In this study, effects of four kinds of redox mediators on denitrification performance, nitrate and nitrite reductase activities were investigated, including anthraquinone (AQ), 2-methyl-1,4-naphthoquinone (ME), lawsone (LAW) and anthraquinone-2,6-disulphonate (AQDS). Experimental results demonstrated that the optimum dosing concentrations of AQ ME and LAW were 75, 25 and 75 mu M. The maximum total nitrogen removal rates increased to 26.02, 20.16 and 33.50 mg-N/g-VSS/h at each optimum dosing concentration, which were approximately 1.60, 1.25 and 2.08 times higher compared to that without redox mediator addition. Unlike AQ and ME, addition of LAW appeared to have suppressed the accumulation of nitrite with peak vales about 21.60 mg/L compared to that of 121.9 mg/L with AQ and 180 mg/L with ME addition. Among four kinds of redox mediators, ME increased the nitrate reductase activity about 1.97 folds, and AQDS enhanced the nitrite reductase activity about 2.08 folds compared to the controls. The unbalance of affected nitrate reductase and nitrite reductase activities by redox mediator was considered as the main reason for nitrite accumulation in this study. (C) 2014 Elsevier B.V. All rights reserved.