removal and recycling of dyes and lignosulfones, poly- and monosaccharides where BOD can be reduced by90%, COD by 98%, and color by95%.

The RO treatment of aqueous solutions containing phenol and its derivatives has been carried out by the combination of the RO with the oxidation of organic substances by hydrogen peroxide in the presence of FeCl. Salt as a catalyst was shown to lead to the effective removal of phenol and its derivatives from aqueous solutions and a possibility for the recirculation of water.

In the food industry, RO is used for milk concentration, concentration and demineralization of whey, concentration of liquid egg white, concentration of fruit and vegetable juices like orange juice, tomato juice, apple juice, watermelon juice, onion juice, alcohol removal from beer, alcohol removal from wine concentration of wine, concentration of coffee, concentration of tea; concentration of sugar beet press water and concentration of potato fruit water.

Reverse osmosis is also used for specific purposes such as space missions, submarine and military purposes, and emergency during catastrophes. For space mis-sions, the following RO membranes were considered to recycle water: BW30 Dow, 'ESPA Hydranautics, NTR729HF Hydranautics, ATFRO AMT, ATFRO-HR AMT, HR95 DDS, and ACM4 TriSep Corp. Even though the urea molecule is larger than ions such as Na+, Cl-, and NH4 +, the rejection of urea is lower. This indicates that the chemical interaction between the solutes and the membrane is more important than the size exclusion effect. LPRO membranes appear to be the most desirable because of their high permeate flux and rejection.

During disasters or military operations, obtaining a supply of potable water is essential in saving lives. Often getting water to those who need it is difficult as the local infrastructure may be destroyed. This means that equipment should be light and robust and mobile. The portability of the equipment is, therefore, the key factor in its design. Small-size RO is not the only way to obtain water, particularly from supplies with dissolved salts. The "pore" size of an RO membrane is so fine that it can exclude bacteria, viruses, toxins, hydrocarbons, and dissolved salts. This makes it particularly versatile and capable of accepting almost any feed water, if only for a short period. Low

power requirement RO equipment draws much less power than a comparable distillation plant; therefore, smaller generators are needed. and again leading to a smaller overall size. This in turn leads to lower fuel demand and a reduced logistic burden. This is often a very significant factor. However, RO membranes are expensive, but provided they are stored correctly, they can last a long time. Typically, a membrane lasts 2-3 years during full production. As emergency equipment is run infrequently and for relatively short periods, the membranes can be expected to last longer.

The selected applications of RO

Application Membrane

Water recycle in sugar industry Osmonics Inc., OSMO 411T-MS 10, spiral wound, polyamide

Sugar concentration TFC flat sheet polysulfone membrane

Juice concentration PCI (Peterson Candy International, 18 AFC99 tubular polyamide)

Must concentration Desal RO

Aroma concentration Separem, MSCB 2521 R99 spiral wound aromatic, polyamide

Essential fruit and vegetable oils Osmonicsnc, SG, CG, AG, thin-film

Wastewater in meat industry Cellulose triacetate / diacetate blend

Yoghurt production Verind s.p.a., Italy, Y 4242 SD spiral wound cellulose acetate

Edible vegetable oil Permionics, Perma PPT-9908, spiral wound, TFC polyamide

Fermentation products recovery Film Tech. SW 30-2514 thin-film composite spiral wound

Organic acids Torary: TR70:2514F spiral wound, composite polamide

Lactic acid purification Nitto Denko Co., NTR7199, flat sheet composite polyamide

Sodium gluconate Film Tec, SW30-2514 cellulose diacetate thin-

film composite

Acetic acid Sempas Membrantechnik GmbH, composite aromatic polyamide

Carboxylic acids Cellulose acetate

Valeric acid PC1 Membrane Systems AFC99, tubular, polyamide

ULTRAFILTRATION

UF Theory

Ultrafiltration is a pressure-driven membrane process (1-10 bars) that enables the macromolecules to be separated, such as proteins, polysaccharides, polymers within the range of molecular weight (M = 1000-1000000 Da), as well as emulsions and micelles. Ultrafiltration enables the colloidal particles, emulsion, and microorganism to be removed. The commercial applications of the UF process were commenced just after the invention of asymmetric membranes in 1968.

