## 1 Introduction  
Carbon neutrality requires intensified research on sustainable nitrogen management strategies in wastewater treatment plants (WWTPs). The traditional nitrification/denifrification process requires a significant amount of aeration for nitrification and an external organic carbon source for denitrification [1], leading to high operating costs and energy requirements. To overcome these challenges, anaerobic ammonia oxidation (anammox)-based processes have been developed, offering substantial economic advantages, and these processes convert NH\({}_{4}\)\({}^{+}\) and NO\({}_{2}\)\({}^{-}\) directly into N\({}_{2}\) gas under anaerobic conditions [2; 3; 4; 5; 6]. The partial nitritation (PN)-anammox (PN/A) process, which combines PN (Equation (1)) and anammox (Equation (2)), involves ammonia-oxidizing bacteria (AOB) converting half of the NH\({}_{4}\)\({}^{+}\) to NO\({}_{2}\)\({}^{-}\), followed by anammox bacteria using the remaining NH\({}_{4}\)\({}^{+}\) and NO\({}_{2}\)\({}^{-}\)[7; 8]. This innovative process has demonstrated benefits, including a 60% reduction in aeration requirement, the elimination of the need for organic carbon sources, and a 90% reduction in sludge production when compared to traditional nitrification/denifrificationprocesses [9; 10; 11]. Currently, more than 200 full-scale facilities employing the PN/A process are successfully operating across Europe, Asia, and North America [12], predominantly treating high-strength NH4+ wastewater (>500 mg/L) in sidestream applications [13; 14]. \[1.0\text{NH}_{4}^{+}+0.75\text{O}_{2}+\text{HCO}_{3}^{-}\to 0.5\text{NH}_{4}^{+}+0.5 \text{NO}_{2}^{-}+\text{CO}_{2}+1.5\text{H}_{2}\text{O} \tag{1}\]  
\[1.0\text{NH}_{4}^{+}+1.32\text{NO}_{2}^{-}+0.066\text{HCO}_{3}^{-}+0.13\text{H} ^{+}\to 1.02\text{N}_{2}+0.26\text{NO}_{3}^{-}+\]  
\[0.066\text{CH}_{2}\text{O}_{2}\text{N}_{0.15}+2.03\text{H}_{2}\text{O} \tag{2}\]  
The widespread application of mainstream PN/A is challenged by the proliferation of nitrite-oxidizing bacteria (NOB), which disrupt the stable supply of NO2-[15; 16]. Various methods have been employed to suppress NOB in sidestream processes, including oxygen control, intermittent aeration, solid retention time (SRT) control, and inhibition by free ammonia (FA) and free nitrous acid (FNA) [17]. For instance, [PERSON] et al. (2013) maintained PN at a dissolved oxygen (DO) concentration of 0.3-0.6 mg/L, while [PERSON] et al. (2015) achieved PN in a sequencing batch reactor (SBR) at a DO concentration of 0.15-0.18 mg/L [18; 19]. Intermittent aeration strategies, such as alternating 15 min of aerobic time with 15 min of anoxic time in PN/A reactors, have also been effective in suppressing NOB [20]. Controlling SRT by adjusting the sludge in the settling tank has been another effective strategy. At high temperatures of 28-30 \({}^{\circ}\)C, a short SRT of 2.5 days has successfully facilitated partial nitritation in wastewater treatment plants, although this method proved ineffective at lower temperatures of 20 \({}^{\circ}\)C [21]. FA, being more inhibitory to NOB than to AOB, can suppress NOB activity at concentrations of 1-150 mg/L without adversely affecting AOB [22]. Various strategies combining these methods have been successfully applied to operate PN by suppressing NOB, with FA strategies being particularly effective in sidestream processes [23]. Despite these success in sidestream applications, there is limited information on stable AOB enrichment methods for initiating nitrification in mainstream nitrogen treatment. Additionally, it is necessary to confirm nitritation by analyzing the proportions of AOB, including representative genera such as _Nitrosomonas_, _Nitrosospira_, and _Nitrosococcus_, and NOB, including representative genera such as _Nitrospira_, _Nitrobacter_, _Nitrotoga_, and _Nitrococcus_[24; 25; 26; 27]. This highlights the need for further research to develop reliable strategies for mainstream applications.