The carbonate and short chain fatty acid (SCFA) buffer subsystems govern the pH and thus the required alkalinity in high-rate anaerobic wastewater treatment systems. By controlling the SCFA concentration through a pH-controlled feed-on-demand system, the required alkalinity is only a function of the partial pressure of carbon dioxide (PCO2), and for non-alkalinity producing substrates, independent of the organic loading rate. This makes the pH-controlled feed-on-demand system a simple and very effective method for minimising the start-up time and improving the reliability of high-rate anaerobic digestion. For maximum sensitivity the controlling pH was measured in a pH region near the bicarbonate equivalence point. This condition was obtained by measuring the pH in a biogas stripped side stream. The concept of pH-controlled feed-on-demand was successfully applied for the start-up and operation of an upflow anaerobic sludge bed (UASB) reactor, using SCFAs as substrate.