## 3 Results  
### Toxicity of BPA in Activated Sludge  
#### 3.1.1 BPA Reduces the Settling Performance and Biomass of Activated Sludge  
EPSs are complex polymers that are mainly composed of protein and polysaccharide. The content and composition of EPSs have a significant impact on the physical and chemical properties of activated sludge, including the sludge structure, surface charge, flocculation, settling performance, dehydration performance, and adsorption capacity [43]. This study found that the concentrations of polysaccharide and protein in the control group reactor remained basically stable at about 11.8 \(\pm\) 0.4 mg\(\cdot\)g\({}^{-1}\) volatile suspended solids (VSS), and 4.2 \(\pm\) 0.1 mg\(\cdot\)g\({}^{-1}\) VSS, respectively. In different stages of the BPA group reactor, the EPS content decreased, and the total secretion of proteins and polysaccharides was lowest in Stage III. The polysaccharide concentration increased slightly, and the protein concentration decreased significantly. The results showed that BPA negatively affected the secretion and accumulation of EPSs in sludge with reduced protein content (Figure 1A,B). When the ratio of polysaccharide to protein in EPS increases, the flocculation and settling properties of activated sludge will deteriorate [44]. The ratio of polysaccharide to protein in the EPSs of the control group was between 0.37-0.43 (Figure 1C), and the ratio during each stage of the reactor in the BPA group was between 0.44-0.89. In stage III, the detection value of polysaccharide content was 5.65 mg\(\cdot\)g\({}^{-1}\) VSS, and the detection value of protein content was 6.7 mg\(\cdot\)g\({}^{-1}\) VSS. The ratio of polysaccharide to protein in EPSs changed the most, resulting in poor flocculation and sedimentation performance of the activated sludge, which was consistent with the detected SVI value. It has been reported that the binding interaction between BPA and EPS is spontaneous, and BPA mainly binds to the protein of EPSs through hydrophobic association [45], which may be one of the reasons for the decreased protein content in the BPA reactor. The electronegativity and hydrophobicity of the protein surface are conducive to particle flocculation and sedimentation, and the sedimentation effect of sludge decreases with the decrease of protein content [46]. PSs have high hydrophilicity, which is not conducive to the coagulation of microbial flocs. In the present study, BPA significantly reduced the protein content of activated sludge and increased the ratio of polysaccharide to protein, resulting in a decrease in the sedimentation performance of the sludge, and a decrease in the coagulation ability of microbial flocs. The mixed liquid suspended solid value of the domesticated sludge was largely stable, and ranged from 3000-3200 mg/L. Under the influence of BPA, the concentration of activated sludge in the reactor gradually decreased, ranging from 2700 to 2300 mg/L (Figure 1D). The results showed that BPA reduced the biomass of activated sludge, and this effect was exacerbated with increasing BPA concentrations. Studies have shown that biomass concentration is positively correlated with the biodegradation rate of pollutants, and toxic pollutants can inhibit the growth of microorganisms, and ultimately lead to a decrease in the concentration of activated sludge [47]. The activated sludge process relies on the biodegradation or biological metabolism of pollutants by the sludge active biomass, and the decrease of mixed liquid suspended solid will lead to a decrease in the biological removal rate of BPA by activated sludge. The average SVI value of activated sludge in urban sewage treatment plants is generally between 50 and 150. The higher the SVI value, the worse the sludge settling performance. Compared with the control group reactor, the SVI in the BPA group reactor showed an increasing trend.