. It can be noticed that at the beginning of the sixth period, the important OLR modification generated a significant decrease of efficiency during about 25 days. The TCOD and SCOD concentrations were then about twice higher than in precedent periods. Nevertheless, these values progressively decreased till reaching the precedent levels after a period of about 30 days. Such a modification of system performances was linked to the rough appearance of high VFA concentrations in acidogenic phase and corresponded to punctual pH decrease in methanogenic reactor before adding of soda solution.