Prior to the adoption of the uniform national treatment standards by 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, for 30 years the process was the workhorse of the wastewater treatment industry. The popularity of the tricking filter process was due to 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 BOD and suspended solids. 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 trickling filters with other processes such as activated sludge. In retrospect, the inferior performance of the trickling filter was largely due to the poor efficiency of the secondary sedimentation process and not to the trickling filters themselves; the trickling filters were in fact doing an outstanding job of treatment. Research conducted Brown and Caldwell and the City of Corvallis, Oregon, has shown that conventional trickling filters can achieve both secondary treatment (monthly average BOD5 and SS less than 30 mg/l) and advanced waste treatment (monthly average BOD5 and SS less than 10 mg/l) with relatively simple design modifications to the secondary sedimentation process. The new process is termed the trickling filter/solids contact process (TF/SC).