BACKGROUND: Enriching hydrogen (H-2)-producing bacteria by suppressing methane-producing microorganisms is a critical step in continuous biologicalH(2) production. Selective inhibition ofmethanogenscanbeachievedby varying the organic loading rate (OLR) in continuous reactors. In this study, continuous H-2 production was examined using mixed anaerobic cultures fed glucose in upflow anaerobic sludge blanket reactors (UASBRs) operating at 37 degrees C and at pH 5.0 by varying the OLR at a constant hydraulic retention time. RESULTS: A stable H-2 yield of 1.64 +/- 0.04 molmol(-1) glucose was obtained at OLRs from 8.6 to 12.8 g COD L-1 d(-1). Increasing the OLR increased the hydrogenase flux and suppressedmethanogenesis. At high OLRs, high acetate and solvent production were associated with Clostridiaceae and Ruminococcaceae while at lowOLRs, the presence of Synergistaceae and Propionibacteriaceae were related to the production of propionate and other reduced byproducts. CONCLUSION: This study successfully demonstrated continuous H-2 production using bench-scale UASBRs fed glucose. Understanding the OLR effect on H-2 production and microbial interaction is important in the full-scale operation of H-2 production facilities using low value feedstocks such as switch grass, corn stover and shorghum. (C) 2016 Society of Chemical Industry