http://www.waterfilters.net/WP25BB97P-StringWound-Water-Filters.html
The Pentek WP25BB97P is a nominal 25 micron polypropylene String-Wound sediment filter. The string-wound design reduces fine sediment from a variety of fluids. These water filter cartridges are wound in a precise pattern around the core providing greater surface area. The result is higher dirt-loading capacity and greater efficiency than standard wound cartridges. They are an economical solution to reduce fine sediment, including sand, silt, rust and scale particles.