Destabilization of an anaerobic reactor by wash-out episode: Effect on the biomethanization performance

In anaerobic processes, the volatile fatty acid profile (VFA) is closely related with the methanogenic activity and, hence, the knowledge of the relationship between these two parameters (VFA evolution and methane production) is fundamental to characterize the behavior of anaerobic reactors for treatment of organic wastes.In this study, a destabilization episode in a 5-l continuously stirred tank reactor (CSTR) at laboratory scale was induced to evaluate the influence of a wash-out episode in the biomethanization performance/efficiency (at thermophilic-dry conditions) of the organic fraction of municipal solid waste coming from a full-scale mechanical biological treatment (MBT) plant.To do this, from a stable reactor operating at 15-day solids retention time (SRT), the organic loading rate was increased in two stages: firstly from 2.93 gVS/LR day−1 (15-day SRT) to 3.66 gVS/LR day−1 (12-day SRT) during 36 days and secondly from 3.66 gVS/LR day−1 to 4.39 gVS/LR day−1 (10-day SRT) along 30 days.The results indicate that acetate-utilizing methanogenic archaea are practically washed-out during the destabilization episode at 10-day SRT. Thus, the VFA and methane profiles indicate that this group is the first population of microorganisms unbalanced on the system, since the methane production drops rapidly near to zero and an important accumulation of VFA (acetic acid mainly), from 314 mg/l to 552 mg/l (increment of 75.8%), was observed.Finally, after the destabilization episode at 10-day SRT and in order to estimate the capacity of the system to recover the biomethanization performance, the SRT was increased from 10 to 15 days and then it was gradually decreased in two stages again: first from 15 to 13 days and finally from 13 to 12 days (initial value of SRT tested in this study). The results have shown that when the same SRT (12 days) is imposed to the reactor twice and a destabilization episode by wash-out occurs between these periods, it is not possible a full recovery of the biomethanization performance.

► The effects of a transient wash-out episode on biomethanization performance were investigated.
► The acetate-utilizing methanogenic archaea are washed-out at 10-day SRT.
► An important accumulation of acetic acid is observed at 10-day SRT.
► The methane production drops near to zero at 10-day SRT.
► The biomethanization performance is restored when 15-day SRT is imposed.