The ultrafiltration term was introduced in 1907 by Bechold, who used membranes with graded porosities prepared by an impregnating filter paper with acetic acid collodion. The theory was developed by Ferry. Reid and Breton also proposed an interesting mechanism for membrane transport. It was postulated that the passage of water took place via hydrogen bonding to carbonyl oxygen in CA thereby filling membrane voids with bound water. Ions and molecules, not capable of hydrogen bonding, could then pass through the membrane by "hole-type diffusion" only. Solvent flux determines the economic efficiency of the process; it is important to obtain as high a flux rate, which is dependent on operating pressure, temperature, solute concentration, time, and boundary layer conditions. A number of models are available in the literature that attempt to describe the mechanism of transport through UF membranes, which depends not only on the _ membrane itself but also on phenomena at membrane vicinity when concentration polarization and cake formation take place. In addition, membrane fouling must also be included.

The resistance-in-series model relates the permeate flux to the pressure-driving force and the resistances encountered.

1 ∆P 1 ∆P

J= -- -- = --- ------------------------------

µ RT µ RM + RCP + RC + RF

where RT is the total resistance and µ is the solution viscosity. According to the resistance-in-series model, the total resistance can be considered as a sum of various components, e.g., RM - resistance of the membrane, Rcp - resistance of the concentration polarization of dissolved solids, Rc - resistance of the cake layer, and Rp - resistance of the fouling.

UF Membranes

Early efforts were directed toward application CA UF membranes marketed by Schleicher and Schuell. The breakthrough in membrane development was the invention of asymmetric membranes by Loeb and Surrirayan, who carried out a series of successful attempts to this end. They casted membranes from CA polymers obtained from Eastman Chemical Products dissolving polymer samples in acetone with the addition of aqueous magnesium perchlorate as a swelling and pore forming agent. This structure was later confirmed by electron microscopy. One of the most common polymers for membrane manufacturing is the aromatic polyamide, which has the advantages of a high rejection capability for salts, organics, and resistance to biodegradation However, aromatic polyamide membranes are also attacked by chlorine, so the feed stream must first be de-chlorinated. Current memo branes are made from hydrophobic polymers such as PSF, PES, polypropylene, polyethylene, and poly vinylidenfluoride due to their excellent chemical resistance and thermal and mechanical properties.

UF Membrane Fouling

When UF membrane systems are used for the treatment of biological process effluents, the main aim is to obtain the required water purification as economical as possible at acceptable flux rates. The realization of this goal in the direct treatment of effluents is, however, severely hampered by membrane fouling. The fouling problem has been previously approached from a number of angles, which included the optimization of flow conditions, the pretreatment of the effluent, the production of membranes with reduced adsorptive properties, the optimization of operational factors,

and the use of high-quality rinse water. All of these methods yielded moderately satisfactory results, but at a relatively high cost. An alternative approach to the fouling problem would be to reduce the pretreatment to minimal acceptable levels and to introduce extensive, but simple, membrane cleaning protocols. Membranes are normally cleaned by a combination of mechanical, chemical and /or biological methods using alkalis, acids, surface-active agents, sequestering agents, disinfectants, and enzymes as cleaning agents. Biological cleaning can be broadly described as the use of cleaning mixtures, which contain bioactive agents (microorganisms or enzymes) to enhance the removal of foulants. Some attempts were also made with ultrasonic cleaning.

