In a recent work, a new regulator of the output of a continuous autocatalytic bioprocess, by means of recirculation loop, has been presented. It was shown that controlling the recirculation flow rate allows the stabilization of the output in presence of an uncontrolled input flow rate. In the present paper, we extend this result when the input substrate concentration is unknown. For this purpose, we propose the design of an observer of the input concentration which, coupled with a slightly different control law reminiscent of the one used in the case where the input concentration is known, guarantees the regulation of the output.