The performance of a single-chamber microbial fuel cell (SCMFC), with air cathodes surrounding a series of graphite discs anodes, is reported. The materials used in the MFC are low-cost options for its construction. The air cathode, made from an activated carbon, was supported on a low-cost polypropylene/silica composite membrane. The system was operated in continuous mode and fed with wastewater from the primary clarifier of a wastewater treatment plant. The optimal external load was 100 Ω, with a current output of 2.2 mA and a power density of 50 mW m−2, defined with regard to the anode surface area. In the treatment of the wastewater, the chemical oxygen demand (COD) loading rate was a key factor in the performance of the SCMFC: lower organic loadings gave higher Coulombic efficiency with a maximum of 63.4 ± 4.2% when the system was fed with 0.055 kg COD m−3 d−1, meaning that the electro-active biofilm was responsible for the majority of the COD removal. The performance was compared with an alternative anode configuration made of graphite granules. The use of granules led to a better treatment of the wastewater but did not improve the performance of the SCMFC with regard to the current and power output. In particular, the power output was approximately three times lower compared to the graphite disk anode (0.17 mW vs 0.484 mW), and the maximum Coulombic efficiency was only 28%.