A new bioelectrochemical system (BES), a membrane-free, continuous feeding up-flow biocatalyzed electrolysis reactor (UBER) was developed to reduce oxidative toxic chemicals to less- or non-toxic reduced form in cathode zone with oxidation of electron donor in anode zone. Influent was fed from the bottom of UBER and passed through cathode zone and then anode zone. External power source (0.5 V) was provided between anode and cathode to enhance electrochemical reactions. Granular graphite and carbon brush were used as cathode and anode, respectively. This system was tested for the reduction of nitrobenzene (NB) using acetate as electron donor and carbon source. The influent contained NB (50-200 mg L-1) and acetate (1000 mg L-1). NB was removed by up to 98% mainly in cathode zone. The anode potential maintained under -480 mV. The maximum NB removal rate was up to 3.5 mol m(-3) TV d(-1) (TV=total empty volume) and the maximum aniline (AN) formation rate was 3.06 mol m(-3) TV d(-1). Additional energy required was less than 0.075 kWh mol(-1) NB. The molar ratio of NB removed vs acetate consumed varied from 4.3 +/- 0.4 to 2.3 +/- 0.1 mol mol(-1). Higher influent phosphate or acetate concentration helped NB removal rate. NB could be efficiently reduced to AN as the power supplied of 0.3 V. (C) 2011 Elsevier B.V. All rights reserved.