## 1 Introduction  
In recent years, the presence of contaminants of emerging concern (CECs) from municipal wastewater effluents in the receiving water bodies has increased [1]. These anthropogenic organic substances are present at trace levels; however, they can represent a severe risk to the environment and human health [2]. As conventional wastewater treatment plants (WWTPs) are not designed to degrade them [3, 4], upgrading them with further steps is necessary to avoid widespread CECs. In response to this concern, the European Commission (EC) established a watchlist of those CECs that are suspected to pose a hazard to the environment (Directives 2000/60/EC, 2008/105/EC and 2013/39/EU). This regulation is updated periodically, setting progressively more restrictive limits for their discharge. Like in other EU Member States, the Water Management Authority of Luxembourg (Administration de la Gestion de l'Eau) is adopting in its own regulation based on those European Directives, applying more severe regulations regarding the mitigation of CEC emissions with the attempt to achieve a better status of surface water bodies. The Luxemburgish guidelines to implement a post-treatment step were released in June 2020. They state the need to achieve 80% elimination of four selected CECs between the inlet of the WWTP and the outlet. Advanced Oxidation Processes (AOPs) are powerful technologies for the purification of secondary treatment effluents, which are based on the generation of hydroxyl radicals (\({}^{\bullet}\)OH). They have a very short lifetime (10\({}^{-9}\) s [5]) but are highly reactive in a non-selective way with organic molecules; thus, they can degrade recalcitrant and persistent compounds and even mineralise them. Among all the AOPs, photo-Fenton has shown promising results due to its efficiency in removing CECs [6]. It is based on an upgrade of the Fenton reaction (Equation (1)), using photons to regenerate the Fe\({}^{3+}\) to Fe\({}^{2+}\), simultaneously producing a higher amount of \({}^{\bullet}\)OH (Equation (2)). \[\mathrm{Fe}^{2+}+\mathrm{H}_{2}\mathrm{O}_{2}\rightarrow\mathrm{Fe}^{3+}+ \mathrm{OH}^{-}+{}^{\bullet}\mathrm{OH} \tag{1}\]  
\[\mathrm{Fe}^{3+}+\mathrm{H}_{2}\mathrm{O}+\mathrm{hv}\rightarrow\mathrm{Fe}^{2+ }+{}^{\bullet}\mathrm{OH}+\mathrm{H}^{+} \tag{2}\]  
The main drawback of this technology is the low pH required to keep iron in the solution (optimal pH 2.8) and avoid slurry generation. As a consequence, the research focused on working at a neutral pH. The main advantages are avoiding the costs of acidification and later neutralisation and the increase in the effluent's salinity [7]. As a downside, a neutral pH requires chelating agents [8] to maintain the iron dissolved. Ethylenediamine-N,N-disuccinic acid (EDDS) offers a suitable solution in terms of chelate stability and treatment time, it allows achieving more than 80% removal in a short treatment time at low iron doses [9]. Its optimal pH range is between 3 and 7, but even in a weak basic medium, it is possible to reach high removal rates, e.g., almost 80% of cyclophosphamide and up to 90% of 5-fluorouracilacil [10], both cytostatics with high ecotoxicological potential. Recently, nitrilotriacetic acid (NTA) has also been proposed as a chelating agent. Via using NTA, the removal rate for carbamazepine, ibuprofen, or croatmiton reaches more than 92% when illuminated with a UVA lamp [11]. The removal rate when using NTA compared with the use of EDDS at a continuous flow and medium scale (around 100 L) is demonstrated to be lower, despite the fact that at a small scale (until 20 L), similar behaviour is reported. This drawback is, however, compensated by the NTA's remarkably lower price. Despite the successful results with photo-Fenton, it still has high costs in terms of reagents and energy consumption. In this context, combining the photo-Fenton process with other technologies is currently considered [12].