Fluidized bed bio filters are flooded cylinders or tanks, partially filled with plastic bean bio media. Water down flows through this media bed and is aerated from bottom, causing the bio media to expand and fluidize. The large numbers of bacteria that colonize this expanded media serve to remove the nutrients that are present in the flowing water. The filter bed is no longer solidly packed, but is in dynamic motion. The most obvious benefit is that all of the filter's large surface area is available for bacterial growth (40m2/m3~1200m2/m3).