Design of controllers and optimization of plants using biofilm reactors require dynamic models and efficient methods of simulation. Continuously stirred biofilm reactors (CSBRs) are useful model units in modeling a variety of different types of biofilm reactors. Often the reaction kinetics in the biofilm is described by a Monod expression. With standard modeling assumptions the equations describing the fast dynamics of a CSBR will then, for each substrate, be one nonlinear partial differential equation coupled with one linear ordinary differential equation. Here, it is shown how a few nonlinear ordinary first order differential equations, which may be solved with standard integration methods, can be used as a close approximation for both dynamic responses and steady state solutions. These low order models can conveniently be used in simulation software and for controller design.