The ratio of autotrophic denitrification (R-auto) is proposed as a metric to better evaluate the nitrate removal efficiency of bioelectrochemical systems (BESs). Two methods for computing R-auto were compared, one based on electron charge transfer and the other on a controlled reaction. The experiment was conducted in a greenhouse with a temperature of 30 +/- 2 degrees C. A reaction with a constant voltage supply was used as a BES and open circuit control was used for the controlled reaction. The results indicated that calculating the R-auto based on electron charge transfer was more suitable for evaluating the performance of BESs. Further discussion about the factors influencing the result of this computing method is presented. (c) 2013 Elsevier Ltd. All rights reserved.