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=Membranes=

From Wikipedia, the free encyclopedia

For the purposes of University of Kentucky course WRD 204

Overview
Membranes are thin porous materials that use small channels, known as pores, to separate solid solutes from a solution. Membranes often use size and well as surface charge to selectively permit substances to pass through. Gede Wenten in the Research Gate article "Recent Development In Membrane and Its Industrial Applications" defines a membrane as "a thin  flexible sheet  or  film,  acting  as  a selective  boundary between two  phases because of its  semi permeable properties."

Properties of Membranes
Classification

Membranes are generally classified by pore size which determines the functionality of the membrane. As discussed by a report by the Texas Water Development Board titled "Fundamentals of Membranes for Water Treatment", Microfiltration (MF) removes particles larger than 0.1 micrometers. Ultrafiltration (UF) removes particles larger than about 5 nanometers. Nanofiltration (NF) can remove particles large than about a nanometer. Reverse Osmosis (RO) removes particles larger than about 0.1 nanometers.

Types

Most membranes, especially those seen in industry, are made of the same thin film carbon materials which are configured differently to fit the process. As discussed by Dr. Sirshendu De in his NPTEL paper "Membrane Based Separation Processes": The most common of these are spiral wound membrane modules. These are flat sheets of membranes would in a cylinder with a flexible spacer separating them to provide a flow channel and maximize surface area. The feed solution is sent through the outside of one end of the module while the permeate travels through the membrane and into the center of the cylinder. These spiral wound modules can be connected to each other and easily operated in series. Hollow fiber membranes are also common and use many hollow fibers bound together to form the selective barrier. These fibers and the solution are pressurized and, depending on the configuration, the solution can flow from the outside of the fibers into the vessel or from the vessel into the fiber and produce a pure permeate. Tubular and flat sheet modules are similar in that they simply use a perpendicular flow through a tube or vacuum system to pass through solution. Flat sheet membranes often require cleaning and back washing.

Flow

Flow is generally a function of pressure and pore size. The rate at which a solution permeates through a membrane is known as flux. Membranes with larger pores generally have higher flux and therefore require lower pressures and less energy to output the same amount of permeate. Flux of a membrane can also be affected by a process called fouling. Fouling is the deposition and aggregation of particles of solute on the surface of the membrane. Fouling can be caused by organic matter, chemical deposits, and large particles.

Uses of Membranes
Water Purification

Membranes are widely used for water purification as they are by far the most cost effective and thermodynamically efficient method to create usable and even drinkable water from various contaminated sources. Grey water and waste water can be purified and sanitized through the membrane process for further use in industry or public use.

Medical Applications

Medical blood dialysis uses membranes to remove toxins from the blood in a similar way to human kidneys. Membranes are also used in similar processes for removal of waste from the body.

Food Production

The agricultural industry uses membranes to separate proteins in milk to make nutritional supplements and for the commercial production of milk.

Precious Metal Recovery

Membranes are often used to recover trace precious metals and materials from industrial waste streams. For example, rare earth metals and Lanthanide metals can be recovered from the ash of coal power plants.