## 3 Results
### Bio-Augmented Activated Sludge
#### 3.2.3 Enhanced Degradation Efficiency  
Before bioaugmentation, the BPA removal rate of activated sludge was low (Figure 8). It was reported that the removal of BPA by a small sewage treatment plant in the city of Rome was 59% [70]. In another previous study, the average removal rate of BPA and its analogs in municipal WWTPs was found to be 69.3%, and the removal efficiencies of BPA were very similar using conventional activated sludge units or membrane bioreactors [71]. The accumulated research results suggest that several factors may lead to different BPA degradation efficiencies, including temperature, BPA concentration, mixed liquid suspended solids, hydraulic retention time, and pH [72].  
Bioaugmentation is a feasible and environmentally friendly solution to increase the efficiency of biological sludge treatment. Within 96 h, the residual concentration of BPA in the biofortified reactor was much lower than that in the control reactor. During the entire reaction time, the BPA removal efficiency of the control group was 51.1-80.2%, and the BPA removal efficiency of the biofortified group of reactors was 71-96.3%. The removal rate of the biofortified group reached 83.1% in 72 h, showing a high BPA removal efficiency. In addition, 96.3% of the BPA in the wastewater was removed in 96 h, and the BPA removal efficiency of the biofortified group was increased by 15% compared with the control group. The results showed that Req-001 had a good ability to remove BPA, and the addition of Req-001 quickly removed BPA in municipal wastewater. Due to the adsorption of microorganisms, a small amount of BPA was removed, and biodegradation was the main reason for the reduction of BPA. Studies have shown that biofortified bacteria can improve the biodegradation and wastewater treatment performance of biofortified systems [73]. Req-001 showed good BPA detoxification ability, ensured the water purification performance of the reactor and the removal of toxic substances, and had a good application potential for bio-augmentation.  
Our previous experimental results showed that the addition of _Rhodococcus_ would have an effect on the microbial community of activated sludge, causing changes in the abundance of some other microorganisms. Especially when the abundance of _Rhodococcus_ is relatively high, some substrates used by _Rhodococcus_ are the same or similar, and growth and reproduction will produce a competitive relationship. The abundance of these competitive bacteria decreased to a certain extent. When the abundance of _Rhodococcus_ decreases, it means that _Rhodococcus_ cannot compete with indigenous bacteria, but it still has an influence on the microbial population structure. With a mutually beneficial symbiotic relationship with _Rhodococcus_, the abundance will increase in a short period of time. After the addition of _Rhodococcus_, the abundance of _Zogloea_ and _Methylobacillus_, which have functions such as COD digestion and extracellular polymer secretion, increased, indicating that _Rhodococcus_ changed the microbial population structure and bio-enhanced the sludge system. The increased abundance of bacteria and _Rhodococcus_ together resulted in improved water quality and reduced organic matter concentrations. The addition of BPA affects the water treatment capacity of the reactor because of the effect of BPA on certain microbial communities. When the bio-enhanced _Rhodococcus_ was added, the concentration of BPA was significantly reduced, because the toxic effect of BPA on activated sludge was alleviated and recovered, and the affected functional flora began to restore the original population niche.