This paper presents a saturated proportional controller that achieves depollution of wastewater in a continuous anaerobic digester. This goal is reached by defining a region of the state-space where the depollution is achieved and forcing attractivity and invariance of this region. The control variable is the dilution rate and the controlled variable is a linear combination (S(lambda)) of the substrates concentrations, that could be the chemical oxygen demand or the biological oxygen demand, depending on the value of A. No measurement of the substrates concentrations in the input flow is required: the only necessary measurement is S(lambda).