## 2 Advanced Treatment Processes for Emerging Contaminants  
The classes of processes examined below are termed advanced treatment processes (ATPs), since they are capable of achieving degradation of specific constituents in solution, not normally achieved by other treatment options. This class covers all those unit operations that do not act on mechanical or biological principles: Coagulation-flocculation-precipitation, stripping, filtration (all types), ion exchange, absorption, electroflotation, biological and bioelectrochemical methods, etc. They may, however, be combined with these and other treatment units to improve overall pollutants' removal efficiency, i.e., in MBT schemes [9]. ATP in combination with common or modified biotereatment technologies can be the key for successful emission control for many CECs, as in many cases, they significantly increase the biodegradability of recalcitrant or inhibitory pollutants, as discussed later. One crucial factor for assessing the feasibility of a particular treatment process is represented by the kinetic rates of the reactions involved in the degradation of target compounds. Advance treatment technologies make use of highly reactive minimally selective reagents that will achieve transformation of contaminants in solution into simpler (e.g., partial decomposition of non-biodegradable organic pollutants into biodegradable intermediates) or innocuous (e.g., mineralization to CO2, water, and residual elements) products, even though the formation of hazardous byproducts cannot be excluded a priori. Advanced oxidation processes are aqueous phase oxidation methods, investigated and developed since the 1970s, and well established in the water sector, as they are currently considered the state-of-the-art in commercially available advanced treatment processes for many contaminants [14]. AOPs are based on the generation of highly reactive radicals (predominantly OH, hydroxyl radical) for the oxidation of organic pollutants within a solution treated by means of different activation processes (ultraviolet photolysis, Fenton processes, electrochemical oxidation, sonochemical processes, microwave, supercritical water and wet air oxidation, and others), often combined with chemicals (e.g., O3, H2O2) and/or catalysts (e.g., TiO2) [15; 16]. OH is among the strongest oxidants applicable in water processes, and can unselectively oxidize virtually any susceptible compound present through the following basic pathways: Radical addition, hydrogen abstraction, electron transfer, and radical combination. The chain of reaction that is generated leads to further generation of reactive species, such as H2O2 and super oxide (-O2-). The mechanisms of radical generation in the most common \begin{table}
\begin{tabular}{l l l} \hline \hline
**CEC Class** & **Example Compound** & **Definition/Use** \\ \hline Reproductive hormones & Dihydrotestosterone (DHT), & Chemicals, usually steroids, that stimulate \\  & Progesterone, Estronte, & reproductive functions \\  & Estradiol & \\ \hline Solvents & Ethanol, kerosene & Chemical solutions (other than water) capable of \\  & & dissolving other substances \\ \hline Steroids & Cholesterol, Coprostanol, & Fat-soluble organic compounds with characteristic \\  & Estronte, Progesterone & molecular structure, including natural and synthetic \\  & & hormones \\ \hline Surfactants & Sodium Lauryl Sulfate & Substances affecting the surface of a liquid \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Cont. AOPs for wastewater treatment were summarized in other works [14; 17]. Figure 1 summarizes the various technologies available in the AOP and in other ATP classes.