## 3 Results
### Toxicity of BPA in Activated Sludge
#### 3.1.2 BPA Reduces the Water Purification Capacity of Activated Sludge  
Due to the long-term acclimation of the inoculated sludge, the control reactor had stable removal efficiency of COD, ammonia nitrogen, and TP. The concentration of COD in the effluent was kept between 20-31 mg/L, the ammonia nitrogen in the effluent was between 2.3-2.9 mg/L, the TP in the effluent was 0.75 mg/L; the removal rates were all above 92%. In the effluent of the BPA group reactor, the COD concentration was 62.6-138.3 mg/L, and the removal rate was also 65.4-84.3% (Figure 2A). The results indicated that BPA had a negative effect on the digestion of COD in the effluent of the reactor. This is because of  
Figure 1: Effects of bisphenol A (BPA) on extracellular polymeric substances (EPSs) and the settling properties of activated sludge. (**A**). Change in protein content in EPS; (**B**). Change in polysaccharide content in EPS; (**C**). Ratio of polysaccharide to protein in EPS; (**D**). Sequencing batch reactor (SBR) sludge concentration and the ratio of SV (mL) to sludge dry weight (g) (SVI). MLSS: mixed liquid suspended solids. the addition of BPA. The microorganisms are not adapted to the increased external carbon, resulting in an increase in the COD content of the system. Moreover, as BPA is an organic compound, its addition increased the COD in the system. In stages II and III of the BPA group reactor, the COD digestion capacity gradually recovered to a moderate level. The activated sludge used in the SBR process has a strong resistance to shock load, and the microorganisms can self-regulate and adapt, maintaining the stability of the reactor, and resisting the adverse environment by increasing biodiversity or secreting EPSs. However, when BPA damage reaches a certain level, the self-healing ability of microorganisms will be limited. Under BPA stress, the COD digestion capacity in the effluent of the reactor did not return to the original level at the end of the experiment. Similar results were obtained by [PERSON] et al., who found that aerobic activated sludge in an SBR also resulted in an increase in effluent COD when 1 mg/L BPA was treated [12]. This result can be explained by the addition of BPA leading to the destruction of the microbial community, the decrease of microbial abundance associated with organic matter degradation, and finally to a higher effluent COD concentration. The response of BPA to the water purification of the SBR was reflected in the effluent quality. At the beginning of stage I in the BPA group reactor, the ammonia nitrogen concentration in the effluent fluctuated greatly, and the concentration reached 30.9 mg/L on the first day (Figure 2B). This phenomenon may have occurred because the domesticated sludge had already adapted to the environmental conditions of artificially simulated sewage, and the microbial community was unable to adapt to the addition of an excess carbon source, which inhibited the growth and reproduction of ammonia oxidation-related bacteria. When the microbial flora adapted to the long-term existence of BPA, the ammonia nitrogen content in the effluent was significantly reduced, but there was still a gap in comparison with the sludge from the control group due to the toxic effect of BPA itself on the sludge. In stages II and III of the BPA group reactor, with the increase of the BPA concentration, the ammonia nitrogen effluent content of the reactor remained very high, even reaching 122.8 mg/L. Although the ammonia nitrogen content decreased with the continuous operation of the reactor, it remained at a relatively high level, and it was difficult to restore the ammonia nitrogen content to its initial state. The results showed that the toxic effect of BPA resulted in a low ammonia nitrogen removal efficiency in the effluent of the reactor.