FILTROX surface filter cartridges can be made of various materials such as polypropylene, polyester, glass fibers and steel fibers. Particles which are bigger than the filter pores cannot penetrate into the filter medium. This effect is known as the ‘sieve effect’ and occurs primarily in surface filters. The filter medium is installed in the filter cartridges in pleated form and removes residue particles individually on the surface of the filter medium. Use of glass fiber materials brings natural adsorption forces into effect.