Membrane

A membrane is a selective barrier that allows certain substances to pass through while blocking others. This concept applies to both biological and synthetic membranes, which have various applications in nature and industry[1].

Types of Membranes

There are two main categories of membranes:

1. Biological membranes:

• Cell membranes (outer coverings of cells or organelles)
• Nuclear membranes (covering cell nuclei)
• Tissue membranes (such as mucosae and serosae)

1. Synthetic membranes:

• Created by humans for laboratory and industrial use

Historical Context

The concept of membranes has been known since the 18th century, but their widespread use outside laboratories began after World War II. Initially, membrane filters were used to test water safety in Europe due to compromised drinking water supplies during the war[1].

Membrane Classification

Membranes are classified based on their pore size and the particles they can filter:

Membrane Processes

Microfiltration (MF)

• Removes particles larger than 0.08-2 μm
• Used for removing suspended solids, bacteria, and as a pre-treatment for reverse osmosis
• Applied in membrane bioreactors (MBR) for biological treatment[1]

Ultrafiltration (UF)

• Removes particles larger than 0.005-2 μm
• Used for similar applications as microfiltration
• Can remove high molecular weight compounds, viruses, and some endotoxins[1]

Nanofiltration (NF)

• Also known as “loose” RO
• Used for removing selected dissolved constituents from wastewater
• Developed as a membrane softening process
• Serves as a pre-treatment for reverse osmosis[1]

Reverse Osmosis (RO)

• Commonly used for desalination and removal of dissolved constituents
• Operates at high pressures (850-7000 kPa)
• Utilizes thin-film composite (TFC) membranes with three layers: polyamide, polysulphone, and polyester[1]

Nanostructured Membranes

An emerging class of membranes uses nanostructure channels for molecular-scale separations:

• Carbon nanotube membranes
• Graphene membranes
• Membranes made from polymers of intrinsic microporosity (PIMS)
• Membranes incorporating metal-organic frameworks (MOFs)[1]

Membrane Configurations

Membranes are arranged in various configurations within modules:

• Tubular
• Hollow fiber
• Spiral wound
• Plate and frame
• Ceramic and polymeric flat sheet[1]

Membrane Process Operation

Key elements affecting permeate flux:

• Membrane permeability (k)
• Operational driving force (Trans Membrane Pressure, TMP)
• Fouling and cleaning of the membrane surface

Operational modes:

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1. Constant TMP
2. Constant flux[1]

Dead-end and Cross-flow Operation Modes

1. Dead-end filtration: All feed passes through the membrane
2. Cross-flow filtration: Feed flows tangentially to the membrane, producing concentrate and permeate streams[1]

Fouling

Fouling is the accumulation of constituents on the membrane surface, leading to decreased performance. Main fouling mechanisms include:

• Build-up of feed constituents
• Formation of chemical precipitates (scaling)
• Colonization by microorganisms (biofouling)[1]

Fouling Control and Mitigation

Techniques to prevent and remove fouling:

1. Physical cleaning:

• Membrane relaxation
• Membrane backwashing
• Back pulsing

1. Chemical cleaning:

• Chemical enhanced backwash
• Chemical cleaning with agents like sodium hypochlorite and citric acid

1. Optimizing operation conditions:

• Reducing flux
• Using cross-flow filtration
• Pre-treatment of feed water

1. Membrane alteration:

• Modifying surface chemistry to reduce foulant adhesion[1]

Recycling of RO Membranes

As RO membranes have a short service life of 5-10 years, efforts are being made to recycle and reuse them:

• Waste prevention through improved design and anti-fouling techniques
• Direct reapplication in less stringent separation processes
• Converting RO membranes to porous membranes for MF or UF applications[1]

In conclusion, membranes play a crucial role in various separation processes, from biological systems to industrial applications. Ongoing research and development in membrane technology continue to improve their efficiency, longevity, and environmental impact.

Membrane (Wikipedia)

This article is about selective barrier membranes. For other uses, see Membrane (disambiguation).

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. Membranes can be generally classified into synthetic membranes and biological membranes. Biological membranes include cell membranes (outer coverings of cells or organelles that allow passage of certain constituents); 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 microfiltration and ultrafiltration technologies. Since the 1980s, these separation processes, along with electrodialysis, are employed in large plants and, today, several experienced companies serve the market.

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.

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