Developments in string wound filter cartridges
String wound filter cartridges are the most commonly used filtration media for reducing contamination in water and other liquids. Filtration applications include reverse osmosis and water softening prefiltration, boiler feed water for steam generation, cooling towers and heat exchangers, bottled water, edible oils, oil and gas production, process water for textiles, electroplating and anodizing solutions, as well as in the beverage, pharmaceutical and chemical industries. Standard cartridges are made from ‘roving’ or ‘friction spun’ yarn and factors such as media migration and chemical leaching were amongst the major drawbacks to the cartridges effectiveness. With a new innovative development in the filter media for making string wound cartridges, problems such as media migration and chemical leaching have been eliminated.
Filter cartridges, made from cotton yarn media and a metal core were first introduced in the mid-1930s. By the late 1970s cartridges with a polypropylene (PP) core and yarn had become popular as they had a wide range of chemical resistance and could be used in many applications. Before, the PP string media was a ‘roving’ – an intermediate product stage in the final textile yarn forming process. In the later years it was replaced to a great extent by ‘friction spun’ yarn that is only similar in appearance to a roving but is relatively bulkier, giving improved dirt-holding capacity and reduced resistance to the flow of liquids.
Media Migration Problem
Despite their great popularity, standard string wound cartridges have a number of shortcomings. Both the roving and friction spun media are made from short chopped fibres, which are usually 50 to 75mm in length. Many of the short fibres on the yarn’s surface are not fully locked into the main body, making them susceptible to media migration, as they tend to come loose with the flow of liquid and increasing pressure differential. Figure 1 shows the ‘roving’ and ‘friction spun’ yarns used in making standard string wound cartridges.
Loose ends of the cut fibres can be seen protruding from the surface of the yarns. The normal textile yarn forming process of fibre bale opening, carding, drawing, and spinning, by which these yarns are made, breaks some of the fibres into even shorter lengths, further aggravating the media migration problem.
Standard Cartridge made from friction-spun compact round yarn with typical diamond shaped open spaces
Chemical Leaching Problem
Another major problem with cartridges made from roving or friction spun yarn is of chemical leaching. In the manufacturing process of these yarns, ‘spin-finish’ is applied on the surface of the fibres. Spin-finish contains a number of chemicals like lubricants, surfactants, antistatic agents, antioxidants, emulsifiers and bactericides, etc. The quantity of these chemicals can be from 0.5% to as much as 2% by weight of the cartridge. Unless the filter is pre-washed, these chemicals start to leach out and can often be observed as foaming in the filtrate. The leaching of these chemicals are detrimental for the downstream processes like the activated charcoal filter, water softening resin, reverse osmosis membrane (RO) etc. The chemicals can also pose possible health problems when used for filtration of drinking water. For example, carbon filter is used to remove a range of chemicals for aesthetic and health contaminants. Carbon is also used as a pre-RO filter to remove chlorine. Chemicals leaching out from the cartridge filter are both adsorbed and deposited on the surface of the carbon, reducing the life and the effectiveness of the carbon to work properly.