The vessels belonging to the first group were defined stable andthe vessels belonging to the second group were defined unstable,because only the former group successfully maintained the acidogenic conditions (i.e., the inhibition of VFA consumption) until theend of the experiment. In other words, in stable vessels acidogenicfermentation occurred, whereas in unstable vessels a completeanaerobic process occurred. Fig. 1 presents the average evolutionof the DA over time for stable and unstable vessels, respectively.The final results were consistent within each group, irrespective ofF/M and initial alkalinity.