Advances in String-Wound Sediment Filter Cartridges
In the mid-1930s, the first string-wound depth filter cartridge entered the U.S. market. It was made from a woven wire mesh core surrounded by a cotton yarn, which was wound to form the depth filtration medium. The yarn itself was a tightly twisted medium and the major filtration essentially took place at the points of crossings of the yarns- which formed a diamond-shaped pat-tern-rather than through the yarn.
Roving & friction-spun yarns
By the late 1960s and early 1970s, polypropylene core and other synthetic media became popular as they offered resistance to growth of microorganisms and offered a wide range of chemical compatibility-or resistance. Typical tex-tile yarns were replaced by 'rovings'-a slightly twisted roll or strand of usually textile fibers-due to lower cost and improved filter life as more of the liquid could pass through the roving itself.
In the following years, roving, which is an intermediate stage before the final textile yarn forming process, was replaced by friction-spun yarns in the winding of string-wound cartridges. Friction-spun yarn is more economical to produce, is relatively bulkier and offers reduced resistance to flow of liquids leading to further improvement in life and performance of string-wound cartridges. See Figure 1 for a typical string wound cartridge.
Despite their great popularity, string-wound cartridges have many major drawbacks. Rovings and friction-spun yarns (see Figure 2) are made from short chopped fibers. Such yarns containing short fibers are inherently prone to media migration, as all the fibers are not completely locked in place. Media migration is further aggravated by the tremendous amount of aggressive handling involved in the textile yarn forming process. As the fibers go through the bale opening, carding and drawing processes involved in the automated systems of a textile plant, some get broken into shorter lengths. These add to inconsistent performance factors for filters because of both movement and particles that may escape.
Leaching of chemicals
String-wound cartridges made from synthetic yarns produced by any stan-dard textile process have another major drawback-they comprise about 1 percent by weight of chemicals that start to leach out as soon as the filter is put to use. These chemicals may be applied to the surface of the fibers to enable processing on textile machines. They consist of lubricants, surfactants, anti-static agents, antioxidants, bactericides, emulsifiers, etc. Unless the filter has been pre-washed, instructions on the cartridge normally say that after install-ing a new cartridge, let the water flow for 'so many minutes' to precondition the filter.
The problem is that the sediment cartridge is the first stage of most systems and flushed water containing the chemicals goes downstream into other treatments like the carbon filter, UV lamp, water softening resin, reverse osmosis (RO) membrane, etc. These chemicals can be detrimental to performance of all such downstream processes in addition to any possible adverse health effects. For example, a carbon filter is used to remove a range of chemical for aesthetic and health reasons. It's also used as a pre-RO filter to remove chlorine. Chemicals leaching out of a cartridge filter are both adsorbed and deposited on the surface of the carbon, reducing the life and ability of the carbon filter to work efficiently.