## 3 Results and Discussion  
### The First Step of Experiment--Biological Nitrification  
The optimization for biological nitrification via alkalinity and pH has been performed in the first step of the experiment. The pH value of the mixed liquid has been controlled and increased gradually to levels of 7.0, 7.2, 7.4, 7.6, and 7.8, respectively. At each pH level, the lab-scale SBR reactor operated for eight cycles (two cycles of warmup and no sampling). The duration of each phase within one operation circle was as follows:  
* Filling time: 12 min
* Nitrification phase: 3.0 h
* Settling: 30 min.
* Discharge: 15 min.
* Filling rate: 30% volume of reactor (conventional value in Vietnamese WWTP).  
Sampling results demonstrate a decrease in the concentration of pollutants in SBR and their characteristic functions (Figures 7-11).  
Nitrification is carried out by autotrophic bacteria that use inorganic materials as a source of nutrients and photosynthesis or chemosynthesis as a source of energy.  
Therefore, organic matter removal has been related to the amount of alkalinity used to intensify nitrification in ASRs. Based on the concentrations of BOD and COD in effluent wastewater and MLSS in the reactor, the reduction of organic matter could be divided into two groups. The first group included biological reactions with a pH value of 7.2. However, the effectiveness of nitrogen removal only increased by 3-5%, and the concentration of ammonia in treated wastewater was reduced by 1-2 mg/L.  
Figure 8: The COD concentration in influent and effluent of model SBR under various pH conditions.  
Figure 9: The total nitrogen (TN) concentration in influent and effluent of model SBR under various pH conditions.  
After the pH value reached 7.4, the nitrification rate increased. The concentration of N-NH\({}_{4}\) in the effluent was in inverse proportion to the amount of alkali. From a pH value of 7.4 to 7.8, the concentration of pollutants in treated wastewater reached the Vietnamese discharge standard; the concentration of N-NH\({}_{4}\) was lower than 5 mg/L, and the concentration of N-NO\({}_{3}\) was lower than 30 mg/L. The results were obtained under the MLVSS value of 2.1 g/L, which corresponds to previous results from the authors and other researchers [15; 37; 38].  
N-NH\({}_{4}\) and the MLSS dynamics in the reactor allowed for the calculation of the SNR and to compare it with kinetic theory (Figures 12-14).  
Figure 11: The concentration of N-NO\({}_{3}\) in influent and effluent of model SBR.  
Figure 10: The concentration of N-NH\({}_{4}\) in influent and effluent of model SBR under various pH conditions.  
The SNR is inversely proportional to the loading of ammonia and proportional to the value of pH in the reactor. According to the results of the experiment, with concentrations of MLSS ranging from 2.0 to 2.2 g/L and accounting for 14-16% of the volume of the reactor, the SNR was in the range of 0.16-0.18 kg N\({}_{\rm ox}\)/kg VSS/d for nitrification in municipal wastewater treatment. At 32 \({}^{\circ}\)C, the typical temperature of ASPs in Vietnamese WWTPs, the performance of the biological nitrification was at 50 to 60% when compared with the kinetic theory of pH in reactors in the range of 7.4 to 7.8. The reduction of the SNR by time and its dependence on the pH value in the reactor are presented in Figure 15. The duration of the nitrification in municipal WWTP should be between 1 and 4 h. After 4 h of nitrification and aeration, the amount of ammonia in the influent was almost exhausted, and the rate of reaction tended to zero. Thus, during this stage, empirical data were obtained that differed from the calculated results of similar studies performed earlier. This is primarily due to the ASP temperature, which is considered one of the main limiting factors of the process [30; 36].  
Figure 12: Specific nitrification rate and N–NH\({}_{4}\) load.  
Figure 13: Specific nitrification rate in relation to pH (T = 32 \({}^{\circ}\)C).