was observed during the first 10 days and a steady production was achieved after 25 days. From day 22 onwards the conversion remained stable with an average PC of 1.7 Nm 3 /m 3 d and average C N 2 +CH 4 of 87.7%. On day 65 the GRT was decreased to 59.2 min, the reactor performance remained stable at Preprint this trial with an average PC of 3.65 Nm 3 /m 3 d and average C N 2 +CH 4 of 94.2%. The outlet gas quality was improved during this period as the feed gas ratios were adjusted to reduce the unconverted CO 2 at the reactor outlet. During days 86 to 115 the feed gas flowrate was further increased and GRT was set at 25.7 min. The PC increased to 6.7 Nm 3 /m 3 d initially, however, there was an incomplete conversion of H 2 . The C H 2 at the reactor outlet kept increasing afterward, Figure , which showed that the activity of the hydrogenotrophic methanogens was decreasing. The average PC achieved during this period was 5.37 Nm 3 /m 3 d. As seen in Figure , the pH of the nutrient media sump was within the normal operating range (< 8.5) and the VFA concentration was also insignificant (<0.1 mg/L). The temperature of the reactor was also maintained at an average value of 52 °C during this period. Before the start of the trial at GRT 25.7 min, nutrient media was added two times to maintain the liquid level in the media sump: 350 mL on day 59 and 200 mL on day 79. The liquid level decreases due to the periodic cleaning of the external mesh filter. On day 109, 150 mL of nutrient media was added that relatively improved the H 2 conversion, and C H 2 decreased from 42% to 35%, however, this effect was short-lived and the C H 2 increased again to < 40% on day 114.