## 2 Methods of Eliminating Hormones and Pharmaceuticals
### Chemical Methods
#### 2.2.1 Advanced Oxidation/O\({}_{3}\)/H\({}_{2}\)O\({}_{2}\)
Figure 6 shows the main physicochemical mechanisms and processes,such as UV, O\({}_{3}\), H\({}_{2}\)O\({}_{2}\), Fe\({}^{2+}\), and TiO\({}_{2}\), and their interactions, leading to the formation of hydroxyl radicals. In the O\({}_{3}\)/H\({}_{2}\)O\({}_{2}\) process, ozone (O\({}_{3}\)) in the presence of hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)) undergoes accelerated autodecomposition, leading to the formation of hydroxyl radicals (-OH), which exhibit a much higher oxidising potential than ozone itself and act non-selectively against a wide range of organic pollutants. The initiating reaction follows the equation H\({}_{2}\)O\({}_{2}\) + O\({}_{3}\)\(\rightarrow\) -OH + O\({}_{2}\) + HO\({}_{2}\)-, and the resulting radicals attack the double bonds and aromatic structures present in the molecules of endocrine compounds and drugs, leading to their breakdown and partial or complete mineralisation [50; 51; 54]. Numerous studies have shown that the elimination efficiency of hormones and pharmaceuticals using the O\({}_{3}\)/H\({}_{2}\)O\({}_{2}\) process can exceed 90%, especially for biodegradation-resistant compounds such as EE2 and diclofenac [31; 34]. Compared to classical ozonation, this method provides faster reaction times and higher degradation efficiencies, even with lower ozone consumption, due to the involvement of highly reactive -OH. Another unquestionable advantage of this process is that it can be integrated with other water treatment technologies, including biological treatment and membrane filtration, thus achieving synergistic treatment effects [118]. Despite its high efficiency, the O\({}_{3}\)/H\({}_{2}\)O\({}_{2}\) process has significant limitations. Its main drawbacks include high operating costs due to the need for on-site ozone production (usually using electricity) and the use of hydrogen peroxide as a chemical reagent. Depending on the scale of the plant and the operating parameters, operating costs can reach 0.20-0.50 EUR/m\({}^{3}\) of treated wastewater, which is significantly higher than typical biological treatment costs [71]. In addition, the efficiency of the process is highly dependent on the physicochemical parameters of the wastewater, including the pH value, the presence of competing oxidants, and the contact time. There is also a risk of the formation of intermediates with unknown toxic profiles, which may require further purification in the final stage. Another challenge is the need for precise dosing of reactants--both excess ozone and H\({}_{2}\)O\({}_{2}\) can lead to oxidant losses and even the formation of secondary products with oxidising properties [54]. In conclusion, the O\({}_{3}\)/H\({}_{2}\)O\({}_{2}\) process is an effective but costly technological solution for the advanced elimination of micropollutants from wastewater. Thanks to the generation of hydroxyl radicals, it is possible to remove a wide spectrum of persistent and toxic  
Figure 6: OH generation pathways in AOP; authors’ own elaboration based on [115; 116; 117; 118]. organic compounds, but its implementation on a larger scale requires detailed economic and environmental analysis and control over process parameters [54].