UF Applications

Ultrafiltration technology has received tremendous importance for the concentration, purification, and fractionation of various products in the food industry, e.g., sugar refining, vegetable oils, corn, fruit juices, wine and beer, fluid milk, cheese, and whey. In the dairy industry, UF is used for the removal of enterotoxins from milk, for the fractionation and concentration of whey proteins, for the clarification of whey, for skim milk concentration. In the food industry, UF is used for the production of chito-oligosaccharides; for the removal of polyphenols, color pectins, and glucose from juices; for the degumming of vegetable oils; for the emulsion removal in olive mill wastewater treatment; to concentrate acid/alkaline wash down for water recycling from the palm oil mill effluent; and for the recovery of native protein from potato juice from a pea whey. In the meat industries, UF is used for effluent treatment from abattoirs for crabs, clams, oysters, shrimps, and lobster processing, for fish gelatin manufacturing. Ultrafiltration is also used for the recovery of polyphenols from oil mill wastewaters and from wine wastewaters. In water treatment, UF is used for the arsenic removal from drinking water, for humic acids removal in water treatment.

Ultrafiltration is frequently used in hybrid systems, such as UF enhanced by polymer for the recovery of-cadmium from aqueous effluents, UF enhanced by polymer for the removal of hexavalent chromium, UF/precipitation for the removal of chromium from tannery wastes, UF/complexation for the separation of cobalt from radioactive wastes, UF enhanced by micelles for the separation of Co (II)/Ni(II), UF/complexation for the recovery of copper from wastewater, UF/ powdered IX resins for the separation of Copper, UF/chitosan composite membranes for the separation of copper, UF/complexation and electrolysis for the removal of mercury from water, UF enhanced by polyelectrolyte for the recovery of mercury, UF enhanced by polymer for the recovery of nickel, UF for chromic acid separation on charged membranes, and UF enhanced by micelles for trichloroethylene (TCE) removal. Ultrafiltration is also used for water purification and reuse in hybrid systems such as ROjUF/ozonation and UF/coagulation,

ozonation. In the leather industry, UF is used in soaking step, unhairing step. deliming bating step, degreasing step, and in chromium removal, in tanning exhausted baths, sulfite recovery in unhairing-liming step, for enzyme reco\'erV in enzymatic unhairing-liming step, and the recovery of surfactants in degreasing steps, Ultrafiltration is used for paper machine circulation water, in a large scale for water reuse for the separation of lignin from black liquor. In the pharmaceutical industry, UF is used for protein fractionation and purification, for the recovery of antibiotics from fermentation broths, for virus removal, and for gene diagnosis of poisoning.

Ultrafiltration is used in the pharmaceutical industry for the fractionation, con-centration, and purification (diafiltration) of the products, such as vaccines, antibiotics, interferon, cephalosporin, etc. Separative techniques are systematically required in biotechnological processes in order to harvest microorganisms and purify their produced metabolites and become more attractive in biotechnology for industrial exploitation (enzyme recovery, membrane bioreactors) industries. These processes are isothermal and involve no phase change or chemical agents. The enzymes or cell separation could be employed in the batch mode or in continuous systems such as the coupling with biological reactors, depending on the application.

In the paper pulp production, UF enables the latex, dyes, aperture and lignosulfonate be recovered, which is further used for the manufacturing of vanillin.

In the textile industry, UF is used for the recovery of surfactants, finishing oils, and other chemicals. The finishing agents in wastewaters approach 2000-3000ppm and after concentrating to 5-8% can be totally reused. The permeate usually contains about 50 ppm contaminants, but may also be recycled to the process. The permeate flux is rather low in this case and is around 6Im-2h-l. The membrane material must be carefully selected and is usually made from stainless steel or polyfluoride winidylene.

Ultrafiltration was developed for the treatment of wastewaters and sewage, water treatment (process and drinking water).

A typical example of the successful application of the membrane in clean technology is electrophoretic painting. The paint is flushed out from the elements and is then concentrated and recovered entirely, whereas the permeate (pure water) is reused as a flushing fluid. The value of the paint recovered from one car body is approximately $4, which does not even take into account the savings from waste management elimination. The average flux in this case is around 20Im-2h-1 as given below:

Applications of the ultrafiltration process.

