Optimization of a fixed-bed bioreactor used in wastewater treatment is addressed. The objective of optimization is to maximize the treatment efficiency of the biofilter by manipulating the feed flow rate while satisfying operational constraints. Numerical results indicate that the optimal input is characterized as being on the boundary of the admissible region. Thus, the characterized optimal solution is implemented using a simple feedback control law, which provides the optimal input profile despite variations in substrate inlet concentration and biomass growth rate. (C) 2001 John Wiley & Sons, Inc.