Old rock trickling filters are being upgraded and rehabilitated with plastic sheet media. The greater surface area and higher void volume of structured-sheet media provides improved treatment efficiency, even at the very shallow depths used in old rock filters (typically 3 ft. to 7ft.). In some cases, the walls of the filter beds may be extended upward a few feet for additional increases in the rated capacity of the retrofitted plant.

Compared with rock, plastic sheet media has 2-3 times the specific surface area, which provides proportionally more area for biomass attachment. Also, the increase in the void volume from 50% to 95% improves the airflow and hydraulic loading capacity, decreases the tendency of the system to clog with biomass, and reduces odors associated with anaerobic pockets caused by silting.