This paper summarizes the findings of a thermophilic anaerobic digester study conducted by Brown and Caldwell for the Orange Water and Sewer Authority (OWASA) in June of 1995 for implementation at the Mason Farm WWTP. Mathematical modeling of the pathogen destruction capabilities and preliminary sizing of components for three alternative thermophilic anaerobic digestion configurations were performed. The paragraphs that follow provide system descriptions of the evaluated alternatives, design criteria, and budgeted construction cost for each alternative as well as identifying the selected plan. The costs are compared to that for preliminary stage auto-thermophilic aerobic digestion (ATAD) (the previously determined least cost process to further reduce pathogens (PFRP) presently detailed in the CFR-40, Part 503 regulations). The existing plant has two anaerobic digesters and a dissolved air flotation thickener (DAFT) for thickening waste activated sludge (WAS). Fermented primary sludge (FPS) from the secondary biological phosphorus removal process is sent to the digesters unthickened. The Class-B liquid sludge is currently applied to farmland at approximately 2.4 TSS.