## 2 Materials and Methods
### Experiment Methods  
The experiment has been divided into two steps, corresponding to biological reactions (nirtification and denitrification) and to two discharge standards for treated wastewater (Vietnam and Russia). The bench consisted of two SBRs that operated in parallel under continuous-flow conditions for one month; this guarantees the processes of nitrification and denitrification in each reactor. The experiment plot and the mission of each reactor are shown in Figure 6.  
The first lab-scale SBR was focused on nitrification and its optimization via the alkalinity and pH, respectively. The main mission of this step of the experiment was to determine the efficiency of ammonia removal through the control of the pH value of the reactor. In addition, the quality of treated wastewater should meet the Vietnamese discharge standard. The major challenge of the second reactor was the evaluation of biological nitrogen removal from municipal wastewater with low-source carbon. Due to the anoxic environment in the reactor, denitrification has occurred for treating municipal wastewater. The Russian discharge standard has been used in the second step of the experiment [30; 31; 32; 33]. Figure 4: Layout scheme of SBR pilot model. Figure 5: Block scheme of SBR pilot model. The purpose of the experiment steps was to determine the specific rates of nitrification (SNR) or denitrification (SDNR), respectively, and the efficiency of nitrogen removal in the SBR reactor. The values of these parameters were obtained after the analysis of influent and effluent samples and mixed liquor in the reactor over time. The following analytical parameters were measured [34; 35]:  
* BOD, COD, ammonia (N-NH4), nitrite (N-NO2), nitrate (N-NO3), organic nitrogen (N\({}_{\rm{org}}\)), hourly influent and effluent analysis per cycle of the reactor operation;
* Temperature, pH, dissolved oxygen (DO), and redox potential (ORP) were fixed by sensors and collected by computer;
* Activated sludge: mixed liquor suspended solids (MLSS) and mixed liquor volatile suspended solids (MLVSS) in biological reactor were analyzed at the beginning and end of SBR circle operation. According to the analysis results, the performance and organic loading of the reactor have been determined. SNR and SDNR were measured based on the changes in the concentration of nitrogen components (ammonia, nitrite, or nitrate) in the influent and effluent wastewater (Equations (1) and (2)). In the ASP, these rates characterize the growth of nitrate-oxidizing and reducing bacteria [1; 2; 11; 36]:  
* SNR: \[r_{N}=\bigg{(}\frac{\mu_{Nm}\times N}{K_{N}+N}\bigg{)}\bigg{(}\frac{K_{O}}{K_{O} -DO}\bigg{)}-k_{dN}\] (1)
* SDNR: \[r_{DEN}=-r_{su}\bigg{(}\frac{NO_{3}}{NO_{3}+K_{NO_{3}}}\bigg{)}\bigg{(}\frac{K _{O}^{\prime}}{K_{O}^{\prime}+DO}\bigg{)}\eta\] (2)  
\(\mu_{Nm}\) is the maximum specific growth rate of nitriying bacteria [g N\({}_{\rm{ox}}\)/g VSS d\({}^{-1}\)];  
\(N\) is the nitrogen concentration [mg/L];  
Figure 6: The experiment site and purpose of research. \(K_{N}\) is the halved velocity constant; substrate concentration at one-half the maximum specific substrate utilization rate [mg/L];  
\(k_{dN}\) is the endogenous decay coefficient for nitrifying organisms [g VSS/g VSS/d];  
\(K_{O}\) is half-saturation coefficient for DO [mg/L];  
DO is the dissolved oxygen concentration [mg/L];  
\(K^{\prime}_{O}\) is the DO inhibition coefficient for nitrate reduction [mg/L];  
\(K_{NO3}\) is half-velocity coefficient for nitrate limited reaction [mg/L];  
\(r_{DEN}\)is the maximum specific growth rate of denitrifying bacteria [g N\({}_{\text{red}}\)/g sludge d\({}^{-1}\)];  
\(r_{su}\) is the rate of substrate concentration changes due to utilization [g/m\({}^{3}\) d\({}^{-1}\)];  
\(\eta\) is the fraction of denitrifying bacteria in the biomass [g VSS/g VSS]. Through the simulation of nitrogen removal in the SBR, the SNR and SDNR have been calculated and compared with the typical values in the kinetic theory of the ASP.