During the 1970s, environmental engineering focused on activated sludge and other wastewater treatment systems that produce high quality effluents. However, interest in trickling filtration systems has increased in the pat several years. Recent technical innovations allow trickling filter systems to produce comparable effluents to those from activated sludge systems. Moreover, their economy of operation and relatively low capital cost bring added advantages. The research described here concerns pilot investigations on the performance of trickling filters with plastic media. The city of Garland, Tex., sponsored these investigations to develop unit process relationships for the design of improvements to the city’s Duck Creek Treatment Plant. The trickling filter/solids contact (TF/SC) process as described by Norris et al.1 had been recommended for application at the treatment plant. The TF/SC process can produce effluents of tertiary quality with respect to 5-day biochemical oxygen demand (BOD5) and total suspended solids (TSS). Monthly average effluent requirements for the Duck Creek plant are 10 mg/L BOD5 and 15 mg/L TSS.