Sedifilt - Syntech Fibres (Pvt)

New polypropylene media for string-wound filter cartridges

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Courtesy of Sedifilt - Syntech Fibres (Pvt)

String-wound cartridges are among the most commonly used filtration media for reducing sediments and other suspended impurities in liquids and water. Traditionally these cartridges are made from 'roving' or 'friction-spun' yarn and factors such as media migration and chemical leaching were among the major drawbacks. With a new innovative development in the polypropylene filter media for making string-wound cartridges, problems such as media migration and chemical leaching have been eliminated. Additionally new winding technology combined with the random structure of the new media has further improved the cartridge s effectiveness.

String-wound depth filter cartridges first came on the market in the mid-1930s. These were made from a cotton string media with a metal core. By the early 1980s cartridges made of polypropylene media and core had become popular due to their inertness and wide range of chemical resistance. These cartridges were wound using a polypropylene roving. Roving is a twisted strand of fibers - an intermediate product stage of standard textile yam forming process. In the later years friction-spun yarns largely replaced roving. Compared to roving, these yarns had relatively higher dirt holding capacity and reduced resistance to flow.

Media Migration - a major problem with cartridges made from roving or friction-spun media
Both roving and friction-spun media comprise short fibers, usually about 2 to 3 inches in length. Loose fiber ends protruding from the surface of such yarns can be seen clearly in Figure

Some of the fibers get broken into even shorter lengths as they pass through the various textile processes of bale opening, carding, drawing and spinning. Many of these short fibers on the yam's surface are not properly or fully locked into the loosely twisted mam body, resulting in media migration problem. Fibers on the surface tend to come loose with the flow of liquid and pressure surges in the system.

Chemical Leaching - another major problem with roving or friction-spun media
In the standard textile manufacturing process, by which polypropylene roving and friction-spun yam media are made, a spin-finish has to be necessarily applied on the surface of the fibers. Spin-finish contains a number of chemicals like lubricants, surfactants, antioxidants, antistatic agents, emulsifiers. and bactericides, etc. Without the use of these chemicals polypropylene fibers cannot be made nor converted into yams on the textile machines. The quantity of these chemicals can vary from about 0.5% to as much as 2% by weight of the cartridge media. Unless the media is pre-washed, these chemicals start to leach out and can often be observed as foaming in the filtrate. The leaching out of these chemicals can be detrimental for the filtrate as well as the downstream treatments. These chemicals may also pose possible health problems when used for filtration of drinking water.

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