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The Info List - Ultrafiltration


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ULTRAFILTRATION (UF) is a variety of membrane filtration in which forces like pressure or concentration gradients lead to a separation through a semipermeable membrane . Suspended solids and solutes of high molecular weight are retained in the so-called retentate, while water and low molecular weight solutes pass through the membrane in the permeate (filtrate). This separation process is used in industry and research for purifying and concentrating macromolecular (103 - 106 Da ) solutions, especially protein solutions. Ultrafiltration
Ultrafiltration
is not fundamentally different from microfiltration . Both of these separate based on size exclusion or particle capture. It is fundamentally different from membrane gas separation , which separate based on different amounts of absorption and different rates of diffusion . Ultrafiltration
Ultrafiltration
membranes are defined by the molecular weight cut-off (MWCO) of the membrane used. Ultrafiltration
Ultrafiltration
is applied in cross-flow or dead-end mode.

CONTENTS

* 1 Applications

* 1.1 Drinking water * 1.2 Protein
Protein
concentration * 1.3 Other applications

* 2 Principles

* 3 Membrane fouling

* 3.1 Concentration polarization

* 3.2 Types of fouling

* 3.2.1 Particulate deposition * 3.2.2 Scaling * 3.2.3 Biofouling
Biofouling

* 4 Membrane arrangements

* 4.1 Tubular modules * 4.2 Hollow fibre * 4.3 Spiral-wound modules * 4.4 Plate and frame

* 5 Process characteristics

* 6 Process design considerations

* 6.1 Pre-treatment

* 6.2 Membrane specifications

* 6.2.1 Material * 6.2.2 Pore size

* 6.3 Operation strategy

* 6.3.1 Flowtype * 6.3.2 Flow velocity * 6.3.3 Flow temperature * 6.3.4 Pressure
Pressure
* 6.3.5 Multi-stage, multi-module

* 6.4 Post-treatment * 6.5 Cleaning

* 7 New developments * 8 References * 9 External links

APPLICATIONS

Industries such as chemical and pharmaceutical manufacturing, food and beverage processing, and waste water treatment , employ ultrafiltration in order to recycle flow or add value to later products. Blood dialysis also utilizes ultrafiltration.

DRINKING WATER

Drinking water treatment 300 m³/h using ultrafiltration in Grundmühle waterworks (Germany)

Ultrafiltration
Ultrafiltration
can be used for the removal of particulates and macromolecules from raw water to produce potable water. It has been used to either replace existing secondary (coagulation, flocculation, sedimentation) and tertiary filtration (sand filtration and chlorination) systems employed in water treatment plants or as standalone systems in isolated regions with growing populations. When treating water with high suspended solids, UF is often integrated into the process, utilising primary (screening, flotation, filtration) and some secondary treatments as pre-treatment stages. UF processes are currently preferred over traditional treatment methods for the following reasons:

* No chemicals required (aside from cleaning) * Constant product quality regardless of feed quality * Compact plant size * Capable of exceeding regulatory standards of water quality, achieving 90–100% pathogen removal

UF processes are currently limited by the high cost incurred due to membrane fouling and replacement. Additional pretreatment of feed water is required to prevent excessive damage to the membrane units.

In many cases UF is used for pre filtration in reverse osmosis (RO) plants to protect the RO membranes.

PROTEIN CONCENTRATION

UF is used extensively in the dairy industry; particularly in the processing of cheese whey to obtain whey protein concentrate (WPC) and lactose-rich permeate. In a single stage, a UF process is able to concentrate the whey 10–30 times the feed. The original alternative to membrane filtration of whey was using steam heating followed by drum drying or spray drying. The product of these methods had limited applications due to its granulated texture and insolubility. Existing methods also had inconsistent product composition, high capital and operating costs and due to the excessive heat used in drying would often denature some of the proteins. Compared to traditional methods, UF processes used for this application:

* Are more energy efficient * Have consistent product quality, 35–80% protein product depending on operating conditions * Do not denature proteins as they use moderate operating conditions

The potential for fouling is widely discussed, being identified as a significant contributor to decline in productivity. Cheese whey contains high concentrations of calcium phosphate which can potentially lead to scale deposits on the membrane surface. As a result substantial pretreatment must be implemented to balance pH and temperature of the feed to maintain solubility of calcium salts. A selectively permeable membrane can be mounted in a centrifuge tube . The buffer is forced through the membrane by centrifugation , leaving the protein in the upper chamber.

OTHER APPLICATIONS

* Filtration
Filtration
of effluent from paper pulp mill * Cheese manufacture, see ultrafiltered milk * Removal of pathogens from milk * Process and waste water treatment * Enzyme recovery * Fruit juice concentration and clarification * Dialysis
Dialysis
and other blood treatments * Desalting and solvent-exchange of proteins (via diafiltration) * Laboratory grade manufacturing

PRINCIPLES

The basic operating principle of ultrafiltration uses a pressure induced separation of solutes from a solvent through a semi permeable membrane. The relationship between the applied pressure on the solution to be separated and the flux through the membrane is most commonly described by the Darcy equation: J = T M P R t {displaystyle J={TMP over mu R_{t}}}

where J is the flux (flow rate per membrane area),TMP is the transmembrane pressure (pressure difference between feed and permeate stream), μ is solvent viscosity, Rt is the total resistance (sum of membrane and fouling resistance).

MEMBRANE FOULING

Main article: Membrane fouling

CONCENTRATION POLARIZATION

When filtration occurs the local concentration of rejected material at the membrane surface increases and can become saturated. In UF, increased ion concentration can develop an osmotic pressure on the feed side of the membrane. This reduces the effective TMP of the system, therefore reducing permeation rate. The increase in concentrated layer at the membrane wall decreases the permeate flux, due to increase in resistance which reduces the driving force for solvent to transport through membrane surface. CP affects almost all the available membrane separation process. In RO, the so