Some metals are essential trace elements for microorganisms. Consequently, they are essential for the optimal operation of biotechnological treatment processes. In anaerobic wastewater treatment systems, these metals have to be present in sufficient amounts and in a bioavailable form to ensure that the conversion of the pollutant takes place at the maximum rate. In some cases they are also needed to enable specific conversions. So far, little is known about the optimal dosage of these trace elements in anaerobic wastewater treatment bioreactors. [Aim:] The main goal of this research is to get insight into the required dosing and dosing strategy of trace metals for anaerobic wastewater treatment systems. [Research:] The work program is carried out at three departments, Microbiology, Environmental Technology and Physical and Colloidal Chemistry. At Environmental Technology, the research concentrates on the specific need and dosage strategies for trace metals in reactor systems. The trace metals dosed to bioreactors precipitate, for instance with sulfide, in this way becoming less bioavailable. By chelating or complexing the trace metals before adding them to the reactor, the trace elements might become better available to the microorganisms. In this way the dosing can be optimized. The need for the different trace metals differs considerably between the different trophic groups (e.g. methanogenic, sulfidogenic and acetogenic microorganisms). Thus by optimizing the metal dosage a trophic group can be favored. Consequently, the process can be steered towards desired end products or degradation pathways. [Future research]: The effect of different dosing strategies will be studied using nickel and cobalt deprived sludge obtained in a previous reactor experiment. The effect of continuous dosing with chelated nickel and cobalt will be compared to unchelated metal addition. The effect of the presence of sulfide on the above-described conditions will also be investigated.