## 1 Introduction  
The activated sludge process (ASP) is a method used worldwide for biological treatment of municipal and industrial wastewater [1; 2]. ASP is normally held in the activated sludge reactors (ASR). Their simplest version aims at the removal of biodegradable organic matter and organic nitrogen-containing matter by converting ammonia to nitrate under aerobic conditions [3; 4; 5; 6]. However, enhanced treatment (e.g., nitrate removal) requires more sophisticated technologies. According to the report of the United States Environmental Protection Agency (USEPA), ASRs are relatively small and require less space than other technologies. However, ASRs may require significant operation costs due to the higher energy demand needed to run the aeration system [7; 8; 9; 10]. The effectiveness of the ASP can be impacted by elevated levels of toxic compounds in wastewater that require an industrial pretreatment program to control complex industrial chemicals. On the other hand, the limitation of conventional ASPs is the enhanced removal of nutrients. Enhanced removal refers to the simultaneous removal of only 5 mg of nitrogen and 1 mg of phosphorus per 100 mg of organic matter, which is usually expressed as a biological oxygen demand (BOD) or chemical oxygen demand (COD). In the conventional aerobic process, the mass of nutrients removed in 100 mg of activated sludge was 2.3 mg of phosphorus and 12.2 mg of nitrogen [2; 11]. Therefore, the removal of nutrients at wastewater treatment plants (WWTPs) is a major challenge because of public health and economic concerns [12; 13; 14]. In the biological wastewater process with activated sludge, ammonia removal is accomplished through nitrifying bacteria, which oxidize ammonia to nitrite and then to nitrate. After that, denitrifying bacteria reduce nitrate to gaseous nitrogen through heterotrophic microorganisms [11; 15]. Through the nitrogen cycle in nature, denitrification can take place in terrestrial or marine ecosystems. However, ASP denitrification occurs in anoxic conditions, which means the presence of nitrate and the absence of dissolved oxygen. Biological nitrogen removal in wastewater treatment systems and its control reactions are shown in Figure 1. However, a number of barriers may influence forward progress in implementing nutrient removal processes and achieving reductions in aquatic ecosystems, including costs, limitations on physical expansion, and advanced operations and control. In urban areas, the production of waste from human activities added to the water during its communal use will end up as municipal wastewater [13; 14; 15; 16]. Especially in developing countries with combined sewage systems, human waste, together with the water used for flushing toilets, has been mixed with rainwater and industrial wastewater in local areas before being treated in municipal WWTPs [17]. Due to the frequent use of septic tanks in households, there is a decrease in available carbon for heterotrophic denitrification [18]. Table 1 demonstrates the urban wastewater and sanitation management activities in Vietnam, which is a typical developing country with combined sewage systems [19; 20]. According to the overview of urban sanitation in Table 1, ASRs have been widely applied in municipal WWTPs. Based on the limitations and disadvantages of this technology, the Vietnamese discharge standard [21] regulates the concentration of nutrients in treated wastewater, especially ammonia and nitrates; these should not be higher than 5 mg/L or 30 mg/L, respectively. However, when compared with the Russian standard for  
Figure 1: The mechanism of nitrogen transformation within activated sludge process at wastewater treatment plants. treated wastewater, the amount of nitrogen released into the environment in Vietnam is unacceptable and leads to damage to the environment [22].