The reactor started producing CH 4 on the second day and on day 5 its concentration reached a maximum value of 28.5 mole% (C CH 4 at the reactor outlet. This showed that the enrichment of the hydrogenotrophic methanogens has started. After that N 2 feed to the reactor was started along with the start of GRT reduction trial. The results of the performance parameters are shown in Figure . In the first step, the GRT was 82.5 min and the H 2 concentration at the outlet kept decreasing indicating increased activity of the hydrogenotrophic methanogens. From day 22 there was an almost complete conversion of H 2 , this shows that the 7 days of enrichment period was insufficient, and it took 22 days for the development of sufficient biofilm that resulted in the complete conversion of H 2 . These results are in agreement with the direct inoculation method reported in , where a rapid increase in CH 4