The vessels belonging to the first group were defined stable and the 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 the end of the experiment. In other words, in stable vessels acidogenic fermentation occurred, whereas in unstable vessels a complete anaerobic process occurred. Fig. 1 presents the average evolution of the DA over time for stable and unstable vessels, respectively.The final results were consistent within each group, irrespective of F/M and initial alkalinity.