This paper deals with the control of fixed bed bioreactors. The dynamics of these processes are described by a set of partial differential equations, which are reduced to ordinary differential equations by using a finite difference method. The task of this work is to demonstrate that a standard proportional-integral (PI) control law, which is based an the dynamics at the bioreactor output, can be used to regulate the operation of fixed bed bioreactors (eg, fermenters, biofilters, etc). The performance of the resulting control law is illustrated by simulation results on a fixed bed anaerobic waste water treatment process.