The use of high rate fixed film biological wastewater treatment processes is gaining considerable interest as an alternative to activated sludge treatment. Much of the interest is related to the potential for smaller reactor volumes due to the ability to maintain a high concentration of biological solids, the elimination of sludge control problems associated with activated sludge clarifiers, and the potential for simple operation. The Biological Aerated Filter (BAF) is a high rate packed bed fixed film system marketed by the Eimco Process Equipment Company and recently developed by OTV of Paris, France. The purpose of the paper is to describe a BAF system, the project objectives, the system start-up experience, and initial performance. (Copyright (c) 1983.)