A 2-year, large-scale pilot investigation was conducted at the City of Newburgh Water Pollution Control Plant, Newburgh, NY, to demonstrate the application of the fluidized bed bioreactor process to the treatment of municipal wastewaters. The experimental effort investigated the ability of the process to treat municipal wastewater to secondary levels. Additionally, the studies evaluated the stability of the process under high hydraulic peak loads typical of combined sewer systems. An organic loading rate of 0.2 to 0.3 gm BOD/gm volatile suspended solids/day is suggested in order to meet secondary treatment levels. Suspended solids removals were relatively low through the reactor; the results indicate that final clarification would be required for the plant to meet a 30 mg/L TSS limitation. The system exhibited good stability under high short-term peak hydraulic loadings. Maximum to average raw flow ratios up to 5 could be handled without bed washout. Special reactor modifications to yield an expanded upper section allowed a ratio as high as 10 to 1 without loss of the bed.