Application Membrane

Algae harvesting IRIS. ardis, Miribel. France. PVDF; PES; PAN. 30.000 Da:20nm

Catalyst recovery UPM.200, polysulfonamide membranes

Colloids of gold AMlCON, PM30 polysulfone, XM300, poly-co(acrylonitrile-vinyl chloride)

Colloid monomethyl mercury Amicon spiral wound regenerated cellulose

Colloids of marine Millipore PreprScalee-TFF PLAC

organic matter regenerated cel1ulose

Dyes (tannery) IRIS, (PES), MWCO of 40, 10,5 and 3 kDa

Colored waters ROMICON, Co. HFl. 1-43.PMI0, polypropylene

Colored waters PLEIADE RAYFLOW polysulfone 3000 Da MW

Oil/water emulsion Hoechst Co, TS 6V 205 polyethersulfone (PES)

Oil/water emulsion Tech-Sep tubular Carbosep 300 kDa

Oil/water emulsion Amicon YM30 and YMI00 cellulose acetate PM30 polysulfone

Olive wastewaters 30 kDa. polysulfone, tubular

Soybean oil Whatman (USA) porous alumina anodic 0.02 µm

Soybean oil NETZSCH, 027.06-1Cl/07-000YAI, Shumacher.Ti 01070,AIGl00

Sugarcane juice Hydranautics 15 kDa cellulosic

Sugarcane juice CellporeC!> (0.8 mm) Inchema, Switzerland Modified polysulfone, 20 kDa

Sugarcane juice TechSep. PPEOI06 20 (PES), PPUOI05 50 polysulfone (PSF)

Application Membrane

Sugarcane juice Perm Ionics, spiral wound, UF 20 kD

Decolorization of cane sugar Membralox (SeT). 20n):l1, 5 and 1 kDa

Decolorization of cane sugar Membralox (SeT). Amicon YM30 polysulfone, PM3 cellulose

Molasses Osmonics. Desai EW E500 PS 300 kDa

Honey Nitto Denko. GR61PP. GR60PP. GR51PP. GR40PP. polysulfone

Apple juice Carbosep 15 kDa (M2) and 50 kDa (M8) ZrolC

Apple juice Carbosep. M2. M8. 15 and 50 kDa. 5iOrTiO, on carbon support

Pectins from apple juice Carbosep. Techsep.

Depectinized apple juice Tech-Sep, Carbosep. M9. M8. M'" ZrO{fiO/C 30. 50, 300 kD.

Apple juice PCI-PVDF (FP200), FP100), PES (E5625, ES209), PS (PU120)

Star fruit juice MILIPORE 25 kDa

Grape juice Osmonics, Desai PW 10 kDa PES

Pineapple juice NGK Filtech, 0.01 pm

Passion fruit juice Amicon YM 30, 30 kDa polysulfone

Fruit pulp 4-polyvinylidene fluoride 200 kDa

Pectin PCI, UK Model B1-ES625, polyethersulfone membrane 25 kD

Wine stabilization D5S Denmark, F540PP. fluor polymer 100kDa

Wine stabilization Amicon. YM 100, YM30, YMIO. YM5. YM3, YM1

Skim milk Os monies, SEPA CF. HG-19 5 kDa. polysulfone

Whey PCI. ES 625. tubular polyethersulfone

Whey protein Millipore. 30 and 100kDa

Milk protein Arnicon YMI0, 10kDa

Spray-dried milk Koch spiral wound 10 kDa

Extracts of defatted soy flour Stork Wafilin, WFBX0121, 10kDa

Fish gelatin l>1EMBRALOX Tl-70, 50kDa, tubular

Plasma proteins Rhone-Poulenc 40 and 100 kDa

Proteins from seafood Carbosep. M2 ofZr02-Ti02 on a carbon support, 15 kDa

Proteins from a pea Koch, Rornicon PM30

Protein recovery from potato 2D-I00kD H-PES

Activated sludge Nitto Denko Co., Japan, NTU-3150, 50 kDa, polysulfone

Chondroitin sulfate SeT, (Membraiox) M2-M4

Membrane Fouling

Contamination of membranes causes a higher energy use, a higher cleaning frequency and a shorter life span of the membrane. Membrane contamination is usually called fouling. The International Union of Pure and Applied Chemistry defines fouling as follows: The process that results in a decrease in performance of a membrane, caused by the deposition of suspended or dissolved solids on the external membrane surface, on the membrane pores, or within the membrane pores. (Koros 1996) When clean water is filtered, the membrane material is the only resistance caused (Rm). The flux is than called the clean water-flux. As a result of the accumulation of

