The development of continuous very-high-gravity (VHG) fermentation is hindered by ineffective glucose uptake in order to result in zero discharge in the effluent stream. To overcome the problem, we proposed a continuous redox potential-controlled fermentation configuration, consisting of a Chemostat vessel connected with two ageing vessels installed in parallel, and the relevant design criteria are also specified. The Chemostat vessel is subjected to redox potential control to maintain yeast viability, and the ageing vessels are used to completely utilize glucose before discharging to next process unit. Two ageing vessels are scheduled alternatively, resulting in continuously-like operation. The size of ageing vessel is governed by the Chemostat size, dilution rate and filling time. The guideline to choose proper dilution rate is provided and the selection criterion of the proposed continuous configuration over batch fermentation is derived. The excess ethanol produced by the proposed continuous configuration over batch fermenter is quantified. As an illustration, a bio-ethanol plant is typically operated 8000 h per annum and the downtime between batches is 6 h. Given that the fermenter size of 100 m(3) for both batch fermenter and Chemostat vessel, and glucose fed at 300 g/l, if the proposed continuous redox potential-controlled fermentation configuration (operated at 0.028 h(-1) and controlled at -50 mV) is selected, it will take 191 h for this configuration to outperform the batch counterpart, and the excess amount of ethanol being produced will be 1142 t. (C) 2010, The Society for Biotechnology, Japan. All rights reserved.