Dairy wastewater was evaluated for biological hydrogen (H-2) production in conjugation with wastewater treatment in a suspended growth sequencing batch reactor (AnSBR) employing sequentially pretreated [heat-shock (100 degrees C, 2h) and acid (pH 3.0, 24h)] mixed consortia. The bioreactor was operated at mesophilic (room) temperature (28 +/- 2 degrees C) under acidophilic conditions (pH 6.0) with a total cycle period of 24 h consisting of FILL (15 min), REACT (23 h), SETTLE (30 min), and DECANT (15 min) phases at three different organic loading rates (OLR) of 2.4, 3.5, and 4.7 Kg COD/m(3)-day, respectively. H evolution rate differed significantly with the substrate/OLR of wastewater used as substrate [OLR 2.4 Kg COD/m(3)-day - volumetric H, production rate: 0.3683 mmol H-2/m(3)-min; specific H-2 production rate: 0.0184 mmol H,/min-g CODL; OLR 3.5 Kg COD/m(3)-day - volumetric H-2 production rate: 1.105 mmol H-2/m(3)-min; specific H, production rate: 0.0245 mmol H-2/min-g CODL and OLR 4.7 Kg COD/m3-day - volumetric H, production rate: 0.7367 mmol H-2,/m(3)-min; specific H2 production rate: 0.0107 mmol H2/min-g CODL. Substrate (COD) removal efficiency of 64.7 (substrate degradation rate (SDR): 1.577 Kg COD/m(3)-day), 60 (SDR: 3.168 Kg COD/m(3)-day), and 50% (SDR-3.2 Kg COD/m(3)-day), respectively, was observed at operating OLR of 2.4, 3.5, and 4.7 Kg COD/m(3)-day, respectively. The system showed rapid stabilization tendency (2.4 Kg COD/m(3)-day: 39 days; 3.5 Kg COD/m(3)-day: 14 days; 4.7 Kg COD/m(3)-day: 24 days) with respect to H? generation and COD reduction. A surge in pH values from 5.8 to 4.5 (2.4 Kg COD/m(3)-day), 5.82 to 4.62 (3.5 Kg COD/m(3)-day), and 6.28 to 4.56 (4.7 Kg COD/m(3)-day) was observed during stabilized phase of operation. (c) 2007 Elsevier Inc. All rights reserved.