A continuous attached growth process for the production of biohydrogen from crude glycerol was developed. The process consisted of an anaerobic up-flow column bioreactor (UFCB), packed with cylindrical ceramic beads, which constituted the support matrix for the attachment of bacterial cells. The effect of crude glycerol concentration, pH and hydraulic retention time on glycerol conversion, hydrogen yield and metabolite distribution was investigated. It was shown that the most critical parameter for the efficient bioconversion was the pH of the influent, whereas the hydrogen yield increased with an increase in feed glycerol concentration and a decrease in the hydraulic retention time. The main soluble metabolite detected was 1,3-propanediol in all cases, followed by butyric and hexanoic acids. The latter is reported to be produced from glycerol for the first time. Acidification of the waste reached 38.5%, and the maximum H-2 productivity was 107.3 +/- 0.7 L/kg waste glycerol at optimal conditions. (C) 2015 Elsevier Ltd. All rights reserved.