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Electric current stimulation has been shown to have a positive influence on heterotrophic denitrifying microbial viability and has the potential to improve wastewater denitrification performance.This study investigated the effects of varying current densities on microbial activity and NO3-removal efficiency under heterotrophic conditions.NO3-removal rate was highest at an applied current density of 400A/m2.However,the optimum removal efficiency of total inorganic nitrogen (TIN;99％) was achieved when the current density was fixed at 200 mA/m2.Accumulation of NH4+-N and NO2--N byproducts were also minimized at this current density.The activity of heterotrophic denitrifying microorganisms was much higher at both 200 and 400 mA/m2.Moreover,the average adenosine-5’-triphosphate (ATP)content (an indicator of cell metabolism) at a current density of 1600 mA/m2 was lower than that under no current,indicating heterotrophic denitrifying microbial activity can be inhibited at high current densities.Hence,direct electrical stimulation on the activity of heterotrophic denitrifying microorganisms in the developed system should be lower than 1600 mA/m2.This study improves the understanding of electric current influence on heterotrophic denitrifying microorganisms and promotes the intelligent application of direct electrical stimulation on wastewater treatment processes.