The decade of the seventies in the United States witnessed the emergence of advanced wastewater treatment (AWT) oriented to nutrient removal and polishing steps for enhanced removal of BOD5 and suspended solids. The decade of the eighties has brought a resurgent interest in reducing costs and energy usage in all forms of wastewater treatment. This paper concentrates on a relatively new secondary treatment process that typifies the interest in reduced costs, the trickling filter/solids contact (TF/SC) process. Initially developed in 1979, the TF/SC process has proven to be lower in capital, operating, and maintenance costs than either activated sludge, rotating biological contractors, or coupled trickling filter/activated sludge processes.1,2 The FT/SC process is capable of meeting a spectrum of design objectives from a monthly average of BOD5 and suspended solids (SS) of 10-10 to higher values in the 45-45 range.