## 1 Introduction
This work proposes the combination of photo-Fenton with activated carbon filtration to reduce the dosage of reagents and the hydraulic retention time (HRT). Technologies based on separation are also very effective for water treatment. Granular activated carbon (GAC) filtration has been deeply investigated since it does not generate toxic by-products and is easy to implement, operate and maintain [13]. The adsorption of CECs depends on the dissolved organic matter, the physical-chemical properties of each CEC, the contact time and the physical-chemical properties of the influent [14; 15]. Nonetheless, the use of GAC revealed two main disadvantages: the high cost of its replacement and the environmental impact produced in terms of carbon footprint [16]. To minimise both GAC drawbacks, regenerated GAC is considered as a replacement when the fresh material is depleted [17]. Different regeneration methods have been investigated but, in all cases, the initial GAC's properties changed, reducing its efficiency. [PERSON] et al. (2022) [18] reported that a 21% reduction in efficiency was obtained in the first regeneration cycle and 64% in the fourth cycle. However, this efficiency reduction is compensated by the cost and environmental impact reductions [19]. Commonly, adsorption experiments testing GAC require a long operation time for their correct evaluation, since a fixed bed column usually contains a large amount of material with a high adsorption capacity, even at the laboratory scale. Some CECs, such as metaldehyde and isoproturon, achieve the breakthrough after between 50,000 and 100,000 bed volumes (BV) with fresh GAC [18]. Aiming at reducing experiment duration but still providing reliable data, [PERSON] et al. (1986) [19] designed a \"Rapid Small-Scale Column (RSSC)\" concept, minimising the volume of GAC and water used in every experiment,which allows the process to reach the breakthrough within days of operation rather than months. The obtained results can be transferred to scale up accurately and reliably. The combination of photo-Fenton and GAC could be an attractive option for CEC depration since it would allow reducing the treatment time and reagent dosage of the photo-Fenton process, and it could extend the lifetime of the subsequent GAC step while still providing a high removal rate. The combination of the two processes has been rarely studied. In the few studies available, photo-Fenton was operated at an acidic pH and at a laboratory scale [20; 21], far away from the conditions and needs of a WWTP. Further, in those studies [20; 21], GAC adsorption was studied in batch mode in contrast to the filtration performed in this work, which is the usual operation in bigger scales. The results showed the removal of up to 95% [20] and 90% [21] for antibiotics and cytostatics (i.e., Flutamide), respectively. Therefore, the combined treatment could be a suitable alternative when tested under realistic conditions: operating at a natural pH and real matrix, using both fresh and regenerated GAC sources as the scope of this work. The objective is to implement it as a post-treatment step in the WWTP of Heiderscheidergrund (12,000 population equivalent, Esch-sur-Sore, North of Luxembourg). Considering the complexity of the individual technology, the combination of both processes was calibrated with indigo carmine as a model compound for organic pollutants [22; 23], known to be a suitable dye and an \({}^{\bullet}\)OH probe in photocatalysis [24]. Its use to compare the adsorption capacity of different GAC types has been widely reported [25]. Indigo carmine is one of the most used dyes, from the textile industry to the food industry; up to 40 thousand tons of indigo carmine are produced annually [26]. It is a toxic dye that can affect human health; it can cause various pathologies such as hypertension, respiratory tract diseases and skin irritations [27]. The management of wastewater containing indigo carmine poses a challenge due to the intricate aromatic molecular structures.