## 3 Results and Discussion
### Changes in Microbial Communities According to FA Concentration  
The microbial communities in PN1 and PN2 showed a decrease in diversity indices, such as ASVs, Chao1, and Shannon, during the operation period (Table 3). This decrease suggests that specific microbial communities became dominant during the enrichment of AOB. PN1, affected by low FA, exhibited relatively minor changes in microbial community diversity and maintained a higher species diversity compared to PN2. In contrast, PN2, influenced by high FA concentrations, experienced a rapid decrease in ASVs and species diversity indices by day 6, the initial stage of the nitrification reaction. These results indicate that under high FA concentration conditions, which suppress NOB, the microbial community was simplified in the early stages. Previous studies support these results; for instance, when AOB enrichment was initiated with an influent NH\({}_{4}\)+-N concentration of 175 mg/L, the OTUs decreased from 3870 to 1430, Chao1 from 3879 to 1450, and Shannon from 6.796 to 3.854 after 34 days. This demonstrates that the diversity indices decreased as NOB was suppressed and AOB became dominant [30]. The microbial diversity and dominant microbial communities underwent significant changes during the AOB enrichment process influenced by FA (Figure 3). At the phylum level, the inoculum primarily comprised _Acidobacteria_ (12.6%), _Frimicutes_ (12.3%), _Bacteroidetes_ (11.0%), and _Proteobacteria_ (10.2%), with other microbial communities (less than 1%) accounting for 45.1%. On day 6 of the nitrification reaction in PN1, the proportions of _Acidobacteria_ and _Frimicutes_ had decreased to 8.4% and 7.2%, respectively, while _Bacteroidetes_ had sharply increased to 36.2%. _Proteobacteria_ showed a slight increase to 12.2%, and the proportion of other microbial communities (less than 1%) had significantly decreased to 24.6%. By day 50, under continued operation with 10 mg/L FA, _Nitrospirane_ and _Chloroflexi_ increased to 28.5% and 14.8%, respectively, whereas _Acidobacteria_, _Frimicutes_, and _Bacteroidetes_ had decreased to 6.4%, 1.5%, and 16.0%. Figure 3: Changes in phylum-level microbial communities in PN1 and PN2 according to FA concentration. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Reactor**} & **Sample Name** & **FA & \multirow{2}{*}{**ASVs**} & \multirow{2}{*}{**Chao1**} & \multirow{2}{*}{**Shannon**} & **Good’s Coverage** \\  & & **Name** & **(mg/L)** & & & & **Coverage** \\ \hline \multicolumn{2}{c}{Inoculum} & 0 & 1299 & 1308 & 8.66 & 0.99 \\ \hline \multirow{3}{*}{PN1} & Day 6 & 10 & 1018 & 1024 & 7.46 & 0.99 \\  & Day 33 & 10 & 942 & 853 & 7.06 & 0.99 \\  & Day 50 & 10 & 826 & 836 & 6.74 & 0.99 \\ \hline \multirow{3}{*}{PN2} & Day 6 & 50 & 763 & 764 & 6.41 & 0.99 \\  & Day 13 & 50 & 763 & 764 & 6.21 & 0.99 \\ \cline{1-1}  & Day 22 & 35 & 651 & 652 & 6.37 & 0.99 \\ \cline{1-1}  & Day 33 & 18 & 625 & 625 & 6.72 & 0.99 \\ \cline{1-1}  & Day 50 & 10 & 550 & 552 & 6.52 & 0.99 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Microbial diversity changes in PN1 and PN2 according to FA concentration. The microbial community dynamics in PN2 also showed significant changes during the nitrification process. On day 6 of the nitrification reaction, the proportion of _Acidobacteria_ and _Frimicutes_ decreased to 6.4% and 5.8%, respectively, while _Bacteroidetes_ and _Proteobacteria_ dramatically increased to 45.5% and 20.6%. After gradually reducing the influent NH4+-N concentration to 50 mg/L (FA of 10 mg/L), by day 50, _Bacteroidetes_ and _Frimicutes_ had decreased to 27.8% and 1.9%, respectively, whereas _Acidobacteria_ and _Proteobacteria_ increased to 9.3% and 35.3%. In PN1, _Nitrospire_ was the dominant phylum due to the predominance of NOB over AOB, as indicated by the low NAE resulting from NH4+ oxidation [31; 32; 33]. In contrast, in PN2, _Proteobacteria_ became the dominant phylum. The high proportion of _Nitrospire_ in PN1 was due to the predominance of NOB over AOB.