## 3 Results and Discussion
### The Second Step of Experiment--Biological Denitrification  
As far as the carbon source is required for denitrification, heterotrophic microorganisms use organic matter as a source of energy. The filling rate and operation cycle of the SBR have been changed in the second step of the experiment. Changes were made according to recommendations from previous studies [25; 28]. The filling rate of the pilot has been increased from 30% (for nitrification) to 50% in order to make use of organic sources in municipal wastewater. The pre-anoxic phase has been applied in the operation cycle for 15 min. The purpose of this phase is to complete the mixing of influent with a high concentration of organic matter and effluent with a high concentration of nitrates. The duration of the denitrification phase is 4 h, and the ORP in the mixed liquor has been controlled. Based on the ORP value in the reactor, the second step of the experiment has been divided into two periods. In the first period, the ORP reduced from 100 to \(-\)75 mV, and in the second period, the ORP reduced from 75 to \(-\)130 mV. After that, the nitrification process was applied with optimal results from the first step of the experiment. In each period of the experiment, the lab-scale SBR reactor operated for 8 cycles (2 circles of warmup and no sampling), with  
Figure 14: The Specific nitrification rate with adding alkalinity. Figure 15: BOD concentration within biochemical Nitrogen Removal. the MLVSS remaining at 2.1 g/L and the percentage concentration of activated sludge increasing by 20-22%. The phase division within the operation circle of the biological nitrogen removal process in pilot was as follows:  
* Filling: 15 min
* Pre-Anoxic phase: 15 min
* Denitrification phase: 4.0 h
* Nitrification phase: 1.5 h
* Settling: 30 min
* Discharge: 15 min
* Filling rate: 50% volume of reactor--ensure carbon feed for denitrification reaction (from organic matter in raw wastewater)  
From the results of hourly sampling (after pre-anoxic phase) and analysis, the reduction of the pollutants concentration in the SBR is shown in Figures 15-17. According to the theory of biological denitrification, in an anoxic environment, heterotrophic microorganisms use organic matter as a source of nutrients and metabolic synthesis as a source of energy. The environmental conditions for biological denitrification reactions are an ORP between 0 and \(-\)200 mV and a dissolved oxygen concentration between 0.2 and 2 mg/L. Therefore, the efficiency of the process of removing organic matter expressed in BOD\({}_{5}\) and COD in the second step of the experiment decreases by 5-10% when compared with the first step. The concentration of BOD\({}_{5}\) in the effluent also reaches the discharge standard. On the other hand, the effect of ORP in the reactor is evident in the nitrate concentration after the anoxic phase. Based on the results of the analysis, after the anoxic phase, the average concentration of nitrate in the reactor was 6 and 0.8 mg/L in each period of the experiment, respectively. In the last 1.5 h of biological wastewater treatment, when nitrification occurs in the reactor, the concentration of nitrate in the reactor increases due to the oxidation of ammonia. After settling, only the effluent in the second period of the experiment reached the discharge standard of the Russian Federation. Figures 18 and 19 present the effect of environmental conditions on the performance of reactors. In this experiment, the growth of biomass takes at least 1 h from the beginning of the anoxic step for rapid denitrification. The DO concentration in the reactor reached denitrification's conditions after 15 min, but the ORP of MLVSS required 1.5 to 2.0 h to achieve optimal values. Therefore, due to the same efficiency of organic removal, the denitrification efficiency of each period in the experiment is different, resulting from the decrease of ORP in the anoxic phase of the reactor.