organic acids viz., acetate and butyrate were evaluated as primary substrates for the production of biohydrogen (H(2)) through photo-fermentation process using mixed culture at mesophilic temperature (34 degrees C). Experiments were performed by varying parameters like operating pH, presence/absence of initiator substrate (glucose) and vitamin solution, type of nitrogen source (mono sodium salt of glutamic acid and amino glutamic acid) and gas (nitrogen/argon) used to create anaerobic microenvironment. Experimental data showed the feasibility of H(2) production along with substrate degradation utilizing organic acids as metabolic substrate but was found to be dependent on the process parameters evaluated. Maximum specific H(2) production and substrate degradation were observed with acetic acid [3.51 mol/Kg COD(R)-day; 1.22 Kg COD(R)/m(3)-day (92.96%)] compared to butyric acid [3.33 mol/Kg COD(R)-day; 1.19 Kg COD(R)/m(3)-day (88%)]. Higher H(2) yield was observed under acidophilic microenvironment in the presence of glucose (co-substrate), mono sodium salt of glutamic acid (nitrogen source) and vitamins. Argon induced microenvironment was observed to be effective compared to nitrogen induced microenvironment. Combined process efficiency viz., H(2) production and substrate degradation was evaluated employing data enveloping analysis (DEA) methodology based on the relative efficiency. Integration of dark fermentation. with photo-fermentation appears to be an economically viable route for sustainable biohydrogen production if wastewater is used as substrate. (C) 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.