The effect of organic loading rate (OLR) on a pressurized anaerobic filter was studied in a laboratory two-phase anaerobic digestion system. The anaerobic filter was operated successively at two working pressures (9 bar and 1.5 bar). The OLR
(COD) for each pressure was increased from 5 to 17.5 kg·m
−3·day
−1. The best performance of the reactor at 9 bar was observed at OLR
(COD) of 12.5 kg·m
−3·day
−1 and hydraulic retention time (HRT) of 1.8 day, with specific biogas productivity (SBP) of 5.3 L·L
−1·day
−1 and COD degradation grade of 90.6%. At higher OLRs and shorter HRTs, the process became unstable. In contrast, there was no indication of digester failure during the experiments at 1.5 bar. The SBP peaked at OLR
(COD) of 17.5 kg·m
−3·day
−1 with 8.2 L·L
−1·day
−1, where COD degradation grade was 90.4%. The biogas collected from the reactor at 9 bar and 1.5 bar contained approximately 74.5% CH
4 and 66.2% CH
4, respectively, regardless of OLR variation. At OLR
(COD) of 5–12.5 kg·m
−3·day
−1, the reactor at 9 bar had the same specific methane yield as at 1.5 bar, which was in the range of 0.31–0.32 L
N·g
−1COD. Raising the working pressure in the reactor resulted in an increase of methane content of the produced biogas. However, the low pH value (approximately 6.5) inside the reactor, induced by high CO
2 partial pressure seemed to limit the reactor performance at high OLRs and short HRT.
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