Microorganisms catalyse the reaction and in this study, mainly the effect of different concentration of biomass on COD removal was investigated. Three sets of two-compartment reactors were established. The cation exchange membrane (CEM) was employed in each reactor and 0.5 V of electricity was supplied. Graphite rod employed in cathodic part and a combination of graphite rod and graphite granules were used in anodic chamber. The highest rate of COD removal (40 +/- 2.0 ppm/h) was achieved in the reactor which had initial VSS at 6130 mg/l, whereas the slowest rate of 23 +/- 1.2 ppm/h in the reactor started with 3365 mgVSS/l. Some ammonia removal was also noticed during the operation. Further understanding and improvement is needed to be competitive against traditional wastewater treatment processes. (C) 2010 Elsevier Ltd. All rights reserved.