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Filtration

Filtration technology is primarily utilised to remove suspended solids. These solids can be dirt, silt or other particulate materials that may affect the downstream treatment process or the final use of water. The process involves passing the entire water stream through a filter medium, with (hopefully) all the contaminants retained in (or on) the medium.

Available filter systems include:

Media Filters

These consist of a tank containing granular media such as sand, anthracite or garnet which capture suspended solids and retain them inside the bed until it is taken offline and backwashed. Media filters are typically capable of removing suspended solids down to 10-20 microns in size.

A typical media filter is also known as bed filters.

Cartridge filters

Cartridge filters are replaceable “inserts,” usually cylindrical in configuration that are placed into housings. They are typically replaced when the pressure drop across the housing becomes unacceptable (usually above 10psig ie 0.68 bar) due to the collected suspended solids. Offered in many different designs and micron removal ratings (down into the submicron range), they provide an excellent array of choices to the knowledgeable design engineer.

A cartridge filter is illustrated below.

For filter cartridges used in water environments, the medium can be composed of one or more of the following materials:

  • Cellulosic polymers
  • Polypropylene
  • Polyethylene
  • Nylon
  • PTFE
  • PVDF
  • Polysulfone
  • Ceramics
  • Activated carbon

Removal Mechanisms

Suspended solids removal by a filter cartridge is accomplished by surface filtration and depth filtration.

Surface filtration

Largely involves a sieving process in that particles which are too big to pass through the openings (pores) are simply retained on the surface of the filter. These filters are also known as “screen” or “membrane” filters and as the structure is uniform; pore sizes can be accurately controlled.

Depth filtration

Utilises a more porous medium, which contains non-uniform openings that do not extend straight through the medium. This “tortuous” path, and the fact that these pores are of varying diameters throughout the depth of the medium, entraps the particles within the body of the cartridge. Another phenomenon known as “adsorption” contributes to the entrapment of particles by utilising chemical attraction forces which result in particles adhering to the medium inside the pores. Contaminant removal is accomplished by one or more of the following mechanisms: direct interception, inertial impaction and diffusion.

In reality, no filter is either a pure surface or depth filter, in that the membranes of surface filters have a finite thickness in which some depth filtration can occur, and depth filters also retain some particulate matter on their surfaces.

Construction

Surface filters are generally manufactured by laminating or casting a thin membrane coating on a backing material. The coating is produced with pores tailored to meet the specifications of the filter cartridge manufacturer.   Depth filters are produced through a number of manufacturing techniques. Probably the oldest is “string-wound”, which involves winding a length of string or yarn around a porous core to produce cartridges with the desired properties. “Melt-blown” cartridges are produced by extruding polymeric fibres onto a core. “Felt-type” cartridges are produced by winding or otherwise adhering layers of felt to a core.   There are many products manufactured by utilising more than one of these technologies to produce hybrid designs.

Not all filter cartridges are designed to simply remove sediment or particulate material. Carbon filters constitute a large family of cartridges primarily designed to remove chlorine, trace organic contaminants and certain gases from water supplies.

Activated Carbon

Activated carbon is typically manufactured from a carbon source such as coconut husks, nutshells, or coal. It is typically processed by heating the carbon at high temperatures in the absence of air, significantly increasing the porosity of the carbon and thus its ability to absorb many gases and organic compounds. In filter cartridges, activated carbon is usually granular, powdered, or in a block form.

Granular Activated Carbon (GAC) consists of loose carbon granules contained in a plastic housing that allows water to flow through the granular carbon from one end of the housing to the other. GAC filters usually exhibit good adsorption of various contaminants, but are not particularly effective at particulate reduction, due to the relatively large spaces between the carbon granules.

Powdered Activated Carbon (PAC) cartridges consist of finely ground activated carbon deposited on a fibrous backing material (in sheet or fibre form) and fabricated into a cylinder with a hollow internal core and with molded plastic end caps to ensure proper flow through the cartridge. PAC cartridges exhibit filtration properties of the substrate material (typically spun-bonded or melt-blown fibres) combined with the adsorption properties of the carbon.

Carbon block cartridges are powder activated carbon to which a thermoplastic binder has been added and which are compressed either by molding or an extrusion operation to yield a product with very small pores to effect fine filtration, again with the adsorption properties of activated carbon. Carbon block cartridges are capable of the finest filtration of any of the types of carbon cartridges.

Another type of filter cartridge also capable of fine filtration characteristics is the ceramic cartridge. It is a molded product, usually from an aluminum oxide base, but these products do not have the versatility of the molded carbon blocks.

An important characteristic of filter cartridges is their contaminant removal rating, usually expressed in microns. The industry has generally settled on two removal categories: nominal and absolute. Unfortunately, there is no agreement among manufacturers and various industry users as to the definitions of these terms.

Bag Filters

These are similar to cartridge filters except that the medium is fabricated into a bag through which the water flows. Although not available with a micron rating as small as cartridge filters, bag filters are generally “tighter” than media filters. The bag filter medium selection is more limited than for cartridges.

Both cartridge and bag filtration technologies can be modified with the addition of a fine “enhancing” medium, such as diatomaceous earth (DE), to provide finer filtration.

Continuous Filters

To meet the demands of continuous processing, a number of filters have been developed which operate continuously. These range from moving belt filters to sophisticated rotating cylinders with automatic backwashing capabilities. Most of these devices cannot filter below about 2-3 microns.

There are membrane micro-filter and ultra-filtration filters capable of much finer filtration.

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Punjab SWM drive

The Punjab state government has chalked out a comprehensive solid waste management programme, where garbage will be collected from each house, segregated and processed at a central place. This waste would be turned into fuel for power generation or compost for agricultural use and thereafter residue will be disposed scientifically. For the effective implementation of the project, the state has been divided into eight clusters – Amritsar, Jalandhar, Ludhiana, Ferozepur, Bathinda, Pathankot, Patiala and Greater Mohali Area. The sites for setting up solid waste processing plants have already been identified and the department has also secured clearance to start the project from October.

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Filtration technology is primarily utilised to remove suspended solids. These solids can be dirt, silt or other particulate materials that may affect the downstream treatment process or the final use of water. The process involves passing the entire water stream through a filter medium, with (hopefully) all the contaminants retained in (or on) the medium. Available filter systems include: Media Filters These consist of a tank containing granular media such as sand, anthracite or garnet which capture suspended solids and retain them inside the bed until it is taken offline and backwashed. Media filters are typically capable of removing suspended…

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