A study of the effect of organic loading rate (OLR) and hydraulic retention time (HRT) on the performance, stability and microbial communities of a laboratory-scale completely stirred tank anaerobic reactor treating two-phase olive mill solid residue (OMSR) was carried out at mesophilic temperature (35 °C). The reactor operated at a fixed influent substrate concentration of 162 g total chemical oxygen demand (COD)/L and 126 g volatile solids (VS)/L. The OLR and HRT varied in the ranges of 0.8–11.0 g COD/L day and 108–15 days, respectively. COD removal efficiencies in the range of 97–77% were achieved for OLRs and HRTs in the ranges of 1.5–9.2 g COD/L day and 108–17 days, respectively. The maximum methane production rate was found to be 1.7 L CH4 STP/L day and it was achieved for an OLR of 9.2 g COD/L day and HRT of 17 days. The methane yield coefficient was 0.244 ± 0.005 L methane at STP conditions/g COD removed. The results obtained demonstrated that an OLR of 11.0 g COD/L day and a HRT of 15 days brought about a decrease in the pH and total volatile fatty acids (TVFA)/alkalinity ratios up to values of 5.3 and 1.5 (mequiv. acetic acid/mequiv. CaCO3), respectively, causing the destabilization of the reactor and process failure. Microbial communities, both Bacteria and Archaea, were studied by molecular fingerprinting methods, cloning and sequencing. Molecular fingerprints of bacterial communities showed higher number of major bands at increasing OLR. Firmicutes, mostly represented by the genus Clostridium, were the predominant bacteria at low OLR. Other bacterial communities such as Gammaproteobacteria, Actinobacteria, Bacteroidetes and Deferribacteres were the most abundant at high OLR. The Archaea were mainly represented by four phylotypes belonging to the genus Methanosaeta independently of the OLR. This study remarks the interest of relating OMSR decomposing bioreactor performance with the microbial communities carrying out the process in order to better understand and monitor this anaerobic digestion.

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