To study whether trace element limitations are present in methanogenic granular sludge from full-scale anaerobic bioreactors, four different anaerobic granular sludges (Nedalco, Eerbeek, Hoogeveen and Heineken) were screened for their metal content and their response to the addition of a metal cocktail and more specifically to cobalt. Three different methanogenic substrates (methanol, acetate and H-2/CO2) were used and the response to trace metal addition was monitored by on-line measurement of the changes in the specific methanogenic activity (SMA) of the sludge. A significant increase of the SMA due to trace metal addition was observed only with the substrate methanol, especially addition of only cobalt had a great effect: the SMA with methanol of the Nedalco and Hoogeveen sludge increased from, respectively, 306 and 155 mg CH4-COD g VSS-1 d(-1) to 535 and 334 mg CH4-COD g VSS-1 d(-1) upon the addition of solely 5 mu M cobalt. In the Heineken sludge, a limitation for another element was present as well. The cobalt concentrations in the sludges were low compared to the other trace elements and could not be directly related to the response of the SMA. With acetate as the substrate and in the presence of cobalt (5 mu M) a reduction of the lag-phase was observed for the Nedalco, and more pronounced for the Heineken sludge. This indicates that cobalt is also important for the onset of the activity/growth of the acetotrophic methanogenic population. Screening of the trace metal content of anaerobic granular sludge in combination with the response of the SMA to trace metal addition is therefore a valuable tool to prevent and foresee trace metal limitation in anaerobic bioreactors. (c) 2006 Elsevier Inc. All rights reserved.