Fermentative hydrogen (H-2) production process in concurrence with feeding pH [aciodophilic (pH 6.0) and neutral (pH 7.0)] and reactor operation mode (continuous and fed-batch) was evaluated in a biofilm configured reactor [upflow mode; retention time, 24 h; operating temperature, 28 +/- 2 degrees C; organic loading rate, 3.4 kg COD/m(3) day] using anaerobic mixed consortia. Acidophilic pH showed relatively effective performance with respect to H-2 production compared to neutral operation. Neutral pH illustrated effective substrate removal efficiency over the corresponding acidophilic operation. Fed-batch mode of operation with acidophilic pH showed highest H-2 production among the studied experimental variations. The pattern of soluble metabolites distribution showed the persistence of acid-forming metabolic flow associated with acidogenesis which may be considered as optimum microenvironment for effective H-2 production. Bio-electrochemical behavior of mixed anaerobic consortia (whole cell) during H-2 production process was evaluated employing cyclic voltammetry (CV) in electrochemical cell [platinum as working electrode; Ag/AgCl as reference electrode; graphite rod as counter electrode; wastewater as electrolyte] to gain insight into the possible mechanism based on intracellular electron transfer involved in the fermentative metabolic process. Voltammogram profiles visualized well defined redox pairs in forward and reverse scans at both pH conditions and the signals corresponded to intracellular electron carrier, NADH/NAD(+) (E-0, -0.32 V). Relatively higher energy output was observed in acidophilic operation which might be attributed to the possibility of efficient proton (H+) transfer between metabolic intermediates. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.