## 3 Results and Discussion  
### Industrial Wastewater Treatment by Microbial Fuel Cells  
The physicochemical parameters of the wastewater used for the MFC experiments, baseline and control, are shown in Table 1, including initial and final values, which are common for all operational conditions, and final values after 217 h (test time). The wastewater analysis shows that the pH shifts from an initial value over 8 in the anode chamber to neutral values at the end time for all systems (MFCs, baseline and control). In MFC systems, the pH of the anodic chamber greatly influences the metabolic activity of microbes affecting the mechanism of proton and electron generation from electroactive bacteria. Since the final pH values are neutral, protons produced are not accumulated during time operation in the anodic chamber in MFCs but transferred through the ionic liquid membrane to the cathode at which they combine with oxygen. Accordingly, alkalinity also decreases in MFCs from 67.60 mg/L to 19.60 mg/L in the MFC anode chamber. Another significant parameter for wastewater treatment is the evolution of the COD loading. As can be seen from Table 1, a high removal rate is achieved in the MFC system after nine days of operation. COD loading reduces from 1760 mg/L to 334 mg/L, which accounts for 81.0% in terms of COD removal. The digestion of the wastewater performed in the presence of graphite granules only (baseline) and the absence of graphitic components (control) for comparison purposes, offers removal rates of only 31.8% and 37.9%, respectively. These values show that the degradation of soluble organic matter is clearly promoted in MFC operation versus conventional anaerobic digestion. Regarding the levels of TSS, higher values of these parameters were found in MFC and baseline conditions in comparison with the control. The increase in this parameter for such conditions is related to the presence of graphite granules in the two first configurations, which would increase the level by adding solids from the graphite phase to the anode bulk solution. Regarding the analysis of the ion species present in the wastewater used, different tendencies can be observed depending on the case. Nitrite, sulphate and ammonium show a decreasing tendency in all conditions, but generally more accentuated under MFC operation. Considering the complexity of the anode conditions in MFCs, it is likely that several mechanisms are involved for each ion species. In the absence of oxygen, nitrites and sulphates can act as electron donors and be employed as nutrients by bacteria. The higher decrease rates in MFC when compared with baseline and control conditions can be attributed to the growth of the biofilm, and the faster water oxidation rate as previously commented. Given that the initial concentration of sulphate is significantly high (263.50 mg/L), it is worth noting that the reduction of this ion species in the MFC systems reaches 51.23%. The concentration of ammonium also decreases in comparison with the initial values. Ammonium is another nutrient used by microorganisms. This reduction is also higher in MFCs than in the baseline and control. This trend is also observable for Kheldahl nitrogen, which is reduced by a 50% in MFCs, showing higher removal rates compared with the rest of the conditions. Regarding the concentration of total phosphates, it can be observed that there is a significant increase in their concentrations in comparison with their initial values (0.51 mg/L) in those systems in which graphite are present (MFC and baseline). The final values for these two parameters are in the interval 14.41-19.27 mg/L. In previous works, it has also been also reported that these ion species can be released to the medium from the class of graphite used in this work due to its composition [25]. In the case of chloride, this parameter clearly increases in the case of MFCs. This is caused by the release to the media of a small amount of ionic liquid that forms the membrane as discussed in the following section.