Prior to the adoption of the uniform national treatment standards by the U.S. Environmental Protection Agency (EPA) on August 17, 1973, the trickling filter process was the secondary treatment process most widely used in plants where oxidation ponds were inappropriate. In fact, the process was the workhorse of the wastewater treatment industry for 30 years. The popularity of the trickling filter process came from its economy, low power use, reliability, stability, and ease of operation. However, effluent quality was not consistently high, typically containing 20 to 40 mg/l of biochemical oxygen demand (BOD) and suspended solids (SS). Where this effluent quality was inadequate, the engineering profession typically added tertiary treatment in the form of chemical addition or granular media filtration, or replaced the tricking filters with other processes, such as activated sludge. In retrospect, the inferior performance of the trickling filter process was largely because of the poor efficiency of the secondary sedimentation process, and not the trickling filters themselves; the trickling filters were, in fact, doing an outstanding treatment job. Research conducted for the City of Corvallis, Oreg., has shown that conventional trickling filters can achieve both secondary treatment (monthly BOD5 and SS averages less than 30 mg/l) and advanced waste treatment (monthly BOD5 and SS averages less than 10 mg/l), with relatively simple design modifications of the secondary sedimentation process. The new process is termed the trickling filter/solids contact process (TF/SC).