particles on the membrane through the filtration of water with a certain level of suspended solids, a cake will form on the membrane (Rc; particles). When particles block the membrane pores, this is called pore plugging (Rpb; scaling). Resistance as a consequence of adsorption in or on the membrane is called biofouling (Ra).

Resistances: Rm = membrane resistance Ra = adsorption, bio-fouling

Rpb = pore plugging Rc = cake layer

Particles, bio-fouling and scaling are three main groups of pollutants that can be distinguished from membrane fouling. These will cause the need of a higher workload, to keep the filtration capacity at a certain level. At a certain point the pressure has increased so much that it is no longer economical. For the control of membrane fouling there are many different techniques. One way to predict fouling is by using the Silt Density Index (SDI) of the feed water. The SDI, which is based on experience, can be defined as the time that is needed to filtrate an amount of water with a noted concentration of salts through a standard 0.45 mm micro filtration membrane. When the SDI is high, one can conclude that the feed water contains a high amount of membrane plugging matter.

Membrane System management

Membrane systems can be managed either through dead-end filtration or through cross-flow filtration.

Dead-end filtration:

When dead-end filtration takes place, all the water that enters the membrane surface is pressed through the membrane. Some solids and components will stay behind on the membrane while water flows through. This depends on the pore size of the membrane. Consequentially, the water will experience a greater resistance to passing through the membrane. When feed water pressure is continual, this will result in a decreasing flux. After a certain amount of time the flux has decreased to such an extent, that the membrane will need cleaning.

Dead-end management is applied because the energy loss is less than when one applies a cross-flow filtration. This is because all energy enters the water that actually passed the membrane. The pressure that is needed to press water through a membrane is called Trans Membrane Pressure (TMP).

The TMP is defined as the pressure gradient of the membrane, or the average feed pressure minus the permeate pressure. The feed pressure is often measured at the initial point of a membrane module. However, this pressure does not equal the average feed pressure, because the flow through a membrane will cause hydraulic pressure losses.

During cleaning of a membrane, components are removed hydraulically, chemically or physically. When the cleaning process is performed, a module is temporarily out of order. As a result, dead-end management is a discontinuous process. The length of time that a module performs filtration is called filtration time and the length of time that a module is cleaned is called cleaning time. In practise one always tries to make filtration time last as long as possible, and apply the lowest possible cleaning

time.

When a membrane is cleaned with permeate, it does not have a continuous production of water. This results in a lower production. The factor that indicates the amount of production is called recovery.

Dead-end filtration

Cross-flow filtration:

When cross-flow filtration takes place, feed water is recycled. During recirculation the feed water flow is parallel to the membrane. Only a small part of the feed water is used for permeate production, the largest part will leave the module. Consequentially, cross-flow filtration has a high energy cost. After all, the entire feed water flow needs to be brought under pressure.

The water speed of the feed water flow parallel to the membrane is relatively high. The purpose of this flow is the control of the thickness of the cake. Consequentially to the flow speed of the water, flowing forces are high, which enables the suspended solids to be carried away in the water flow. Cross-flow management can achieve stable fluxes. Still, the cleaning of cross-flow installations needs to be applied from time to time. Cleaning is performed by means of backward flushing or chemical cleaning. The cross-flow system is applied for Reverse osmosis, nanofiltration, ultrafiltration and microfiltration, depending on the pore size of the membrane.

Cross-flow filtration