## 3 Results and Discussion
### Photo-Fenton
#### 3.1.2 Simultaneous Application of MP and LP Lamps  
Expecting to improve the previous scenarios, the simultaneous use of both lamps was tested, searching for synergies between the two systems. The addition of the 40 W LP lamp was added as a preprocessing stage to increase the treatment efficiency, aiming at better usage of the reagents.  
The simultaneous use of both lamps (500 W MP + 40 W LP lamp) was carried out under the optimal conditions for the 500 W MP lamp: 5 mg L\({}^{-1}\) of iron and 40 mg L\({}^{-1}\) of H\({}_{2}\)O\({}_{2}\). The time to achieve the target, 80% removal, was reduced to 15 min, meaning 0.3 min of relative contact time (Figure 4a). As the required treatment time was diminished, a reduction in the dosage of reagents, mainly iron, was also tested to minimise their discharge into the environment. With 3 mg L\({}^{-1}\) of iron maintaining the 40 mg L\({}^{-1}\) of H\({}_{2}\)O\({}_{2}\) the time to reach 80% removal increased to 20 min, while, for 5 mg L\({}^{-1}\) of iron, 80% removal was achieved in 15 min. The difference in irradiation time between both scenarios was 0.1 min--this increase is acceptable for the scale-up. Therefore, to find a compromise between iron dosage and treatment time, 3 mg L\({}^{-1}\) of iron, maintaining the 40 mg L\({}^{-1}\) of H\({}_{2}\)O\({}_{2}\), was selected as the most suitable.  
1.3 Comparison of the Photo-Fenton Process: LP lamp, MP Lamp and Simultaneous Application of MP and LP Lamps  
The removal of the model compound by photo-Fenton was compared across the three different lamp alternatives (Figure 4b) to select the most suitable scenario. The optimised working condition for each lamp was selected from the previous test as follows: 5 mg L\({}^{-1}\) of iron and 40 mg L\({}^{-1}\) for the 500 W MP lamp; 6.5 mg L\({}^{-1}\) of iron and 30 mg L\({}^{-1}\) for the 40 W LP lamp; and 3 mg L\({}^{-1}\) of iron and 50 mg L\({}^{-1}\) for the combination of both lamps.  
Figure 3: _Cont._  
Figure 4b shows that the simultaneous application of the lamps and the 500 W MP lamp as a stand-alone had similar yields despite the treatment with both lamps being slightly quicker. On the contrary, the 40 W LP lamp alone shows remarkably slower removal rates; due to its lower power, it has a lower energy consumption.  
Nevertheless, the 40 W LP lamp was the best option if the removal vs. energy consumption (on nominal values) was considered, as the lower power of the LP lamp has a lower energy consumption. More than 5 times the energy consumption was needed to achieve the 80% removal of the reference compound by the 500 W MP lamp or the combination (30 Wh on the 40 W LP lamp and 167 Wh on the 500 W MP lamp) compared to the 40 W LP lamp. The slight increase of the target compound removal of the combination of lamps, with respect to the 500 W MP lamp stand-alone, did not justified the use of the combination of lamps due to the increment of energy consumption.  
Figure 4: Indigo carmine removal trend during photo-Fenton process. [indigo carmine]\({}_{0}\) = 25 mg L\({}^{-1}\), pH\({}_{0}\) = 7.1–7.2, t\({}_{0}\) = 19–20 \({}^{\circ}\)C. **(a)** Combination LP and MP lamps for the different initial H\({}_{2}\)O\({}_{2}\) and iron concentrations. **(b)** Comparison of different lamp possibilities: 500 W MP lamp, iron = 5 mg L\({}^{-1}\), H\({}_{2}\)O\({}_{2}\) = 40 mg L\({}^{-1}\); LP lamp, iron = 6.5 mg L\({}^{-1}\), H\({}_{2}\)O\({}_{2}\) = 30 mg L\({}^{-1}\); simultaneous use of lamps, iron = 3 mg L\({}^{-1}\), H\({}_{2}\)O\({}_{2}\) = 50 mg L\({}^{-1}\) (c) Comparison of two different chelating agents: 500 W MP lamp, iron = 5 mg L\({}^{-1}\), H\({}_{2}\)O\({}_{2}\) = 40 mg L\({}^{-1}\).  
Thus, considering that long HRT is required for the 40 W LP lamp, the 500 W MP lamp was selected.