The objective of this work was to evaluate the production of hydrogen in a continuous system as a function of hydraulic retention time (HRT). The intermediates accumulated and other parameters of pH, oxidation-reduction potential were quantified. The heat treatment (103 degrees C for 24 h) of the compost from a cattle dung composting facility was able to select H-2-producing spores; this product was used as a seed for continuous systems. The brewery waste was used as substrate. For the eight runs with combinations of five HRTs and four pHs, the results indicate that at pH = 5.5, a maximum H-2 production of 47% H-2 concentration, 43 ml H-2/g CODadded, and 3.11 H-2/l reactor d was achieved at HRT = 18 h. Nevertheless, at HRT = 18 h, pH 5.5 was also the optimum pH for the maximum H-2 production among four pHs evaluated from 5 to 6.5. There was a significant accumulation of volatile acid and alcohols during the entire study. (c) 2005 Elsevier Ltd. All rights reserved.