A significant innovation in trickling filtrating technology in recent years has been the use of synthetic (plastic) media in place of traditional rock media. One of the principal reasons for increased use of plastic media is that it has a large specific surface area for greater slime surface per unit volume. Coupled with ample voids for air movement, this allows a higher loading biochemical oxygen demand (BOD) per unit volume than can be achieved with rock media. Another reason for its increased use is that the light weight of the media permits construction of filters up to approximately 25 ft. (7.65 m) deep; the depth for high rate rock filters is limited to approximately 5 to 6 ft (1.52 to 1.83 m). The high loading capacities and great depth make the use of plastic media advantageous in plant expansions where space is often limited. In addition, the high allowable organic loading makes plastic media particularly applicable to the treatment of strong industrial wastes.