A membraneless bioelectrochemical system - rotatable bio-electrochemical contactor (RBEC) consists of an array of rotatable electrode disks was developed to convert the chemical energy from wastewater organics (acetate) directly into electricity. Each rotatable electrode disk had an upper-air exposing and a lower-water submerging halves. Intermittent rotation (180 degrees) enabled each halve to alternately serve as anode and cathode. Removal of chemical oxygen demand (COD) was increased by 15% (from 0.79 to 0.91 kg COD m(-3) d(-1)) by allowing electron flow from the lower to the upper disk halves. Coupling with a potentiostat could alleviate cathodic limitation and increased COD removal to 1.32 kg COD m(-3) day(-1) (HRT 5 h). About 40% of the COD removed was via current, indicating that the biofilm could use the lower half disk as electron acceptor. The RBEC removed COD more energy-efficiently than conventional activated sludge processes as active aeration is not required (0.47 vs. 0.7-2.0 kW h kg COD-1). (C) 2011 Elsevier Ltd. All rights reserved.