## 2 Methods of Eliminating Hormones and Pharmaceuticals
### Chemical Methods
#### 2.2.2 Ozonation Process  
Studies on the evaluation of ozonation efficiency for bisphenol A and deionised water as a matrix have been reported in the literature. The authors of [68] chose bisphenol A because of its lowest susceptibility to chemical oxidation processes. Figure 7 shows that the efficiency of BPA removal significantly depends on the ozone dose used. At a dose of 1 mg/dm\({}^{3}\), an efficiency of about 40% was obtained, which may indicate an initial oxidation step of BPA; however, this is not sufficient for its full degradation. Increasing the dose to 5 mg/dm\({}^{3}\) results in a significant increase in efficiency to 70%, suggesting a more advanced mineralisation of the compound and an intensification of the reaction with hydroxyl radicals formed by ozone degradation. Surprisingly, a further increase in the ozone dose to 10 mg/dm\({}^{3}\) results in a decrease in efficacy to 60%. This may be due to several possible reasons, for example, an excessive ozone dose may lead to the oxidation of readily reactive organic compounds present in the wastewater, resulting in competition for ozone and reducing its availability to react with BPA. There is also the possibility of the formation of more stable and more difficult-to-remove intermediates as a result of the incomplete decomposition of BPA [48, 50, 68]. In light of the data obtained, it can be concluded that the optimum ozone dose for effective BPA degradation is approximately 5 mg/dm\({}^{3}\). This value provides the highest efficiency while minimising the side effects associated with excessive ozone dosing [68]. The efficiency of the ozonation process is higher under neutral and alkaline conditions than under acidic conditions (Figure 8). This is due to the different reaction mechanism between the compound to be removed and ozone depending on the pH of the solution. In an alkaline environment, radicals are often rapidly removed by hydroxyl anions or other substances, limiting their participation in the reaction [68]. The efficiency of the BPA ozonation process is strongly dependent on the pH value, which affects both the stability of ozone and the oxidation reaction mechanism. Under strongly acidic conditions (pH = 1), ozone has high stability, which limits its rapid decomposition and allows for oxidative activity to be maintained for a longer period in the reaction medium. However, BPA degradation reactions mainly occur by direct oxidation by molecular ozone without the involvement of hydroxyl radicals (-OH), which translates into low BPA removal efficiencies--estimated at 20-30% [119]. An additional limitation of using an acidic environment is the need for aggressive acidifying agents (e.g., HCl), which generates additional costs and technological risks, and requires a final adjustment of the effluent pH [50]. Figure 7: Effect of ozone dose on elimination of bisphenol A; authors’ own elaboration based on [68]. On the other hand, under strongly alkaline conditions (pH = 12), there is an intensive autodecomposition of ozone and the generation of hydroxyl radicals, which have high oxidising potential and are the main factor responsible for BPA degradation. As a result, BPA removal efficiencies in this pH range can reach up to 75-85% [50]. Nevertheless, excessive degradation can lead to ozone losses and requires very precise dosing. Furthermore, as in the case of acidic pH, the use of additional reactants (e.g., NaOH) and the neutralisation of the effluent prior to further management are necessary. The formation of secondary oxidation products with unknown toxicological properties cannot be excluded either [31]. In conclusion, although an alkaline environment is conducive to the effective elimination of BPA, from a technical point of view, the two extreme pH ranges (acidic and alkaline) carry important technological and economic constraints that must be taken into account when designing and optimising the ozonation process [31; 50]. Table 5 illustrates the removal efficiency of Biphenol A as a function of the ozone dose, reaction time, and pH.