Some heavy metals are essential as trace element for the optimal functioning of microorganisms in biotechnological processes. Moreover, the metal requirements can differ significantly from organism to organism. Therefore, the presence and maintenance of the required amount in an available form is a prerequisite to obtain the maximal conversion rate of a particular biotechnological process. To overcome problems because of lack of nutrients, excess is often dosed in practice. Although this method is effective, it is a waste of natural resources, results in unnecessary release of heavy metals in the environment and increases the operation costs. Thus, a targeted dosing of heavy metals can lead to an optimisation of the process performance. [Research]: Experiments will be conducted in three phases: -The first one will be oriented towards the study of the depletion of Co, Ni and Fe on the treatment performance of a continuously operated methanogenic UASB. -The second phase will be focused on the effect of different metal ligands and their bioavaliability on anaerobic sludge activity. -During the third phase the frequency and dosing concentration of trace metals will be inbestigated.