In BTF the conversion of CO 2 and H 2 to CH 4 is carried out by the hydrogenotrophic methanogens (HM), a class of anaerobic archaea found in the anaerobic digester for the biogas production. The HM are enriched and immobilized on a packing material in the form of biofilm and the substrate gases pass over it. The optimal supply of the nutrients is essential for the continuous operation of BTF because the methanogens are dependent on their availability. Major elements found in the methanogenic microorganism are C, H, N, Na, K, S, P, Ca, and trace elements are Fe, Ni, Co, Mn, Zn, Cu, and Mo . Central to the hydrogenotrophic methanogenesis pathway are enzyme complexes that catalyze redox reactions, and these enzymes contain transition metals as cofactors for electron transport or as catalytic centers at active sites. Important transition metals for methanogenic archaea are Fe, Ni, Co, Mn, and Zn and low bioavailability of transition metals limits the methanogenesis in pure and mixed cultures . The depletion of trace metals results in process imbalances because they are essential in enzyme complexes . Specifically, for the hydrogenotrophic methanogenesis, the hydrogenases are Ni-Fe enzymes that oxidize H 2 and reduce ferredoxin (coenzyme F 420 ) . These energy converting hydrogenases are abundant in Fe and contain a cluster of Fe 4 S 4 with a Ni-Fe active site. For example, the membrane-associated hydrogenases contain three Fe 4 S 4 clusters and a Ni-Fe active site , and can contain additional polyferredoxin subunits with 6, 10, or 14 Fe 4 S 4 clusters and a Ni-Fe active site . Another type of hydrogenase contains four Fe 4 S 4 clusters and a Ni-Fe active site . Physiological studies of pure cultures have shown that optimal metal concentrations for microbial metabolism are orders of magnitude higher than in situ concentrations . In the case of biofilm reactors, such as BTF, the nutrient concentration required could be higher due to the diffusion resistance in the water and biofilm. In addition, for the biofilm reactors the supply method of the nutrients over the packing materials plays a vital role.