## 3 Results and Discussion
### The Second Step of Experiment--Biological Denitrification
In the first period (ORP from 100 to \(-\)75 mV), the maximum efficiency of the removal of nitrate in a biological reactor was only 25%. Besides, the performance of process denitrification was in the range of 20-25% in the last 2 h (ORP from \(-\)50 mV to \(-\)100 mV). However, the efficiency of the removal of nitrate in the second period of the experiment has increased to 70-75% at the end of the denitrification reaction. For 3 h, when the reactor was in anoxic conditions, the quantity of nitrate removed from the reactor reached 50%. Moreover, in this period, the reduction of ORP in the reactor has been supplied for biological denitrification. After 1 h from the pre-anoxic phase, the ORP in the reactor has been decreased gradually to levels of \(-\)100 mV and \(-\)150 mV at the end of the reaction. Based on the conditions of growth zones, the relationship between the SDNR and the quantity of organic matter used for biological reactions is shown in Figures 20 and 21. The SDNR was also affected by anoxic conditions as a result of the varying efficiency of nitrate removal in each period of the experiment. When compared with the theoretical kinetics of food to mass (F/M) ratio (rate of organic matter applied per unit volume of mixed liquor) and SDNR, a quarter of the points in the first period of the experiment were located in the working zone of SDNR. The maximum efficiency of nitrate removal in this period was only 25%, with a SDNR of the reactor lower than 0.25 g N\({}_{\text{red}}\)/g VSS/d; this is consistent with some previous research [39]. On the other hand, with the optimal conditions of DO and ORP, most of the SDNR values in the second period of the experiment were located in the working zone by theory and tend to peak at 0.5 g N\({}_{\text{red}}\)/g VSS/d. After that, the optimal F/M ratio for biological municipal wastewater treatment is in the range of 0.4-0.6 g BOD/g VSS/d. Figure 21 presents the ratio for reducing 1 g of nitrate per unit of BOD in the denitrification reaction. The value of SDNR is proportional to the amount of organic materials used, which were a source of nutrients in the biochemical reaction of nitrogen oxidation [36; 40; 41]. Therefore, from the first to the second period of the experiment, when the SDNR of the reactor rose from 0.2 to 0.4 g N\({}_{\text{red}}\)/g VSS/d, the ratio N\({}_{\text{red}}\)/BOD\({}_{\text{used}}\) in the biological reactor increased from 0.05 to 0.1. In the second period of the experiment with optimal anoxic conditions, the SDNR of the reactor was in  
Figure 20: Specific denitrification rate with food to mass ratio in lab-scale SBR. the range 0.35-0.45 g N\({}_{\text{red}}\)/g VSS/d, and the ratio N\({}_{\text{red}}\)/BOD\({}_{\text{used}}\) was from 0.05 to 0.11. These parameters are the fundamentals for applying biological nitrogen removal to ASP in municipal wastewater treatment without using methanol or another chemical reagent in denitrification [42]. With the addition of the denitrification process in SBR, the mixing process between the mixed liquor and wastewater is very important to ensure the reactor achieves the conditions of denitrification [28]. When the time of the first aerobic phase is 10-15 min and the pH value is 8.3-8.5, the specific rate of denitrification reaches a maximum of 0.43 kg N/kg VSS/day, and the efficiency of the denitrification process is more than 75%. The use of floating feed material has been considered as a tool to improve the efficiency of removing nitrogen compounds in the reactor. According to research [42], the use of feed material in SBR increases the MLSS by 20% under optimal conditions. The rate of substrate utilization and biomass growth in the reactor increases by 10-20%, which leads to a reduction of the biological load on the batch reactor by 15-20% [31]. To consider the efficiency of the SBR technology, several aspects may be focused on in further studies. Aeration improvement is a way to reduce the energy consumption and cost of the WWTP [43].