The main issue of this paper is the synthesis of robust nonlinear PID control law for the regulation of the outlet substrate concentration in fixed bed bioreactor. The controller design is realized via a linearizing control law, which is coupled with a proportional-derivative reduced order observer to infer online the unknown kinetic terms related with the bioreaction rate. This controller is analogous to PID structure, where novel tuning rules are given in terms of the estimation and closed-loop time constants and the system structure. This control strategy is robust against noisy substrate concentration measurements, sustained disturbances and model uncertainties. Performance of the outlet substrate concentration in the fixed bed bioreactor is compared by means of numerical simulations, when the proposed control scheme and a standard PID controller are used.