2.5.1. Calculation Method

Many previous studies have shown that nitrite, ammonia, and nitrogen are the main products in the process of nitrate reduction, while NO<sup>x</sup> and N2O are basically not found [45,46], the same result was also found in the preliminary experiment of this study. Therefore, in this experiment, only nitrite nitrogen, ammonia nitrogen, and nitrogen were

considered in the reduction products. The removal efficiency of NO<sup>3</sup> −-N (η) and the product conversion ratio (Sproduct) are calculated as follows:

$$\eta = \frac{\text{C}\_0 - \text{C}\_{\text{NO}\_3^-}}{\text{C}\_0} \times 100\% \tag{12}$$

$$\mathbf{S}\_{\text{NH}\_4^+} = \frac{\mathbf{C}\_{\text{NH}\_4^+}}{\mathbf{C}\_0 - \mathbf{C}\_{\text{NO}\_3^-}} \times 100\% \tag{13}$$

$$\text{S}\_{\text{NO}\_2^-} = \frac{\text{C}\_{\text{NO}\_2^-}}{\text{C}\_0 - \text{C}\_{\text{NO}\_3^-}} \times 100\% \tag{14}$$

$$\mathbf{S}\_{\rm N\_2} = \frac{\mathbf{C}\_0 - \mathbf{C}\_{\rm NO\_3^-} - \mathbf{C}\_{\rm NO\_2^-} - \mathbf{C}\_{\rm NH\_4^+}}{\mathbf{C}\_0 - \mathbf{C}\_{\rm NO\_3^-}} \times 100\% \tag{15}$$

where, C<sup>0</sup> is the initial concentration of NO<sup>3</sup> <sup>−</sup>-N, mg/L; CNO<sup>3</sup> <sup>−</sup>, CNH<sup>4</sup> <sup>+</sup> and CNO<sup>2</sup> − are the concentration of NO<sup>3</sup> −-N, NH<sup>4</sup> + -N and NO<sup>2</sup> − after reaction, mg/L.
