From day 191 to 281 nutrient concentrations were routinely measured to identify the essential nutrients and to estimate the minimum or critical concentration of these nutrients. The flushing routine was changed from once every second day to once every week. The media sump was cleaned and refilled twice during this period. First, on day 191 the media sump was cleaned and refilled with a fresh batch of media. The feed gas flowrate and all other parameters were kept the same. From day 211 the concentrations of NH 4 + , K + , PO 4 3-, Fe, Ni, Co, and Mn were measured routinely, the results are shown in Figure . As can been seen in Figure , right after the media replacement activity on day 191, there was an almost complete conversion of H and Fe could be the significant parameter for the methanogens in BTF. However, this does not mean that other trace metals are not essential; their significance for the methanogens and for the overall anaerobic digestion process have been reviewed in detail .