## 3 Results and Discussion
### Characteristic microorganisms in the PSD and AnMBR  
To identify the microorganisms present in the PSD and AnMBR, we conducted 16S rRNA gene amplicon sequencing on three samples from each bioreactor. After quality control, denoising and chimer removal, a total of 148,306 high-quality reads were generated from the 16S rRNA V4 sequences from 6 samples, ranging from 16,126 to 41,310 reads, with an average of 24,718 (Table S2). The sequence numbers were normalized to 16,126 reads in each sample to describe the alpha diversity and composition characteristics. All the sequences were clustered into OTUs at 97% sequence similarity. The number of OTUs per sample ranged between 386 and 608 (Table S2). Good's coverage ranged from 0.85595 to 0.93668 in all samples (Table S2), suggesting that major microbial diversity had been captured. The average Chao1 index values were \(2217\pm 295\) and \(1459\pm 748\) for the PSD and AnMBR samples, respectively. The PD indices were \(57.4\pm 1.82\) for the PSD samples and \(49.5\pm 7.36\) for the AnMBR samples. The Shannon and Simpson diversity indices were significantly higher in the AnMBR (Shannon: \(7.012\pm 0.330\); Simpson \(0.978\pm 0.009\)) than in the PSD samples (Shannon: \(5.973\pm 0.040\); Simpson \(0.942\pm 0.05\)) (\(p<0.05\)) However, the number of observed OTUs, Chao1, ACE, and Faith PD index did not differ significantly between the PSD and AnMBR (Table S3). The 16S rRNA gene sequences were classified at both the phylum and family levels. At the phylum level, _Proteobacteria_ (\(27.11\pm 0.46\%\)), _Euryarchaeota_ (\(13.72\pm 0.41\%\)), _Firmicutes_ (\(11.44\pm 0.92\%\)), _Synergistetes_ (\(11.22\pm 1.02\%\)), and _Bacteroidetes_ (\(10.89\pm 2.51\%\)) were the predominant phyla in the PSD samples, accounting for 74.38% of the total microbiota. However, a large proportion of _Proteobacteria_ (\(75.53\pm 0.46\%\)), _Firmicutes_ (\(6.37\pm 0.92\%\)), and _Bacteroidetes_ (\(5.76\pm 2.51\%\)) were observed in the AnMBR samples (Figure 2). Samples of the PSD and AnMBR also harbored different family-level profiles, with _Methanobacteriaceae_ (\(12.99\pm 0.35\%\) in the PSD and \(2.08\pm 0.35\%\) in the AnMBR) and _Syntrophaceae_ (\(4.24\pm 0.10\%\) in the PSD and \(3.61\pm 0.01\%\) in the AnMBR) representing the most abundant \begin{table}
\begin{tabular}{l c c} \hline \hline
**Parameter** & **AnMBR** & **PSD** \\ \hline Hydraulic retention time & 24 h & 40 days \\ solid retention time & \(\sim\)200 days & \(\sim\)200 days \\ Organic loading rate & 0.71 (g COD/L/d) & 0.16 (g VSS/L/d) \\ Biogas production a  & 122 (L/kg COD) & 374 (l/kg VSS) \\ CH\({}_{4}^{\circ}\) & 80–85 & 60–65 \\ pH & 7.11 & 7.25 \\ Temperature (\({}^{\circ}\)C) & 20 & 37 \\ TSS (\%) b  & 4.11 & 4.24 \\ VSS (\%) c  & 2.87 & 2.87 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Anaerobic membrane bioreactor (AnMBR) and sludge anaerobic reactor (PSD) system operating parameters. genera in the PSD and AnMBR samples, respectively (Table S4). At the phylum level, the relative abundances of most predominant phyla were significantly different between the two types of samples (all \(p<0.05\)) (Figure 2). Moreover, _Methanobacteriaceae_ (12.99 \(\pm\) 0.35% in the PSD and 2.08 \(\pm\) 0.35% in the AnMBR) and _Syntrophaceae_ (4.24 \(\pm\) 0.10% in the PSD and 3.61 \(\pm\) 0.01% in the AnMBR) were identified as the families that most significantly differed in abundance between the PSD and AnMBR samples (all \(p<0.05\)) (Table S4). This result suggested a distinct difference in the microbial community structure between the PSD and AnMBR samples, which was also supported by the weighted PCoA results. The weighted PCoA showed that the PSD samples grouped together in one cluster, while the AnMBR samples grouped in another cluster (Figure S1). _Methanobacteriaceae_ and _Syntrophaceae_ were identified as the genera that significantly differed in abundance between the PSD and AnMBR samples.