Studies were carried out with carbon dioxide absorber (CA) to evaluate the usage of carbon dioxide (CO2) in the biogas as an acidifying agent by Up-flow Anaerobic Sludge Blanket (UASB) reactor. Investigation on the 51 absorber revealed that ratio of brewery wastewater (BW) flow rate to biogas flow rate of 4.6-5.2 was optimum for minimum consumption of CO2 for acidification. The acidified BW after the absorber was treated in UASB reactor with optimum organic loading rate (OLR) of 23.1 kg COD/m(3)/day and hydraulic retention time (HRT) of 2h. UASB reactor exhibited good performance with respect to reduction of chemical oxygen demand (COD) and methane yield. The implications of the present study on the full scale anaerobic reactor of medium scale brewery revealed that sufficient cost savings could be made if CO2 in the biogas or CO2 that was being wasted (let out to the atmosphere) can be used instead of sulfuric acid (H2SO4) for pH control. (c) 2006 Elsevier Ltd. All rights reserved.