## 1 Introduction  
Water availability is a global issue requiring pressing solutions. Currently, around 40% of the world population is facing a scarcity of clean water, and this situation is likely to worsen in the next decades. The lack of access to safe water and sanitation causes serious health problems and has large unquantified impacts on economic productivity. Moreover, over 80% of the world's wastewater is discharged into the environment without any treatment, damaging ecosystems and triggering the spread of communicable diseases [1]. Microbial fuel cells (MFCs) are a promising technology that could help to tackle this problem. MFCs are systems in which organic matter is oxidized by microbes under anaerobic conditions in the anodic compartment. Protons and electrons are released due to the oxidation process. While electrons are transferred to the cathode by an external circuit, protons travel from the anode to the cathode typically through a separator. At the cathode, electrons and protons combine to form water according to the schematic representation showed in Figure 1. Electricity can be produced from a single substrate (e.g., acetate, lactate or glucose), but what is truly innovative is that electricity can also be generated from complex substrates such as domestic sewage and industrial wastewater [2; 3]. When these complex substrates are used to feed MFCs, this technology offers the two-fold benefit of energy production and wastewater treatment. Compared with other available options for wastewater treatment, MFCs offer prominent advantages. These include direct electricity generation, energy savings due to the lack of aeration sources and low sludge production compared with conventional wastewater treatment with no downstream processes. Regarding this, nearly half of the cost of current standard wastewater processes is due to the energy input, especially for the needs of aeration and the treatment of the sludge generated during the treatment process [4]. Other benefits are operational advantages such as the possibility of using many types of wastes, low operational costs and resistance to environmental stress [5; 6]. MFCs usually include a separator between the anode and the cathode that prevents short circuits due electrode separation and improves Coulombic efficiency (CE) since it hinders the diffusion of oxygen to the anode [7; 8]. Very recently, polymer inclusion membranes based on ionic liquids have shown to be efficient separators in MFCs acting as proton exchange membranes. These membranes showed a good performance when compared with conventional materials such as Nafion and their cost are lower compared with this commercial membrane [9; 10; 11]. Ionic liquids (ILs) are organic salts that remain liquid near room temperature. They consist of an organic cation such as imidazolium, pyridinium, pyrrolinium, phosphonium, ammonium, and a polyatomic inorganic anion or organic anion such us tetrafluoroborate, hexafluorophosphate, chloride, trifluoromethylsulfonate, and bis[(trifluoromethyl)sulfonyl)imide]. Some of their advantages include task-specific functionality since these compounds can be tailored by selecting the proper cations and anions in their structure for a given purpose. They generally have high viscosities, negligible vapor pressure and non-flammability in addition to good electrochemical features such as high ion conductivity and wide electrochemical stability window [12]. Due to these properties, it can be adventurous to use ionic liquid-based separators for wastewater treatment in MFCs. However, the study of the performance of MFC devices including such types of membranes is scarce in the literature. Several types of complex substrates have been investigated for bioenergy generation in these devices, including domestic, food processing and swine wastewater [13; 14; 15; 16; 17; 18]. Brewery wastewater of high organic matter loading has been a preferable option for feeding MFCs.