essential metal depletion in an anaerobic reactor the effect of the absence of trace elements on the conversion of a mixture of volatile fatty acids by a distillery anaerobic granular sludge was investigated two uasb reactors were operated under identical operational conditions except for the influent trace metal concentrations during 140 days experiments were carried out in three periods where different organic loading rates olr were applied to the reactors the total trace metal concentration steadily decreased at a rate of 48 microg metalg tsd in the deprived reactor down to 35 of their initial value in contrast trace metals accumulated in granules present in the control reactor at the end of the experiment the cod removal efficiencies were 99 and 77 for the control and deprived reactors respectively due to the lack of propionate conversion cobalt sorption experiments were carried out in order to study its speciation and its effects on the speciation of other metals as well a paper mill wastewater treating granular sludge was also included in the study as a comparison results obtained showed that the principal metal forms normally associated with any sludge are a function of each soluble metal concentration in the system and the characteristics of the particular sludge