This paper presents a case history of a “rating study” that was performed at the Central Valley Water Reclamation Facility (CVWRF). The CVWRF utilizes the Trickling Filter/Solids Contact (TF/SC) treatment process and, since startup in 1988, has provided an average final effluent quality of 8 mg/l TSS and 5 mg/l CBOD. The objectives of the study were to (1) verify appropriateness of the original design parameters, based on both operational experience and process unit testing; (2) if appropriate, establish higher parameters consistent with the plant’s effluent requirements; and (3) provide a capital improvements staging plan based on actual¾not theoretical¾criteria. The results of the rating study indicated that the liquid stream unit process performance exceeded expectations. The following revisions to operating criteria were considered justified: Increase of rated plant capacity from 62 mgd to 75 mgd, primary clarifier overflow rate increase of 20 percent, trickling filter organic loading rate increase of 8 percent, and secondary clarifier overflow rate increase of 50 percent.