Ahlstrom Grade 909 Medium Speed Wet-Strengthened Filter Paper. Wet-Strengthened Grades: Qualitative wet-strengthened grades are manufactured for applications using filter paper in a system or apparatus that operates under pressure or vacuum conditions. These systems require the use of extremely strong filter paper such as those in Ahlstrom's Binder Series. These grades contain a small amount of chemically stable, insoluble binding resin that strengthens the natural fiber-to-fiber bonds under wet conditions. Grades containing wood fibers or resins containing nitrogen, are not recommended for use in quantitative ashing techniques and in Kjeldahl estimations. Qualitative wet-strengthened filter papers are also used in industrial laboratories where technicians require a strong and economical filter paper with consistent quality and performance. Grade 909, 3.0 µm - Medium Speed. This medium flow rate qualitative wet-strengthened grade filter paper with very good retention is a tighter version of Ahlstrom 962. This grade has a rough surface and is often found in the food industry for routine tests.