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A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Biological membranes include cell membranes (outer coverings of cells or organelles that allow passage of certain constituents);[1] nuclear membranes, which cover a cell nucleus; and tissue membranes, such as mucosae and serosae. Synthetic membranes are made by humans for use in laboratories and industry (such as chemical plants).

This concept of a membrane has been known since the eighteenth century but was used little outside of the laboratory until the end of World War II. Drinking water supplies in Europe had been compromised by the war and membrane filters were used to test for water safety. However, due to the lack of reliability, slow operation, reduced selectivity and elevated costs, membranes were not widely exploited. The first use of membranes on a large scale was with micro-filtration and ultra-filtration technologies. Since the 1980s, these separation processes, along with electrodialysis, are employed in large plants and, today, several experienced companies serve the market.[2]

The degree of selectivity of a membrane depends on the membrane pore size. Depending on the pore size, they can be classified as microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) membranes. Membranes can also be of various thickness, with homogeneous or heterogeneous structure. Membranes can be neutral or charged, and particle transport can be active or passive. The latter can be facilitated by pressure, concentration, chemical or electrical gradients of the membrane process. Membranes can be generally classified into synthetic membranes and biological membranes.[3]

## Membrane processes classifications

### Microfiltration (MF)

Microfiltration removes particles higher than 0.08-2 µm and operates within a range of 7-100 kPa.[4] Microfiltration is used to remove residual suspended solids (SS), to remove bacteria in order to condition the water for effective disinfection and as a pre-treatment step for reverse osmosis.

Relatively recent developments are membrane bioreactors (MBR) which combine microfiltration and a bioreactor for biological treatment.

### Ultrafiltration (UF)

Ultrafiltration removes particles higher than 0.005-2 µm and operates within a range of 70-700kPa.[4] Ultrafiltration is used for many of the same applications as microfiltration. Some ultrafiltration membranes have also been used

This concept of a membrane has been known since the eighteenth century but was used little outside of the laboratory until the end of World War II. Drinking water supplies in Europe had been compromised by the war and membrane filters were used to test for water safety. However, due to the lack of reliability, slow operation, reduced selectivity and elevated costs, membranes were not widely exploited. The first use of membranes on a large scale was with micro-filtration and ultra-filtration technologies. Since the 1980s, these separation processes, along with electrodialysis, are employed in large plants and, today, several experienced companies serve the market.[2]

The degree of selectivity of a membrane depends on the membrane pore size. Depending on the pore size, they can be classified as microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) membranes. Membranes can also be of various thickness, with homogeneous or heterogeneous structure. Membranes can be neutral or charged, and particle transport can be active or passive. The latter can be facilitated by pressure, concentration, chemical or electrical gradients of the membrane process. Membranes can be generally classified into synthetic membranes and biological membranes.[3]

Microfiltration removes particles higher than 0.08-2 µm and operates within a range of 7-100 kPa.[4] Microfiltration is used to remove residual suspended solids (SS), to remove bacteria in order to condition the water for effective disinfection and as a pre-treatment step for reverse osmosis.

Relatively recent developments are membrane bioreactors (MBR) which combine microfiltration and a bioreactor for biological treatment.

### Ultrafiltration (UF)

Ultrafiltration removes particles higher than 0.005-2 µm and operates within a range of 70-700kPa.[4] Ultrafiltration is used for many of the same applications as microfiltration. Some ultrafiltration membranes have also been used to remove dissolved compounds with high molecular weight, such as proteins and carbohydrates. Also, they can remove viruses and some endotoxins.

The wall of an ultrafiltration hollow fiber membrane, with characteristic outer (top) and inner (bottom) layers of pores.

### Nanofiltration (NF)

Nanofiltration is also known as “loose” RO and can reject particles smaller than 0,002 µm. Nanofiltration is used for the removal of selected dissolved constituents from wastewater. NF is primarily developed as a membrane softening process which offers an alternative to chemical softening.

Likewise, nanofiltration can be used as a pre-treatment before directed reverse osmosis. The main objectives of NF pre-treatment are:[5] (1). minimize particulate and microbial fouling of the RO membranes by removal of turbidity and bacteria, (2) prevent scaling by removal of the hardness ions, (3) lower the operating pressure of the RO process by reducing the feed-water total dissolved solids (TDS) concentration.

### Reverse osmosis (RO)

Nanofiltration is also known as “loose” RO and can reject particles smaller than 0,002 µm. Nanofiltration is used for the removal of selected dissolved constituents from wastewater. NF is primarily developed as a membrane softening process which offers an alternative to chemical softening.

Likewise, nanofiltration can be used as a pre-treatment before directed reverse osmosis. The main objectives of NF pre-treatment are:[5] (1). minimize particulate and microbial fouling of the RO membranes by removal of turbidity and bact

Nanofiltration is also known as “loose” RO and can reject particles smaller than 0,002 µm. Nanofiltration is used for the removal of selected dissolved constituents from wastewater. NF is primarily developed as a membrane softening process which offers an alternative to chemical softening.

Likewise, nanofiltration can be used as a pre-treatment before directed reverse osmosis. The main objectives of NF pre-treatment are:[5] (1). minimize particulate and microbial fouling of the RO membranes by removal of turbidity and bacteria, (2) prevent scaling by removal of the hardness i

Likewise, nanofiltration can be used as a pre-treatment before directed reverse osmosis. The main objectives of NF pre-treatment are:[5] (1). minimize particulate and microbial fouling of the RO membranes by removal of turbidity and bacteria, (2) prevent scaling by removal of the hardness ions, (3) lower the operating pressure of the RO process by reducing the feed-water total dissolved solids (TDS) concentration.

Reverse osmosis is commonly used for desalination. As well, RO is commonly used for the removal of dissolved constituents from wastewater remaining after advanced treatment with microfiltration. RO excludes ions but requires high pressures to produce deionized water (850-7000 kPa).

### Nanostructured Membranes

An emerging cl

An emerging class of membranes rely on nanostructure channels to separate materials at the molecular scale. These include carbon nanotube membranes, graphene membranes, membranes made from polymers of intrinsic microporosity (PIMS), and membranes incorporating metal-organic-frameworks (MOFs). These membranes can be used for size selective separations such as nanofiltration and reverse osmosis, but also adsorption selective separations such as olefins from paraffins and alcohols from water that traditionally have required expensive and energy intensive distillation.

## Membrane configurations

In the membrane field, the term module is used to describe a complete unit composed of the membranes, the pressure support structure, the feed inlet, the outlet permeate and retentate streams, and an overall support structure. The principal types of membrane modules are:

• Tubular, where membranes are placed inside a support porous tubes, and these tubes are placed together in a cylindrical shell to form the unit module. Tubular devices are

The key elements of any membrane process relate to the influence of the following parameters on the overall permeate flux are:

• The membrane permeability (k)
• The operational driving force per unit membrane area (Trans Membrane Pressure, TMP)
• The fouling and subsequent cleaning of the membrane surface.

### Flux, pressure, permeability

The total permeate flow from a membrane system is given by following equation:

${\displaystyle Q_{p}=F_{w}\cdot A}$

Where Qp is the permeate stream flowrate [kg·s−1], Fw is the water flux rate [kg·m−2·s−1] and A is the membrane area [m2]

The permeability (k) [m·s−2·bar−1] of a membrane is given by the next equation:

${\displaystyle k={F_{w} \over P_{TMP}}}$