In this study a new reactor design is proposed to control the pH during the bio-electrochemical denitrification process. A previously developed UBER was modified by including a pumparound system. With the pumparound system a portion of the treated water is continuously withdrawn from the UBER into a CO2 sparging bottle to decrease its pH to about 6.1 +/-0.1, before being returned to the cathode zone where denitrification process takes place. Continuous denitrification was studied with a HRT of 24 h applying an electric current in the range of 15-25 mA. The effects of circulation flow rate (F-c) on the pH and on the concentrations of nitrate and nitrite ions in the effluent were investigated. The pumparound system succeeded to stabilize the cathode pH around 7-8 through alteration of circulation flow rate (F-c). Complete denitrification with no trace of nitrite was therefore achieved at circulation flow rate of 0.7 mL/min and electric current 25 mA. A further investigation in absence of bicarbonate sodium resulted in a satisfactory nitrate treatment showing that the carbon dioxide gas dissolved in the CO2 sparging bottle supplied enough carbon for the autohydrogenotrophic microorganisms. (C) 2010 Elsevier B.V. All rights reserved.