The use of synthesis gas fed sulfate-reducing bioreactors to simultaneously remove both oxidized sulfur compounds and metals shows great potential to treat wastewaters generated as a result of flue gas scrubbing, mining activities and galvanic processes. Detailed information about the phylogenetic and functional composition of the microbial communities within these bioreactors however was limited prior of this study. In order to understand and enable the effective control of performance of these bioreactors, an increased understanding of the microbial aspects of sulfate-reducing synthesis gas fed bioreactorsis required. 16S rRNA gene analysis demonstrated that the bacterial communities were dominated by the sulfate-reducing genera Desulfovibrio and Desuifomicrobium. Archaeal communities were comprised of microorganisms belonging to the methanogenic genus Methanobacterium. Synthesis gas fed bioreactors however were also able to sustain a diverse bacterial community, not limited to hydrogenotrophic microorganisms. Abundant 16S rRNA clones were also found which showed affiliation to the proteolytic microorganism Proteiniphilum acetatigenes and uncultured Thermotogales, while other clones clustered within the Chloroflexi subphylum I. A putative role for these organisms as scavengers of dead microbial cells was hypothesized. Due to the relatively short sludge retention time in these bioreactors hydrogen threshold concentrations are not reached, and instead Monod kinetic parameters controls hydrogen competition. As a result of fluctuations in operating conditions at full-scale a continuous state of hydrogen limitation may not be reached, resuiting in suppression of methanogenesis being a slow process. Limiting the carbon dioxide feed rate appeared to be a very effective selective tool to control methanogenesis at full scale, although it did not lead to the complete removal of methanogens from the system.Heterotrophic SRB and homoacetogens were able to coexist when H2, CO2 and sulfate were supplied as the sole substrates as they were not limited by the same substrate; homoacetogens being hydrogen limited, while heterotrophic SRB were acetate limited. This consortium was able to compete effectively with methanogens, as the growth rate of even autotrophic methanogens is negatively affected by the lack of acetate. Even though homoacetogens are reported to have a high hydrogen threshold and a low ?™, they posses a high affinity (K5), which gives them a kinetic advantage over methanogens with intermediate growth rates (0.15 -0 .50 day"1) at 300CAs growth of methanogens is affected by the availability of acetate, limiting the addition of acetate to the feed of synthesis gas fed bioreactors may provide an additional tool to selectively control methanogenesis. In conclusion,the increased understanding of the microbial communities ofsu!fate-reducing synthesis gas fed bioreactors has provided greater insight into the competition for hydrogen and possibilities to control unwanted methanogenesis. Furthermore, the performance and stability of a full-scale reactor over a period of 128 weeks, demonstrate that this technology can be used successfully at full scale to treat sulfate and metal rich wastewaters.