We propose a design of robust dynamic output feedback for the global partial stabilization of a class of continuous stirred bioreactors. Our approach is robust with respect to exogenous disturbances and functional uncertainties of the kinetics, considered as bounded unknown inputs. The method is based on a design of dynamic guaranteed bounds on the unmeasured variables, conditioned to past measurements and given bounds on the initial condition. This approach is suitable to systems which are not detectable for unknown inputs. The flexibility in the design of the parameters allows the consideration of saturation constraints on the control variable.