World-wide, there are many trickling filter plants that have given excellent service for decades, yet are in danger of abandonment in favor of newer technologies. Except in a few instances, trickling filter plants have produced only a semi-polished effluent and have not obtained the low TSS and BOD levels possible with the activated sludge process and other newer attached growth systems. However, since 1979, a process modification known as the TF/SC process (Trickling Filter/Solids Contact process) has allowed the production of effluents from existing trickling filters that have equaled those of any other secondary treatment process. In fact, the TF/SC process has shown itself to be capable of producing monthly average BOD and TSS of 10 mg/l and 10 mg/l respectively. These results have been obtained with both the newer plastic media, as well as with plants that incorporate older rock trickling filters.