Trickling filters using rock media are a widely accepted form of biological treatment in Utah and other western states. Popular in both small- and medium-sized communities because of stable, reliable, and economical performance, these plants are capable of removing substantial amounts of those organic contaminants in wastewater that are conventionally removed — biochemical oxygen demand (BOD) and suspended solids (SS). Recently adopted effluent limitations, however, dictate the addition of unit processes designed not only to enhance BOD and SS removal efficiency but to reduce the emissions of other pollutants such as ammonia (NH3), coliform organisms, and chlorine toxicity as well.