In this paper, anaerobic-process control based on bicarbonate alkalinity addition and regulation during start-up and overload operation is described. The experiments were performed with laboratory-scale hybrid digesters fed on olive-mill effluents. An automatic instrument monitored bicarbonate concentration and controlled its value in each reactor by alkali addition. Two different start-up procedures were tested: step-increases of substrate concentration at constant feedflow rate and step-increases of flow rate at constant feed-concentration, keeping in-reactor bicarbonate alkalinity constant by automatic control. Two overload experiments were carried out. In the first case, the volumetric loading rate was increased, as described above, up to 8 kg TOC m3 d keeping both reactors under automatic control. No major differences were observed in the performance of the reactors. In the second case, overload was induced by a 100% step-increase of the substrate concentration as acetic acid and alkalinity was controlled in one reactor only. As expected, the system without control became unstable and sour conditions developed.